Ataxia-televangelist mutated (ATM)/ ATR serine/threonine kinase (ATR)-mediated RAD51 recombinase (RAD51) promotes osteogenic differentiation and inhibits osteoclastogenesis in osteoporosis

Abstract

Osteoporosis is a metabolic bone disease that significantly affects the quality of life and can even lead to death. In this study, we aimed to investigate the role of RAD51 recombinase (RAD51) in osteoblast and osteoclast differentiation. We analyzed differentially expressed genes using microarray analysis. The osteogenic differentiation capability was analyzed by alkaline phosphatase (ALP) staining and alizarin red staining assays. Osteogenesis and osteoclast related genes expression was detected using quantitative real-time PCR (qPCR) and Western blotting. The phosphorylation of Ataxia-telangiectasia mutated (ATM) and ATR serine/threonine kinase (ATR) was tested using Western blotting. The effect of RAD51 on osteoporosis was also explored in vivo. The results showed that RAD51 was downregulated in osteoporosis, but upregulated in differentiated osteoblasts. Overexpression of RAD51 enhanced the differentiation of osteoblasts and suppressed the formation of osteoclasts. Furthermore, p-ATM and p-ATR levels were upregulated in osteoblasts and downregulated in osteoclasts. RAD51 expression was reduced by the ATM/ATR pathway inhibitor AZ20. AZ20 treatment inhibited osteoblastogenesis and promoted osteoclastogenesis, whereas RAD51 reversed the effects induced by AZ20. Moreover, RAD51 improved bone microarchitecture in vivo. Taken together, ATM/ATR signaling-mediated RAD51 promoted osteogenic differentiation and suppressed osteoclastogenesis. These findings reveal a critical role for RAD51 in osteoporosis.

Keywords: ATM/ATR; Osteoporosis; RAD51; osteoblastogenesis; osteoclastogenesis.

Figure 1.

Downregulated RAD51 in osteoporosis. (a) Microarray analysis was performed to investigate the abnormal expression of mRNAs between the control and osteoporosis groups. Red indicates high levels, and green indicates low levels. (b) The expression levels of RAD51 in the serum of patients with osteoporosis and healthy individuals were determined by qPCR. ***P < 0.001.

Figure 2.

RAD51 promoted osteogenic differentiation. (a) The expression levels of RAD51 in undifferentiated and differentiated cells were determined by qPCR. (b) Transfection efficiency was assessed by qPCR. (c) The effects of RAD51 on ALP activity were assessed by ALP staining. (d) Calcium deposition was assessed by an ARS assay. (e) The expression levels of ALP, RUNX2, and BMP4 were determined using qPCR. (f) The expression levels of ALP, RUNX2, and BMP4 were determined using Western blot. ***P < 0.001. **P < 0.01. *P < 0.05.

Figure 3.

RAD51 suppressed osteoclastogenesis. (a) The expression level of RAD51 was determined in RAW264.7 cells treated with or without RANKL by qPCR. (b) The mRNA expression levels of TRAP, NFATC1, cathepsin K, and Itgb3 were determined using qPCR. (c) The protein expression levels of TRAP, NFATC1, and cathepsin K were determined using Western blot. ***P < 0.001. **P < 0.01. *P < 0.05.

Figure 4.

RAD51 level was positively regulated by ATM/ATR signaling. (a) The protein expression levels of p-ATM and p-ATR in the osteoblasts were determined using Western blot. (b) The protein expression levels of p-ATM and p-ATR in the osteoclasts were determined using Western blot. (c) Expression levels of RAD51 in osteoblasts treated with AZ20. (d) Expression levels of RAD51 in osteoclasts treated with AZ20. **P < 0.01.

Figure 5.

ATM/ATR-RAD51 axis facilitate osteoblastogenesis and suppress osteoclastogenesis. (a) Osteoblastogenesis was assessed by ALP staining assay. (b) Calcium deposition was assessed by an ARS assay. (c) The levels of ALP, RUNX2, and BMP4 were determined by Western blot. (d) The levels of TRAP, NFATC1, and cathepsin K were determined by Western blot. ***P < 0.001. *P < 0.05.

Figure 6.

RAD51 suppressed osteoporosis in vivo. (a) The representative micro-CT sectional femoral images. (b-g) The bone microarchitecture parameters, such as BMD, BV/TV, Tb.N, Tb.Th, Tb.Sp, and Tb.PF, were analyzed by micro-CT. ***P < 0.001. **P < 0.001. *P < 0.05.