Phorbol esters up-regulate p55 and down-regulate p75 expression of interleukin-2 receptors in human T cells

We examined the effects of phorbol esters on the expression of interleukin 2 receptors (IL-2R) in T cell clones. By flow cytometric analysis, we found that phorbol esters up-regulated IL-2R p55 expression, while they down-regulated p75 expression. The expression of IL-2R p55 in T cell clones treated with phorbol esters showed an initial transient and marginal decrease, which was followed by a progressive increase after 6 h of incubation. On the other hand, the expression of IL-2R p75 progressively decreased to a minimum plateau level. Down-regulation of p75 was also revealed in cells treated with diacylglycerols instead of phorbol esters, but there was no up-regulation of p55 in these cells. Moreover, in the presence of cycloheximide, phorbol esters down-regulated p75 expression but did not up-regulate p55 expression. Therefore, it seems that a transient activation of protein kinase C (PKC) is sufficient to down-regulate p75, but not to up-regulate p55, and that a novel protein synthesis is required to increase p55 expression.