Suppression of apoptotic cell death of IL-3-dependent cell lines by ER/SR Ca2+-ATPase inhibitors upon IL-3 deprivation.

An ER/SR Ca2+-ATPase inhibitor, cyclopiazonic acid (CPA), was found to suppress apoptotic cell death of IL-3-dependent cell lines, FDC.P2, IC-2, and Ba/F3, upon IL-3 deprivation. Structurally unrelated ER/SR Ca2+-ATPase inhibitors, thapsigargin and 2,5-di(tert-butyl)-1,4-benzohydroquinone also maintained cell viability in the absence of IL-3. In the Ca2+-free medium CPA failed to suppress apoptosis, suggesting that the anti-apoptotic activity of CPA is dependent on extracellular calcium. The culture supernatant of CPA-treated cells was able to prolong cell survival of FDC.P2 in the absence of IL-3. An anti-IL-4 antibody almost completely eliminated the anti-apoptotic activity of CPA. Indeed, a significant amount of IL-4 was detected in the supernatant of cells treated with ER/SR Ca2+-ATPase inhibitors. Thus, our present data clearly demonstrate not only that ER/SR Ca2+-ATPase inhibitors induce the secretion of IL-4 in IL-3-dependent cell lines, but also that IL-4 can replace IL-3 in protecting these cells from apoptotic cell death in an autocrine manner.