Endoplasmic Reticulum Stress signalling - from basic mechanisms to clinical applications

The endoplasmic reticulum (ER) is a membranous intracellular organelle and the first compartment of the secretory pathway. As such, the ER contributes to the production and folding of approximately one-third of cellular proteins, and is thus inextricably linked to the maintenance of cellular homeostasis and the fine balance between health and disease. Specific ER stress signalling pathways, collectively known as the unfolded protein response (UPR), are required for maintaining ER homeostasis. The UPR is triggered when ER protein folding capacity is overwhelmed by cellular demand and the UPR initially aims to restore ER homeostasis and normal cellular functions. However, if this fails, then the UPR triggers cell death. In this review, we provide a UPR signalling-centric view of ER functions, from the ER's discovery to the latest advancements in the understanding of ER and UPR biology. Our review provides a synthesis of intracellular ER signalling revolving around proteostasis and the UPR, its impact on other organelles and cellular behaviour, its multifaceted and dynamic response to stress and its role in physiology, before finally exploring the potential exploitation of this knowledge to tackle unresolved biological questions and address unmet biomedical needs. Thus, we provide an integrated and global view of existing literature on ER signalling pathways and their use for therapeutic purposes

SLC37A1 and SLC37A2 Are Phosphate-Linked, Glucose-6-Phosphate Antiporters

Blood glucose homeostasis between meals depends upon production of glucose within the endoplasmic reticulum (ER) of the liver and kidney by hydrolysis of glucose-6-phosphate (G6P) into glucose and phosphate (Pi). This reaction depends on coupling the G6P transporter (G6PT) with glucose-6-phosphatase-α (G6Pase-α). Only one G6PT, also known as SLC37A4, has been characterized, and it acts as a Pi-linked G6P antiporter. The other three SLC37 family members, predicted to be sugar-phosphate:Pi exchangers, have not been characterized functionally. Using reconstituted proteoliposomes, we examine the antiporter activity of the other SLC37 members along with their ability to couple with G6Pase-α. G6PT- and mock-proteoliposomes are used as positive and negative controls, respectively. We show that SLC37A1 and SLC37A2 are ER-associated, Pi-linked antiporters, that can transport G6P. Unlike G6PT, neither is sensitive to chlorogenic acid, a competitive inhibitor of physiological ER G6P transport, and neither couples to G6Pase-α. We conclude that three of the four SLC37 family members are functional sugar-phosphate antiporters. However, only G6PT/SLC37A4 matches the characteristics of the physiological ER G6P transporter, suggesting the other SLC37 proteins have roles independent of blood glucose homeostasis

A biological and pharmacological investigation of the unfolded protein response in models of breast cancer

Breast cancer is the most common malignancy in women worldwide. At the molecular level, breast cancer is a heterogeneous disease. For example, the claudin low subtype of breast cancer is characterised by epithelial-mesenchymal transition (EMT). EMT is a process by which epithelial cells lose cell-cell adhesion and their cell polarity, and gain migratory and invasive properties to become mesenchymal cells. The claudin-low subtype is also driven by specific oncogenes, such as MYC or KRAS. Both EMT and oncogene-driven tumorigenesis have been associated with the unfolded protein response (UPR). The UPR is a cellular stress response pathway which activates an adaptive mechanisms to overcome stress and restore endoplasmic reticulum (ER) homeostasis. The UPR comprises three signalling pathways which are activated by three transmembrane sensors of the ER: inositol requiring enzyme 1 (IRE1), protein kinase RNA‐activated (PKR)‐like ER kinase (PERK) and activating transcription factor 6 (ATF6). In breast cancer, IRE1, PERK and ATF6 contribute to cancer cell proliferation, drug resistance and EMT. Chapter III shows that the pharmacological block of IRE1 has no effect on EMT in breast cancer. The knockout of XBP1 was also not sufficient to induce an epithelial phenotype in the MDA-MB-231 claudin low breast cancer cell line. However, Chapter IV shows that plakoglobin, a component of the cell cell contacts, was down regulated by IRE1 in MDA-MB-231 cell line thus affecting cell migration and adhesion in an EMT independent manner. In Chapter V, the relationship between UPR and oncogenes in the regulation of apoptosis was also investigated by overexpressing the oncogenes MYC and KRASG12V in the MCF10A non-tumorigenic mammary epithelial cell line. We observed that the UPR was not activated by MYC and KRASG12V although these oncoproteins decreased cell viability and induced the activation of caspase-3. Finally, in Chapter VI, a cell free drug screening for IRE1 identified compound NUIG10 as a promising inhibitor of IRE1. Altogether, this thesis has evaluated and broadened our knowledge of the role that the UPR and more specifically IRE1 plays in breast cancer.2024-12-1

Inhibition of IRE1 RNase activity modulates the tumor cell secretome and enhances response to chemotherapy

IRE1/XBP-1 activation has a major role in Triple negative breast cancer (TNBC). Here, the authors show that inhibition of IRE1’s RNase activity attenuates autocrine and paracrine signaling of pro-tumorigenic cytokines and synergizes with paclitaxel to confer potent anti-tumor effects in TNBC

Endoplasmic reticulum stress signalling - from basic mechanisms to clinical applications

The endoplasmic reticulum (ER) is a membranous intracellular organelle and the first compartment of the secretory pathway. As such, the ER contributes to the production and folding of approximately one-third of cellular proteins, and is thus inextricably linked to the maintenance of cellular homeostasis and the fine balance between health and disease. Specific ER stress signalling pathways, collectively known as the unfolded protein response (UPR), are required for maintaining ER homeostasis. The UPR is triggered when ER protein folding capacity is overwhelmed by cellular demand and the UPR initially aims to restore ER homeostasis and normal cellular functions. However, if this fails, then the UPR triggers cell death. In this review, we provide a UPR signalling-centric view of ER functions, from the ER's discovery to the latest advancements in the understanding of ER and UPR biology. Our review provides a synthesis of intracellular ER signalling revolving around proteostasis and the UPR, its impact on other organelles and cellular behaviour, its multifaceted and dynamic response to stress and its role in physiology, before finally exploring the potential exploitation of this knowledge to tackle unresolved biological questions and address unmet biomedical needs. Thus, we provide an integrated and global view of existing literature on ER signalling pathways and their use for therapeutic purposes.status: publishe