The biological effects of retinoids on cell differentiation and proliferation

The retinoids are the natural and synthetic analogues of retinol or vitamin A. These molecules are able to modulate differentiation and proliferation processes in several cell types. The presence of retinoids is essential for inducing or maintaining the differentiation of epithelial cells. On the other hand, when excess retinol is added to the culture medium, the differentiation of some mesenchymal cells is impaired. Retinoids also promote the differentiation of carcinoma cells of various origins: embryonal carcinoma, leukaemic and melanoma cells. While their effect on the proliferation of normal cells appears variable, these molecules inhibit the growth of cells treated with a tumoural promoter, or spontaneously transformed cells, and they prevent tumour promotion. Various mechanisms of action might be involved: retinoids could act at the level of the genome with or without the participation of their binding proteins. They might also influence glycoconjugate biosynthesis and interact, through certain glycosylation reactions, with growth factor receptors. The role of vitamin A and its structural analogues in cell differentiation and growth is of great therapeutic interest.