Integrins: roles in cancer development and as treatment targets

1During the last 10 years, novel insights into the mechanisms that regulate cell survival as well as cell migration and invasion have led to the development of novel integrin-based therapeutics for the treatment of cancer. Several integrins play important roles in promoting cell proliferation, migration and survival in vitro and in vivo. Antagonists of these integrins suppress cell migration and invasion of primary and transformed cells and also induce apoptosis of primary cells. Integrin antagonists also block tumour angiogenesis and tumour metastasis. Currently, humanised antibody antagonists of integrins α5β1 and αvβ3 as well as peptide inhibitors of integrins αvβ3/αvβ5 are under evaluation as angiogenesis-inhibiting therapeutics in cancer clinical trials.

2The invasion and survival of cells in vivo controls embryonic development, angiogenesis, tumour metastasis and other physiological processes. Cell surface receptors for the extracellular matrix (ECM), such as the integrins, play key roles in the regulation of normal and tumour cell migration and survival. The integrin family of cell adhesion proteins controls cell attachment to the ECM (Figure 1). While some integrins selectively recognise primarily a single ECM protein ligand (e.g., α5β1 recognises primarily fibronectin), others can bind several ligands (e.g., integrin αvβ3 binds vitronectin, fibronectin, fibrinogen, denatured or proteolysed collagen, and other matrix proteins). Several integrins recognise the tripeptide Arg–Gly–Asp (e.g., αvβ3, α5β1, αIIbβ3), whereas others recognise alternative short peptide sequences (e.g., integrin α4β1 recognises EILDV and REDV in alternatively spliced CS-1 fibronectin). Inhibitors of integrin function include function-blocking monoclonal antibodies, peptide antagonists and small molecule peptide mimetics matrix (reviewed in; ).

3Although integrins mediate cellular adhesion to ECM proteins found in intercellular spaces and basement membranes, they also transduce intracellular signals that promote cell migration (reviewed in; ) as well as cell survival. However, unlike growth factor receptors, integrins have no intrinsic enzymatic activity but activate signalling pathways strictly by coclustering with kinases and adaptor proteins in focal adhesion complexes. The association of integrins with polyvalent or crosslinked ECM proteins clusters integrins and their associated cofactors, thus activating integrin-regulated signalling pathways. For example, integrin ligation suppresses apoptosis by activating suppressors of apoptosis and by inhibiting caspase activation. Integrins also stimulate cell migration by activating Rho and Rac GTPases and by anchoring actin filaments to the membrane. These adhesion proteins promote cell cycle entry by stimulating expression of cyclins. Integrin ligation, therefore, supports signal transduction cascades that promote cell proliferation, cell survival and cell migration. In contrast, inhibition of cell integrin–ligand interaction inhibits cell migration and proliferation and induces apoptosis.

4Studies from several groups showed that cell attachment is required for the survival of normal cells. Complete loss of cell contact with the substratum (e.g., suspension culture) or adhesion to a nonspecific substratum such as poly-L-lysine induces apoptosis (‘anoikis’) of primary cells such as fibroblasts, endothelial cells and epithelial cells (; Boudreau et al, 1996; ). In contrast, loss of contact with the substratum does not necessarily kill tumour cells. Anchorage-independent tumour cells survive loss of contact with the substrate because they accumulate mutational changes in survival factors, such as upregulation of Bcl 2 expression and/or loss of p53 activity, which render the cells independent of integrin-mediated survival signals.

5Recent studies have shown that cell death can also occur when a subset of integrins in a cell fail to bind their ECM ligands. For example, expression of αvβ3 or α5β1 can inhibit cell survival in cells attached to the matrix through other integrins. The expression of αvβ3 inhibits cell survival in cells attached to native collagen through integrin α2β1. As integrin αvβ3 does not bind native collagen, these results indicate that the unligated integrin αvβ3 induces cell death. In a similar manner, inhibition of integrin α5β1 activity with antibody antagonists induces apoptosis of endothelial cells that are attached to vitronectin through αv integrins. In addition, expression of dominant negative integrins (e.g., Tac-β3, the IL-2 receptor fused with the integrin beta 3 subunit cytoplasmic tail) also inhibits survival by impairing normal integrin-mediated survival signalling. Integrin ligation suppresses caspase 8 activation, while unligated integrins facilitate caspase 8 activation in a stress response and death receptor independent manner (; Kim et al, 2002). Additional studies suggest that unligated integrins activate membrane-associated protein kinase A (PKA), which itself can activate caspase 8 in endothelial cells. Thus, in normal cells, some integrins regulate survival when ligated and induce apoptosis when unligated.

