Increasing age is a major risk factor for calcific aortic valve disease (CAVD). Interestingly, SIRT6 knockout mice have a marked progeroid phenotype, and we recently reported that sirtuin enzyme expression is dramatically reduced with aging and that SIRT6 expression is reduced further in valves from patients with end-stage CAVD. It is unknown, however, whether experimentally reducing SIRT6 promotes osteogenic signaling in the valve and ultimately accelerates progression of CAVD. Thus, we used cultured mouse aortic valve interstitial cells and ldlr-deficient, apolipoprotein B100-only mice (LA) that were SIRT6 wild-type (LA-SIRT6
+/+
) or heterozygous (LA-SIRT6
+/-
) and fed a Western diet for 3 or 12 months to determine the role of SIRT6 in valve calcification.
In vitro
, reduction of SIRT6 increased histone acetylation and significantly increased mRNA and protein levels of the osteogenic genes Runx2 and Sp7 in response to bone morphogenetic protein 2 (100ng/ml BMP2 for 18 hours), and siRNA knockdown of SIRT6 increased mRNA levels of Sp7 even in the absence of exogenous bone morphogens. Using high-resolution ultrasound to evaluate aortic valve function
in vivo,
we found that 3 month old LA-SIRT6
+/-
mice did not have significant impairments in valve function compared to LA-SIRT6
+/+
mice. In contrast, 12 month old LA-SIRT6
+/-
had dramatically worsened aortic valve dysfunction and stenosis compared to LA-SIRT6
+/+
mice, which was also associated with reductions in left ventricular ejection fraction. Collectively, our data strongly suggest SIRT6 plays a critical role in the tonic repression of osteogenic signaling in the aortic valve, and that age-related reductions in SIRT6 are likely to increase susceptibility to valve calcification in response to risk factors for CAVD such as hypercholesterolemia. Collectively, increasing activity of SIRT6 or reducing acetylation of its targets may serve as viable therapeutic strategies to slow progression of age-related valvular calcification and stenosis.
Bicuspid aortic valve disease: the role of oxidative stress in Lrp5 bone formation
