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.
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