NO increases protein tyrosine phosphatase activity in smooth muscle cells: relationship to antimitogenesis

We investigated the mechanisms of NO-induced antimitogenesis in primary aortic smooth muscle cells from newborn rats. S-nitroso-N-acetylpenicillamine (SNAP), an NO-releasing agent, decreased basal and growth factor-stimulated DNA synthesis with a threshold effectiveness of 0.3-3 microM. A second NO-releasing agent, 3-morpholinosydnonimine-N-ethylcarbamide, a hydrolysis-resistant cyclic nucleotide, 8-bromo-guanosine 3',5'-cyclic monophosphate (8-BrcGMP), and atrial natriuretic peptides elicited a similar effect, whereas 8-bromo-adenosine 3',5'-cyclic monophosphate (8-BrcAMP) was ineffective, supporting the view that NO and cGMP, but not cAMP, mediated at least some of SNAP's antimitogenic effect. SNAP and 8-BrcGMP decreased the levels of phosphotyrosine, especially in proteins of 70-85 kDa and approximately 215 kDa molecular mass. SNAP decreased protein phosphotyrosine levels with a threshold effectiveness similar to that of its antimitogenic effect. Moreover, SNAP increased protein tyrosine phosphatase (PTPase) activity in cell homogenates, indicating that phosphotyrosine dephosphorylation was likely to be the result of increased PTPase activity. Peroxovanadate, a selective PTPase inhibitor, blocked the antimitogenic effect of 8-BrcGMP, suggesting that loss of protein phosphotyrosine and antimitogenesis were causally linked. These findings describe a potential mechanism for NO-induced antimitogenesis in aortic smooth muscle cells in primary culture.