Autophagy, insulin signaling, and insulin resistance in skeletal muscle tissue
Crosstalk between autophagy and insulin signaling is of particular significance. It was demonstrated that the inhibition of autophagy by chloroquine, bafilomycin, or the inactive ATG5 mutant diminished glucose uptake in the L6 skeletal muscle stimulated by insulin. In addition, considerable reductions were observed in the phosphorylation levels of Akt and IRS-1 [
86]. Numerous studies have described the importance of autophagy in the insulin-resistant skeletal muscle in vivo. In HFD-fed mice and animal models of polycystic ovary syndrome (PCOS) with hyperinsulinemia and insulin resistance, autophagy reduced in skeletal muscle [
86,
185]. However, six weeks of HFD feeding resulted in enhanced autophagy markers (LC3II and beclin), indicating that autophagy increased as a compensatory response to early insulin resistance [
86]. Interestingly, genetic animal models with continuous hyperactive autophagy manifest more insulin sensitivity on the HFD challenge, as evidenced by increased phosphorylated Akt in the gastrocnemius skeletal muscle [
93].
It is noted that insulin sensitizers, such as adiponectin, promote the autophagy pathway in skeletal muscle [
86]. Exercise, which is known to improve insulin resistance, partly functions through autophagy [
186]. In addition, Song et al. showed that dehydroepiandrosterone activates mTORC1 and inhibits autophagy, leading to insulin resistance in the skeletal muscle of mice with PCOS [
185]. Shi et al. reported that dihydromyricetin, a flavonoid component of herbal medications, improves the skeletal muscle insulin sensitivity by inducing autophagy via the AMPK–PGC-1α–sirtuin 3 (Sirt3) signaling pathway [
187]. The specific knockout of
ATG7 in skeletal muscle unexpectedly improved glucose uptake and insulin resistance following HFD feeding [
54].
The skeletal muscle of T2DM patients with hyperinsulinemia exhibited a reduction in autophagy, which was demonstrated by the reduced expression of
SQSTM1/p62,
ATG14,
GABARAPL1, RB1 inducible coiled-coil 1
(RB1CC1)/ family-interacting protein of 200 kDa
(FIP200), and WD repeat domain phosphoinositide-interacting protein 1 (
WIPI1)genes, decreased levels of SQSTM1/p62, LC3BII, and ATG5 proteins, as well as the elevated phosphorylation of FOXO3a [
188]. These data indicate that insulin resistance is associated with autophagy inhibition in muscles while increased insulin sensitivity had inverse effects [
189]. Conversely, Kruse et al. did not find any changes in the mRNA and protein levels of the autophagic markers of the skeletal muscle in T2DM patients, suggesting that markers of autophagy are probably adapted to hyperglycemia in skeletal muscle in T2DM patients [
189]. Regarding the in vivo impact of insulin on autophagy, it is noted that the glycemic status, as a crucial factor, as along with insulin affect autophagy in patients with diabetes [
190]. Insulin infusion led to a decreased LC3BII/I ratio in obese and lean individuals, but not in diabetic patients, indicating that insulin is capable of inhibiting autophagy in conditions other than diabetes [
189]. In this context, the results of the study by Lv et al. are noteworthy. They investigated autophagy alterations in hyperglycemic conditions with or without hyperinsulinemia, by using both animal models of hyperglycemia, i.e., hyperglycemia by glucose infusion and hyperglycemia induced by STZ. In STZ-treated rats, which present hyperglycemia and low insulin, autophagy proteins (LC3, ATG5, ATG12, and BECN1) were significantly elevated in the gastrocnemius muscle, which contributed to the increase of p-FOXO3a and the subsequent repression of mTOR. In contrast, in animals with glucose infusion hyperglycemia, which present hyperinsulinemia, autophagy markers, including LC3-II/I, ATG7, and BECN1 decreased in gastrocnemius muscle, indicating that insulin in the lack of insulin resistance can suppress autophagy in skeletal muscle [
191]. It appears that the status of insulin and glycemia, as well as insulin sensitivity, altogether create a unified response that determines the stimulation or inhibition of the autophagy process.
