Angiotensin II Stimulates p90rsk2 in vascular smooth muscle cells: A potential Na+/H+ exchanger kinase

Angiotensin II (AngII) is a multi-functional agonist for vascular smooth muscle cells (VSMC), stimulating increases in signal events, cell growth, and ion flux. We previously defined protein kinase C (PKC)-dependent and independent mechanisms by which AngII stimulated activity of the Na+/H+ exchanger isoform-1 (NHE-1), and identified a 90 kD kinase that exhibited increased activity in VSMC isolated from genetically hypertensive rats. To determine whether this 90 kD kinase was p90rsk2 (RSK2), VSMC were stimulated with 100 nM AngII and kinase activity measured by phosphorylation of recombinant NHE-1 (a glutathione S-transferase fusion protein containing ammo acids 516-815 (NHE-1[516-815])). The activity of a 90 kD kinase that phosphorylated NHE-1[516-815] was markedly decreased by immunodepletion of RSK2. Characterization of RSK2 activation by AngII revealed many similarities to the 90 kD NHE-1 kinase: 1) the time course of activation of RSK2 and 90kD NHE-1 kinase was the same, 2) both RSK2 and 90kD NHE-1 kinase phosphorylated recombinant NHE-1 containing amino acids 670-714 to the greatest extent; and 3) both RSK2 and 90kD NHE-1 kinase were regulated by MAP kinase and Ca2+, but not by PKC. These findings establish RSK2 as a putative NHE-1 kinase and potential mediator of increased Na+/H+ exchange in hypertension.