Contribution of Mn-SOD to the redox regulation of human lymphocyte function: Proliferation versus apoptosis

A shift in oxidative status is associated with lymphocyte stimulation. The oxidative balance in lymphocytes seems to be very delicate. Induction of proliferation triggers the production of free radicals, which changes the oxidative status of the cells. The increase in oxidative status is at least partially countered by antioxidant compounds such as glutathione and enzymes such as glutathione peroxidase, superoxide dismutase, and catalase. Mild oxidative stress induces the cells to proliferate. On the other hand, too much oxidative stress may cause cells to die or suffer irreversible damage. I investigated the role of Mn-superoxide dismutase (Mn-SOD) in the redox regulation of human lymphocyte proliferation, The mitochondrial Mn-SOD plays an important role in controlling intracellular levels of superoxide anions, maintaining a proper balance of oxidants in aerobic organisms. My results show that the increase in oxidative status by superoxide in, resting human peripheral blood lymphocytes induces cells to proliferate; however, higher concentrations of superoxide are harmful, inducing apoptosis and necrosis. In contrast, the Jurkat human T-cell leukemia-derived cell line showed peculiar unexpected behavior. Low concentrations of thiols caused growth arrest and intermediate concentrations of thiols (0.1 mM to 3.0 mM) induced apoptosis. Jurkat cells contains little Mn-SOD activity, with a different electrophoretic mobility from that of normal lymphocytes. Single strand conformational polymorphism analysis of the Jurkat Mn-SOD gene revealed a different pattern than the wild type gene, suggesting a mutation in the Sod2 gene. This was confirmed by cloning and sequencing this gene. We found a new mutation, L60F, in exon 3 of the mature protein. Characterization of the mutant L60F Mn-SOD recombinant protein revealed sensitivity to inactivation by thiols and lower thermal stability than wild type Mn-SOD. These results suggest the possibility that the L60F and similar mutations in the Mn-SOD gene may be tumorogenic and carcinogenic, consistent with the leukemic phenotype of the Jurkat T-cell line and with the proposed role of Mn-SOD as a tumor suppressor gene.