Testosterone Synthesis

Androgens are synthesized from cholesterol in testicular Leydig cells, the adrenal cortex, and ovarian thecal cells (see Fig. 38-1). In the adult gonads the principal regulator of testosterone synthesis and secretion is luteinizing hormone (LH), which is produced by the anterior pituitary gland (Chapter 38). The precursor cholesterol is synthesized in the Leydig cells from acetate and stored as cholesterol esters in lipid droplets. A cholesterol ester hydrolase mobilizes free cholesterol from the lipid droplets, which in turn is transferred to the inner mitochondrial membrane. Stimulation of this transfer represents a major action of LH and is mediated by the steroidogenic acute regulatory (StAR) protein. Leydig cells can convert a small fraction of testosterone to estradiol (see Chapter 40 and Fig. 38-1). LH increases the level of enzymes in this synthetic pathway.

Unlike the peptide hormones, the intracellular storage of steroid hormones, which can be mobilized and secreted, is minimal. The amount of testosterone in the human testis is approximately 300 ng/g of wet tissue. Assuming a normal, adult testis in humans weighs 15 g, total testicular testosterone is approximately 9 µg, or 0.1% of its daily production (5 to7 mg).

During human development, testosterone synthesis begins during the first trimester of pregnancy and is regulated by placental chorionic gonadotropin (hCG). At this stage of fetal development, male sexual differentiation is dependent on placental hCG until fetal pituitary gonadotrophs become functional at the end of the first trimester. During the second trimester, gonadotrophs become able to provide adequate gonadotropin-releasing hormone (GnRH) to stimulate gonadotropin formation. Gonadotropin secretion and sex steroid production decline late in fetal life, followed by a prominent postnatal surge lasting 2 to 3 months; by 3 or 4 months of age, testosterone secretion is significantly reduced. At puberty, gonadotropin secretion increases and again stimulates the Leydig cell to produce testosterone. The neurotransmitters gamma aminobutyric acid, neuropeptide Y, and kisspeptins (ligands of the orphan G-protein-coupled receptor GPR54) have each been proposed to influence puberty by regulating GnRH.

Gonadotropin secretion in early puberty follows a diurnal rhythm, with elevated concentrations of LH and testosterone at night. In adult men the diurnal rhythm for LH is less demonstrable. Specifically, testosterone levels in the early morning are approximately 25% higher than in the late afternoon. LH secretion fluctuates every 1 to 2 hours, resulting from intermittent stimulation of gonadotrophs by GnRH. GnRH secretory episodes in turn are coupled to excitatory discharges of a neural oscillator system. Intermittent GnRH secretion is required for the pituitary to function normally, and testosterone is released into the circulation in pulses in response to the pulsatile stimulation of Leydig cells by LH.

Androgen Production by the Adrenal Glands

Although glucocorticoids and mineralocorticoids are the principal products of the adult adrenal gland, dihydroepiandrosterone (DHEA), androstenedione, and testosterone, as well as some DHEA sulfate and estrone, can be also secreted (see Fig. 38-1). The concentrations of DHEA, DHEA sulfate, and androstenedione in the circulation increase between 7 to 10 years of age. This process has been termed adrenarche, to distinguish it from gonadarche, which is the onset of adult gonadal function at puberty. Adrenal androgen secretion declines in the elderly and during severe illness.

Regulation of Testosterone Synthesis and Secretion

The major site of testosterone synthesis and secretion is the Leydig cell, which has cell-surface LH receptors that associate with the Gs subunit of adenylyl cyclase. Steroidogenesis mediated by LH requires mobilization of intracellular Ca⁺⁺ and the Ca⁺⁺-binding protein calmodulin, and activation of phospholipase C (see Chapter 1). The effects of LH involve rapid stimulation of testosterone production (within minutes), which is mediated by the StAR. Other hormones that influence testosterone synthesis include prolactin, cortisol, insulin, insulin-like growth factors, estradiol, activin, and inhibin. There is a growing appreciation of the multiple factors involved in testosterone synthesis that are produced within the seminiferous tubules by germ cells and Sertoli cells or peritubular myoid cells. These factors maintain the serum concentration of testosterone in adult men at 0.3 to 1.0 mg/dL (10 to 30 nM). During illness, LH production declines, and cytokines suppress testosterone production.

