Systemic Mode of Action of Vitamin A

Abstract

This chapter reviews the effects of vitamin A deficiency. Vitamin A is essential for several physiological functions in animals—namely, (1) vision, (2) growth, (3) reproduction, and (4) proper maintenance of the integrity of epithelial cells. The chapter also discusses the requirement of vitamin A in specific biological events, such as in the regeneration of the liver cells in an adult rat after partial hepatectomy and in the estrogen-induced development of chick oviduct. The effects of vitamin A deprivation on biosynthesis of RNA and proteins and on glycosylation of proteins are described. The chapter examines the transport, delivery, and mode of action of steroid hormones and the transport and delivery of retinol. An overview of the similarities between the mode of action of retinol and steroid hormones is presented. The chapter concludes that the most pronounced and characteristic effects are invariably on the epithelial cells. It is recognized that one of the typical properties of the epithelial cells is that they undergo constant in situ regeneration. Therefore it follows that vitamin A is required for their controlled division and differentiation. The chapter also focuses on the fact that the steroid hormones circulate in blood bound to proteins, from which they are removed by specific receptor proteins present in the target cells. The steroid–receptor complex in the target cells is translocated into the cell nucleus, where it binds to the chromatin and induces characteristic gene expression.