CD14+ Monocytes Will Become a New Target for the Treatment of Osteoporosis: Based on Mendel Randomization, Clinical Analysis and Cell Experiment Verification.

To explore the causal relationship between monocytes and osteoporosis by Mendel randomization, and to verify it through subsequent experiments. Data regarding osteoporosis and immune cell phenotypes were sourced from the GWAS-Catalogue database. We utilised several Mendelian randomization methods, including the inverse variance weighted method, MR-Egger, weighted median method, and simple median method, complemented by Cochran's Q, MR-Egger regression and Leave-One-Out analysis. Clinical samples were classified into healthy and osteoporosis groups, and blood samples from both cohorts were analysed using flow cytometry. In vitro cell experiments were performed to investigate the effect of si-CD14 on the differentiation of monocytes into osteoclasts, employing western blotting, qPCR and TRAP staining techniques. In addition, we assessed the impact of CD14+ monocytes on the proliferation and mineralisation of osteoblasts through western blotting, qPCR and Alizarin Red staining, and further investigated the underlying mechanisms. Cochran's Q results indicated that the Mendelian randomization findings exhibited heterogeneity; therefore, the conclusions of this study were derived from the inverse variance weighting method. The weighted results of this method demonstrated a positive causal relationship between CD14+ monocyte count and osteoporosis (β = 0.096599, 95% CI: 1.06246, 1.141806, p = 1.46E-07). Additionally, the CD14+/CD16- monocyte count was found to have a positive causal relationship with osteoporosis (β = 0.097927, 95% CI: 1.065098, 1.142008, p = 3.67E-08). Mouse monocytes are activated through the NF-kB pathway under RANKL stimulation, leading to their differentiation into osteoclasts; however, si-CD14 transfection can inhibit this differentiation. Similarly, glucocorticoid stimulation can inhibit the proliferation and mineralisation of osteoblasts, while co-culturing with CD14+ monocytes exacerbates the glucocorticoid-induced biological activity, which is regulated by the TGF-β/SMAD3 pathway. Increased levels of CD14+ monocytes or CD14+/CD16- monocytes are recognised as risk factors for osteoporosis. CD14 plays a crucial role in this process. Inhibition of CD14 expression in monocytes can prevent their differentiation into osteoclasts by suppressing the NF-kB pathway. Additionally, the co-culture of CD14+ monocytes with osteoblasts has been shown to inhibit the TGF-β/SMAD3 pathway, thereby suppressing the proliferation and mineralisation of osteoblasts.