Do androgens support bone density in premenopausal women via direct receptor action and aromatization to estradiol?

In premenopausal women, circulating androgens support bone mineral density (BMD) through complementary direct and indirect pathways within the skeletal microenvironment.

Direct androgen receptor signaling

Androgens, primarily testosterone and dihydrotestosterone (DHT), exert direct anabolic actions on the skeleton by binding to androgen receptors (AR) expressed on bone cells.

• Osteoblast stimulation: Direct AR activation on osteoblasts and osteocytes promotes osteoblast differentiation, proliferation, and bone mineralization, particularly within cortical bone where AR expression is highest.
• Inhibition of resorption: Direct signaling through AR on osteoblasts and osteocytes downregulates osteoclastogenesis, while direct AR binding on mature osteoclasts further limits active bone resorption.

Indirect action via local aromatization

The primary mechanism by which androgens support bone density is indirect, relying on enzymatic conversion within the bone tissue.

• Local estradiol production: Circulating androgens (testosterone and androstenedione) are converted to estradiol by the enzyme aromatase (CYP19A1), which is locally expressed by osteoblast-lineage cells.
• Estrogen receptor activation: This locally produced estradiol acts in a paracrine or autocrine manner on estrogen receptors (mainly ER-alpha) to inhibit osteoclastogenesis and promote osteoclast apoptosis. This pathway represents the dominant endocrine regulator of bone remodeling, maintaining both trabecular and cortical microarchitecture.

Clinical and therapeutic implications

The physiological significance of these pathways is highlighted in distinct clinical states:

• Hormone suppression: In premenopausal women undergoing ovarian suppression, the administration of aromatase inhibitors prevents the conversion of androgens to estradiol. Despite normal or elevated circulating androgen levels, this block leads to rapid bone loss and microarchitectural deterioration.
• Hyperandrogenic states: In conditions such as polycystic ovary syndrome (PCOS), elevated circulating androgens correlate with normal or elevated BMD, driven by the combination of direct AR stimulation and increased substrate availability for local aromatization.

Bottom line

• Premenopausal bone density is preserved through dual mechanisms: directly via androgen receptor binding to promote bone formation, and indirectly—and more dominantly—via local aromatization of androgens to estradiol, which prevents bone resorption.