Regulation of Male Germ Cell Apoptosis

Testicular tissue is extremely sensitive to external stress such as cancer treatments, which induce inappropriate germ cell apoptosis leading to defects in reproductive health (1, 2). For postpubertal men, cryopreservation of sperm or testicular tissue is a proven method for preserving fertility against the effects of these treatments (3). Unfortunately, no such method exists for prepubertal boys, whose spermatogenesis has not yet begun (3). For these boys, the best way to preserve fertility would be to protect the spermatogenic stem cells against apoptosis induced by cancer treatments in vivo. Strategies aimed at cell protection should be directed at inhibiting the early signals in apoptosis cascades (4). Sphingolipids are powerful mediators of diverse cellular processes, such as apoptosis and cellular differentiation, and thus hold great promise in different fields of cancer research (5). The sphingolipid metabolites ceramide and sphingosine-1-phosphate (S1P) represent early mediators of apoptosis (6). Ceramide plays an important role in the induction of apoptosis, whereas S1P is considered to be a survival factor. S1P was recently demonstrated to block mouse oocyte apoptosis induced by cancer therapies in vitro and in vivo (7), suggesting that modulation of the sphinoglipid pathway is a reasonable attempt to protect germ cells from unwanted cell death. Little, however, is known of how S1P exerts its antiapoptotic effects. Initially, S1P was suggested to act as an intracellular signaling molecule and the balance between the intracellular levels of S1P and ceramide to be crucial in determining whether the cell will survive or die. The issue is, however, complicated by the latest findings that S1P not only acts as an intracellular second messenger but also as a ligand to its receptors, recently renamed as S1P 1, S1P 2, S1P 3, S1P 4, and S1P 5. In the present study, we aimed at evaluating the inhibitory potential of S1P against male germ cell apoptosis induced by external stress in vitro and in vivo and at characterizing the mechanisms, by which S1P inhibits this apoptosis. The in vitro studies, in which culture of human seminiferous tubules was used as a model for external stress situations, showed that during male germ cell apoptosis testicular ceramide levels increase rapidly before initiation of caspase 3 activation and apoptotic DNA fragmentation and that in cultured human seminiferous tubules, germ cell death can be inhibited by exogenous S1P. Consistent with 6 SUMMARY