Hormones That Act Mainly on the Genetic Machinery of the Cell
Steroid Hormones Increase Protein Synthesis
Another means by which hormones act—specifically, the steroid hormones secreted by the adrenal cortex, ovaries, and testes—is to cause synthesis of proteins in the target cells. These proteins then function as enzymes, transport proteins, or structural proteins, which in turn provide other functions of the cells.
The sequence of events in steroid function is essen-tially the following:
1. The steroid hormone diffuses across the cell membrane and enters the cytoplasm of the cell, where it binds with a specific receptor protein.
2. The combined receptor protein–hormone then diffuses into or is transported into the nucleus.
3. The combination binds at specific points on the DNA strands in the chromosomes, which activates the transcription process of specific genes to form mRNA.
4. The mRNA diffuses into the cytoplasm, where it promotes the translation process at the ribosomes to form new proteins.
To give an example, aldosterone, one of the hor-mones secreted by the adrenal cortex, enters the cyto-plasm of renal tubular cells, which contain a specific aldosterone receptor protein. Therefore, in these cells, the sequence of events cited earlier ensues. After about 45 minutes, proteins begin to appear in the renal tubular cells and promote sodium reabsorption from the tubules and potassium secretion into the tubules. Thus, the full action of the steroid hormone is charac-teristically delayed for at least 45 minutes—up to several hours or even days. This is in marked contrast to the almost instantaneous action of some of the peptide and amino acid–derived hormones, such as vasopressin and norepinephrine.
Thyroid Hormones Increase Gene Transcription in the Cell Nucleus
The thyroid hormones thyroxine and triiodothyronine cause increased transcription by specific genes in the nucleus. To accomplish this, these hormones first bind directly with receptor proteins in the nucleus itself; these receptors are probably protein molecules located within the chromosomal complex, and they likely control the function of the genetic promoters or operators.
Two important features of thyroid hormone func-tion in the nucleus are the following:
1. They activate the genetic mechanisms for the formation of many types of intracellular proteins—probably 100 or more. Many of these are enzymes that promote enhanced intracellular metabolic activity in virtually all cells of the body.
2. Once bound to the intranuclear receptors, the thyroid hormones can continue to express their control functions for days or even weeks.