Bone remodeling is tightly regulated by secreted and membrane-bound factors within the bone multicellular unit (BMU).It is well established that the osteoblasts regulate osteoclast differentiation via cell signaling molecules such as RANKL.However,secreted factors produced by osteoclasts to influence osteoblast differentiation remain to be vigorously discovered.Here we identify that high temperature requirement protease A 1 (HtrA1) is produced during osteoclastogenesis as a secreted protein,and negatively regulates osteoblast differentiation.By microarray and qPCR analyses,we found that HtrA1 transcripts are highly upregulated during RANKL-induced osteoclastogenesis when compared to other members of HtrA subfamily genes HtrA2,HtrA3 and HtrA4.Western blot analysis showed that HTAR1 is detected in the supernatant of osteoclast cultures,and increased during RANKL stimulation.Noticeably,treatment with HtrA1 recombinant proteins inhibits osteoblast like cells differentiation and BMP2-induced ERK and p38 phosphorylation,but did not significantly alter the OPG/RANKL gene expression in osteoblasts.Conversely,treatment of recombinant HtrA1 did not change bone marrow culture-mediated osteoclastogenesis.Our data revealed that HtrA1is produced during RANKL-induced osteoclast differentiation and negatively mediates osteoblast differentiation in bone microenvironment.