Régulations et perturbations des fonctions testiculaires par la vitamine A

Abstract

In addition to a fundamental role in very diverse processes such as vision, growth and differentiation of numerous cell types, vitamin A (retinol) and its principal biologically active derivative, retinoic acid, are clearly involved in the regulation of testicular functions in the rodent. Indeed, an excess of retinol leads to testicular lesions and spermatogenetic disorders. Conversely, a deficiency in vitamin A induces early cessation of spermatogenesis and altered testosterone secretion. Further, RAR a or RXR b mutant mice are sterile. Retinoids seem to exert an action on the three main testicular cell types (Sertoli, germinal, Leydig). Indeed, they act on the signaling pathways and Sertoli cell metabolism and modify the secretion of numerous Sertoli factors. They seem to be required for the proliferation and differentiation of spermatogonia A, as well as for spermiogenesis. Lastly, a deficiency in vitamin A leads to an atrophy of the internal genital organs subsequent to a decreased testosterone production. Recent studies conducted by our group have shown that retinoids affect these three cell types as early as the fetal stage. Until now, research into the effects, the physiological involvement and the mode of action of retinoids in testicular functions has been carried out exclusively in the rodent. Surprisingly, no study has yet been conducted in human, in whom nevertheless both deficiency in, and excess of, vitamin A are common occurrences. Severe vitamin A deficiency can be observed in most of the developing countries. In industrialized countries, severe hypovitaminosis A has practically disappeared but conversely, hyperretinoidemia is frequently provoked with therapeutic intent, retinoids being used at high doses to treat skin diseases and numerous forms of cancer. A study of the role of retinoids in testicular function in human thus appears a necessity.