Integrins within the Tumor Microenvironment: Biological Functions and Advances in Therapeutic Targeting

Many cellular functions important for tumor initiation and progression are mediated by members of the integrin family, a diverse family of cell adhesion receptors. With recent studies emphasising on the role of the tumor microenvironment (TME) in tumor initiation and progression, it is not surprising that a lot of attention is being given to integrins. Several integrins are under trial with many demonstrating appealing activity in patients with different cancers. A deeper knowledge of the functions of integrins within the tumor microenvironment is still required, and might lead to better inhibitors being discovered. Integrin expression is commonly dysregulated in many tumors with integrins playing key roles in signaling as well as promotion of tumor cell invasion and migration. This review report new data on the differential expression of integrins within solid tumors using two publicly available resources: The Cancer Genomic Atlas (TCGA) and Gene Expression Profiling Interactive Analysis (GEPIA). In this analysis, I investigated the expression of integrin alpha 2 (ITGα2), ITGα3, ITGβ4 and ITGβ6 in tumor tissues versus adjacent normal tissues. This analysis showed that integrins were differentially expressed in cervical squamous cell carcinoma (CESC), head and neck squamous cell carcinoma (HNSC), esophageal carcinoma (ESCA) and lung adenocarcinoma (LUAD). This analysis showed that ITGα2 and ITGβ6 expression are upregulated in CESC and ESCA, ITGα3 is upregulated in HNSC and ESCA whilst ITGβ4 is highly expressed in CESC, HNSC, ESCA and LUAD tumor tissues compared to adjacent normal tissues. Integrins also play a major role in adhesion of circulating tumor cells to new sites and the resulting formation of secondary tumors. Furthermore, integrins have demonstrated the ability to promoting stem cell-like properties in tumor cells as well as drug resistance. Anti-integrin therapies rely heavily on the doses or concentrations used as these determine whether the drugs act as antagonists or as integrin agonists. This review offers the latest synthesis in terms of current knowledge of integrins functions within the tumor microenvironment and potential targets for different cancers.


Tumorigenesis involves the transformation of normal cells into cancer cells via mechanisms that
include activation of many signaling cascades. Several studies have shown that integrins are directly involved in tumor initiation and progression via influencing cancer cell proliferation, migration, invasion and survival as well as partaking in cellular signaling [1,2]. Most studies on integrins have focussed on their role in cellular migration and invasive behaviour. This is mostly due to integrins binding to extracellular matrix (ECM) proteins and aiding in creating traction needed by cells for migration and invasion. In addition, several tumor microenvironment (TME) components including biological molecules such as growth factors and proteases owe their localisation to certain regions to integrins [3,4]. Thus, integrins can influence cellular proliferation and signaling through 'capturing' growth factors and proteases in certain regions of the TME [5,6]. In cancer, integrins play a significant role in controlling cellular growth through binding to ECM proteins such as collagens and fibronectin and relay extracellular signals into the cell to affect gene expression [7][8][9]. It is important to point out that integrins relay both pro-and anti-tumorigenic signals, with the balance determining tumor growth and inhibition. Advances in TME biology have revealed that integrins, bound to ECM proteins or not, influence tumor growth via influence on cancer cell stemness as well as chemoresistance [10,11]. Understanding the behavior of integrins in normal tissues and cancer has huge implications for their therapeutic targeting.
Beside integrins found on cancer cells, integrins are also present on cells associated with tumors and play a great role in influencing stromal cell-tumour cell interactions [12][13][14]. These stromal cells range from cancer associated-fibroblasts (CAFs), -macrophages (CAMs), -endothelial cells (CANs) as well as pericytes [15][16][17][18]. Most of these stromal cells are directly linked to processes occurring during tumor development and metastasis such as inflammation, angiogenesis and desmoplasia [19][20][21]. Integrins traverse the lipid bilayer and are the link between the extracellular space and the cytoplasm [22,23]. Integrins relay extracellular cues through receptors for growth factors and cytokines and thus promote cellular signaling. Being present in both cancer cells and stromal cells, integrins influence tumor development as well as metastasis. New findings show that specific integrins are required by certain growth factors and oncogenes during tumor initiation and growth [5,6]. This makes it critical to delineate the crosstalk between integrins and growth factors and oncogenes during drug development. To date, several studies both preclinical and clinical trials have been done using integrin inhibitors and these have demonstrated various levels of efficacy at blocking tumor progression [24,25]. In addition, integrin inhibitors have shown little or no known side effects in some cancer patients [26][27][28]. The success demonstrated with integrin inhibitors must be augmented by further research into the role of integrins in different cancers as well as in tumor development and growth.

