Introduction
Currently, esophageal cancer is the seventh most common worldwide malignant tumors. More than 456,000 new cases of esophageal cancer and 300,000 deaths were recorded globally in 2018, accounting for 3.2% and 5.3% of all cancers, respectively (Brayet al. , 2018). According to histopathological classification, esophageal cancer is composed of two main histologic subtypes, oesophageal squamous cell carcinoma (ESCC) and esophageal adenocarcinoma (EA).
East Asia is a region with a high incidence of esophageal cancer, and China accounts for approximately half of the world’s esophageal cancer cases (Ferlay et al. , 2010). Notably, more than 90% patients are diagnosed with ESCC in China (Zhao et al. , 2012; Liang et al. , 2017). The major treatment for ESCC patients is surgical resection, and the prognosis is favorable in the early stages. But the early symptoms of ESCC are more hidden, and invasive and aggressive, cause most diagnosed ESCC patients are in moderate and advanced stages (Mariette et al. , 2007). Therefore, for these patients, chemotherapy helps to reduce the primary tumor bulk, to increase the possibility of radical resection and to treat micro-metastatic disease and prolong the survival period of patients (Lordick et al. , 2016).These chemotherapeutic drugs include combinations of platinum-based drugs, 5-fluorouracil, paclitaxel and doxorubicin. However, conventional chemotherapeutics is often seriously hindered by a number of limitations, including high-systemic toxicity, chemoresistance and short in vivo circulation half-life.
Currently, targeted therapy was recognized as promising strategy for cancer treatment due to its high specificity and minimal side effects. The ubiquitin-proteasome system (UPS) plays a critical role in regulating 80-90% protein degradation and turnover as well as regulation of multiple cellular events, including the cell cycle, signal transduction, response to oxidative stress, cell proliferation and apoptosis (Ciechanover, 1994; Hochstrasser, 1995; Jana, 2012). As the critical endpoint for the UPS, the 26S proteasome is the chief proteolytic effector responsible for recognized and degraded the ubiquitylated protein, which consists of a core particle 20S proteasome, and two 19S regulatory particles (Bardet al. , 2018). In the past decade, proteasome as a target for cancer treatment, has gained increasing attention. Among them, the proteasome inhibitor (e.g., bortezomib, carfilzomib) has been confirmed to show excellent therapeutic effects in the treatment of multiple myeloma and other tumors (Johnson, 2015; Manasanch et al. , 2017).
Ixazomib, an oral 20S subunit-selective inhibitor being developed used for treatment of a broad range of human malignant tumor. Compared with bortezomib, ixazomib has a shorter proteasome dissociation half-life which is a critical role in improving the distribution of drug in solid tumor tissues (Kupperman et al. , 2010). Currently, ixazomib was approved for treatment of multiple myeloma patient combination with lenalidomide and dexamethasone by the US FDA (Shirley, 2016). Besides haematological neoplasms, a series of studies have suggested that ixazomib also have therapeutic anticancer functions in solid tumor, including colorectal cancer, bladder cancer, osteosarcoma and breast cancer (Kupperman et al. , 2010; Sato et al. , 2017; Yue et al. , 2019; Harris et al. , 2020). However, the detailed mechanisms underlying ixazomib induced cancer cell death remain unclear.
In recent years, endoplasmic reticulum stress (ERS) pathway is discovered regulated cellular processes, including protein folding, sorting, secretion, and play a crucial role in the process of apoptosis (Oakes et al. , 2015; Minchenko et al. , 2016). Early ERS is a response promoting survival, while the unfolded protein response reduces the accumulation of unfolded protein and restores the function to the endoplasmic reticulum (Oakeset al. , 2015). However, when the unfolded protein response is not sufficient to protect cell survival, the endoplasmic reticulum will act as the trigger point of apoptosis signals to induce apoptosis and promote the expression of apoptosis-inducing factors, such as the Bcl-2 homology 3 (BH3)-only pro-apoptotic protein NOXA, a key mediator for ERS-induced apoptosis (Kelly et al. , 2012; Cano-Gonzalez et al. , 2018). Proteasome inhibitor MG-132 was found induces apoptosis via activation of NOXA signaling pathways in hepatic stellate and chronic lymphocytic leukemia cell lines (Baouet al. , 2010; Sosa Seda et al. , 2010). Furthermore, Many studies have found evidence of sustained and high-level activation of ERS in esophagus cancer cells (Rosekrans et al. , 2015; Hu et al. , 2019). These studies suggest that ixazomib could potentially be used for induce ESCC cell apoptosis via ER stress-mediated NOXA induction.
In the present study, we evaluate the preclinical efficacy of ixazomib in ESCC cell lines and animal xenograft models by a series of in vitro and in vivo experiments. Our results demonstrate that that ixazomib is strongly apoptotic and blocks ESCC cell growth and survival. We further show that ixazomib exerts its pro-apoptotic action via a mechanism involving the activation of the NOXA-c-Myc pathway. These results indicate that the ixazomib may be an additional useful strategy to improve the treatment outcome for ESCC patient.