Cellular Senescence and Extracellular Vesicles in the Pathogenesis and Treatment of Obesity-A Narrative Review

Abstract

This narrative review explores the pathophysiology of obesity, cellular senescence, and exosome release. When exposed to excessive nutrients, adipocytes develop mitochondrial dysfunction and generate reactive oxygen species with DNA damage. This triggers adipocyte hypertrophy and hypoxia, inhibition of adiponectin secretion and adipogenesis, increased endoplasmic reticulum stress and maladaptive unfolded protein response, metaflammation, and polarization of macrophages. Such feed-forward cycles are not resolved by antioxidant systems, heat shock response pathways, or DNA repair mechanisms, resulting in transmissible cellular senescence via autocrine, paracrine, and endocrine signaling. Senescence can thus affect preadipocytes, mature adipocytes, tissue macrophages and lymphocytes, hepatocytes, vascular endothelium, pancreatic β cells, myocytes, hypothalamic nuclei, and renal podocytes. The senescence-associated secretory phenotype is closely related to visceral adipose tissue expansion and metaflammation; inhibition of SIRT-1, adiponectin, and autophagy; and increased release of exosomes, exosomal micro-RNAs, pro-inflammatory adipokines, and saturated free fatty acids. The resulting hypernefemia, insulin resistance, and diminished fatty acid β-oxidation lead to lipotoxicity and progressive obesity, metabolic syndrome, and physical and cognitive functional decline. Weight cycling is related to continuing immunosenescence and exposure to palmitate. Cellular senescence, exosome release, and the transmissible senescence-associated secretory phenotype contribute to obesity and metabolic syndrome. Targeted therapies have interrelated and synergistic effects on cellular senescence, obesity, and premature aging.

Keywords: AMPK; DDR; ER stress; NEFA; ROS; SASP; SIRT-1; VAT; Western-type diet; adiponectin; aging; autophagy; cellular senescence; epigenome; exosomes; extracellular vesicles; hypertrophic obesity; insulin resistance; lipotoxicity; metabolic syndrome; miRNA; obesogenic environment; p53; senolytic.

Figure 1

The Randle and reverse Randle effects in the cyclical inhibition of substrate metabolism during feeding (glucose oxidation) and fasting/exercise (β-oxidation of FFAs), which is dysregulated by insulin resistance, hyperglycemia, and hypernefemia. Glucose is transported (stimulated by insulin) and partially metabolized through glycolysis prior to conversion of pyruvate in the mitochondrion to acetyl-CoA by pyruvate dehydrogenase (PDH). Acetyl-CoA is oxidized through the Krebs cycle to generate reducing equivalents (NADH and FADH₂). Uptake of fatty acids is primarily dictated by plasma concentrations, and fatty acid metabolism through β-oxidation in the mitochondrial matrix contributes the largest contribution to ATP production under normal conditions. NADH and FADH₂ generated from substrate oxidation are used in the process of oxidative phosphorylation through the coupled reactions of the electron transport chain and an ATP synthase. Several points of regulation are depicted; red dashed lines indicate points of negative feedback (inhibition), and green dashed lines indicate points of positive feedback (see text). FA, fatty acids; OM, outer membrane (mitochondrial); IM, inner membrane. Reproduced from Berthiaume 2016 [48].

Figure 2

Schema of pathways producing and utilizing endoplasmic reticulum (ER)-derived H₂O₂. Reduced thiols (-SH) of cysteine residues in nascent proteins are oxidized by catalytically competent protein disulfide isomerase (PDI) to promote protein folding with native disulfide bonds (S-S). PDI can be re-oxidized by ERO1, with flavin adenine dinucleotide (FAD) embedded in its active site, which reduces molecular oxygen (O₂) to hydrogen peroxide (H₂O₂). ER-resident antioxidants PRDX4, GPX7, and GPX8 oxidize their catalytic domain by utilizing H2O2 (-SOH = sulfenic acid) and mediate “the disulphide relay” to PDI. NOX4 produces H₂O₂ over superoxide on the ER membrane, unlike other NOX family proteins. Reproduced from Konno et al. (2021) [70].

