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Am J Transl Res. 2013; 5(2): 224–234.
Published online Mar 28, 2013.
PMCID: PMC3612517
Exposure to fine airborne particulate matter induces macrophage infiltration, unfolded protein response, and lipid deposition in white adipose tissue
Roberto Mendez,1 Ze Zheng,1 Zhongjie Fan,5 Sanjay Rajagopalan,3 Qinghua Sun,3,4 and Kezhong Zhang1,2
1Center for Molecular Medicine and Genetics, Wayne State University School of Medicine, Detroit, MI 48201, USA
2Department of Immunology and Microbiology, Wayne State University School of Medicine, Detroit, MI 48201, USA
3Division of Cardiovascular Medicine, Davis Heart and Lung Research Institute, College of Medicine, Columbus, OH 43210, USA
4Division of Environmental Health Sciences, College of Public Health, Ohio State University, Columbus, OH 43210, USA
5Department of Cardiology, Peking Union Medical College Hospital, Chinese Academy of Medical Science and Peking Union Medical College, Beijing, China
Address correspondence to: Dr. Kezhong Zhang, Center for Molecular Medicine and Genetics, Wayne State University School of Medicine, 540 E. Canfield Avenue, Detroit, MI 48201, USA. Phone: 313-577-2669; Fax: 313-577-5218; E-mail: kzhang/at/med.wayne.edu; Dr. Qinghua Sun, Division of Environmental Health Sciences, College of Public Health, Ohio State University, Columbus, OH 43210, USA. Phone: 614-247-1560; Fax: 614-688-4233; E-mail: sun.224/at/osu.edu
Received December 17, 2012; Accepted February 15, 2013.
Recent epidemiological studies have suggested a link between exposure to ambient air-pollution and susceptibility to metabolic disorders such as Type II diabetes mellitus. Previously, we provided evidence that both short- and long-term exposure to concentrated ambient particulate matter with aerodynamic diameter <2.5 μm (PM2.5) induces multiple abnormalities associated with the pathogenesis of Type II diabetes mellitus, including insulin resistance, visceral adipose inflammation, brown adipose mitochondrial adipose changes, and hepatic endoplasmic reticulum (ER) stress. In this report, we show that chronic inhalation exposure to PM2.5 (10 months exposure) induces macrophage infiltration and Unfolded Protein Response (UPR), an intracellular stress signaling that regulates cell metabolism and survival, in mouse white adipose tissue in vivo. Gene expression studies suggested that PM2.5 exposure induces two distinct UPR signaling pathways mediated through the UPR transducer inositol-requiring 1α (IRE1α): 1) ER-associated Degradation (ERAD) of unfolded or misfolded proteins, and 2) Regulated IRE1-dependent Decay (RIDD) of mRNAs. Along with the induction of the UPR pathways and macrophage infiltration, expression of genes involved in lipogenesis, adipocyte differentiation, and lipid droplet formation was increased in the adipose tissue of the mice exposed to PM2.5. In vitro study confirmed that PM2.5 can trigger phosphorylation of the UPR transducer IRE1α and activation of macrophages. These results provide novel insights into PM2.5-triggered cell stress response in adipose tissue and increase our understanding of pathophysiological effects of particulate air pollution on the development of metabolic disorders.
