Skeletal muscle inflammation and insulin resistance in obesity (original) (raw)

Despite the evidence for increased SM inflammation in obesity, the underlying mechanisms remain largely unexamined. Below, we detail potential roles for various mediators in SM inflammation.

Chemokines, adhesion molecules, and immune cell infiltration. Similar to what is observed in visceral AT (61, 74, 75), inflammation, including immune cell infiltration, starts early in SM during obesity development (35, 53, 56, 57, 60). Macrophage infiltration precedes T cell infiltration (35). Infiltration of leukocytes from the circulation into tissues requires attractant signals such as chemokines, and chemokines such as MCP-1 increase early in SM and visceral AT of mice fed a HFD. In visceral AT, the increase in MCP-1 appears to precede the increases in macrophages and the activation marker TNF-α (74, 75), suggesting that the initial increase in chemokines may derive from tissue-resident cells. Adipocytes and myocytes, the main resident cells in AT and SM, respectively, can express chemokines including MCP-1 (9, 32, 35–37, 61). Under stimulation with inflammatory molecules or FFAs or in obesity, adipocytes and myocytes secrete more chemokines (9, 32, 34–37, 61), which induce immune cell migration (9, 37, 61). Therefore, chemokines secreted by myocytes or adipocytes may play crucial roles in immune cell infiltration and inflammation in SM and visceral AT. MCP-1 overexpression in myocytes or adipocytes increases inflammation with enhanced immune cell infiltration in SM or visceral AT in mice (37, 76), while MCP-1 knockout prevents HFD-induced increases in muscle or AT macrophages (53). The RANTES/CCR5 pathway is also upregulated in SM and visceral AT in obesity (9, 35, 77) and may play a role in obesity-linked inflammation in visceral AT (77). The initiating signals that trigger SM or AT inflammation are not well known and may include FAs, particularly HFD-derived saturated FAs, which can induce expression of inflammatory molecules including chemokines in myocytes and adipocytes (34, 37). In addition to myocyte or adipocyte secretion of chemokines, as obesity progresses, recruited immune cells may also secrete chemokines, which may further increase inflammation in SM and AT.

The arachidonic acid–derived leukotriene LTB4, which is increased in SM, visceral AT, and liver of obese mice, also contributes to macrophage infiltration of visceral AT in obesity (78). Interactions of adhesion molecules on immune cells and their ligands on endothelial cells are crucial for immune cell migration. Lymphocyte function–associated antigen-1 (LFA-1), a β2 integrin mainly expressed on immune cells, plays an essential role in T cell accumulation and inflammation in SM and visceral AT of obese mice, likely by interacting with ICAM-1 on endothelial cells or antigen-presenting cells (35, 79).

While infiltration of circulating Ly-6Chi monocytes is important in obesity-linked inflammation and accumulation of proinflammatory CD11c+ macrophages in AT in mice (48, 80), the role of Ly-6Clo monocytes remains to be determined. In the circulation, Ly-6Clo, but not Ly-6Chi, monocytes express CD11c (81, 82). Circulating CD11c+/Ly-6Clo monocytes are increased with obesity and hyperlipidemia, infiltrate into atherosclerotic aortas, become CD11c+ macrophages/dendritic cells, and contribute to atherogenesis in mice (46, 81–83). Infiltration of CD11c+/Ly-6Clo monocytes likely also plays a role in CD11c+ macrophage accumulation and inflammation in visceral AT and SM in obesity. In addition, macrophages and T cells proliferate in visceral AT (79, 84, 85), and potential proliferation in SM warrants investigation.

Immune cell activation. Macrophages and T lymphocytes not only are increased in number but also display proinflammatory phenotypes in SM and visceral AT in obesity. The tissue inflammatory milieu, including increased cytokines, macrophage/T cell interactions, and increased FFAs and metabolites, may play key roles in immune cell proinflammatory activation in obesity (Figure 1).

