Figure 7. Secreted factors from preosteoclasts stimulate osteoblast differentiation through COX2-PGE2 signaling.

(A–D) Representative images (A, C) and the quantified CFU-F frequency (B), and CFU-OB frequency (D) of bone marrow stromal cells treated with control conditioned medium (Con-CM) or senescent conditioned medium (SnC-CM). (E) qRT-PCR analysis of the relative levels of Alpl, Col1a1, Bglap, Runx2 mRNA expression in bone marrow stromal cells cultured in the mixture of osteoblast differentiation medium (DM) and Con-CM or SnC-CM (DM:CM = 1:1). **p<0.01, and ***p<0.001. Statistical significance was determined by unpaired, two-tailed Student’s t-test. (F) Relative levels of Cox2 mRNA expression in BMSCs cultured in growth medium (GM), osteoblast DM, Con-CM, or SnC-CM. (G) PGE2 protein levels from GM, DM, Con-CM, or SnC-CM were calculated. Results are expressed as mean ± standard deviations. (H) qRT-PCR analysis of the relative levels of Alpl, Col1a1, Bglap mRNA expression in BMSCs cultured in the mixture of osteoblast DM and Con-CM or SnC-CM (DM:CM = 1:1), or together with celecoxib (40 μM). (I) Schematic model for the role of preosteoclast secretome-COX2/PGE2 axis in mediating subchondral bone formation during metabolic syndrome (MetS). Under MetS, preosteoclasts in subchondral bone marrow undergo cellular senescence and secrete SASP factors, which acts on both osteoclast precursors to suppress osteoclast differentiation and osteoblast precursors to activate COX2-PGE2 signaling to promote osteoblast differentiation for bone formation.