Retinoid-specific induction of differentiation and reduction of the DNA synthesis rate and tumor-forming ability of a stem cell line from a rat mammary tumor.

Differentiation of the stem cell line rat mammary (Rama) 25 to alveolar-like cells can be monitored by the increase in production of domes (hemispheric blisters) in the cell monolayer and immunoreactive casein in the tissue culture medium. This step was accelerated not only by the synthetic inducer dimethyl sulfoxide (DMSO) but also by all-trans-retinol, all-trans-retinal, all-trans-retinoic acid (RA), and all-trans-retinyl acetate (concentration range, 0.04-4 microM) in the presence of the hormones prolactin, hydrocortisone (HC), insulin, and 17 beta-estradiol; 9-cis-all-trans-retinal was without effect. A combination of RA and HC was active in producing doming, whereas RA, all four hormones, and serum were required for maximum production of immunoreactive casein. The retinoids in the same concentration range also caused a reduction in the DNA synthetic rate in a similar time period. When Rama 25 cells were treated with RA and the four hormones yielding the droplet and doming cultures, subsequent injection of these cells into young, female inbred nu/nu (nude) mice led to a reduced incidence of tumors compared with injections of untreated cells. Tumorigenic variant cell lines were selected previously from Rama 25 that were either elongated and failed to differentiate at all to doming and casein-secreting cultures (Rama 521) or that did so spontaneously but whose rates were not accelerated by addition of DMSO (Rama 259). Both Rama 521 and Rama 259 failed to respond to the retinoids and hormones in producing domes and immunoreactive casein, in decreasing DNA synthetic rates, and in lowering the incidence of tumors induced by injection of the cell lines into nude mice. Thus the anticancer activity of the retinoids in rat mammary gland carcinogenesis may be due in part to their differentiation-inducing properties.