Figure 3

Maladaptive UPR and endoplasmic reticulum (ER) stress-mediated inflammation. Under ER stress, the UPR is activated, and this leads to activation of the three principal UPR signaling transmembrane receptor proteins, including IRE-1, PERK, and ATF6. Activation of the IRE-1 leads to the splicing of the mRNA of a transcription factor XBP1 and subsequent expression of sXPB1, a highly active transcription factor for the release of ER-resident enzymes and molecular chaperones. As a result, it leads to activation of NF-κB and CHOP, resulting in increased expression of proinflammatory gene products. Likewise, activated IRE-1 recruits TRAF2, and this complex causes activation of downstream signaling of kinases, including JNK and NF-κB, which induce production of inflammatory cytokines and trigger inflammation. These inflammatory kinases then phosphorylate and activate downstream mediators of inflammation. Phosphorylation of PERK/eIF2α downstream signaling pathway results in uncoupling of NF-κB from IkB. As a result, NF-κB translocates into the nucleus, where it activates expression of genes encoding proinflammatory cytokines, including IL-1, IL-6, and TNF-a, resulting in persistent inflammatory response. On the other hand, autophosphorylation of PERK initiates activation of eukaryotic initiation factor 2 (eIF2α), which further undergoes phosphorylation, resulting in translational attenuation of protein synthesis. Similarly, this downstream phosphorylation of eIF2α leads to increased expression of ATF4 and translocation into the nucleus, where it binds to the UPRE, resulting in transcriptional modification of CHOP, a proapoptotic gene transcription factor that initiates inflammation as well as apoptosis. IRE-1 recruits TRAF2 and causes activation of downstream signaling of kinases, including JNK and NF-κB, inducing the production of inflammatory cytokines. PERK phosphorylates eIF2α, which leads to the activation of NF-κB and CHOP to further promote the expression of the inflammatory gene. ER stress leads to dissociation of TRAF from TRAF2-procaspase 12 complex, which is located on the ER membrane, leading to activation of caspase 12. At the same time, IRE1–JNK complex recruits TRAF2 leading to the formation of the IRE1–JNK–TRAF2 complex. The ATF6 pathway also activates NF-κB, further intensifying the expression of inflammatory genes, which secrete more cytokines. Reproduced from Amen et al. (2019) [14].

Figure 4

Unfolded protein response signaling with their regulating microRNAs. miRNAs have a crucial role in shaping the UPR, while miRNA expression is also regulated by UPR. miR-181, miR-30, miR-199a, miR-495, and miR-375 negatively regulate GRP78. On the other hand, miR-322 suppresses IRE1. XBP1 and RIDD signaling, members of IRE1 branch in UPR, regulate miR-153, miR-346, miR-34a, miR-17, miR-96, and miR-125b. Additionally, miR-34c, miR-665, and miR-30c are known to target XBP1, while miR-26a suppresses eIF2α, reducing the protein translation. Certain miRNAs also form a link between UPR branches. In this direction, PERK-mediated miR-30c activation inhibits XBP1 pathway and establishes a negative crosstalk between PERK and IRE1 branches of UPR. Reproduced from Demirel-Yalciner (2022) [80].

Figure 5

Unifying hypothesis for transmissible ER stress, UPR, adipocyte SASP, NLRP3 inflammasome activation and inhibition of human antigen R (HuR) RNA-binding protein, SIRT-1, heat shock factor 1 (HSF-1), adiponectin (APN), and impaired heat shock (HS) response during chronic metabolic stress in obesity. Under conditions of nutrient excess, insulin signaling, hypoxia, and ER stress, hypertrophic adipocytes fail to undergo apoptosis and thus develop a senescence-associated secretory phenotype (SASP). This is associated with loss of HuR-SIRT1-HSF1 pathways, impaired HS response, prolonged activation of the NLRP3 inflammasome, and release of detrimental exosomal miRNA. The WAT SASP and ER stress can spread to other tissues, including pre-adipocytes, WAT-infiltrating bone marrow-derived macrophages (BMDMΦ), skeletal muscle, vascular endothelium, pancreatic β cells, and Kupffer cells/hepatocytes. This leads to accelerated obesity, atherosclerosis, insulin resistance, T2DM, and NASH/liver cirrhosis. A key feature of SASP in hypertrophic adipocytes is the recruitment of BMD monocytes and their polarization to metabolically activated macrophages via cytokine, chemokine, and exosomal microRNA (miR-34a, miR-155) release and formation of crown-like structures (CLS) in WAT. Lean WAT produces exosomal adiponectin (APN), which, in the presence of a functional heat shock response, inhibits vascular, pancreatic β-cell, hepatocyte, and WAT metaflammation and senescence. In obese VAT, exosomal adiponectin is decreased by 40-fold. Impaired multimerization of adiponectin due to vitamin C deficiency contributes to ER stress and UPR in hypertrophic obesity and loss of HMW adiponectin secretion. Hypoxia in hypertrophic obesity also impairs oxygen and ascorbate-dependent ER protein folding, leading to the UPR response and further ER stress. During adipocyte hypertrophy, ER stress can be improved by vitamin C, tauro-ursodeoxycholic acid (TUDCA), or 4-phenylbutyric acid (4-PBA) administration [22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89].

