Angiotensin II inhibits inducible nitric oxide synthase in tubular MCT cells by a posttranscriptional mechanism.

Abstract

Expression of the inducible isoform of nitric oxide synthase (iNOS) and generation of nitric oxide (NO) have been recently described, in addition to mesangial and medullary thick ascending limb cells, in proximal tubular cells, including MCT, a mouse proximal tubular epithelium cell line. Because vasoconstrictors may interfere with the induction of iNOS and the subsequent generation of NO, in the study presented here, whether exogenous angiotensin II (ANG II) influences bacterial lipopolysaccharide (LPS)/gamma-interferon (gamma-IF)-stimulated NO synthesis and iNOS protein and mRNA expression in MCT cells was tested. LPS/gamma-IF readily stimulated nitrite synthesis in MCT cells, as one measured parameter of NO synthesis. Coincubation of cells with 10(-9)-10(-6) M ANG II attenuated this LPS/gamma-IF-stimulated induction of nitrite. This effect was reversed by the AT1-receptor blocker losartan, but not by an AT2-receptor antagonist, indicating signal transduction through AT1-receptors. Western blot analysis applying a specific monoclonal antibody generated against mouse iNOS revealed that 10(-8)-10(-6) M ANG II significantly reduced LPS/gamma-IF-induced iNOS protein expression. However, ANG II had no effect on LPS/gamma-IF-induced iNOS mRNA as assessed by Northern blots. Moreover, transient transfection studies using a chimeric gene construct, in which iNOS regulatory elements are linked to the CAT reporter gene, showed no effect of ANG II on the LPS/gamma-IF-stimulated transcriptional activity. The study presented here demonstrates that ANG II influences LPS/gamma-IF-stimulated NO generation in MCT cells, most likely at a posttranscriptional level, by influencing iNOS protein expression. Whether proximal tubular cells in vivo express iNOS remains to be established, but this study suggests a mechanism for how iNOS activity is influenced by ANG II in cultured proximal tubular cells.