Hypertension-linked Immune Activation Regulates Osteoclastogenesis

Osteoporosis is an age-related disease affecting 18% of adults. Hypertension is associated with increased risk for fragility fracture and accelerated bone loss in adults. We hypothesized that experimental hypertension would induce osteoporosis and examined the mechanisms involved. We used two preclinical models in 12-week-old male C57Bl/6J mice: angiotensin (ang) II infusion (490 ng/kg/min) and deoxycorticosterone acetate (DOCA)-salt. After 6 weeks of ang II or 3 weeks of DOCA-salt, the left femur and sixth lumbar vertebral body were studied. Mechanical testing showed that ang II reduced femoral ultimate strength (170.7±4.794 vs 157.3±3.670 MPa, p=0.0325; 154.1±7.403 vs. 119.9±4.375 MPa, p=0.0026) and peak moment (39.09±1.660 vs. 32.77±0.9380 Newton millimeters (Nmm), p=0.0018; 29.15±1.291 vs 19.58±1.308 Nmm, p=0.0004) with cortical and trabecular bone loss compared to controls. μCT showed that hypertension reduced vertebral body bone volume fraction (30.1±1.41 vs 25.1±0.992 %, p=0.0068; 25.9±2.03 vs 18.5±1.29 %, p=0.0119) and trabecular thickness (62±1.6 vs 57±1.4 μm, p=0.0105; 60±2.1 vs 51 ±1.6 μm, p=0.0069). Flow cytometry of bone marrow from ang II-treated mice revealed that CD4+ T cells and γδ T cells produce increased IL-17A which is known to induce osteoclastogenesis. Colony stimulating factor-1 (CSF1) is also known to stimulate osteoclast formation. Using in situ hybridization, we observed increased CSF1 mRNA in endothelial cells (ECs) of transcortical arterioles of hypertensive mice. Subsequent studies of cultured bone marrow derived ECs showed that CSF1 mRNA expression increased with 0% and 10% compared to 5% stretch. 5% stretch treated with endothelial nitric oxide synthase inhibitor, L-NAME, increased expression comparable to 0% and 10%, indicating that nitric oxide inhibits CSF1 production. Thus, T-cell activation and nitric oxide-dependent EC dysfunction in hypertension may skew the bone marrow to favor osteoclastogenesis, leading to bone loss. Understanding this mechanism will elucidate novel pharmacological targets to mitigate hypertension-induced bone loss.