Is Inflammatory Bowel Disease a Vascular Disease? Targeting Angiogenesis Improves Chronic Inflammation in Inflammatory Bowel Disease

See "Endothelial caveolin-1 regulates pathologic angiogenesis in a mouse model of colitis" by Chidlow JH, Greer JJM, Anthoni C, et al on page 575; and "VEGF-A links angiogenesis and inflammatory bowel disease pathogenesis," by Scaldaferri F, Vetrano S, Sans M, et al, on page 585. See "Endothelial caveolin-1 regulates pathologic angiogenesis in a mouse model of colitis" by Chidlow JH, Greer JJM, Anthoni C, et al on page 575; and "VEGF-A links angiogenesis and inflammatory bowel disease pathogenesis," by Scaldaferri F, Vetrano S, Sans M, et al, on page 585. "Next to leukocytes, the microvasculature and its endothelial lining play the most important role in inflammation."—Elie Metchnikoff, 1886 Nobel Laureate Although the importance of the microvasculature in inflammation was first described >120 years ago by the Nobel Laureate Elie Metchnikoff, advances in endothelial and vascular biology have only begun to define the molecular and cellular basis for this understanding over the past 2 decades.1Hatoum O.A. Binion D.G. The vasculature and inflammatory bowel disease: contribution to pathogenesis and clinical pathology.Inflamm Bowel Dis. 2005; 11: 304-313Google Scholar This issue of Gastroenterology includes complementary manuscripts by 2 outstanding inflammatory bowel disease (IBD) vascular biology research groups who have provided insight into angiogenic mechanisms and the role of these pathophysiologic processes involving excess blood vessel growth in both human IBD and animal models of chronic gut inflammation. These 2 papers highlight the emergence of a more sophisticated understanding of chronic inflammation, where all cellular components of the intestinal tissues, including the microvascular endothelium, are being targeted for investigation as well as defining new potential routes for therapy. Over the past 2 decades, the initial exploration of the vascular biology of IBD focused on the role of the microvascular endothelium as a "gatekeeper" of the inflammatory process, through the regulated interaction and recruitment of inflammatory cells into foci of inflammation. Endothelial cells will undergo activation in response to cytokines and bacterial products, thereby leading to cell adhesion molecule expression, enhanced leukocyte interaction, and the influx of the inflammatory infiltrate into tissues. Early exploration of these pathways in IBD revealed an increased expression of cell adhesion molecules, and selective recruitment of leukocytes binding the α4 integrin in the chronically inflamed intestine.2Koizumi M. King N. Lobb R. et al.Expression of vascular adhesion molecules in inflammatory bowel disease.Gastroenterology. 1992; 103: 840-847Crossref Scopus (211) Google Scholar Inhibition of α4 leukocyte interaction with ligands on the gut microvasculature in the Cotton-Top Tamarin, a new world primate that develops a spontaneous colitis in captivity, formed the basis for the development of selective leukocyte adhesion therapy.3Podolsky D.K. Lobb R. King N. et al.Attenuation of colitis in the cotton-top tamarin by anti-alpha 4 integrin monoclonal antibody.J Clin Invest. 1993; 92: 372-380Google Scholar The anti-α4 inhibitor natalizumab ultimately received US Food and Drug Administration (FDA) approval for the treatment of refractory Crohn's disease in 2008.4Sandborn W.J. Colombel J.F. Enns R. et al.Natalizumab induction and maintenance therapy for Crohn's disease.N Engl J Med. 2005; 353: 1912-1925Google Scholar, 5Targan S.R. Feagan B.G. Fedorak R.N. et al.Natalizumab for the treatment of active Crohn's disease: results of the ENCORE trial.Gastroenterology. 2007; 132: 1672-1683Google Scholar Additional observations regarding leukocyte adhesion in human IBD have demonstrated enhanced and altered patterns of leukocyte binding, where naïve leukocytes are preferentially recruited into the IBD microvasculature, which is exposed to chronic inflammatory stress in vivo.6Salmi M. Granfors K. MacDermott R. et al.Aberrant binding of lamina propria lymphocytes to vascular endothelium in inflammatory bowel diseases.Gastroenterology. 1994; 106: 596-605Crossref Scopus (111) Google Scholar, 7Burgio V.L. Fais S. Boirivant M. et al.Peripheral monocyte and naive T-cell recruitment and activation in Crohn's disease.Gastroenterology. 