Update on adrenarche

Abstract

Purpose of review:

Adrenarche is the pubertal maturation of the innermost zone of the adrenal cortex, the zona reticularis. The onset of adrenarche occurs between 6–8 years of age when dehydroepiandrosterone sulfate (DHEAS) concentrations increase. This review provides an update on adrenal steroidogenesis and the differential diagnosis of premature development of pubic hair.

Recent findings:

1) The complexity of adrenal steroidogenesis has increased with recognition of the alternative “backdoor pathway” and the 11-oxo-androgens pathways. 2) Traditionally, sulfated steroids such as DHEAS have been considered to be inactive metabolites. Recent data suggest that intracellular sulfated steroids may function as tissue specific intracrine hormones particularly in the tissues expressing steroid sulfatases such as ovaries, testes, and placenta.

Summary:

The physiologic mechanisms governing the onset of adrenarche remain unclear. To date, no validated regulatory feedback mechanism has been identified for adrenal C19 steroid secretion. Available data indicate that for most children, premature adrenarche is a benign variation of development and a diagnosis of exclusion. Patients with premature adrenarche tend to have higher BMI values. Yet, despite greater knowledge about C19 steroids and zona reticularis function, much remains to be learned about adrenarche.

Introduction

Adrenarche represents the peri-pubertal maturation of the adrenal cortex. The onset of adrenarche is arbitrarily defined as occurring when dehydroepiandrosterone sulfate (DHEAS) concentrations are detectable using standard laboratory techniques, usually, between 6–8 years of age. The physical manifestation of adrenarche, pubarche, is characterized by the development of pubic or axillary hair, adult apocrine odor, increased oiliness of the skin and hair, and acne. In the National Health and Nutrition Survey (NHANES), the mean ages for pubic hair development among girls were 9.5 years for non-Hispanic blacks, 10.3 years for Mexican-Americans, and 10.5 years for non-Hispanic whites [1]. For boys, means ages for pubic hair development were 11.1 years for non-Hispanic blacks, 12.3 years for Mexican-Americans, and 12.0 years for non-Hispanic whites [1].

Adrenarche occurs in individuals with hypogonadotropic hypogonadism and in those with primary gonadal failure [2]. The occurrence of adrenarche in patients with hypothalamic-pituitary-gonadal (HPG) axis disorders indicates that the regulatory mechanisms responsible for adrenarche and HPG axis function differ [3]. Adrenarche appears to be a phenomenon limited to humans and higher primates [4].

Adrenal steroidogenesis

Steroidogenesis is the process through which cholesterol is converted to steroid hormones (Figure 1). The adrenal cortex and the gonads are the primary sources of steroid hormones [5]. Only minimal amounts of steroid hormone are stored within the adrenal gland. Enzymes expressed in liver, adipose tissue, and other peripheral tissues can modify steroid hormones; such tissue specific modifications activate and inactivate the secreted steroid hormones.

Figure 1.

Steroid Pathways. A. Classical/Canonical Pathway; B. 11-oxo-Androgen Pathway; C. Alternative Backdoor Pathway. Abbreviations: CYP11A1: cholesterol side chain cleavage; HSD3B2, 3-beta hydroxysteroid dehydrogenase type 2; CYP17A1, 17-alpha hydroxylase/17,20 lyase; CYP21A2, 21-hydroxylase; CYP11B1, 11beta-hydroxylase; CYP11B2, aldosterone synthase; SRD5A1: 5a-reductase type 1, SRD5A2: 5a-reductase type 2, AKR1C1: Aldo-keto reductase type 1 C1, AKR1C2: Aldo-keto reductase type 1 C2, AKR1C3: Aldo-keto reductase type 1 C3, AKR1C4: Aldo-keto reductase type 1 C4, HSD17B3: 17B-hydroxysteroid dehydrogenase type 3, HSD17B6: 17B-hydroxysteroid dehydrogenase type 6

The adrenal cortex consists of three distinct zones. The outer zone, the zona glomerulosa, synthesizes mineralocorticoids and is principally regulated by the renin-angiotensin pathway and serum potassium concentrations. Expression of the enzyme aldosterone synthase, encoded by CYP11B2, is limited to the zona glomerulosa. The middle zone, the zona fasciculata, synthesizes cortisol and is regulated by pituitary adrenocorticotrophin (ACTH) secretion. Acting through the ACTH receptor, encoded by the melanocortin-2 receptor (MC2R) gene, ACTH has chronic and acute actions. Its chronic actions promote transcription and translation of the adrenal steroidogenic enzymes. Acutely, ACTH stimulates cortisol secretion. The hypothalamic-pituitary-adrenal (HPA) axis governs cortisol secretion by negative feedback inhibition to limit ACTH secretion. ACTH and cortisol manifest diurnal variation with the highest concentrations in the early morning.

