Veterinary Physiology-III

Mechanism of steroid (Lipophilic) hormone action

MECHANISM OF ACTION OF STEROID HORMONES

• Being lipids, steroid hormones enter the cell by simple diffusion across the plasma membrane.
  • The receptors for these hormones exist either in the cytoplasm or nucleus, where the receptors bind with the hormone.
  • Steroids stimulate their target cells by the intracellular action by entering the cytoplasm. Receptors for steroid hormones are present within the cytoplasm of the target cells.
  • When hormone binds to intracellular receptor, a characteristic series of events occurs.
• Receptor activation is the conformational change occurring in the receptor induced by the binding hormone. The major consequence of activation is that the receptor becomes competent to bind DNA. The activated hormone-receptor complex moves into the nucleus.
• Activated receptors bind to hormone response elements, which are short specific sequences of DNA, which are located in promoters of hormone-responsive genes.
• Transcription The gene to which the receptor has been bound is affected. Most commonly, receptor binding stimulates transcription. The hormone-receptor complex thus functions as a transcription factor.
• Gene transcription stimulates the formation of a specific messenger RNA which leaves the nucleus and stimulates the synthesis of specific protein necessary for steroid hormone functions. Changes in the mRNA levels effect a change in the rate of synthesis of various proteins translated.
• The biological response to a protein/peptide hormone receptor interaction is more rapid than for steroid, since, in the former case, the pre existing enzymes are activated, whereas the steroid / thyroid hormone action requires transcription and synthesis of new proteins (enzymes).