A Pump and a Signaling Receptor All in One

Abstract

Evidence is mounting that the Na + ,K + -ATPase is not just a regulator of ionic balance, but also a signal-transducing receptor for such ligands as ouabain, which were originally identified as inhibitors of the ion pump activity of Na + ,K + -ATPase. Wang et al. revisited the question of whether Na + ,K + -ATPase was located in caveolar membranes by using a detergent-free isolation method and found that Na + ,K + -ATPase, caveolin-1, and the nonreceptor tyrosine kinase Src were abundant in these membrane microdomains from LLC-PK1 cells. Na + ,K + -ATPase interacted with caveolin-1 in glutathione S -transferase pull-down experiments. Ouabain treatment of LLC-PK1 cells stimulated the association of Na + ,K + -ATPase and caveolin-1 based on coimmunoprecipitation. In the response to ouabain, a Na + ,K + -ATPase-caveolin-1-Src complex was detected by coimmunoprecipitation, and the tyrosine phosphorylation of caveolin-1, along with that of multiple other proteins, was stimulated in isolated caveolar membrane fractions. Exposure of cells to cholesterol-depleting agents promoted the redistribution of Na + ,K + -ATPase and Src out of caveolin-1-positive membranes. Cholesterol depletion or depletion of caveolin-1 by RNA interference inhibited ouabain-stimulated increase in phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2). Thus, Na + ,K + -ATPase appears to serve a dual role as an ion pump and a signaling receptor, with ouabain both inhibiting ion pumping activity and stimulating downstream signaling cascades mediated by protein kinases. H. Wang, M. Haas, M. Liang, T. Cai, J. Tian, S. Li, Z. Xie, Ouabain assembles signaling cascades through the caveolar Na + /K + -ATPase. J. Biol. Chem. 279 , 17250-17259 (2004). [Abstract] [Full Text]