Figure 6

Alterations in lipid metabolism are associated with insulin-resistant states [108]. Obesity and T2DM are associated with increased lipolysis in adipose tissue owing to the action of or resistance to multiple hormones and to the increased production of cytokines (tumor necrosis factor (TNF) and interleukin-6 (IL-6)) by ATMφs. The release of TNF and IL-6 from macrophages is potentiated by the adipocyte secretion of adipocytokines, such as retinol-binding protein 4 (RBP4). FFAs, including long-chain fatty acids (LCFAs) that are released by lipolysis, are taken up by muscle and liver via the fatty acid transporter scavenger receptor class B member 1 (SRB1). In muscle, LCFA thioesters (LCFA-CoAs) are imported into mitochondria for β-oxidation via the carnitine shuttle, in which LCFA-CoAs are converted into long-chain acylcarnitines (LCACs). Incomplete β-oxidation in insulin-resistant states causes accumulation of acylcarnitines (ACs) of varying lengths, which are associated with insulin resistance and hyperglycemia. In the liver, LCFAs are imported into mitochondria and oxidized to generate acetyl-CoA, which activates pyruvate carboxylase, leading to increased production of phosphoenolpyruvate (PEP) from pyruvate. Glycerol generated from lipolysis, in addition to PEP, is converted into glucose-6-phosphate (G6P), resulting in increased hepatic glucose production. Overall, the increased flux of metabolic substrates into liver causes insulin resistance and hyperglycemia. Although an increase in AC muscle content correlates with insulin resistance, a causative effect has not been established in vivo. Hypoxia and stabilization of HIF-1α blocks mitochondrial fatty acid β-oxidation by inhibiting LCAD, MCAD, and CPT1 and leads to lipotoxicity and insulin resistance. Reproduced from Yang et al. (2018) [108].

Figure 7

Schematic representation of adiponectin signal transduction [118]. Implicating crosstalk with the insulin signaling pathway: insulin and adiponectin interact with their respective receptors, which trigger a cascade of signaling events. Insulin actions are mainly carried out by PI3K/AKT pathway, resulting in increased protein synthesis, lipogenesis, glucose uptake and utilization, glycogen synthesis, and reduced lipolysis and gluconeogenesis. Interaction of adiponectin with its receptors (Adipo R1 and R2) results in the activation of multiple signaling pathways, including IRS1/2, AMPK, and p38 MAPK. Activation of IRS1/2 by adiponectin signaling is a major mechanism by which adiponectin sensitizes insulin action in insulin-responsive tissues. Adiponectin drastically increases the expression and activity of PPARα, which upregulates acetyl-CoA oxidase (ACO), CPT1, and uncoupling proteins (UCPs), promoting FAO and energy expenditure. Reproduced from Achari et al. (2017) [118].

Figure 8

Biogenesis and release of exosomes and recognition and endocytosis by recipient cell. The transfer of multivesicular bodies (MVBs) to lysosomes for degradation (autophagy) or their fusion with the plasma membrane and ILV exocytosis as exosomes (which is increased in cellular senescence) is depicted. The enlarged exosome diagram shows exosome cargo and membrane molecules and receptors. The formation of microvesicles by outward plasma membrane budding (ectosomes) is also shown. Exosomal membrane lipids include cholesterol, ceramides, sphingomyelin, phosphatidylinositol (PI), phosphatidylserine (PS), phosphatidylcholine (PC), phosphatidylethanolamine (PE), and gangliosides (GM) [133,135,139].

Figure 9

Exosome number and size, adiponectin secretion, and relationship to fat depot source and obesity. (A) Lean (Ctrl) and obese (Ob) mouse VAT-derived large extracellular vesicles (lEVs) and small extracellular vesicles (sEVs) mode size distribution curve comparison. Ctrl and Ob EV subtype size distribution curves are represented by plain or dashed lines, respectively, and are presented as the mean ± SEM. (B) lEV and sEV secretion from ob/ob mouse obese VAT, SAT, and BAT explants. EV number secreted per gram of AT per 24 h is presented. (C) Increased lEV and sEV secretion from ob/ob mouse VAT with obesity. EV secreted by total mouse VAT is presented. (D) EV secretion of human omental, mesenteric (Mesent.), and subcutaneous (Subcut.) AT collected from human subjects with obesity. Secretion of EVs is presented as the number of EVs secreted per gram of human AT. (E) VAT-derived lEVs and sEVs (8 mg each) from lean mice reveal a particular enrichment of adiponectin in sEVs. Each color represents an independent experiment, with lEV and sEV preparations derived from the same VAT. n = 6, mean ± SEM, * p ≤ 0.05 (t test for matched pairs). (F) ELISA quantification of the adiponectin content of VAT-derived sEVs isolated from lean or obese VAT explants. Obese VAT-derived sEVs, isolated from either ob/ob (Ob) or HFD mice, are depleted of adiponectin (Adpn) compared to sEVs produced from VAT collected from their respective lean control mice (Ctrl and SD). The results are presented as box and whisker plots with the mean from four sEV-independent samples measured for each condition (Ctrl, Ob, SD, and HFD). Data are presented as the mean ± SEM for (A–F), with dot plots representing independent samples. Statistical significance is indicated for each panel as follows: * p ≤ 0.05 and ** p ≤ 0.01. Reproduced from Blandin et al. (2023) [83].

