Ovariectomy fails to augment bone resorption and marrow B lymphopoiesis in granulocyte colony-stimulating factor...

The mechanism of pathological bone loss induced by estrogen deficiency has not been fully elucidated. It has been shown in recent animal studies that increased B lymphopoiesis induced by estrogen deficiency is involved in the mechanism of stimulated bone resorption. Mice transgenic for granulocyte colony-stimulating factor (G-CSF) (G-Tg) exhibit generalized osteopenia with an increase in osteoclast number and enhancement of bone resorption, which coexists with enhanced hematopoiesis. When ovariectomy was performed on G-Tg, it did not further reduce bone mass as revealed by radiography, dual-energy X-ray absorptiometry, and peripheral quantitative computed tomography. Ovariectomy increased the amount of colony-forming units of interleukin 7 (CFU-IL-7) by threefold in the marrow of normal mice in association with an increase in the number of B220-positive cells expressing the receptor activator of nuclear factor-kappaB ligand (RANKL). In contrast, the number of B220-positive cells expressing RANKL and CFU-IL-7 remarkably decreased in the marrow of G-Tg. Ovariectomy induced neither CFU-IL-7 nor B220-positive cells expressing RANKL in the marrow of G-Tg. Strong inhibition of B lymphopoiesis by G-CSF resulted in depletion of B cells expressing RANKL from the marrow, which may lead to resistance to bone loss due to ovariectomy. This observation suggests that B lymphopoiesis plays a possible role in bone loss in a condition of acute estrogen deficiency.