Faculty Bibliography

BACKGROUND:Aortic valve stenosis characteristically progresses due to cuspal calcification, often necessitating valve replacement surgery. The present study investigated the hypothesis that TGF-beta1, a cytokine that causes calcification of vascular smooth muscle cells in culture, initiates apoptosis of valvular interstitial cells as a mechanistic event in cuspal calcification. METHODS:Noncalcified and calcified human aortic valve cusps were obtained at autopsy or at the time of cardiac surgery. The distributions within cusps of TGF-beta1, latent-TGF-beta1-associated peptide, and TGF-beta receptors were studied using immunohistochemistry. The effects of TGF-beta1 on mechanistic events contributing to aortic valve calcification were also investigated using sheep aortic valve interstitial cell (SAVIC) cultures. RESULTS:Immunohistochemistry studies revealed that calcific aortic stenosis cusps characteristically contained within the extracellular matrix qualitatively higher levels of TGF-beta1 than noncalcified cusps. Noncalcified normal valves demonstrated only focal intracellular TGF-beta1. Addition of TGF-beta1 to SAVIC cultures led to a cascade of events, including: cellular migration, aggregation, formation of apoptotic-alkaline phosphatase enriched nodules, and calcification of these nodules. The time course of these events in the SAVIC culture system was rapid with nodule formation with apoptosis by 72 hours, and calcification after 7 days. Furthermore, ZVAD-FMK, an antiapoptosis agent (caspase inhibitor), significantly inhibited calcification and apoptosis induced by TGF-beta1, but had no effect on nodule formation. However, cytochalasin D, an actin-depolymerizing agent, inhibited nodule formation, but not calcification. CONCLUSIONS:TGF-beta1 is characteristically present within calcific aortic stenosis cusps, and mediates the calcification of aortic valve interstitial cells in culture through mechanisms involving apoptosis.

Apoptosis is a highly orchestrated form of programmed cell death, and this is believed to contribute to continuous decline of ventricular function in heart failure. However, the apoptotic cascade is not completed in failing myocardium and DNA damage is prevented due to abolition of DNA fragmentation factors. The extranuclear apoptotic program is interrupted secondary to inhibition of activated caspase-3 by upregulated inhibitors of apoptotic process. During the apoptotic process, upstream step comprising extensive mitochondrial loss of cytochrome c may contribute to systolic dysfunction of heart. Intactness of nuclear blueprint underscores the likelihood of reverse remodeling that has been demonstrated in the post-LVAD myocardial specimens.

Clinical disorders associated with increased serotonin [5-hydroxytryptamine (5-HT)] levels, such as carcinoid syndrome, and the use of serotonin agonists, such as fenfluoramine have been associated with a valvulopathy characterized by hyperplastic valvular and endocardial lesions with increased extracellular matrix. Furthermore, 5-HT has been demonstrated to up-regulate transforming growth factor (TGF)-beta in mesangial cells via G-protein signal transduction. We investigated the hypothesis that increased exposure of heart valve interstitial cells to 5-HT may result in increased TGF-beta1 expression and activity because of serotonin receptor-mediated signal transduction with activation of Galphaq, and subsequently up-regulation of phospholipase C. Thus, in the present study we performed a clinical-pathological investigation of retrieved carcinoid and normal valve cusps using immunohistochemical techniques to detect the presence of TGF-beta1 and other proteins associated with TGF-beta expression, including TGF-beta receptors I and II, latent TGF-beta-associated peptide (LAP), and alpha-smooth muscle actin. Carcinoid valve cusps demonstrated the unusual finding of widespread smooth muscle actin involving the interstitial cells in the periphery of carcinoid nodules; these same cells were also positive for LAP. Normal valve cusps were only focally positive for smooth muscle actin and LAP. In sheep aortic valve interstitial cell cultures 5-HT induced TGF-beta1 mRNA production and increased TGF-beta1 activity. 5-HT also increased collagen biosynthesis at the dosages studied. Furthermore, TGF-beta1 added to SAVIC cultures increased the production of sulfated glycan and hyaluronic acid. In addition, overexpression of Galphaq using an adenoviral expression vector for a constitutively active Galphaq mutant (Q209L-Galphaq) resulted in increased phospholipase C activity as well as up-regulation of TGF-beta expression and activity. These results strongly support the view that G-protein-related signal transduction is involved in 5-HT up-regulation of TGF-beta1. In conclusion, 5-HT-associated valve disease may be, in part, because of TGF-beta1 mechanisms.