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. Antioxid. Redox Signal. 11, 2307-2316.

GRP78/BiP has recently emerged as a novel biomarker for aggressive prostate cancer. Here, we report that homozygous deletion of Grp78 specifically in mouse prostate epithelium suppresses prostate tumorigenesis without affecting postnatal prostate development and growth. Mouse prostates with double conditional knockout of Grp78 and Pten exhibit normal histology and cytology, in contrast to the invasive adenocarcinoma in mouse prostates with Pten inactivation. AKT activation in Pten null prostate epithelium is inhibited by Grp78 homozygous deletion, corresponding with suppression of AKT phosphorylation by GRP78 knockdown in prostate cancer cell line. Thus, inactivation of GRP78 may represent a previously undescribed approach to stop prostate cancer and potentially other cancers resulting from the loss of PTEN tumor suppression and/or activation of the oncogenic AKT.cancer suppressor ͉ chaperone gene ͉ inactivation P rostate cancer is the most common cancer in men and develops through successive stages including prostatic intraepithelial neoplasia (PIN), carcinoma in situ, invasive adenocarcinoma, and metastatic disease. Although local surgery, radiation, or hormonal ablation provide initial response at early stages of the disease, tumor cells often develop resistance and relapse. Thus, the identification of new therapeutic targets for prostate cancer is of critical importance. The 78-kDa glucose regulated protein (GRP78) was initially linked to prostate cancer progression and metastasis through epitope mapping of humoral immune response from cancer patients, and identified as a functional molecular target for circulating ligands (1). Recent studies further revealed that Ϸ2/3 of human prostate cancers expressed high level of GRP78, associating with recurrence, development of castration-resistance, and poor survival (2, 3). These studies provide the first hints that GRP78 may have a critical role in prostate cancer development and therapeutic resistance.GRP78, also referred to as BiP or HSPA5, is a member of the HSP70 protein family. As a major endoplasmic reticulum (ER) chaperone, GRP78 facilitates protein folding and assembly, protein quality control, ER-associated protein degradation, Ca 2ϩ binding, and regulation of transmembrane ER stress inducers (4, 5). GRP78 is encoded by a single copy gene in rodents and humans. It is expressed as early as at the 2-cell stage of embryonic development, and is essential for proliferation and survival of embryonic cells (6). GRP78 is highly induced in a wide range of tumors through intrinsic factors such as altered glucose metabolism of cancer cells, compounded by extrinsic factors such as glucose deprivation, hypoxia, and acidosis in the microenvironment of poorly-perfused solid tumor (7). In a wide variety of cancer cell lines and xenograft models, GRP78 has emerged as having a critical role in cancer cell survival, tumor progression, and resistance to therapy (7-10). Despite these advances, it remains unknown whether GRP78 could also be essential for the genesis of tumor. I...

Purpose The ER chaperone GRP78 translocates to the surface of tumor cells and promotes survival, metastasis, and resistance to therapy. An oncogenic function of cell surface GRP78 has been attributed to the activation of phosphoinositide 3-kinase (PI3K) pathway. We intend to use a novel anti-GRP78monoclonal antibody (MAb159) to attenuate PI3K signaling and inhibit tumor growth and metastasis. Experimental Design MAb159 was characterized biochemically. Anti-tumor activity was tested in cancer cell culture, tumor xenograft models, tumor metastasis models, and spontaneous tumor models. Cancer cells and tumor tissues were analyzed for PI3K activity. MAb159 was humanized and validated for diagnostic and therapeutic application. Results MAb159 specifically recognized surface GRP78, triggered GRP78 endocytosis, and localized to tumors but not normal organs in vivo. MAb159 inhibited tumor cell proliferation and enhanced tumor cell death both in vitro and in vivo. In MAb159 treated tumors, PI3K signaling was inhibited without compensatory MAPK pathway activation. Furthermore, MAb159 halted or reversed tumor progression in the spontaneous PTEN loss driven prostate and leukemia tumor models, and inhibited tumor growth and metastasis in xenograft models. Humanized MAb159, which retains high affinity, tumor specific localization, and the anti-tumor activity, was non-toxic in mice and had desirable pharmacokinetics. Conclusions GRP78 specific antibody MAb159 modulates PI3K pathway and inhibits tumor growth and metastasis. Humanized MAb159 will enter human trials shortly.

The unfolded protein response (UPR) is an evolutionarily conserved mechanism to allow cells to adapt to stress targeting the endoplasmic reticulum (ER). Induction of ER chaperone GRP78/BiP increases protein folding capacity; as such it represents a major survival arm of UPR. Considering the central importance of the UPR in regulating cell survival and death, evidence is emerging that cells evolve feedback regulatory pathways to modulate the key UPR executors, however, the precise mechanisms remain to be elucidated. Here, we report the fortuitous discovery of GRP78va, a novel isoform of GRP78 generated by alternative splicing (retention of intron 1) and alternative translation initiation. Bioinformatic and biochemical analyses revealed that expression of GRP78va is enhanced by ER stress and is notably elevated in human leukemic cells and leukemia patients. In contrast to the canonical GRP78 which is primarily an ER lumenal protein, GRP78va is devoid of the ER signaling peptide and is cytosolic. Through specific knockdown of endogenous GRP78va by siRNA without affecting canonical GRP78, we showed that GRP78va promotes cell survival under ER stress. We further demonstrated that GRP78va has the ability to regulate PERK signaling and that GRP78va is able to interact with and antagonize PERK inhibitor P58IPK. Our study describes the discovery of GRP78va, a novel cytosolic isoform of GRP78/BiP, and the first characterization of the modulation of UPR signaling via alternative splicing of nuclear pre-mRNA. Our study further reveals a novel survival mechanism in leukemic cells and other cell types where GRP78va is expressed.