The Role of Integrins in Colorectal Cancer

ABSTRACT: Integrins have direct effects in stimulating proliferation and preventing apoptosis in cancer cells and mediating proangiogenic interactions between endothelial cells and extracellular matrix. Alterations of expression of various integrins and their receptors have been observed in various cancers in which angiogenesis is known to play a role, including colorectal cancer. Inhibition of specific integrins might thus inhibit both direct effects of integrins on cancer cells and tumor angiogenesis. Inhibitory peptides and anti-integrin monoclonal antibodies are currently being investigated in clinical trials in patients with solid tumors, with early evidence suggesting clinical benefit in disease stabilization with use of an anti-αvβ3 antibody in the settings of colorectal cancer, renal cell carcinoma, and melanoma. Integrin inhibition alone and with other targeted therapeutic approaches should be further investigated in clinical trials in patients with colorectal cancer.

Integrins are transmembrane glyccoprotein receptors consisting of α and β subunits that mediate cell-matrix and cell-cell interactions. The extracellular environment plays a major role in cellular behavior. Integrins act as receptors to mediate attachment and migration of cells via recognition of variable extracellular matrix molecules, and thus may play an important role in tumor angiogenesis by mediating interaction of tumor cells and endothelial cells. The integrins also have a direct effect on cancer cells by stimulating intercellular signaling that can affect gene expression and alter the regulation of cell survival, differentiation, and proliferation. The development of agents capable of inhibiting the integrins involved in particular types of cancer may permit simultaneous targeting of both tumor cells and endothelial cell involvement in cancer.

Role of Integrins in Tumor Angiogenesis and Tumor Cells

There is substantial evidence for the role of angiogenesis in the development of colon cancer, and much of the available data indicate that markers of angiogenesis predict prognosis in this setting (Table 1).[1-5] The roles of growth factors such as vascular endothelial growth factor (VEGF) and VEGF receptors as important components in development of tumor angiogenesis are well established, and investigation into these roles has permitted the development of anti-VEGF monoclonal antibodies and VEGF receptor inhibitors that can prevent tumor angiogenesis (Figure 1).[6] It is also known that in addition to secretion of growth factors in the area surrounding the tumor, the activities of invasion and adhesion molecules are important in angiogenesis. Survival of cancer cells is dependent on cell-cell contact and on cell contact with the extracellular matrix, and integrins act to allow such direct contact. For example, the integrin α_vβ₃ acts to permit contact of tumor cells and endo-thelial cells with the extracellular matrix; in addition to permitting cellular interaction, this activity permits both tumor cells and endothelial cells to retain their shape and functional integrity. There is evidence that blocking of this interaction of α_vβ₃ with the extracellular matrix results in tumor cell and endothelial cell death.

There are many potential mechanisms by which interaction of the extracellular matrix with cancer cells can promote cancer cell proliferation, survival, and invasion; potential effects of the tumor microenvironment on cancer cells are shown in Figure 2, including those mediated by matrix metalloproteinase 3 (MMP-3) and integrins.[7] The integrin-mediated interaction of the rigid matrix with the cancer cell may result in activation of the Rho signaling pathway, stimulating contractility and epithelial mesenchymal transition, and the Erk pathway, inducing cell proliferation. As also shown in Figure 2, integrins α_vβ₃/α_vβ₅ can exert proangiogenic effects by interacting with vitronectin, fibrinogen, and other molecules in the endothelial cell milieu after activation by MMP-2 or thrombospondin-1. In addition, integrins can influence apoptosis and anoikis in endothelial cells in a variety of ways (Figure 2), including the "classical" model involving activation of the PI3K/AKT pathway, the unligation model (activation of caspase 8 by unoccupied integrins), and the caspase model (activation of caspase 8), all resulting in loss of apoptosis/anoikis.[7]

There are thus many ways in which integrins may be involved in angiogenesis and cancer cell proliferation and survival. Numerous modifications of integrin expression and cellular organization have been described in various carcinomas, raising the possibility of therapeutic intervention with agents that modulate integrin expression or activity. This possibility is bolstered by accumulating evidence in the clinical setting that integrin expression in tumors is associated with disease course. As shown in Figure 3, for example, a recently reported study in nearly 500 metastatic colorectal cancer samples showed that patients with the highest levels of α_vβ₆ protein expression on immunostaining had the poorest survival.[8]

