Novel Mechanism of Na/K Pump Inhibition by Chelerythrine (CET), a PKC Inhibitor, Uncovers Potential Early Signal Transducers of Apoptosis.

Abstract

In human lens epithelial cells (HLECs), CET abolishes Na/K pump (NKP) activity, concomitantly inhibits K fluxes via the Na‐K‐2Cl isoform 1 and stimulates K channels (Lauf et al. Cell Physiol Biochem 2012). Since in HLECs neither ATP, nor phosphorylation of the α1 subunit or FXYD2 proteins were altered, nor ouabain binding to pig renal Na/K ATPase, an alternate direct action of CET on NKP was explored. CET is a BH3‐mimetic for proapoptotic Bcl‐2 proteins interacting with their BH1 domains. Thus we hypothesized that the NKP might also possess BH1 domains to which a) the BH3‐mimetic CET and b) BH3‐domain type Bcl‐2 proteins would bind and hence modify NKP activity crucial for cellular homeostasis. Indeed, MAFT sequence alignments between Bcl‐2 and BclXl revealed the presence of at least one BH1‐like domain within the NKP potentially interacting with CET or Bcl‐2 BH3 proteins. This site consists of some 13 amino acids (aa) within the N‐terminal 100 aa of the actuator (A) within the NKP (and other P‐type pumps). We propose that CET inhibits the NKP by conformational changes of its A domain thus disrupting the α1 subunit catalytic activity. Consequently, by interacting with the putative NKP CET‐binding BH1‐like site, Bcl‐2 BH3 domain proteins may be intermediary cytosolic sensors of normal NKP function thus constituting important signal transducers in the initial phases of apoptosis.