Maintenance of endoplasmic reticulum homeostasis in 5-FU resistant Hela cells
The changes of chaperone-mediated folding play an important role in tumorigenesis and the evolution of drug resistance mechanisms (Jarosz, 2016). Based on proteomic profiling, we found that the most significant difference in protein synthesis, processing and transport between 2D monolayer cultures and 3D MTSs was mainly enriched in the ER, which was dominated by the heat shock protein family, including HYOU1, HSPA5, HSP90B1, HSPG2 (Figure. 5A-D ). Compared with 2D monolayer cultures, changes in the tumor microenvironment and metabolic rearrangement of tumor cells in 3D MTSs would affect protein processing (Mischiati et al., 2015). It was shown that hypoxia areas in tumor, nutritional deficiency, pH gradient, and disorder of calcium ion homeostasis could promote unfolded protein response (UPR) to activate the ER stress response (Ron & Walter, 2007). UPR signaling can contribute to maintaining proper protein folding function under the ER stress, but the mechanism of apoptosis would be triggered when the cells were exposed to ER stress for a long time and the ER function was impaired (Hoyer-Hansen & Jaattela, 2007). We observed an up-regulation of protein folding, which contributes to the maintenance of ER homeostasis (Table S3 and Table S4 ). Pavan et al. showed that disrupting ER homeostasis by increasing the concentration of calcium ion could induce the death of tumor cells (Pavan Grandhi, Potta, Nitiyanandan, Deshpande, & Rege, 2017). Thus, the maintenance of ER homeostasis in 3D MTSs may be one of the major reasons for the increased drug resistance to 5-FU.