Ouabain induces inhibition of the progression phase in human T-cell proliferation

Ouabain, a specific inhibitor of the Na-K ATPase, has been shown to exert immunosuppressive effects. The goals of this study were to define the stage of the proliferative response which is sensitive to ouabain and to correlate the inhibitory action of ouabain on cell proliferation with its effect on Na-K ATPase activity. We found that ouabain inhibited T-cell proliferation in a dose-dependent manner and this inhibition was similar in CD4+ and CD8+ T cells. To define the role of the Na-K ATPase in early activation of T lymphocytes, we examined the effects of ouabain on the induction of competence (acquisition of responsiveness to interleukin (IL)-2 or IL-4) by phytohemagglutinin (PHA) or the combination of phorbol dibutyrate/ionomycin. Ouabain, at concentrations that completely inhibited the enzyme activity, did not interfere with the induction of competence, suggesting that although activated cells express increased activity of Na-K ATPase, this enzyme activity does not play a role in early activation pathways. In contrast, ouabain inhibited the progression phase to DNA synthesis in a dose-dependent manner even at concentrations that had little or no effect on Na-K ATPase activity. This inhibition was not due to a decrease in the production of IL-2 but rather to an inhibition of the expression of the p55 and p75 subunits of the IL-2 receptor (IL-2R). The inhibition of p55 appeared to occur at the mRNA level. These results indicate that the activity of the Na-K ATPase is not essential for the induction of competence or early activation. On the other hand, inhibition of cell proliferation and transcription of IL-2R subunits by low concentrations of ouabain may be related to changes in intracellular K+ concentrations or to inhibition of membranal phospholipid metabolism secondary to alteration in Na-K ATPase activity.