Role of TRAIL and its transcriptional regulation in stimulated vascular smooth muscle cells

Abnormal migration of vascular smooth muscle cells (VSMCs) from the media to the intima of the vessel and subsequent proliferation are key underlying events contributing to not only the development of atherosclerosis, but also to several clinical conditions, including in-stent restenosis and transplant vasculopathy. Platelet derived growth factor-BB (PDGF-BB) is a potent mitogen and chemoattractant for VSMCs. It is released following vascular injury, and contributes to atherosclerotic lesion development. Recently, we demonstrated that TNF-related apoptosis-inducing ligand (TRAIL) can induce VSMC proliferation and intimal thickening after vascular injury; however, the molecular mechanism(s) mediating this is not fully established.The first study has explored the role of TRAIL in PDGF-BB-induced VSMC proliferation and migration. TRAIL siRNA reduced both PDGF-BB-inducible VSMC proliferation and migration. Using VSMCs isolated from aortas of TRAIL-/- mice, PDGF-BB-inducible VSMC proliferation was completely abolished in TRAIL-/- VSMC, whilst inducible migration was reduced compared to WT VSMC. Interestingly, TRAIL is regulated by PDGF-BB. Therefore, in the second part of this thesis examined the molecular mechanisms regulating TRAIL transcription and expression in VSMCs.TRAIL mRNA, protein, and transcription were induced by PDGF-BB. PDGF-BB-inducible TRAIL promoter activity was inhibited by dominant negative Sp1, suggesting the involvement of this transcription factor. Four Sp1 binding sites on the human TRAIL promoter were required for PDGF-BB-induced TRAIL promoter activity by Sp1. Interestingly, histone deacetylase inhibitors, suberoylanilide hydroxamic acid (SAHA) and trichostatin A (TSA), and p300, a transcriptional coactivator with histone acetyltransferase activity, increased TRAIL mRNA and promoter activity in a Sp1-dependent manner. PDGF-BB induced acetylated histone-3 (ac-H3) protein levels, similar to SAHA, and also increased p300 levels. In co-immunoprecipitation assays, PDGF-BB enhanced the physical interaction of Sp1, ac-H3 and p300 and chromatin immunoprecipitation studies revealed that Sp1, ac-H3 and p300 were enriched on the TRAIL promoter.This present work provides the first evidence of the contribution of TRAIL in PDGF-BB-induced VSMC proliferation and migration, and has revealed the molecular mechanism(s) behind this. These findings have the promising potential for targeting TRAIL as a novel therapeutic of vascular proliferative disorders