Cytokines and signaling pathways in immune cell activation. Cytokines play central roles in immune cell activation. IFN-γ and TNF-α are crucial for macrophage polarization into M1 proinflammatory phenotypes, while IL-4, IL-13, and IL-10 are crucial for macrophage polarization into alternatively activated (M2) phenotypes (86). IL-12 is critical for T cell polarization to Th1, whereas IL-4 is critical for T cell polarization to Th2 phenotypes. TNF-α, the signature cytokine of M1-polarized macrophages, and IFN-γ, the signature cytokine of Th1, are both increased in SM and visceral AT in obesity and are involved in obesity-linked AT inflammation, including macrophage activation (35, 58, 87). These cytokines may also induce immune cell activation and play crucial roles in muscle inflammation. IL-10 is reduced in SM in obesity, and overexpression of IL-10 in SM attenuates obesity-induced macrophage activation in muscle (60).

TNF-α exerts proinflammatory effects mainly by activating IκB kinase/NF-κB (IKK/NF-κB) and JNK pathways. The IKK complex, which consists of the catalytic subunits IKKα and IKKβ and the regulatory subunit IKKγ, activates NF-κB transcription activity by phosphorylating and degrading the inhibitory protein IκB. Ablation of IKKβ in myeloid cells protects mice from obesity-induced inflammation (88). Activation of NF-κB in obesity also leads to increases in IKKε, a non-canonical IKK, in macrophages, adipocytes, and liver. Knockout or inhibition of IKKε in mice attenuates obesity-linked inflammation including reductions in accumulation and M1 polarization of macrophages in visceral AT and liver (89, 90).

Obesity increases JNK activity in muscle and AT (89, 91) and increases phosphorylated JNK levels in circulating monocytes (47). Ablation of JNK1 alone or both JNK1 and JNK2 in hematopoietic cells or myeloid cells dramatically decreases obesity-induced inflammation in mice (92, 93). Tissue culture studies support a crucial role of JNK in macrophage polarization to M1, but not M2, phenotypes (47, 92, 93).

IFN-γ exerts proinflammatory effects primarily through activating the JAK/STAT1 pathway. Upon binding its receptor, IFN-γ mainly activates JAK1 and JAK2, which phosphorylate and activate STAT1. STAT1 plays a pivotal role in M1 polarization and Th1 polarization (86). Short-term treatment of obese mice with a JAK1/JAK2 inhibitor decreases inflammation in SM (35), supporting an important role of the JAK/STAT pathway in obesity-linked muscle inflammation.

Cytokines may be the main mediators by which macrophages and T lymphocytes influence each other’s inflammatory status. For example, knockout of LFA-1 in mice reduces obesity-induced T cell infiltration and Th1 polarization, along with decreased IFN-γ levels, but does not change total macrophage content, in SM and visceral AT. However, macrophage expression of proinflammatory markers such as MCP-1 and TNF-α is decreased (35, 79), possibly because of reduced induction of macrophage activation by decreased Th1 cytokine in muscle.

T cells, particularly CD8+ memory T cells including those in AT, may become activated and proliferate under the stimulation of cytokines IL-12 and IL-18, which are mainly expressed by macrophages and dendritic cells and are increased in obesity (79). In addition, macrophages and dendritic cells can activate T cells through the MHC/antigen/TCR pathway. MHC-II and CD11c, which are mainly expressed on M1-like macrophages/dendritic cells, play important roles in macrophage/dendritic cell–induced T cell activation in obese AT (46, 84). Moreover, MHC-II is upregulated on obese adipocytes, which also contribute to T cell activation in obese AT (94). The potential role of these pathways in obesity-linked SM inflammation remains to be examined.

FFAs and signaling pathways in immune cell activation. In addition to increased cytokines, increased influx of FFAs (derived from lipolysis in AT or from a HFD; see below) usually occurs in SM in obesity. FAs, particularly long-chain saturated FAs, have been consistently shown to induce inflammation, thereby also likely contributing to immune cell activation in SM in obesity. Palmitic acid or a mixture of long-chain FAs increases macrophage expression of proinflammatory molecules and induces M1 polarization, possibly via engagement of TLR2 and TLR4 and subsequent activation of NF-κB and JNK pathways (47, 92, 93, 95). In addition, palmitic acid and its metabolite ceramide activate the NLRP3 inflammasome, a cytosolic multiprotein complex that activates caspase-1, leading to maturation and secretion of the proinflammatory cytokines IL-1β and IL-18 (96). Consistently, in addition to NF-κB and JNK, TLR2/4 and the inflammasome play crucial roles in obesity-linked macrophage proinflammatory activation and inflammation (47, 95, 96).