Keywords: Ambient particulate matter, PM2.5, unfolded protein response, lipid metabolism, white adipose tissue
Air pollution is a continuing world-wide challenge to public health. Epidemiologic and experimental investigations have consistently demonstrated a link between fine particulate matter (aerodynamic diameter <2.5 μm, PM2.5) and susceptibility to cardiovascular disease [1]. The mechanisms by which PM2.5 predisposes to cardiovascular events are via PM2.5 interaction with pro-inflammatory pathways, hyper-coagulability, alterations in autonomic tone, and vasomotor alterations [1]. Indeed, the pathways mediating the effects of air pollution are indistinguishable from those triggered by other classic risk factors for cardiovascular disease [1]. We have recently demonstrated that PM2.5 exposure mediates early alterations in insulin resistance, visceral inflammation, and structural and functional alterations in brown adipose tissue [2-4]. We have also shown that PM2.5 exposure induces endoplasmic reticulum (ER) stress and Unfolded Protein Response (UPR) characterized by activation of double-strand RNA-activated protein kinase-like ER kinase (PERK), leading to phosphorylation of translation initiation factor eIF2α and induction of C/EBP homologous transcription factor CHOP/GADD153 in liver tissue [5]. PM2.5 exposure stimulates inflammatory responses, disrupts insulin signaling, and represses peroxisome proliferator-activated receptor α (PPARα) and PPARγ in the liver, leading to hepatic glycogen depletion, insulin resistance, and steatohepatitis [6]. Taken together, it has been shown the pathophysiologic effects of PM2.5 may occur via activation of intracellular stress responses and innate immune pathways and synergize with other triggers or risk factors, such as high-fat diet, leading to modulation of cell metabolism or death programs [2,4,7].
In eukaryotic cells, the ER is primary recognized as a compartment for protein folding and assembly [8]. A variety of biochemical, physiological, or pathological conditions can directly or indirectly interrupt the protein folding process, causing the accumulation of unfolded or misfolded proteins in the ER lumen −a condition referred to as “ER stress”. The UPR pathways are activated to help the cell adapt to ER stress conditions by remodeling transcriptional and translational programs. The basic UPR pathways are mediated through three primary ER-localized protein stress sensors: PERK (double-strand RNA-activated protein kinase-like ER kinase), IRE1α (inositol-requiring 1α), and ATF6 (activating transcription factor 6). The UPR signaling is known to intersect with a variety of inflammatory pathways as well as oxidative stress responses, all of which may influence lipid and glucose metabolism [9-12]. In this study, we demonstrated that long-term exposure to environmentally relevant PM2.5 induces macrophage infiltration and activation of distinct UPR pathways mediated through IRE1α, including ER-associated Degradation (ERAD) and Regulated IRE1-dependent mRNA Decay (RIDD) [13], in mouse white adipose tissue. Along with activation of the UPR pathways and infiltration of macrophages, expression of the genes involved in lipogenesis, adipocyte differentiation, and lipid droplet formation was significantly increased in the adipose tissue of the mice exposed to PM2.5. These results provide important mechanistic evidence that PM2.5 modulates inflammatory stress responses and lipid metabolism in fat tissue, which may partially explain the link between air pollution and development of metabolic syndrome.
Ethics statement
All animal works have been conducted according to relevant national and international guidelines. All the experimental procedures were performed in accordance with the recommendations of the Weatherall report, “The use of non-human primates in research”. The Committees on Use and Care of Animals at Ohio State University and Wayne State University approved all experimental procedures.
Animal model and ambient PM2.5 exposure
C57BL/6 male mice at six-weeks of age were purchased from the Jackson Laboratories (Bar Harbor, ME) and were housed in cages with regular chow in an Association for Assessment and Accreditation of Laboratory Animal Care-accredited animal housing facility. The Committees on Use and Care of Animals at the Ohio State University approved all experimental procedures. Mice were randomly assigned a group and were exposed to concentrated ambient PM2.5 or filtered air (FA) for 6 hours/day, 5 days/week from April 2009 to January 2010 in an exposure facility “Ohio’s Air Pollution Exposure System for the Interrogation of Systemic Effects” (OASIS)-1 in Columbus, OH, USA, as previously described [3]. The PM2.5 components to which the animals were exposed are primarily attributed to long-range transport [3]. The control mice in the experiment were exposed to an identical protocol with the exception of a high-efficiency particulate-air filter positioned in the inlet valve position to remove all of the PM2.5 in the filtered air (FA) stream. On the final day of the exposure, the mice were euthanized and tissue samples were collected for further studies.
