Targeting protein homeostasis with nelfinavir/salinomycin dual therapy effectively induces death of mTORC1 hyperactive cells

// Elaine A. Dunlop 1 , Charlotte E. Johnson 1 , Marie Wiltshire 1 , Rachel J. Errington 1 and Andrew R. Tee 1 1 Division of Cancer and Genetics, Cardiff University, Heath Park, Cardiff, CF14 4XN, UK Correspondence to: Andrew R. Tee, email: // Keywords : mTORC1, TSC, therapy, nelfinavir, cell death Received : February 28, 2017 Accepted : March 03, 2017 Published : March 15, 2017 Abstract Uncontrolled cell growth in Tuberous Sclerosis Complex occurs due to inappropriate activation of mechanistic (mammalian) target of rapamycin complex 1 (mTORC1). The current therapy, rapamycin, produced promising clinical trial results, but patient tumours regrow if treatment is discontinued, revealing rapamycin has cytostatic properties rather than a cytotoxic effect. Taking advantage of the enhanced levels of endoplasmic reticulum (ER) stress present in TSC2 -null cells, we investigated drug combinations producing a cytotoxic response. We found a nelfinavir and salinomycin combination specifically killed TSC2 -deficient, mTORC1 hyperactive cells. Cytotoxicity was rescued by reducing protein synthesis, either through mTORC1 inhibition or cycloheximide treatment. This indicates that the drug combination targets the cells by tipping the protein homeostasis balance of the already metabolically stressed TSC2 -deficient cells in favour of cell death. Furthermore, this drug combination also inhibited tumour formation in TSC2 -deficient cell models and caused tumour spheroid death in 3D culture. Importantly, the 3D assay could differentiate the cytostatic agent, rapamycin, from the cytotoxic nelfinavir/salinomycin combination. Sporadic cancer cell lines with hyperactive mTORC1 signalling were also susceptible to this nelfinavir/salinomycin drug combination. This work indicates that the protein homeostasis pathway is an attractive therapeutic target in both Tuberous Sclerosis Complex and mTORC1-driven sporadic cancers.