1995; 109: 1029-1038Google Scholar The ability to isolate human intestinal microvascular endothelial cells (HIMEC) has furthered our understanding of specific endothelial contribution to chronic inflammation in IBD.8Haraldsen G. Rugtveit J. Kvale D. et al.Isolation and longterm culture of human intestinal microvascular endothelial cells.Gut. 1995; 37: 225-234Google Scholar, 9Ogawa H. Rafiee P. Fisher P.J. et al.Sodium butyrate inhibits angiogenesis of human intestinal microvascular endothelial cells through COX-2 inhibition.FEBS Lett. 2003; 554: 88-94Google Scholar HIMEC have been isolated routinely from normal margins of control patient bowel as well as from involved and uninvolved segments of IBD, and expanded in vitro to allow for specific experimentation and a mechanistic examination of endothelial activation in IBD.10Binion D.G. West G.A. Ina K. et al.Enhanced leukocyte binding by intestinal microvascular endothelial cells in inflammatory bowel disease.Gastroenterology. 1997; 112: 1895-1907Abstract Full Text Full Text PDF Scopus (183) Google Scholar HIMEC isolated from both Crohn's disease and ulcerative colitis chronically inflamed bowel segments demonstrated an increased ability to bind leukocytes following inflammatory activation (interleukin-1β, tumor necrosis factor [TNF]-α, and lipopolysaccharide) compared with control HIMEC. This increase in leukocyte binding was an acquired phenomenon; endothelial cultures generated from paired segments of involved and uninvolved intestine demonstrated enhanced leukocyte binding only in areas exposed to chronic inflammation in vivo.11Binion D.G. West G.A. Volk E.E. et al.Acquired increase in leucocyte binding by intestinal microvascular endothelium in inflammatory bowel disease.Lancet. 1998; 352: 1742-1746Google Scholar This finding is all the more remarkable because the isolation strategies for human intestinal microvascular cultures can only generate small numbers of pure endothelial cells at the start of the culture, which will be passaged over the course of 6–8 weeks in vitro before having sufficient cell numbers for experimentation. Nevertheless, these disease-specific differences are maintained throughout the lifespan of the HIMEC culture, and have been present in multiple cell lines generated from multiple, unique patients over the past decade. In addition to their central role in leukocyte recruitment, angiogenesis has emerged as an additional vascular mechanism contributing to chronic inflammation in IBD.12Chidlow Jr, J.H. Langston W. Greer J.J. et al.Differential angiogenic regulation of experimental colitis.Am J Pathol. 2006; 169: 2014-2030Google Scholar, 13Chidlow Jr, J.H. Shukla D. Grisham M.B. et al.Pathogenic angiogenesis in IBD and experimental colitis: new ideas and therapeutic avenues.Am J Physiol Gastrointest Liver Physiol. 2007; 293: G5-G18Google Scholar, 14Danese S. Sans M. de la Motte C. et al.Angiogenesis as a novel component of inflammatory bowel disease pathogenesis.Gastroenterology. 2006; 130: 2060-2073Google Scholar The growth of new blood vessels in chronic intestinal inflammation has been always clinically apparent; surgeons operating on patients with Crohn's disease are guided by the neovascularization on the serosal surface of the bowel, which effectively demarcates "involved" segments of bowel. This neovascularization has been also readily apparent on imaging techniques, including microbubble enhanced ultrasound15Di Sabatino A. Ciccocioppo R. Armellini E. et al.Serum bFGF and VEGF correlate respectively with bowel wall thickness and intramural blood flow in Crohn's disease.Inflamm Bowel Dis. 2004; 10: 573-577Google Scholar as well as computed tomographic enteroclysis, where increased vascular perfusion is a hallmark feature of increased bowel inflammation.16Bodily K.D. Fletcher J.G. Solem C.A. et al.Crohn disease: mural attenuation and thickness at contrast-enhanced CT Enterography—correlation with endoscopic and histologic findings of inflammation.Radiology. 2006; 238: 505-516Google Scholar However, the increased blood supply does not automatically equate with adequate perfusion of the intestine in IBD. One of the potential confounding areas in our understanding of the role of the microvasculature in IBD has been a series of observations from both human and animal models of IBD that suggest microvascular dysfunction and impaired perfusion of the mucosal surfaces affected by chronic inflammation. Intraoperative perfusion studies,17Hulten L. Lindhagen J. Lundgren O. et al.Regional intestinal blood flow in ulcerative colitis and Crohn's disease.Gastroenterology. 1977; 72: 388-396Abstract Full Text PDF Scopus (130) Google Scholar endoscopic ultrasound,18Angerson W.J. Allison M.C. Baxter J.N. et al.Neoterminal ileal blood flow after ileocolonic resection for Crohn's disease.Gut. 1993; 34: 1531-1534Google Scholar and intraoperative ultrasound19Tateishi S. Arima S. Futami K. Assessment of blood flow in the small intestine by laser Doppler flowmetry: comparison of healthy small intestine and small intestine in Crohn's disease.J Gastroenterol. 1997; 32: 457-463Google Scholar of mucosal surfaces have all demonstrated impaired perfusion of the mucosal surfaces in chronically inflamed segments in Crohn's disease patients. This relative ischemia has also been observed in animal studies, where diminished perfusion at the level of the microcirculation has been demonstrated.20Mori M. Stokes K.Y. Vowinkel T.L. et al.Colonic blood flow responses in experimental colitis: time course and underlying mechanisms.Am J Physiol Gastrointest Liver Physiol. 