ABSTRACT PARTICULATE MATTER COMPOSITION AS A DETERMINANT OF ROS PRODUCTION IN ALVEOLAR MACROPHAGES: A FLUORESCENCE-BASED MICROPLATE AND FLOW CYTOMETRIC BASED STUDY Previous studies have established a link between Particulate matter (PM) exposure and adverse health effects including pulmonary and heart disease. PM is divided into three categories: coarse or PM10 (<10μm), fine or PM2.5 (< 2.5μm), and ultrafine (< 0.1μm) The chemical composition of PM2.5 results from a complex mixture of solid and liquid particles: and can be highly variable depending on the time and location of a sample collection. Redox-active components of PM2.5 such as transition metals (Cu 2+ and Fe 2+ ) and quinones (9,10-phenanthraquinone and 1,2-naphthaquinone) are believed to play a major role in PM2.5 toxicity, due to their ability to trigger high intracellular reactive oxygen species (ROS) production. To better understand the relative contribution of each PM2.5 constituent to intracellular ROS production, a fluorescence-based microplate and flow cytometric approach were developed to determine the oxidative capabilities of each constituent on alveolar macrophages (AM). The assays revealed a variety of findings. ROS production induced by quinones in AM required concentrations 2-3 orders of magnitude lower than that required to obtain a similar response with transition metals. Upon further analysis, PM2.5 site-specific filters also revealed that Fe 2+ and 9,10-PQ were the compounds principally responsible for the majority of ROS production at each collection site. Finally, between the two transition metals analyzed through flow cytometry, iron produced the greater generation of ROS intracellularly. David Flores May 2021
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ABSTRACT
PARTICULATE MATTER COMPOSITION AS A DETERMINANT OF ROS PRODUCTION IN ALVEOLAR MACROPHAGES: A
FLUORESCENCE-BASED MICROPLATE AND FLOW CYTOMETRIC BASED STUDY
Previous studies have established a link between Particulate matter (PM) exposure
and adverse health effects including pulmonary and heart disease. PM is divided into
three categories: coarse or PM10 (<10µm), fine or PM2.5 (< 2.5µm), and ultrafine (<
0.1µm) The chemical composition of PM2.5 results from a complex mixture of solid and
liquid particles: and can be highly variable depending on the time and location of a
sample collection. Redox-active components of PM2.5 such as transition metals (Cu2+ and
Fe2+) and quinones (9,10-phenanthraquinone and 1,2-naphthaquinone) are believed to
play a major role in PM2.5 toxicity, due to their ability to trigger high intracellular reactive
oxygen species (ROS) production. To better understand the relative contribution of each
PM2.5 constituent to intracellular ROS production, a fluorescence-based microplate and
flow cytometric approach were developed to determine the oxidative capabilities of each
constituent on alveolar macrophages (AM). The assays revealed a variety of findings.
ROS production induced by quinones in AM required concentrations 2-3 orders of
magnitude lower than that required to obtain a similar response with transition metals.
Upon further analysis, PM2.5 site-specific filters also revealed that Fe2+ and 9,10-PQ were
the compounds principally responsible for the majority of ROS production at each
collection site. Finally, between the two transition metals analyzed through flow
cytometry, iron produced the greater generation of ROS intracellularly.
David Flores May 2021
PARTICULATE MATTER COMPOSITION AS A DETERMINANT OF
ROS PRODUCTION IN ALVEOLAR MACROPHAGES: A
FLUORESCENCE-BASED MICROPLATE AND FLOW
CYTOMETRIC BASED STUDY
by
David Flores
A thesis
submitted in partial
fulfillment of the requirements for the degree of
Master of Science in Biology
in the College of Science and Mathematics
California State University, Fresno
May 2021
APPROVED
For the Department of Biology:
We, the undersigned, certify that the thesis of the following student meets the required standards of scholarship, format, and style of the university and the student's graduate degree program for the awarding of the master's degree. David Flores
Thesis Author
Laurent Dejean (Chair) Biology
Jason Bush Biology
Alam Hasson Chemistry
For the University Graduate Committee:
Dean, Division of Graduate Studies
AUTHORIZATION FOR REPRODUCTION
OF MASTER’S THESIS
X I grant permission for the reproduction of this thesis in part or in its
entirety without further authorization from me, on the condition that
the person or agency requesting reproduction absorbs the cost and
provides proper acknowledgment of authorship.
