Muscle insulin resistance: assault by lipids, cytokines and local macrophages

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Muscle insulin resistance: assau
lt by lipids, cytokines and
local macrophagesGirish Kewalramani�, Philip J. Bilan� and Amira Klip

Cell Biology Program, The Hospital for Sick Children,Toronto, Ontario, Canada

Correspondence to Amira Klip, PhD, Cell BiologyProgram, The Hospital for Sick Children, 555University Avenue, Toronto, ON M5G 1X8, CanadaTel: +1 416 813 6392; fax: +1 416 813 5028;e-mail: [email protected]

�Girish Kewalramani and Philip J. Bilan contributed

equally to the writing of the article.

Current Opinion in Clinical Nutrition and

Metabolic Care 2010, 13:382–390

Purpose of review

The present review outlines possible mechanisms by which high fatty acids, associated

with high-fat diet and obesity, impose insulin resistance on glucose uptake into skeletal

muscle.

Recent findings

It is well established that muscle insulin resistance arises in conditions of high-fatty acid

availability, and correlates with accumulation of triglycerides within skeletal muscle

fibres. However, it is debated whether triglycerides or other lipid metabolites such as

diacylglycerols and ceramides are directly responsible. These lipid metabolites can

activate serine kinases that impair insulin signalling. Accumulation of acylcarnitines and

reactive oxygen species could be additional causative agents of insulin resistance.

Further, the precise defects in insulin signalling in muscle caused by high intramuscular

lipid (i.e. lipotoxicity) remain unclear. In parallel, proinflammatory activation within the

adipose tissue of obese and high-fat fed animals or humans causes muscle insulin

resistance, and is ascribed to circulating inflammatory cytokines. Recent evidence also

shows proinflammatory macrophages infiltrating muscle tissue and/or intermuscular

adipose tissue, and there is growing evidence that fatty acids trigger macrophages to

secrete factors that directly impair insulin actions. These factors are postulated to

activate stress-signalling pathways in muscle that act on the same insulin-signalling

components affected by lipotoxicity.

Summary

Altered intramuscular lipid metabolism, circulating cytokines, and inflammatory

macrophage infiltration of muscle tissue have been recently linked to muscle insulin

resistance provoked by fatty acids. Each is analysed separately in this review, but they

may act simultaneously and synergistically to render skeletal muscle insulin-resistant.

Keywords

cytokines, fatty acids, macrophages, muscle inflammation, muscle insulin resistance

Curr Opin Clin Nutr Metab Care 13:382–390� 2010 Wolters Kluwer Health | Lippincott Williams & Wilkins1363-1950

Introduction

Skeletal muscle is the primary site of dietary glucose

disposal in vivo, and muscle insulin resistance is an

obligatory element of the metabolic syndrome [1].

High-fat feeding is a major cause of insulin resistance

that is characterized by inability of muscle to store

carbohydrate, along with inability of the adipose tissue

and liver to store fat and curb glucose output, respect-

ively. Insulin stimulates glucose uptake into muscle

through an elaborate signal transduction cascade that

mobilizes vesicles containing glucose transporter 4

(GLUT4) to the muscle surface [2]. A body of research

proposes that abnormal handling of lipid metabolites

contributes to muscle insulin resistance [3,4,5��]. A paral-

lel school proposes a contribution from circulating inflam-

matory cytokines to muscle insulin resistance [6–9]. This

is based on the findings that immune cells infiltrate

opyright © Lippincott Williams & Wilkins. Unautho

1363-1950 � 2010 Wolters Kluwer Health | Lippincott Williams & Wilkins

adipose tissue during high-fat feeding, and these cells

become locally activated to initiate a proinflammatory

cross-talk cycle with adipocytes. The result of this low-

grade inflammation in adipose tissue is secretion to the

circulation of macrophage inflammatory cytokines, along

with free fatty acids (FAs) produced by uncontrolled

lipolysis in fat cells [7,10]. Skeletal muscle thus becomes

a target of both FAs and cytokines. Finally, recent studies

also show that macrophages directly infiltrate skeletal

muscle, potentially contributing to local inflammation of

this tissue [11��]. We analyse these three mechanisms

whereby high-fat feeding causes insulin resistance:

altered intramuscular lipid metabolism, circulating cyto-

kine and macrophage infiltration of muscle tissue.

Whereas they are treated separately, it is clear that they

may act simultaneously and synergistically to render

glucose uptake into skeletal muscle resistant to insulin,

and consequently, to bring about whole-body insulin

rized reproduction of this article is prohibited.

DOI:10.1097/MCO.0b013e32833aabd9

mailto:[email protected]

http://dx.doi.org/10.1097/MCO.0b013e32833aabd9

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Muscle insulin resistance Kewalramani et al. 383

resistance. We emphasize the recent literature in this

analysis, and existing review articles are quoted for

succinctness.

Fatty acid entry and dynamic storage asintra-myocellular triglyceridesDuring prolonged dietary excess, adipose tissue exceeds

its storage capacity and loses its sensitivity to insulin-

mediated suppression of lipolysis. Hence FAs are

released to the circulation and accumulate in nonadipose

tissues, primarily skeletal muscle [12,13]. This increased

lipid accumulation is not passive, since FA uptake is

elevated and this is associated with increased expression

of fatty acid transporter proteins CD36 and FATP1 and

acyl-CoA synthetase activity [14–16].

Within muscle cells, FAs become activated to acyl-CoA

derivatives and undergo several fates. A major portion

then becomes esterified as triglycerides. Intra-myocellu-

lar triglyceride (IMTG) accumulation tracks with insulin

resistance in rodents and humans [4,17,18]. Although

earlier studies suggested that IMTG may be a cause of

insulin resistance, more recent studies show that they

represent a storage form for FAs, and that other fat

metabolites such as diacylglycerols (DAG), ceramides,

long-chain fatty acyl CoAs (LCFA-CoA) and acylcarni-

tine products of incomplete FA oxidation, are more

directly responsible for insulin resistance [5��]. IMTG

is thus a dynamic store that can lower the level of these

metabolites, as well as break down into FA for energy

procurement.

Supporting the concept that IMTG is not a direct cause of

insulin resistance are studies that have manipulated both

the rate of IMTG synthesis and the rate of FA oxidation

to change IMTG levels.

Intra-myocellular triglyceride synthesis

Overexpression in muscle of diacylglycerol acyltransfer-

ase-1 (DGAT1) drives IMTG accumulation yet it pro-

motes insulin sensitivity, and concomitantly reduces the

levels of DAG and ceramides and increases FA oxidation

efficiency. Conversely, DGAT1 knockout mice have

lower IMTG, higher DAG and ceramide levels and are

more susceptible to high-fat diet-induced insulin resist-

ance [19,20,21�]. Understanding the underlying mechan-

isms is of great interest, because this model is in line with

the ‘athletes’ paradox’ whereby endurance athletes have

high-insulin sensitivity in muscle along with higher oxi-

dative capacity and elevated IMTG levels [22].

