[Lack of serum causes apoptosis of thymocytes not requiring protein synthesis and ATP generation]

Incubation of rat thymocytes in serum-free media was found to result in their apoptotic death characterized by internucleosomal DNA fragmentation, nuclear pyknosis and subsequent irreversible plasma membrane damage. As in the case of glucocorticoid (hydrocortisone)-induced apoptosis, DNA fragmentation under serum withdrawal was suppressed by endonuclease inhibitors (Zn2+ and spermine). At the same time, protein synthesis inhibitors (cycloheximide and puromycin) failed to block the apoptosis induced by serum withdrawal but inhibited the hydrocortisone-induced apoptosis. Various inhibitors of oxidative phosphorylation (uncoupler, rotenone, oligomycin), causing sharp decrease in cellular ATP did not suppress DNA fragmentation, whereas thymocyte plasma membrane damage accelerated under their effect. The results obtained indicate that intact thymocytes contain all the components of the apoptotic system; however, in the absence of apoptotic stimuli (e.g., hydrocortisone) the system is blocked by some growth factors of serum origin. Serum withdrawal is sufficient by itself to induce apoptosis and does not require the synthesis of special proteins.