Figure 10

Lipids as signaling molecules that regulate metabolism. Several signaling lipids, including fatty acids, fatty acid esters of hydroxy fatty acids (FAHFAs), diacylglycerol (DAG), and ceramides, regulate insulin sensitivity. (a) Saturated fatty acids (SFAs) activate Toll-like receptor 4 (TLR4), possibly via its co-receptor, myeloid differentiation protein 2 (MD2), which, through the adaptor proteins TIR domain-containing adaptor-inducing interferon-β (TRIF) and myeloid differentiation primary response protein MYD88, increases the activity of pro-inflammatory transcription factors. TRIF activation promotes the nuclear translocation of interferon regulatory factor 3 (IRF3) to increase the expression of cytokines. MYD88 activation increases phosphorylation of inhibitor of nuclear factor-κB (NF-κB) kinase (IKKβ), which further phosphorylates inhibitor of NF-κB (IκB), leading to nuclear translocation of NF-κB to increase pro-inflammatory cytokine expression. MYD88 also activates Jun N-terminal kinase (JNK) to increase the activity of transcription factor activator protein 1 (AP1), thereby altering the expression of cytokines. Proinflammatory cytokines, through their receptors, further activate these proinflammatory transcription factors to establish a positive feedback loop for sustained inflammation. SFA-activated TLR4 and cytokine production impair insulin signaling through IKKβ and JNK activation. SFA-mediated activation of endoplasmic reticulum (ER) stress, the SRC–JNK pathway, and the incorporation of SFAs into DAG and ceramides also impair insulin signaling by inhibiting phosphorylation of the insulin receptor, insulin receptor substrate 1 (IRS1) or AKT. Polyunsaturated fatty acids (PUFAs) exert anti-inflammatory effects by activating G-protein-coupled receptor 120 (GPR120), which recruits β-arrestin 2 and sequesters TAK1-binding protein 1 (TAB1) to inhibit the TAK1-mediated activation of JNK and IKKβ. PUFA, FAHFAs, and resolvins may exert anti-inflammatory effects by activating G-protein-coupled receptors (GPCRs) in antigen-presenting cells such as macrophages to inhibit cytokine production. (b) DAG and ceramide may induce insulin resistance. DAG accumulates ectopically in insulin-resistant muscle and liver. In muscle, DAG-activated protein kinase Cθ (PKCθ) promotes the phosphorylation of IRS1 on Ser1101 in mice, impairing IRS1 phosphorylation on tyrosine and attenuating insulin signaling. In the liver, DAG-activated PKCε promotes phosphorylation of IR on Thr1160 to suppress insulin signaling. Elevated DAG impairs glucose uptake via reduced insulin-responsive glucose transporter 4 (GLUT4) in muscle, and glucose output via GLUT2 from the liver is increased; these changes induce hyperglycemia. Ceramides contribute to hyperglycemia by activating protein phosphatase 2 A (PP2A), which dephosphorylates AKT, stimulating PKCλ and PKCζ, which prevent AKT membrane association, thereby inhibiting AKT activity. Reproduced from Yang et al. (2018) [108].

Figure 11

Schematic representation of glycerolipid metabolism in the synthesis of diacylglycerol (DAG) and triacylglycerol (TAG) from glycerol and acyl-CoA [246]. PC, phosphatidylcholine; PE, phosphatidylethanolamine; PS, phosphatidylserine.

Figure 12

mRNA targets of microRNAs correlated to altered insulin resistance after surgery. Green color indicates predicted negative correlation of mRNA transcripts (i.e., positive correlation of microRNAs) to HOMA change following surgery, while red color indicates the reverse relationship (r > 0 for mRNA targets and r < 0 for microRNAs). Genes in grey and marked with an asterisk (*) represent targets of multiple microRNAs that have different correlational directions in relation to HOMA change. Reproduced from Hubal et al. (2017) [299].