Furthermore, the glucagon level, which is elevated in the presence of high insulin and insulin resistance state, is another main factor that regulates autophagy [
192]. The effects of glucagon on the autophagy pathway are the opposite of insulin’s effects, i.e., glucagon activates autophagy. It has been found that chronic exposure to insulin diminishes the stimulating effects of glucagon on autophagy in hepatocytes [
101], indicating that insulin is more important than glucagon in autophagy regulation. However, more research is required to clarify the in vivo effects of different variables on skeletal muscle autophagy. Overall, the autophagy function is balanced under normal conditions, whereas it is inhibited in the presence of insulin. Nevertheless, it may be increased as a compensatory response to maintain homeostasis. Figure
3 shows insulin signaling in modules under physiological and pathological conditions (3.3.a and 3.3.b).
Insulin resistance is associated with elevated levels of glucose, circulating fatty acids, and TG, as well as lipid accumulation, which is linked to metabolism abnormalities in the skeletal muscle [
193]. It is well established that an HFD results in the accumulation of lipid derivatives, such as triacylglycerol, diacylglycerol, and ceramide, in skeletal muscle, leading to insulin resistance [
194]. Several studies support the role of autophagy in lipid metabolism, suggesting that it is impaired in pathological conditions such as insulin resistance [
86,
185,
195]. It was demonstrated that p62, an autophagy receptor targeting cargo to autophagosomes, is found on LDs, which indicates that lipophagy participates in the removal of TG from muscle cells such that the treatment of fatty acid-exposed muscle cells with rapamycin (an autophagy activator) reduced lipid accumulation while bafilomycin (an autophagy inhibitor) showed the opposite effect [
196]. On the other hand, lipids can modulate the autophagy process in skeletal muscle cells. It was demonstrated that HFD globally reduces the expression of autophagy genes in the soleus (SOL) muscle [
197]. Palmitate, a saturated fatty acid with high concentrations in circulation, and known to promote insulin resistance in skeletal muscle, impaired the autophagic flux in C2C12 cells [
195]. These data suggest that autophagy impairment contribute to lipid accumulation and thereby insulin resistance in muscles. In contrast, Morales-Scholz concluded that autophagy is neither implicated in lipid accumulation nor in the development of insulin resistance in skeletal muscle. In this study, feeding rats with a high saturated-fat diet (HSFD) for 16 weeks did not change the autophagic markers in their skeletal muscle, but autophagy defects were seen in the liver [
198]. The difference between oxidative muscles and glycolytic muscles in autophagy is of particular significance. While autophagic proteins (LC3-I, LC3-II, and SQSTM1) were higher in the oxidative muscle compared to the glycolytic muscle, autophagic flux was low in the oxidative muscle [
199]. In female mice fed an HFD, p62 and ATG12-5 accumulation was induced in the oxidative muscle (SOL) but not in the glycolytic muscle (plantaris) [
200]. It is noted that HFD induces the accumulation of diacylglycerols and ceramides in oxidative muscles SOL and extensor digitorum longus (EDL) but not in the glycolytic muscle epitrochlearis (Epit) [
201]. It appears that the importance of autophagy with regard to lipid accumulation in muscles depends on the muscle type.
In addition, it has been shown that autophagy activity is related to glucose homeostasis. It was found that the glycogen stores of the skeletal muscle were mobilized through the autophagy degradation pathway in newborn rats [
109]. The hexokinase (HK) enzyme, which catalyzes the first reaction of glycolysis, exerts a modulatory effect on autophagy [
202]. Roberts et al. demonstrated that HKII suppresses the autophagy inhibitor mTORC1, thereby activating autophagy [
203]. In C2C12 cells under the condition of low glucose concentrations, HKII was upregulated, which is associated with the induction of autophagy [
204]. It appears that HKII, as a regulatory molecule, plays a crucial role in the metabolic switch from glycolysis to autophagy during glucose starvation. Given that HKII is regulated by insulin, its possible downregulation during insulin resistance leads to impairments in the autophagy pathway.
It should be noted that skeletal muscle autophagy may be altered in response to pathological conditions manifesting metabolic syndrome. For example, in NAFLD, myocellular autophagy is enhanced as a metabolic response to hyperammonemia [
205], or in animals with hypertension, autophagy markers, including LC3I, ATG7, and lysosome-associated membrane protein 2 (LAMP2), are increased in skeletal muscle [
206]. It can be concluded that the metabolism of muscle cells is influenced by insulin resistance, which is associated with autophagy dysregulation [
205,
206].
