Can valvular interstitial cells become true osteoblasts? A side-by-side comparison

Aortic valve calcification is believed to involve the differentiation of valvular interstitial cells (VICs) into either a myofibroblastic or an osteoblast-like phenotype. Despite purported similarities between diseased VICs and osteoblasts, few studies have directly compared VICs and osteoblasts in side-by-side experiments. In the present study, VICs were compared against multiple osteoblastic cell types at different stages of differentiation. These findings may help to resolve whether VICs progress through a myofibroblastic phenotype before reaching an osteoblast-like stage.

While the addition of mineralization medium generally increased the expression of osteogenic markers and decreased the expression of myofibroblastic markers, VICs displayed different levels and patterns of expression than the osteoblastic cell types used for comparison. Additionally, the lack of an alpha-SMA increase at any point after the addition of mineralization medium to VICs indicated that these cells may not need to progress through a myofibroblastic stage before reaching an osteoblast-like gene expression profile.

Three cell types representing a range of osteoblastic lineage commitment and differentiation were used in the phenotypic comparison against VICs. Specifically, VICs, embryonic fibroblasts (C3H10T1/2), pre-osteoblasts (MC3T3-E1), and mature primary osteoblasts were cultured on tissue-culture polystyrene in control or mineralization medium, and harvested for qPCR, DNA, and protein analysis at time points ranging from one to eight days.

Culture of VICs in mineralization medium decreased the expression of alpha-smooth muscle actin (alpha-SMA; a myofibroblast marker), with no peak in alpha-SMA gene or protein expression in mineralization medium at any time point. The application of a mineralization medium led to increased expression levels of alkaline phosphatase (ALP; an early mineralization marker) for all cell types, although the magnitude of the increase in ALP was drastically smaller for VICs than for the osteogenic cell types. Only the osteogenic cell types demonstrated an appreciable increase in osteocalcin (an indicator of later-stage mineralization).