Ovariectomy increases vascular calcification via the OPG/RANKL cytokine signalling pathway

Background: Observational studies suggest a strong relationship between menopause and vascular calcification. Receptor activator of nuclear factor-κB ligand (RANKL) and osteoprotegerin (OPG) are critical regulators of bone remodelling and modulate vascular calcification. We assessed the hypothesis that ovariectomy increases vascular calcification via the OPG/RANKL axis. Materials and methods: Age-matched sexually mature rabbits were randomized to ovariectomy (OVX, n = 12) or sham procedure (SHAM, n = 12). One month post-procedure, atherosclerosis was induced by 15 months 0.2%-cholesterol diet and endothelial balloon denudations (at months 1 and 3). Aortic atherosclerosis was assessed in vivo by magnetic resonance imaging (MRI) at months 9 and 15. At sacrifice, aortas were harvested for ex vivo microcomputed tomography (μCT) and molecular analysis of the vascular tissue. Results: Vascular calcification density and calcific particle number were significantly greater in OVX than SHAM (8.4 ± 2.8 vs. 1.9 ± 0.6 mg cm^-3, P = 0.042, and 94 ± 26 vs. 33 ± 7 particles cm^-3, P = 0.046, respectively). Calcification morphology, as assessed by the arc angle subtended by the largest calcific particle, showed no difference between groups (OVX 33 ± 7°vs. SHAM 33 ± 5°, P = 0.99). By Western blot analysis, OVX increased the vascular OPG:RANKL ratio by 66%, P = 0.029, primarily by decreasing RANKL (P = 0.019). At month 9, MRI demonstrated no difference in atheroma volume between OVX and SHAM, and no significant change was seen by the end of the study. Conclusions: In contrast to bone, vascular OPG:RANKL ratio increased in response to ovariectomy with a corresponding fourfold increase in arterial calcification. This diametrical organ-specific response may explain the comorbid association of osteoporosis with calcifying atherosclerosis in post-menopausal women.