Angiotensin II activates extracellular signal-regulated kinase independently of receptor tyrosine kinases in renal smooth muscle cells: implications for blood pressure regulation.

Angiotensin II can cause hypertension through enhanced vasoconstriction of renal vasculature. One proposed mechanism for reduction of angiotensin II-induced hypertension is through inhibition of the mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase cascade. MEK/ERK has been shown to phosphorylate the regulatory subunit of myosin light chain at identical positions as myosin light chain kinase. There are multiple mechanisms proposed regarding angiotensin II-mediated ERK activation. We hypothesized that renal microvascular smooth muscle cells (RmuVSMCs) signal through a unique pathway compared with thoracic aorta smooth muscle cells (TASMCs), which is involved in blood pressure regulation. Use of epidermal growth factor (EGF) and platelet derived growth factor (PDGF) receptor-specific inhibitors 4-(3-chloroanilino)-6,7-dimethoxyquinazoline (AG1478) and 6,7-dimethoxy-3-phenylquinoxaline (AG1296), respectively, demonstrates that angiotensin II activates ERK in TASMCs, but not RmuVSMCs, through transactivation of EGF and PDGF receptors. In addition, inhibition of Src with its specific inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo[3,4-d]pyrimidine (PP2) abolishes angiotensin II-, but not EGF-or PDGF-, mediated phosphorylation of ERK in RmuVSMCs, yet it has no effect in TASMCs. The physiological significance of transactivation was examined in vivo using anesthetized Wistar-Kyoto rats with 15 mg/kg 2'-amino-3'-methoxyflavone (PD98059), an MEK inhibitor, as well as 20 mg/kg AG1478 and 1.5 mg/kg AG1296 in an acute model of angiotensin II-mediated increase in blood pressure. None of the inhibitors had an effect on basal blood pressure, and only PD98059 reduced angiotensin II-mediated increase in blood pressure. Moreover, in RmuVSMCs, but not TASMCs, angiotensin II localizes phosphorylated ERK to actin filaments. In conclusion, angiotensin II signals through a unique mechanism in the renal vascular bed that may contribute to hypertension.