Role of PDK4 in insulin signaling pathway in periadrenal adipose tissue of pheochromocytoma patients.

Studies have shown that pheochromocytoma (PHEO) is associated with glucose intolerance and decreased insulin sensitivity. In adipocytes, pyruvate dehydrogenase kinase 4 (PDK4) is involved in glucose uptake. However, very little is known about the role of PDK4 in the insulin signaling pathway in the adipose tissue of PHEO patients. We analyzed the expression of adipokines, oxidative stress-related genes, PDK4, phosphorylated AMPK (pAMPK) and phosphorylated IRS1 (pIRS1) in the periadrenal adipose tissue (peri-A) of patients with PHEO and non-functioning adrenal adenoma (NFA). We also investigated the effects of epinephrine on PDK4, pAMPK and pIRS1 in human stromal vascular fraction (SVF) cells, mouse 3T3-L1 preadipocytes and brown preadipocytes. PHEO patients had higher mRNA levels of PGC1α, C/EBPα, C/EBPβ, COXII and AP2 and lower mRNA levels of PPARγ in their peri-A than NFA patients. Decreased pAMPK and increased PDK4 and pIRS1 were observed in the peri-A of PHEO patients. PHEO patients also had significantly higher NOX4 protein expression and lower Nrf2 and HO-1 protein expression in their peri-A than NFA patients. In vitro, epinephrine treatment upregulated PDK4 expression, inhibited AMPK phosphorylation and enhanced IRS1 phosphorylation. The knockdown of PDK4 by siRNA upregulated pAMPK and downregulated pIRS1. In conclusion, PDK4 may play an essential role in hypercatecholamine-induced insulin resistance in the periadrenal adipose tissues of PHEO patients.