Insulin regulates glucose homeostasis but can also promote vascular smooth muscle (VSMC) proliferation and migration important in atherogenesis. We recently showed that TNF-related apoptosis-inducing ligand (TRAIL) promotes intimal thickening, via accelerated growth of VSMCs; whether insulin's effects on VSMCs is dependent on TRAIL is unknown.
TRAIL and TRAIL-receptor expression in response to insulin and glucose in VSMCs were determined by PCR. Transcriptional activity was assessed. Transient transfections using wild-type and site-specific mutations of the human TRAIL promoter were assayed. Chromatin immunoprecipitation studies were performed. VSMC proliferation and apoptosis was assessed.
Acute insulin and glucose exposure for 24 h stimulated TRAIL mRNA expression in VSMCs. This was also evident at the transcriptional level. Importantly, both insulin and glucose-inducible TRAIL transcription was blocked by dominant-negative Sp1 overexpression. TRAIL has 5 functional Sp1-binding elements (Sp1-1, Sp1-2, Sp-5/6 and Sp1-7). While insulin required Sp1-1 and Sp1-2 sites, glucose needed all Sp1-binding sites on the TRAIL promoter to induce transcription. Furthermore, insulin, but not glucose was able to promote VSMC proliferation over time, associated with increased DcR2 expression. In contrast, chronic 5 d insulin treatment to VSMCs repressed TRAIL and DcR2 expression. Chronic insulin exposure also reduced Sp1 enrichment on the TRAIL promoter and promoted VSMC apoptosis.
Here we have identified new mechanistic insight into how TRAIL is regulated by insulin. Our findings may have significant implications at different stages of diabetes associated cardiovascular disease; TRAIL may offer a novel therapeutic solution to combat insulin induced vascular pathologies. This article is protected by copyright. All rights reserved.