How do vitamin A and retinoids exert their effect at a molecular level?

Vitamin A exerts its effect on cells by a mechanism similar to that of corticosteroids; some authorities have suggested that it should be classified as a hormone. Vitamin A acts on cells by binding to nuclear retinoic acid receptors (RAR) and/ or other retinoid X receptors (RXR) that are closely related to the steroid and thyroid hormone receptors. Each of these receptors demonstrates three distinct receptor subtypes, which have been named α, β, and γ. Retinoids vary in their affinity for these six receptors, which partially accounts for the different pharmacologic effects produced by different retinoids. Another important factor is that different tissues appear to vary in the expression of receptor subtypes. In human keratinocytes, RAR-γ is the major retinoid receptor expressed. Tissues appear to regulate their requirement for vitamin A and retinoids by changing the concentration of the binding proteins.