Hormones That Bind to Nuclear Receptor Proteins

Unlike the water-soluble hormones, the lipophilic steroid and thyroid hormones do not travel dissolved in the aqueous portion of the plasma; rather, they are transported to their target cells attached to plasma carrier proteins. These hormones must then dissociate from their carrier proteins in the blood in order to pass through the lipid component of the plasma membrane and enter the target cell, within which their receptor proteins are located (fig 11.4).

The receptors for the lipophilic hormones are known as nuclear hormone receptors because they function within the

Chapter Eleven

Chapter Eleven

Hormones Receptors Target Cells
Blood Target cell

■ Figure 11.4 The mechanism of action of a steroid hormone on the target cells. Some steroids bind to a cytoplasmic receptor, which then translocates to the nucleus. Other steroid hormones enter the nucleus and then bind to their receptor. In both cases, the steroid-receptor complex can then bind to a specific area of DNA and activate specific genes.

cell nucleus to activate genetic transcription (production of mRNA). The nuclear hormone receptors thus function as transcription factors that first must be activated by binding to their hormone ligands. The newly formed mRNA produced by the activated genes directs the synthesis of specific enzyme proteins that change the metabolism of the target cell in ways that are characteristic of the effects of that hormone on that target cell.

Each nuclear hormone receptor has two regions, or domains: a ligand (hormone)-binding domain and a DNA-binding domain (fig. 11.5). The receptor must be activated by binding to its hormone ligand before it can bind to a specific region of the DNA, which is called a hormone-response element. This is a short DNA span, composed of characteristic nucleotide bases, located adjacent to the gene that will be transcribed when the nuclear receptor binds to the hormone-response element.

The nuclear hormone receptors are said to constitute a su-perfamily composed of two major families: the steroid family and the thyroid hormone (or nonsteroid) family. In addition to the receptor for thyroid hormone, the latter family also includes the receptors for the active form of vitamin D and for retinoic acid (derived from vitamin A, or retinol). Vitamin D and retinoic acid, like the steroid and thyroid hormones, are lipophilic molecules that play important roles in the regulation of cell function and organ physiology.

Endocrine Glands 293

Steroid hormone

Ligand-binding domain

DNA-binding domain

Ligand-binding domain

— Receptor protein for steroid hormone

— Receptor protein for steroid hormone

Half-sites

JJ- DNA

Hormone-response element

Target gene

Hormone-response element

Target gene

Steroid hormone

Hormone Response Element

Genetic transcription mRNA

Steroid hormone

Genetic transcription mRNA

■ Figure 11.5 Receptors for steroid hormones. (a) Each nuclear hormone receptor protein has a ligand-binding domain, which binds to a hormone molecule, and a DNA-binding domain, which binds to the hormone-response element of DNA. (b) Binding to the hormone causes the receptor to dimerize on the half-sites of the hormone-response element. This stimulates genetic transcription (synthesis of RNA).

Modern molecular biology has ushered in a new era in endocrine research, where nuclear receptors can be identified and their genes cloned before their hormone ligands are known. In fact, scientists have currently identified the hormone ligand for only about half of the approximately seventy different nuclear receptors that are now known. The receptors for unknown hormone ligands are called orphan receptors. For example, the receptor known as the retinoid X receptor (abbreviated RXR) was an orphan until its ligand, 9-cis-retinoic acid (a vitamin A derivative) was discovered. The significance of this receptor will be described shortly.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • maximilian
    What hormone activates a gene to transcribe messenger rna?
    5 years ago
  • elen yonatan
    Which hormones do not bind to nuclear receptors of the target cells?
    5 years ago
  • antje
    What hormones dont bind to nuclear receptors of the target cells?
    5 years ago
  • florian egger
    Which hormones do not bind to nuclear receptors of the target cell?
    5 years ago
  • Teagan
    Is retinoic acid a steroid hormone?
    5 years ago
  • SUSAN
    When steroid hormones bind to their receptors?
    5 years ago
  • luwam
    What binds to nucleo recepters?
    5 years ago
  • armaan
    How hormones activate by binding to cytoplasmic/nuclear receptors?
    5 years ago
  • AMARANTH
    Do water soluble hormones bind to nuclear receptors?
    5 years ago
  • Karri
    Where do protein and steroid hormones bind on its target cell?
    5 years ago
  • henriikka p
    Do steroid hormones activate a gene to transcribe messenger rna?
    5 years ago
  • olli
    When a steroid hormone binds to its target cell receptor it?
    4 years ago
  • Sara
    Can steroids bind to receptor proteins in the nucleus?
    4 years ago
  • Pansy Goodbody
    Which of these hormones can enter the target cell and bind to receptors in the nucleus?
    4 years ago
  • manfredo
    What hormone binds to receptors in the nucleus?
    4 years ago
  • carola
    Do Thyroid hormones alter the transcription of mRNA in the target cell?
    4 years ago
  • Efrem
    Which hormones do not bind to nuclear receptor of the target cells?
    3 years ago
  • doreen bauer
    What binds the DNA of a hormonal protein?
    3 years ago
  • jaiden
    Where in a target cell are the receptors for lipophilic hormones located?
    2 years ago
  • yusef
    Why thyroid hormones are lipophilic?
    9 months ago
  • jo
    Why hormones entering to nucleus and the function is?
    8 months ago
  • Principio
    What is hormone response elements?
    6 months ago
  • amber bradley
    Which harmone does not bind with cytoplasmic receptor?
    2 months ago
  • gemma
    Which hormone does not bind with cytoplasmic receptor?
    2 months ago

Post a comment