Mitochondrial regulation of Leydig cell steroid synthesis. Implications for the decline in male testosterone production with advancing age

Testicular Leydig cells are the principal source of the steroid hormone testosterone, synthesized from cholesterol precursors in response to a luteinizing hormone (LH)/cyclic adenosine 3', 5' (cAMP) signal. This signal activates a multi-step, multi-compartment steroidogenic pathway, one of whose compartments, the mitochondria, comprise a key control point in steroid biosynthesis. This step consists of transport of cholesterol from the outer to the inner mitochondrial membrane. Though the molecular mechanisms of this cholesterol transport process has garnered substantial research interest, comparatively fewer investigations have pursued the relationship between mitochondrial energetic production to the steroidogenic process and general Leydig cell biology. Moreover, mitochondria have been proposed to play key roles in biological aging, and previous studies have suggested mitochondrial compromise in aged Leydig cells. To this end, the relationship between mitochondrial energetic dynamics and steroidogenesis was explored in primary Brown-Norway rat Leydig cells. Initial studies demonstrated that the mitochondrial electron transport chain (ETC), which drives the formation of the mitochondrial membrane potential (Deltapsi) utilized for ATP synthesis, was critical for LH-mediated testosterone production as a number of steroidogenic steps were suppressed upon ETC inhibition. Conversely, it was found that ETC disruption stimulated non-hormonal (basal) testosterone production in a calcium-mediated manner. Further investigation of the effects of mitochondrial energetics on hormonally-mediated steroidogenesis demonstrated primary Leydig cells derive nearly all of their cellular ATP from mitochondrial oxidative phosphorylation. Moreover, Leydig tumor cell models commonly used to study steroidogenic energetics were found to derive a significant proportion of their ATP pool from glycolysis. Despite divergent sources of ATP, primary and tumor Leydig cells exhibited similar hormone-mediated steroidogenic kinetics in the face of mitochondrial perturbation, demonstrating the critical role of mitochondrial fidelity in hormone-mediated steroid synthesis. Additional investigations of mitochondrial stimulation of basal steroidogenesis took a different course, though one that again demonstrates a critical role in Leydig cell function. These studies suggested that mitochondria are involved in Leydig cell ion homeostasis and in the proposed model, ATP depletion mediated by mitochondrial dysfunction disrupts mechanisms of cytoplasmic calcium, [Ca2+]c, clearance, resulting in elevated [Ca2+]c and stimulation of basal steroidogenesis. Finally, the key role played by mitochondria in Leydig cell function supports a hypothesized role in the age-related decline of hormone-mediated testosterone synthesis. While decreases in Deltapsi and ATP measurement were observed in correlation to decreases in cAMP and testosterone synthesis, findings collectively point to a model in which submaximal production of cAMP upon hormonal stimulation, mediated in part by mitochondrial dysfunction, results in diminished steroidogenic enzyme expression and consequent diminished maximal testosterone synthesis.