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 ex ert 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 proliferatio n with its effect on Na-K ATPase activity. We found that ouabain inhib ited T-cell proliferation in a dose-dependent manner and this inhibiti on 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 eff ects of ouabain on the induction of competence (acquisition of respons iveness to interleukin (IL)-2 or IL-4) by phytohemagglutinin (PHA) or the combination of phorbol dibutyrate/ionomycin. Ouabain, at concentra tions 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, ouabai n inhibited the progression phase to DNA synthesis in a dose-dependent manner even at concentrations that had little or no effect on Na-K AT Pase activity. This inhibition was not due to a decrease in the produc tion 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 a ppeared to occur at the mRNA level. These results indicate that the ac tivity of the Na-K ATPase is not essential for the induction of compet ence or early activation. On the other hand, inhibition of cell prolif eration and transcription of IL-2R subunits by low concentrations of o uabain may be related to changes in intracellular K+ concentrations or to inhibition of membranal phospholipid metabolism secondary to alter ation in Na-K ATPase activity. (C) 1995 Wiley-Liss, Inc.