Fatty acid oxidation

During high-fat diets, reduced FA oxidation is suggested

from the observed lower expression of genes regulating

mitochondrial content and function such as peroxisome

opyright © Lippincott Williams & Wilkins. Unauth

proliferator-activated receptor-g coactivator-1a (PGC-

1a) [23–25]. Although these findings have led to the

suggestion that reduced FA oxidation is causally related

to insulin resistance, this thinking has been challenged.

In fact, recent studies report elevations in muscle FA

oxidation, in several oxidative enzyme activities, protein

expression of PGC-1a, and mitochondrial respiratory

chain subunits [26–28]. The reason for the discrepancies

among studies regarding the response of mitochondrial

activity to obesity/high-fat diet is unclear, but differences

as dietary composition, diet duration and muscle type

examined are important variables to consider.

In summary, lipid accumulation inside muscle cells is

coincident with the development of muscle insulin resist-

ance though the causative forms are still debated, as is the

intramuscular mechanism whereby they interfere with

insulin action (see next section).

Diacylglycerols, ceramides and acylcarnitinesas generators of muscle insulin resistanceWhen not stored as IMTG, acylated FA can esterify into

DAG, metabolize into ceramides (only in the case of

saturated FA), or enter b-oxidation as acylcarnitine con-

jugates [29]. If the latter process is incomplete, acylcar-

nitines of diverse size will accumulate [30]. Each of these

forms has recently been considered as a viable causative

agent of insulin resistance. In particular, some FA metab-

olites have been linked to interruptions in insulin signal-

ling at distinct levels, and to consequently reducing

insulin-dependent GLUT4 translocation that mediates

stimulation of glucose uptake. In muscle cells this entails

insulin signalling through insulin receptor substrate-1

(IRS1), phosphatidylinositol 3-kinase and bifurcation

towards Akt and Rac activation [2,31–33]. Activated

Akt phosphorylates the Rab-GAP AS160 allowing signal

transmission via Rabs (Rab8A in muscle cells). Together

with Rac activation and consequent actin remodelling,

both signalling arms are required to induce GLUT4

translocation and glucose uptake [2,31–33]. We briefly

discuss next the FA metabolites that have been proposed

to interfere with this signalling cascade.

Diacylglycerols and protein kinase Cs

Excess LCFA-CoA in muscle cells from high-fat-fed

animals esterify to glycerol-3-phosphate to generate

DAG [34]. DAG accumulates in muscles of rodent

models of high-fat-induced insulin resistance and of

obese individuals [18]. In muscle, DAG can activate both

conventional and novel protein kinase C (PKC), and this

activation is associated with inhibition of early steps in

the insulin signal cascade [3,35,36]. In particular,

increased activity of the novel-type PKCu and PKCecorrelates with inhibition of muscle insulin action in

response to high-fat feeding and elevated FA [37–39],

orized reproduction of this article is prohibited.

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384 Genes and cell metabolism

and PKCu knockout mice are protected from muscle

insulin resistance induced by lipid infusion [40]. Both

PKCu and PKCe act upstream of the stress kinases IkBa

kinase b (IKKb) and c-Jun NH2-terminal kinase (JNK),

and through them mediate phosphorylation of IRS1 on

Ser307 [35]. Activated PKCu also directly phosphorylates

muscle IRS1 at Ser1101 [41]. Phosphorylation of IRS1 at

these serine residues reduces its tyrosine phosphorylation

and downstream propagation of the insulin signal. In

addition, PKCe may also promote the degradation of

insulin receptors [42].

Despite ample evidence for a role of novel PKC in

triggering impaired insulin-stimulated glucose uptake

in muscle, several studies could not verify predictions

of this hypothesis [35,43,44�], specifically when expres-

sing a dominant-negative mutant of PKCu or when

infusing lipid and insulin acutely. Further, impaired

insulin signalling at the level of IRS1, widely held as

the cause of insulin resistance, may not necessarily

translate into downstream insulin resistance. Indeed, in

an oxidative stress model of insulin resistance, the defect

in IRS1 was corrected by several manipulations, yet

downstream insulin resistance persisted [45]. Moreover,

various conditions evoking insulin resistance at the level

of IRS1 and GLUT4 translocation also cause resistance

to platelet-derived growth factor (PDGF)-stimulated

GLUT4 translocation, yet PDGF does not signal via

IRS [46]. Finally, knock-in mice expressing alanine

instead of serine at position 307 of IRS1 developed more

rather than less insulin resistance upon high-fat feeding

[47�]. These findings suggest that insulin resistance of

GLUT4 translocation may be elicited by actions on steps

other than IRS1. Thus, additional research is needed to

solidify the connections between DAG, PKC and lipid-

induced muscle insulin resistance.

Ceramides

Ceramides can be generated by hydrolysis of plasma

membrane sphingomyelin or by de-novo synthesis from

long-chain saturated FAs [29]. Numerous studies have

implicated ceramides in the development of impaired

insulin-stimulated muscle glucose uptake and indicate

that these lipid species act downstream of IRS1 to reduce

muscle glucose uptake [48,49�]. Ceramides can impair

insulin signalling in muscle by reducing Akt activation

[50,51]. Possible explanations are the ceramide-mediated

activation of protein phosphatases that dephosphorylate

Akt, and the atypical PKCz-mediated phosphorylation of

the pleckstrin homology domain of Akt, precluding Akt

migration to the plasma membrane to become activated

[50,52,53]. However, the moderate reductions observed

in Akt activity may not be the cause of insulin resistance

of muscle glucose uptake, since GLUT4 translocation

can proceed with as little as 20% of Akt activation [54].

This suggests that targets other than Akt may contribute


to the ceramide-induced insulin resistance. In this regard,

the cell-permeating short chain C2-ceramide inhibits

insulin-dependent Rac activation and its consequent

actin remodelling in cultured L6 myotubes, which is

essential for GLUT4 translocation [55]. Interestingly,

muscle ceramide levels are only elevated when muscle

is exposed to saturated FA (palmitate) and not unsatu-

rated FA (linoleate), and inhibition of ceramide biosyn-

thesis via serine palmitoyl transferase with myriocin

prevents palmitate-induced insulin-resistant glucose

uptake, but not the insulin resistance caused by linoleate

[48]. Compellingly, treatment of Zucker diabetic fatty

rats with myriocin prevented the onset of diabetes,

correlating with lower ceramide levels in muscle [48].