Energy-dispersive x-ray fluorescence (ED-XRF)
All particle samples for gravimetric and elemental analyses were collected on filters. Filter masses were measured on a microbalance (model MT5, Mettler-Toledo Inc., Highstown, NJ). Analyses for major elements followed by nondestructive XRF (model EX-6600-AF, Jordan Valley) using five secondary fluorescers (Si, Ti, Fe, Ge, and Mo) and spectral software XRF2000v3.1 (U.S. EPA and ManTech Environmental Technology, Inc.) as described elsewhere [14].
In vitro exposure to PM2.5
For PM2.5 in vitro exposure experiments, the Teflon filters used for gravimetric and elemental analyses were placed downstream of the cyclone inlet of the “OASIS-1” exposure system to collect ambient particulates as previously described [14-16]. PM2.5 was collected during mouse exposure. Mouse monocyte/macrophage cell line RAW264.7 was treated with PM2.5 at the concentration of 5 μg/ml, and the same volume of PBS was added in the control group.
Hematoxylin and eosin (H&E), immunohistochemsitry or immunofluorescence staining
H&E staining was performed on paraffin-embedded tissue sections (5 μm) of mouse epididymal adipose tissue. Sections were placed on slides, deparaffinized in xylene, hydrated in graded ethanol, and rinsed in distilled water as previously described [11]. Slides were incubated in Harris Hematoxylin (Sigma) for 3 minutes and dipped in Scott’s Tap Water (Sigma) 5 times. Sections were counterstained in eosin (Sigma) for 10 minutes. Slides were dehydrated in 100% ethanol and cleared in xylene. Permount (Fisher) was used to mount slides. For immunohistochemistry staining, the 5 μm adipose tissue sections were blocked with 0.5% H2O2 in methanol to reduce endogenous peroxidase activity. The sections were incubated with anti-BiP/GRP78 antibody (1:100) overnight at 4°C. The slide sections were washed and then incubated with the second antibody conjugated with HRP at room temperature for 2 hours. Sections were developed in VECTASTAIN peroxidase substrate (Vector laboratories, Burlingame, California) and counterstained with hematoxylin. Immunofluorescent staining of mouse monocyte/macrophage cell line RAW264.7 was performed based on the standard protocol. Briefly, RAW264.7 cells cultured on glass cover-slips were fixed with ice-cold methanol for 20 minutes, blocked with 1% BSA in PBS, and then incubated with anti-F4/80 (1:200) antibody in blocking solution overnight at 4°C. The nucleus was stained with DAPI. The cells were then incubated with Alexa fluor 488 (Invitrogen) for 1 hour at room temperature before being mounted and examined by fluorescent microscopy.
Quantitative real-time reverse-transcription (RT-PCR)
Epididymal mouse adipose tissues from the mice were excised, and RNA was isolated using Trizol reagent according to the manufacturer’s instructions. The cDNAs were synthesized from mRNA templates using SuperScript III First-Strand Synthesis SuperMix for qRT-PCR (Invitrogen). Real-time PCR was performed using a reaction mix of cDNA, primers, and Fast SYBR Green Master Mix (Applied Bioscience). Samples were denatured at 95°C for 20 seconds then cycled between 95°C (3 seconds) and 60°C (30 seconds) using the 7500 Fast Real-Time PCR System (Applied Biosystems). The sequences of primers for examining the regulated IRE1-dependent decay (RIDD) were previously as previously described [13]. The other real-time PCR primer sequence information is shown in Supplemental Information. Fold changes of mRNA levels were determined after normalization to internal control β-actin RNA levels.
