Antiangiogenic Gene Therapy in Cancer

Abstract

One of the most recent and exciting approaches in cancer gene therapy is the ability to target the developing blood supply of the tumor. An appealing feature of antiangiogenic gene therapy is that the tumor vasculature is a readily accessible target, particularly when the carrier and its gene are administered systemically. This is in contrast to several other gene therapy approaches in which the tumor vasculature represents a major obstacle to achieving high levels of transfection of the tumor cells. Several gene-based viral or non-viral therapies that target tumor angiogenesis have shown efficacy in pre-clinical models. Genes that encode antiangiogenic polypeptides such as angiostatin and endostatin have significantly inhibited tumor growth, inducing a microscopic dormant state. The products of these genes are thought to act extracellularly to inhibit angiogenesis. An alternative approach that investigators have used successfully in tumor-bearing mice is to target angiogenic growth factors or their receptors that are essential for tumor growth. Levels of angiogenic factors such as vascular endothelial growth factor (VEGF) have been reduced by either antisense methods or the use of genes encoding truncated angiogenic decoy receptors. Despite these promising findings of tumor reduction with antiangiogenic gene therapy, advances in the viral and/or non-viral delivery systems are essential for this therapy to have clinical utility. In this review, we will discuss the mechanisms of angiogenesis/antiangiogenesis, and the current status and future directions of antiangiogenic gene therapy.

Figure 1.. Promotion of tumor growth by angiogenesis.

In order for tumors to progress from in situ to large tumors, increases in the proangiogenic factors coupled with decreases in the antiangiogenic factors are required (see Table I for list of activators or inhibitors). The thickness of the arrow represents the respective gradients of activators/inhibitor molecules.

Figure 2.. Models of VEGF Expression in Tumor Growth.

A) In tissues with low vascularity, marked increases in VEGF and angiogenesis occur when the tumor size reaches 2 mm in diameter. B) and C) demonstrate a second model of VEGF expression in tumors that have metastasized to vascular tissues. In early stages of tumor growth (B), increases in VEGF and angiogenesis do not initially occur when the tumor is 2 mm in size. Instead, the tumor co-opts the vessels of the organ. At a later stage (C) when the tumor reaches several centimeters in size in the vascular tissue, VEGF, angiopoietin 2, and angiogenesis at the periphery increase, accompanied by central necrosis of the tumor.

Figure 3.. Functions of various angiogenic regulators:

Two VEGF receptors, VEGFR1 and VEGFR2, are localized to mitogenic endothelial cells. VEGF, by binding to VEGFR1, induces migration of endothelial cells, while activation of VEGFR2 is essential for mitogenesis. In addition to VEGF/VEGFR, angiopoietin 1 (Ang1) and angiopoietin 2 (Ang2) appear to have important physiologic roles in vessel formation and stabilization. Although angiopoietins 1 and 2 have similar binding affinity to the endothelium-specific receptor, Tie2, the consequences of their binding are different. By activating Tie 2, Ang1 stabilizes vessels. In contrast, Ang2 does not activate Tie2. Rather, up-regulation of Ang2 stimulates angiogenesis in the presence of VEGF, but induces apoptosis in the absence of VEGF.

Figure 4. Proposed mechanisms for inhibition of angiogenesis by p53.

Two important mechanisms by which P53 may inhibit angiogenesis in neoplasms are induction of the antiangiogenic protein, TSP1, and decreased expression of the pro-angiogenic protein, VEGF. After wild-type p53 transcriptionally up-regulates TSP1 in the tumor cell, the secreted TSP1 binds to and activates the endothelial surface receptor, CD36. Several proteins, including p59fyn, caspase-3 like proteases, and p38 mitogen-activated protein kinase (p38MAPK), are then sequentially activated leading to apoptosis of the endothelial cell. Many of the mediators of the apoptotic signal transduction pathway downstream of p38MAPK have not yet been identified. A second mechanism by which p53 may inhibit angiogenesis is by decreasing the pro-angiogenic factor, VEGF. It is not clear whether down-regulation of the VEGF by p53 occurs at the transcriptional or at the post-translational stage. Pharmacological delivery of the p53 gene may result in the inhibition of angiogenesis by additional mechanisms (see section IIA).