This study investigates the effects of ER stress-inducing agents on the dynamics of melanoma cell death within a complex tumour microenvironment. Utilising the fluorescent ubiquitination-based cell cycle indicator (FUCCI), we have demonstrated in vitro and in vivo through real-time cell cycle imaging that melanomas are composed of subcompartment-specific, differentially cycling tumour cells (Haass et al. 2014, Pigment Cell Melanoma Res). Further, we have shown that targeting the endoplasmic reticulum with Bortezomib (26S proteasome inhibitor) induces cell cycle arrest and G2-phase-dependent Noxa-mediated apoptosis of metastatic melanoma cells in vitro (Beaumont et al. 2016, J Dermatol Res).
Here, we have confirmed Noxa-mediated apoptosis in multiple melanoma cell lines and have observed compensatory changes in the anti-apoptotic proteins indicating that the cells have a high propensity to survive. The combination of ER stress and apoptosis in melanoma cells raised the question whether Bortezomib could induce immunogenic cell death (ICD), as induction of ICD by Bortezomib has been demonstrated in other cancers but not yet in melanoma. Indeed, we found upregulation of some ICD markers (calreticulin, HSP70, HSP90 and HMGB1) in Bortezomib-treated melanoma cells. Further, ER stress is required for an ICD response, thus we aimed to enhance ER stress as a therapeutic approach by combining Bortezomib with Fenretinide (synthetic retinoid). Single agent low dose Bortezomib and Fenretinide, both in adherent and in 3D cell culture, lead to cell cycle arrest but only limited cell death, while the combination resulted in synergistic cell death. We transduced our cells with the F-XBP1ΔDBD-venus reporter construct, to visualise ER stress. Indeed, single agent Bortezomib or Fenretinide induced ER stress and the combination resulted in apoptosis. Taken together, our data suggest that bortezomib combined with fenretinide is a strategy worth investigating to improve melanoma therapy by a more efficacious apoptotic response and enhanced through ICD induction.