Insulin is involved in transcriptional regulation of nkcc and the cftr cl-channel through pi3k activation and erk inactivation in renal epithelial cells

Abstract

It is is well known that insulin stimulates glucose transport and epithelial Na? channel (ENaC)-mediated Na? reabsorption; however, the action of insulin on Cl-secretion is not fully understood. In this study, we investigated the action of insulin on Na+-K+-2 Cl-cotransporter (NKCC)-mediated Cl-secretion in epithelial A6 cells. Interestingly, insulin treatment remarkably enhanced the forskolin-stimulated Cl-secretion associated with an increase in apical Cl-conductance by upregulating mRNA expression of both CFTR and NKCC, although insulin treatment alone had no effect on the basal Cl-secretion or apical Cl-conductance without forskolin application. We next elucidated a role of phosphoinositide 3-kinase (PI3K) in the insulin-induced enhancement of the Cl-secretion, since insulin actually activated PI3K, resulting in activation of Akt, a downstream molecule of PI3K. LY294002 (a PI3K inhibitor) reduced the Cl-secretion by suppressing mRNA expression of NKCC, whereas insulin still had a stimulatory action on mRNA expression of CFTR even in the presence of LY294002. On the other hand, we found that a MEK inhibitor (PD98059) further enhanced the insulin-stimulated CFTR mRNA expression and the Cl-secretion in forskolinstimulated A6 cells and that insulin induced slight, transient activation of ERK followed by significant inactivation of ERK. These observations suggest that: (1) insulin respectively upregulates mRNA expression of NKCC and CFTR through activation of PI3K and inactivation of ERK; (2) insulin signals on mRNA expression of NKCC and CFTR are not enough to stimulate transepithelial Cl-secretion, but enhance the stimulatory action of cAMP on transepithelial Cl-secretion.