Increased mesenchymal podoplanin expression is associated with calcification in aortic valves.

BACKGROUND AND AIM OF THE STUDY: Calcific aortic valve disease (CAVD) is a progressive disease starting from mild valvular sclerosis and progressing to severe aortic stenosis (AS) with calcified valves. The origin of the calcification is proposed to be mesenchymal cells which have differentiated towards an osteoblastic phenotype. Podoplanin is a glycoprotein expressed in the endothelium of lymphatic vessels and in osteoblasts and osteocytes, mesenchymal cells, as well as in many carcinomas and aortic atherosclerotic lesions. In CAVD, its expression has been evaluated only as a marker of the lymphatic vasculature.

MATERIALS AND METHODS: We determined podoplanin expression in human aortic valves in four patient groups: control (C, n=7), aortic regurgitation (AR, n=8), aortic regurgitation and fibrosis (AR + f, n=15) and AS (n=49) by immunohistochemistry and quantitative real-time PCR (RT-PCR).

RESULTS: Immunohistochemically, podoplanin expression was significantly increased in AR + f and AS groups when compared with the control and AR groups and the level of expression positively correlated with the extent of calcification and vascularity. Podoplanin mRNA levels were 1.7-fold higher in the AS group as compared with the control group (P=.05). Podoplanin-positivity was present not only in lymphatic vessel endothelium but also in osteoblasts, osteocytes, chondrocytes, macrophages and extracellular matrix. The majority of the podoplanin-positivity was in spindle cells with a myofibroblastic phenotype, often associated with calcifications. Tricuspid valves had more calcification-associated podoplanin than bi/unicuspid valves (median 1.52 vs 1.16, P<.001).

CONCLUSIONS: CAVD is characterized by an increased expression of podoplanin; this is associated with the differentiation of valvular interstitial cells into calcium-producing, myofibroblast-like cells. In addition, tricuspid valves express relatively more podoplanin than bi/unicuspid valves.