Endothelin-1 and insulin induce cellular inactivation of protein kinase F(A)/glycogen synthase kinase-3α in a common signaling pathway

In this study, we investigate the effects of endothelin-1 (ET-1) and insulin on the cellular activity of protein kinase F(A)/glycogen synthase kinase-3α (kinase F(A)/GSK-3α) in rat adipocytes. The cellular activity of kinase F(A)/GSK-3α is inhibited to ~50% of control within 30 min when cells are treated with 1 nM ET-1 at 37°C; in addition, significant inhibition to ~60% of control is observed at as low as 1 pM ET-1. Conversely, ET-1 at concentrations up to 1 nM has no direct effect on purified kinase F(A)/GSK- 3α in vitro. Immunoblotting analysis further reveals that the protein level of this kinase is not significantly changed when treated with 1 nM ET-1 for 30 min. Similar to ET-1, insulin as low as 10 nM can also induce inactivation of kinase F(A)/GSK-3α to ~50% of control in adipocytes when processed under identical conditions. Most importantly, when treated with both insulin and ET-1, the activity of kinase F(A)/GSK-3α can be decreased only to ~50% of control. Taken together, the results provide initial evidence that ET-1 and insulin may regulate this important multisubstrate/multifunctional protein kinase in a common signaling pathway in cells.