Heterogeneity of Signal Transduction by Na/K‐ATPase α‐Isoforms: Role of α2 Na/K‐ATPase

Abstract

Na/K-ATPase α1 subunit is a ubiquitous transmembrane ion transporting enzyme. More and more studies provide evidences that α1 Na/K-ATPase mediates signal transduction through Src tyrosine kinases upon binding to ligands like cardiotonic steroid. Among four isoforms expressed in human, α2 Na/K-ATPase is predominantly expressed in cardiac myocytes and plays an important role in the regulation of muscle contraction. To understand whether α2 isoform participates in signal transduction, we generate a mammalian cell line expressing α2 isoform only (LX-α2-4), by transfecting α2 N/K-ATPase into α1 knockdown pig kidney cells (PY-17). LX-α2-4 cells exhibited normal ouabain-sensitive ATPase activity, but failed to effectively regulate Src and ERKs. Consistently, ouabain was unable to stimulate Src kinase and downstream effectors such as ERK and Akt in these cells. Additionally, α2 cells were unable to rescue the expression of caveolin. Furthermore, while α1 Na/K-ATPase is known to regulate Src via the NaKtide sequence, the corresponding α2 NaKtide was unable to inhibit Src in vitro. Finally, LX-α2-4 cells grew much slower than these of α1-expressing AAC-19 cells. In short, α2 Na/K-ATPase does not regulate Src as α1, implying that α1 and α2 isoforms play distinct roles in mediating contraction and signaling in myocytes. This suggests potential therapeutics utilities may be achieved through targeting different isoforms of Na/K-ATPase in various diseases. Figure 1. Possible signaling transduction mechanisms by isoforms of Na/K-ATPase