The unfolded protein response (UPR) is an evolutionarily conserved mechanism that activates both proapoptotic and survival pathways to allow eukaryotic cells to adapt to endoplasmic reticulum (ER) stress. Although the UPR has been implicated in tumorigenesis, its precise role in endogenous cancer remains unclear. A major UPR protective response is the induction of the ER chaperone GRP78/BiP, which is expressed at high levels in a variety of tumors and confers drug resistance in both proliferating and dormant cancer cells. To determine the physiologic role of GRP78 in in situ-generated tumor and the consequence of its suppression on normal organs, we used a genetic model of breast cancer in the Grp78 heterozygous mice where GRP78 expression level was reduced by about half, mimicking anti-GRP78 agents that achieve partial suppression of GRP78 expression. Here, we report that Grp78 heterozygosity has no effect on organ development or antibody production but prolongs the latency period and significantly impedes tumor growth. Our results reveal three major mechanisms mediated by GRP78 for cancer progression: enhancement of tumor cell proliferation, protection against apoptosis, and promotion of tumor angiogenesis. Importantly, although partial reduction of GRP78 in the Grp78 heterozygous mice substantially reduces the tumor microvessel density, it has no effect on vasculature of normal organs. Our findings establish that a key UPR target GRP78 is preferably required for pathophysiologic conditions, such as tumor proliferation, survival, and angiogenesis, underscoring its potential value as a novel therapeutic target for dual antitumor and antiangiogenesis activity. [Cancer Res 2008; 68(2):498-505]

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 tumor vasculature is essential for tumor growth and survival and is a key target for anticancer therapy.