6While integrin ligation by the ECM positively regulates migration, antagonising integrins inhibits cell migration. Although blocking integrin ligation can prevent cell attachment to the ECM and thus inhibit migration, recent studies show that antagonised integrins actively inhibit signal transduction leading to cell migration. For example, the inhibition of integrin α5β1 negatively regulates fibroblast, endothelial cell and tumour cell migration even when other integrin receptors for provisional matrix proteins are ligated. Antagonists of integrin α5β1 suppress cell migration on vitronectin, but not cell attachment to vitronectin, indicating that these antagonists affect the migration machinery rather than integrin receptors for vitronectin. In fact, α5β1 antagonists activate PKA, which then inhibits cell migration by disrupting the formation of stress fibres. Direct activation of PKA by forskolin or by overexpression of the catalytic, active subunit of PKA also inhibits cell migration. Thus, integrins regulate cell migration by making contact with the substratum and by promoting signal transduction cascades that support migration.

7Angiogenesis is the process by which new blood vessels develop from pre-existing vessels. The growth of new blood vessels promotes embryonic development, wound healing and the female reproductive cycle, and also plays a key role in the pathological development of solid tumour cancers, haemangiomas, diabetic retinopathy, age-related macular degeneration, psoriasis, gingivitis, rheumatoid arthritis and possibly osteoarthritis and inflammatory bowel disease (reviewed in ). New advances in understanding the mechanisms regulating angiogenesis, such as those that promote cell migration and invasion, are leading to the development of novel therapeutics for cancer.

8Growth factors released by hypoxic tissues or pathological tissues such as tumours stimulate new blood vessel growth. New vessels grow by sprouting from pre-existing vessels (reviewed in ) or by recruitment of bone marrow-derived endothelial progenitor cells. While growth factors and their receptors play key roles in angiogenic sprouting, adhesion to the ECM also regulates angiogenesis (Figure 2). Adhesion promotes endothelial cell survival (; Stupack and Cheresh, 2002), as well as endothelial cell proliferation and motility during new blood vessel growth. One ECM protein in particular, fibronectin, is associated with vascular proliferation; it is expressed in provisional vascular matrices and provides proliferative signals to vascular cells during wound healing, atherosclerosis and hypertension. Notably, fibronectin-null mice die early in development from a collection of defects, which include an improperly formed vasculature. Recent experimental studies showed that fibronectin regulates angiogenesis, as antibody inhibitors of fibronectin block angiogenesis.

9Studies in experimental angiogenesis models and in mutant mice indicate that several integrins play key roles in regulating angiogenesis. Embryonic deletion of integrin α5β1 induces early mesenchymal abnormalities, which include defects in the organisation of the emerging vasculature and defects in the ability of endothelial cells to form vessel-like structures ex vivo. Similarly, loss of integrin α4β1 leads to aorta, heart and other vascular malformations. Deletion of the αv subunit causes 80% of embryos to die early in development from uncertain causes, while the few surviving embryos die a few hours after birth with significant defects in brain development, including failure of blood vessels to form properly. In contrast, individual loss of the β3orβ5 subunit during embryogenesis does not cause noticeable defects in the formation of the cardiovascular system. In fact, one study show that loss of the β3 or the β3 and β5 subunits promotes tumour angiogenesis. These studies led to the controversial conclusion that the αvβ3/αvβ5 integrins are not required for angiogenesis, but instead may suppress angiogenesis. As many studies have shown that αvβ3 and αvβ5 inhibitors block angiogenesis by inducing apoptosis in proliferating endothelial cells, it is possible that loss of the integrin-mediated death mechanism can lead to enhanced angiogenesis. In fact, loss of either the β3 or β5 subunits does block angiogenesis induced by the angiomatrix protein Del-1. Thus, integrins appear to have diverse roles in the establishment of the cardiovascular system, with integrin α5β1 clearly playing a major role during development of the vascular system.

