Venous thromboembolism (VTE), which encompasses pulmonary embolism and deep vein thrombosis, is a frequent disease that is associated with vein wall fibrosis. Endothelial cells undergo phenotypical changes named endothelial-to-mesenchymal transition (EndMT), characterized by the loss of endothelial markers and the acquisition of mesenchymal markers involved in matrix deposition and fibrosis. Transforming growth factor (TGFβ) is the most potent inducer of EndMT. In chronic thromboembolic pulmonary hypertension, TGFβ induces EndMT and impairs thrombus resolution. However, the molecular mechanisms implicated in TGFβ signaling in the context of VTE are unknown. We hypothesized that epigenetic processes regulate the TGFβ signaling pathway in endothelial cells promoting EndMT and recurrent venous thromboembolism.

To study the role of histone deacetylase 6 (HDAC6) in EndMT, endothelial cells were transfected with a siRNA against HDAC6 or treated with a pharmacological inhibitor (TSC 20b). Endothelial cells were also treated with TGFβ and thrombin during 3 to 5 days. Real time PCR were performed to analyze endothelial and mesenchymal marker expression.

Expression of the mesenchymal markers, calponin and α-smooth muscle actin (SMA), is increased by TGFβ and thrombin. Interestingly these changes are inhibited in presence of siRNA or TSC 20b. Inhibition of HDAC6 also decreases the expression of TGFβ and Alk1 suggesting a role of HDAC6 in the regulation of the TGFβ pathway (Fig. 1).

We found that treatment of endothelial cells with TGFβ and thrombin is associated with EndMT. Our preliminary data suggest that HDAC6 contribute to EndMT. Deciphering the mechanisms by which epigenetic pathways are regulating EndMT might lead to the discovery of novel biomarkers or new therapeutic targets would be instrumental in guiding decisions of treatment for patients with a high risk of recurrent VTE.