Permission to reproduce this thesis in part or in its entirety must be
obtained from me.
Signature of thesis author: David Flores
ACKNOWLEDGMENTS
There are many individuals that have contributed to the completion of this work.
First, I would like to thank my parents and family for always supporting me on any path I
decided to take in life, I love you guys. To Dr. Laurent Dejean, who has been my mentor
for the past three years. He has showed me not only skills needed to be a successful
scientist, but he has also taught me many skills to be successful in life. His patience and
guidance when problems arose were fundamental in my progression through this journey.
My committee members: Dr. Alam Hasson and Dr. Jason Bush have also served as
mentors and have helped me expand my knowledge in other fields of science. Their
influence on my professional career is incalculable. I would also like to thank Dr. Krish
Krishnan and Ms. Ronna Crews for their outstanding leadership and advice throughout
my graduate program as leaders of the NIH Bridges to Doctorate. In that same spirit I
thank the Bridges to Doctorate for accepting me as a member of the program, which
helps underrepresented students reach doctorate programs in STEM. The amount of
resources provided by the program allowed me to gain opportunities that I would
otherwise not have experienced, such as an internship within the UC system. To all the
members of my lab that had a hand on this project: Anthony Waterston, Kosha Raval,
and Micah Olivas thank you for helping with the completion of this project. And finally,
to the two best colleagues/confidants/and friends Devon Lee and Mandeep Kaur. They
have been the best support system anyone could ask for. Not only were they fundamental
in the completion of this project, but without them I am not sure my graduate program
would have been successful. They have given me immeasurable advice both
professionally and personally and I cannot thank them enough. And finally, to Fresno
State for being a great campus and providing me with the resources necessary to be
successful.
TABLE OF CONTENTS
Page
LIST OF TABLES ...................................................................................................... viii
LIST OF FIGURES ....................................................................................................... ix
RESULTS AND DISCUSSION .................................................................................... 17
Dose-Response Assays as a Multi-Component Assessment of PM2.5 Toxicity ........ 17
Construction of Site-Specific Breakdown of PM2.5 Compound ROS Generation Contribution ............................................................................................... 21
Statistical Confirmation of Significance within Microplate Experiments ................ 24
Development of Flow Cytometry-based Assay to Detect ROS Generation in AM When Exposed to PM2.5 Constituents ................................................... 26
APPENDIX: TUKEYS MULTIPLE COMPARISONS AMONG THE MEANS OF THE RANGES................................................................................................... 40
LIST OF TABLES
Page
Table 1. List of the PM2.5 filter samples collected from Claremont and Fresno .........6
Table 2. Concentration ranges used for the treatment of the cells ............................ 13
Table 3. The contribution of each component to the filter samples collected............ 23
Table 4. Linear regression statistics for the ranges of each component of PM2.5 ...... 24
Table 5. One-way ANOVA statistics for ranges of each component of PM2.5 ........... 26
Figure 2. Mass dependent PM2.5 filter treatments produce variable cell responses associated with ROS production ..................................................5
Figure 3. The global protocol of the microplate assay................................................ 13
Figure 5. Cell response, (RFU) of the cells treated with the four compounds ........... 18
Figure 6. Comparisons of slopes within the orange highlighted regions from Figure 5 of the 4 testes PM2.5 components .................................................. 20
Figure 7A and 7B. Cell response to each PM2.5 component treatment from different filters collected of Claremont and Fresno regions ...................... 23
Figure 8. Iron (Fe2+) dose response in flow cytometry with DCF probe. ................... 27
Figure 9. Copper (Cu2+) dose response in flow cytometry with DCF probe. ............. 28
Figure 10. Flow cytometry treatment concentration vs. DCF mean detected for Iron treatments ............................................................................................ 29
Figure 11. Flow cytometry treatment concentration vs. DCF mean detected for Copper treatments ....................................................................................... 30
INTRODUCTION
Air pollution presents an incredibly pervasive issue to public health, particularly
in the Central Valley of California. According to State of the air report conducted by the
American lung association, the most polluted cities classified by annual particle
concentration were Fresno-Madera-Hanford, CA (reported as an aggregated group) and
Bakersfield,CA, along with other cities in the San Joaquin Valley (State of the Air 2020).
