Using Na₃PO₄ to Enhance In vitro Animal Models of Aortic Valve Calcification

The pathogenesis of calcific aortic valvular disease (CAVD) involves an active inflammatory process of valvular interstitial cells (VICs) characterized by the activation of specific osteogenic signaling pathways and apoptosis. This process can be studied by analyzing certain molecular markers and gene expression pathways of spontaneous calcification. The purpose of our study is to investigate the role of sodium phosphate (Na3PO4) as a calcification promoter, with the aim of improving in vitro animal models for testing potential calcification inhibitors.

VICs were extracted from 6 healthy 6-month-old fresh porcine hearts by serial collagenase digestion. Quantitative polymerase chain reaction (qPCR) was used to quantify trans-differentiation of genes of interest during spontaneous calcification of VICs. Spontaneous calcification of VICs was increased by adding Na₃PO₄ (3 mM, pH 7.4). The degree of calcification was estimated by Alizarin Red staining for calcium deposition, and Sirius Red staining for collagen. Colorimetric techniques were used to determine calcium and collagen deposition quantitatively. Additionally, the enzymatic activity of alkaline phosphatase (ALP) was measured by a kinetic assay. For statistical analysis we used SPSS and Microsoft Office Excel 2013.

Porcine VICs calcify spontaneously with demonstrable calcium and collagen deposition. In this study we observed an increase of calcium and collagen deposition from day 0 to day 14 (calcium: 376%; P<0.001, collagen: 3553%; P<0.001). qPCR analysis of mRNA by day 14 showed the following results: α-actin, a marker of myoblast phenotype, was increased to 1.6-fold; P<0.001. Runx2, an osteoblast marker, rose to 1.3 fold; P<0.05, TGF-β, a promoter of osteogenesis, increased to 3.2-fold; P<0.001, and RhoA, a regulator of nodular formation in myoblasts, increased to 4.5-fold; P<0.001, compared to their levels at day 0. RANKL mRNA and calponin did not change significantly. Treatment of porcine VICs with Na3PO4 (3 mM, pH 7.4) led to a marked increase in calcium deposition by day 14 (522%; P<0.001), and a significant increase in ALP activity by day 7 (228%; P<0.05). There were no significant changes in ALP activity between the groups by day 14.

This study has demonstrated the upregulation of some specific molecules during spontaneous calcification of aortic VICs with an active increase of calcium, collagen and ALP activity. In this in vitro model it was possible to increase spontaneous VICs calcification with Na₃PO₄ (3 mM, pH 7.4) to a level in which inhibitors of calcification could be tested to identify a novel potential therapeutic strategy against calcific aortic stenosis.