2005; 289: G1024-G1029Google Scholar Hatoum et al21Hatoum O.A. Binion D.G. Otterson M.F. et al.Acquired microvascular dysfunction in inflammatory bowel disease: loss of nitric oxide-mediated vasodilation.Gastroenterology. 2003; 125: 58-69Google Scholar demonstrated in a series of ex vivo experiments on microvessels immediately isolated from surgically resected IBD bowel a profound loss of nitric oxide generation from the resistance arterioles, which have been exposed to chronic inflammation in vivo, thereby leading to microvascular dysfunction.21Hatoum O.A. Binion D.G. Otterson M.F. et al.Acquired microvascular dysfunction in inflammatory bowel disease: loss of nitric oxide-mediated vasodilation.Gastroenterology. 2003; 125: 58-69Google Scholar This corresponds with observations in IBD HIMEC that have demonstrated that nitric oxide generation is impaired, and excess and sustained oxyradical generation is a commonly encountered finding in IBD endothelial cells.22Binion D.G. Rafiee P. Ramanujam K.S. et al.Deficient iNOS in inflammatory bowel disease intestinal microvascular endothelial cells results in increased leukocyte adhesion.Free Radic Biol Med. 2000; 29: 881-888Google Scholar In addition to a loss of NOS2, there is increased expression of arginase II isoforms, which leads to a depletion of the substrate arginine, which is necessary for the generation of nitric oxide by the endothelium, and plays a central role in vasorelaxation.23Horowitz S. Binion D.G. Nelson V.M. et al.Increased arginase activity and endothelial dysfunction in human inflammatory bowel disease.Am J Physiol Gastrointest Liver Physiol. 2007; 292: G1323-G1336Google Scholar When one considers that tissue ischemia is one of the most potent stimuli for angiogenesis, then the increased vascularization on the outer surfaces of the bowel in conjunction with impaired perfusion at the mucosal surface owing to microvascular dysfunction may represent a compensatory process. Angiogenesis is now recognized to play a critical role in various human disease processes, including cancer and chronic inflammation.24Heidemann J. Ogawa H. Dwinell M.B. et al.Angiogenic effects of interleukin 8 (CXCL8) in human intestinal microvascular endothelial cells are mediated by CXCR2.J Biol Chem. 2003; 278: 8508-8515Google Scholar The initial reports of angiogenesis being a component of human and animal models of IBD appeared during the past decade.12Chidlow Jr, J.H. Langston W. Greer J.J. et al.Differential angiogenic regulation of experimental colitis.Am J Pathol. 2006; 169: 2014-2030Google Scholar, 13Chidlow Jr, J.H. Shukla D. Grisham M.B. et al.Pathogenic angiogenesis in IBD and experimental colitis: new ideas and therapeutic avenues.Am J Physiol Gastrointest Liver Physiol. 2007; 293: G5-G18Google Scholar, 14Danese S. Sans M. de la Motte C. et al.Angiogenesis as a novel component of inflammatory bowel disease pathogenesis.Gastroenterology. 2006; 130: 2060-2073Google Scholar Just how angiogenesis helps to perpetuate chronic intestinal inflammation is currently being defined. The concept that chronic inflammation requires an increased metabolic supply in the affected tissues, which must be provided by the augmented vascular supply, has formed one of the core concepts regarding angiogenesis and cancer, and may also be a component of angiogenesis in chronic inflammation. Likewise, the pathophysiologic neovascular beds arising from angiogenesis may contribute to altered leukocyte recruitment capacity for the chronically inflamed tissues. A third proposed mechanism regarding the contribution of angiogenesis to chronic inflammation is based on the fact that the endothelium is a dynamic cell population that can contribute to the milieu of cytokines and growth factors present in the chronically inflamed bowel.25Sans M. Danese S. de la Motte C. et al.Enhanced recruitment of CX3CR1+ T cells by mucosal endothelial cell-derived fractalkine in inflammatory bowel disease.Gastroenterology. 2007; 132: 139-153Google Scholar An additional mechanism that will be integrally linked with vascular endothelial growth factor (VEGF)-A–mediated angiogenesis has been explored by Scaldaferri et al26Scaldaferri F. Vetrano S. Sans M. et al.VEGF-A links angiogenesis and inflammatory bowel disease pathogenesis.Gastroenterology. 