Similarly, in L6 myotubes the palmitate-induced

reduction in insulin-stimulated Akt and glucose uptake

was relieved by myriocin, which also reduced ceramide

levels. Surprisingly, longer treatment with myriosin

diverted palmitate towards DAG synthesis and reduced

IRS1-directed insulin signalling [49�]. Insulin resistance

could also be relieved upon incubating human muscle

cells with palmitate along with the unsaturated FA oleate,

which redirected palmitate away from ceramides to

IMTG storage [56]. These results suggest that palmitate

generates insulin resistance primarily via ceramides, pro-

vided its metabolism is not diverted to DAG or IMTG.

Whether ceramides directly influence insulin sensitivity in

human skeletal muscle in vivo remains unclear [57–59].

In summary, ceramide build-up may be a contributor to

insulin resistance as a result of a diet high in saturated

FAs and further studies should clarify the influence of

specific FAs on muscle ceramide content and insulin

resistance and their mode of action.

LCFA-CoAs and acylcarnitines

In addition to generating IMTG, DAG and ceramides,

the high availability of acyl-CoAs in muscle of high-fat-

fed animals may induce upregulation of b-oxidative

metabolism. However, downstream pathways such as

the TCA cycle or the electron transport chain activity

may not increase accordingly, eventually leading to the

accumulation of LCFA-CoAs and acylcarnitine conju-

gates that were not completely oxidized [5��]. Indeed,

muscle and serum samples from obese humans or high-

fat-fed rats have increased levels of acylcarnitines [5��]

and muscle also exhibits elevated levels of LCFA-CoAs

in conjunction with muscle insulin resistance [60�,61],

but the latter lipid species may impair muscle insulin

action indirectly by acting as precursors to other lipid

intermediates such as ceramides and DAG. To date, a

direct target of LCFA-CoAs in the insulin-signalling

pathway has not been identified. Whether oxidative

metabolism is slowed down in insulin resistance or is

insufficient to handle the excess LCFA-CoA, the

improved muscle insulin action in rat muscle with modest


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overexpression of carnitine palmitoyltransferase-1 pro-

vides proof-of-concept that FA oxidation is a suitable

target to improve muscle insulin action [60�]. Indeed,

carnitine supplementation itself can improve insulin

action in high-fat-fed animals, suggesting that carnitine

availability may be limiting for appropriate fat oxidation

[5��]. It is unknown whether acylcarnitines themselves

are toxic or whether the incomplete FA oxidation

generates oxidative radicals that on their own intersect

with the insulin-signalling cascade [62]. In this regard, it

is interesting that direct exposure of muscle cells to low

levels of peroxide causes insulin resistance that in the

short term affects insulin-induced Akt phosphorylation

and Rac activation [55], and in the long term reduces

tyrosine phosphorylation of IRS1 [63��].

In summary, FAs induce muscle cell-autonomous

defects that may be mediated by aggregate changes in

the levels of DAG, ceramide, LCFA-CoA, acylcarnitines

and possibly oxidative radicals, each of which has the

capacity to interfere with distinct steps in the insulin-

signalling cascade, reducing GLUT4 translocation to the

muscle membrane.

Inflammatory cytokines propagating muscleinsulin resistanceIn addition to muscle cell-autonomous metabolic

responses, FAs trigger activation of inflammatory signals

in adipocytes, in innate immune system cells such as

macrophages [64] and even in muscle cells (reviewed in

[54]), leading to the release of proinflammatory cytokines

directly from the targeted cells. Cytokines can impact on

insulin action, and tumour necrosis factor-a (TNFa),

interleukin (IL)-6 and IL-10 affect insulin responses

directly in skeletal muscle. In vivo, each cytokine may

act directly as well as through other tissues to secondarily

affect insulin action in skeletal muscle. Moreover, the

balance between proinflammatory and anti-inflammatory

cytokines may well be fundamental in the development

of muscle insulin resistance. However, to dissect the

direct effects of cytokines on muscle, it is easier to rely

on studies in cell cultures, in order to eliminate contri-

bution from other tissues on which the cytokine may also

act. The effects of TNFa, IL-6 and IL-10 are briefly

reviewed next for their potential action to modulate

insulin-dependent GLUT4 translocation in muscle cells

in culture, so as to evaluate their potential direct action on

this tissue. More comprehensive reviews on the topic of

cytokine action on skeletal muscle are recommended

[11��,65,66].

Tumour necrosis factor-a

Through stimulation of PKCu and activation of nuclear

factor kappa-light-chain-enhancer of activated B cells

(NF-kB), palmitate augments TNFa gene expression


in muscle cells [67,68]. Exposure of primary human

muscle cell cultures to TNFa causes insulin resistance

of glucose uptake [69,70]. This may be mediated by

several mechanisms, including activation of extracellular

signal-regulated kinase-1/2 (ERK1/2), JNK, or the NF-

kB pathway. Indeed, silencing IkBa kinase b (IKKb)

prevented TNFa-induced alterations in Akt, AS160 and

glucose transport [69], as did RNAi-mediated knockdown

of MAP4K4 (which lies downstream of the TNFa recep-

tor and upstream of JNK and ERK1/2) [70]. The negative

effect of TNFa on muscle insulin signalling might also

be attributed to activation of sphingomyelinase which

produces ceramides [71], or to reducing expression of

sortilin [72], a protein that appears to control GLUT4

sorting in adipocytes [73]. Collectively, these studies

suggest that TNFa is elevated in muscle tissue of obese

humans and rodents and induces muscle insulin resist-

ance via multiple mechanisms, possibly acting in an

autocrine or paracrine manner to alter muscle insulin-

stimulated glucose uptake.

Interleukin-6

In mouse C2C12 myoblasts, palmitate activates NF-kB

and augments IL-6 production in parallel to reducing

insulin-dependent glucose uptake, and notably these

effects are reversed upon addition of anti-IL-6 antibody

to the cultures [74]. It has been argued that acutely, IL-6

may have insulin-potentiating actions on muscle glucose

uptake, but may cause insulin resistance upon prolonged

exposure [75]. The positive effects were ascribed to

phosphorylation of AS160 via AMPK, and the negative

ones to activation of JNK1/2, suppressor of cytokine

signalling-3 expression and activation of the IRS1 tyro-

sine phosphatase, protein tyrosine phosphatase-1B. How-

ever, in vivo the distinct effects of IL-6 are less clearly

segregated. Human muscle cells in culture produce con-

siderable amounts of IL-6 in response to palmitate, but

recombinant IL-6 polypeptide did not alter insulin sig-

nalling and glycogen synthesis in these cultures [76,77].