Integrin Biological Properties
Integrins are receptors that traverse the lipid bilayer of the cell membranes and their main function is to relay extracellular cues to cells by binding to the ECM and other immunoglobulin molecules ( Figure 2) [23,[29][30][31]. A minimum of 24 different heterodimer combinations from 18 α-subunits and 8 β-subunits are formed. Each combination of α and β subunits specifically bind to certain ECM proteins and immunoglobulin molecules [23,31]. The formation of the integrins dimers occur in the endoplasmic reticulum with post-translational changes occurring in the Golgi apparatus before the inactive integrins are transferred to the cell surface [32]. Different cells express different integrins and this greatly influence adhesion to different surfaces as well as ability to migrate [20,23]

Integrin Contribution to Tumorigenesis
Several pieces of evidence have shown that integrins contribute toward cancer growth and progression ( Figure 3). With many cancers starting in epithelial cells, integrins expressed by these cells play key roles during cancer growth and progression. Integrins increase cellular signaling via interactions with FAK, PI3K-Akt and the MEK-ERK pathways [3,4]. In addition, stromal cells including caner-associated fibroblasts synthesize increased amounts of ECM proteins and these display enhanced interaction with integrins. Several reports show that integrins are also involved in metastasis and the priming and colonization of new sites for secondary tumors to form.
Furthermore, cancer therapy has been shown to induce integrins expression leading to drug resistance.

The Significance of Integrins in Tumorigenesis
Examples of integrins expressed by epithelial cells include α2β1, α6β4 and α3β1 and may display dysregulated expression in tumors. Whilst data strongly show these integrins role in cellular attachment to the ECM and the basement membrane, recent data also showed their participation in cellular proliferation, migration and metastatic behaviours [52][53][54]. Integrins expression has also been shown to change when epithelial cells transform to cancer cells. For example, αvβ6 and αvβ3 integrins are known to be highly upregulated in tumors such as ovarian and colon compared to normal cells [55][56][57][58]. In addition, these integrins are also associated with increased metastasis and tumor cell proliferation in ovarian cancer [59,60]. Several reports also show that some integrins decrease in expression in some tumors. For example, α2β1 integrin expression is decreased in breast tumors [61][62][63]. Decreased integrin expression in cancer cells has been linked with invasion and metastatic behavior [64][65][66]. Several studies have linked integrin expression in tumors to patient survival and disease progression, with integrins αvβ3, α6β4 and αvβ6 among the most studied. As new data is generated, the picture become clear on which integrins are important in different tumors. McCabe and colleagues demonstrated that αvβ3 integrin is associated with metastatic behaviour of prostate cancer cells to the bone matrix [67]. In addition, the expression of αvβ3 and αvβ6 is linked to decreased cervical cancer patients survival [68,69].
Integrin α5β1 expression is associated with metastasis and decreased cancer patient survival in melanoma and lung carcinoma [70][71][72]. The expression of αvβ3 is linked to pancreatic cancer cells metastasizing to the lymph node [73].

Bioinformatic Analysis of Integrins' RNA-seq Data
Increasing data indicate that integrins represent an important component of the tumor

Statistical Analysis
GraphPad Prism software (version 6, San Diego, USA) was used for the analysis with both Student's t-test and one way analysis of variance test used to analyse significance of differences.
Statistical significance was set at p < 0.05.

