Thyroid hormones genes affected

Hormonal actions on target neurons are classified in terms of cellular mechanisms of action. Hormones act either via cell-surface or intracellular receptors. Peptide hormones and amino-acid derivatives, such as epinephrine, act on cell-surface receptors that do such things as open ion-channels, cause rapid electrical responses and facilitate exocytosis of hormones or neurotransmitters. Alternatively, they activate second-messenger systems at the cell membrane, such as those involving cAMP, Ca2+/ calmodulin or phosphoinositides (see Chs 20 and 24), which leads to phosphorylation of proteins inside various parts of the target cell (Fig. 52-2A). Steroid hormones and thyroid hormone, on the other hand, act on intracellular receptors in cell nuclei to regulate gene expression and protein synthesis (Fig. 52-2B). Steroid hormones can also affect cell-surface events via receptors at or near the cell surface. 

Retinoids are classed as morphogens, small molecnles released from one type of cells that can affect the growth and differentiation of neighboring cells. Their normal roles in the hnman body are to indnce differentiation of some cells, stop the differentiation of others, and both snppress and indnce apoptosis in different cell types. Their diverse actions come from the diversity of their receptors. The two classes of retinoid receptors are retinoid X receptors (RXRs) and retinoic acid receptors (RARs), each with a, P, and y snbclasses. RXRs are versatile they bind to RARs and to other nnclear receptors such as thyroid hormone receptors. Once activated, the receptors act as transcription factors that in turn regulate the expression of genes that control ceUnlar growth and differentiation. ... 

The effect of the thyroid hormone on brain development has been reviewed by Bernal (2002), who has indicated the many neurobiological actions of the hormone on developing neural tissue, in addition to documenting some of the genes affected by T3 (Table 49.4). 

In the adult brain, receptors have been identified, but the specific genes affected by thyroid hormones have not yet been located. However, in the developing brain of the fetus and neonate, the effects of... 

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-... 

Small lipophilic (lipid-soluble) hormones diffuse across the plasma membrane and then interact with intracellular receptors in the cytosol or nucleus. The resulting hormone-receptor complex often binds to regions of the DNA and affects the transcription of certain genes (see Topic G7). Small lipophilic hormones with intracellular receptors include the steroid hormones which are synthesized from cholesterol (see Topic K5) (e.g. the female sex hormones estrogen and progesterone), thyroxine which is produced by thyroid cells and is the principal iodinated compound in animals, retinoic acid which is derived from vitamin A, and vitamin D which is synthesized in the skin in response to sunlight (see Topic K5). 

The effects of steroids involve longer-term changes, such as the conversion of a resting cell to a growing cell. Steroids and related hormones (i.e., thyroid, vitamin D, and retinoic acid hormones) act intracellularly, within the nucleus, where they regulate transcription of specific genes (usually positively). Table 23.6 lists several proteins whose synthesis is affected by these hormones. 

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