Targeting HDAC5 to improve cancer chemotherapy

Chemotherapy resistance and dose-limiting adverse effects remain major challenges for tumor management. It has been shown that cytoplasmic translocation of HDAC5 plays a critical role in cell adaptation to metabolic and pathological stress. This is particularly the case in cancer cells where metabolic stress triggers AMPK activation, which leads to phosphorylation and subsequent nuclear export of HDAC5. I hypothesized that chemotherapeutic stress triggers similar adaptive mechanisms as the metabolic stress, thus eliciting innate chemoresistance through cytoplasmic translocation of HDAC5. In this thesis study, I firstly explored the cancer types that may involve cytoplasmic localization of HDAC5, expecting to identify cancer models that may benefit from therapies based on targeting HDAC5 (Chapter II). Using tissue arrays and immunohistochemistry, I investigated the protein level and subcellular localization of HDAC5 and its paralog HDAC4 in normal tissues. Using cancer arrays, I found that HDAC5 expression was significantly higher in tumors originated from cervical, rectal, liver, ovarian, skin, esophageal, lymph, and intestinal carcinomas than corresponding normal tissues. Importantly, in normal cells HDAC5 was predominantly located in the nucleus whereas in cancer cells it was located predominantly in the cytoplasm. Next, I assessed if specific targeting of HDAC5 possesses advantage in safety aspect over other anti-HDAC based strategies (Chapter III). Currently used panHDACi is associated with various adverse effects, among which thrombocytopenia is the most severe, limiting dose and application. I showed that specifically targeting HDAC5 avoided panHDACiassociated thrombocytopenia, which is caused by inhibition of megakaryocyte maturation. I further show that the impairment of megakaryocyte maturation was due to degradation of a key transcription factor GATA-1, but not increased tubulin acetylation. Next, focusing on cancer mechanism, I show that chemotherapy (doxorubicin and cisplatin) stress leads to an increase in cytosolic calcium, which correlates with the activation of CaMKII[delta] kinase, HDAC5 phosphorylation (at S259) and cytoplasmic localization (Chapter IV). Disruption of CaMKII[delta] by either chemical inhibitor, siRNA knockdown or CRISPR/CAS9-based genome editing abolishes chemotherapy-triggered HDAC5 phosphorylation and cytosolic localization, whereas re-expressing a wild type CaMKII[delta] restore the chemotherapy triggered cytoplasmic localization of HDAC5, demonstrating the critical role of CaMKII[delta] in mediating the adaptive pathway. Finally, I evaluated the potential application of HDAC5 inhibition in sensitizing cancer cells to classical chemotherapy drugs (Chapter V). I show that inhibition of HDAC5 sensitizes hepatoma and osteosarcoma cells to doxorubicin and cisplatin, which allowed dose reduction of these drugs dramatically but maintained efficacy. In addition, I show that targeting HDAC5 resensitizes chemoresistant ovarian adenocarcinoma to doxorubicin. Findings from this thesis research pave the way towards the development of HDAC5/CaMKII[delta] inhibitors to sensitize cancer cells to chemotherapy and metabolic therapies, making it possible to reduce doses of classical chemotherapeutical drugs and provide hope to minimize adverse effects on normal tissues.