Steroid hormones are chemical messengers produced from cholesterol and able, like all other hormones, to influence the activity of more or less large groups of target cells. Due to this common origin, steroid hormones have the tetracyclic structure ( cyclopentanperidrofenantrene ) characteristic of cholesterol.
Classification
In relation to the receptors with which they bind and interact, steroid hormones can be grouped into five categories: glucocorticoids, mineralocorticoids, androgens, estrogens and progestins.
Secretion and mode of action
Steroid hormones, unlike peptide ones, are synthesized only by a few organs and are not stored in reserve tissues, but rather produced when necessary and readily released in plasma. Here, due to their poor solubility in water, they must necessarily bind themselves to transport proteins:
specifics, such as SHBG (sex hormone transport proteins) and CBG (corticosteroid-binding globulins)
and aspecific, like albumin.
Only the free altitude, separated from such proteins or bound to them in a bland manner, represents the bioavailable and active fraction of the hormone.
Transport proteins maintain a circulating reserve of hormones, important in case of sudden changes in their concentration. They also have the ability to prolong their average life, protecting them from degradation; on the other hand, however, they limit its action, blocking their entry into the cells (only the free altitude, thus separated from these carriers is biologically active). Since the catabolism of steroid hormones is entrusted to the liver, the circulating levels depend on:
from the speed of synthesis;
from exogenous contributions (hormonal therapies);
from the concentration of transport proteins;
from liver function.
As the liver removes the steroid hormones from the plasma, the transport proteins obey the law of mass action and release the hormone in a proportional way, keeping the relationship between the free hormone and the bound hormone constant. The shares of circulating steroid hormones are particularly small, in the order of a millionth / billionth of a gram per milliliter of blood.
Thanks to their lipophilicity, steroid hormones are able to cross the plasma membrane in both directions (that's why they cannot be stored), and bind to specific cytoplasmic and nuclear receptors to carry out their genomic action (they activate transcription and genetic translation, stimulating the synthesis of new and specific proteins); however, in more recent times - for some steroid hormones such as testosterone, estrogen and aldosterone - specific membrane receptors have also been identified, able to bind to them and trigger cellular responses much faster than the previous ones.
Steroid hormones in brief
MINERALCORTICOIDS: steroid hormones that increase the reabsorption of water and sodium, favoring the excretion of potassium and hydrogenions. They are therefore important in the presence of hypotension (low blood pressure), because they increase the blood volume. The mineralocorticoid synthesis takes place in the glomerular area (most external) of the adrenal cortex. The best known and most active mineralocorticoid is aldosterone.
Major excess pathology: Conn syndrome Main defect pathology: primitive adrenal insufficiency.
GLUCOCORTICOIDS: steroid hormones that aim to raise blood sugar levels, increasing the production of glucose starting from some amino acids and decreasing its peripheral use. These hormones also stimulate the deposit of glycogen in the liver and reduce the inflammatory and immune response. They are particularly important in the process of adapting to intense physical stress. Glucocorticoid synthesis occurs in the fasciculate and reticular areas of the adrenal cortex. The best known and most active is cortisol. Major excess pathology: Cushing's syndrome. Main defect pathology: Addison's disease.
ANDROGENES: steroid hormones that influence male sexual characteristics and abilities; they have an anabolic action especially in the musculature and bone tissue. In women, they are secreted in much lower quantities than men. Androgens are synthesized from interstitial cells of Leydig (testicle) and in small quantities also from the adrenal cortex (important for the female organism). The most known and active is testosterone, which is then transformed into the powerful androgen dihydrotestosterone in some tissues - such as prostate, external genitals and skin - that contain the enzyme 5α-reductase.
ESTROGENES: steroid hormones that influence female sexual characteristics and abilities; they have an anabolic action especially in the adipose, bony and muscular tissue. In humans they are secreted in much lower quantities than women. They are synthesized starting from androgens by the aromatase enzyme, especially in the ovary, in the placenta during pregnancy and to a lesser extent in various tissues (especially in adipose tissue, which makes an important contribution to men and women after menopause ). The best known and most active is estradiol.
PROGESTINICS: steroid hormones that influence female sexual characteristics and abilities; they are important for the proper performance of the menstrual cycle and pregnancy. The most known and active is progesterone, synthesized in many tissues, because it is the precursor of the other classes of steroid hormones.