Western blot analysis
Total cell lysates were prepared from cultured RAW264.7 cells using NP-40 lysis buffer (1% NP-40, 50 mM Tris-HCl, pH 7.5, 150 mM NaCl, 0.05% SDS, 0.5 mM Na vanadate, 100 mM NaF, 50 mM β-glycerophosphate, and 1 mM phenylmethylsulfonyl fluoride) supplemented with protease inhibitors (EDTA-free Complete Mini, Roche). Denatured proteins were separated by SDS-PAGE on 10% Tris-glycine polyacrylamide gels and transferred to a 0.45-mm PVDF membrane (GE Healthcare). The membrane was incubated with a rabbit polyclonal anti-phosphorylated IRE1α antibody (Abcam, 1:1000), or rabbit polyclonal anti-IRE1α antibody (Cell Signaling Technologies, 1:1000) as the primary antibody, and a HRP-conjugated anti-rabbit secondary antibody. Membrane-bound antibodies were detected by an enhanced chemiluminescence detection reagent (GE Healthcare). Levels of α-tubulin were determined as loading controls. The signal intensities were determined by Quantity One 4.6.7 (Bio-Rad Life Science, CA). The ratios of signal intensities of phosphorylated IRE1α to that of total IRE1α were first determined. The fold changes of IRE1α phosphorylation in PM2.5-exposed RAW264.7 cells were determined by comparing IRE1α phosphorylation ratio in PM2.5-exposed RAW264.7 cells to that in vehicle-exposed control RAW264.7 cells (100%). All in vitro experiments were repeated at least three times independently.
Statistics
Experimental results are shown as mean ± SEM (for variation between animals or experiments). The mean values for biochemical data from the experimental groups (PM2.5 exposure verse filtered air) were compared by a paired or unpaired, 2-tailed Student’s t test. Statistical tests with P<0.05 were considered significant.
PM2.5 exposure leads to macrophage infiltration in white adipose tissue
To elucidate in vivo effect of long-term PM2.5 exposure, male C57BL/6J mice were exposed to concentrated ambient PM2.5 for 10 months in the “OASIS-1” exposure system composed of the Midwestern regional background in Columbus, OH, where most of the PM2.5 is attributed to long-range transport [5,17,18]. The “OASIS-1” is a versatile aerosol concentration enrichment system through which PM2.5 fine and ultrafine particles are concentrated and exposed to the animals in the chamber [19,20]. It has been demonstrated that the distribution and size of concentrated PM2.5 collected from the exposure chamber air truly reflect that of non-concentrated PM2.5 present in the ambient air [19-21]. The mobile OASIS-1 system allows us to perform the studies on animal models that recapitulate true personal, long-term exposure to environmental relevant PM2.5. During the exposure time period, the mean daily PM2.5 concentration was 12.7 (s.d., 8.4) μg/m3. The mean concentration of PM2.5 in the exposure chamber was 94.4 μg/m3 (~ 7-fold of ambient PM2.5 levels). The control mice in the experiment were exposed to an identical protocol with the exception of a high-efficiency particulate-air filter positioned in the inlet valve position to remove all of the PM2.5 in the filtered air stream. The X-ray fluorescence spectroscopic analysis of PM2.5 composition in the exposure chamber revealed higher concentration of a range of metals [18]. During the exposure, the major composition of PM2.5 included alkali metals (K and Na), alkaline earth metals (Mg and Ca), transition metals (Fe and Zn), and non-metals (S) (Table 1).
Table 1
Table 1
Elemental concentrations of PM2.5 particle during the exposure period from April 2009 to January 2010*
Our previous studies demonstrated that long-term ambient PM2.5 exposure induces inflammation and mitochondrial alteration in mouse adipose tissues [3,22]. White adipose tissue (WAT) has been recognized as an endocrine and secretory organ that plays key roles in the development of metabolic syndrome. To further delineate the impact of PM2.5 exposure on WAT, we first examined whether long-term PM2.5 exposure causes morphological changes in mouse WAT. H&E staining revealed increased cell infiltration in the WAT of the mice exposed to PM2.5, compared to that in the mice exposed to FA (Figure 1A). Based on the morphology of infiltrated tissue, the invading cells in WAT upon PM2.5 exposure are likely macrophages. To further characterize this observation, we stained the mouse WAT for mature macrophage cell surface F4/80. The F4/80 staining result confirmed activated macrophages invading the WAT of the PM2.5-exposed mice (Figure 1B).
Figure 1
Figure 1
Inhalation exposure to PM2.5 leads to macrophage infiltration and activation in WAT. A: H&E staining of WAT sections from mice after PM2.5 or FA exposure for 10 months. Magnifications: 100× (upper panel) and 200× (lower panel). (more ...)