The role of myokines in autophagy-related insulin resistance
Myokines are peptides or cytokines that are synthesized and secreted by skeletal muscle, with autocrine, paracrine, and endocrine activities. They include myonectin, myostatin, apelin, fibroblast growth factor 21 (FGF21), brain-derived neurotrophic factor (BDNF), irisin, IL-15, IL-6, and other similar factors [
207]. Myokines are potentially involved in regulating metabolism, autophagy, and insulin resistance as well as crosstalk between skeletal muscle and other tissues [
208].
Myostatin is known as a negative regulator of skeletal muscle mass. In addition, it plays a role in the pathogenesis of insulin resistance. Its circulating levels are increased in obese patients with insulin resistance, and it has a negative correlation with adiponectin levels and insulin sensitivity indices [
209]. In C2C12 skeletal muscle cells, it was found that myostatin exerted deleterious effects on the factors involved in the insulin signaling pathway, including IRS-1, glycogen synthase kinase-3 (GSK3), Akt, GLUT4, AMPK, and PGC1α [
210]. Furthermore, it was reported that myostatin potentiates the autophagy process in C2C12 cells, accounting for muscle mass loss and muscle atrophy in these cells [
211]. Studies have shown that myostatin inhibition by gene silencing [
212], anti-myostatin oligomers [
213], or by anti-myostatin antibodies [
214] improves insulin resistance. It appears that the improvement of insulin sensitivity by myostatin suppression is due to the increased muscle mass and insulin signaling reinforcement rather than autophagy regulation.
Irisin is primarily produced by the skeletal muscle. Both autophagy and irisin increase in skeletal muscle following exercise and it seems that both mediate the beneficial effects of exercise on skeletal muscle insulin resistance [
215]. In insulin-resistant C2C12 skeletal muscle cells, it was found that irisin could induce the autophagy process through the p38 MAPK/PGC-1α axis, improving insulin signaling and glucose uptake [
216].
Several studies have reported that CTRP15 is related to insulin resistance in patients with metabolic syndrome [
217], coronary artery disease [
218], T2DM [
219], and PCOS [
220]. Its serum levels significantly increased in individuals during exercise [
221]. It has been demonstrated that CTRP15 negatively regulates liver autophagy via Akt/mTOR signaling, as evidenced by reduced
Atg expression and p62 degradation [
222].
IL-15 and IL-6 are cytokines secreted by skeletal muscle cells, addition to the immune cells during immune responses, and are involved in the pathogenesis of autoimmune diseases and insulin resistance [
223]. IL-15 has been shown to regulate skeletal muscle mass by suppressing the ubiquitin proteolytic process. In addition, it directly stimulates glucose uptake in muscle cells [
224,
225]. High levels of IL-15 contribute to the improvement of insulin resistance in postmenopausal women with metabolic syndrome [
226]. IL-15 triggers autophagy in the immune cells, mediating homeostasis and the survival of these cells [
227,
228]. IL-6 is a pleiotropic cytokine with respect to insulin resistance; while numerous studies have indicated its association with insulin resistance [
226‐
229], others have reported its insulin-sensitizing effects [
230,
231]. It has been found that IL-6 inhibits starvation-stimulated autophagy in pro-monocytic U937 cells [
184]. Nevertheless, it is still unclear whether IL-15 and IL-6 trigger autophagy in order to regulate insulin signaling in skeletal muscle cells.
Overall, myokines provide crosstalk between skeletal muscle and other tissues and serve functions in all tissues. The available data are not sufficient to fully describe their contribution to autophagy-related insulin resistance, and since skeletal muscle mass is crucial in insulin sensitivity and a high autophagy rate leads to muscle loss, it should be considered to gain a correct understanding of the role of myokines in autophagy-related insulin resistance.
Autophagy, mitochondrial dysfunction, ER stress, and oxidative stress in skeletal muscle tissue and their links with insulin resistance
A growing body of evidence links mitochondrial dysfunction and oxidative stress to insulin resistance [
229]. Particularly, in the muscle, it has been shown that elevated ROS levels stimulate the p38 mitogen-activated protein kinase (MAPK)/JNK/ERK pathway, which is involved in insulin resistance. Furthermore, increased levels of ROS activate mitophagy in the skeletal muscle such that the treatment of myotubes with H
2O
2 augments the levels of phosphorylated ULK1 and LC3 proteins [
230]. Autophagy can mutually modulate mitochondria function and oxidative stress. In the skeletal muscle of
ATG7-deficient mice, the protein carbonyl level, as a biomarker of oxidative stress, was increased and the ATP content and cytochrome oxidase activity were reduced, suggesting that autophagy deficiency leads to oxidative stress and mitochondrial impairments in the muscle, which is associated with reduced insulin resistance [
54].
