Targeting Endoplasmic Reticulum Stress as a Therapeutic Strategy in Cancer

Abstract

Endoplasmic Reticulum (ER) is a vital organelle involved in the synthesis and folding of proteins. Upon excessive cellular demands, this machinery escalates its production resulting in accumulation of several unfolded and misfolded proteins inside ER lumen. This condition is termed as Endoplasmic Reticulum Stress. To overcome this, a cytoprotective mechanism known as Unfolded Protein Response (UPR) is launched by the cell. However, upon prolonged ER stress, it morphs into terminal UPR and cells undergo apoptosis. Baseline activity level of ER stress response system is elevated in cancer cells than normal cells. ER stress is now found to be closely associated with several hallmarks of cancer and therefore it bears the potential to emerge as the new Achilles’ heel in cancer. However, there are two specific challenges: (a) development of sub-cellular ER targeting tools and (b) development of ER stress inducers. To address these, we have engineered ER specific nanoparticles and spatially targeted anti-apoptotic Bcl-2 present there to induce ER stress in HeLa cells. Cellular internalization studies revealed that these nanoparticles internalized into the ER. Western blot analysis and cell viability studies revealed that ER stress induction was followed by autophagy. We then studied the location-function-relationship of Bcl-2 on ER and mitochondria. Subsequently, we explored graphene oxide (GO) and doxorubicin and cisplatin as ER stress inducers to engineer ER-GO-NPs. Fluorescence confocal microscopy revealed that these ER targeting GO-NPs successfully localized into ER in HeLa cervical cancer cells. Concomitant increase in ER stress marker CHOP indicated that these nanoparticles successfully induced ER stress and induced autophagy in HeLa cells. Combinatorial treatment with chloroquine (autophagy inhibitor) further improved the cell killing ability of the ER targeted nanoparticles. These ER-GO-NPs exhibited excellent cell killing ability over other cancer cell lines as well. Finally, to develop novel small molecule-based ER stress inducers, we synthesised a library of sulfonohydrazide-hydrazone based molecules and screened them against HeLa cell lines. Four molecules were identified as novel potential ER stress inducers, one of them being fluorescent. Further studies with the fluorescent derivative revealed its localization in ER within 3 h. Cell viability studies, immunofluorescence and western blot assays revealed that the fluorescent molecule induced ER stress in HeLa cells that was accompanied by autophagy induction. We anticipate that here presented approaches can serve as a tool to exploit ER stress as an alternative in future cancer therapy.