Roles of RSK1 in LPA5R‐induced NHE3 regulation

Abstract

Na+/H+ exchanger 3 (NHE3) mediates bulk of Na+ and fluid absorption in the small intestine and colon. We have previously shown that lysophosphatidic acid (LPA) receptor LPA5R regulates NHE3 by two distinct signaling pathways involving Pyk2 and Erk1/2. The goal of this study is to identify downstream effectors of Pyk2 and Erk1/2. In Caco‐2bbe cells stably expressing NHE3 and LPA5R, Caco‐2bbe/NHE3V/LPA5R, LPA treatment increased phosphorylation level of PDK1. Knockdown of Pyk2 blocked LPA‐induced phosphorylation of PDK1, suggesting that PDK1 is dependent on Pyk2. Knockdown of PDK1 subsequently abrogated LPA‐induced NHE3 activation demonstrating a role of PDK1 in NHE3 regulation. RSK1 is a known target of Erk1/2. We found that LPA activates RSK1 in Caco‐2bbe cells. To determine whether RSK1 is a downstream target of PDK1 or Erk1/2, we used two distinct antibodies to RSK1 that recognize phosphorylation at S221 by PDK1 or at S380 by Erk1/2. Phosphorylation at S221 and S380 was attenuated by PDK1 knockdown and PD98059, respectively, suggesting that both PDK1 and Erk1/2 are involved in LPA‐induced RSK1 activation. In addition, LPA induced colocalization of RSK1 and NHE3 at the plasma membrane, and knockdown of RSK1 diminished LPA‐induced NHE3 activation. In summary, RSK1 is the converging target of Pyk2 and Erk1/2 and is involved in NHE3 activation by LPA5R.