Mechanism of action of retinoids.

In several recent reviews, we have suggested that the mechanism of action of retinoids in controlling cell differentiation is related to their effects on the expression of oncogenes and peptide growth factors. It is currently believed that oncogenes control metabolic pathways that involve peptide growth factors and their receptors, as well as postreceptor signaling mechanisms. Retinoids, therefore, have been valuable probes to study the function of oncogenes and peptide growth factors. In several tumor cells, including human promyelocytic leukemia, human and murine neuroblastoma, and murine teratocarcinoma, retinoic acid induces terminal differentiation, accompanied by suppression of the expression of either the c-myc or the N-myc gene. Many studies have indicated that retinoic acid can markedly increase the number of cellular receptors for epidermal growth factor, which is partially encoded by another oncogene, erb-B. We have shown that retinoic acid greatly inhibits the anchorage-independent growth of a rat fibroblast cell line that has been transfected with the c-myc gene, particularly when these cells are stimulated by the combination of platelet-derived growth factor and transforming growth factor-beta. At present, the mechanisms by which retinoids control oncogene and growth factor expression are unknown. A wide range of new compounds, including the retinoidal benzoic acid derivatives, are now available to study these mechanisms, and will necessitate the identification of a high-affinity receptor for retinoids and the elucidation of the interaction of this receptor with the genome of the cell. The recent synthesis of new terephthalic acid anilides and chalcone carboxylic acid derivatives, which have retinoid-like activity, offers a particularly useful approach to this problem.