Previous studies have established a direct link between chronic particulate matter (PM)
exposure and adverse health effects such as asthma, chronic obstructive pulmonary
disease, lung cancer, heart disease, and increased mortality rates in the U.S. (Zanobetti
and Schwartz 2009). PM is classified into three main categories, coarse or PM10
(<10µm), fine or PM2.5 (< 2.5µm), and ultrafine (< 0.1µm) (Tao, Gonzalez-Flecha et al.
2003).
PM2.5 relates to a class of particulate matter 2.5 microns or less in diameter, and
refers to a complex mix of liquid and solid particles of varying composition dependent on
a multitude of factors such as combustion source, time, and location. Typically, however,
PM2.5 components include a variety of different transition metals, organic compounds,
combustion particles, and other pollution constituents (Aust, Ball et al. 2002). Among
some of the most nefarious components of PM2.5, however, are the transition metals and
organic quinones that may be responsible for the majority of PM2.5’s toxic qualities.
PM2.5 is believed to be more harmful to health compared to the larger, coarse
PM10 (Valavanidis, Fiotakis et al. 2008). PM10 due to its larger size becomes deposited
into the nasopharynx regions, where they will eventually become cleared by respiratory
defense mechanisms like nasal hairs, mucociliary clearance or swallowing (Tao,
Gonzalez-Flecha et al. 2003). The ability of PM2.5 to bypass these defenses of the
respiratory tract and penetrate deeply into the lower airways, eventually reaching the
2 2
distal ends of the alveoli where they can remain for longer periods make it especially
dangerous, particularly in high concentrations (Brauer, Avila-Casado et al. 2001,
Davidson, Phalen et al. 2005). Chronic exposure to air pollution has been linked to health
conditions such as cardiovascular disease, respiratory issues; and some studies have even
associated prolonged exposure to air pollution and the occurrence of certain types of
degenerative neurological diseases (Chen, Zhong et al. 2010).
While the exact pathophysiological mechanism behind PM2.5’s deleterious effects
on biological systems remains to be defined, it is understood that the generation of
reactive oxygen species (ROS) leads to the subsequent formation of oxidative stress, and
if occurring in severe enough quantities, can ultimately lead to cellular death and tissue
damage in mammalian cells (Sawyer, Mundandhara et al. 2009). Reactive oxygen species
are free radical structures and natural byproducts of oxidative metabolism; ROS generally
maintains a homeostatic relationship with antioxidants, however when the concentration
of ROS begins to exceed the capacity of the cell to contain and combat its formation,
oxidative damage begins to occur (Pizzino, Irrera et al. 2017).
This increase of ROS production due to exposure of PM2.5 has previously been
hypothesized to play a direct role in pulmonary inflammation and disturbing the natural
balance between antioxidants and reactive oxygen species within the lungs (Castro and
Freeman 2001, Landreman, Shafer et al. 2008). The imbalance between antioxidant and
ROS within the cell will eventually lead to oxidative stress of the cell causing DNA
damage and lipid peroxidation.
Transition metals such as iron can stimulate the production of H2O2 that can
eventually generate hydroxyl radicals through redox reactions like the Fenton reaction.
These reactions can then cause an oxidative burst within the cell that leads to oxidative
damage to macromolecules (Valavanidis, Salika et al. 2000). In a study conducted by
(Jung, Guo et al. 2006), they investigated the generation of hydroxyl ions from a few
3 3
ambient PM samples collected from Davis, CA. What they found was that transition
metals contributed to a significant portion of hydroxyl ions found in the ambient particles
(Paulson, Hasson et al. 2016). In another study conducted by (Vidrio, Phuah et al. 2009)
they also used PM2.5 particles collected in Davis, CA throughout the course of the year.