The possibility remains that IL-6 may need to interact

with other proinflammatory and anti-inflammatory cyto-

kines to modulate muscle insulin action. In rats, sustained

systemic IL-6 administration enhanced muscle insulin

signalling at the level of IRS1 and Akt [78], whereas

overexpression of IL-6 in mouse skeletal muscle for

several days by electroporation gene transfer impaired

insulin-stimulated muscle glucose uptake [79]. Overall,

an improved understanding of IL-6 in muscle insulin

resistance may lead to a better strategy to enhance

insulin sensitivity.

Interleukin-10

IL-10 is a classical anti-inflammatory cytokine expressed

in muscle that can modulate lipid-induced insulin resist-

ance. IL-10 co-treatment protects mice from lipid and

IL-6-induced muscle insulin resistance [80]. More


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specifically, IL-10 partially prevents the harmful effects

that conditioned media from palmitate-treated macro-

phages has on insulin-induced GLUT4 translocation

in L6 muscle cells [81]. Along with in-vivo studies of

IL-10 administration [80] or transgenic overexpression

in muscle [82�], these studies support the hypothesis

that IL-10 reduces insulin resistance in muscle cells;

however, the major cellular targets (muscle or macro-

phages) and the intramuscular mechanisms responsible

for this effect remain uncertain and should be investi-

gated.

In summary, TNFa, IL-6 and IL-10 impact directly on

muscle cells to alter insulin signalling and the stimulation

of glucose uptake. Whereas TNFa generally has negative

effects and IL-10 has positive actions, the response to

IL-6 varies depending on dose and time of exposure. The

studies summarized illustrate the potential of muscle

cells in culture to respond to cytokines, which in vivomay stem from inflamed adipose tissue, innate immune

cells or even from autonomous production by the muscle

cells themselves.

Role of macrophages in skeletal muscleinsulin resistanceAs in other organs, resident macrophages are an essential

component of skeletal muscle. Major positive functions

of macrophages in muscle include contribution to regen-

eration and revascularization when in their M2 (anti-

inflammatory) phenotypic polarization [11��]. However,

macrophages can also become M1 polarized (inflamma-

tory), and if this process occurs within the immediate

vicinity of muscle fibres, there is the potential for local,

paracrine action of macrophage-released cytokines onto

the muscle cells. We and others have hypothesized that

this may occur in response to circulating FA, activating

macrophages locally and creating local inflammation of

skeletal muscle, akin to that occurring in adipose tissue

[11��,54]. Such macrophages may abut the skeletal

muscle fibres directly, or may be drawn into the tissue

by infiltrating adipose cells in the context of obesity.

Indeed, adipocytes can be found between muscle bun-

dles, and high-circulating FA and obesity provoke expan-

sion of this depot [83]. We review next the emerging

evidence that macrophages infiltrate skeletal muscle

during obesity, their possible proximity to muscle tissue

adipocytes, and the possible cross-talk that may occur

between macrophages and skeletal muscle cells in the

context of FA administration.

Activated macrophages within the muscle bed

Weisberg et al. [84] first reported a three-fold increase in

macrophages in muscle tissue adipose depots of obese

mice, by immuno-histochemical detection of the macro-

phage marker F4/80. Similarly, Nguyen et al. [85] used


fluorescence-activated cell sorting analysis to show

increased numbers of M1-activated (CD11c-positive)

macrophages in muscle tissue from mice fed a high-fat

diet. Interestingly, conditional ablation of CD11c-

positive macrophages in mice improved whole-body

and muscle insulin sensitivity during high-fat feeding

and, compellingly, lowered the CD11c signal and

reduced mRNA expression of inflammatory markers

(TNFa, CD68 and monocyte chemoattractant protein-

1) within muscle tissue [86]. Further, mice with deletion

of the anti-inflammatory gene PPARg in macrophages

showed elevated CD11c and F4/80 signal in muscles from

mice fed a high-fat diet, concomitant with impaired

muscle insulin signalling [87]. Conversely, transgenic

expression of IL-10 within muscle reduced the rise in

macrophage markers in this tissue of mice fed a high-fat

diet [82�]. In humans, Varma et al. [88��] documented an

increased infiltration of macrophages in skeletal muscle

of obese insulin resistant individuals. In spite of this

collective evidence, two studies found very low expres-

sion of macrophage-specific markers (CD68 and CD14)

in human muscle biopsies from severely obese patients

[89,90], and the mRNA levels of these markers in muscle

did not change when these individuals underwent a

lifestyle intervention programme that improved insulin

action [89]. Nonetheless, the majority of studies to date,

using distinct and quantitative approaches, support the

notion that activated macrophages increase within

muscle from insulin resistant rodents and humans.

The above studies, whereas correlating the presence of

activated macrophages in muscle with muscle insulin

resistance in conditions of high-fat feeding, do not rule

out contribution of macrophage action within adipose

tissue that secondarily affects muscle insulin sensitivity.

In-vitro studies could add proof-of-concept that FAs

activate macrophages to secrete cytokines that in turn

render muscle cells resistant to insulin. To this end, we

showed an M1-type activation of murine RAW264.7

macrophages exposed to palmitate for 6 h. Medium col-

lected subsequently from such preactivated macrophages

rendered L6 myoblasts resistant to insulin. Although this

conditioned medium contained TNFa and IL-6, neither

cytokine alone was responsible for the resistance, which

was manifest as reduced GLUT4 translocation to the cell

membrane with concomitant reduction of the stimulation

of glucose uptake [81]. Insulin-dependent activation of

Akt [81] and of Rac (unpublished observation) was

diminished, likely contributing to the dampened

GLUT4 translocation. Using a more elaborate in-vitro

system of co-culture of primary human skeletal muscle

myotubes with THP-1 macrophages, Varma et al. [88��]

showed that palmitate elevated the expression of a

variety of cytokines and chemokines (TNFa, IL-6,

IL-10, IL-1b and MCP-1) in the myotubes, caused IkBa

protein degradation, elevated JNK phosphorylation and


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diminished insulin-dependent phosphorylation of Akt.

Toll-like receptors have been suggested as FA sensors

that trigger immune responses in macrophages, but what

about direct FA effects on lipid metabolism? A recent

study suggests that lipotoxicity can be relieved in macro-

phages by DGAT1 overexpression, blunting the proin-

flammatory M1 activation of macrophages exposed to

excess levels of fatty acids [91�].


