Murine 3T3 cells arrest in a quiescent, nondividing state when transferred into medium containing little or no serum. Within the first day after transfer, fibroblasts can be activated to proliferate by platelet-derived growth factor (PDGF) alone; cells starved longer than 1 day, however, are activated only by serum. We demonstrate that endogenous vitamin A (retinol) or retinol supplied by serum prevents cell death and that retinol, in combination with PDGF, can fully replace serum in activating cells starved longer than 1 day. The physiological retinol derivative 14-hydroxy-4,14-retroretinol, but not retinoic acid, can replace retinol in rescuing or activating 3T3 cells. Anhydroretinol, another physiological retinol metabolite that acts as a competitive antagonist of retinol, blocks cell activation by serum, indicating that retinol is a necessary component of serum. It previously has been proposed that activation of 3T3 cells requires two factors in serum, an activation factor shown to be PDGF and an unidentified survival factor. We report that retinol is the survival factor in serum.Mammalian sera contain 1-2 M vitamin A (retinol) (1). Retinol is metabolized by cells to physiological derivatives (2), including 11-cis-retinal involved in vision (3), retinoic acids that induce differentiation in a variety of systems (4), and three signaling molecules, 14-hydroxy-4,14-retro-retinol (14-HRR) (5, 6), anhydroretinol (AR) (7-9), and 13,14-dihydroxyretinol (DHR) (10). Of the retinoids, 14-HRR, DHR, and retinol, at least one is required in T lymphocyte activation (6, 10, 11) and for the growth of B lymphoblastoid (5, 6, 10, 12, 13) and HL-60 (8) cells when these cells are cultured in serum-free medium. AR competitively inhibits the growth-supportive effects of 14-HRR, DHR, or retinol (7-10). The enzyme, retinol dehydratase, which converts retinol to AR in the moth Spodoptera frugiperda recently has been purified, cloned, and expressed (14).When deprived of serum, NIH 3T3 cells arrest in a nondividing, quiescent state (15, 16). Proliferation of cells starved for 1 day is reportedly stimulated by platelet-derived growth factor (PDGF) (17), fibroblast growth factor (18), epidermal growth factor (EGF) (19), and phorbol ester (20); however, cells starved longer can be activated only by serum (21). We show in this study that retinol in serum and its intracellular derivative, 14-HRR, play an essential role in 3T3 cell activation. Whereas PDGF and EGF are activation factors and initiate the cell cycle, retinol and 14-HRR ensure cell cycle progression by preventing cell death.
METHODSCell Activation Assays. NIH 3T3 cells (American Type Culture Collection) plated in 96-well microtiter plates in 100 l͞well of DMEM containing 10% calf serum (Colorado Serum, Denver) were grown to almost confluency, then were arrested by starvation in 150 l͞well of DMEM containing 0.5% calf serum. Cells were starved for 2 days before the assay unless mentioned otherwise. Resting cells were treated with assay reagents in 200 l͞well of RPMI med...