PM2.5 exposure induces ER stress in white adipose tissue
In addition to macrophage infiltration, we also observed enlargement of adipocytes in WAT of the PM2.5-exposed mice. H&E staining indicated that sizes of adipocytes in WAT of the mice after 10-months PM2.5 exposure were significantly enlarged, compared to those in the mice after FA exposure (Figure 2A). The expansion of adipocytes may be partially due to inflammation and oxidative stress in fat tissue upon PM2.5 exposure, as we previously addressed [3]. To gain further understanding of the molecular basis responsible for the phenotype observed in the WAT of the PM2.5-exposed animals, we asked whether inhalation exposure to PM2.5 (whole-body PM2.5 exposure) can trigger ER stress response, a fundamental intracellular stress signaling that has profound impact in cell pathophysiology, in mouse WAT. We examined induction of the ER chaperone protein GRP78/BiP, a hall marker of ER stress and the master regulator of the UPR signaling, in the WAT of the PM2.5-exposed mice. Immunohistochemical staining showed that induction of BiP was increased in the WAT of the PM2.5-exposed animals, compared to that in the FA-exposed animals (Figure 2B), suggesting activation of ER stress response in the WAT upon PM2.5 exposure.
Figure 2
Figure 2
Exposure to PM2.5 enlarges adipocytes and induces BiP expression in mouse WAT. A: H&E staining of WAT sections from mice after PM2.5 or FA exposure for 10 months. Magnification: 200×. The graph beside the image shows fold change of adipocyte (more ...)
PM2.5 exposure activates ERAD- and RIDD- UPR pathways in white adipose tissue
To delineate the UPR pathways in WAT triggered by PM2.5 exposure, we performed quantitative real-time RT-PCR analysis of gene expression involved in ER stress and UPR signaling in WAT from the mice exposed to PM2.5 or FA. Levels of spliced Xbp1 mRNA, a target of ER stress sensor IRE1α RNase activity and a potent trans-activator of ERAD [8], were increased in the WAT of the mice exposed to PM2.5 (Figure 3A). Expression levels of the mRNAs encoding key components in ERAD including ER degradation enhancer, mannosidase alpha-like 1 (Edem1) and Der1-like domain family member 1 (Derl1), which are regulated by the UPR pathway through IRE1α/XBP1, were also increased in the WAT of PM2.5-exposed mice (Figure 3A). EDEM1 is known to extract misfolded glycoproteins from the calnexin cycle for their degradation in an N-glycan-independent manner [23,24]. DERL1 acts in the ERAD pathway by forming a channel that allows the retrotranslocation of misfolded proteins into the cytosol where they are ubiquitinated and degraded by the proteasome. Up-regulation of spliced Xbp1, Edem1, and Derl1 mRNAs in PM2.5-exposed mouse WAT suggests a distinct effect of PM2.5 exposure on accumulation of misfolded proteins in the ER and activation of IRE1α/XBP1-regulated ERAD pathway in WAT.
Figure 3
Figure 3
Exposure to PM2.5 induces expression of genes involved in UPR pathways and lipid metabolism in mouse WAT. Quantitative real-time PCR analysis of expression of genes involved in ER-associated degradation (A), IRE1α-dependent mRNA decay (B), lipogenesis, (more ...)
PM2.5 exposure also induces RIDD, a pathway mediated by the UPR transducer IRE1α to promote rapid turnover of mRNAs encoding membrane and secreted proteins [13]. Quantitative real-time RT-PCR analysis demonstrated that levels of the RIDD targets, including the Pmp22, Col6, HgNat, Blos1, Scara3, and PdgfR mRNAs [13], were all decreased in the WAT of mice exposed to PM2.5 (Figure 3B), indicating the activation of RIDD pathway upon PM2.5 exposure.
