These studies describe inhibitory effects of N-acetylcysteine on several biochemical events associated with the activation of extracellular signal-regulated kinases (ERK) by angiotensin II in the cardiac fibroblast and compare these effects with those of the nitric oxide donor, S-nitroso-N-acetylpenicillamine, an agent we showed previously to inhibit angiotensin II-induced ERK activation and the concomitant phosphorylation of proline-rich tyrosine kinase 2 (Wang, D., Yu, X., and Brecher, P. (1999) J. Biol. Chem. 274, 24342-24348). The transactivation of the epidermal growth factor receptor by angiotensin II, a process required for the activation of ERK, was inhibited by N-acetylcysteine but not by nitric oxide. The transactivation of the epidermal growth factor receptor by angiotensin II was shown to be independent of intracellular calcium increases. Nitric oxide, but not N-acetylcysteine, inhibited the angiotensin II-induced increase in intracellular Ca
2؉. Neither nitric oxide nor N-acetylcysteine inhibited either phospholipase C activation or inositol triphosphate generation in response to angiotensin II. N-Acetylcysteine did inhibit the phosphorylation of the calcium sensitive tyrosine kinases PYK2 and Src, effects that also occurred using nitric oxide. These studies describe a novel effect of N-acetylcysteine on cross-talk between a G proteinlinked receptor and a tyrosine kinase receptor and offer additional molecular insight to explain how N-acetylcysteine and nitric oxide act at different sites and might have an additive effect on specific hormonal responses.The diverse response of target cells to angiotensin II (Ang II) 1 with respect to activation of signaling pathways has been well documented. Most studies have focused on either the vascular smooth muscle cell or the cardiac fibroblast to dilineate the intracellular steps involved in the activation of the MAP kinase system, a characteristic response now associated with hypertrophy, proliferation, or apoptosis (1-3). This kinase cascade has been shown to be under complex regulation, and its activation by Ang II has been used as a model for the study of how G protein-linked receptors can influence systems normally activated by tyrosine kinase-linked receptors (4 -6). One suggested mechanism for this response was proposed to be the transactivation of the EGF receptor by Ang II and the requirement for that receptor tyrosine kinase in the ultimate activation of MAP kinases (7,8).In a preceding study (9), we have characterized the acute activation of MAP kinases in quiescent cardiac fibroblasts by Ang II, and shown that nitric oxide (NO), a substance known to functionally oppose the physiological effects of Ang II in several organ systems, including the heart (10), can attenuate the activation of ERK by Ang II. This effect of NO on ERK activation also occurred in the same cell type when either of the other major MAP kinases (JNK or p38) were activated by Ang II, suggesting regulation at a site, or sites, upstream of Ras or proteins within the Ras superfamil...