Reactivation of DRP1 plays a functional role in resistance to MEK inhibition in pancreatic cancer cells

Background In RAS-mutant tumours, ERK phosphorylates the mitochondrial fission GTPase DRP1 to promote mitochondrial fission. DRP1 activity is tumour-promoting in pancreatic and other RAS-driven cancers, but its role in therapeutic resistance is unknown. Methods We developed a panel of patient-derived pancreatic cancer cell lines resistant to the MEK inhibitor trametinib. We used immunofluorescence imaging, in vitro growth assays and orthotopic xenografts to determine the role of DRP1 in trametinib resistance. Results We find that trametinib-resistant cells exhibit increased expression and phosphorylation of DRP1 compared to sensitive counterparts. Quantitative analysis of mitochondrial structure reveals that mitochondria in resistant cells are morphologically distinct and relatively smaller than sensitive cells treated with trametinib. Genetic and pharmacological inhibition of both c-Myc and CDK6 are sufficient to block DRP1 phosphorylation in resistant cells, suggesting that activation of a c-Myc-CDK6 signalling axis drives reactivation of mitochondrial fission in the absence of MAPK signalling. Importantly, deletion of DRP1 leads to either growth inhibition or re-sensitisation to trametinib in resistant lines. Conclusion These findings suggest DRP1 contributes to drug resistance, and that inhibition of mitochondrial fission might be a promising therapeutic strategy to combat resistance to MAPK and RAS inhibitors.