Retinoids hormonal

By selectively affecting gene transcription and the consequent production of appropriate target mRNAs, the amounts of specific proteins are changed and metabolic processes are influenced. The influence of each of these hormones is quite specific generally, the hormone affects less than 1% of the genes, mRNA, or proteins in a target cell sometimes only a few are affected. The nuclear actions of steroid, thyroid, and retinoid hormones are quite well defined. Most evidence sug-... 

Retinoid Hormones Retinoids are potent hormones that regulate the growth, survival, and differentiation of cells via nuclear retinoid receptors. The prohormone retinol is synthesized from vitamin A, primarily in liver (see Fig. 10-21), and many tissues convert retinol to the hormone retinoic acid (RA). 

The retinoids share with certain steroid hormones the dis-I inetion of belonging to the few classes of substances capable 111 powerful positive influence on cell growth and differentia-... 

The antagonist-induced conformation of nuclear hormone receptors attracts co-repressors like Nco/SMRT (nuclear hormone receptor co-repressor/silencing mediator of retinoid and thyroid receptors) which further recruit other nuclear proteins with histone deacetylase activity. Their action leads to chromatin condensation, thus preventing the general transcription apparatus from binding to promoter regions. 

A most important function of vitamin A is in the control of cell differentiation and mrnover. PsA-trans-retinoic acid and 9-cw-retinoic acid (Figure 45-1) regulate growth, development, and tissue differentiation they have different actions in different tissues. Like the steroid hormones and vitamin D, retinoic acid binds to nuclear receptors that bind to response elements of DNA and regulate the transcription of specific genes. There are two families of nuclear retinoid receptors the retinoic acid receptors (RARs) bind all-rrijw-retinoic acid or 9-c -retinoic acid, and the retinoid X receptors (RXRs) bind 9-cw-retinoic acid. 

Signals that enter the cell (steroids, vitamin D, thyroid hormone, and retinoids)... 

The signal is what starts everything off. Signals take a variety of forms, but for our purposes there are only two. The first type are signals that go into the cell, bind to internal receptors, and exert their effects. Steroid hormones, vitamin D, thyroid hormone, and retinoids are the only members of this class. All of the intracellular receptors ultimately activate the transcription of regulated genes. The common feature of signals that enter the cell is that they are all small lipophilic molecules that can cross the cell membrane. 

The superfamily of nuclear receptors includes about 150 proteins in the human genome. These are the targets of all principal classes of steroids as well as those of retinoids, vitamin D, and thyroid hormones. 

Retinoids are small molecule hormones that elicit pleiotropic biological responses by activating two families of nuclear receptors which are structurally... 

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