Glucose-regulated proteins in cancer: molecular mechanisms and therapeutic potential

Abstract

The glucose-regulated proteins (GRPs) are stress-inducible chaperones that mostly reside in the endoplasmic reticulum or the mitochondria. Recent advances show that the GRPs have functions that are distinct from those of the related heat shock proteins, and they can be actively translocated to other cellular locations and assume novel functions that control signalling, proliferation, invasion, apoptosis, inflammation and immunity. Mouse models further identified their specific roles in development, tumorigenesis, metastasis and angiogenesis. This Review describes their discovery and regulation, as well as their biological functions in cancer. Promising agents that use or target the GRPs are being developed, and their efficacy as anticancer therapeutics is also discussed.

Figure 1. GRPs in UPR and the stress response

ER luminal GRP78 acts as a UPR signaling regulator by binding to and maintaining the ER stress sensors (PERK, ATF6 and IRE1) in inactive forms. It also binds to and suppresses the activation of ER associated caspase-7 and -12 (C7/12). Upon ER stress, GRP78 is titrated away through binding to mal-folded proteins. This triggers the UPR, as exemplified by dimerization of PERK and IRE1, and activation of their downstream signaling pathways, leading to arrest of translation and ER associated protein degradation (ERAD). The UPR also generates the active nuclear form of ATF6, as well as ATF4, and the spliced form of XBP (XBP-1s), which act in concert with other transcriptional factors including YY1, NF-Y and TFII-I and chromatin modifiers, to activate the ER stress response element (ERSE) present on the promoters of ER stress responsive genes. A major UPR response is to induce the transcription of ER folding proteins such as the GRPs to booster the ER protein folding capacity, as well as the mitochondrial (Mito) chaperone GRP75. Stressed cells actively promote re-localization of GRP78 and GRP94 to the plasma membrane (PM), and in some instances, their secretion, and generate a cytosolic isoform of GRP78 (78va) through alternative splicing. Nonetheless, UPR can also induce transcription of the pro-apoptotic transcription factor CHOP, and following release from GRP78, caspase-7 and caspase-12 are activated, triggering apoptosis. Thus, the UPR regulates the balance between survival and cell death in stressed cells, and the up-regulation of the GRPs represents a major adaptive, protective measure through maintenance of cellular homeostasis.

Figure 2. GRPs in survival and immunity

The majority of GRP78, GRP94 and GRP170 are located in the ER lumen serving as ER folding proteins, and GRP75 is primarily a mitochondrial chaperone. Under ER stress or pathological stress conditions, a subfraction of GRP78 and GRP94 translocate to the cell surface and their secreted forms can be detected. Cell surface GRPs control critical growth and apoptotic signaling functions, as well as immune functions notably antigen presentation. ER stress also induces GRP78 translocation into the nucleus and mitochondria, and alternative splicing of GRP78 mRNA leading to the generation of a cytosolic isoform. The pro-survival functions of the GRPs in the various subcellular locations are indicated in blue, and the immune functions in red.

Figure 3. Summary of agents targeting the GRPs

Stress-induced GRP expression can be suppressed at the transcriptional level by inhibiting transcription factors required for the stress induction of the GRP promoter. Several microRNAs have been identified that suppresses the translation of GRP78 mRNA in cancer cells. Since chaperone function of the GRPs depends on the ATPase catalytic activity, compounds or peptides that bind to their ATP binding domains or alter their ATPase activity are effective in suppressing GRP function. A BAG1 peptide binds to the GRP78 substrate binding domain and inhibits its protein refolding activity. GRP78 can be cleaved specifically by the bacterial toxin SubAB, which renders it non-functional. Cell surface GRP78 can be effectively targeted by specific peptides in conjugated or non-conjugated forms and the human plasminogen factor Kringle 5 (K5). Antibodies against cell surface GRP78 are able to suppress GRP78-mediated oncogenic signaling and induce cancer cell death by multiple mechanisms. On the other hand, the inducible GRP promoter containing the ER stress responsive elements (ERSEs) can be used to direct cytotoxic gene expression in cancer cells. The GRP inhibitors listed have shown a wide range of anti-cancer effects in vitro and in vivo.

Box 1 Figure Legend (Optional). Functional domains of the GRPs

The locations of the signal sequence targeting the proteins into the ER (GRP78, GRP94 and GRP170) or the mitochondria (GRP75) are shown. The ATPase and substrate binding domains are indicated for all the GRPs. The location and the ER retention motifs for GRP78, GRP94 and GRP170 are shown. The Ca²⁺ binding, receptor binding and dimerization domains for GRP94 are denoted.