Borrelidin Induces OSCC Cell Death via the Unfolded Protein Response

Oral squamous cell carcinoma (OSCC) associated morbidity and mortality rates have remained largely un-improved since the introduction of cisplatin in 1978. We have previously reported that pharmacological exacerbation of the apoptotic arm of the unfolded protein response (UPR) with small molecules and natural products can effectively and selectively induce cell death in OSCC. The 18-membered macrolide borrelidin is a known threonyl-tRNA synthetase inhibitor, and has exhibited antitumor, antibacterial and antimalarial properties in murine models.  Objective: In an ongoing effort to identify bioactive UPR-inducing natural products, we sought to determine whether borrelidin could activate the UPR and cell death in OSCC.  Method: Quantitative real-time PCR analysis of UPR-related transcripts and luminescent proliferation and caspase-3/7 assays were performed with a panel of OSCC cells and genetically modified (for UPR genes) murine embryonic fibroblasts (MEFs). Result: Borrelidin significantly reduced the proliferation of five out of six OSCC cell lines examined, and potently induced mRNA transcripts for the UPR genes CHOP, ATF4, IRE1alpha, BIP and GADD34. Luminescent caspase 3/7 assays and an accumulation of cell death mRNA transcripts for NOXA, PUMA, DR5 and BIM revealed that borrelidin could induce apoptosis in OSCC. Finally, MEF’s null for PERK and ATF4, were significantly resistant to borrelidin in a dose-response fashion. Conclusion:  Borrelidin could induce the UPR and cell death in a panel of OSCC cells. Importantly, intact UPR signaling through the PERK-eIF2alpha-ATF4 axis of the UPR was required for efficient cell death. The fact that one OSCC cell line was resistant to borrelidin through an unknown mechanism suggests that it is not generally toxic and might be used therapeutically to treat OSCC.  Studies currently underway will elucidate whether or not borrelidin can induce the UPR and reduce the tumor burden of OSCC xenografts in a murine model.