Role of endothelial nitric oxide synthase in estrogen-induced osteogenesis

It is well recognized that high-dose estrogen induces a marked osteogenic r esponse in long bones of female mice. In light of evidence which suggests t hat nitric oxide synthase (NOS) plays a role in regulation of osteoblast ac tivity, we analyzed whether NOS is involved in mediating this response. Int act female mice were administered 17 beta -estradiol (E-2) either alone or in combination with NG-nitro-L-arginine methylester (IL-NAME) or aminoguani dine (AG), over 24 days. The former inhibits both constitutive and inducibl e isoforms of NOS, whereas the latter is a selective inhibitor of inducible NOS. Bone mineral density (BMD) of the femur was subsequently measured by dual-energy X-ray absorptiometry (DXA), and histomorphometry performed at t he proximal metaphysis on longitudinal tibial sections. As expected, E-2 gi ven alone led to a marked accumulation of cancellous bone at the proximal t ibial metaphysis, associated with a significant gain in femoral BMD, and an increase in cancellous mineralizing surfaces as assessed by histomorphomet ry. Neither L-NAME nor AG affected cancellous histomorphometric indices whe n given alone. However, when administered in combination with L-NAME, the m agnitude of the skeletal response to E2 was significantly reduced. The tend ency for L-NAME to reduce estrogen-induced bone formation within the proxim al tibial metaphysis was more marked distally compared with proximally. In contrast, AG showed no tendency to suppress the osteogenic response to E-2. Subsequently, we examined the effect of E2 administration on expression wi thin mouse femoral bone marrow of endothelial NOS (eNOS), which is the pred ominant constitutive isoform of NOS within bone. No change in eNOS mRNA lev els was observed following E2 administration, as assessed by reverse transc ription-polymerase chain reaction (RTPCR). Taken together, our results sugg est that eNOS plays a role in mediating estrogen-induced bone formation in intact female mice, possibly as a consequence of posttranscriptional regula tion of eNOS activity by estrogen. (C) 2001 by Elsevier Science Inc. All ri ghts reserved.