They also found that soluble Fe contributed the most to hydroxyl ions production
(Paulson, Hasson et al. 2016).
Quinones (Q) found in PM2.5 (1,2- and 1,4-napthoquinones, 9,10-
phenanthrenequinone) can also produce reactive oxygen species (ROS). As shown in
Figure 1, the quinone is reduced to a semiquinone (SQ) radical anion that is
accomplished by the addition of one electron/proton pair. Following this first reduction
an additional electron/proton reduction occurs to convert the semiquinone (SQ) into a
hydroquinone (HQ). These are all reversible redox conversions between quinone and
hydroquinone. These redox steps may be further followed by the reductions of oxygen to
superoxide anion and further reduction to hydrogen peroxide (Paulson, Hasson et al.
2016). Also, processes such as redox cycling typically occur within the electron transport
chain of the mitochondria (Valavanidis, Fiotakis et al. 2008).
Figure 1. Quinone redox chemistry. The Quinone redox chemistry scheme as a
mechanism for the generation of superoxide anions and the subsequent reduction to
hydrogen peroxide (Paulson, Hasson et al. 2016)
4 4
Mitochondria are a major source of intracellular ROS production, specifically as a
by-product of aerobic metabolism in the electron transport chain (Forman, Torres et al.
2002, Xiong, Ru et al. 2017). Recent studies suggest that exposure to different pollutants
such as PM can lead to mitochondrial dysfunction, which as a result contributes to
various health issues (Bhargava, Tamrakar et al. 2018). These pollutants may interact
directly or indirectly with the mitochondria, influencing mitochondrial ROS production
and ultimately leading to mitochondrial DNA damage. The mechanisms involved with
mitochondrial dysfunction caused by PM exposure are not yet fully understood, however
a possible mechanism is the interference of complex III in the electron transport chain by
pro-oxidative PM components such as quinones (Xia, Kovochich et al. 2007). This
mitochondrial dysregulation by exposure to PM is therefore a potential mechanism
leading to multiple human pathologies (Bhargava, Tamrakar et al. 2018).
The innate defense mechanism of the lungs responds to PM2.5 exposure by
recruiting alveolar macrophages into the lung and activating tissue resident macrophages
(Miyata, van Eeden et al. 2011). These alveolar macrophages (AM) proceed to
phagocytose the PM2.5 particles, producing ROS and therefore hydrogen peroxide as a
byproduct to clear the foreign particles from the alveolar space (Becker, Soukup et al.
1996, Goldsmith, Frevert et al. 1997, Geng, Meng et al. 2006). Due to the imperative role
of alveolar macrophages in acting as the primary defense mechanism of the lungs against
foreign pathogens, the cell line NR8383 corresponding to alveolar macrophages obtained
from rats, was chosen to act as the model organism for experimentation; the specific
ability of alveolar macrophages to withstand oxidative stress in far greater proportions
than other cells of the body, combined with the more predictable, less variable response
during ROS induction cause the line to be optimal for the assessment of ROS production
(Landreman, Shafer et al. 2008).
5 5
Previous work conducted by Landreman et. al 2008 helped establish a
fluorescence macrophage-based assay for intracellular ROS detection. Building upon the
protocol used by Landreman et al. 2008, research in our laboratory was focused on the
study of the mass dependence of PM2.5 samples collected from Fresno (Fr) and Claremont
(Cl) , CA during summer (Sum) and winter (Win) seasons on intracellular ROS
production as is shown in Figure 2. Alveolar macrophages were treated with the ambient
PM2.5 collected samples, which contained various defined quantities of PM2.5 mass shown
in Table 1.
