**2. Steroid hormones**

Steroid hormones are classified according to the number of carbon (C) atoms in the molecule deriving from the pregnane (C-21), androstane (C-19) or estrane (C-18) nucleus. C-21 hormones include progesterone, cortisol, corticosterone and aldosterone; C-19 testosterone, androstenedione (A4) and dihydrotestosterone; and C-18 estradiol, estrone and estriol. Based on its functional actions, steroid hormones are classified into five principal classes: estrogens (estradiol, estrone, estriol), progestins (progesterone), androgens (testosterone, A4,

Hormonal and Neural Mechanisms Regulating Hormone Steroids Secretion 5

*vitro* studies show that treating Leydig cells with testosterone decreases the expression of SR-BI, TSPO, StAR and cytochrome P-450scc (Kostic et al., 2011). In the adrenals, ACTH induces StAR synthesis by stimulating the synthesis of cyclic adenosin mono phosphate (cAMP), while the early steps in steroidogenic synthesis are mediated by posttranscriptional and post-translational changes in the StAR protein (Spiga et al., 2011). In the gonads, gonadotropic hormones transcriptionally control StAR gene expression via a cAMP second messenger (Sugawara et al., 1997). A characteristic feature of steroidogenic cells is the presence of numerous cytoplasmic lipid droplets containing cholesterol esters. Cholesterol esters in these droplets are synthesized by acyl coenzymeA-cholesterol acyltransferase, an endoplasmic reticulum-enzyme. The esters synthesized by cholesterol acyltransferase accumulate within the endoplasmic reticulum membranes and bud off as lipid droplets. Cholesterol esters in lipid droplets are hydrolyzed by a soluble sterol ester hydrolase. Gonadotropins stimulate the activation of cAMP-dependent-protein kinase that activates this enzyme by phosphorylating specific serine residues, thus promoting binding

of the sterol esterase to lipid droplets and the hydrolysis of cholesterol esters.

2. The access of the inner mitochondrial membranes to cholesterol;

flavoprotein and iron sulfur protein electron transport chain;

enzyme proteins, as well as cholesterol delivery (Straus & Hsue, 2000).

**4. Enzymes participating in the synthesis of steroid hormones** 

1. The rate of cholesterol delivery to the mitochondria;

messenger (Auchus & Miller, 2000; Straus & Hsue, 2000).

**4.2 3β-Hydroxysteroid dehydrogenase/∆5-4-isomerases** 

4. The catalytic activity of P-450scc.

aromatase cytochrome P450

**4.1 Cytochrome P-450scc** 

determined by:

Cholesterol is converted into pregnenolone, and the rate of pregnenolone synthesis is

3. The available quantity of cholesterol side chain cleavage enzyme and, secondarily its

Acute alterations in steroidogenesis generally result from changes in the delivery of cholesterol to P-450scc, whereas long-term alterations involve changes in the quantity of

The synthesis of the numerous enzymes participating in steroid hormones synthesis is under the stimulatory effects of hormones secreted by the pituitary ACTH, LH and follicle stimulating hormone (FSH). Other pituitary hormones, such as growth hormone (GH) and

Based on the progression of changes occurring to the cholesterol molecule, four enzymes seem to be crucial for hormone synthesis: cytochrome P-450scc, 3β-hydroxysteroid dehydrogenase/Δ5-4-isomerases (3β-HSDs), 17 α-Hydroxylase/17,2O-desmolase, and

The cytochrome P-450scc side chain cleavage is an enzyme present in the inner mitochondrial membranes. The enzyme is involved in three catalytic cycles on the cholesterol molecule: the introduction of hydroxyl groups at positions C-22 and C-20, and the subsequent scission of the side chain between these carbons. ACTH and LH stimulate cytochrome P-450scc synthesis in a mechanism that depends on cAMP synthesis as a second

3β-Hydroxysteroid dehydrogenase/Δ5-4-isomerases (3β-HSDs) are located at the microsomal fraction, and are expressed in the adrenal cortex and in steroidogenic cells of the

prolactin (PRL), also play a role regulating the synthesis of some of these enzymes.

dihydrotestosterone), glucocorticoid (cortisol, corticosterone), and mineralocorticoids (aldosterone, deoxicorticosterone).

All steroid hormones derive from cholesterol in a process that includes:

