Rewiring tumor cells to undergo drug-induced apoptosis is a promising way to overcome chemoresistance. Therefore, identifying causative factors for chemoresistance is of high importance. Unbiased global proteome profiling of sensitive, early, and late cisplatin-resistant oral squamous cell carcinoma (OSCC) lines identified CMTM6 as a top-ranked upregulated protein. Analyses of OSCC patient tumor samples demonstrated significantly higher CMTM6 expression in chemotherapy (CT) nonresponders as compared with CT responders. In addition, a significant association between higher CMTM6 expression and poorer relapse-free survival in esophageal squamous cell carcinoma, head and neck squamous cell carcinoma, and lung squamous cell carcinoma was observed from Kaplan-Meier plot analysis. Stable knockdown (KD) of CMTM6 restored cisplatin-mediated cell death in chemoresistant OSCC lines. Upon CMTM6 overexpression in CMTM6-KD lines, the cisplatin-resistant phenotype was rescued. The patient-derived cell xenograft model of chemoresistant OSCC displaying CMTM6 depletion restored the cisplatin-induced cell death and tumor burden substantially. The transcriptome analysis of CMTM6-KD and control chemoresistant cells depicted enrichment of the Wnt signaling pathway. We demonstrated that CMTM6 interaction with membrane-bound Enolase-1 stabilized its expression, leading to activation of Wnt signaling mediated by AKT–glycogen synthase kinase-3β. CMTM6 has been identified as a stabilizer of programmed cell death ligand 1. Therefore, as CMTM6 facilitates tumor cells for immune evasion and mediates cisplatin resistance, it could be a promising therapeutic target for treating therapy-resistant OSCC.

M6A RNA methylation, which serves as a critical regulator of transcript expression, has gathered tremendous scientific interest in recent years. From RNA processing to nuclear export, RNA translation to decay, m6A modification has been studied to affect various aspects of RNA metabolism, and it is now considered as one of the most abundant epitranscriptomic modification. RNA methyltransferases (writer), m6A-binding proteins (readers), and demethylases (erasers) proteins are frequently upregulated in several neoplasms, thereby regulating oncoprotein expression, augmenting tumor initiation, enhancing cancer cell proliferation, progression, and metastasis. Though the potential role of m6A methylation in growth and proliferation of cancer cells has been well documented, its potential role in development of therapy resistance in cancer is not clear. In this review, we focus on m6A-associated regulation, mechanisms, and functions in acquired chemoresistance, radioresistance, and resistance to immunotherapy in cancer.

Cisplatin alone or in combination with 5FU (5-fluorouracil) and docetaxel (TPF) are common regimen chemotherapeutics for treatment of advanced oral squamous cell carcinoma (OSCC). Despite the initial positive response, several patients experience relapse due to chemoresistance. The potential role of Bcl-2 antiapoptotic members in acquired chemoresistance is yet to be explored. To address this, we designed two different relevant OSCC chemoresistant models: (i) acquired chemoresistant cells, where OSCC lines were treated with conventional chemotherapy for a prolonged period to develop chemoresistance, and (ii) chemoresistant patient-derived cells, where primary cells were established from tumor of neoadjuvant-treated OSCC patients who do not respond to TPF. Among all Bcl-2 antiapoptotic members, Mcl-1 expression (but not Bcl-2 or Bcl-xL) was found to be upregulated in both chemoresistant OSCC lines and chemoresistant tumors when compared with their respective sensitive counterparts. Irrespective of all three chemotherapy drugs, Mcl-1 expression was elevated in OSCC cells that are resistant to either cisplatin or 5FU or docetaxel. In chemoresistant OSCC, Mcl-1 mRNA was upregulated by signal transducer and activator of transcription 3 (STAT3) activation, and the protein was stabilized by AKT-mediated glycogen synthase kinase 3 beta (GSK3β) inactivation. Genetic (siRNA) or pharmacological (Triptolide, a transcriptional repressor of Mcl-1) inhibition of Mcl-1 induces drug-mediated cell death in chemoresistant OSCC. In patient-derived xenograft model of advanced stage and chemoresistant OSCC tumor, Triptolide restores cisplatin-mediated cell death and facilitates significant reduction of tumor burdens. Overall, our data suggest Mcl-1 dependency of chemoresistant OSCC. A combination regimen of Mcl-1 inhibitor with conventional chemotherapy deserves further clinical investigation in advanced OSCC.