ER stress as a trigger of UPR and ER-phagy in cancer growth and spread

Abstract

Tumors can survive environmental and metabolic stress by triggering homeostatic responses that re-establish the pre-stress status and permit them to grow and thrive. The endoplasmic reticulum (ER) is the organelle where proteins undergo post-translational modifications and are folded and exported to the secretory pathway. Its environment and activity are therefore fundamental for proteostasis, i.e., the plethora of mechanisms controlling protein formation, folding, degradation, and secretion, needed to assure protein balance and cellular health. In different tumor-related conditions, such as after the activation of oncogenes or under hypoxia and nutrient deprivation, the ER experiences stress, triggered by a high load of proteins to be folded compared to the limited folding capacity of the organelle. As a consequence, three ER membrane sensors and the related unfolded protein response (UPR) are activated. The UPR comprises a complex interconnection between signal transduction pathways that promote a homeostatic response that acts by increasing the amount of protein chaperones and of proteins involved in ER-associated protein degradation (ERAD) on one hand and attenuating protein translation on the other. ER-phagy, literally "eating" the ER, is part of another homeostatic response consisting of the clearance of non-functional ER portions including misfolded proteins. This response is also activated by a set of dedicated ER-phagy receptors after ER stimuli, which overlap the stimuli generating ER stress. Thus, the UPR and ER-phagy are two closely related homeostatic mechanisms that cooperate in re-establishing ER homeostasis. However, while the role of the UPR in favoring cancer growth and thriving by promoting angiogenesis, metastasis, chemotherapy resistance, and epithelial-to-mesenchymal transition is consolidated, that of ER-phagy is still in its infancy. This essay provides an overview of emerging concepts on ER stress, the UPR, and ER-phagy and their crosstalk in tumorigenesis. We also critically review new findings on their pharmacological targeting in cancer.

Keywords: ER stress; ER-phagy; ERO1 alpha; UPR; cancer; hypoxia.

Figure 1

Endoplasmic reticulum (ER) stressors in cancer. Different factors/conditions related to cancer are considered ER stressors. The epidermal growth factor receptor family, composed of four members HER1, HER2, HER3, and HER4, is highly expressed in some cancers (for example, HER1/HER2 in gastric cancer and HER2 in breast cancer). The activation of HER2 and HER1 results in the activation of intracellular pathways including RAS/RAF/MEK/ERK, PI3K/AKT/TOR, Src family kinases, and STAT transcription factors that modulate survival, proliferation, mobility, and cancer cell invasiveness. Some tumors express RAS mutations that activate downstream signal transduction independently from upstream receptor activation. The increased cell proliferation relies on increased protein translation that is a source of ER stress in these tumors. On the same line, the lack of the activity of oncosuppressors such as p53 triggers uncontrolled cell proliferation and growth, which might induce ER stress. Extrinsic factors, i.e., independent from the genetic makeup of the tumor, as a low concentration of oxygen, hypoxia, might induce ER stress in tumors. Indeed, this is a common condition of solid tumors, which not only leads to the assembly of the two components HIF1alpha and HIF1beta of HIF1, rendering it an active transcription factor of genes involved in angiogenesis and cell proliferation, but also impairs the formation of post-translational disulfide bonds in proteins, triggering ER stress.

Figure 2

Unfolded protein response (UPR) and ER-phagy in cancer. UPR is a homeostatic response to ER stress that is present in many cancer types. UPR is activated by three different sensors on the ER membrane: IRE1, PERK, and ATF6. IRE1 dimerizes following ER stress, activating an RNAse domain that promotes the unconventional splicing of XBP1 mRNA (XBP1s). The translated XBP1s acts as a transcription factor of genes involved in ER-associated degradation (ERAD) and chaperones. ER stress–activated PERK phosphorylates eIF2 alpha, promoting the attenuation of protein translation while also promoting the phosphorylation of NRF2, thereby the transcription of genes with an antioxidant function. The PERK signal also favors the selective translation of ATF4, which regulates the redox control, the genes involved in autophagy, and the CHOP-ERO1 axis. Regarding the axis CHOP-ERO1, we have seen that in breast cancer cells under hypoxic conditions, ERO1 is not downstream to CHOP. However, the lack of ERO1 converges and activates the PERK signal (21). ATF6 translocates in the Golgi where it is cleaved by SP1 and SP2 proteases and acts as a transcription factor of ER chaperones. ER stress also activates ER-phagy, a mechanism that leads to the clearance of ER portions containing misfolded proteins. FAM134B is an ER-phagy receptor that, through a physical interaction with a protein adaptor, such as calnexin, might sense unfolded proteins and starts the autophagy of the ER. SEC62 is another ER-phagy receptor that leads to the ER-phagy of ER portions containing ERAD-insensitive unfolded proteins.

Figure 3

Drugs targeting UPR and ER-phagy. Many selective modulators or inhibitors of UPR sensors and downstream mediators are available. However, a large number of these modulators/inhibitors fail to enter clinical practice because of off-target effects. At the moment, only MKC8866, an inhibitor of the RNAse activity of IRE1, is under clinical trial. It is worth mentioning that ISRIB, an inhibitor of the activity of P-eIF2 alpha in repressing protein translation, thus reactivates protein translation. ISRIB has a good safety profile in the preclinical models of cancer; at the moment no off-target effects are reported, and it was proven to be a valid drug in counteracting prostate cancer, KRAS-positive lung xenografts, hypoxic breast xenografts, and ERO1-devoid TNBC xenografts. We still lack the selective inhibitors/modulators of ER-phagy. Recently, vitexin, a plant-derived flavone O-glycoside, was shown to disrupt the complex FAM134B-BIP and inhibit breast cancer (MCF7-derived) xenografts.