10Studies in experimental models of angiogenesis also indicate that several integrins can play important roles in regulating angiogenesis in normal animals. The expression of both integrins αvβ3 and α5β1 are significantly upregulated on endothelium during angiogenesis. The expression of integrins αvβ3 and α5β1 partially controls angiogenesis; neither integrin is expressed by quiescent endothelium and both are expressed in response to angiogenic growth factors. Their expression is controlled by the transcription factor Hox D3. Hox D3 is a Homeobox gene expressed by ECs that may regulate an angiogenic switch. When expressed in vivo, Hox D3 promotes a haemangioma-like proliferation of blood vessels; this transcription factor promotes the expression of integrin αvβ3, α5β1 and uPA, molecules with established roles in angiogenesis. Thus, Hox D3 may provide a switch to activate a program of angiogenesis. Once integrins α5β1 and αvβ3 are expressed, angiogenesis depends on each integrin as antagonists of each can block angiogenesis in vivo. Antibody and peptide antagonists of integrins αvβ3 and α5β1 inhibit growth factor as well as tumour angiogenesis, tumour growth and tumour metastasis. These studies indicate that these integrins function in part by promoting survival in proliferating endothelial cells in vivo. Studies of the signals transduced when integrins are antagonised indicate that unligated integrins activate PKA, which then activates caspase 8 and induces apoptosis.

11In addition, other integrins have been shown to regulate angiogenesis. Integrin αvβ5 promotes VEGF-, but not bFGF-, mediated angiogenesis. Integrin receptors for laminin and collagen also play roles in regulating blood vessel formation as antagonists of α2β1 and α1β1 suppressed VEGF-mediated angiogenesis. Thus, integrins play key roles in regulating tumour angiogenesis, and integrin antagonists hold promise as future therapeutics for cancer.

12Tumour metastasis promotes the spread of tumours to local and distant sites away from primary tumours. Metastasis is the leading cause of the morbidity and mortality associated with cancer. Tumour cells isolated from metastases are highly migratory and invasive. Therefore, understanding the mechanisms regulating cell migration may be helpful in developing new modes of therapy for metastatic cancer.

13Increased levels of expression of integrins αvβ3 is closely associated with increased cell invasion and metastasis. Notably, integrin αvβ3 is expressed on invasive melanoma but not benign nevi or normal melanocytes. Additionally, increased αvβ3 expression levels correlate with increased rates of melanoma metastases.

14Integrin α6 expression is also significantly upregulated in numerous carcinomas, including head and neck cancers and breast cancer. Integrin α6β4 expression enhances tumour cell invasiveness and metastasis, particularly in breast carcinomas. Thus, antagonists of these integrins may be useful to prevent the spread of tumour cells.

15Several integrin inhibitors are currently under investigation as therapeutics for cancer. Antibody and peptide inhibitors of integrins αvβ3 and αvβ5 (for review, see ) and of α5β1 are currently in clinical trials for the inhibition of angiogenesis in cancer. A humanised anti-αvβ3 antibody, Vitaxin, is currently in Phase II trials for cancer, while a humanised anti-α5β1 antibody is in Phase I trials for cancer (Varner, personal communication; www.pdl.com). A cyclic peptide inhibitor of integrin αvβ3/αvβ5, Cilengitide, is in Phase I/II trials for glioblastoma and other cancers. Other promising integrin α5β1- and αvβ3-blocking peptides with antitumour angiogenesis and tumour metastasis activities are currently in preclinical development. As Avastin, the antibody inhibitor of VEGF, has recently shown promise as a therapeutic for colon cancer in Phase III clinical trials, these integrin-based antiangiogenesis therapeutics hold great promise as powerful therapeutics for the treatment of cancer.

16The studies reviewed here indicate that integrin promote cellular migration and survival in tumour and primary cells. Antagonists of integrins αvβ3, α5β1, αvβ5 and α6β4 show great promise as potential inhibitors of tumour growth and metastasis as well as tumour angiogenesis. Clinical trials are currently underway to evaluate inhibitors of integrin αvβ3, αvβ5 and α5β1 for their usefulness in the treatment of cancer.