ATG7 deficiency in the muscle is associated with the induction of FGF21. FGF21 is a mitokine that increases browning WAT and fatty acid oxidation, thereby improving insulin sensitivity [
54]. Kim et al. reported that mitochondrial dysfunction in the muscle with
ATG7 deficiency improves insulin resistance. This finding is in contrast with the results of previous studies, which stated that mitochondrial dysfunction and autophagy impairment exacerbate insulin resistance [
47,
184,
185]. It appears that the outcomes of autophagy deficiency depend on (1) where it occurs and (2) the component of the autophagy mechanism that is disrupted.
Considering the importance of muscle mass in whole-body responses to insulin, individuals who are prone to muscle mass loss are more likely to develop insulin resistance [
231]. Oxidative stress in muscles initiates the process of muscle atrophy via activating autophagy, thereby decreased insulin resistance [
232]. For example, neuronal nitric oxide synthase (nNOS) reduces muscle mass by activating oxidative stress and increases the expression of atrophy-related ubiquitin ligase genes (atrogin 1 and
MuRF1) [
233]. Moreover, MAPK induces autophagy during oxidative stress [
234]. The formation of autophagosomes in myofibers is associated with atrophy and muscular dystrophy [
235]. In addition, it has been indicated that the overexpression of PGC-1α (a mitochondrial biogenesis inducer) in skeletal muscle improves age-associated insulin resistance and reduces age-induced autophagy, thereby preventing muscle mass loss during aging [
238]. However, the muscle-specific knockouts of autophagy genes, including
Atg5−/− and
Atg7
−/−, lead to muscle atrophy, presenting a myopathy-like phenotype [
236,
237]. All the data demonstrated that proper muscle function requires the precise regulation of autophagy.
ER stress was observed in skeletal muscle with insulin resistance. It has been shown that ER stress disrupts insulin signaling in skeletal muscle and therefore reduces glucose uptake, which is mediated by tribbles-like protein 3 (TRBP3) [
156]. ER stress is also known as an autophagic inducer, since the three main arms of ER stress, i.e., PERK, IRE1α, and ATF6α, trigger autophagy by activating JNK, X-box binding protein 1 (XBP1), and ATF4 [
238]. It has been found that the treatment of C2C12 skeletal muscle cells with an ER stress inducer augments the conversion of LC3-I to LC3-II and decreases p62. In these cells, PKC-ϴ is demonstrated to mediate the ER stress-induced effects on autophagy [
239]. The activation and membrane localization of PKC-ϴ might contribute to identifying membrane vesicles that form autophagosomes. Another candidate molecule that links ER stress to autophagy is skeletal muscle- and kidney-enriched inositol polyphosphate 5-phosphatase (SKIP). This phosphatase, which is upregulated under ER stress, negatively regulates insulin signaling by dephosphorylating PI3, and therefore, stimulating autophagy [
240]. In general, ER stress is followed by autophagy activation, as a protective mechanism to remove misfolded proteins and thus relieve ER stress [
238], providing cell survival. Additionally, the alleviation of ER stress is one of the main mechanisms by which autophagy lowers insulin resistance [
93]. It was found that the genetic hyperactivation of autophagy improved HFD-induced insulin resistance in insulin-responsive tissues such as skeletal muscle, which was attributed to relieved ER stress, as evidenced by reduced levels of ER stress markers CHOP and ATF-6 factor in the skeletal muscle, liver, and adipose tissue [
93]. In support of these findings, it was shown that ER stress inducers reversed the beneficial effects of autophagy hyperactivation in the aforementioned tissues [
93].
However, autophagy acts as a defense response against ER stress, and depending on the degree of insulin resistance, this process is affected in different ways [
61]. It should be noted that autophagy dysregulation may contribute to ER stress-mediated insulin resistance. In this regard, it is demonstrated that autophagy inducers (rapamycin or adiponectin) relieved ER stress and insulin resistance in skeletal muscle L6 cells while autophagy suppression had the opposite effect [
241]. Therefore, it is concluded that under insulin resistance conditions, dysfunction of autophagy makes cells more vulnerable to ER stress.
