Effects of retinoids, β‐carotene, and canthaxanthin on UV‐ and x‐ray‐induced transformation of C3H10t1/2 cellsin vitro

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Abstract
We observed that various retinoids (including all‐trans‐retinoic acid, 13‐cis‐retinoic acid, and the synthetic retinoid Ro‐11–1430) have approximately the same ability to suppress ultraviolet light‐induced transformation of C3H10T1/2 cells in vitro. Retinoids also suppress X‐ray induced transformation in vitro. 0Ro‐11–1430 has no effect when present for only 1 day after the X‐ray exposure but does have a suppressive effect on radiation transformation when present for 5 or 10 days after irradiation. Ro‐11–1430 has its major suppressive effect on X‐ray transformation when present in irradiated cultures in the confluent stationary phase of growth. Natural β‐carotene (type IV) from carrots, but not synthetic β‐carotene, has the ability to suppress radiation (X‐ray)‐induced transformation when present for the entire transformation assay period. Natural β‐carotene is without effect on the transformation process when present in irradiated cultures only during confluence. For these retinoids, as well as ‐carotene and canthaxanthin, there is a highly significant suppressive effect on radiation transformation and radiation transformation enhanced by 12‐O‐tetradecanoylphorbol‐13‐acetate when the compounds are present at toxic levels; when nontoxic levels are utilized, these compounds have the ability to suppress the yield of transformed cells to approximately one‐half of that observed in irradiated cultures in the absence of these compounds. A selective toxicity for transformed cells appeared to exist for the β‐carotene‐treated F‐17 cells. This apparent selective toxicity was not observed in another line of transformed cells, Cl 16 cells, or in human cells. We observed different uptake patterns of β‐carotene by nontransformed C3H10T1/2 cells, F‐17 cells, and Cl 16 cells that may account for the observed apparent selective toxicity of one line of transformed cells (F‐17 cells) to β‐carotene.