The inner zone, the zona reticularis secretes C19 steroids such as DHEA and DHEAS. DHEAS is the steroid hormone circulating in the greatest abundance, has a long half-life, and shows minimal diurnal variation. Evidence is accumulating that intracellular sulfated steroids may function as intracrine hormones in tissues expressing steroid sulfatases [6,7*]. Despite attempts to characterize an adrenal androgen stimulating factor, no proposed substance has been validated [8]. Further, no feedback loops regulating zona reticularis C19 androgen secretion have been substantiated. Hence, the physiologic mechanisms governing zona reticularis C19 secretion remain an enigma.

Detailed description of the classic/canonical pathway for adrenal steroidogenesis is beyond the scope of this review (Figure 1A)). The reader is referred to reviews of adrenal steroidogenesis [5,9]. However, two additional pathways for androgen biosynthesis warrant mention: the alternative “backdoor pathway” and the 11-oxo-androgen pathway. In the “backdoor pathway”, 17-hydroxyprogesterone can be converted to dihydrotestosterone (DHT) without prior conversion to testosterone (Figure 1B). This pathway is particularly relevant in individuals with 21-hydroxylase and P450-oxidoreductase deficiencies [10].

In the 11-oxo-androgen pathway, the 11β-hydroxylase enzyme, encoded by CYP11B1, participates in the synthesis of 11-ketoandrostenedione, 11β-hydroxyandrostenedione, 11β-hydroxytestosterone, and 11-ketotestosterone (Figure 1C) [11**,12]. Among girls with adrenarche and premature adrenarche, DHEA, DHEAS, 11-ketotesterone, testosterone, and 5-androstenediol-3-sulfate (Adiol-S) concentrations rise with adrenarche [13]. DHEAS and Adiol-S concentrations rise in parallel [14].

While testosterone and DHT are the most potent androgens, 11-ketotestosterone and 11-ketodihydrotestosterone are also potent androgen receptor agonists [15]. In peripheral tissues such as the testes, 11-ketotestosterone can be converted to 11-ketodihydrotetosterone by 5α-reductase type 2. The other 11-oxo-androgens, 11β-hydroxytestosterone, 11-ketoandrostenedione and 11β-hydroxyandrostenedione are less potent androgens but have in vitro androgen activity. Androstenedione, DHEA, and DHEAS do not show significant androgen receptor agonist activity.

Using LC-MS/MS, DHEA, DHEAS, and 11-oxo-androgen concentrations were measured in serum samples obtained from 18 different species. As anticipated, DHEA, DHEAS, and 11-oxo-androgen concentrations were an order of magnitude higher in humans and non-human primates. These studies also confirmed that the adrenal gland is the primary source of circulating 11-ketotestosterone among humans [16].

Adrenarche

During fetal life, the fetal adrenal cortex secretes DHEA and DHEAS, which serve as substrates for placental estrogen biosynthesis [5]. Following birth, the fetal adrenal cortex involutes and measurable DHEA and DHEAS concentrations decline. With onset of adrenarche, DHEA and DHEAS concentrations rise with DHEAS concentrations peaking during the second decade of life.

Traditionally, adrenarche is considered to occur between 6–8 years of age. However, this notion is being questioned because urinary excretion of DHEA and its metabolites begins to increase as early as age 3 years [17]. One small study of healthy Finnish children found that DHEAS concentrations were measurable at 1 year of age and correlated with DHEAS concentrations at 6 years of age [18*].

The molecular trigger for adrenarche remains unclear. Environmental exposures such as stress and nutrition may modulate the timing of adrenarche [19]. Studies in monozygotic and dizygotic twins showed heritability to be 58%−61% for adrenarche [20,21]. However, no distinct genetic markers were identified in these small case-control candidate gene studies [22]. At adrenarche, DHEAS concentrations rise independently of ACTH and cortisol concentrations. Yet, the lack of adrenarche in patients with MC2R mutations or ACTH deficiency indicates a partial or permissive role for ACTH in adrenarche. Nevertheless, despite the availability of new tools to investigate the serum and urine steroid metabolome profiles, the proximate signals to initiate adrenarche remain unknown [23].

Differential Diagnosis of Premature Adrenarche

The appearance of pubic or axillary hair before age 8 years in females and 9 years in males is arbitrarily defined as premature pubarche. The most common etiology is premature adrenarche. Other causes of early pubarche need to be eliminated before premature adrenarche is diagnosed (Figure 2).

Figure 2.

Algorithm to evaluate adrenarche. This algorithm provides guidance for evaluation of children with premature pubarche. For patients with CPP, breast development in girls and testicular enlargement in boys usually precedes pubic hair development. Rare forms of CAH include 3-hydroxysteroid dehydrogenase deficiency due to HSD3B2 mutations and 11-hydroxylase deficiency due to CYP11B1 mutations. Another rare genetic disorder is apparent cortisone reductase deficiency due to H6PD mutations.