Figure 2. Mass dependent PM2.5 filter treatments produce variable cell responses
associated with ROS production. The scatter plot represents the PM2.5 filter extracts
from both Claremont (Cl) and Fresno (Fr) collection sites during summer (Sum) and
winter (win). The filters were collected in 12 hour (12hr) and 6 hour (6hr) periods. The
data shows a variable cellular response based on the different filters which were used to
treat the AM, suggesting that PM2.5 mass is not the only variable for cytotoxicity
(Waterston 2019).
6 6
Table 1. List of the PM2.5 filter samples collected from Claremont and Fresno. The
collections took place on the rooftop sites at Harvey Mudd College, Claremont and
California State University, Fresno during summer and winter seasons. There were three
collection times a day, for approximately 6 or 12 hours. The times were from 7 AM – 1
PM, 1 PM - 7 PM, and 7 PM – 7 AM. Also shown are the PM2.5 mass loading in each of
the filters collected. (Paulson, Hasson et al. 2016) (Waterston 2019)
Filter ID Location Time PM2.5 mass
loading(μg/m3)
Cl-Sum-Ni-12hr-A Claremont Night 12.11
Cl-Sum-Ni-12hr-B Claremont Night 12.42
Cl-Sum-Ni-6hr Claremont Afternoon 14.18
Fr-Win-Ni-12-hr-A Fresno Night 15.15
Fr-Win-Ni-12-hr-B Fresno Night 24.97
Fr-Win-Ni-12-hr-C Fresno Night 15.87
Fr-Win-Ni-12-hr-D Fresno Night 17.77
Following the treatment, a comparison between Fr-Win-Ni-12hr-B and Cl-Sum-
Ni-12hr-A was made. These two samples were chosen due to the similarity in collection
time both being night and 12-hour collection times, however the Fresno sample was
during the winter and Claremont was during the summer (Waterston 2019). After
comparison of the mass-normalized cellular ROS response for both Claremont and
Fresno samples, there was a significant difference observed in the average slope of
cellular ROS response from Figure 2 between Fr-Win-Ni-12hr-B and Cl-Sum-Ni-12hr-A
which are shown in detail in (Waterston 2019). This analysis led us to suggest that the
absolute amount of PM2.5 in the sample was not the only determinant for intracellular
ROS production. It was then suggested that the chemical composition of the PM2.5 might
also be an important determinant of macrophages intracellular ROS production.
While it is generally recognized that PM2.5 induces cytotoxic responses in
mammalian cells, compound-specific studies relating individual components of PM2.5
7 7
pollution like transition metals (Cu2+ and Fe2+) and quinones (9,10-Phenanthroquinone
and 1,2-Naphthoquinone) to the formation of ROS have only briefly been investigated.
Imperative to improving our collective understanding of the stress dynamics of PM2.5
exposure is knowledge of the harmful components, and to what degree they pose threat to
cells. Therefore, a main question formulated for this work was to determine whether the
cells were triggered to generated ROS production when exposed to elements such as
transition metals (Fe2+ and Cu2+) and quinones (9,10-Phenanthroquinone and 1,2-
Naphthoquinone) that are found within the complex of PM2.5. And if these PM2.5
components are triggering the increase of ROS intracellularly, to what extent are
transition metals and quinones contributing to the overall increase of ROS intracellularly.
If transition metals (Cu2+ and Fe2+) and quinones (9,10-Phenanthroquinone and 1,2-
Naphthoquinone) are important factors in the increase of ROS intracellularly and in PM2.5
toxic effects, then we expect to see a strong correlation between these compounds present
in PM2.5 and the induced ROS response from the cells treated.
Microplate experimentation aims to provide an evaluation and multi-component
analysis of oxidative stress potentials among the primary constituents of PM2.5. This was
accomplished using a fluorescence-based approach utilizing the probe 2’,7’-
Dichlorofluorescin diacetate (DCF) to quantify the presence of ROS within cells,
producing relative fluorescent units (RFU) able to be related directly to the formation of
oxidative stress. Furthermore, by incorporating the data gleaned from previous iterations
of the study, we were able to construct balances for the site-specific samples collected.
