Apoptosis induced by IL-2 withdrawal is associated with an intracellular acidification

It is known that phorbol esters can protect IL-2-dependent lymphocytes against apoptosis induced by IL-2 withdrawal. However, the mechanism of this effect remains unclear. In this article we show that apoptosis induced by IL-2 withdrawal in the CTLL-2 cell line correlates with a decrease in intracellular pH (pHi). Supplementing the incubation medium with phorbol esters during IL-2 deprivation protects CTLL-2 cells against both apoptosis and intracellular acidification. Interestingly, IL-4 also supports short-term cell survival and maintenance of normal pHi. The protein kinase inhibitor staurosporine prevents the protective effects of IL-2, PMA, and IL-4 on apoptosis and intracellular acidification. In contrast, inhibition of the Na+/H+ antiporter by 5-N-ethyl-N-isopropyl amiloride reverts the protective effects of PMA and IL-4, but only weakly affects IL-2-mediated suppression of apoptosis. Taken together, these results indicate that intracellular acidification may be an important event during apoptosis induced by IL-2 deprivation in the CTLL-2 cell line. Moreover, they suggest a key role for protein kinase C activation both in the maintenance of pHi and in the suppression of apoptosis, through mechanisms which rely on the activation of the Na+/H+ antiporter to a different extent, depending on the rescuing factor employed.