Figure 1 Lipotoxicity, cytokines and macrophage inflammatory respo

uptake

High-fat diet leads to obesity due to excess storage of lipid in adipose tissueproinflammatory responses from resident tissue macrophages and recruitmeproinflammatory macrophages and stressed adipocytes release TNFa, IL-6 aFA, these secreted cytokines impinge on muscle to cause insulin resistance, snumber of adipocytes and macrophages may increase in the muscle tissue beand FA release [83]. Insulin resistance of muscle glucose uptake may resultlong-chain fatty acyl-CoA (LCFA-CoA) and their metabolites diacylglycerolsLCFA-CoA positively correlate with obesity and insulin resistance, its cause mleading to glucose uptake at the level of IRS1 or Akt, respectively, or by additiPKC isoforms to engage IKKb and JNK which phosphorylate IRS1 on S307 odownstream effectors [35,41]. Ceramides are thought to act through protein(e.g. TNFa and IL-6) act on muscle cell surface receptors to activate IKKb

In summary, whereas there is growing support for the

infiltration or activation of macrophages within the

muscle bed, and in-vitro studies support a FA-mediated

negative cross-talk from macrophages towards muscle

cells, thorough examination of this phenomenon in

obesity is needed, as is exploration of its causes and

consequences towards inflammation and possible contri-

bution to muscle insulin resistance in vivo.


nses may conspire to cause muscle insulin resistance of glucose

that elicits increased fatty acid (FA) release from adipocytes, initiation ofnt of macrophages from the circulation (as monocytes) [92]. Activatednd other cytokines and adipokines to the circulation [92]. Together withpecifically impaired insulin-stimulated glucose uptake [7]. In addition, thetween fibre bundles and establish microenvironments of proinflammationfrom increased accumulation of FA as intramuscular triglyceride (IMTG),(DAGs), ceramides, and acylcarnitines Whereas the levels of IMTG anday lie with the metabolites DAG and ceramide that impair insulin signallingonal means (dotted arrow). DAG is thought to act primarily through novelr PKC may directly phosphorylate IRS1 on S1101 reducing signalling tophosphatase 2A or PKCz (not shown) [53,93] and Rac [55]. Cytokines

and JNK; TNFa can also activate MAP4K4 to induce insulin resistance.

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ConclusionSubstantial evidence supports the concept that FA, cyto-

kines and macrophages each can directly induce insulin

resistance in isolated muscle cells. The underlying pro-

posed mechanisms are summarized briefly in Fig. 1,

which also shows that in the context of the whole body,

FA, cytokines and macrophages likely act simultaneously

and synergistically to negatively impact on the muscle

insulin response.

With this exciting background, new questions arise, and

some are amenable to scrutiny through in-vitro exper-

imental systems, for example: What are the specific

effects that FA alongside macrophage products have

on the insulin sensitivity of muscle cells? Do proinflam-

matory factors affect lipid metabolism within muscle

cells, elevating deleterious metabolite levels and reactive

oxygen species, emulating lipotoxicity? Conversely, do

FAs cause a state of lipotoxicity within innate immune

cells? Do cytokines activate elements of the inflamma-

tory pathways within muscle cells to produce autocrine/

paracrine cycles that propagate insulin resistance? What

specific macrophage factors cause insulin resistance in

muscle cells? Which are the steps in insulin signalling

responsible for the reduction in insulin-stimulated glu-

cose uptake when IRS1 and Akt cannot fully account for

this defect?

In vivo, a key unanswered question is whether proin-

flammatory factors reach muscle through the circulation

or are they also produced locally by intermuscular macro-

phages and adipocytes. As well, recognizing that in the

whole body there is interplay of several tissues linking

inflammation and insulin resistance, do FAs potentiate

the inflammatory response of innate immune cells to

pathogens? The answers to these and other questions

will be useful for the treatment of insulin resistance and

other obesity-linked disorders.

AcknowledgementsWe thank Dr M. Constantine Samaan for helpful discussions.

Dr Klip’s laboratory is supported by grants from the Canadian DiabetesAssociation and the Canadian Institutes of Health Research (MOT12601).

References and recommended readingPapers of particular interest, published within the annual period of review, havebeen highlighted as:� of special interest�� of outstanding interest

Additional references related to this topic can also be found in the CurrentWorld Literature section in this issue (p. 491).

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A broad-ranging review of lipid metabolism in muscle and various causes of insulinresistance by fatty acid excess including a good description of acylcarnitineaccumulation and their role in lipotoxicity.

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7 Guilherme A, Virbasius JV, Puri V, Czech MP. Adipocyte dysfunctions linkingobesity to insulin resistance and type 2 diabetes. Nat Rev Mol Cell Biol 2008;9:367–377.

8 Zeyda M, Stulnig TM. Obesity, inflammation, and insulin resistance: a mini-review. Gerontology 2009; 55:379–386.

9 Hotamisligil GS. Inflammation and metabolic disorders. Nature 2006; 444:860–867.

10 Schenk S, Saberi M, Olefsky JM. Insulin sensitivity: modulation by nutrientsand inflammation. J Clin Invest 2008; 118:2992–3002.

11

��Olefsky JM, Glass CK. Macrophages, inflammation, and insulin resistance.Annu Rev Physiol 2010; 72:219–246.

An exceptional review of the role of innate immunity in inflammation and in insulinresistance observed in obesity. Includes discussion of how proinflammatorymacrophages interact with muscle.

12 Storlien LH, Jenkins AB, Chisholm DJ, et al. Influence of dietary fat composi-tion on development of insulin resistance in rats. Relationship to muscletriglyceride and omega-3 fatty acids in muscle phospholipid. Diabetes 1991;40:280–289.

13 Kraegen EW, Clark PW, Jenkins AB, et al. Development of muscle insulinresistance after liver insulin resistance in high-fat-fed rats. Diabetes 1991;40:1397–1403.

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15 Marotta M, Ferrer-Martnez A, Parnau J, et al. Fiber type- and fatty acidcomposition-dependent effects of high-fat diets on rat muscle triacylglycerideand fatty acid transporter protein-1 content. Metabolism 2004; 53:1032–1036.

16 Cameron-Smith D, Burke LM, Angus DJ, et al. A short-term, high-fat diet up-regulates lipid metabolism and gene expression in human skeletal muscle. AmJ Clin Nutr 2003; 77:313–318.

17 Kiens B. Skeletal muscle lipid metabolism in exercise and insulin resistance.Physiol Rev 2006; 86:205–243.

18 Moro C, Galgani JE, Luu L, et al. Influence of gender, obesity, and musclelipase activity on intramyocellular lipids in sedentary individuals. J ClinEndocrinol Metab 2009; 94:3440–3447.

19 Watt MJ. Storing up trouble: does accumulation of intramyocellular triglycer-ide protect skeletal muscle from insulin resistance? Clin Exp PharmacolPhysiol 2009; 36:5–11.

