Synthesizing fatty acids in skeletal muscle would not be predicted to be relevant to HFD-induced insulin resistance. Since FAS is decreased by exogenous fat in lipogenic tissues (19
), one might also anticipate that excess dietary fat would decrease FAS in muscle. Unexpectedly, we found that HFD feeding increased FAS activity in skeletal muscle of control mice in concert with the induction of insulin resistance. FASKOS mice, with skeletal muscle–specific inactivation of FAS, were partially protected from diet-induced insulin resistance, but also prone to diet-induced muscle weakness.
The underlying mechanism involves an effect of FAS on the modulation of cytosolic calcium. We found that FAS, thought to be exclusively cytosolic, was also present in the SR fraction of skeletal muscle, where it is physically associated with SERCA proteins. FAS deficiency decreased SERCA activity by altering the phospholipid composition of the SR, which resulted in increased cytosolic calcium. Decreased uptake of cytosolic calcium led to CaMKKβ-dependent activation of AMPK to improve glucose metabolism. However, altered calcium handling prevented normal muscle relaxation, leading to disruption of subsequent contraction manifested as decreased overall muscle performance. At the sarcomere, cytosolic calcium binds troponin to release tropomyosin from actin, thus permitting myosin-induced ATP-dependent muscle contraction. During normal relaxation, cytosolic calcium is taken up by the SR, leading to actin-myosin dissociation. Calcium cycling between SR and the cytosol is strictly regulated in striated muscles, and its disruption causes muscle dysfunction (43
). Figure depicts the potential relationship among FAS, insulin sensitivity, and weakness in skeletal muscle.
FAS-driven modulation of SERCA activity and cytosolic calcium leads to increased insulin sensitivity and muscle weakness.
FAS is linked to the generation of phospholipid species that activate nuclear receptors (17
), consistent with the possibility that FAS is part of a synthetic pathway leading to the production of phospholipids, such as certain PE species, at the SR. However, our data did not establish that FAS enzyme activity is required for effects on SR composition. It is possible that FAS interacts with the SR or SERCA to alter the PC/PE ratio independent of its capacity to convert malonyl-CoA into palmitate.
Lipids are implicated in the pathogenesis of diet-induced skeletal muscle insulin resistance. This phenomenon is incompletely understood, but a popular explanation is the accumulation of lipid species such as DAG and ceramide that may induce stress kinases, leading to decreased insulin-stimulated Akt phosphorylation (24
). Perhaps underappreciated is reduced muscle AMPK activity in insulin resistance (29
). Skeletal muscles from FASKOS mice were not protected from HFD-induced suppression of insulin-stimulated Akt phosphorylation, but they were protected from a HFD-induced reduction in AMPK activity. DAG, ceramide, and the activities of certain stress kinases were not different in muscles from HFD-fed control and FASKOS mice, which suggests that these mediators are not likely contributors to the insulin sensitivity of FASKOS muscle. Our observations indicate that the insulin-sensitizing phenotype with FAS deficiency is mediated by AMPK, not Akt, and suggest that multiple pathways may be involved in muscle insulin resistance.
FAS affects activity of the nuclear receptor PPARα in liver (17
), brain (46
), and macrophages (47
), and overexpression of PPARα in skeletal muscle causes insulin resistance and inactivates AMPK (18
), which led us to consider PPARα as a mediator of the FASKOS phenotype. However, FAS inactivation in muscle did not affect activity of a PPARα reporter gene, and treatment with a potent PPARα agonist did not rescue the effect of FAS deletion on AMPK signaling. Similarly, inactivation of FAS in cardiac muscle did not affect PPARα target genes, but instead activated calcium signaling pathways (33
) through incompletely defined mechanisms. The calcium-regulatory mechanism shown here in skeletal muscle may also be involved in the phenotype of mice with FAS knockout in heart (33
), another tissue adapted for contractile function, but its relevance to other tissue-specific FAS knockouts is uncertain. Inactivation of pancreatic β cell FAS has no effect on insulin secretion, a process requiring dynamic regulation of intracellular calcium (46
), which suggests that in this model, the loss of FAS does not have a major impact on the calcium flux required to release insulin-secretory granules. However, it is possible that the loss of FAS in other nonmuscle tissues alters calcium signaling.
Our data suggest that the induction of FAS by HFD feeding may contribute to skeletal muscle insulin resistance. Although not extensively studied in muscle, FAS regulation involves insulin signaling, the induction of the transcription factor SREBP1c (also known as SREBF1), and other factors (48
). In humans, skeletal muscle expression of SREBP1c is highest in individuals that are most insulin resistant (49
). This observation is consistent with our present findings in mice, supporting the plausibility of SREBP1c-driven genes such as FAS being increased in muscle in the setting of diet-induced insulin resistance. HFD feeding has been reported to increase SERCA activity in soleus muscle under certain conditions (50
), also consistent with our observations linking HFD feeding, FAS, and SERCA.
It is tempting to speculate that HFD-induced FAS expression in muscle represents a selective advantage for mammals. Skeletal muscle’s raison d’être is contraction, required for acquiring food, escaping predators, and finding a mate. A HFD-induced increase in muscle FAS may promote insulin resistance, but it may also optimize contractile function. When evolutionary pressures were substantial, mammalian meals were infrequent but high in fat, especially if they consisted of other mammals. Our ancestors probably fasted for prolonged periods while hunting, then feasted for prolonged periods on high-fat prey. Disruption of metabolic homeostasis induced by HFD feeding after fasting may have prompted the selection of animals with the capacity to increase FAS activity in muscle and maintain SERCA activity in order to escape other predators attracted by the same high-fat prey. It has been suggested that insulin resistance is a normal physiological response that limits obesity through mobilization of substrates (51
). Our results suggest that, in addition to defending against obesity, insulin resistance maintains muscle strength in the setting of nutrient excess. A HFD-induced increase in skeletal muscle FAS would preserve SERCA activity and normalize force-generating capacity, perhaps selected by evolutionary pressures. It would also increase insulin resistance, probably unrelated to evolutionary pressures since the complications of insulin resistance become manifest well after the next generation is spawned.
Diabetes may have complex effects on skeletal muscle calcium metabolism, since SERCA is decreased in the diabetic soleus muscle (52
) and insulin receptor substrate proteins interact with SERCA (53
). FAS inhibitors may be useful for treating type 2 diabetes (54
). Our findings suggest that these agents might lower blood sugar at the expense of muscle function, identifying a possible side effect of a new class of medication, but also representing a potentially reasonable exchange for modern humans who need to escape the consequences of abnormal glucose metabolism instead of another mammal looking for a meal.