JNK modulates the effect of caspases and NF-κB in the TNF-α-induced down-regulation of Na+/K+ATPase in HepG2 cells

An inhibition Of the Na+/K- ATPase was previously shown to accompany and potentiate apoptosis in different experimental models. Since TNF-alpha is known to be a pro and anti-apoptotic cytokine, this work was undertaken to study the effect of TNF-a on the Na+/K(+)ATPase in HepG2 cells and to determine the signaling pathway involved. Cells were incubated for I h with TNF-a in presence and absence of PDTC, SP600125 and FK009, respective inhibitors of NF-kappa B, c-JNK, and caspases. The activity of the pump was assayed by measuring the ouabain-inhibitable release of inorganic phosphate, and changes in its expression were monitored by western blot analysis. TNF-alpha decreased significantly the activity and protein expression of the Na+/K+ ATPase. NF-KB and caspases were found to be the main effectors of the cytokine, mediating respectively down-regulation and up-regulation of the pump. Their activity was however modulated at I h by c-JNK, which stimulated the caspases and inhibited NF-KB, resulting in a net inhibition of the ATPase, and probably favoring the apoptotic pathway.