Published OnlineFirst September 7, 2011; DOI:10.1158/1078-0432.CCR-11-0642 Clinical Cancer Molecular Pathways Research b1-Integrin: A Potential Therapeutic Target in the Battle against Cancer Recurrence Dalit Barkan1 and Ann F. Chambers2 Abstract Primary cancer treatment, involving both local and often systemic adjuvant therapy, is often successful, especially if the cancer is detected at an early stage of progression. However, for some patients, the cancer may recur either locally or as distant metastases, in some cases many years after apparently successful primary treatment. Significant tumor dormancy has been documented in several cancers, such as breast, melanoma, and renal cancer. Tumor dormancy has long been recognized as an important problem in management of cancer patients. Recent work has clarified biologic aspects of tumor dormancy and has shown that dormant tumor cells may be resistant to cytotoxic chemotherapy and radiation. This work has led to recognition of a key role for b1-integrin in regulating the switch from a dormant state to active proliferation and metastasis. Here we discuss the role of b1-integrin and its signaling partners in regulating the dormant phenotype. We also consider possible therapeutic approaches, such as small molecules or antibodies (ATN-161, volociximab, and JSM6427), directed against b1-integrin signaling to target dormant cancer cells and to prevent metastatic recurrence. Clin Cancer Res; 17(23); 7219–23. Ó2011 AACR. Background in refs. 1, 2, 11) and may be resistant to therapies that target actively dividing cells (3, 10). Hence, the persistent pres- The major fear of cancer patients is the propensity of the ence of these dormant tumor cells, even after systemic cancer to recur, sometimes after long latency periods, as treatment therapy, may lead to outgrowth of the cells years metastatic disease. Most therapies fail in the metastatic later (14, 15). setting. Thus, it is vital to develop new strategies to confront During the course of metastatic spread, disseminated metastatic disease before it emerges. tumor cells encounter new and foreign microenvironments Recurrence of cancer after very long latency is explained that will determine their fate. The majority of disseminated clinically by the persistence of disseminated dormant tumor cells will likely meet their death upon colonization of tumor cells that are not clinically manifested. Several exper- a "nonpermissive" site, because of the well-recognized imental mouse metastasis models using a variety of cancer inefficiency of the metastatic process (2). In contrast, a cell lines showed the presence of dormant cancer cells, small proportion of tumor cells may persist and enter a often coexisting in a metastatic site with actively growing quiescent state of cellular dormancy that can last for years metastases (1–9). Furthermore, these 2 populations of cells or decades (12, 16). A subset of these cells may eventually have been shown to be biologically distinct, with, for escape their dormant state and form metastases. Reciprocal example, very different responses to cytotoxic chemother- interactions between these cells and their surrounding apy (2, 3, 10). Importantly, accumulating evidence shows microenvironment lead to intracellular signaling in the that dormant disseminated solitary tumor cells may also be tumor cells that dictates their survival, growth arrest, and present in patients with no clinical evidence of relapsing resumption of proliferative growth. These interactions tumors, even in some patients who were diagnosed in early involve adhesion molecules on the surface of the tumor stages of the disease (1, 11–13). Although breast and renal cells that sense and engage with the new surroundings. cancers, as well as melanoma, are well documented to have In this review, we focus on one such adhesion molecule, late recurrences, patients with other cancer types can also be b1-integrin (Intb1), which has been shown to play an at risk (11). Moreover, several studies have shown that important role in the switch from cellular dormancy to disseminated solitary dormant cells are quiescent (reviewed metastatic growth and in mediating resistance of tumor cells to adjuvant therapy and ionizing radiation. Targeting Intb1 Authors' Affiliations: 1Department of Biology, Faculty of Natural Sciences, as potential therapy and translational advances in the field University of Haifa, Haifa, Israel; 2London Regional Cancer Program, Department of Oncology, University of Western Ontario, London, Ontario, are discussed. Canada b Corresponding Author: Dalit Barkan, Department of Biology, Faculty of 1-integrin downstream signaling in cellular Natural Sciences, University of Haifa, Haifa, Israel. Phone: 972-48288789; dormancy and the switch to