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 potentiation, we found dose-dependent binding of PA (0.01 to 100 microg/ml PA) to the mice splenic lymphocytes. Interestingly, treatment of 1 microg PA/20 g mice increased the splenic lymphocyte number approximately 5-fold over control but at a 10-microg dose the cell number was decreased compared with a 1-microg dose. Flow cytometric analysis of cell-cycle phase distribution of nuclear DNA in splenic lymphocytes showed that at a 1-microg dose, PA shifted the cell-cycle phases from G0/G1 to S and G2/M supporting the pro-proliferative role of PA. In contrast, the same inducer increased the sub-G1 cell population at a 10-microg dose indicating the breakdown of cellular DNA. These findings were supported by DNA ladder formation and nuclear breakdown at this higher dose. Further studies revealed that at a 1-microg dose, the level of the pro-proliferative/anti-apoptotic protein bcl-2 was increased in splenic lymphocytes whereas at a 10-microg dose it showed a decreasing trend. In contrast, concentrations of proapoptotic proteins, p53 and bax, were increased at a 10-microg dose. A search of the mechanism(s) of such differential action of PA at these 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 beneficial 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 cells. All these findings indicated that the cross-talk between all these pro- and anti-apoptotic factors may contribute to maintain a balance between growth and death of cells and may be one of the important factors deciding whether a cell would follow a proliferative pathway or an apoptotic pathway.
Das
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Immunotechnology Section, Bose Institute, Calcutta, India.
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Sinha
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Ray
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D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D002453:Cell Cycle / Q000187:drug effects; D002455:Cell Division / Q000187:drug effects*; D016207:Cytokines / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D005434:Flow Cytometry; D008214:Lymphocytes / Q000187:drug effects*; D008297:Male; D051379:Mice; D011518:Proto-Oncogene Proteins / Q000187:drug effects; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000187:drug effects; D013154:Spleen / Q000166:cytology; D013205:Staphylococcal Protein A / Q000494:pharmacology*; D016159:Tumor Suppressor Protein p53 / Q000187:drug effects; D051028:bcl-2-Associated X Protein
C494827:Bax protein, mouse; D016207:Cytokines; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D013205:Staphylococcal Protein A; D016159:Tumor Suppressor Protein p53; D051028:bcl-2-Associated X Protein