Spontaneous and Controllable Activation of Suicide Gene Expression Driven by the Stress-InducibleGrp78Promoter Resulting in Eradication of Sizable Human Tumors

Abstract

GRP78 is a stress-inducible chaperone protein with antiapoptotic properties that is overexpressed in transformed cells and cells under glucose starvation, acidosis, and hypoxic conditions that persist in poorly vascularized tumors. Previously we demonstrated that the Grp78 promoter is able to eradicate tumors using murine cells in immunocompetent models by driving expression of the HSV-tk suicide gene. Here, through the use of positron emission tomography (PET) imaging, we provide direct evidence of spontaneous in vivo activation of the HSV-tk suicide gene driven by the Grp78 promoter in growing tumors and its activation by photodynamic therapy (PDT) in a controlled manner. In this report, we evaluated whether this promoter can be applied to human cancer therapy. We observed that the Grp78 promoter, in the context of a retroviral vector, was highly activated by stress and PDT in three different types of human breast carcinomas independent of estrogen receptor and p53. Complete regression of sizable human tumors was observed after prodrug ganciclovir treatment of the xenografts in immunodeficient mice. In addition, the Grp78 promoter-driven suicide gene is strongly expressed in a variety of human tumors, including human osteosarcoma. In contrast, the activity of the murine leukemia virus (MuLV) long-terminal repeat (LTR) promoter varied greatly in different human breast carcinoma cell lines, and in some cases, stress resulted in partial suppression of the LTR promoter activity. In transgenic mouse models, the Grp78 promoter-driven transgene is largely quiescent in major adult organs but highly active in cancer cells and cancer-associated macrophages, which can diffuse to tumor necrotic sites devoid of vascular supply and facilitate cell-based therapy. Thus, transcriptional control through the use of the Grp78 promoter offers multiple novel approaches for human cancer gene therapy.

Patterns of GRP78 and MTJ1 expression in primary cutaneous malignant melanoma

“…Robust induction of GRP78 occurs in many malignancies. 46 Furthermore, GRP78 has been shown to be involved in a number of physiological events that are key to tumor growth and survival. 47,48 Once upregulated or transcriptionally active, GRP78 can promote tumor proliferation through a variety of mechanisms.…”

Glucose-regulated protein 78 regulates multiple malignant phenotypes in head and neck cancer and may serve as a molecular target of therapeutic intervention

“…Previously, Grp78 has been implicated in the regulation of tumorigenesis. It has been shown that spontaneous activation of a reporter gene driven by the Grp78 promoter was found to be highly expressed in xenograft tumors in mice (31). Inhibition of Grp78 in xenografted fibrosarcoma suppressed tumor growth in mice presumably due to a cytotoxic T-cell response (32).…”

The Unfolded Protein Response: A Novel Component of the Hypoxic Stress Response in Tumors

“…One promising therapeutic avenue entails the expression of a virally delivered cytotoxin under control of the ER stress-activated GRP78 promoter, which robustly eradicates human tumors in mouse xenograft experiments (99). Another approach is the development of small molecule-based therapies that directly inactivate key upstream activators of the UPR, such as PERK and Ire1, which are transmembrane kinases amenable to high-throughput screening for chemical inhibitors.…”