Retinoid pathway and cancer therapeutics

Abstract

The retinoids are a class of compounds that are structurally related to vitamin A. Retinoic acid, which is the active metabolite of retinol, regulates a wide range of biological processes including development, differentiation, proliferation, and apoptosis. Retinoids exert their effects through a variety of binding proteins including cellular retinol-binding protein (CRBP), retinol-binding proteins (RBP), cellular retinoic acid-binding protein (CRABP), and nuclear receptors i.e. retinoic acid receptor (RAR) and retinoid x receptor (RXR). Because of the pleiotropic effects of retinoids, understanding the function of these binding proteins and nuclear receptors assists us in developing compounds that have specific effects. This review summarizes our current understanding of how retinoids are processed and act with an emphasis on the application of retinoids in cancer treatment and prevention.

Figure 1. Retinoid Pathway

Retinoids absorbed from food are converted to retinol and bound to CRBP in the intestine. Then, retinol is converted to retinyl esters and enters into blood circulation. The liver up takes retinyl esters, which are converted to retinol-RBP complex in the hepatocyte. In the serum, the retinol-RBP complex is bound to transthyretin (TTR) in a 1:1 ratio to prevent elimination by the kidney and to ensure retinol is delivered to the target cell. The uptake of retinol by the target cell is mediated by a trans-membrane protein named “stimulated by retinoic acid 6” (STRA6), which is a RBP receptor. In the target cell, retinol either binds to CRBP or is oxidized to retinaldehyde by retinol dehydrogenase (RDH) in a reversible reaction. Then, retinaldehyde can be oxidized by retinaldehyde dehydrogenase (RALDH) to RA. In the target cell, RA either binds to CRABP or enters the nucleus and binds to nuclear receptors to regulate gene transcription. Alternatively, RA can mediate via nongenomic mechanism and regulate cellular function. Hepatocytes not only process retinoids, but also are the target cells. In addition, hepatocytes located next to the storage site (stellate cell). Thus, retinoid-mediated signaling must have a profound effect in regulating hepatocyte function and phenotype [36, 190, 191]