PM2.5 exposure stimulates lipogenesis, lipid deposition, and adipocyte differentiation
It has been demonstrated that enhanced lipogenesis and lipid droplet formation is associated with ER stress response in the liver or adipose tissues [12,25-27]. To determine whether PM2.5 exposure affects lipid metabolism in WAT, we examined expression levels of genes regulating lipogenesis, lipid transport, and lipid droplet formation in WAT of PM2.5 or FA-exposed mice. A significant increase of the lipogenic gene acetyl-CoA carboxylase (Acaca) was observed in PM mice (Figure 3C). ACACA facilitates the conversion of acetyl-CoA to malonyl-CoA, the rate limiting step of fatty acid synthesis [28]. Expression of the long-chain fatty acid transporter, Fat/Cd36, was also significantly increased in WAT of PM2.5-exposed mice (Figure 3C), indicating an increase in fatty acid uptake and deposition [29]. Additionally, expression of the key enzyme that catalyzes TG synthesis, diglyceride acyltransferase 2 (Dgat2), and hormone sensitive lipase (Lipe), which catalyzes the rate-limiting step of triglyceride breakdown, was increased in WAT upon PM2.5 exposure (Figure 3C). These increases may represent an adaptive response in adipose tissue to increased levels of triglycerides [30,31].
White adipocytes contain a large single lipid droplet surrounded by a layer of cytoplasm. Consistent with enlarged cell sizes and increased lipogenesis in WAT upon PM2.5 exposure, expression of genes encoding functions in adipocyte differentiation and lipid droplet formation, including small adipocyte factor 1 (Smaf1), Carcinoembryonic antigen-related cell adhesion molecule 1 (Ceacam1), adipocyte-specific protein fat-specific protein 27 (Fsp27), Perilipin 1 (Plin1), and fat-inducing transcript 2 (Fit2), was increased in mouse WAT upon PM2.5 exposure (Figure 3D). Additionally, induction of Adipsin, an adipokine that plays important roles in autoimmunity, glucose transport, and triglyceride accumulation in fat tissues [32], was also increased in WAT of the PM2.5-exposed mice (Figure 3E). Taken together, these results indicate the stress responses triggered by PM2.5 facilitate de novo lipogenesis, lipid accumulation, and adipocyte differentiation. This is consistent with the enlarged adipocytes observed in mouse WAT upon PM2.5 exposure (Figure 2A).
PM2.5 exposure activates macrophages and the UPR transducer IRE1α in vitro
Our previous studies demonstrated that circulating or resident macrophages are primary targets of whole-body PM2.5 exposure in mediating inflammatory stress responses [5,6]. As our animal experiments have shown that PM2.5 exposure stimulates macrophage infiltration and activation in WAT (Figure 1), we tested whether PM2.5 can activate macrophages in vitro. Cultured mouse monocyte/macrophage cell line RAW264.7 was exposed to 5μg/ml of PM2.5 for 4, 8, and 12 hours, respectively. Activation of macrophages, as evidenced by F4/80 staining, was increased upon PM2.5 exposure in a time-dependent manner (Figure 4A). Next, we tested whether PM2.5 can induce UPR signaling in macrophages in vitro. Upon 12 hours of PM2.5 exposure, levels of phosphorylated IRE1α, an indicator of the IRE1α-mediated UPR signaling, were significantly increased in RAW264.7 cells (Figure 4B). This result supports the effect of PM2.5 on activation of the UPR signaling pathways mediated through IRE1α in the macrophage-infiltrated WAT from PM2.5-exposed animals (Figure 3A and and3B3B).
Figure 4
Figure 4
PM2.5 exposure activates macrophages and the UPR transducer IRE1α in vitro. A: Immunofluorescent analysis of F4/80 induction in RAW264.7 cells cultured in the presence of vehicle (PBS) or 5 μg/ml PM2.5 particles for 4, 8, and 12 hours, (more ...)
