Based on indirect evidence, it has often been assumed that the zona reticularis of the adult human adrenal cortex is the source of the adrenal androgens, dehydroepiandrosterone (DHEA) and DHEA sulfate (DHEAS), but direct tests of this concept have been few. Using the techniques of cell culture, Northern blotting, and RIA, we compared the properties of separated adult zonal cells to those of fetal zone cells, a cell type well known to secrete large amounts of DHEA(S) due to its low expression of 3 beta-hydroxysteroid dehydrogenase (3 beta HSD). In nine glands from donors of a wide age range, the zona fasciculata and zona reticularis were separated and dissociated, and the cells were placed in culture. After 5 days, serum was removed by a 24-h period in serum-free defined medium followed by a 24-h exposure to cAMP analogs, with the optional addition of insulin, also in serum-free medium. The separated fasciculata and reticularis cells showed large differences in the DHEA(S)/cortisol (F) production ratios from added pregnenolone precursor, consistent with the synthesis of only F and essentially no DHEA(S) by fasciculata cells and with the synthesis of mostly DHEA(S) with little or no F by both reticularis cells and fetal zone cells. The different patterns of steroidogenesis were accompanied by a much lower level of expression of type II 3 beta HSD in reticularis cells, similar to that in fetal zone cells. In contrast, other genes were similarly regulated in the two adult zones and in the fetal zone by both cAMP and insulin. The levels of messenger ribonucleic acids for 17 alpha-hydroxylase, cholesterol side-chain cleavage enzyme, 21-hydroxylase, and 11 beta-hydroxylase responded to cAMP and insulin in both reticularis cells and fetal zone cells in the same pattern as that previously established in fasciculata cells. The central role of the limited expression of 3 beta HSD in the DHEA(S)-synthesizing property of reticularis cells was established by inhibition of 3 beta HSD in fasciculata cells with trilostane, which caused them to increase their DHEA/F production ratio to a level exceeding even that in fetal zone cells. There did not appear to any age-related changes in gene expression that could account for the large age-related decline in DHEA(S) biosynthesis in humans in either reticularis or fasciculata cells. Thus, the most likely cause of the age-related decline in adrenal androgen biosynthesis is an age-related decline in the number of functional reticularis cells, without a major change in the differentiated properties of the zonal cells as a function of age.

The zona reticularis is the site of biosynthesis of dehydroepiandrosterone and dehydroepiandrosterone sulfate in the adult human adrenal cortex resulting from its low expression of 3 beta-hydroxysteroid dehydrogenase

Control of secretion and the function of C19-Δ5-steroids of the human adrenal gland

Dehydroepiandrosterone sulfate (DHEAS) is the most abundant product of the young adult human adrenal cortex and fetal adrenal gland.'-' DHEAS secretion shows a dramatic cycle of high and low points over the life span in humans, as discussed in several other articles in this volume. Secretion from the adrenal cortex is very high in the fetus, drops sharply after birth to very low levels in childhood, rises again before puberty, a phenomenon termed adrenarche, and reaches high levels once again in young adulthood. Thereafter, DHEAS secretion progressively declines, often reaching very low or negligible levels after age 70. Plasma DHEAS levels reflcct synthesis by the adrenal cortex, because the adrenal is normally the overwhelming source of DHEAS and the major contributor to unconjugated dehydroepiandrosterone (DHEA), which is present at much lower circulating levels and also is synthesized by the gonads. Synthesis of the adrenal androgens DHEA and DHEAS in adult humans and other primates presents several puzzles to the researcher. First, why do the adrenal glands synthesize large amounts of these steroids in young adulthood, after a period from birth to pre-puberty during which little or none of these steroids is synthesized? Second, why does the high rate of secretion in young adulthood show such a strong age-related decline? Third, is the age-related decline in the adrenal androgens equivalent to deprivation of an essential hormone, and, therefore, is there a role for replacement therapy of DHEA(S) in later life as has successfully been achieved with estrogens? The question of the function of DHEA(S) in young adulthood and approaches to the concept of replacement therapy in older individuals are thc subjects of several articles in this volume and will not be addressed here. Rather, the aim of this review is to account for the high rate of synthesis of DHEA(S) by the young adult adrenal gland at a cellular and molecular level, to examine the basis for the differences between humans and nonprimate species in DHEA(S) biosynthesis, and to offer some ideas for the mechanism of the age-rclatcd decline in secretion.

Biosynthesis of DHEAS by the Human Adrenal Cortex and Its Age‐Related Decline

The 24-h profiles of plasma cortisol (F), 11-β-hydroxyandrostenedione (11OHAD), androstenedione (AD), dehydroisoandrosterone (DHEA) and testosterone (T) were obtained simultaneously in 11 normal males sampled at 15-min intervals. The data were submitted to a detailed quantitative analysis including the estimation of the circadian rhythm and of the episodic variations as well as the evaluation of the concomitance of episodic pulses of different hormones. A bimodal circadian rhythm was detected in the various individual profiles. The major acrophase occurred in the morning earlier for T (around 04:00 h) than for the hormones of totally or partially adrenal origin (around 07:00 h); the secondary acrophase (around 17:00 h) and the main midnight nadir were common to all hormones. The amplitude of the rhythm was highest for purely adrenal hormones (F and 11OHAD), averaging 79 and 75%, respectively, lower for hormones of mixed origin (DHEA and AD), averaging 44 and 42%, respectively, and minimal for T (22%). The possible relationship between the circadian and pulsatile variations of the various steroids was estimated in each individual by calculating Pearson’s standard coefficient of variation on all pairs of hormonal profiles. A very tight relationship (r > 0.75; p 0.30; p 0.30; p < 0.01). The pulsatility of the corticotrophic axis was readily transmitted to the secretory pattern of 11OHAD, DHEA and AD. Ninety-six percent of the F pulses were reflected in at least one other hormonal profile. Finally, we showed that concomitant pulses common to the five adrenal and gonadal patterns were more frequent than would be expected on the basis of chance. These results: (1) demonstrate a total parallelism between the long-lasting secretory events and the episodic bursts of the 4 adrenal hormones showing that the reticular and fascicular zones of the adrenal respond to pituitary control as an homogeneous structure; (2) demonstrate the existence of a partial synchronization of adrenal and testicular pulsatile variations; (3) suggest that, throughout the afternoon, a common mechanism may influence the slow variations of adrenal hormones and of testicular testosterone.

Control of circadian and episodic variations of adrenal androgens secretion in man

Publisher Summary The regulation of the structure of the mammalian adrenal cortex is an intrinsic component of the regulation of the synthesis of its steroid hormones. The two important structural factors to be considered are (1) adrenocortical mass and (2) zonation. Adrenocortical structure is involved in the regulation of the synthesis of the mineralocorticoid, glucocorticoid, and androgenic steroids secreted by the adrenal under both normal and pathological conditions. The adrenal cortex exhibits a unique system of physiological regulation, incorporating three separately regulated systems of hormone synthesis—mineralocorticoid, glucocorticoid, and androgen—into the life history of a single cell type. The adrenal glands vary in shape from almost spherical, in fetal life and in the adult of some species, through somewhat flattened to more or less leaf-like, for example, in humans and cattle. Although the size of the gland varies with the size of the animal, the thickness of the cortex remains remarkably constant. The mass of the zona glomerulosa is a major factor in the long-term regulation of aldosterone synthesis, and the inner zone (zona reticularis or fetal zone) is of major importance for the regulation of adrenal androgen synthesis.

M. Groom

Papers

Steroid biosynthesis by cultures of normal human adrenal tissue.