Differential Expression of Integrins in Tumors
Several pieces of evidence points to integrins contributing to tumor initiation and development of malignant phenotypes, therefore it is important to investigate the significance of integrins in several solid tumors using The Cancer Genomic Atlas dataset. In this analysis, four cancers were used as examples to investigate the relationship between integrin expression and cancer malignance phenotype. The cancers analysed are cervical squamous cell carcinoma (CESC), head and neck squamous cell carcinoma (HNSC), esophageal carcinoma (ESCA) and lung adenocarcinoma (LUAD). Expression of integrins in tumor tissues was compared to that in adjacent normal tissues (as a box plot) based on TCGA/GEPIA database.
Bioinformatic analysis showed that ITGα2 expression was significantly upregulated in CESC and ESCA tumor tissues compared to adjacent normal tissues ( Figure 4A). The increased ITGα2 expression observed in ESCA was observed in our previous report [20]. Consistent with the above results, 10 out of 11 cervical cancer specimens showed medium to high ITGα2 protein expression based on immunohistochemistry data available at The Human Protein Atlas database.
There were no significant differences in ITGα2 expression in HNSC and LUAD tumor tissues compared to adjacent normal tissues ( Figure 4A). ITGα3 expression was upregulated in HNSC and ESCA tumor tissues versus adjacent normal tissues whilst there were not significant differences in ITGα3 expression in CESC and LUAD tumor tissues versus normal tissues ( Figure   4B). Consistent with the data for HNSC, 3 out of 4 HNSC cancer specimens showed medium to high ITGα3 protein expression based on data available on The Human Protein Atlas database.

Integrin Promotion of Tumor Cell Survival
Integrins have been shown to act in both a pro-and anti-tumorigenic manner, depending on signals within the tumor microenvironment. Tumor cell-expressed integrins promote cell survival via involvement in proliferation, invasion and metastasis among several processes. In the case of invasion, integrins binding to different ECM proteins generates the needed traction for movement and invasive behavior. Formation of focal adhesions and the involvement of MMPs at the leading edge allow cells to invade new territories and tissues [74]. Several reports however point to the involvement of some integrins in preventing tumor cell migration. For example Kren and colleagues demonstrated that the absence of integrin B1 increased cancer cell metastatic behaviour [75]. Integrins can promote tumor cell survival though participation in several signaling mechanisms whilst integrins have also been shown to play in a role in the process of apoptosis. How integrins behave depends on whether they are free or bound to their ligands. A critical balance is maintained through integrins interacting with ligands and this helps to maintain cell and organs integrity. For example, the binding of integrins to ligands can activate survival pathways such as MEK-ERK, nuclear factor-kβ (NF-kB) and PI3-K-Akt pathways [3,4,[76][77][78]. Interaction of integrins and the ECM upregulates BCL-2 expression in cancer cells [79,80]. The interaction between integrins and growth factor receptors also play a role in preventing apoptosis in cells [81,82]. Deletion of integrins genes or inhibition of integrin-mediated signaling has been shown to activate a compensatory upregulation of vascular endothelial growth factor receptor-2 expression, complicating the targeting of integrins in pathological conditions such as cancer [83][84][85]. Several data show that integrin function in tumors may be context dependent, with data showing that integrins such as αvβ3 is pro-tumorigenic but can inhibit tumor growth in cancers such as melanoma and glioblastoma [52,[86][87][88].
A subpopulation of cells known as cancer stem cells (CSCs) that is thought to give rise to all tumor cells present in tumors [89][90][91][92]. Several pieces of evidence point to integrins playing a part in tumorigenesis and as markers of CSCs [11,93]. For example, Asselin-Labat and colleagues demonstrated that stem cell-like cells within the mammary ductal epithelium express αvβ3 integrin and it can be used a marker of these cells [94]. Vaillant and colleagues revealed that β3 integrin as well as WNT1 were both markers of mammary CSCs [95]. Luo and colleagues demonstrated that removal of integrin-associated focal adhesion kinase signaling reduced the number of CSCs in mammary tumors in mice [96]. Samanna and colleagues showed that αvβ3 expression was linked to tumorigenicity and MMP-2 activity in melanoma cells [97]. Specifically, the authors showed that integrins influence the expression of CSCs markers such as CD44 [97].

