Beyond the endoplasmic reticulum: atypical GRP78 in cell viability, signalling and therapeutic targeting

Abstract

GRP78 (glucose-regulated protein of 78 kDa) is traditionally regarded as a major ER (endoplasmic reticulum) chaperone facilitating protein folding and assembly, protein quality control, Ca(2+) binding and regulating ER stress signalling. It is a potent anti-apoptotic protein and plays a critical role in tumour cell survival, tumour progression and angiogenesis, metastasis and resistance to therapy. Recent evidence shows that GRP78 can also exist outside the ER. The finding that GRP78 is present on the surface of cancer but not normal cells in vivo represents a paradigm shift on how GRP78 controls cell homoeostasis and provides an opportunity for cancer-specific targeting. Cell-surface GRP78 has emerged as an important regulator of tumour cell signalling and viability as it forms complexes with a rapidly expanding repertoire of cell-surface protein partners, regulating proliferation, PI3K (phosphoinositide 3-kinase)/Akt signalling and cell viability. Evidence is also emerging that GRP78 serves as a receptor for viral entry into host cells. Additionally, a novel cytosolic form of GRP78 has been discovered prominently in leukaemia cells. These, coupled with reports of nucleus- and mitochondria-localized forms of GRP78, point to the previously unanticipated role of GRP78 beyond the ER that may be critical for cell viability and therapeutic targeting.

Figure 1. Summary of GRP78 functions in different subcellular compartments

GRP78 is traditionally recognized as a major ER chaperone facilitating protein maturation and degradation, Ca²⁺ binding and regulating ER stress signaling. In non-stressed cells, GRP78 is primarily located in the ER lumen with a subfraction detected as a transmembrane protein. GRP78 also exists outside the ER and acts in multifaceted cellular activities. For example, ER stress promotes cell surface expression of GRP78 and generation of a cytoplasmic isoform resulting from alternative splicing. Cell surface GRP78 emerges as an important receptor in cell signaling, viability and anti-cancer therapeutic targeting. The cytoplasmic GRP78 isoform is a newly identified regulator of the ER stress signaling pathway, in addition to the function of canonical GRP78 in the cytoplasm. Beyond the ER, the mitochondrial, nuclear and secreted forms of GRP78 have been linked to cellular homeostasis and therapeutic resistance. Abbreviations: N, nucleus; ER, endoplasmic reticulum; C, cytoplasm; M, mitochondria.

Figure 2. Examples of GRP78 on the cell surface serving as a receptor and regulator of cell signaling

Cell surface GRP78 forms complexes with a variety of extracellular ligands (e.g., activated α₂-macroglobulin, Kringle 5, Par-4) and cell surface anchored (▼) proteins (e.g., Cripto, T-cadherin) in tumor and endothelial cells leading to pro-survival or pro-apoptotic pathways. It also regulates the coagulation cascade through interaction with integral membrane protein (Tissue factor) and facilitates fungal (R. oryzae) and viral entries (e.g., Coxsackie virus A9, Borna disease virus and dengue virus serotype 2) in the respective host cells. 78: GRP78; α₂M*: activated α₂-macroglobulin.

Figure 3. Role of GRP78 and its isoform GRP78va in regulating UPR signaling pathway and cell survival

ER stress induces Grp78 transcription, resulting in upregulation of GRP78 and GRP78va, the latter is generated by alternative splicing. In the ER lumen, GRP78, in association with P58^IPK and other co-chaperones, enhances protein folding and degradation of misfolded proteins (ERAD). Release of GRP78 from PERK, IRE1 and ATF6 induces the UPR signaling. PERK activation leads to eIF2α phosphorylation and translation attenuation, contributing to cell survival. In the cytosol, GRP78va enforces PERK signaling by inactivation of the cytosolic P58^IPK.