20 Liu L, Zhang Y, Chen N, et al. Upregulation of myocellular DGAT1 augmentstriglyceride synthesis in skeletal muscle and protects against fat-inducedinsulin resistance. J Clin Invest 2007; 117:1679–1689.

21

�Liu L, Shi X, Choi CS, et al. Paradoxical coupling of triglyceride synthesis andfatty acid oxidation in skeletal muscle overexpressing DGAT1. Diabetes 2009;58:2516–2524.

This work builds on the authors’ study in 2007 showing the DGAT1 enzyme isupregulated by endurance exercise training and DGAT1 overexpression leads toaccumulation of IMTG, yet not muscle insulin resistance. In this study, an un-expected role for DGAT1 overexpression to elevate muscle fatty acid oxidation isrevealed. This strongly links DGAT1 to the reduced lipotoxicity and improvedinsulin sensitivity brought on by exercise training.

22 Goodpaster BH, He J, Watkins S, Kelley DE. Skeletal muscle lipid content andinsulin resistance: evidence for a paradox in endurance-trained athletes. J ClinEndocrinol Metab 2001; 86:5755–5761.

23 Crunkhorn S, Dearie F, Mantzoros C, et al. Peroxisome proliferatoractivator receptor gamma coactivator-1 expression is reduced in obesity:potential pathogenic role of saturated fatty acids and p38 mitogen-activated protein kinase activation. J Biol Chem 2007; 282:15439–15450.

24 Sparks LM, Xie H, Koza RA, et al. A high-fat diet coordinately downregulatesgenes required for mitochondrial oxidative phosphorylation in skeletal muscle.Diabetes 2005; 54:1926–1933.


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25 Richardson DK, Kashyap S, Bajaj M, et al. Lipid infusion decreases theexpression of nuclear encoded mitochondrial genes and increases the ex-pression of extracellular matrix genes in human skeletal muscle. J Biol Chem2005; 280:10290–10297.

26 Turner N, Bruce CR, Beale SM, et al. Excess lipid availability increasesmitochondrial fatty acid oxidative capacity in muscle: evidence against a rolefor reduced fatty acid oxidation in lipid-induced insulin resistance in rodents.Diabetes 2007; 56:2085–2092.

27 Hancock CR, Han DH, Chen M, et al. High-fat diets cause insulin resistancedespite an increase in muscle mitochondria. Proc Natl Acad Sci U S A 2008;105:7815–7820.

28 Garcia-Roves P, Huss JM, Han DH, et al. Raising plasma fatty acid concen-tration induces increased biogenesis of mitochondria in skeletal muscle. ProcNatl Acad Sci U S A 2007; 104:10709–10713.

29 Consitt LA, Bell JA, Houmard JA. Intramuscular lipid metabolism, insulinaction, and obesity. IUBMB Life 2009; 61:47–55.

30 Koves TR, Ussher JR, Noland RC, et al. Mitochondrial overload and incom-plete fatty acid oxidation contribute to skeletal muscle insulin resistance. CellMetab 2008; 7:45–56.

31 Huang C, Thirone AC, Huang X, Klip A. Differential contribution of insulinreceptor substrates 1 versus 2 to insulin signaling and glucose uptake in l6myotubes. J Biol Chem 2005; 280:19426–19435.

32 Randhawa VK, Ishikura S, Talior-Volodarsky I, et al. GLUT4 vesicle recruitmentand fusion are differentially regulated by Rac, AS160, and Rab8A in musclecells. J Biol Chem 2008; 283:27208–27219.

33 Ishikura S, Klip A. Muscle cells engage Rab8A and myosin Vb in insulin-dependent GLUT4 translocation. Am J Physiol Cell Physiol 2008; 295:C1016–C1025.

34 Timmers S, Schrauwen P, de Vogel J. Muscular diacylglycerol metabolism andinsulin resistance. Physiol Behav 2008; 94:242–251.

35 Gao Z, Wang Z, Zhang X, et al. Inactivation of PKCtheta leads to increasedsusceptibility to obesity and dietary insulin resistance in mice. Am J PhysiolEndocrinol Metab 2007; 292:E84–91.

36 Ragheb R, Shanab GM, Medhat AM, et al. Free fatty acid-induced muscleinsulin resistance and glucose uptake dysfunction: evidence for PKC activa-tion and oxidative stress-activated signaling pathways. Biochem Biophys ResCommun 2009; 389:211–216.

37 Frangioudakis G, Cooney GJ. Acute elevation of circulating fatty acids impairsdownstream insulin signalling in rat skeletal muscle in vivo independent ofeffects on stress signalling. J Endocrinol 2008; 197:277–285.

38 Yu C, Chen Y, Cline GW, et al. Mechanism by which fatty acids inhibit insulinactivation of insulin receptor substrate-1 (IRS-1)-associated phosphatidyli-nositol 3-kinase activity in muscle. J Biol Chem 2002; 277:50230–50236.

39 Corcoran MP, Lamon-Fava S, Fielding RA. Skeletal muscle lipid depositionand insulin resistance: effect of dietary fatty acids and exercise. Am J Clin Nutr2007; 85:662–677.

40 Kim JK, Fillmore JJ, Sunshine MJ, et al. PKC-theta knockout mice are protectedfrom fat-induced insulin resistance. J Clin Invest 2004; 114:823–827.

41 Li Y, Soos TJ, Li X, et al. Protein kinase C theta inhibits insulin signaling byphosphorylating IRS1 at Ser(1101). J Biol Chem 2004; 279:45304–45307.

42 Ikeda Y, Olsen GS, Ziv E, et al. Cellular mechanism of nutritionally inducedinsulin resistance in Psammomys obesus: overexpression of protein kinasecepsilon in skeletal muscle precedes the onset of hyperinsulinemia andhyperglycemia. Diabetes 2001; 50:584–592.

43 Serra C, Federici M, Buongiorno A, et al. Transgenic mice with dominantnegative PKC-theta in skeletal muscle: a new model of insulin resistance andobesity. J Cell Physiol 2003; 196:89–97.

44

�Hoy AJ, Brandon AE, Turner N, et al. Lipid and insulin infusion-induced skeletalmuscle insulin resistance is likely due to metabolic feedback and not changesin IRS-1, Akt, or AS160 phosphorylation. Am J Physiol Endocrinol Metab2009; 297:E67–75.

In this study, lipid infusion of rats was performed while co-infusing insulin to mimicconditions of a meal. Whereas insulin resistance of muscle glucose uptake andglycogen synthesis occurred, insulin signalling was not affected as it can be withlipid infusion alone. Of the lipid metabolites, only LCFA-CoA accumulated in themuscle, whereas triglycerides, DAG and ceramide did not. An observed rise in theprotein levels of PDK4 would reduce pyruvate dehydrogenase activity and mayresult in reduced glucose uptake.

