Abstract SY06-01: Critical role of the stress chaperone and cell signaling regulator GRP78 in cancer
Authors: Lee Amy S. 
Published: July 2012

Mammalian cells have evolved multiple adaptive pathways that allow for the response to perturbations in endoplasmic reticulum (ER) homeostasis. One major pro-survival mechanism is mediated by the 78,000 dalton glucose-regulated protein (GRP78), also referred to as BiP/HSPA5. In addition to assisting in the assembly and quality control of ER proteins, GRP78/BiP regulates cell signaling in response to physiologic and pathologic stress (1). Tumor cells are subjected to ER stress caused by altered glucose metabolism in the transformed cells, the hypoxic tumor microenvironment, and nutrient deprivation due to inadequate vasculature. GRP78 is overexpressed in a wide variety of human cancers and is commonly associated with poor prognosis and resistance to therapy (2, 3).

While studies in cell lines have established that GRP78 promotes cancer cell survival and confers resistance to therapeutic agents (4-6), mouse models with genetically altered GRP78 expression are powerful systems to interrogate the role of GRP78 in tumorigenesis and metastasis. While homozygous knockout of GRP78 results in early embryonic lethality, Grp78 heterozygous mice are phenotypically normal, implying that GRP78 haploinsufficiency is well tolerated under normal developmental conditions. In contrast, in transgene-driven endogenous mammary tumors, Grp78 heterozygosity lengthens the latency period and inhibits tumor growth via suppression of tumor cell proliferation and increase in apoptosis (7). Strikingly, Grp78 heterozygosity also potently suppresses tumor angiogenesis while having no effect on the microvessel density of normal organs.

To test directly whether GRP78 regulates the function of the host vasculature within the tumor microenvironment, we created conditional knockout of GRP78 in the host endothelial cells. Upon injection of wild-type syngeneic tumor cells into the wild-type and mutant mice, we observed severe reduction in tumor angiogenesis and metastatic growth in the mutant mice with minimal effect on normal tissue microvessel density. In vitro studies using constitutively activated human endothelial cells further revealed that GRP78 regulates endothelial cell proliferation, as well as survival and migration, all of which are angiogenic functions required to support tumor growth and metastasis. These observations reinforce the emerging notion that the biological response of the tumor stroma can predispose the same tumor to different outcomes. While this has been appreciated with direct targets of angiogenic signaling such as VEGFR receptors, our work reveals another type of modulator, i.e., the ER-stress protein GRP78. Further, GRP78 may have many additional biological variables intrinsic to the tumor microenvironment. For example, perfusion, metabolic status, pH, chemotherapeutics, as well as radiation might alter GRP78 status of the tumor endothelium and thereby alter tumor response. Considering the recent discovery that antiangiogenic drugs that alter VEGF levels in patients result in severe blood vessel regrowth and drug resistance, suppression of GRP78 may represent a novel dual antitumor, antiangiogenic approach to arrest tumor progression and metastasis without these complications.

While traditionally GRP78 is regarded as protective against hypoxia and nutrient starvation which are prevalent in the microenvironment of solid tumors, its role in hematologic malignancies has not been documented. We observed elevated GRP78 expression in leukemic blasts of adult patients, leukemia cell lines and early relapse of childhood acute lymphocytic leukemia. To directly elucidate the requirement of GRP78 in leukemogenesis, we created a biallelic conditional knockout mouse model of GRP78 and PTEN in the hematopoietic system. Strikingly, heterozygous knockdown of GRP78 in PTEN-null mice is sufficient to restore the hematopoietic stem cell population to the normal range and suppress PTEN-null mediated leukemic blast cell expansion. AKT/mTOR activation in PTEN-null bone marrow cells is inhibited by Grp78 heterozygosity, which corresponds with suppression of AKT phosphorylation by GRP78 knockdown in the leukemia cell line. PTEN mutation is a major cause of human cancers. These results reinforce the critical role played by GRP78 in PTEN-null driven tumorigenesis and AKT activation, since conditional knockout of GRP78 in the prostate epithelium potently blocks PTEN-null mediated prostate cancer development and AKT activation without affecting normal prostate morphology or development (8).

