In mice with Mx-Cre-mediated excision of floxed FN in the liver, injury still induces a robust increase in ColI, ColIII, Ltbp-1, Ltbp-3, and Ltbp-4 incorporation into the extracellular matrix [33, 34]
In mice with Mx-Cre-mediated excision of floxed FN in the liver, injury still induces a robust increase in ColI, ColIII, Ltbp-1, Ltbp-3, and Ltbp-4 incorporation into the extracellular matrix [33, 34]. We examined the expression of Fibrillin-1, Fibrillin-2, Ltbp-1, Ltbp-3 and Ltbp-4 by immunofluorescence in FN-deleted RIP-Tag tumors. pancreatic adenocarcinomas. In the nearly complete absence of fibronectin, no differences in vascular density or the deposition of basement membrane laminins, ColIV, Nid1, Nid2, or the TGF binding matrix proteins, fibrillin-1 and -2, could be observed. Our results reveal that fibronectin and the endothelial fibronectin receptor subunits, 5 and v, are dispensable for tumor angiogenesis, suggesting that the inhibition of angiogenesis induced by antibodies or small molecules may occur through a dominant MBX-2982 negative effect, rather than a simple functional block. Introduction Extracellular matrix proteins and their adhesion receptors are enticing targets for the regulation of tumor angiogenesis. The recruitment of new blood vessels by tumors is an important bottleneck in tumor development, without which tumors fail to grow. Thus, targeting tumor angiogenesis has been a therapeutic goal. Endothelial cell migration and survival is strongly regulated by adhesion to extracellular matrix, mediated by integrin receptors on the endothelium. Since the endothelium and its underlying matrix are readily targeted with small molecules and antibodies, disrupting matrix-integrin interactions would seem to be a useful method of inhibiting tumor angiogenesis. Interactions between the extracellular matrix protein Fibronectin (FN) and its integrin receptors were some of the first such proposed targets, since FN and its receptors are strongly expressed around the tumor vasculature, and both are essential for developmental angiogenesis. Embryos and embryoid bodies deficient in FN fail to form vascular networks, despite proper endothelial cell specification and vasculogenesis of the dorsal aorta and cardinal vein [1C3]. MBX-2982 The FN binding integrins include 51, 41, 81, 91, v1, v3, v5, v6 and v8 . Embryos deficient in the 5 subunit (Itga5) of 51, considered the primary PLAUR FN receptor, are embryonic lethal with vascular defects . Combined deletion of integrin v (Itgav) and 5 results in a more severe phenotype than deletion of v alone, yielding a spectrum of defects resembling the FN-null embryos and suggesting that these two alpha subunits contribute to the primary FN receptors in embryonic vascular development . Indeed, mutating the RGD motif in FN critical for binding of both 51 and v/35 integrin receptors also results in embryonic lethality with vascular phenotypes . Thus, several lines of genetic evidence suggest that binding of FN by 5- and v-based integrins is critical for mammalian angiogenesis. One of the critical processes regulated by the FN-binding integrins is the assembly of soluble FN into insoluble FN fibrils . experiments suggest this is an essential step in incorporation of other matrix proteins, such as the fibrillins, MBX-2982 latent-TGF-binding proteins, collagens, and elastin, and the subsequent development of the MBX-2982 endothelial basement membrane . Blocking FN assembly also disrupts vascular network formation and in collagen plugs assembly during angiogenesis remain unclear. Although early preclinical studies supported the utility of inhibitors of the FN- 51 and FN- v3/5 interactions, the clinical results thus far have been disappointing. The most advanced study to date, a Phase III clinical trial of the selective v3 and v5 integrin inhibitor Cilengitide revealed no treatment benefit . A competitive inhibitor of the 51 synergy site important in FN binding, ATN-161, also moved to Phase II clinical trials, but there are no ongoing studies with this drug . Antibodies targeting 51 more specifically have been no more successful. Volociximab, designed to bind 51 and block interactions with FN, did not result in significant therapeutic benefits in several clinical trialssome of which were discontinued for failing to reach primary thresholds . PF-04605412, also designed to bind 51, failed to reach primary thresholds, despite effective suppression of tumor growth when used in preclinical xenografts . It is difficult to know whether such treatments would have worked if the inhibition obtained were complete. In fact, low doses of Cilengitide have been shown to promote, rather than suppress, tumor angiogenesis . Higher and more consistent doses are possible in pre-clinical models, suggesting the possibility that the level of inhibition achieved, rather than the target, may be the reason behind the potent pre-clinical effects and disappointing clinical results. Genetic mutation of the genes.