45 Potashnik R, Bloch-Damti A, Bashan N, Rudich A. IRS1 degradation andincreased serine phosphorylation cannot predict the degree of metabolicinsulin resistance induced by oxidative stress. Diabetologia 2003; 46:639–648.

46 Hoehn KL, Hohnen-Behrens C, Cederberg A, et al. IRS1-independent defectsdefine major nodes of insulin resistance. Cell Metab 2008; 7:421–433.


47

�Copps KD, Hancer NJ, Opare-Ado L, et al. Irs1 serine 307 promotes insulinsensitivity in mice. Cell Metab 2010; 11:84–92.

Phosphorylation of serine 307 on IRS1 by JNK has been shown to decreasepropagation of insulin signals through IRS1, in vitro, and this is often cited as thecause for insulin resistance via JNK in multiple tissues of various animal models ofhigh-fat induced insulin resistance. However, the current study demonstrates micewith homozygous knock-in mutations of alanine for serine 307 develop more severewhole-body, muscle and liver insulin resistance when placed on a high-fat diet.

48 Holland WL, Brozinick JT, Wang LP, et al. Inhibition of ceramide synthesisameliorates glucocorticoid-, saturated-fat-, and obesity-induced insulin resis-tance. Cell Metab 2007; 5:167–179.

49

�Watson ML, Coghlan M, Hundal HS. Modulating serine palmitoyl transferase(SPT) expression and activity unveils a crucial role in lipid-induced insulinresistance in rat skeletal muscle cells. Biochem J 2009; 417:791–801.

This study demonstrates that prolonged inhibition of serine palmitoyl transferase, akey enzyme in the synthesis of ceramide from palmitic acid, shunts FA metabolismtowards DAG accumulation in cultured muscle cells shifting the site of impairedinsulin signalling from Akt alone to IRS1. Overall, ceramide and DAG can beconsidered lipotoxic metabolites that cause muscle insulin resistance.

50 Thrush AB, Brindley DN, Chabowski A, et al. Skeletal muscle lipogenic proteinexpression is not different between lean and obese individuals: a potentialfactor in ceramide accumulation. J Clin Endocrinol Metab 2009; 94:5053–5061.

51 Summers SA. Ceramides in insulin resistance and lipotoxicity. Prog Lipid Res2006; 45:42–72.

52 Holland WL, Summers SA. Sphingolipids, insulin resistance, and metabolicdisease: new insights from in vivo manipulation of sphingolipid metabolism.Endocr Rev 2008; 29:381–402.

53 Powell DJ, Turban S, Gray A, et al. Intracellular ceramide synthesis and proteinkinase Czeta activation play an essential role in palmitate-induced insulinresistance in rat L6 skeletal muscle cells. Biochem J 2004; 382:619–629.

54 Bilan PJ, Samokhvalov V, Koshkina A, et al. Direct and macrophage-mediatedactions of fatty acids causing insulin resistance in muscle cells. Arch PhysiolBiochem 2009; 115:176–190.

55 JeBailey L, Wanono O, Niu W, et al. Ceramide- and oxidant-induced insulinresistance involve loss of insulin-dependent rac-activation and actin remodel-ing in muscle cells. Diabetes 2007; 56:394–403.

56 Pickersgill L, Litherland GJ, Greenberg AS, et al. Key role for ceramides inmediating insulin resistance in human muscle cells. J Biol Chem 2007;282:12583–12589.

57 Boden G. Ceramide: a contributor to insulin resistance or an innocentbystander? Diabetologia 2008; 51:1095–1096.

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59 Straczkowski M, Kowalska I, Baranowski M, et al. Increased skeletal muscleceramide level in men at risk of developing type 2 diabetes. Diabetologia2007; 50:2366–2373.

60

�Bruce CR, Hoy AJ, Turner N, et al. Overexpression of carnitine palmitoyl-transferase-1 in skeletal muscle is sufficient to enhance fatty acid oxidationand improve high-fat diet-induced insulin resistance. Diabetes 2009;58:550–558.

This study demonstrates overexpression of CPT1 in rat skeletal muscle improvesits capacity to oxidize FA when the animals were placed on a high-fat diet. Insulinsignalling is improved in the muscle, as is insulin-stimulated glucose uptake. DAGaccumulation and PKCu activity are reduced in the muscle. However, the levels ofacylcarnitines or the percentage of incomplete FA oxidation did not change withCPT1 overexpression. The results are consistent with increased FA oxidation bymuscle is a means to alleviate insulin resistance caused by high-fat feeding.

61 Wein S, Wolffram S, Schrezenmeir J, et al. Medium-chain fatty acids ame-liorate insulin resistance caused by high-fat diets in rats. Diabetes Metab ResRev 2009; 25:185–194.

62 Samocha-Bonet D, Heilbronn LK, Lichtenberg D, Campbell LV. Does skeletalmuscle oxidative stress initiate insulin resistance in genetically predisposedindividuals? Trends Endocrinol Metab 2010; 21:83–88.

63

��Bashan N, Kovsan J, Kachko I, et al. Positive and negative regulation of insulinsignaling by reactive oxygen and nitrogen species. Physiol Rev 2009; 89:27–71.

A broad-ranging review on the physiology and pathophysiology of ROS, thecellular mechanisms of how they are generated and degraded and how theyimpact insulin signalling.

64 Shi H, Kokoeva MV, Inouye K, et al. TLR4 links innate immunity and fatty acid-induced insulin resistance. J Clin Invest 2006; 116:3015–3025.

65 Sell H, Dietze-Schroeder D, Eckel J. The adipocyte-myocyte axis in insulinresistance. Trends Endocrinol Metab 2006; 17:416–422.


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66 Wei Y, Chen K, Whaley-Connell AT, et al. Skeletal muscle insulin resistance:role of inflammatory cytokines and reactive oxygen species. Am J PhysiolRegul Integr Comp Physiol 2008; 294:R673–680.

67 Jove M, Planavila A, Sanchez RM, et al. Palmitate induces tumor necrosisfactor-alpha expression in C2C12 skeletal muscle cells by a mechanisminvolving protein kinase C and nuclear factor-kappaB activation. Endocrinol-ogy 2006; 147:552–561.

68 Hommelberg PP, Plat J, Langen RC, et al. Fatty acid-induced NF-kappaBactivation and insulin resistance in skeletal muscle are chain length depen-dent. Am J Physiol Endocrinol Metab 2009; 296:E114–120.