The most frequent differential diagnosis is congenital adrenal hyperplasia (CAH). The virilizing congenital adrenal hyperplasias encompass a group of autosomal recessive disorders. These disorders are characterized by impaired glucocorticoid biosynthesis, loss of negative feedback inhibition, increased ACTH secretion, and subsequent increased adrenal C19 steroid secretion [24]. The most common form is 21-hydroxylase deficiency due to mutations in the 21-hydroxylase (CYP21A2) gene. The phenotypic spectrum ranges from severe to mild reflecting the consequences of the specific mutation. The current estimate regarding prevalence of non-classic 21-hydroxylase deficiency in American Caucasians is 1 in 200 [25]. Whether measured by radioimmunoassay or LC-MS/MS, early morning 17-OHP values greater than 200 ng/dl have greater than 96% sensitivity and 96% specificity to detect children with non-classic 21-hydroxylase deficiency [26,27]. Nevertheless, when symptoms suggest CAH, ACTH stimulation tests should be performed despite normal morning 17-OHP concentrations [28*].

Typically, patients with GnRH-dependent precocious puberty (CPP) follow the normal sequence of pubertal development with breast development in girls and testicular enlargement in boys preceding the appearance of sexual hair. Secondary CPP can occur in patients with advanced skeletal maturation attributed to untreated CAH or androgen secreting tumors.

Apparent cortisone reductase and PAPS synthase 2 (PAPSS2) deficiencies are very rare genetic disorders associated with premature pubarche. Apparent cortisone reductase deficiency is due to mutations in the hexose-6-phosphate dehydrogenase (H6PD) gene [29]. Loss of function PAPSS2 mutations impair conversion of DHEA to DHEAS leading to low DHEAS concentrations and elevated unconjugated adrenal C19 steroids [30]. Patients with PAPSS2 mutations may also manifest spondyloepimetaphyseal dysplasia characterized by short stature and mild brachydactyly [31].

Androgen secreting adrenal or gonadal tumors are very rare causes of premature pubarche and virilization. The tempo for pubertal changes is often rapid and accompanied by accelerated linear growth and skeletal maturation. Adrenal cortical, Leydig cell, and ovarian androgen secreting tumors can present with premature pubarche or virilization [32,33,34,35]. Testicular, brain, and hepatic tumors may secrete β-human chorionic gonadotropin (β-hCG) which stimulates testicular LH receptors to secrete testosterone; affected boys typically present with virilization and testicular enlargement. Girls generally do not present with precocious puberty associated with β-hCG secreting tumors because estradiol is not synthesized in the absence of FSH-induced aromatase expression.

Exposure to exogenous androgens in creams or gels containing testosterone, DHT, or androstenedione can lead to premature pubarche and/or virilization. Transfer of exogenous androgens can occur by direct application or skin-skin contact [36].

Laboratory Data

In premature adrenarche DHEA and DHEAS concentrations are elevated, typically above 50 mcg/dl, and consistent with the stage of pubic hair development. However, circulating DHEA, DHEAS, and androstenedione concentrations may not correlate with the clinical features. Traditionally, steroid hormones have been measured using radioimmunoassays. Limitations of this methodology include being able to measure only a single hormone and lack of specificity. Newer methods include LC-MS/MS and GC/MS. Advantages of these methods include the ability to measure multiple hormones simultaneously while providing greater specificity [37]. Knowing the specific reference ranges for the laboratory is important to avoid the misinterpretation of the laboratory results [38]. Since some hormones show diurnal variation, early morning samples, i.e. between 7:30–8:30 AM, are preferred.

Bone age

Premature adrenarche can be associated with advanced skeletal maturation and tall stature [39]. Skeletal maturation, also known as bone age, represents the expected change in ossification centers over time. Determination of bone age typically involves an X-ray of the left hand and wrist. The current methods of evaluating bone age are imperfect because existing standards are largely derived from a white population and are not always applicable to children of other ethnicities [40,41,42].

When the bone age is advanced, concerns arise regarding the potential for adult short stature. However, available data from outcome studies conflict. Some studies report that predicted adult heights are decreased in this population whereas other studies indicate that most children achieve an appropriate adult height [43,44,45,46]. Importantly, significantly advanced bone age results (> 2SD) should prompt evaluation for other disorders, e.g., non-classic CAH [47].

