Insulin inhibits angiotensinogen gene expression via the mitogen-activatedprotein kinase pathway in rat kidney proximal tubular cells

The present study aimed to investigate the molecular mechanism(s) of insuli n action on angiotensinogen (ANG) secretion and gene expression in kidney p roximal tubular cells exposed to high levels of glucose. Immortalized rat p roximal tubular cells (IRPTC) were cultured in monolayer. The levels of rat ANG and ANG messenger RNA in the IRPTC were quantified by a specific RIA f or rat ANG (RIA-rANG) and by an RT-PCR assay. Insulin inhibited the stimula tory effect of a high level of glucose (25 mM) and phorbol la-myristate 13- acetate, an activator of protein kinase C) on the secretion of ANG and the expression of the ANG messenger RNA in IRPTC. This inhibitory action of ins ulin on the ANG secretion and gene expression was blocked by PD98059 tan in hibitor of mitogen-activated protein kinase kinase) but not by Wortmannin t an inhibitor of phosphatidylinositol-3-kinase). PD98059 was effective in in hibiting the phosphorylation of MEK 1/2 and p44/42 MAP kinase in IRPTC stim ulated by insulin. These studies demonstrate that insulin prevents the stim ulatory effect of high levels of glucose on the expression of the renal ANG gene in IRPTC, at least in part, via the MAPK kinase signal transduction p athway, subsequently inhibiting the activation of the local renal renin-ang iotensin system.