Synergic action of insulin and genistein on Na +/K +/2Cl − cotransporter in renal epithelium

Abstract

Transepithelial Cl − secretion in polarized renal A6 cells is composed of two steps: (1) Cl − entry step across the basolateral membrane mediated by Na +/K +/2Cl − cotransporter (NKCC) and (2) Cl − releasing step across the apical membrane via cystic fibrosis transmembrane conductance regulator (CFTR) Cl − channel. We estimated CFTR Cl − channel activity and transcellular Cl − secretion by measuring 5-nitro 2-(3-phenylpropylamino)benzoate (NPPB, a blocker of CFTR Cl − channel)-sensitive transepithelial conductance (Gt) and short-circuit current (Isc), respectively. Pretreatment with 1 μM insulin for 24 h had no effects on NPPB-sensitive Gt or Isc. On the other hand, in A6 cells treated with carbobenzoxy- l-leucyl-leucyl- l-leucinal (MG132; 100 μM for 2 h) that inhibits endocytosis of proteins at the plasma membrane into the cytosolic space, insulin pretreatment increased the NPPB-sensitive Isc with no effects on NPPB-sensitive Gt. Genistein (100 μM) induced sustained increases in NPPB-sensitive Gt and Isc, which were diminished by brefeldin A (a blocker of protein translocation to Golgi apparatus from endoplasmic reticulum). Co-application of insulin and genistein synergically stimulated the NPPB-sensitive Isc without any effects on NPPB-sensitive Gt. These observations suggest that: (1) insertion and endocytosis of NKCC are stimulated by insulin, (2) the insulin-induced stimulation of NKCC insertion into the basolateral membrane is offset by the stimulatory action on NKCC endocytosis from the basolateral membrane, (3) genistein stimulates insertion of both CFTR Cl − channel into the apical membrane and NKCC into the basolateral membrane, and (4) insulin and genistein synergically stimulated NKCC insertion into the basolateral membrane.