Effects of Dietary Retinoids upon Growth and Differentiation of Tumors Derived from Several Murine Embryonal Carcinoma Cell Lines

We have examined the effects of dietary retinoids upon the growth and differentiation of seven embryonal carcinoma lines in mice. The control diet contained 4000 IU/mg retinyl palmitate; the other diets contained 2 x 105 IU/mg retinyl palmitate, 50 mg/kg all-frans-retinoic acid (RA) 100 mg/kg RA, and no retinoid. The RA-containing diets had little influence on tumor latency or incidence but did suppress growth of many of the tumors. Decreased tumor mass was, in most but not all instances, accompanied by an increased proportion of differentiated cells. Increased differentiation was most commonly quantitative rather than qualitative; i.e., there was a larger proportion of the same types of differentiated cells seen in tumors from the control diet group rather than an increase in the spectrum of cell types observed. Notably, tumors from two differentiation-defective embryonal carcinoma lines were refractory to both the differentiation-inducing and growth-suppressing properties of dietary RA. Taken together, our results suggest that dietary RA can reduce teratocarcinoma growth in part by promoting differentiation but that other mechanisms are likely to be involved. The therapeutic benefits that we observed with dietary RA were compromised by adverse effects, including failure of the mice to gain weight as effectively as those on the control diet. The effects of elevated levels of retinyl palmitate, or its omission from the diet, were much less striking than that of RA. Both modifications tended to decrease tumor latency but had little effect, if any, upon ultimate tumor mass. Elimination of retinoid from the diet failed to significantly reduce degree of differentiation in tumors which normally differentiate extensively in animals on retinoid-containing diets. Excess retinyl palmitate led to a marginal increase in differentiation in F9 tumors and a statistically significant increase in differentiation in OC15-S1 tumors. Tumors from other embryonal carcinoma lines did not contain elevated levels of differentiated cells. The interpretation of these results is complicated by our observations that although our dietary alteration in levels of palmitate were dramatic, they resulted in much more modest differences in circulating retinoid levels when compared with mice on the control diet.