metastatic growth Fax: 972-48247181; E-mail: [email protected] Integrins are a family of cell adhesion molecules com- doi: 10.1158/1078-0432.CCR-11-0642 prising 18a and 8b subunits that combine into at least 24 Ó2011 American Association for Cancer Research. heterodimers. Intb1 partners with a subunits to form 12 www.aacrjournals.org 7219 Downloaded from clincancerres.aacrjournals.org on December 1, 2011 Copyright © 2011 American Association for Cancer Research Published OnlineFirst September 7, 2011; DOI:10.1158/1078-0432.CCR-11-0642 Barkan and Chambers potential integrin receptors, which bind to a wide array of contrast, high uPAR expression induced a5b1 integrin, and, arginine-glycine-aspartic acid (RGD)–containing extracel- in turn, this complex recruited EGFR and FAK, which in a lular matrix (ECM) molecules such as collagens, laminin, fibronectin-dependent manner induced sustained ERK and fibronectin (17, 18). Intb1 consists of a large extracel- activation. lular domain, a single transmembrane stretch, and a short Hence, Intb1 plays an important role in the cross-talk cytoplasmic domain devoid of an intrinsic enzymatic activ- between disseminated tumor cells and their microenviron- ity. The cytoplasmic domain transduces bidirectional sig- ment, thus dictating the fate of the tumor cells. Fig. 1 nals from inside the cell by regulating the conformation and summarizes the intracellular functions of Intb1 in cellular ligand affinities of the extracellular domain of Intb1 (inside- dormancy and their potential role in the switch to meta- out signaling), while mediating downstream signaling and static growth. interactions with the cytoskeleton (outside-in signaling). The outside-in signaling is initiated upon ligand binding to Intb1, followed by the formation of adhesion complexes b1-integrin and resistance to adjuvant and ionizing assembled from signaling molecules, such as tyrosine radiation therapy kinases, serine-threonine kinases, phospholipid kinases, Intb1 may also play an important role in resistance of phosphatases, Ras superfamily proteins, and various adapt- tumor cells to chemotherapy and ionizing radiation. Resis- er proteins (17–19). tance of dormant solitary cells to chemotherapy has been Bidirectional signals of Intb1, together with cross-talk shown previously in vivo (3, 8, 28) and in the 3D model with other cellular receptors (18), have been shown to play system (D. Barkan; unpublished data). This resistance is due a crucial role in cell adhesion, survival, differentiation, and to the quiescent nature of the dormant cells. Resistance of proliferation. Furthermore, several studies have under- dormant tumor cells to radiotherapy and or chemotherapy scored the important role of Intb1 in tumor initiation may lead to persistence of the cells and recurrence of the (20, 21), reversion (22), survival (23), tumor progression, cancer following treatment. Indeed, low expression and/or and metastasis (20, 24, 25). function of Intb1, as was shown in dormant tumors cells, We and others have shown that Intb1 activation is a key have been reported in tumor cells that have acquired mul- regulator in the switch from cellular dormancy to meta- tidrug resistance (29). On the other hand, several lines of static growth in vitro and in vivo (5, 6, 26, 27). In vitro evidence show that Intb1 signaling plays a significant role in studies used a 3-dimensional (3D) culture system, con- mediating resistance to cytotoxic chemotherapies by stituted from growth factor–reduced basement mem- enhancing the cell survival pathway mediated by phosphoi- brane to model dormancy, and found that dormant nositide 3-kinase (PI3K) and the serine-threonine kinase versus proliferative behavior in this model mimicked the (Akt) pathway (30, 31). Intb1 has also been implicated in dormant versus metastatic behavior of multiple cell lines mediating resistance to ionizing radiation through activa- in vivo (5). By using this 3D system, it was shown that tion of the PI3K/AKT pathway (32, 33), and it was shown to supplementation of the basement membrane with either be upregulated upon exposure to clinical doses of ionizing fibronectin or type I collagen induces Intb1downstream radiation (33). Importantly, clinical evidence shows that signaling (5, 26), leading to activation of focal adhesion increased Intb1 expression, which is also linked to fibro- kinase (FAK) by Src. This activation results in downstream nectin levels, is associated with decreased survival in inva- activation of the extracellular signal–regulated kinase sive breast cancer (34). (ERK), a key regulator in cell-cycle and cytoskeletal Therefore, given that Intb1 plays an important role in the reorganization. ERK, in turn, induces phosphorylation transition from cellular dormancy to metastatic