The molecular mechanism by which the endothelin-A receptor (EtAR) induces proliferation in fibroblasts was studied by examining the involvement of the mitogen-activated protein (MAP) kinase athway, phosphoinositide hydrolosis and cylcic guanosine monophosphate (cGMP) fomation and hydrolysis in a cell line stably transfected with EtAR. Previous studies show ligand concentration dependency on cGMP formation. We show here that endothelin-1 (ET-1) at a low concentration (1 pM) induced an increase in cGMP formation but did not induce proliferation, whereas at a high concentration (100 nM) it induced cCMP hydrolosis, resulting in cell proliferation. Interestingly, proliferation was not induced by cCMP formation without the hydrolosis or by the hydrolosis without cGMP formation. The EtAR-induced proliferation was mediated by the MAP kinase pathway and cGMP. The MAP kinase pathway was not affected by cGMP stimulators or inhibitors, suggesting either a simultaneous co-regulation of proliferation by the two pathways or a regulation of cGMP by the MAP kinase pathway. We conclude that both cGMP formation and cGMP hydrolosis are needed for cell proliferation and that cGMP does not regulate proliferation via MAP kinase pathway activation.
