Mechanical strain-mediated reduction in RANKL expression is associated with RUNX2 and BRD2

Area of Science:

• Bone biology and mechanotransduction
• Epigenetics in skeletal regulation
• Osteoblast and osteoclast differentiation

Background:

• Mechanical loading influences bone remodeling by modulating osteoblast and osteoclast activity.
• Osteocytes secrete sclerostin (SOST) to inhibit osteoblasts and express RANKL to recruit osteoclasts.
• The transcription factor RUNX2 is a master regulator of osteoblast differentiation and targets both SOST and RANKL.

Purpose of the Study:

• To investigate the role of RUNX2 and epigenetic factors in mediating the effects of mechanical strain on SOST and RANKL expression in osteoblasts.
• To elucidate the molecular mechanisms by which mechanical loading regulates bone remodeling processes.

Main Methods:

• Utilized human osteoblastic Saos-2 cells subjected to mechanical strain via four-point bending.
• Employed RUNX2 knockdown, chromatin immunoprecipitation, and RT-qPCR to analyze gene expression and protein interactions.
• Investigated the expression and interaction of epigenetic regulators, specifically BRD2, with RUNX2 and the RANKL promoter.

Main Results:

• Mechanical strain down-regulated SOST and RANKL expression in Saos-2 cells without altering RUNX2 levels.
• RUNX2 knockdown increased basal SOST but did not affect strain-induced SOST down-regulation; it prevented RANKL down-regulation by strain.
• Strain and RUNX2 knockdown reduced BRD2 expression, and BRD2 was found to interact with RUNX2 and occupy the RANKL promoter, with occupancy decreasing upon strain exposure.

Conclusions:

• RUNX2 suppresses basal SOST expression and facilitates strain-induced RANKL down-regulation.
• A mechanosensitive epigenetic loop involving BRD2 and RUNX2 mediates the regulation of RANKL expression by mechanical strain.
• These findings highlight a novel mechanism for mechanical loading in bone remodeling through epigenetic regulation.