Linear growth during childhood is driven by growth plate chondrogenesis [48]. Androgens and estrogens affect skeletal maturation and bone health through their actions on osteoblasts, osteocytes, and osteoclasts [49]. In premature adrenarche, the rising DHEAS, testosterone and androstenedione concentrations are positively correlated with bone age advancement [50,51]. Obesity can further accelerate the rate of skeletal maturation in premature adrenarche; increased insulin, IGF-1, and leptin concentrations likely contribute to this acceleration [52,53]. Genetics, nutritional status, hormones, medications, and various disease states influence the tempo of skeletal maturation.

Neurobiology

Both DHEA and DHEAS are also considered to be neurosteroids because they can be synthesized de novo in the brain. The temporal proximity of increasing DHEAS secretion and cerebral cortex maturation suggests the hypothesis that these hormones enhance brain development and enable the pre-adolescent brain to adapt to the social challenges of approaching adulthood [54,55*].

What are the evolutionary benefits of adrenarche and the long childhood pause before achieving reproductive maturity? Potential developmental tasks occurring during this time frame include learning about gender specific activities, socialization, and cultural awareness [56,57,58*]. Earlier onset of adrenarche appears to be associated with increased risk to develop anxiety symptoms during puberty and young adulthood [59**,60]. Available data suggest that the pattern and timing of hormone exposure during adrenarche interacts with brain development, biologic sex, and psychological stress to influence risk for mental health issues during adolescence [61,62].

Polycystic ovary syndrome

Polycystic ovary syndrome is a common heterogeneous disorder characterized by clinical or biochemical hyperandrogenism and irregular menses [63*,64]. The relationship of premature adrenarche to PCOS remains unresolved. Inquiry into this question is problematic because the diagnostic criteria for PCOS include features typical for peri-pubertal development such as irregular menses, polycystic ovary morphology, and mild hyperandrogenism [65**].

Based on a report that 45% of a cohort of Catalan girls with a history of premature pubarche had developed functional ovarian hyperandrogenism, it was suggested that girls with premature pubarche might have an increased risk to develop PCOS [66]. Subsequent data led to the concept of a developmental sequence beginning with prenatal growth restriction (SGA infants), rapid post-natal growth in early childhood, hyperinsulinemia, increased hepato-visceral fat stores, and premature pubarche that culminated in functional ovarian hyperandrogenism [67,68,69*,70]. Yet, available data are inconsistent. Not all girls with PCOS have a history of premature adrenarche. Similarly, not all girls with premature adrenarche will eventually develop PCOS. Since definitive answer remains unclear, longitudinal follow-up of girls with premature adrenarche might help to identify the risk factors associated with development of PCOS [65].

Obesity/metabolism

The molecular basis of the hyperinsulinemia and/or insulin resistance noted in children with premature adrenarche is likely multi-factorial. Obesity exacerbates insulin resistance and is often accompanied by compensatory hyperinsulinemia [71]. Hyperinsulinemia promotes increased adrenal and gonadal steroid secretion. One prospective study involving young Finnish adult women with a history of premature adrenarche concluded that premature adrenarche was not associated with dyslipidemia or metabolic syndrome [72*]. Rather adiposity was associated with insulin resistance or metabolic syndrome independent of pubertal history [73].

The interrelationships between insulin resistance, hyperinsulinemia, and hyperandrogenism are being scrutinized through urinary steroid metabolome by GC-MS analysis of 24-hour urine collections. Specific patterns of the urinary steroid metabolome have been associated with non-syndromic obesity [74]. When the urinary steroid metabolome of children with insulin resistance was compared to those without insulin resistance, children with insulin resistance showed increased excretion of C19 androgens, glucocorticoids, and mineralocorticoid metabolites [75*].

Evaluation and Treatment

Premature adrenarche is a diagnosis of exclusion and is the most common of prepubertal androgen excess [76*]. Following diagnosis, no specific intervention with medication is generally needed. Nevertheless, regular re-evaluations to monitor linear growth velocity, weight gain, skeletal maturation, and pubertal progression are helpful. Annual monitoring of adrenal C19 steroids can be considered. Given the potential consequences of premature adrenarche monitoring physical and psychological development with a focus on healthy lifestyle interventions may be beneficial.

Conclusion

The proximate physiologic mechanisms governing the onset of adrenarche remain unclear. Although DHEAS is the most abundant circulating steroid hormone, its function remains to be established. Available data indicate that for most children premature adrenarche is a benign variation of development. The importance of the 11-oxo-androgen pathway to adrenal function has become increasingly apparent. Despite greater knowledge about zona reticularis function, much remains to be learned about adrenarche.

Key Points.

• Adrenarche represents adrenal pubertal maturation and is accompanied by increased adrenal C19 steroid secretion

• Three pathways for C19 steroid biosynthesis have been characterized and likely play roles in adrenarche

• Premature adrenarche is a diagnosis of exclusion

• Long-term longitudinal follow-up is important to assess for metabolic and reproductive consequences associated with premature adrenarche