Tumor necrosis factor-alpha and insulin interactions: Effects on leptin production and Akt expression in adipocytes

TNF-alpha acutely increases leptin production in animals and humans. Insulin-mediated glucose uptake potently regulates leptin production in adipocytes. Yet, TNF-alpha inhibits insulin signaling and decreases Akt expression, which was demonstrated to regulate leptin production in adipocytes. Three studies were undertaken to determine how TNF-alpha stimulates leptin production in vivo and to determine the mechanism of TNF-alpha-induced decreases in Akt expression in cultured adipocytes. The first study tested the hypothesis that TNF-alpha stimulates leptin production via enhancing adipocyte glucose utilization. Prolonged exposure to TNF-alpha increased glucose uptake but inhibited leptin production in isolated adipocytes. Thus, the increase in glucose uptake due to TNF-alpha did not stimulate leptin production. The second study tested the hypothesis that hyperinsulinemia mediates TNF-alpha-induced increases in plasma leptin. Rats were made insulin deficient with streptozotocin and then administered insulin implants to prevent fluctuations of plasma insulin. Injection of TNF-alpha increased plasma insulin and prevented leptin levels from decreasing in fasted nondiabetic rats, which suppressed food intake upon refeeding. TNF-alpha injection did not affect circulating leptin concentrations or the refeeding response in the STZ-diabetic rats. Thus, increases in plasma insulin were necessary in order for TNF-alpha to maintain plasma leptin during fasting and thereby suppress the refeeding response. The final study tested the hypothesis that TNF-alpha-induced decreases in Akt expression in adipocytes are mediated by the ubiquitin/proteosome pathway. TNF-alpha-induced the caspase-mediated cleavage of Akt1 but not Akt2. TNF-alpha enhanced Akt1 and Akt2 ubiquitination, which was suppressed by caspase inhibition. Caspase and proteosome inhibition attenuated the effect of TNF-alpha to decrease Akt1 and Akt2 expression. Adipocytes preexposed to TNF-alpha and then stimulated with insulin exhibited decreased levels of Akt, and phosphorylated Akt and Mdm2. Caspase and proteosome inhibition increased Akt, and phosphorylated Akt and Mdm2 protein levels. Thus, caspase and proteosome-mediated degradation of Akt due to TNF-alpha impairs Akt-dependent insulin signaling. Therefore, TNF-alpha induces the caspase-dependent degradation of Akt via two mechanisms: the cleavage of Akt1, and the ubiquitination of Akt1 and Akt2, which results in their degradation by the 26S proteosome. Collectively, these three studies reveal novel mechanisms by which TNF-alpha regulates insulin-mediated responses of adipocytes.