Influx of FAs into SM and triglyceride-rich lipoproteins. In obesity, elevated levels of circulating FFAs, mainly derived from lipolysis in adipocytes, lead to increased FA influx into SM, which not only induces inflammation in immune cells (see above) and myocytes in muscle, but also causes insulin resistance in myocytes (see below). In addition, obesity is usually associated with hypertriglyceridemia, with elevated levels of triglyceride-rich lipoproteins (TGRLs), including enterocyte-derived chylomicrons and hepatocyte-derived VLDLs, which may also release more FAs into SM and contribute to muscle inflammation and insulin resistance. Indeed, hypertriglyceridemia correlates with and may be a causal factor for insulin resistance and T2D (97). Diets enriched with saturated fat or carbohydrates tend to cause increased levels of TGRLs (98). Besides a diet high in saturated fat, a diet high in carbohydrates, particularly fructose, also induces inflammation in muscle (39, 99). In addition to the potential direct effect of high carbohydrates, elevated levels of TGRLs may contribute to muscle inflammation induced by a high-carbohydrate diet.

Under physiologic conditions, triglyceride in TGRLs is hydrolyzed by lipoprotein lipase (LPL) and releases FFAs, which are transferred into SM mainly as an energy source and into adipocytes, where they are re-esterified into triglyceride for storage (100). Increased blood TGRL levels (with no or modest changes in LPL activity; ref. 100) in obesity or increased LPL activity is expected to enhance TGRL-derived FA transfer into SM, leading to increased muscle lipid deposition and eliciting muscle inflammation. Indeed, obesity or muscle-specific overexpression of LPL increases muscle triglyceride content, with increased FA metabolites, including diacylglycerol (DAG) and ceramide, while muscle deletion of LPL decreases lipid content in SM (101–103). LPL-mediated lipid transfer appears to involve apolipoprotein E (apoE), as apoE deficiency impairs FA delivery, leading to less lipid content and decreased inflammation in muscle (58).

The renin-angiotensin system in immune cell activation. In addition to cytokines and FAs, the renin-angiotensin system (RAS), which is activated locally in SM and AT and systemically in obesity (104, 105), has been involved in regulation of inflammation including immune cell inflammation (106–108). The classical RAS involves cleavage of angiotensinogen by renin in the circulation and formation of angiotensin I (ANG I). ANG I is converted to active ANG II by angiotensin-converting enzyme (ACE), which is mainly expressed on endothelial cells in pulmonary circulation. The nonclassical RAS involves generation of ANG 1–7 from ANG I or II by ACE2 (109, 110).

By interacting with ANG II receptors (ATRs), ANG II plays important roles in regulating blood pressure and fluid and electrolyte balance (109, 110). In addition, ANG II plays pathologic roles in fibrosis, oxidative stress, and inflammation, which all occur in obesity, via hemodynamic (blood flow reduction) or non-hemodynamic effects (109, 110). ANG II can induce activation of NF-κB, expression of MCP-1, TNF-α, and VCAM-1, and production of ROS (which activates p38 MAPK) in monocytes, endothelial cells, and cultured myocytes (106–108, 111–113). ACE inhibitors and ATR blockers (ARBs) reduce inflammation, including SM and AT inflammation induced by obesity or fructose feeding (39, 114), indicating a crucial role of ANG II in SM and AT inflammation in obesity. In contrast, ANG 1–7 exerts cellular effects mainly through the Mas receptor (109, 110) and has antiinflammatory effects including inhibition of macrophage infiltration and proinflammatory activation in AT induced by HFD or high-fructose diet (115, 116).