Breakdowns of each sample filter for analysis of PM2.5’s chemical constituents’
contribution to the total fluorescent cell responses were developed. This allowed us to
gain better insight into what PM2.5 components present the most toxic combinations of
PM2.5 pollution and the extent of such. As described in paragraphs 6 and 7 of this
introduction, previous studies have observed transition metals such as iron and quinones
8 8
to be involved in the generation of ROS intracellularly. For these reasons we continued
our study, in conjunction with statistical confirmation and chemical analysis of the filter
data collected from (Paulson, Hasson et al. 2016) (Waterston 2019). This allowed for the
identification of correlations between particular concentrations of principal contributors
(Fe2+, Cu2+, 9,10-Phenanthroquinone, 1,2-Naphthoquinone), detrimental health effects,
and filter origins. With these findings, better strategies to abate pollution and combat the
P value summary ns ns ns ns Are SDs significantly different (P < 0.05)? No No No No
Development of Flow Cytometry-based Assay to Detect ROS Generation in AM When Exposed to PM2.5
Constituents
PM2.5 induced ROS generation in alveolar macrophages was measured by
observing the magnitude of difference among fluorescent (gate R11) and non-fluorescent
(gate R12) population means along the DCF-A detection scale (x-axis shown in Figures 8
and 9). As a result of the positive shift along the x-axis proceeding the production of
ROS, the magnitude of the distance along this axis between both populations serves as an
indicator of the degree of ROS production generated from each treatment. Shown in
Figure 8, a positive trend in shift magnitude is displayed amongst increasing
concentrations of Iron 39-156 ng/mL. This is additionally supported by the increase in
shift between control populations and the treatment populations, suggesting that a
positive correlation between treatment concentration and ROS generation exists. In
contrast, we see less definable, more ambiguous trends in the copper treatments already
stated to have low positive correlation with RFU generation, as is shown by Figure 9.
27 27
Figure 8. Iron (Fe2+) dose response in flow cytometry with DCF probe. The graphs
from left to right show an increasing shift on the x-axis labeled as DCF-A as there is an
increase in treatment concentration. The same increasing trend is observed with the
controls (CP vs. Z50 vs. Z100). Iron showed the greatest DCF mean shift on the X-axis
as the concentration increased shown in Figure 10.
28 28
Figure 9. Copper (Cu2+) dose response in flow cytometry with DCF probe. The three
concentrations of copper that were used to treat the cells (39,78, 156 ng/mL) showed no
significant shift within the x-axis representing the DCF detected within the cell.
29 29
To demonstrate flow cytometry as a validation technique and reaffirm
relationships determined in microplate experimentation, Figures 10 and 11 show a trend
of DCF mean shift between both copper and Iron within their respective relevant
concentrations. Similarly, to the microplate results, Iron displays a clear, positive
correlation with RFU generation and subsequent DCF mean shift, whereas copper and its
weak correlation are unable to replicate or produce results similar to that of iron.
CP
Z100
ug/mL
Z50 u
g/mL
Fe 15
6 ng/m
L
Fe 78
ng/m
L
Fe 39
ng/m
L
0
500
1000
1500
DC
F-A
mean
Treatment concentration vs DCF mean
Figure 10. Flow cytometry treatment concentration vs. DCF mean detected for Iron
treatments. Shows the iron treatments having a positive trend in DCF detection between
the three concentrations (39,78, 156 ng/mL) and the control treatments. Fe 156 ng/mL is
observed to have the biggest impact on ROS increase based on the amount of DCF that is
being detected by the flow cytometer. Iron was observed to possess a clear, definitive
relationship between shift on the x-axis of Figure 8 and increasing concentrations.
30 30
CP
Z100
ug/mL
Z50 u
g/mL
Cu 1
56 n
g/mL
Cu 7
8 ng/m
L
Cu 3
9 ng/m
L
0
200
400
600
800
DC
F-A
mean
Treatment concentration vs DCF mean
Figure 11. Flow cytometry treatment concentration vs. DCF mean detected for
Copper treatments. Shows the copper treatments having a no substantial effect on the
amount of detected DCF mean through the increase of concentrations. The zymosan
positive controls are observed to produce the highest amount of DCF mean detection. The
copper treatments had a very similar level of DCF detection to the cells that were only
treated with DCF and Sytox blue probes. Copper was shown to possess a negligible effect
on cell response, RFU. This relationship is much consistent with previous microplate data
implying a weak or insignificant relationship with the effects of copper on ROS
generation.
