Inhibition of bone resorption reduces osteoarthritis in mice with high bone turn over

Abstract

Mechanical adaptation of bone is a cellular process that allows bone to adjust its mass and structure to its mechanical environment. Such remodeling occurs by recruitment and activation of osteoclasts and osteoblasts in relation to strain distribution over bone tissue. These mechanical changes are detected by osteocytes, which results in production of signalling molecules such as nitric oxide (NO). We have shown earlier that mechanically stimulated osteocytes inhibit osteoclast formation via soluble factors, thus affecting bone resorption. We hypothesize that mechanically stimulated osteocytes modulate osteoclast formation by changing expression of osteoclastogenesis-related genes. Therefore, we investigated whether mechanical stimulation affects osteoclastogenesis-related gene expression in osteocytes. MLO-Y4 osteocytes were seeded at 10 cells/cm on glass slides in α-MEM with 5% FBS and 5% FCS. Osteocytes were mechanically stimulated by 1 h pulsating fluid flow (PFF;0.7±0.3 Pa,5 Hz), and post-incubated for 1 h and 6 h without PFF. Controls were kept under static culture conditions. NO production was determined as parameter of bone cell responsiveness. After 1 h PFF and 1 h and 6 h postincubation, cells were lysed for total RNA isolation. Gene expression of MEPE, PHEX, sclerostin, RANKL, and OPG was studied using real time-PCR. Within 5 min PFF increased NO production by 2.0-fold. PFF upregulated MEPE gene expression by 23.0-fold after 1 h, but not at 1 h and 6 h post-incubation. PFF did not affect PHEX expression. Sclerostin expression was not detectable. PFF upregulated RANKL expression by 2.1 fold, and OPG expression by 2.7 fold. PFF decreased the RANKL/OPG ratio by 20% after 1 h, but not at 1 h and 6 h post-incubation. Our data suggest that mechanical loading induces changes in osteoclastogenesis-related gene expression in osteocytes, which likely contributes to the inhibition of osteoclastogenesis after mechanical loading of bone. Since mechanical loading upregulated gene expression of MEPE but not of PHEX, possibly resulting in the upregulation of OPG gene expression, we speculate that MEPE might be a key soluble factor produced after mechanical loading of osteocytes leading to the inhibition of osteoclastogenesis.