Additive effects of estrogen and mechanical stress on nitric oxide and prostaglandin E₂ production by bone cells from osteoporotic donors

Mechanical loading is thought to provoke a cellular response via loading-induced flow of interstitial fluid through the lacuno-canalicular network of osteocytes. This response supposedly leads to an adaptation of local bone mass and architecture. It has been suggested that loss of estrogen during menopause alters the sensitivity of bone tissue to mechanical load, thereby contributing to the rapid loss of bone. The present study aimed to determine whether estrogen modulates the mechanoresponsiveness of bone cells from osteoporotic women. Bone cell cultures from nine osteoporotic women (aged 62–90 years) were pre-cultured for 24 h with 10⁻¹¹ mol/l 17β-estradiol (E2) or vehicle, and subjected to 1 h of pulsating fluid flow (PFF) or static culture. E2 alone enhanced prostaglandin E₂ (PGE₂) and nitric oxide (NO) production by 2.8-fold and 2.0-fold, respectively, and stimulated endothelial nitric oxide synthase protein expression by 2.5-fold. PFF, in the absence of E2, stimulated PGE₂ production by 3.1-fold and NO production by 3.9-fold. Combined treatment with E2 and PFF increased PGE₂ and NO production in an additive manner. When expressed as PFF-treatment-over-control ratio, the response to fluid shear stress was similar in the absence or presence of E2. These results suggest that E2 does not affect the early response to stress in bone cells. Rather, E2 and shear stress both promote the production of paracrine factors such as NO and PGE₂ in an additive manner.