Endothelin-stimulated Ca2+ mobilization by 3T3-L1 adipocytes is suppressed by tumor necrosis factor-alpha.

The cytokine tumor necrosis factor-alpha (TNFalpha) contributes to metabolic changes in disease states such as insulin resistance. However, the mechanism by which TNFalpha alters cellular function in these conditions is poorly understood. Because changes in intracellular calcium concentration plays a critical role in hormone action we investigated the effect of TNFalpha on calcium homeostasis in 3T3-L1 adipocytes. In these studies we show that TNFalpha causes a concentration- and time-dependent decrease in Na+/myo-inositol cotransporter (SMIT) mRNA levels and myo-inositol accumulation as well as a decrease in myo-inositol incorporation into phosphoinositides. These changes coincided with a decrease in endothelin-1-induced phosphatidylinositol (PI) cycle activity in 3T3-L1 adipocytes chronically exposed to TNFalpha. Endothelin-1-induced mobilization of calcium from intracellular stores was also diminished by TNFalpha. The effect of TNFalpha on endothelin-1-induced PI cycle activity and calcium mobilization was not due to a decrease in endothelin receptors. However, TNFalpha did cause a moderate decrease in phosphatidylinositol 4,5-bisphosphate (PIP2)-specific phospholipase C (PLC) activity in 3T3-L1 adipocytes. Combined, a decrease in phosphoinositide production and PIP2-specific PLC activity could be responsible for altering PI cycle activity and the generation of the second messenger myo-inositol 1,4,5-trisphosphate, thereby reducing calcium mobilization. Such changes in intracellular signaling may contribute to the pathophysiology of insulin resistance associated with TNFalpha.