The recent discovery that GRP78 is an upstream regulator of the AKT oncogenic signaling system raises the issue of how GRP78, an ER chaperone, might regulate cell surface signaling. Evidence is accumulating that in select cell types such as cancer cells and stressed endothelial cells, a subfraction of GRP78 relocalizes from the ER to the cell surface. ER stress promotes cell surface expression of GRP78, which is regulated by the KDEL retrieval mechanism (9). In tumor cell lines, cell surface GRP78 acts by co-receptor binding with various ligands on the plasma membrane, transducing signals for proliferation, survival and apoptosis (10). Importantly, on a therapeutic level, the preferential expression of GRP78 on the cell surface of tumor cells provides a new approach for selective targeting of tumor cells while sparing normal organs. Additionally, targeting cell surface GRP78 may simultaneously suppress multiple cell signaling pathways, therefore making it much more difficult for the tumor and tumor supporting host cells to mount compensatory measures to overcome this. Thus, the combination of the new role of GRP78 as a general cell signaling regulator with the traditional role of GRP78 as protector of ER homeostasis suggests that GRP78 is a prime candidate for therapeutic targeting. Current advances in therapeutic approaches against GRP78 in cancer will be discussed.

References:

1. Pfaffenbach KT, Lee AS. The critical role of GRP78 in physiologic and pathologic stress. Curr Opin Cell Biol (in press).

2. Lee AS. GRP78 induction in cancer: therapeutic and prognostic implications. Cancer Res 67:3496-3499, 2007.

3. Wang M, Wey S, Zhang Y, Ye R, Lee AS. Role of the unfolded protein response regulator GRP78/BiP in development, cancer, and neurological disorders. Antioxid Redox Signal 11:2307-2316, 2009.

4. Li J, Lee AS. Stress induction of GRP78/BiP and its role in cancer. Curr Mol Med 6:45-54, 2006.

5. Pyrko P, Schonthal AH, Hofman FM, Chen TC, Lee AS. The unfolded protein response regulator GRP78/BiP as a novel target for increasing chemosensitivity in malignant gliomas. Cancer Res 67:9809-9816, 2007.

6. Baumeister P, Dong D, Fu Y, Lee AS. Transcriptional induction of GRP78/BiP by histone deacetylase inhibitors and resistance to histone deacetylase inhibitor-induced apoptosis. Mol Cancer Ther 8:1086-1094, 2009.

7. Dong D, Ni M, Li J, Xiong S, Ye W, Virrey JJ, Mao C, Ye R, Wang M, Pen L, Dubeau L, Groshen S, Hofman FM, Lee AS. Critical role of the stress chaperone GRP78/BiP in tumor proliferation, survival and tumor angiogenesis in transgene-induced mammary tumor development. Cancer Res 68:498-505, 2008.

8. Fu Y, Wey S, Wang M, Ye R, Liao CP, Roy-Burman P, Lee AS. Pten null prostate tumorigenesis and AKT activation are blocked by targeted knockout of ER chaperone GRP78/BiP in prostate epithelium. Proc Natl Acad Sci USA 105:19444-19449, 2008.

9. Zhang Y, Liu R, Ni M, Gill P, Lee AS. Cell surface relocalization of the endoplasmic reticulum chaperone and unfolded protein response regulator GRP78/BiP. J Biol Chem 285:15065-15075, 2010.

10. Ni M, Zhang Y, Lee AS. Beyond the endoplasmic reticulum: atypical GRP78 in cell viability, signaling and therapeutic targeting. Biochem J (in press).

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr SY06-01. doi:10.1158/1538-7445.AM2011-SY06-01