Positive effect of human adipocyte-secreted factors on human osteoblast proliferation and the OPG/RANKL ratio in vitro
Markus C Kühn, Werner A Scherbaum & Sven Schinner
Objectives: Reduced bone mass and compromised bone quality characterise osteoporosis. A positive relationship of the body-mass-index or the body fat mass and the bone density has been reported by several clinical studies. However, it is not clear whether this observation is attributable to higher mechanical stress or whether adipocytes secrete products that directly affect the bone structure.
Methods: Isolation of human primary adipocytes to generate fat-cell conditioned medium (FCCM). Isolation of primary human preosteoblasts. Local Ethical Committee approval has been obtained. Measurement of cell proliferation using 3H-thymidine incorporation assay. Real-time reverse transcription PCR to assess gene expression.
Results: The stimulation of MC3T3-E1 cells with FCCM led to a 2.8-fold increase in proliferation (P< 0.05) after 48 h. Similarly, FCCM treatment increased the proliferation of primary human preosteoblastic cells (1.5-fold, P<0.05). This effect of FCCM on the proliferation could be reduced with inhibitors of the receptor tyrosine kinase, of the FGFR1 and of the PI3K. Primary human preosteoblasts treated with FCCM for 14 days showed an increase in OPG mRNA expression and a decrease in RANKL mRNA expression.
Conclusions: These data demonstrate the direct influence of human adipocyte-secreted factors on preosteoblasts in vitro. Human adipocyte-secreted factors stimulate the proliferation of preosteoblasts leading to more cells capable of differentiating into mature osteoblasts. Furthermore, these factors might reduce the paracrine stimulation of osteoclasts by osteoblasts as they increase the OPG/RANKL ratio in preosteoblasts in vitro. These findings could explain the higher bone mass in obese people and attribute it to a direct effect of adipocyte-secreted factors.