Targeting Integrins in Anticancer Therapy

Inhibition of integrin-mediated interactions with the extracellular matrix may have direct effects in inhibiting proliferation and promoting apoptosis in cancer cells and anti-angiogenic effects via abrogation of the interaction between the matrix and endothelial cells. Two current approaches to therapeutic inhibition involve use of RGD (arginine-glycine-aspartate) motif peptides, which mimic the extracellular matrix proteins involved in integrin binding, and mono-clonal antibodies to particular αβ heterodimers. Mechanistic studies have indicated that these agents can induce apoptosis of cancer cells and endo-thelial cells, prevent cell migration and metastasis, and delay tumor growth.

Peptide antagonists of α_vβ₃/α_vβ₅ and monoclonal antibodies directed against α_vβ₃ have been developed on the rationale that receptors for these integrins are expressed by angiogenic but not resting endothelial cells.

The cyclic RGD peptide cilengitide (EMD 121974) is an antagonist of α_vβ₃/α_vβ₅ that was shown to induce tumor regression in preclinical studies. In a phase I dose-finding study, 37 patients with solid tumors received cilengitide doses of 30 to 1,600 mg/m2 without dose-limiting toxicities.[9] Stable disease for greater then 5 months was observed in three patients, consisting of two patients with renal cell carcinoma and one with colorectal cancer. The most common adverse events were nausea, anorexia, diarrhea, and fatigue. A phase II study of cilengitide in non–small-cell lung cancer recently has been completed, and phase II trials in other settings are ongoing.

The first studies with recombinant human monoclonal antibodies directed against α_vβ₃ were reported in 2000. The monoclonal antibody (Vitaxin) was shown to inhibit angiogenesis and induce endothelial apoptosis and tumor regression in animal models. In a phase I trial, 17 patients with solid tumors received doses of 0.1 to 4.0 mg/kg per week. Among 14 evaluable patients, 1 had a partial response, 7 had stable disease, and 6 had progressive disease, with stable disease persisting for up to 22 months.[10] The major adverse events were fever, chills, nausea, and flushing.

A second generation of anti-α_vβ₃ monoclonal antibodies has now entered clinical evaluation; second-generation monoclonal antibodies have an improved pharmacokinetics profile, with a prolonged half-life permitting maintenance of adequate drug levels over time with repeated injections. In a recently reported phase I study, 25 patients with various metastatic solid tumors, including 7 with renal cancer and 5 with colorectal cancer, received the anti-α_vβ₃ monoclonal antibody MEDI-522 at doses of 2 to 10 mg/kg per week.[11] Grade 3 toxicity was observed in only one patient (fatigue), with the most common toxicities consisting of fatigue, nausea, and myalgia. Dynamic computed tomography imaging showed increases in contrast mean transit time between baseline and 8 weeks after start of treatment, suggesting an effect of treatment in reducing tumor perfusion. No partial or complete responses were observed. However, three patients with renal cell carcinoma had prolonged periods of stable disease (34 weeks, > 1 year, and > 2 years). Less-prolonged stabilization was observed with other tumor types, including colorectal cancer.

Conclusion

Integrins play an important role in initiation, progression, and development of metastasis in colorectal cancer, and expression of integrins may be associated with poor prognosis in this setting. Although physiologic interactions involving integrins are complex, pharmacologic interactions often result in tumor growth inhibition by direct effects against cancer cells and/or inhibition of tumor angiogenesis. Integrins appear to be promising targets for drug discovery, and a number of inhibitory peptides and anti-integrin antibodies currently are in development. MEDI-522, an antibody directed against the α_vβ₃ integrin, and EMD 121974, a small molecule that inhibits α_vβ₃ and α_vβ₅ integrins, display a good safety profile with evidence of anticancer activity in colorectal cancer in phase I trials. Integrin inhibition alone and with other targeted therapeutic approaches should be further investigated in clinical trials in patients with colorectal cancer.

Disclosures:

The authors have no significant financial interest or other relationship with the manufacturers of any products or providers of any service mentioned in this article.

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