TGF-beta1-Induced MAPK activation promotes collagen synthesis, nodule formation, redox stress and cellular senescence in porcine aortic valve interstitial cells

Abstract

Background and aim of the study: Aortic valve stenosis is a major cause of valve replacement, particularly in the elderly. TGF-beta1 is upregulated in stenotic valves and induces calcification and collagen synthesis in cultured valve interstitial cells. It has been shown previously that TGF-beta1 increases reactive oxygen species (ROS) in these cells in association with calcifying nodule formation, but the cellular signaling pathways responsible for these TGF-beta1-induced effects are not well defined.

Methods: Cultured porcine aortic valve interstitial cells were used to investigate the effects of inhibitors of TGF-beta1 signaling pathways on 3H-proline incorporation into the extracellular matrix, the peak number of calcifying nodules formed, redox stress as dichlorofluorescein diacetate (DCF-DA) fluorescence, and senescence-associated beta-galactosidase staining.

Results: Nodule formation and proline incorporation were inhibited by SB431542, implicating the Smad pathway, by SB203580, implicating the P38 MAPK pathway, and by U0126, implicating the Mekl/2/Erk1/2 pathway in both processes. Fasudil, an inhibitor of the Rho kinase pathway, was selective in inhibiting nodule formation but not proline incorporation. It was verified that Smad2 phosphorylation, Erk1/2 phosphorylation and p38 MAPK phosphorylation were all induced by TGF-beta1, with Smad 2 phosphorylation peaking at 1-2 h and MAPK phosphorylation at 24-48 h. The effect of TGF-beta1 on phosphorylation of Smad 2 was inhibited by SB431542, on the phosphorylation of p38 MAPK was inhibited by SB203580, and on the phosphorylation of Erk1/2 was inhibited by U0126. ROS generation in response to TGF-beta1, measured as 2,7-dichlorofluorescein-diacetate fluorescence, was inhibited significantly by SB203580 and U0126, implicating both the p38 MAPK and Mekl/2/Erk1/2 signaling pathways. Both pathways also mediated TGF-beta1-induced cellular senescence which was localized to cellular aggregates and mature nodules.

Conclusion: These data imply that the inhibition of either Smad or MAPK signaling pathways may have a therapeutic benefit in ameliorating the adverse pathological changes associated with aortic valve stenosis.