Dysfunctional APPL1-Mediated Epigenetic Regulation in Diabetic Vascular Injury

APN (adiponectin) and APPL1 (adaptor protein, phosphotyrosine interacting with PH domain and leucine zipper 1) are potent vasculoprotective molecules, and their deficiency (eg, hypoadiponectinemia) contributes to diabetic vascular complications. However, the molecular mechanisms that govern their vasculoprotective genes as well as their alteration by diabetes remain unknown.

Diabetic medium-cultured rat aortic endothelial cells, mouse aortic endothelial cells from high-fat-diet animals, and diabetic human aortic endothelial cells were used for molecular/cellular investigations. The in vivo concept-prove demonstration was conducted using diabetic vascular injury and diabetic hindlimb ischemia models.

In vivo animal experiments showed that APN replenishment caused APPL1 nuclear translocation, resulting in an interaction with HDAC (histone deacetylase) 2, which inhibited HDAC2 activity and increased H3Kac27 levels. Based on transcriptionome pathway-specific real-time polymerase chain reaction profiling and bioinformatics analysis, Angpt1 (angiopoietin 1), Ocln (occludin), and Cav1 (caveolin 1) were found to be the top 3 vasculoprotective genes suppressed by diabetes and rescued by APN in an APPL1-dependent manner. APN reverses diabetes-induced inhibition of Cav1 interaction with APPL1. APN-induced Cav1 expression was not affected by Angpt1 or Ocln deficiency, whereas APN-induced APPL1 nuclear translocation or upregulation of Angpt1/Ocln expression was abolished in the absence of Cav1 both in vivo and in vitro, suggesting Cav1 is upstream molecule of Angpt1/Ocln in response to APN administration. Chromatin immunoprecipitation-qPCR (quantitative polymerase chain reaction) demonstrated that APN caused significant enrichment of H3K27ac in Angpt1 and Ocln promoter region, an effect blocked by APPL1/Cav1 knockdown or HDAC2 overexpression. The protective effects of APN on the vascular system were attenuated by overexpression of HDAC2 and abolished by knocking out APPL1 or Cav1. The double knockdown of ANGPT1/OCLN blunted APN vascular protection both in vitro and in vivo. Furthermore, in diabetic human endothelial cells, HDAC2 activity is increased, H3 acetylation is decreased, and ANGPT1/OCLN expression is reduced, suggesting that the findings have important translational implications.

Hypoadiponectinemia and dysregulation of APPL1-mediated epigenetic regulation are novel mechanisms leading to diabetes-induced suppression of vasculoprotective gene expression. Diabetes-induced pathological vascular remodeling may be prevented by interventions promoting APPL1 nuclear translocation and inhibiting HDAC2.