Angiotensin II stimulates p90(rsk) in vascular smooth muscle cells: A potential Na⁺-H⁺ exchanger kinase

Angiotensin II is a multifunctional agonist for vascular smooth muscle cells (VSMCs), stimulating increases in signal events, cell growth, and ion flux. We previously defined protein kinase C (PKC)-dependent and -independent mechanisms by which angiotensin II stimulated activity of the Na⁺-H⁺ exchanger isoform-1 (NHE-1) and identified a 90-kD kinase that exhibited increased activity in VSMCs isolated from genetically hypertensive rats. To determine whether this 90-kD kinase was p90(rsk) (RSK), VSMCs were stimulated with 100 nmol/L angiotensin II, and NHE-1 kinase activity was measured by phosphorylation of recombinant NHE-1 (a glutathione S-transferase fusion protein containing amino acids 516 to 815 of the cytoplasmic carboxyl tail) in vitro. NHE-1 kinase (90 kD) activity was markedly decreased by immunodepletion of RSK. Characterization of RSK activation by angiotensin II revealed many similarities to the 90-kD NHE-1 kinase, including time course and NHE-1 domain phosphorylation, as well as regulation by extracellular signal-regulated kinases (ERK1/2), intracellular Ca²⁺, and PKC. Specifically, angiotensin II stimulated a rapid and transient (peak, 5 minutes) increase in RSK activity. Analysis of several NHE-1 fusion proteins revealed that only proteins containing amino acids 670 to 714 were phosphorylated by RSK. Inhibiting ERK1/2 (30 μmol/L PD098059 for 30 minutes) or chelating intracellular Ca²⁺ prevented RSK activation. In contrast, downregulating PKC (1 μmol/L phorbol dibutyrate for 24 hours) had little effect. These findings establish RSK as a putative NHE-1 kinase and potential mediator of increased Na⁺-H⁺ exchange in hypertension.