Leydig cells: formation, function, and regulation

Abstract

Herein we summarize important discoveries made over many years about Leydig cell function and regulation. Fetal Leydig cells produce the high levels of androgen (testosterone or androstenedione, depending upon the species) required for differentiation of male genitalia and brain masculinization. Androgen production declines with loss of these cells, reaching a nadir at postpartum. Testosterone then gradually increases to high levels with adult Leydig cell development from stem cells. In the adult, luteinizing hormone (LH) binding to Leydig cell LH receptors stimulates cAMP production, increasing the rate of cholesterol translocation into the mitochondria. Cholesterol is metabolized to pregnenolone by the CYP11A1 enzyme at the inner mitochondrial membrane, and pregnenolone to testosterone by mitochondria and smooth endoplasmic reticulum enzymes. Cholesterol translocation to the inner mitochondrial membrane is mediated by a protein complex formed at mitochondrial contact sites that consists of the cholesterol binding translocator protein, voltage dependent anion channel, and other mitochondrial and cytosolic proteins. Steroidogenic acute regulatory protein acts at this complex to enhance cholesterol movement across the membranes and thus increase testosterone formation. The 14-3-3γ and ε adaptor proteins serve as negative regulators of steroidogenesis, controlling the maximal amount of steroid formed. Decline in testosterone production occurs in many aging and young men, resulting in metabolic and quality-of-life changes. Testosterone replacement therapy is widely used to elevate serum testosterone levels in hypogonadal men. With knowledge gained of the mechanisms involved in testosterone formation, it is also conceivable to use pharmacological means to increase serum testosterone by Leydig cell stimulation.

Figure 1.

Fetal and adult periods of testosterone production. Fetal Leydig cells produce the high levels of testosterone that are required for the differentiation of the male genitalia and for brain masculinization. Testosterone production declines with the postnatal decline in numbers of the fetal Leydig cells, reaching a nadir early in the postpartum period. Thereafter, testosterone gradually increases to high levels with the development of the adult Leydig cells from stem cells of the neonatal testis. LH is not required either for the development of fetal Leydig cells or for their initial testosterone production. Later, however, the fetal Leydig cells express LH receptor and respond to LH stimulation.

Figure 2.

The steroidogenic pathway. In Leydig cells, androgens are derived from cholesterol. Cholesterol is from de novo synthesis, lipoproteins, lipid droplets, or plasma membrane. Lipoproteins, LDL (through the LDL-receptor pathway), HDL (through the SR-BI pathway), and lipid droplets contain esterified cholesterol (yellow) that can be used in steroidogenesis after de-esterification (green). Cholesterol is imported into mitochondria through a large protein complex, the transduceosome, composed of mitochondrial and cytosolic proteins. Cholesterol reaches CYP11A1 in the inner mitochondrial membrane where it is metabolized to pregnenolone. Pregnenolone is converted by 3β-HSD located at the mitochondria and endoplasmic reticulum. Subsequent metabolism to androgen and its metabolites is by Leydig cell species-specific expression of CYPs and HSD.

Figure 3.

Protein–protein interactions driving cholesterol import into mitochondria. Cholesterol import into mitochondria is the result of series of protein–protein interactions. VDAC and TSPO are proteins found in most mitochondria, and ATAD3A is found in many cells. The presence of CYP11A1, adrenodoxin reductase and adenodoxin as well as the extremely high levels of expression of the cholesterol binding protein TSPO are characteristics of steroidogenic cell mitochondria. ACBD1 is a TSPO endogenous ligand. In response to hormone treatment, the outer mitochondrial membrane (OMM) TSPO and VDAC complex recruits ACBD3 which brings PKA to mitochondria. The hormone-induced STAR protein contains a mitochondrial signal sequence and is targeted to the OMM, where it interacts with VDAC and is locally phosphorylated by PKA for maximal activity. 14-3-3 adaptor proteins, binding to either STAR (14-3-3γ) or VDAC1 (14-3-3ɛ), provide negative control of maximally produced steroid formation, thus allowing for sustainable steroid formation. This complex is termed the transduceosome because it transduces the cAMP signal directly at the OMM. The OMM proteins TSPO and VDAC, together with the IMM proteins ATAD3 and CYP11A1, are part of the larger 800-kDa metabolon composed of proteins that bring cholesterol directly to CYP11A1 for metabolism.