Sertoli cells are somatic cells within the seminiferous tubules. Tight junctions between these cells at the base of seminiferous tubules form a blood-testis barrier, which prevents circulating proteins from entering the tubular compartment. Sertoli cells secrete many types of proteins, some of which enter the tubular lumen and are important in spermatogenesis. Others are secreted through the basal end of the cell and enter the circulation. Among these proteins are the androgen-binding protein, transferrin, and inhibin-B. Follicle-stimulating hormone (FSH) is the major regulator of Sertoli cell function. The FSH receptor is also membrane-bound and acts through both cyclic adenosine monophosphate and Ca⁺⁺. Insulin and insulin-like growth factors, testosterone, vitamin A, and β-endorphins also influence Sertoli cell function.

The hormones of the hypothalamus, pituitary, and testes form an internally regulated unit (Fig. 41-1). Not only are the testes stimulated by pituitary gonadotropins, but the testes also regulate LH and FSH secretion through negative-feedback mechanisms. Testosterone suppresses gonadotropin secretion by slowing the pulsatile release of GnRH. Estradiol, which is synthesized from testosterone in the ovary, testes, adipose tissue, liver, and brain, inhibits gonadotropin release through effects on both the hypothalamus and pituitary. Inhibin-B selectively reduces FSH synthesis and secretion.

FIGURE 41–1 Hormonal control of testicular function.

Normally, women produce approximately 0.25 mg/day of testosterone compared with the 5 to 7 mg/day for adult men. Most testosterone circulating in women is derived from the peripheral conversion of androstenedione secreted by the ovaries and adrenals (see Chapter 40). Benign and malignant tumors of the adrenal and ovary, congenital steroidogenic enzyme defects, and disturbances of gonadotropin secretion can be associated with increased androgen production in women.

Androgen Action

Endogenous testosterone or exogenous testosterone derivatives are transported to their target tissues through the blood. The primary mode of circulation of testosterone is a high-affinity interaction with the hepatic glycoprotein, sex hormone binding-globulin (SHBG). Approximately 1% to 3% of circulating testosterone is unbound and available for entering target tissues and altering gene activity.

There is evidence that SHBG binds androgen target cells and may play a role in the action of testosterone. SHBG formation is increased by estrogens and thyroxine and decreased by androgens, growth hormone, and insulin. The higher levels of estrogen in women promote twofold to threefold higher levels of SHBG than in men. Also, patients with hyperthyroidism exhibit higher SHBG levels. Obesity is associated with low concentrations of SHBG, perhaps because of hyperinsulinemia and insulin resistance.

Depending on the tissue, intracellular testosterone or a more active metabolite DHT can interact directly with ARs (Fig. 41-2). When testosterone enters the prostate gland or any tissue with significant 5α-reductase activity, nearly 90% of it is metabolized to DHT. There are two isoenzymes of 5α-reductase, encoded by two different genes. Type I 5α-reductase is found in liver, skin, sebaceous glands, most hair follicles, and prostate, whereas 5α-reductase type II predominates in genital skin, beard and scalp hair follicles, and prostate. The presence of ambiguous genitalia in patients with inactivating mutations of the 5α-reductase type II gene is indicative of the importance of this enzyme in normal development of male external genitalia. The distribution of the isozymes has been exploited to develop tissue-specific inhibitors of 5α-reductase activity.

FIGURE 41–2 Androgen action at target cells.

Androgen binding to ARs and the events that follow are similar to those of other steroid hormones (see Chapter 1). The AR is encoded by a gene on the X chromosome and is expressed in most tissues. When a ligand binds to the AR, the conformation of the receptor is altered, it binds to DNA response elements, multiple coactivator proteins are recruited, and the transcription of messenger RNAs for tissue-specific proteins ensues. Although most actions of androgens are mediated by transcriptional activity of the receptor, others are mediated through second messengers, such as the mitogen-activated protein kinase pathway.

Androgen regulation of target tissues may be positive, as in the stimulation of androgen-dependent proteins within the prostate, or negative, as in the inhibition of pituitary gonadotropin α-subunit gene expression and GnRH release by the hypothalamus. Negative regulation is less well understood but in certain cases has been explained by AR binding to, and interfering with, the action of stimulatory transcription factors.

Antiandrogens and Androgen Receptor Antagonists

There are two basic mechanisms to block the activity of androgen. These include inhibition of androgen formation or antagonism of androgen-AR interactions. Strategies for suppression of androgen formation include blockade of gonadotropin formation using GnRH analogs and the use of spironolactone and ketoconazole to competitively inhibit the activities of steroid 17α-hydroxylase (CYP17) and cholesterol side-chain cleavage enzyme (CYP11A), respectively (see Fig. 38-1). Another agent, finasteride, is a competitive inhibitor of the 5α-reductase isozymes and antagonizes the formation DHT, which has potent androgenic action. All these drugs are substrate analogs and compete with the natural substrates for active sites on the enzymes.