Other Free Encyclopedias » Medicine Encyclopedia » Genetics in Medicine - Part 2 » Hormonal Regulation - Hormone Receptors, Nuclear Receptors And Their Hormones, Nuclear Hormone Receptors And Transcriptional Regulation, The Importance Of Hormone Concentration

Hormonal Regulation - Nuclear Hormone Receptors And Transcriptional Regulation

complex target gene proteins

As noted above, nuclear receptor hormones generally act as transcription factors to increase transcription of their target genes. They do this by increasing the rate at which RNA polymerase binds to the target gene's promoter. This occurs in several steps.

The binding of a hormone to the receptor triggers the assembly of other proteins to form a "coactivator complex." The hormone-receptorcoactivator complex binds to a specific DNA sequence (called the hormone response element, a type of transcriptional enhancer). This complex then alters the local DNA structure by directly or indirectly chemically modifying the histones. These modifications open up the DNA, increasing access to the target genes and thereby allowing RNA polymerase and other (general) transcription factors to reach the gene promoter region. Additionally, Some hormones trigger second-messenger signal cascades, quickly turning on cellular enzymes for a rapid response. the hormone-receptor-coactivator complex can directly interact with general transcription factors to help form a "preinitiation complex" of proteins on the target gene promoter. RNA polymerase then interacts with this complex, and the transcription of the gene into mRNA begins.

All hormone-regulated nuclear receptors activate transcription in this manner. Some, such as the steroid receptors, exist in cells as cytoplasmic complexes with "chaperone proteins," such as HSP90, and are excluded from the nucleus in the absence of the hormone. In the presence of hormone, the complexes dissociate, and the receptors dimerize and are transported to the nucleus, where they activate transcription.

Other receptors, such as the retinoic acid, thyroid hormone, or vitamin D receptors, are always found in the nucleus and interact with their specific target genes in the presence or absence of the hormone. When the hormone is absent, the receptor interacts with "corepressor" proteins. The complex of receptor and corepressor interacts with histone deacetylases, leading to local chromatin condensation and silencing of the target gene. Hormone binding leads to a change in the three-dimensional structure of the receptor, causing dissociation of the corepressor complex and leading to the recruitment of the coactivator complex, which enables the target gene to be transcribed.

Hormonal Regulation - The Importance Of Hormone Concentration [next] [back] Hormonal Regulation - Nuclear Receptors And Their Hormones

User Comments

The following comments are not guaranteed to be that of a trained medical professional. Please consult your physician for advice.

Your email address will be altered so spam harvesting bots can't read it easily.
Hide my email completely instead?

Cancel or