Skeletal muscle insulin resistance is linked to inflammation, and the increase of inflammation enhances skeletal muscle insulin resistance [
242]. IL-1, TNF-α, C-reactive protein (CRP), and monocyte chemoattractant protein-1 (MCP-1) are associated with skeletal muscle insulin resistance [
243]. Insulin signaling is mainly disrupted by inflammation, ER stress, oxidative stress, the activation of the JNK and NF-кB pathways, and the increase of skeletal muscle insulin resistance. The JNK pathway and TNF-α disrupt insulin signaling by phosphorylating IRS-1 on the serine residue [
244,
245]. IL-1 is demonstrated to activate the IKK/NF-кB pathway in the skeletal muscle, decrease IRS-1 activity, and enhance skeletal muscle insulin resistance. Moreover, IL-6 increases the expression of the suppressors of cytokine signaling (SOCS) 1 and SOCS3, leading to the degradation of IRS-1 and the development of insulin resistance [
246]. Inflammation can also increase nitric oxide production, inhibit the PI3K-Akt pathway, and promote insulin resistance [
244]. Overall, it seems that autophagy, mitochondrial dysfunction, ER stress, inflammation, and oxidative stress in the skeletal muscle are closely related to each other as a network and can all be affected by insulin resistance.
Autophagy abnormality in β-cells under metabolic disorders
Metabolic disorders, such as obesity and diabetes, affect β-cell function through various mechanisms, including the dysregulation of autophagy. In these conditions, excess nutrients compel β-cells to secrete more insulin to maintain normal glucose levels. This increased demand on β-cells overloads the ER, leading to ER stress, poly-ubiquitinated protein aggregates, incomplete insulin processing, and/or irregularities in insulin secretion [
247]. It has been established that blocking autophagy through chemicals or short hairpin RNAs (shRNAs) enhances the death of β-cells under ER stress, while treatment with rapamycin, an autophagy inducer, increases the viability of β-cells in the presence of ER stress, which demonstrates the protective role of autophagy against ER stress in β-cells [
248]. Furthermore, evidence indicates that an excess of lipids and the resulting lipotoxicity, which is accompanied by metabolic abnormalities, significantly contribute to the impairment of β-cells. The expression of genes involved in lipid metabolism, oxidative stress, and apoptosis exhibits an abnormal pattern within the β-cells of individuals with type 2 diabetes [
249]. In these cells, the accumulation of lipid droplets is linked to the downregulation of transcription factor EB (TFEB), which is a crucial modulator of autophagy. This, in turn, led to a decrease in the levels of the lysosomal biomarker LAMP2 [
249]. Hyperglycemia is another primary factor that affects β-cells in metabolic disorders. It seems that chronic hyperglycemia or glucotoxicity affects lysosomal degradation in islets [
250]. Lipid overload and high glucose levels reduce the expression levels of TFEB, LAMP1, and LC3, as well as p62 aggregation in INS‐1 cells, indicating that autophagy is dysregulated by hyperlipidemia and hyperglycemia [
249]. Glucotoxicity also leads to increased production of ROS and mitochondrial abnormalities in β-cells [
251]. It has been demonstrated that ROS, including H
2O
2, can induce autophagy as a protective mechanism [
252]. In this context, when β-cells are exposed to amyloid polypeptides, ROS triggers an autophagy response to mitigate cellular injury. Conversely, the inhibition of autophagy worsens oxidative stress and β-cell damage [
253]. Human islet amyloid polypeptide (hIAPP) is another key element involved in the pathogenesis of β-cell dysfunction in patients with type 2 diabetes [
254]. As autophagy is responsible for clearing hIAPP, a deficiency in autophagy results in the accumulation of hIAPP, subsequently contributing to the development of diabetes [
255]. It was found that the inhibition of autophagy exacerbates oxidative stress and β-cell damage induced by amyloid polypeptides [
253]. Overall, reduced autophagy function resulting from genetic predisposition, aging, and obesity may contribute to β-cell impairment and diabetes. Therefore, the modulation of pancreatic β-cell autophagy is a promising approach for addressing diabetes related to hyperlipidemia, hyperglycemia, and the accumulation of islet amyloids.