Bromodomain and Extraterminal Domain (BET) Protein Inhibition Hinders Glioblastoma Progression by Inducing Autophagy-Dependent Differentiation

Abstract

Glioblastoma multiforme (GBM) is the most common and aggressive type of malignant primary brain tumor, and it is characterized by a high recurrence incidence and poor prognosis due to the presence of a highly heterogeneous mass of stem cells with self-renewal capacity and stemness maintenance ability. In recent years, the epigenetic landscape of GBM has been explored and many epigenetic alterations have been investigated. Among the investigated epigenetic abnormalities, the bromodomain and extra-terminal domain (BET) chromatin readers have been found to be significantly overexpressed in GBM. In this work, we investigated the effects of BET protein inhibition on GBM cell reprogramming. We found that the pan-BET pharmacological inhibitor JQ1 was able to promote a differentiation program in GBM cells, thus impairing cell proliferation and enhancing the toxicity of the drug Temozolomide (TMZ). Notably, the pro-differentiation capability of JQ1 was prevented in autophagy-defective models, suggesting that autophagy activation is necessary for BET protein activity in regulating glioma cell fate. Given the growing interest in epigenetic therapy, our results further support the possibility of introducing a BET-based approach in GBM clinical management.

Cytoprotective, Cytotoxic and Cytostatic Roles of Autophagy in Response to BET Inhibitors

“…Upon combining genetic autophagy inhibition with JQ1, β3-tubulin and synaptophysin levels were lower than in the shRNA control samples. These results were confirmed with the pharmacological inhibition of autophagy using CQ, where the JQ1-induced accumulation of β3-tubulin and synaptophysin was suppressed upon CQ treatment, confirming the importance of the autophagic flux in JQ1-induced GBM differentiation [50].…”

“…Colardo et al [ 50 ] studied the relationship between the BET family and autophagy in glioblastoma (GBM) using U87MG (U87), GL15 and the patient-derived GH2 cell lines. These investigators reported that the expression levels of BRD2 and BRD4 were high in these three cell lines, as well as in GBM patient samples.…”

“…With respect to autophagy, the protein levels of autophagy-related markers ULK1, ATG5, ATG7, Beclin1 and p62/SQSTM1 were examined by Western blotting in U87 and GH2 cells [ 50 ]. Here, JQ1 caused the transient upregulation of ULK1, an upstream promoter of autophagy, but without promoting significant differences in ATG5, ATG7 and Beclin1 protein levels.…”

“…Furthermore, a transient elevation in p62/SQSTM1 levels at 24 h after JQ1 treatment was observed, followed by a significant reduction, which was not prevented by CQ, suggesting that there was an impairment in p62/SQSTM1 expression instead of p62/SQSTM1 degradation. Despite these apparent inconsistencies, JQ1 induction of autophagy was confirmed in U87 and GH2 cells by immunofluorescence analysis of endogenous LC3 with the accumulation of LC3 dots [ 50 ].…”

“…The relationship between GBM cell differentiation and autophagy was investigated using Beclin1-directed shRNA in GL15 cells [ 50 ]. Upon combining genetic autophagy inhibition with JQ1, β3-tubulin and synaptophysin levels were lower than in the shRNA control samples.…”