Induction of cell proliferation and apoptosis: Dependence on the dose of the inducer

Protein A (PA) of Staphylococcus aureus is known as an immunomodulator, In a search of the molecular mechanism(s) of PA-induced immunocyte potentiatio n, we found dose-dependent binding of PA (0.01 to 100 mu g/ml PA) to the mi ce splenic lymphocytes. Interestingly, treatment of 1 mu g PA/20 g mice inc reased the splenic lymphocyte number similar to 5-fold over control but at a 10-mu g dose the cell number was decreased compared with a 1-mu g dose. F low cytometric analysis of cell-cycle phase distribution of nuclear DNA in splenic lymphocytes showed that at a 1-mu g dose, PA shifted the cell-cycle phases from G0/G1 to S and G2/M supporting the pro-proliferative role of P A. In contrast, the same inducer increased the sub-G1 cell population at a 10-mu g dose indicating the breakdown of cellular DNA These findings were s upported by DNA ladder formation and nuclear breakdown at this higher dose. Further studies revealed that at a 1-mu g dose, the level of the pro-proli ferative/anti-apoptotic protein bcl-2 was increased in splenic lymphocytes whereas at a 10-mu g dose it showed a decreasing trend. In contrast, concen trations of proapoptotic proteins, p53 and bar, were increased at a 10-mu g dose. A search of the mechanism(s) of such differential action of PA at th ese two doses revealed that the lower dose of PA upregulated the production of interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha ) to the extent which has already been reported by our laboratory to be ben eficial to the host. However, at a larger dose, much higher release of TNF- alpha and interleukin-2 (IL-2) may account for the apoptosis of splenic cel ls. All these findings indicated that the cross-talk between all these pro- and anti-apoptotic factors may contribute to maintain a balance between gr owth and death of cells and may be one bf the important factors deciding wh ether a cell would follow a proliferative pathway or an apoptotic pathway. (C) 1999 Academic Press.