ABSTRACT
Vitamin A and its derivatives regulate fundamental physiological functions, including growth, reproduction, vision, and epithelial differentiation. Retinoic acid (RA) is the transcriptionally active derivative of vitamin A and is required for both embryonic development and adult function. The RARs are ligand-regulated receptors that function similarly to the vitamin D and thyroid hormone receptors; once ligand bound they become transcriptional activators. The ROR/RZRs are a group of related orphan receptors that bind as monomers and hence do not require a direct or palindromic repeat as their response element but bind to a single hexameric motif. Cross-talk between the retinoic acid, thyroid hormone, and vitamin D receptors has been extensively investigated. It has been shown that these interactions can even extend to the estrogen receptors. The peroxisome proliferator-activated receptors are similar to the nuclear receptors acceptor, thyroid hormone and vitamin D receptors, in that they bind with high affinity to their response element as a heterodimer with retinoid X receptor.
