PM 2.5 exposure inhibits osteoblast differentiation by increasing the ubiquitination and degradation of Smad4.

Increasing epidemiological evidence has shown that PM 2.5 exposure is significantly associated with the occurrence of osteoporosis. It has been well demonstrated that PM 2.5 exposure enhanced the differentiation and function of osteoclasts by indirectly causing chronic inflammation, while the mechanism in osteoblasts remains unclear. In our study, toxic effects were evaluated by direct exposure of 20-80 μg/ml PM 2.5 to MC3T3-E1 cells and BMSCs. The results showed that PM 2.5 exposure did not affect cell viability via proliferation and apoptosis, but significantly inhibited osteoblast differentiation in a dose-dependent manner. Osteogenic transcription factors Runx2 and Sp7 and other biomarkers Alp and Ocn decreased after PM 2.5 exposure. RNA-seq revealed TGF-β signaling was involved in PM 2.5 exposure inhibited osteoblast differentiation, which led to P-Smad1/5 and P-Smad2 reduction in the nucleus by increasing the ubiquitination and degradation of Smad4. At last, the inflammation response increased in MC3T3-E1 cells with PM 2.5 exposure. Moreover, the mRNA levels of Mmp9 increased in bone marrow-derived macrophage cells treated with the conditional medium collected from MC3T3-E1 cells exposed to PM 2.5 . Overall, these results indicated that PM 2.5 exposure inhibits osteoblast differentiation and concurrently increases the maturation of osteoclasts. Our study provides in-depth mechanistic insights into the direct impact of PM 2.5 exposure on osteoblast, which would indicate the unrecognized role of PM 2.5 on osteoporosis.