Flow cytometry allows us to incorporate sytox blue into the assay for reliable
separation between oxidatively stressed and non-stressed cells (both categories being live
cells) from dead cells as shown by Figure 4. The sytox blue is able to permeate through
dead cells due to the cell membrane being compromised, on the other with live cell
membranes it is unable to permeate the cell. The ability of sytox blue to distinguish
between dead and live cells while detecting ROS generation produced via DCF, gives an
additional parameter to analyze the effects of oxidative damage produced by the PM2.5
constituents that can potentially lead to cell death in AM.
31 31
We can observe from Figures 10 and 11 that iron seems to have the biggest effect
on DCF detection compared to the copper effect. This is observation is similar to what we
observed for our microplate experiments. However it is opposite of what is reported in in
vitro assays such as the DTT assay. From this we observe that iron produced the highest
cell response associated with ROS generation through the measurement of DCF
intracellularly through both flow cytometry and microplate assay. This is contrary to
what is observed in the literature. In a study conducted by Charrier, McFall et al. 2014
they ran a cell-free surrogate lung fluid to quantify the formation of hydrogen peroxide
from different transition metals. They observed that copper produced the most hydrogen
peroxide from ambient PM and that iron suppresses hydrogen peroxide formation in
chemical solutions (Charrier, McFall et al. 2014). These results help provide a better
understanding of how these components are affecting the increase of intracellular ROS.
As was previously discussed in the introduction of this writing, Iron is hypothesized to be
a significant contributor to ROS generation through redox reactions such as the Fenton
reaction. This capability of iron to produce the response observed in alveolar
macrophages make it necessary to continue the study of iron contribution to ROS
generation intracellularly and localize where these reactions are taking place inside the
cell.
CONCLUSION
A macrophage-based ROS detection assay has been established in this study to
evaluate the potential of intracellular ROS production by PM2.5 constituents: 1,2-
naphthoquinone, 9,10- phenanthrenequinone, Cu2+ and Fe2+. The multi-component
analysis of the oxidative effects of PM2.5 constituents on alveolar macrophages revealed a
variety of findings related to the contribution of each component of PM2.5 to the cell
response, RFU and thus generation of intracellular ROS. Previous studies conducted by
former members of our research group determined that intracellular PM2.5- associated
ROS generation was not strictly dependent on mass exposure but could be more directly
correlated to the composition of the PM2.5 particles.
Expanding upon these initial findings, this study aimed to provide a clearer
understanding of the influence of PM2.5 composition on the overall propensity of AM to
generate ROS. Microplate experimentation revealed that composition was indeed a
significant factor in cell response (ROS generation), RFU and specifically identified the
compounds chiefly responsible. Both in regard to relative abundance and also ROS
generation potential, the compounds capable of producing the greatest responses were
iron and 9,10 PQ.
As reactive oxygen species are primarily responsible for the oxidative stress in
cells and tissues, both the microplate dose-response assay coupled with flow cytometry
were tailored to determine the ROS-generating capabilities of PM2.5 constituents, while
also monitoring the health of cells during the assay. These assays were ultimately
successful in allowing the identification of some primary contributors to the oxidative
stress observed in our alveolar macrophages. These assays also allowed us to narrow
down the possibility that iron and copper induced ROS generation is only due to the
potential cytotoxic effects of the treatments themselves.