Traffic-related airborne PM2.5 is a complex mixture of is a complex mixture of particles and gases from gasoline and diesel engines, together with dust from wear of road surfaces, tires, and brakes [33,34]. Airborne PM2.5 demonstrates an incremental capacity to penetrate into the distal airway units and potentially enter the systemic circulation with diminishing sizes. Recent studies suggested that traffic-related PM may promote cardiovascular and/or metabolic diseases [17,35,36]. Disruption of adipose endocrine and metabolic function is thought to play a key role in dysregulated lipid and energy homeostasis, which is characteristic of disorders, such as obesity and type II diabetes. Recently, our group has reported the effects of chronic air-pollution exposure on the development of metabolic dysfunction. We have provided evidence that exposure to PM2.5 leads to visceral adipose inflammation and oxidative stress along with alterations in levels of circulating adipokines, including adiponectin and leptin [3,22]. PM2.5 exposure also significantly reduces mitochondria in both WAT and brown adipose tissue (BAT) and induces ER stress response in the liver [5]. Inhalation exposure to PM2.5 stimulates hepatic inflammation, depletes glycogen storage, and promotes lipid accumulation in the liver, contributing to glucose intolerance and insulin resistance [6]. The pathophysiologic effects of PM2.5 exposure on the liver apparently account for the development of metabolic syndrome under air pollution. In this study, we delineated the nature of the inflammatory stress response and alteration in lipid metabolic pathways in visceral adipose tissue triggered by PM2.5 exposure. Our study suggests that PM2.5 exposure in WAT induces ER stress and activates ERAD and RIDD, two unique UPR signaling pathways mediated through the UPR transducer IRE1α (Figures 3 and and4).4). Along with ER stress response, expression of the genes involved in lipogenesis, lipolysis, adipocyte differentiation, and lipid droplet formation was increased in WAT of the mice exposed to PM2.5 (Figure 3). These findings represent important new insights into the effects of PM2.5 exposure on metabolic disorders, such as type-II diabetes and obesity.
Based on our previous and current studies, inflammation, oxidative stress, and ER stress response are likely integrated in the liver and adipose tissue where they may form a “stress loop” that contributes to alteration in lipid and energy metabolism under PM2.5 exposure [3,6,9,22,37]. During this process, macrophage is the key player in mediating inflammatory stress responses in both liver and fat tissues under air pollutant exposure. Presumably, infiltrated macrophages, as we observed in WAT under PM2.5 exposure (Figure 1), may be primarily responsible for inflammation and ER stress response in WAT triggered by PM2.5. This interesting scenario needs to be further elucidated d in the future.
Under ER stress, splicing of Xbp1 mRNA by the UPR transducer IRE1α is a prerequisite for production of the functional UPR trans-activator XBP1 [9]. We previously showed that PM2.5 exposure can activate IRE1α, but suppresses splicing of Xbp1 mRNA in liver tissues [5]. However, the current study showed that PM2.5 exposure can induce both IRE1α activation and Xbp1 mRNA splicing in WAT (Figures 3 and and4).4). This discrepancy in Xbp1 mRNA splicing under PM2.5 exposure may be due to the differences in target tissues/cell types as well as PM2.5 exposure times and concentrations. PM2.5 exposure activates IRE1α in WAT, which mediates both Xbp1 mRNA splicing and RIDD pathway (Figures 3 and and4).4). In contrast, PM2.5 only activates IRE1α-mediate RIDD, but not Xbp1 mRNA splicing, in the liver tissue [5]. It is interesting to elucidate the conditions under which PM2.5 induces or suppresses Xbp1 mRNA splicing and its related pathophysiologic impact in the future. Moreover, for future research, whether alterations in ER stress are a cause or consequence of PM2.5 exposure and the temporality of these changes are important subjects to pursue. Whether the response in WAT occurs congruently with hepatic ER stress response that we previously demonstrated [5], and whether hepatic ER stress is a pre-requisite for functional changes in the adipose tissue also deserve further investigation.
Acknowledgement
Portions of this work were supported by National Institutes of Health (NIH) grants DK090313 and ES017829 to KZ, and NIH ES018900 to QS. RM is supported by a training grant provided by NIGMS R25GM058905.
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