Differential regulation of the tumor suppressor molecules, retinoblastoma susceptibility gene product (Rb) and p53, during cell cycle progression of normal human T cells.

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We have activated resting human T lymphocytes to study the roles of the putative cell cycle control gene products, retinoblastoma susceptibility gene product (Rb) and p53, in regulating cell proliferation. After stimulation with phorbol, 12,13, dibutyrate and the calcium ionophore, ionomycin, which triggers a rapid entry of cells into G1 phase, we demonstrated Rb phosphorylation 24 h later, well before the onset of DNA synthesis. This finding, in contrast to reports using proliferating cell lines, implies that Rb phosphorylation is not a proximal event regulating the G1 to S transition. The production of p53 became detectable 3 to 6 h after addition of phorbol, 12,13,-dibutyrate and ionomycin, and peaked at 30 to 42 h. To further delineate the relationship of the synthesis and metabolism of the proteins to cell cycle progression, we used three agents to arrest progression of activated T cells at various points in the cell cycle. Aphidicolin arrested the cells at the G1/S boundary, whereas deferoxamine, an iron chelator, arrested the cells at an earlier stage of the cell cycle. Cyclosporin A blocked T cell activation at the earliest point in the cell cycle. In the presence of aphidicolin, Rb phosphorylation and p53 production proceeded normally whereas cyclosporin A inhibited both events. Although deferoxamine completely prevented Rb phosphorylation, p53 production was unaffected, suggesting a differential regulation of the two molecules. Our results place Rb phosphorylation and p53 production in the hierarchy of genetic events that are thought to regulate T lymphocyte progression through the cell cycle.