Biochemical Mechanisms for Vitamin A Functions

Rando, R.R.  Retinoid isomerization reactions in the visual system. In: Blomhoff R, ed. Vitamin A in Health and Disease, he explained:

Vitamin A functions at two levels in the body. The first is in the visual cycle in the retina of the eye; the second is in all body tissues systemically to maintain growth and the soundness of cells. In the visual system, carrier-bound retinol is transported to ocular tissue and to the retina by intracellular binding and transport proteins.Rhodopsin, the visual pigment critical to dimlight vision, is formed in rod cells after conversion of all-trans retinol to retinaldehyde, isomerization to the 11-cis-form, and binding to opsin. Alteration of rhodopsin through a cascade of photochemical reactions results in ability to see objects in dim light.  

The speed at which rhodopsin is regenerated relates to the availability of retinol. Night blindness is usually an indicator of inadequate available retinol, but it can also be due to a deficit of other nutrients, which are critical to the regeneration of rhodopsin, such as protein and zinc, and to some inherited diseases, such as retinitis pigmentosa.

The growth and differentiation of epithelial cells throughout the body are especially affected by vitamin A deficiency (VAD). Goblet cell numbers are reduced in epithelial tissues. The consequence is that mucous secretions with their antimicrobial components diminish. 

Ross, A.C. & Stephensen, C.B.  Vitamin A and retinoids in antiviral responses. FASEB J., wrote: 

Cells lining protective tissue surfaces fail to regenerate and differentiate, hence flatten and accumulate keratin. Both factors – the decline in mucous secretions and loss of cellular integrity – diminish resistance to invasion by potentially pathogenic organisms. The immune system is also compromised by direct interference with production of some types of protective secretions and cells. 

It is said that: 

Classical symptoms of xerosis (drying or nonwetability) and desquamation of dead surface cells as seen in ocular tissue (i.e., xerophthalmia) are the external evidence of the changes also occurring to various degrees in internal epithelial tissues.

This is what Pemrick, S.M., Lucas, D.A. & Grippo, J.F. The retinoid receptors in Leukemia, said about the mechanism of vitamin A:

Current understanding of the mechanism of vitamin A action within cells outside the visual cycle is that cellular functions are mediated through specific nuclear receptors. These receptors are activated by binding with specific isomers of retinoic acid (i.e., all-trans and 9-cis retinoic acid). Activated receptors bind to DNA response elements located upstream of specific genes to regulate the level of expression of those genes. 

The FAO/WHO concluded this topic by saying:

The synthesis of a large number of proteins vital to maintaining normal physiologic functions is regulated by these retinoid-activated genes. There also may be other mechanisms of action that are as yet undiscovered.

References:

Chambon, P. 1996. A decade of molecular biology of retinoic acid receptors. FASEB J., 10: 940–954.

Ross, A.C. & Stephensen, C.B. 1996. Vitamin A and retinoids in antiviral responses. FASEB J., 10: 979–985.

Pemrick, S.M., Lucas, D.A. & Grippo, J.F. 1994. The retinoid receptors. Leukemia. 3: S1-10.

Rando, R.R. 1994. Retinoid isomerization reactions in the visual system. In: Blomhoff R, ed. Vitamin A in Health and Disease. p.503–529. New York, Marcel Dekker, Inc.