Roles of Hdac6-Mediated Sp1 Deacetylation in the Evolution of Acquired Temozolomide Resistance in Glioblastoma

Rationale: The prognosis of glioblastoma (GBM) is usually poor even following treatment with the first-line chemotherapeutic agent temozolomide (TMZ). One most known resistant mechanism is the presence of DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT). However, compared with MGMT-mediated innate TMZ resistance, the development of acquired resistance is considered more complex with multi-factorial involvement such as the presence of cancer stem cells (CSCs). Preliminary results: We treated the MGMT-negative GBM cells with TMZ to investigate the acquired resistance, and found that both histone deacetylase 6 (HDAC6) and Sp1 are key factors protecting GBM against TMZ. These results include the following: (1) Stemness markers are highly increased in TMZ-resistant GBMs; (2) The activity of HDACs affects the stem-like characteristics and cell survival of GBM stem cells (GSCs); (3) The pan-HDACs inhibitor suberoylanilide hydroxamic acid (SAHA) increases TMZ-sensitivity and induces senescence in TMZ-resistant cells; (4) SAHA suppresses anti-senescence genes (hTERT and BMI1) expression via inhibition Sp1 transactivation; (5) Both HDAC6 and Sp1 are elevated and interact with each other in GSCs and resistant GBM cells; (6) TMZ treatment induces Sp1 deacetylation, but SAHA or HDAC6 attenuates that. Hypothesis: We hypothesize that HDAC6 increases Sp1 activation via protein deacetylation and causes Sp1-downstream target upregulation, which may enrich stemness properties and protect GBM against chemotherapeutic drugs. Specific Aims: Based on this hypothesis, we will investigate the role of the HDAC6/Sp1 pathway in GBM resistance to TMZ. First, we will study mechanisms and functional roles of HDAC6/Sp1 pathway in different resistant cell lines. Second, we will confirm the in vivo chemopotentiating effects of HDAC6/Sp1 pathway inhibition in TMZ-resistant GBM animal models. Third, we will evaluate the relationship between the HDAC6/Sp1 pathway and drug resistance in clinical cancer specimens. Novelty and application: This project will increase the understanding of the mechanism of HDAC6-regulated Sp1 activity and its downstream targets in the resistance developed to anti-cancer treatments, and reveal whether targeting HDAC6 by its selective inhibitors is a good therapeutic strategy for preventing brain tumor resistance to chemotherapy.