Egr-1 Regulates Skeletal Muscle Insulin Signaling Pathway

The maintenance of blood glucose homeostasis is very important for the physiological functions of organs.Disruption of blood glucose homeostasis leads to hypoglycemia or hyperglycemia,such as type 2 diabetes.Insulin is an important hormone that allows your body to absorb blood glucose.Insulin is a hormone made by the pancreas that allows your body to absorb the majority of postprandial glucose from blood during early stages after meal(before 3 hours),and slightly decrease blood glucose after that until next feeding.In insulin-sensitive tissues(liver,fat and skeletal muscle),skeletal muscle insulin sensitivity is crucial because 80%-90%of postprandial glucose is disposed in muscle tissue.Insulin signaling involves a cascade of events initiated by insulin binding to its cell surface receptor,then activates the downstream PI3K/Akt pathway which stimulates the translocation of insulin-mediated GLUT4 from intracellular vesicles to the plasma membrane resulting in glucose uptake.Our preliminary results show that insulin activates the Akt phosphorylation quickly in myotubes,then decrease a lot at about 3 hours after insulin treatment,and decrease to the basal level at about 12 hours after that.Inhibition the transcriptional activity of Egr-1 enhances the Akt phosphorylation,especially at 3 hours and beyond after insulin stimulation.Thus,Egr-1 is very important for the inhibition of muscle insulin pathway during late stages after insulin stimulation.Although the majority of postprandial glucose is absorbed during early stages after postprandial insulin stimulation,overactivated cellular insulin signaling during late stages after postprandial insulin stimulation leads to hypoglycemia.It won’t occur in normal condition,because the organism has its own negative feedback system to prevent the occurrence of such thing.The target cells could desensitize insulin,including decreasing the membrane insulin receptor levels,reducing the affinity between insulin and insulin receptor and lowering the activity of downstream insulin signaling pathway.However,the detailed mechanisms that how insulin pathway during late stages after insulin stimulation maintains blood glucose homeostasis is not very clear.Therefore,the further clarification of the mechanisms provides an excellent way to get a better understanding of glucose homeostasis maintenance.Our investigations revealed that Egr-1 was rapidly activated by insulin stimulation.Activated Egr-1 is very important for the inhibition of muscle insulin pathway during late stages after insulin stimulation.Inhibition the transcriptional activity of Egr-1 enhanced the insulin signaling and glucose uptake in L6 myotubes during late stages after insulin stimulation,and overexpression of Egr-1 further inhibited the insulin signaling.To address the metabolic effects of muscle Egr-1 on glucose homeostasis and insulin sensitivity,we generated mice with specific Egr-1 deletions in their skeletal muscle tissue.Deletion of Egr-1 in the skeletal muscle improved systemic insulin sensitivity and glucose homeostasis in mice especially during the late stages after insulin stimulation.Further mechanistic studies show that Egr-1 inhibits muscle insulin signaling and glucose uptake during late stages after insulin stimulation by activating PTP1B transcription.Knockdown the expression of PTP1B largely prevented the inhibition of insulin signaling and glucose uptake induced by Egr-1 overexpression.These results indicate that Egr-1/PTP1B signaling is a negative feedback pathway in insulin pathway under normal physiological conditions to prevent the excessive absorption of blood glucose.But we found GGPPS expression had no significant change after insulin stimulation and Egr-1 did not regulate GGPPS expression in muscle cells.These results suggest that GGPPS does not mediate the regulation of muscle insulin pathway by Egr-1.To address the metabolic effects of GGPPS activity in skeletal muscle,we generated mice with specific GGPPS deletions in their skeletal muscle tissue.Heterozygous knockout of GGPPS in the skeletal muscle improved systemic insulin sensitivity and glucose homeostasis in mice fed normal chow diet.These alterations were accompanied by activated PI3K/Akt signaling and enhanced glucose uptake in the skeletal muscle.Further investigations showed that GGPPS was able to enhance the geranylgeranylation of RhoA,which further induced the inhibitory phosphorylation of IRS-1(Ser307)by increasing Rho-kinase activity,leading to the development of insulin resistance.Next,we found GGPPS was overexpressed in the skeletal muscles of obese diabetic db/db mice and mice fed a high-fat diet(HFD).Metabolic perturbation due to elevated FFA levels increased GGPPS expression in skeletal muscle.Heterozygous knockout of GGPPS in the skeletal muscle also improved systemic insulin sensitivity and glucose homeostasis in mice fed a high-fat diet.These results implicate a crucial role of the GGPPS/RhoA/Rho-kinase/IRS-1 pathway in skeletal muscle,in which it mediates lipid-induced systemic insulin resistance in obese mice.Collectively,Egr-1 is a key transcription factor responding to insulin in postprandial status.Egr-1 is rapidly activated by insulin stimulation in skeletal muscle,which inhibits the insulin pathway and glucose uptake during late stages after insulin stimulation by activating PTP1B transcription.GGPPS is upregulated responding to free fatty acids stimulation under obese condition.FFAs-stimulate GGPPS expression in the skeletal muscle is able to enhance the geranylgeranylation of RhoA,which further inhibit the insulin pathway and glucose uptake by increasing Rho-kinase activity.Therefore,this study not only provides an excellent way to get a better understanding of the regulation of blood glucose homeostasis by skeletal muscle,but also provides some potential pharmacological targets for the treatment of insulin resistance and type 2 diabetes.