One of the early obligatory responses elicited by mitogenic and hypertrophic factors is the stimulation of protein synthesis which results from changes at both the transcriptional and translational levels (1-3). Although the signaling mechanisms involved in this response remain poorly defined, it is known that phosphorylation/dephosphorylation reactions play a critical role in controlling the overall rate of protein synthesis (1, 4, 5). Signals initiated by growth factors interacting with receptor tyrosine kinases or G protein-coupled receptors are integrated and propagated through an elaborated network of cytoplasmic serine/threonine kinases (6 -8). The best understood of these protein kinase cascades is the mitogen-activated protein (MAP) 1 kinase module leading to activation of the ERK subfamily of MAP kinases (9 -12). Two isoforms of ERKs, referred to as p44 mapk (ERK1) and p42 mapk (ERK2), have been described and found ubiquitously expressed in tissues (13,14). ERK isoforms are activated by phosphorylation on both threonine and tyrosine residues by two dual-specificity MAP kinase kinases termed MEK1 and MEK2 (9,12,15,16). MEKs are in turn activated by serine phosphorylation catalyzed by a number of MAP kinase kinase kinases which include Raf-1 (17-20), B-Raf (21-24), Mos (25), and MEK kinase-1 (26).While the mechanisms of ERKs regulation are relatively well understood, the precise physiological roles of these enzymes remain to be established. The p44 mapk and p42 mapk isoforms are rapidly phosphorylated and activated in response to virtually all growth factors (14, 27). However, the observation that a MAP kinase is activated in a specific process does not demonstrate that this enzyme is functionally essential in vivo. Strong evidence for the critical involvement of ERKs in the regulation of cell proliferation were obtained from studies showing a close correlation between ERKs activation and DNA synthesis (28,29) and from the demonstration that inhibition of cellular ERKs activity blocks cell cycle progression (30, 31). Studies using constitutively active and dominant-negative mutants of MEK1 or thiophosphorylated MAP kinase (32, 33), together with pharmacological blockade experiments (34), also demonstrated the absolute requirement of the ERK pathway for neuronal differentiation. The role of the ERK pathway in the regulation of protein synthesis and in many other growthrelated processes remains to be clarified.The peptide hormone AII provides a good model system to study the signaling pathways by which growth factors regulate the rate of protein synthesis. In vascular SMC, AII induces cellular hypertrophy as a result of increased protein synthesis, but has no effect on cell division (35)(36)(37)(38). The trophic action of AII is initiated by its interaction with the G protein-coupled AT 1 receptor, which stimulates the activity of phospholipase C to produce the second messengers IP 3 and diacylglycerol, and inhibits the activity of adenylyl cyclase (39,40). These early signals ultimately result in the activ...

Angiotensin II (AII) is a growth factor which induces cellular hypertrophy in cultured vascular smooth muscle cells (SMC). To understand the molecular basis of this action, we have examined the role of the 70-kDa S6 kinases (p70S6K) in the hypertrophic response to AII in aortic SMC. AII potently stimulated the phosphotransferase activity of p70S6K, which reached a maximal value at 15 min and persisted for at least 4 h. This response was completely abolished when the cells were incubated in the presence of the AT1-selective receptor antagonist losartan. The enzymatic activation of p70S6K was associated with increased phosphorylation of the enzyme on serine and threonine residues. The immunosuppressant drug rapamycin was found to selectively inhibit the activation of p70S6K by AII, but not the activation of mitogen-activated protein kinase or the induction of c-fos mRNA expression. Treatment of aortic SMC with rapamycin also potently inhibited AII-stimulated protein synthesis with a half-maximal concentration similar to that required for inhibition of p70S6K. These results provide strong evidence that p70S6K plays a critical role in the signaling pathways by which AII induces hypertrophy of vascular SMC.

Angiotensin II (ANG II) is a multifunctional hormone that exerts potent vasoconstrictor and hypertrophic effects on vascular smooth muscle. Here, we demonstrate that the p38 mitogen-activated protein (MAP) kinase pathway is involved in ANG II-induced vascular contraction. Addition of ANG II to rat aortic smooth muscle cells (SMC) caused a rapid and transient increase of p38 activity through activation of the AT(1) receptor subtype. This response to ANG II was strongly attenuated by pretreating cells with antioxidants and diphenylene iodonium and was mimicked by exposure of cells to H(2)O(2). Stimulation of p38 by ANG II resulted in the enzymatic activation of MAP kinase-activated protein (MAPKAP) kinase-2 and the phosphorylation of heat shock protein 27 (HSP27) in aortic SMC. Pretreatment of cells with the specific p38 MAP kinase inhibitor SB-203580 completely blocked the ANG II-dependent activation of MAPKAP kinase-2 and phosphorylation of HSP27. ANG II also caused a robust activation of MAPKAP kinase-2 in the intact rat aorta. Incubation with SB-203580 significantly decreased the potency of ANG II to induce contraction of rat aortic rings and depressed the maximal hormone response. These results suggest that the p38 MAP kinase pathway selectively modulates the vasoconstrictor action of ANG II in vascular smooth muscle.