69 Austin RL, Rune A, Bouzakri K, et al. siRNA-mediated reduction of inhibitor ofnuclear factor-kappaB kinase prevents tumor necrosis factor-alpha-inducedinsulin resistance in human skeletal muscle. Diabetes 2008; 57:2066–2073.

70 Bouzakri K, Zierath JR. MAP4K4 gene silencing in human skeletal muscleprevents tumor necrosis factor-{alpha}-induced insulin resistance. J BiolChem 2007; 282:7783–7789.

71 Haus JM, Kashyap SR, Kasumov T, et al. Plasma ceramides are elevated inobese subjects with type 2 diabetes and correlate with the severity of insulinresistance. Diabetes 2009; 58:337–343.

72 Kaddai V, Jager J, Gonzalez T, et al. Involvement of TNF-alpha in abnormaladipocyte and muscle sortilin expression in obese mice and humans. Diabe-tologia 2009; 52:932–940.

73 Shi J, Kandror KV. Sortilin is essential and sufficient for the formation of Glut4storage vesicles in 3T3-L1 adipocytes. Dev Cell 2005; 9:99–108.

74 Jove M, Planavila A, Laguna JC, Vazquez-Carrera M. Palmitate-inducedinterleukin 6 production is mediated by protein kinase C and nuclear-factorkappaB activation and leads to glucose transporter 4 down-regulation inskeletal muscle cells. Endocrinology 2005; 146:3087–3095.

75 Nieto-Vazquez I, Fernandez-Veledo S, de Alvaro C, Lorenzo M. Dual role ofinterleukin-6 in regulating insulin sensitivity in murine skeletal muscle. Dia-betes 2008; 57:3211–3221.

76 Weigert C, Brodbeck K, Staiger H, et al. Palmitate, but not unsaturated fattyacids, induces the expression of interleukin-6 in human myotubes throughproteasome-dependent activation of nuclear factor-kappaB. J Biol Chem2004; 279:23942–23952.

77 Roher N, Samokhvalov V, Diaz M, et al. The proinflammatory cytokine tumornecrosis factor-alpha increases the amount of glucose transporter-4 at thesurface of muscle cells independently of changes in interleukin-6. Endocri-nology 2008; 149:1880–1889.

78 Holmes AG, Mesa JL, Neill BA, et al. Prolonged interleukin-6 administrationenhances glucose tolerance and increases skeletal muscle PPARalpha andUCP2 expression in rats. J Endocrinol 2008; 198:367–374.

79 Franckhauser S, Elias I, Rotter Sopasakis V, et al. Overexpression of Il6 leadsto hyperinsulinaemia, liver inflammation and reduced body weight in mice.Diabetologia 2008; 51:1306–1316.

80 Kim HJ, Higashimori T, Park SY, et al. Differential effects of interleukin-6 and-10 on skeletal muscle and liver insulin action in vivo. Diabetes 2004;53:1060–1067.

81 Samokhvalov V, Bilan PJ, Schertzer JD, et al. Palmitate- and lipopolysacchar-ide-activated macrophages evoke contrasting insulin responses in musclecells. Am J Physiol Endocrinol Metab 2009; 296:E37–46.


82

�Hong EG, Ko HJ, Cho YR, et al. Interleukin-10 prevents diet-induced insulinresistance by attenuating macrophage and cytokine response in skeletalmuscle. Diabetes 2009; 58:2525–2535.

High-fat diet increases proinflammatory cytokine levels and macrophage numberswithin muscle tissue and this was corrected by transgenic overexpression of IL-10in muscle which also improved whole-body and muscle insulin sensitivity.

83 Vettor R, Milan G, Franzin C, et al. The origin of intermuscular adipose tissueand its pathophysiological implications. Am J Physiol Endocrinol Metab 2009;297:987–998.

84 Weisberg SP, McCann D, Desai M, et al. Obesity is associated with macro-phage accumulation in adipose tissue. J Clin Invest 2003; 112:1796–1808.

85 Nguyen MT, Favelyukis S, Nguyen AK, et al. A Subpopulation of macrophagesinfiltrates hypertrophic adipose tissue and is activated by free fatty acids viatoll-like receptors 2 and 4 and JNK-dependent pathways. J Biol Chem 2007;282:35279–35292.

86 Patsouris D, Li PP, Thapar D, et al. Ablation of CD11c-positive cells nor-malizes insulin sensitivity in obese insulin resistant animals. Cell Metab 2008;8:301–309.

87 Hevener AL, Olefsky JM, Reichart D, et al. Macrophage PPAR gamma isrequired for normal skeletal muscle and hepatic insulin sensitivity and fullantidiabetic effects of thiazolidinediones. J Clin Invest 2007; 117:1658–1669.

88

��Varma V, Yao-Borengasser A, Rasouli N, et al. Muscle inflammatory responseand insulin resistance: synergistic interaction between macrophages and fattyacids leads to impaired insulin action. Am J Physiol Endocrinol Metab 2009;296:E1300–E1310.

This study observes increased macrophages in muscle of obese individuals andshows synergistic actions of excess palmitic acid and macrophage co-culture withhuman myotubes in vitro, to cause muscle inflammation and impair insulin signal-ling to Akt.

89 Bruun JM, Helge JW, Richelsen B, Stallknecht B. Diet and exercise reducelow-grade inflammation and macrophage infiltration in adipose tissue but notin skeletal muscle in severely obese subjects. Am J Physiol Endocrinol Metab2006; 290:E961–967.

90 Di Gregorio GB, Yao-Borengasser A, Rasouli N, et al. Expression of CD68and macrophage chemoattractant protein-1 genes in human adipose andmuscle tissues: association with cytokine expression, insulin resistance, andreduction by pioglitazone. Diabetes 2005; 54:2305–2313.

91

�Koliwad SK, Streeper RS, Monetti M, et al. DGAT1-dependent triacylglycerolstorage by macrophages protects mice from diet-induced insulin resistanceand inflammation. J Clin Invest 2010; 120:756–767.

Excess fatty acids induce proinflammatory M1 activation of macrophages. There isevidence this is mediated by toll-like receptor signalling. However, the importanceof lipid metabolism as a trigger of macrophage M1 activation is highlighted, sinceDGAT1 overexpression in macrophages reduces macrophage inflammatory re-sponses.

92 Odegaard JI, Chawla A. Mechanisms of macrophage activation in obesity-induced insulin resistance. Nat Clin Pract Endocrinol Metab 2008; 4:619–626.

93 Stratford S, Hoehn KL, Liu F, Summers SA. Regulation of insulin action byceramide: dual mechanisms linking ceramide accumulation to the inhibition ofAkt/protein kinase B. J Biol Chem 2004; 279:36608–36615.


Muscle insulin resistance: assault by lipids, cytokines and local macrophages

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