Transforming growth factor-beta1 mechanisms in aortic valve calcification: increased alkaline phosphatase and related events.

Background: Aortic valve stenosis is the most frequent indication for valve replacement surgery, and is commonly associated with pathologic calcification. Previous investigations by our group have shown a strong association of transforming growth factor-beta1 (TGF-beta1)-related mechanisms with calcific aortic stenosis in both cell culture and clinical pathology studies.

Methods: In the present investigations we sought to investigate the sequence of events involved in TGF-beta1-initiated aortic valve interstitial cell calcification in cell culture, and to study related gene expression pattern differences comparing calcific aortic stenosis surgical specimens with normal aortic valve leaflets.

Results: Sheep aortic valve interstitial cells (SAVIC) in culture progressively calcified over 14 days after the addition of TGF-beta1 to a significantly greater extent than non-TGF-beta1 controls. The TGF-beta1-induced SAVIC calcification was associated with maximal levels of alkaline phosphatase by 72 hours. Annexin V positive apoptosis was increased in TGF-beta1-treated SAVIC cultures at 14 days compared with controls. Matrix metalloproteinase 9 per gel zymography was detectable only in SAVIC cultures treated with TGF-beta1 from seven days on. Matrix metalloproteinase 2 was present in all SAVIC cultures per gel zymograms, either with or without TGF-beta1, but the active form of matrix metalloproteinase 2 significantly increased over 14 days in response to TGF-beta1. Quantitative gene expression studies (re: RNA levels) of human aortic valve cusps obtained at cardiac surgery demonstrated a number of related trends, including upregulation of the expression of TGF-beta1, alkaline phosphatase, and matrix metalloproteinase 9 in calcified human aortic valves.

Conclusions: Transforming growth factor-beta1 causes SAVIC to calcify due to an early maximal increase in alkaline phosphatase activity with associated apoptotic events and increased matrix metalloproteinase 9. These TGF-beta1-related mechanistic events may be of clinical relevance based upon the gene expression pattern changes observed in calcific aortic stenosis valve cusps.