Heterogeneity of Signal Transduction by Na/K‐ATPase α Isoforms: Role of α3 Na/K‐ATPase

Abstract

Na/K-ATPase is an ion transporting pump. The a1 subunit of Na/K-ATPase also forms a receptor complex by direct interaction with Src. Na/K-ATPase ligands like ouabain modulate this interaction and thereby activate various downstream protein and lipid kinases. The aim of this study was to test whether this signaling capability also exists in other a Na/K-ATPase isoforms in the absence of a1, focusing on α3 of Na/K-ATPase. To this end, a1 Na/K-ATPase knockdown pig kidney cells (PY-17) were transfected with α3 Na/K-ATPase to generate the first mammalian cell lines expressing virtually only α3 Na/K-ATPase (LM-α3-1). ATPase assay and Rb86+ uptake indicated that ion-pumping function in these cells is normal. These assays also showed that, compared to α1, α3 Na/K-ATPase has less affinity for Na+ (10.72± 3.1 and 27.7± 9.7 mM respectively) and has much more capacity to be activated when intracellular Na+ isincreased. This characteristic may explain the importance of α3 Na/K-ATPase in neuronal cells which are highly dependent on Na+ gradientfor action potential. In LM-α3-1, Na/K-ATPase–Src interaction was disrupted, and ouabain failed to stimulate Src. However, caveolin-1 expression was restored and ouabain-induced ERK and Akt activation was observed. Unlike in AAC-19 cells, ouabain-induced activation of ERK was PI3K- and PKC-dependent and not Src, as indicated by pharmacological inhibition studies. Finally, LM-α3-1 grew slower than AAC-19 but faster than PY-17 cells. Our results suggest that α3 Na/K-ATPase isoforms regulate signaling pathways differently from that of α1. Since α Na/K-ATPase isoforms have specific tissue-distributions, this may provide cell/tissue specific regulation. Figure 1. Possible signaling transduction mechanisms by isoforms of Na/K-ATPase