Integrins Contribution towards Tumor-Host cell interactions
Integrins are also involved in the response of normal cells to the presence of tumor cells.
Reports indicate that most cells including fibroblasts, mesenchymal stem cells, pericytes, immune cells and endothelial cells utilise integrins in cellular processes such as migration, angiogenesis and desmoplasia. The targeting of integrins on stromal cells may aid in limiting the 'support' given to tumor cells by normal cells.
Integrins are involved in every stage of tumorigenesis from initiation to metastasis [98]. Several pieces of evidence points to 'leaky' blood vessels in tumors which causes impaired blood flow and drug delivery [99]. In addition, leaky blood vessels are also known to promote fibrosis in tumors [100]. Brooks and colleagues showed that integrin αvβ3 was upregulated in chick chorioallantoic membrane and associated to angiogenesis [101]. It has been speculated that integrins may enable the interaction between ECM proteins and angiogenic-associated cells such as endothelial cells within the TME [102,103]. Remodelling of the ECM has also been linked to increased integrin binding to degraded ECM proteins such as collagen [104]. Several reports have demonstrated that integrins are important for cellular processes such as migration and angiogenesis [98]. Various compounds or small molecules in addition to genetic manipulation have confirmed the involvement of integrins including α2β1, α6β1 and α6β4 [3,4,98]. Together with growth factor receptors, integrins can relay signals from growth factors to influence angiogenesis in tumors [101,105,106]. In elaborate experiments, Friedlander demonstrated that protein factors, basic fibroblast growth factor and tumor necrosis factor-α require the integrin αvβ3 to induce angiogenesis in corneal [105]. In addition, the same authors also showed that vascular endothelial growth factor and transforming growth factor-α induced angiogenesis through interaction with integrin αvβ5 [105]. Most importantly, integrins can bring together signals from both the ECM and several growth factors in order to influence cellular processes.
Several cells have been shown to play a role in angiogenesis and these include vascular smooth muscle cells and pericytes. Blood vessel maturation for example requires pericytes and smooth muscle cells [107][108][109][110]. Most blood vessels found within the TME are abnormal and leaky, leading to less oxygen reaching all cells. Interaction between pericytes and endothelial cells is via integrin α4β1 and vascular cell adhesion molecule 1 (VCAM1) for example [111]. This may lead to blood vessel formation and stability [111]. The branching of blood vessels within tumors also require pericytes involvement through integrin binding [112]. Thus a combination of therapeutic targeting tumor cells as well as therapeutics targeting angiogenesis and its associated cells to bring about blood vessel normalization may provide a synergistic and durable effect in cancer treatment [113][114][115]. Indeed, it has been suggested that treatment with anti-angiogenic therapeutics first before cytotoxic drugs may be better at treating cancer [115]. Thus today, vascular normalization is part of cancer therapies in many settings and clinics.
One of the observed hallmarks of desmoplasia in tumors is the synthesis and deposition of large amount of ECM proteins such as collagens. Increased collagen within the TME means increased integrin signaling allowing promotion of cancer cell survival and resistance to therapy [116].

Integrins on both cancer cells and stromal cells contribute towards cancer cell growth via
activation of signaling that promote cancer cell growth. For example, Zhu and colleagues demonstrated that in non-small cell lung carcinoma several integrins including α11 is necessary for cancer cell growth [117]. Integrin α11 was instrumental in inducing insulin-like growth factor 2 release in human non-small-cell lung cancer cells [117]. Integrins inhibitors must therefore target both cancer cells and stromal cells for effective and durable cancer treatment.
Several pieces of evidence have shown that bone marrow-derived cells from the bone marrow are found within solid tumors [21,118,119]. Once in the TME, these bone marrow-derived cells can exhibit contrasting effects on cancer cells with some cells having anti-tumorigenic effects whilst others have tumor promoting effects. For example, infiltrating macrophages expressing integrin αvβ3 are able to have anti-tumorigenic effects [120]. In addition, bone marrow derived cells with no functional integrin β3 are not found within tumor sites and cannot therefore promote angiogenesis [121]. Jin and colleagues demonstrated that cells attracted to tumors including endothelial cells require integrin α4β1 [122]. Integrin α4β1 is needed for the binding of bone marrow derived cells to the endothelium within TME [123]. In terms of targeting integrins needed for the homing of bone marrow-derived cells to tumors, a balance must be maintained between anti-tumorigenic and pro-tumorigenic effects.
Tumor metastasis has been associated with the interaction between cancer cells and platelets. For example, Mammadova-Bach and colleagues demonstrated that integrin α6β1 on platelets promotes metastasis via binding to lung cancer cell-derived ADAM9 [124]. In addition, Gay and colleagues showed that platelets can alter tumor cell characteristics and promote metastasis [125].
Furthermore, fibrinogen can behave as act as a connection between platelets and cancer cells via attaching to integrins on both cells and this allows cancer cells to be associated with blood vessels [126,127]. Metastasis to the bone marrow and the lungs is also made possible via interaction between cancer cells and platelets [127,128]. Trikha and colleagues demonstrated that the combined inhibition of integrins on both cancer cells and platelets enhances the antitumorigenic of the inhibitors compared to inhibition of integrins on tumor cells [129].

