PGC-1α is a key regulator of mitochondrial function and oxidative metabolism in skeletal muscle. The loss of PGC-1α shown here via tissue-specific ablation showed definitively that PGC-1α was necessary for normal regulation of mitochondrial gene expression in this tissue. Reduced PGC-1α and OXPHOS levels have been associated with insulin resistance and type 2 diabetes in humans (7
) and mice (31
). We therefore focused our attention on glucose and insulin homeostasis in MKOs and SKM-Hets. Our data show clearly that a reduction of PGC-1α levels in skeletal muscle, and the subsequent reductions in OXPHOS gene expression, were not causally linked to systemic insulin resistance, at least under the conditions studied here. Of course, it cannot be ruled out that decreases of PGC-1α and OXPHOS genes might contribute to insulin resistance in muscle under other experimental conditions, such as in aging or during prolonged periods of hyperglycemia. The data about insulin sensitivity in MKOs are surprising in light of the reduced expression of GLUT4 and various mitochondrial genes in skeletal muscle of the MKOs, suggesting that unidentified compensatory systems might be operating in these mice. These data are also consistent with results observed in mice containing a mutation in PGC-1β. These animals have reduced mitochondrial gene expression in the absence of hyperactivity, but show no evidence of insulin resistance in skeletal muscle (32
). It is possible that the reduced body fat mass increases insulin sensitivity in MKOs above that of control mice. In contrast, SKM-Hets exhibited body fat mass and insulin sensitivity that was not significantly different from that observed in control animals (Supplemental Figure 9 and data not shown). Body weight, fat mass, fat percentage, growth curves, and food intake were not significantly altered in SKM-Hets (Supplemental Figure 9, A–E). Accordingly, basal energy expenditure in SKM-Hets was not significantly different than that of control animals (Supplemental Figure 9, F and G). Finally, no shift in substrate usage or alteration in blood lipid levels was detected in SKM-Hets (Supplemental Figure 10).
Importantly, glucose homeostasis is disturbed with loss of one or both alleles of PGC-1α in skeletal muscle, and the homozygous mutants are severely hypoinsulinemic. Muscle-specific heterozygotes showed a delay in insulin release during a glucose tolerance test, possible because of impaired first-phase insulin secretion. This might contribute to the impaired glucose tolerance observed in these animals. Thus, a reduction of PGC-1α expression results in a surprising crosstalk between muscle and pancreatic β cells. In theory, this crosstalk could be mediated by a variety of molecules, including circulating lipids or peptides, or could even involve CNS signals to the islets. However, the most obvious candidates here are the cytokines. IL-6 is one of the best-studied circulating factors that is produced by skeletal muscle and other metabolic tissues and has a wide range of effects on insulin sensitivity and insulin secretion (33
). In fact, IL-6 has been introduced as the first myokine, a cytokine that is produced and secreted by contracting muscle fibers and exerts effects on other organs (26
). Although IL-6 has been proposed to mediate antiinflammatory effects of exercise, aberrant IL-6 levels have been shown to be proinflammatory in other contexts, and its exact role remains controversial (33
). Importantly, IL-6 has been shown to inhibit glucose-stimulated insulin secretion in isolated murine islets without being cytotoxic (34
). In contrast, an acute increase in IL-6 levels has no significant effect on serum insulin concentration in either rodents or humans in vivo (33
). In fact, similar to our findings in the MKOs, short-term infusion of IL-6 does not induce insulin resistance in rats in vivo despite SOCS3 induction (36
). Moreover, IL-6 knockout animals have normal insulin levels, although they develop obesity at the age of 6–7 months (33
). To our knowledge, no experimental study with chronically and specifically elevated IL-6 exists to date.
Elevation of other cytokines produced and released by skeletal muscle has been implicated in altering systemic insulin sensitivity and insulin release; these cytokines include TNF-α, IL-1β, and IL-15 (37
). Thus, while IL-6 represents a convenient marker to show elevated circulating cytokine levels in MKOs, it is likely that other secreted factors also contribute to the observed phenotype. In fact, several markers for inflammation are elevated in skeletal muscle of MKOs (Figure A). Moreover, recent experiments have shown that there is extensive crosstalk between different cytokines in type 2 diabetes, for example, between IL-1 and IL-6 (41
). A more global study of the levels of secreted factors in the blood of MKOs may untangle this web in the future. Nevertheless, in the present study we showed evidence of an inhibitory action of IL-6 on glucose-stimulated insulin secretion in isolated islets (Figure , C and D). Moreover, a contribution of IL-6 to the islet dysfunction was implied by elevated SOCS3 and endogenous IL-6 mRNA levels in the pancreata of MKOs (Supplemental Figure 11).
A likely explanation for the elevation of several proinflammatory genes in the muscle of MKOs is the stimulatory effect of increased ROS levels secondary to reduced expression of ROS-detoxifying genes in MKOs. In skeletal muscle, IL-6 transcription is stimulated by NF-κB, which in turn can be activated by ROS or calcium signaling in this tissue (42
). Alternatively, increased oxidative stress, low glucose availability, and low glycogen content are capable of inducing heat shock proteins that in turn, via heat shock factors 1 and 2, activate IL-6 synthesis in muscle (42
). The ability of PGC-1α to suppress ROS has been observed in multiple tissues (30
). It is also conceivable that PGC-1α itself could have more direct antiinflammatory effects in skeletal muscle, because it has previously been shown that the closely related PGC-1β attenuates macrophage-mediated inflammation (45
The organ crosstalk shown here suggests a potentially important and novel mechanism contributing to the failure of pancreatic β cells to adequately secrete insulin in human type 2 diabetes. Most current models of β cell desensitization and exhaustion imply that hypoinsulinemia is a late event in type 2 diabetes caused by glucotoxicity and β cell apoptosis (46
). However, β cell function and mass decrease progressively, sometimes at very early stages of the disease (47
). Thus, reduction of PGC-1α levels in skeletal muscle of some predisposed human populations might contribute to a decrease in insulin secretion through the actions of myokines such as IL-6 and others without causing β cell apoptosis. In fact, our analysis showed that IL-6 and TNF-α mRNA levels in human skeletal muscle was strongly and negatively associated with PGC-1α gene expression. Strikingly, BMI, fasting glucose and fasting insulin levels did not correlate with IL-6 and TNF-α levels (Figure and Supplemental Table 1). Once insulin resistance is promoted by factors other than PGC-1α dysregulation, β cell desensitization and apoptosis might be stimulated by other factors, although even here, myokines regulated by PGC-1α might contribute to impaired function. It will be important to study the implication of skeletal muscle–specific dysregulation of PGC-1α on amino acid–induced insulin secretion in the future. PGC-1α is thought to affect protein biosynthesis and degradation rates in skeletal muscle (9
) and thereby could potentially alter the levels of amino acids that change pancreatic insulin secretion. Experiments in isolated islets in vitro showed no inhibitory effect of IL-6 on
-arginine–mediated changes in insulin secretion (Figure , C and D). In summary, our findings highlight the importance of skeletal muscle PGC-1α as a regulator of the myokine expression and suggest further studies of muscle-islet crosstalk in diabetes. In addition, these results also suggest a plausible mechanism for how muscle, particularly in the context of metabolic disease and/or exercise, might talk to other organ and tissue systems, including blood vessels, heart, and brain.