Post-translational reduction of cell surface expression of insulin receptors by cyclosporin A, FK506 and rapamycin in bovine adrenal chromaffin cells

Abstract

Long-term (≥3 h) treatment of cultured bovine adrenal chromaffin cells with cyclosporin A (CsA) decreased cell surface 125I-insulin binding by 62% in a concentration (IC 50=18 μM)- and time ( t 1/2=16 h)-dependent manner, but did not change the K d value. FK506 (1 μM) or rapamycin (3 μM) treatment reduced 125I-insulin binding. Western blot analysis showed that CsA treatment decreased insulin receptor (IR) β-subunit level ( t 1/2=15 h) in membrane fraction, but did not alter total cellular levels of IR precursor and IR β-subunit. Internalization rate of cell surface IR measured by using brefeldin A, an inhibitor of vesicular exit from the trans-Golgi network, was comparable between non-treated and CsA-treated cells. Thus, CsA, FK506 and rapamycin inhibit peptidyl prolyl cis- trans isomerase activities of cyclophilin and FK506-binding protein, and down-regulate IR presumably by reducing cell surface externalization of IR.