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Retinoids, active metabolites of vitamin A (retinol), play critical roles in vision, embryonic
development, cell differentiation and homeostasis. Their receptors can be divided into two classes: the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). Both types of receptor are members of the nuclear hormone receptor superfamily. The receptors are subdivided into six distinct regions denoted the A-F domains. RARs and RXRs are required for RA-induced activation of a retinoic acid response element (RARE). The RAREs are found in many promoters and consist of direct repeats of the same sequence. The expression of target genes can be modulated by interacting with both receptors through RARE in promoter regions of target genes. As either homodimers or heterodimers with RAREs located in the promoter regions of target genes. RA-target gene activation is receptor specific and cell type specific. Either excessive or lacking of vitamin A may result in abnormal development that leads to tumorigenesis process. Some retinoids are being evaluated as chemopreventive and chemotherapeutic agents for a variety of human cancers. Therefore, the current opinoin concerning retinoids and cancers are reviewed.
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