Myeloid‐Specific Inhibition of GHS‐R on Metabolic Dysregulation in Skeletal Muscle During Experimental Type 2 Diabetes

Abstract

Skeletal muscle plays a significant role in insulin-stimulated glucose disposal, which is a focal point in type 2 diabetes (T2D) research.The complex and highly regulated relationship between insulin secretion and insulin-mediated glucose disposal continue to be debated in the literature.Emerging evidence suggests that chronic-low grade inflammation and its associated inflammatory responses play important roles in T2D pathophysiology.Macrophages are a major source of inflammatory cytokines and have been demonstrated to be responsible for the inflammation of many tissues. Growth Hormone Secretagogue receptor (GHS-R), the known receptor of hormone ghrelin, has roles in both glucose homeostasis and inflammation. Insulin resistance is a major risk factor for T2D and muscle is a key tissue associated with insulin resistance. Little is known whether macrophage GHS-R affects the metabolic dysregulation of skeletal muscle in T2D. METHODS This study utilized a high fat diet and multiple low dose streptozotocin injections to generate a T2D model to evaluate the effects of myeloid-specific inhibition of GHS-R (LysM-Cre;Ghsrf/f) on signaling pathways associated with metabolic dysregulation in skeletal muscle.Gastrocnemius muscles were examined for common markers of insulin signaling and protein anabolism.Gene expression and phosphorylation state of proteins were measured using Western blotting. RESULTS Our data indicate that the GHS-R deficiency in myeloid cells resulted in an increase of mTORC1 activity, showing a suppression of phosphorylation on AKT at Ser473 (â25%; p<0.05).AKT is phosphorylated at Ser473 by mTORC2, and the formation/activity of mTORC2 activity is generally suppressed by mTORC1.Although not statistically significant, mTORC1 activity appeared to be elevated, supported by downstream targets of mTORC1 of higher phosphorylation ratios in both S6K ribosomal at Ser235/236 (P=0.11) and 4EBP1 at Thr37/46 (P=0.18).It is to note that upstream canonical AKT activation was not affected by treatment based on the phosphorylation of its substrate GSK3B (P=0.99).In addition, the knockout model also showed a lower phosphorylation of P-ACC suggesting a lower activity of AMPK, an insulin-independent pathway of glucose uptake known to suppresses mTORC1 activity. CONCLUSION Results from the present study suggests that a myeloid-specific knockout of GHSR1a plays a role in the metabolic shift of skeletal muscle towards significant decrease in mTORC2, likely due to augmented mTORC1 activity.