Role of the unfolded protein response regulator GRP78/BiP in development, cancer, and neurological disorders

Abstract

GRP78/BiP is a major endoplasmic reticulum (ER) chaperone protein critical for protein quality control of the ER, as well as controlling the activation of the ER-transmembrane signaling molecules. Through creation of mouse models targeting the Grp78 allele, the function of GRP78 in development and disease has been investigated. These led to the discovery that GRP78 function is obligatory for early embryonic development. However, in adult animals, GRP78 is preferably required for cancer cell survival under pathologic conditions such as tumor progression and drug resistance. The discovery of surface localization of GRP78 in cancer cells reveals potential novel function, interaction with cell-surface receptors, and possible therapeutic implications. Mouse models also reveal that GRP78 controls maturation and secretion of neuronal factors for proper neural migration and offers neuroprotection.

FIG. 1.

GRP78 regulates ER stress–signaling pathways leading to UPR survival and apoptosis responses. In nonstressed cells, the ER-transmembrane signaling molecules (ATF6, IRE1, and PERK) and ER-associated caspases (murine caspase-12/human caspase 4) are maintained in an inactive state through binding to GRP78. After ER stress, such as protein misfolding, GRP78 is titrated away, and the survival pathways are activated to block further damage. However, when the stress is too severe, apoptotic responses are triggered, which eventually lead to cell death.

FIG. 2.

Survival and apoptosis responses to ER stress. ER stress can be induced by multiple adverse physiologic conditions, such as those indicated that alter ER homeostasis. When the ER protein load exceeds the ER protein-folding capacity, the UPR signaling pathways will be activated. The stressed cells will try to survive through reducing the nascent proteins influx by transient translational arrest, exporting the malfolded proteins for degradation in a process referred to as ERAD, as well as increasing the folding capacity of the ER by induction of the chaperones and folding enzymes. However, if the stress is too severe to overcome, the ER stress–induced apoptosis pathways will be activated, leading to JNK and caspase activation; induction of CHOP, PUMA, and NOXA; and BAX and BAK activation.

FIG. 3.

(A) Schematic drawing of the Grp78 WT and genetically modified alleles. The creation of genetically altered Grp78 mice has been described (38). The loxP sites are indicated by black arrows. Excision of exons 5 through 7, which are flanked by the loxP sites, will disrupt the critical ATPase and peptide-binding domains of GRP78 required for its chaperone function. The location and expected size of PCR products for the WT, floxed, and knockout alleles are indicated. (B) Whole-cell lysate from pooled livers of three WT Grp78 (+/+) and three heterozygous (+/−) littermates were subjected to Western blot with anti-KDEL (that recognized the KDEL C-terminal motif of both GRP78 and GRP94) and anti–β-actin. Quantitation of multiple Western blots with the GRP78 level normalized against β-actin as the loading control is shown below. The 50% reduction of GRP78 level in the Grp78^+/− mice is statistically significant (*p value <0.05 by two-sided t test).

FIG. 4.

Inactivation of proapoptotic pathways and activation of prosurvival pathways contribute to cancer cell survival. Cancer cells are subjected to ER stress because of intrinsic factors such as high glucose metabolic rate and extrinsic factors such as nutrient and oxygen deprivation in the tumor microenvironment. This leads to the induction of the GRPs, and, coupled with mutations as a result of tumorigenesis that often inactivate the proapoptotic pathways, shifts the balance to the prosurvival branches of the UPR.

FIG. 5.

GRP78 induction promotes drug resistance of the tumor. As a consequence of UPR resulting from glucose starvation or hypoxia, induction of GRP78 in cancer cells confers the development of drug resistance during cancer therapy.

FIG. 6.

Cell-surface GRP78 is a potential mediator for cancer cell–specific therapy. Preferential expression of GRP78 on the surface of cancer cells provides a mechanism for cancer-specific targeting, with peptides directly and specifically binding to cell-surface GRP78 and transducing proapoptotic agents.