General Mechanism of Steroid Hormone Action
Steroid hormones are lipophilic, allowing them to easily and freely cross the plasma membrane into the cell. Unlike peptide hormones, which require cell-surface receptors, steroids act from within.
Main steps of the molecular mechanism:
- Primary Interaction: Once inside the cell, the hormone finds its specific receptor protein located in the cytosol and binds to it.
- Translocation: The resulting hormone-receptor complex changes its structure and moves into the cell nucleus.
- Genome Interaction: Inside the nucleus, the complex finds a specific DNA region — the enhancer. This is a regulatory zone that enhances gene reading (also called HRE — Hormone Response Element).
- Activation of the Template Process: Binding of the complex causes a conformational change in DNA. As a result, transcription (the primary template process) — the synthesis of mRNA molecules on a DNA template — is sharply accelerated.
- Result: The accumulation of specific mRNA leads to an increased synthesis of corresponding proteins (enzymes or transporters) on ribosomes.
Cortisol and Activation of Gluconeogenesis
Cortisol (a glucocorticoid) is a classic example of a hormone regulating carbohydrate metabolism at the genomic level.
Its main physiological effect is the stimulation of gluconeogenesis, the process of forming glucose molecules from non-carbohydrate precursors such as amino acids and pyruvate.
Upon entering the cell and binding to its cytosolic receptor, cortisol travels to the nucleus, where its complex interacts with a specific enhancer — GRE (Glucocorticoid Response Element). This interaction alters the spatial structure of DNA and dramatically accelerates gene transcription. The result is a sharp increase in the cytoplasmic content of mRNAs encoding key enzymes of glucose synthesis. More mRNA means more gluconeogenesis enzymes and higher glucose levels.
Aldosterone: Regulation of Water and Electrolyte Balance
Aldosterone is another lipophilic steroid hormone, but its targets are renal tubular cells (nephrocytes).
Its mechanism of action (induction) follows the same pattern: membrane penetration $\to$ binding to an intracellular receptor $\to$ transport of the complex to the nucleus $\to$ binding to a DNA region (HRE / enhancer).
Features of template biosyntheses under aldosterone action:
- Acceleration of mRNA synthesis encoding specific $Na^+$ transport proteins and the $Na^+,K^+$-ATPase.
- As a result, the number of these transport proteins on the nephrocyte membrane increases.
- This leads to enhanced reabsorption of $Na^+$ ions from the primary urine back into the blood.
Clinical Significance: In pathological hyperproduction of aldosterone, patients exhibit $Na^+$ retention in the body (hypernatremia — increased blood sodium and decreased urinary sodium) and concurrent loss of $K^+$ ions.