Cell cycle-specific locus of vitamin A inhibition of growth.

Vitamin A addition to logarithmically growing, asynchronous Chinese hamster ovary (CHO) cells inhibits cell doubling in a concentration-dependent manner. The effect of retinol is re versible and is not the result of direct cytotoxicity. Flow micro fluorimetric analysis showed thatthe retinol-exposed cells were arrested in G1 phase. Excursions in omnithinedecarboxylase (ODC) activity in retinol-treated cells were used as an index of the extent of G1 progression to localize the temporal site of vitamin action. ODC, the rate-limiting enzyme in polyamine biosynthesis, varies in activity as a function of the CHO cell cycle. ODC is maximal in mitotic cells, is rapidly lost after mitotic exit to achieve a minimal expressed activity after 1 hm, and then progressively increases during the CHO cell cycle as the result of a transcription-dependent induction which occurs as a temporally discrete event during G1transition. Mitotically synchronous cell studies showed the locus of the vitamin Ainduced arrest to be in mid-G1 phase as indicated by a backof effect on the turnover of ODC in early G1but a block of mid-G1 induction of ODC in retinol-treated cells. This site of action was confirmed by a 2-hr delay in the onset of DNA synthesis induced in cells exposed to a 4-hr G@pulse of retinol. The inhibitory action of vitamin A was cell cycle dependent; cell cycle progression was blocked only when retinol was added prior to Hour 4 of the cycle. These results suggest the presence of a retinol-sensitive restriction or commitment point in CHO cells during mid-01 phase and that ODC induction is associated with passage through or beyond this point.