Integrin Crosstalk with Other Receptors
Cooperation between integrins and oncogenes in promoting tumor growth has been noted. Guo and colleagues demonstrated that integrin α6β4 cooperates with ERBB2 and enhances tumor initiation and invasion in breast cancer [130]. There is also an association between integrin β1 and polyoma middle T oncoproteins in driving breast cancer [131]. Some lung tumors require the involvement of integrin α1 for the initiation and growth of the tumors [132]. The Src oncogene activity has been shown to be enhanced via its interaction with integrin α4β3 [133].
These several pieces of evidence demonstrate that integrin signaling is involved in enhancing the pro-tumorigenic effect of many oncogenes.
Tumor development and growth studies have shown that integrins play crucial roles through interactions with growth factors as well as cytokines. The involvement of integrins in adhesion, migration and invasion of tumor cells is well documented, however integrins have been shown to play more roles including enhancing angiogenesis and signaling. Interaction between integrins and growth factors and cytokines is not always pro-tumorigenic, with several pieces of data showing that integrins interactions can be anti-tumorigenic [134]. The interaction of integrins and growth factors is likely to influence signaling pathways involving kinases to enhance tumor cell survival and invasiveness [135,136]. Growth factors can also influence integrin internalisation and activity and vice-versa [137][138][139][140][141][142].
The cooperation between integrins and growth factors and their receptors has been noted for its role in tumor development, growth and eventual metastasis in several cancers. Involvement of growth factors including the epidermal growth factor receptor family can be through increased expression or activation leading to oncogenic signaling in tumor cells. For example, enhanced expression of ERBB2, also known as v-erb-b2 avian erythroblastic leukemia viral oncogene homolog 2, and its receptors is observed in several cancers including breast and nasopharyngeal carcinoma [143][144][145][146]. Cooperation between integrins and the EGF receptor family has been reported in breast cancer. Guo and co-workers demonstrated that integrin α6β4 cooperates with ErbB2 signaling in breast cancer to promote tumor development and growth [130]. This cooperation also promoted invasion of tumor cells [130]. Furthermore, the authors showed that this cooperation causes the activation of the signal transducer and activator of transcription (STAT3) signaling, a signaling pathway known to cause increased cellular proliferation [130]. The targeting of both α6β4 integrin and the ERBB2 may produce a synergistic therapeutic effect [130]. The interaction between integrins, growth factors and ECM proteins such as collagen and glycoproteins such a vitronectin can promote cancer cell migration and metastasis [147][148][149].
Integrin αvβ5 for example interacts with EGF and vitronectin to promote tumor cell migration [147][148][149]. Pouliot and colleagues demonstrated that EGF cooperates with α3β1 and the ECM protein laminin for example to promote migration of human colon carcinoma cells [150].
Another important study demonstrated that integrins α1β1 and α2β1 interact with EGF to promote the invasive behavior of hepatocarcinoma cells in a fibrotic microenvironment [151].
The cooperation between integrins and the hepatocyte growth factor (HGF) receptor, MET, has been implicated in tumor initiation and invasion. Bertotti and colleagues demonstrated that Integrin β4 cooperates with HGF receptor, MET, to promote fibroblasts transformation and the eventual involvement of such transformed fibroblasts in tumorigenesis [152]. In another study, the same authors also showed that through induction of integrin β4 phosphorylation, MET signaling upregulates anchorage-independent growth [153]. Reports show that integrin αvβ5 can regulates the expression of genes induced by HGF and involved in cell invasion [154]. Overall, HGF-MET signaling cascade involvement in tumorigenesis is dependent on cooperation with integrins. Another growth factor shown to be involved in promoting tumorigenesis is transforming growth factor (TGF-β). For example, TGF-β has been shown to play a key in epithelial-mesenchymal transition (EMT) leading to increased tumor cell movement and invasiveness [155][156][157]. Integrins such as αvβ6 and αvβ8 play a key role in the activation of TGFβ signaling through binding to the RGD sequence of the latency-associated peptide of TGF-β1 [158][159][160][161]. Marsh and colleagues demonstrated that enhanced integrin αvβ6 expression together with increased TGF-β activation leads to aggressive disease in basal cell carcinoma [162].

