Tyrosine kinase receptor activation inhibits NPR-C in lung arterial smoothmuscle cells

We have previously demonstrated that expression of title atrial natriuretic peptide (ANP) clearance receptor (NPR-C) is reduced selectively in the lun g of rats and mice exposed to hypoxia but not in pulmonary arterial smooth muscle cells (PASMCs) cultured under hypoxic conditions. The current study tested the hypothesis that hypoxia-responsive growth factors, fibroblast gr owth factors (FGF-1 and FGF-2) and platelet-derived growth factor-BB (PDGF- BB), that activate tyrosine kinase receptors can reduce expression of NPR-C in PASMCs independent of environmental oxygen tension. Growth-arrested rat PASMCs were incubated under hypoxic conditions (1% O-2) for 24 h; with FGF -1, FGF-2, or PDGF-BB (0.1-20 ng/ml for 1-24 h); or with ANG II (1-100 nM), endothelin-1 (ET-1, 0.1 muM), ANP (0.1 muM), sodium nitroprusside (SNP, 0. 1 muM), or 8-bromo-cGMP (0.1 mM) for 24 h under normoxic conditions. Steady -state NPR-C mRNA levels were assessed by Northern blot analysis. FGF-1, FG F-2, and PDGF-BB induced dose- and time-dependent reduction of NPR-C mRNA e xpression within Ih at a threshold concentration of 1 ng/ml; hypoxia, ANG I I, ET-1, ANP, SNP, or cGMP did not decrease NPR-C mRNA levels in PASMCs und er the above conditions. Downregulation of NPR-C expression by FGF-1, FGF-2 , and PDGF-BB was inhibited by the selective FGF-1 receptor tyrosine kinase inhibitor PD-166866 and mitogen-activated protein/extracellular signal-reg ulated kinase inhibitors U-0126 and PD-98059. These results indicate that a ctivation of tyrosine kinase receptors by hypoxia-responsive growth factors , but neither hypoxia per se nor activation of G protein-coupled receptors, inhibits NPR-C gene expression in PASMCs. These results suggest that FGF-1 , FGF-2, and PDGF-BB play a role in the signal transduction pathway linking hypoxia to altered NPR-C expression in lung.