Role of vitamin K2 and vitamin K cycle enzymes in aortic valvular interstitial cell calcification

Abstract Introduction Calcific aortic valve disease (CAVD) is the most common valve disease. Currently, there is no medical treatment available for CAVD. Vitamin K antagonists promote CAVD while population-based studies suggest that dietary intake of vitamin K reduces the incidence of CAVD in adults. However, randomized trials have so far not proven a clear benefit of vitamin K supplementation on the progression of AVS. Consequently, there is a need for a deeper understanding of the role of vitamin K in valvular calcification. This study aims to further investigate mechanisms of action of vitamin K during valvular calcification. Methods and Results Aortic valve samples were obtained from patients undergoing surgical aortic valve replacement for CAVD and valvular interstitial cells (VIC) were isolated by collagenase II digestion (Fig. 1). VIC expressed typical markers like Vimentin and α-SMA and were cultured in a control medium (CM) or further calcified in a pro-calcifying medium (PCM) containing sodium dihydrogen phosphate and ascorbic acid. PCM led to upregulation of Runt-related transcription factor 2 (RUNX2), alkaline phosphatase (ALP) expression, and ALP-activity after 7 and 21 days of culture. Moreover, alizarin red staining visualized significant calcium mineral deposition after 21 days of culture (Fig. 2). The addition of the most potent vitamin K2 derivative (Menaquinone-7, MK-7) improved cell viability under both control and calcifying conditions (Fig. 3). Moreover, MK-7 protected VIC from ferroptotic cell death (Fig. 4) and increased expression of the vitamin K-dependent Matrix-Gla-protein (MGP), a major inhibitor of ectopic calcification (Fig. 5). To fulfill its function as an activator of vitamin K-dependent proteins like MGP, vitamin K needs to be reduced to vitamin K hydroquinone by the vitamin K epoxide reductase enzymes VKORC1 and VKORC1L1. In VIC, using our calcification protocol, treatment with PCM led to the downregulation of VKORC1, but not of VKORC1L1, an enzyme with known anti-oxidative properties (Fig. 6). We next performed siRNA-mediated knockdown of VKORC1 and VKORC1L1 in VIC and observed calcification after 7 and 21 days of culture by measuring expression of ALP, RUNX2, quantifying ALP activity, and staining for calcium minerals (alizarin red). Intriguingly, VKORC1L1-knockdown sharply increased the expression of ALP. To further elucidate these findings, we will perform overexpression experiments to analyze, if VKORC1L1 can alleviate PCM-induced calcification Conclusion: Vitamin K2 inhibits in vitro calcification of valvular interstitial cells and upregulates the calcification inhibitor Matrix-Gla-Protein. Our results suggest the involvement of the vitamin K cycle enzyme VKORC1L1, which is known to exhibit anti-oxidative and cytoprotective effects. Further studies are warranted to shed light on the regulation of the enzymes to improve our understanding.