Immunohistochemical studies and transcriptional activation experiments confirmed that αvβ6
activates the TGF-β signaling cascade in vivo and EMT leading to tumor growth [163,164].
Our study demonstrated that both tumor cells and stromal cells secrete TGF-β, and it is plausible that the crosstalk between TGF-β and integrins play a key role in tumor growth and progression [21].
Integrins cooperation with growth factor occurs between tumor cells and stromal cells as well.
For example, endothelial cell migration and proliferation require integrin and growth factor interactions during angiogenesis. Several reports have linked specific integrins to specific growth factors during angiogenesis [101,[167][168][169]. Fibroblast growth factor receptor is known to interact with integrin αvβ3 leading to the phosphorylation of serine residues in Raf [170,171].
The resulting complex known as MAP3K5 complex causes inhibition of the apoptotic pathway thus protects endothelial cells from apoptosis [172]. In addition, VEGFR2 has been shown to interact with integrin αvβ3 and αvβ5 to inhibit apoptosis by regulating the Ras-ERK pathway [170]. VEGFR2 also interacts with integrin αv in endothelial cells to increase angiogenesis [172][173][174]. Increased tumor cell proliferation is observed when activated αvβ3 causes secretion of growth factor VEGF [175]. Nikolopoulos and colleagues demonstrated that integrin β4 cooperates with fibroblasts growth factor to induce angiogenesis and increased tumor growth [176]. CXCR4 is expressed by both tumor and stromal cells and its increased expression has been associated with increased expression of several integrins [177][178][179].

