Osteoprotegerin inhibits aortic valve calcification and preserves valve function in hypercholesterolemic mice

Abstract

Background: There are no rigorously confirmed effective medical therapies for calcific aortic stenosis. Hypercholesterolemic Ldlr (-/-) Apob (100/100) mice develop calcific aortic stenosis and valvular cardiomyopathy in old age. Osteoprotegerin (OPG) modulates calcification in bone and blood vessels, but its effect on valve calcification and valve function is not known.

Objectives: To determine the impact of pharmacologic treatment with OPG upon aortic valve calcification and valve function in aortic stenosis-prone hypercholesterolemic Ldlr (-/-) Apob (100/100) mice.

Methods: Young Ldlr (-/-) Apob (100/100) mice (age 2 months) were fed a Western diet and received exogenous OPG or vehicle (N = 12 each) 3 times per week, until age 8 months. After echocardiographic evaluation of valve function, the aortic valve was evaluated histologically. Older Ldlr (-/-) Apob (100/100) mice were fed a Western diet beginning at age 2 months. OPG or vehicle (N = 12 each) was administered from 6 to 12 months of age, followed by echocardiographic evaluation of valve function, followed by histologic evaluation.

Results: In Young Ldlr (-/-) Apob (100/100) mice, OPG significantly attenuated osteogenic transformation in the aortic valve, but did not affect lipid accumulation. In Older Ldlr (-/-) Apob (100/100) mice, OPG attenuated accumulation of the osteoblast-specific matrix protein osteocalcin by ∼80%, and attenuated aortic valve calcification by ∼ 70%. OPG also attenuated impairment of aortic valve function.

Conclusions: OPG attenuates pro-calcific processes in the aortic valve, and protects against impairment of aortic valve function in hypercholesterolemic aortic stenosis-prone Ldlr (-/-) Apob (100/100) mice.

Figure 1. Experimental Strategy.

Figure 2. Lipid in the aortic valve.

Oil Red-O staining is equally abundant in Older vehicle-treated mice (A,C) and OPG-treated Older mice (B,D). Group data for Younger mice (E) and Older mice (F). N = 7–8; p = NS for Veh vs. OPG.

Figure 3. Immunofluorescent staining for osterix in the aortic valve in Young LA mice.

There is abundant staining (green) at the cusp base in a vehicle-treated mouse (A), but less staining in the cusp base from an OPG-treated mouse (B). N = 12. *p<0.05 for Veh vs. OPG; RLU relative light units.

Figure 4. Calcification in the aortic valve.

Alizarin Red staining in a valve from an Older vehicle-treated mouse (A,C) demonstrates bright red staining, indicating valve calcification (arrows). Valve cusps are thickened in an Older OPG-treated mouse, but are minimally calcified (B,D). Dashed borders contain valve cusps, with care taken to exclude the aortic annulus (aa). Group data for valve calcification in Young mice (E) and Older mice (F). *p<0.05 Veh vs. OPG, N = 12.

Figure 5. Immunostaining for osteocalcin in the aortic valve in Older mice.

In vehicle-treated mice (A,C), osteocalcin (dark brown) is abundant near the cusp base (arrows). There is only scant staining in the valve of an OPG-treated mouse (B,D). N = 4. *p<0.05 Veh vs. OPG.

Figure 6. Immunostaining for monocyte chemo-attractant protein-1 (MCP-1) in Older LA mice.

OPG decreased levels of MCP-1 in the aortic valve. N = 11 (Veh) and N = 6 (OPG). *p<0.05 for Veh vs. OPG.

Figure 7. Masson’s Trichrome staining for collagen in Older mice.

Collagen (dark blue) was equally abundant in valves of vehicle-treated mice (A,C) and OPG-treated mice (B,D). N = 12; p = NS for Veh vs. OPG.

Figure 8. Aortic valve function.

A,B: M-mode echocardiograms from Older mice depict aortic valve systolic dimension (arrows). Vertical white bar = 1 mm. C: Aortic systolic valve dimension in Young Mice studied at age 8 mo. N = 12, p = NS. D: Aortic valve systolic valve dimension in Older LA mice before (Pre-Rx, age 6 mo.) and after (Post-Rx, age 12 mo.) treatment with VEH or OPG, N = 12. *p<0.05 for VEH vs. OPG. † p<0.05 for 12 mo. vs. 6 mo. for comparisons within each treatment group.