33 33
The microplate dose-response assay found that quinones had a greater ability to
generate ROS intracellularly compared to transition metals. The quinones also showed
this oxidative potential at a much lower concentration range than both Fe2+ and Cu2+ as
was shown in Figure 6. Quinones have two ketone groups, which allow them to become
biologically active (Yang, Ahmed et al. 2018). 9,10-PQ was found to produce the highest
amount of ROS generated/ ng of compound compared to all other PM2.5 constituents
tested in this study. 9,10-PQ being a redox active quinone has been shown to generate
ROS through redox cycling leading to excessive ROS levels intracellularly and
eventually oxidative damage (Yang, Ahmed et al. 2018).
Further analysis of the PM2.5 site-specific filters revealed that Fe2+ and 9,10-PQ
were the compounds principally responsible for the majority of ROS production at each
collection site as shown in Figure 7. Table 3 shows the remarkably higher environmental
concentrations of each compound and their relatively higher toxicity, the two exhibited
the greatest capacity to generate ROS, especially when compared to their significantly
fewer toxic counterparts Cu2+ and 1,2-NQ.
In a study conducted by Xiong, Yu et al. 2017 using the DTT assay as a method
to measure the rate of ROS production, they found that Cu at a concentration of 2 μM
was the most important metal in DTT consumption (which correlates to higher oxidative
potential), however most of the DTT oxidation did not lead to ROS generation (Xiong,
Yu et al. 2017). Another finding from their study was that iron was inactive in the DTT
assay, however it contributed to the production of hydroxyl radical when in the presence
of other PM components such as quinones, highlighting the potential synergistic ability
of some transition metals to increase ROS (Xiong, Yu et al. 2017). These synergistic or
antagonistic interactions between PM components to potentially increase ROS
intracellularly could explain why copper is observed to be the highest contributor to
34 34
oxidative potential in cell-free assays but have limited contribution in ROS increase with
in vivo assays as was observed in our study.
Coupled with the microplate dose-response assay, flow cytometry served as a
validation for the ROS generation divergence between the copper and iron. Iron was
demonstrably higher in producing oxidative stress within the alveolar macrophages
compared to copper. The flow cytometry assay was successful in scrupulously identifying
cell death real time through detection of SYTOXTM Blue and monitoring health of cells.
The successful ability of flow cytometry to perform a multi-fluorescent probe assay for
ROS and cell death detection illustrates an effective, accurate method of assessing the
degree of ROS generation of air pollution relative to the location of collection.
The assays performed in this study were ultimately revealed to be effective modes
of gauging ROS at a compound-specific level, asserted by both the low levels of
experimental variance and ability to replicate results in a precise manner, the
fluorescence-based approach of gauging oxidative stress was observed to be an
appropriate simulation of cells in vivo.
Some of the limitations of this approach were revealed later in the conduction of
the experiments; many times, throughout the duration of the project experimentation was
delayed due to a variety of problems. Most notably, although alveolar macrophages
represent some of the most resilient cell lines, issues of viability were still often
encountered. To perform these experiments in optimal conditions, cell viability is ideally
above 97%. While cultures were relatively easy to maintain and growth was on occasion
too rapid, ensuring this viability threshold was met prior to experimentation many times
was a challenge.
Future aims of the study involve the identification of subcellular origins of ROS
within the cell. Spoken about previously, there are many hypotheses regarding the
mechanisms and locations of origin for stress signals at the intracellular level, but further
35 35
studies must be conducted to precisely identify them. The prevailing theory of iron-
mediated ROS inception states that Fenton reactions, occurring in the cytosol of the cell,
react iron cations with hydrogen peroxide (H2O2), producing the harmful free radicals as
a byproduct in addition to oxidizing the metal (Bystrom, Guzman et al. 2014). To
confirm this would require the use of MitoSOXTM Red, a probe used in the
identification of harmful agents that facilitate in the formation of mitochondrial stress and
produces a fluorescent signal in the presence of such. The use of flow cytometry in
conjunction with the aforementioned probe have been observed to be successful in
detecting superoxide anions within the mitochondria, allowing researchers to gain further
insight into compounds responsible for inducing such generation (Kauffman, Kauffman
et al. 2016).
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APPENDIX: TUKEYS MULTIPLE COMPARISONS AMONG THE MEANS OF THE RANGES
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43 43
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