Targeting Integrins in Cancer Therapy
Integrins are appealing therapeutic targets for cancer therapy due to their increased expression in several cancers and cooperation with growth factors during tumor development and metastasis. Furthermore, integrins have been shown to play a key role in the development of drug resistance. Our work and that of others have specifically shown the involvement of integrins and their ligands in cancer cell drug resistance [11,20,21,180,181]. Several in vitro and animal studies have shown that inhibition of integrin function through the use of antagonists is effective at blocking tumor development and metastasis. Integrin function blockage can affect both tumor cells and stromal cells [98,[182][183][184]. For example, cilengitide is currently under Phase 3 clinical trial for the treatment of glioblastoma. Cilengitide is also under study for its effectiveness against lung, glioblastoma and prostate cancer [185][186][187][188]. A monoclonal antibody, LM609 or its humanized version Etaracizumab, demonstrated anti-angiogenic activity in many studies including breast cancer studies [189]. Mulgrew and colleagues demonstrated that blocking αvβ3 with the monoclonal antibody Abegrin can result in inhibition of tumor growth [190].
Integrin αvβ3 is one of the major integrins expressed by osteoclasts and its inhibition through the use of monoclonal antibody Vitaxin®, blocks bone metastasis [191]. Vitaxin also demonstrated efficacy in patients with renal cancer and other solid tumors [192,193]. Trikha and colleagues and Chen and co-workers demonstrated that the monoclonal antibody CNTO95 have anti-angiogenic effects and can block cell signaling and invasion in breast cancer cells, respectively [194,195]. The amount of integrin antagonists used in studies is very important as some studies demonstrated that certain concentrations of doses of antagonists can act as integrin agonist fashion [196][197][198]. Most anti-angiogenic drugs increase tumor perfusion which is linked to better drug delivery in tumors [199,200]. Thus, several studies have demonstrated that antiangiogenic therapy targeting integrins works well when combined with chemotherapy. In addition, integrin targeting affects not only tumor cells but several stromal cells within the TME.
Integrin β1 via interaction with several signaling cascades including the Akt and the FAK pathways has been shown to play key roles in development of lung cancer and head and neck drug resistance [201,202]. Hirata and colleagues demonstrated that enhanced integrin β1 signaling via interaction with FAK was associated with resistance to the highly selective inhibitor of B-Raf, PLX4720 [203].
Vascularized tumors including glioblastomas demonstrate increased expression of integrins such as integrin αvβ3 and β8 [204][205][206]. In most cases, these tumors are aggressive and exhibit low patient survival making the development of integrin antagonists necessary to increase patient survival [207][208][209]. It has been shown that tumor microenvironment is a crucial determinant of therapy effectiveness, thus targeting angiogenesis and stromal integrins is a viable option [15,16,20,21,89,206,207]. For example, esophageal carcinoma abundantly express ECM proteins some of which are ligands for several integrins and thus can influence cancer cell survival and migration [20]. Thus, it is possible to target integrins such as α2β1 as the ECM ligands including fibronectin and collagen are abundantly expressed in esophageal cancer [3,4,20,21]. Several clinical trials have been performed or under way to determine the effectiveness of integrin targeting in different cancers [186][187][188].
Park and colleagues demonstrated that an integrin β1 inhibitory, AIIB2, was able to reduce breast cancer survival through activation of apoptosis [210]. In yet another study, Bhaskar and colleagues showed that chimeric integrin α5β1 antibody, Volociximab, displayed anti-angiogenic activity and inhibited tumor growth [211]. Khalili and colleagues also showed that peptide inhibitor of integrin α5β1, ATN-161, was able to inhibit cancer growth as well as metastasis [212]. The same peptide inhibitor, ATN-161, together with commonly used drug fluorouracil were able to inhibit colon metastasis to the liver [213]. A small molecule integrin αvβ3 inhibitor S247 was able to breast cancer cell metastasis to the bone [214]. The same effect was demonstrated by another integrin αvβ3 inhibitor PSK1404 [215]. S137 and S247 are RGD peptide mimetics and have been shown to inhibit metastasis [216]. 6.3G9 is an integrin αvβ6 antibody that has been shown to have anti-cancer activity in pharyngeal carcinoma cells [163].
One major challenge associated with targeting integrins and its associated cellular processes such as angiogenesis is the lack of validated biomarkers. Several markers such as serum levels of fibroblast growth factor and vascular endothelial growth factor have been suggested to indicate the efficacy of anti-integrin therapy. However, few or no reports have shown the efficacy of such biomarkers in the clinical setting. Current investigations involving coupling integrin antagonists and radionuclides aim to identify newly formed blood vessels in tumors [217]. In addition, newly formed vessels can also be identified via the use of ultrasound with microbubbles as shown for αv-integrins by Leong-Poi and colleagues [218]. Angiogenesis in tumors can also be detected via the use of 18 F-labeled PEGylated RGD peptide [219]. The above noted examples and others demonstrate the potential use of labelled integrins antagonists as both treatment options and as diagnostic tools in cancer therapy. Furthermore, therapeutics targeting integrins can also be used to deliver drugs and proapoptotic peptides to tumor cells. For example, Hood and co-workers targeted integrin αvβ3 with a nanoparticle whilst delivering mutant RAF1 gene to blood vessels within the tumor, leading to tumor shrinkage [81]. Nanoparticles can also be coupled to drugs such as doxorubicin and be able to target integrins and tumor vasculature [220].

Conclusion
Whilst most cells within the human body express integrins, those expressed by tumor cells and stromal cells within the TME are involved in several processes along the tumorigenic path.