2009; 135: 585-595Google Scholar in the present paper. Here the authors characterize the potential for increased vessel permeability and vascular leak to be found in chronically inflamed intestine. Using Evans blue dye as a marker of vascular permeability, these investigators demonstrated that transgenic mice overexpressing this molecule had increased leak into the gut tissues at baseline, which was markedly increased after the induction of DSS-induced colitis. When we consider the edema in bowel wall that accompanies chronic gut inflammation, VEGF-A–enhanced permeability will likely play a role in the "stiffness" that accompanies the remodeling of bowel in IBD, particularly Crohn's disease.27Granger D.N. Barrowman J.A. Microcirculation of the alimentary tract. II Pathophysiology of edema.Gastroenterology. 1983; 84: 1035-1049Scopus (44) Google Scholar This increase in interstitial fluid contributes to impaired perfusion dynamics, and potentially underlies the relative ischemia that accompanies chronic bowel inflammation. The contribution of hypoxia-inducible factors have recently received significant attention, and is likely a direct result of impaired perfusion secondary to VEGF-A–induced vascular leak.28Robinson A. Keely S. Karhausen J. et al.Mucosal protection by hypoxia-inducible factor prolyl hydroxylase inhibition.Gastroenterology. 2008; 134: 145-155Abstract Full Text Full Text PDF Scopus (300) Google Scholar, 29Taylor C.T. Colgan S.P. Hypoxia and gastrointestinal disease.J Mol Med. 2007; 85: 1295-1300Google Scholar The role of VEGF-A in IBD pathogenesis and inflammation induced angiogenesis is strengthened and defined in the present work by Scaldaferri et al.26Scaldaferri F. Vetrano S. Sans M. et al.VEGF-A links angiogenesis and inflammatory bowel disease pathogenesis.Gastroenterology. 2009; 135: 585-595Google Scholar These authors show definitively that this angiogenic growth factor is increased in IBD patient tissues, mediates angiogenesis in the relevant population of endothelial cells (HIMEC), and plays a role in leukocyte recruitment through up-regulation of intercellular adhesion molecule 1, in addition to its capacity to increase vascular permeability. Importantly, they also demonstrated that the VEGFR2 seems to be the key receptor for VEGF-A in both human IBD and animal models, which paves the way for future therapies targeting this ligand in the treatment of chronic gut inflammation. Sophisticated rodent models with targeted gene deletion in combination with DSS-induced colitis have allowed Chidlow et al30Chidlow J.H. Greer J.J.M. Anthoni C. et al.Endothelial caveolin-1 regulates pathologic angiogenesis in a mouse model of colitis.Gastroenterology. 2009; 135: 575-584Google Scholar to explore the fundamental mechanisms of endothelial signaling and angiogenesis in response to chronic intestinal inflammation. These authors have examined the function of caveolin-1, the major structural protein found in endothelial caveolae, in angiogenesis induced by chronic inflammation. In an elegant series of experiments using sophisticated rodent models of gene-deleted caveolin-1, they demonstrate that loss of this molecule in the endothelium prevents angiogenesis in the context of DSS colitis, resulting in a diminished chronic inflammation and less disease severity. Bone marrow transplant experiments confirmed that the expression of caveolin-1 must be present in the gut endothelium for angiogenesis to occur in the setting of chronic inflammation. Furthermore, these authors demonstrate that inhibition of angiogenesis significantly ameliorates chronic inflammatory damage to the bowel, linking angiogenesis with severity of the disease process. Although these authors did not specifically measure VEGF-A levels in their models, we can surmise from prior work, and the current studies by Scaldaferri et al, that the chronic inflammatory process would markedly increase this angiogenic factor. Thus, the endothelium must maintain the molecular mechanisms to respond to these signals, ultimately leading to the angiogenic process. Our current IBD treatment approaches do not routinely target angiogenesis for therapy. To date, perhaps the best example of an antiangiogenic therapeutic trial in Crohn's disease has been a series of reports describing the beneficial effect of thalidomide in patients with refractory disease.31Ehrenpreis E.D. Kane S.V. Cohen L.B. et al.Thalidomide therapy for patients with refractory Crohn's disease: an open-label trial.Gastroenterology. 1999; 117: 1271-1277Google Scholar, 32Vasiliauskas E.A. Kam L.Y. Abreu-Martin M.T. et al.An open-label pilot study of low-dose thalidomide in chronically active, steroid-dependent Crohn's disease.Gastroenterology. 1999; 117: 1278-1287Google Scholar In addition to its capacity to function as a transcriptional inhibitor of TNF-α, thalidomide has documented antiangiogenic potential, which is felt to have been a central component of its teratogenicity, leading to phocomelia, severe birth defects, and fetal demise.33D'Amato R.J. Loughnan M.S. Flynn E. et al.Thalidomide is an inhibitor of angiogenesis.Proc Natl Acad Sci U S A. 1994; 91: 4082-4085Google Scholar Although thalidomide returned for clinical use through an extremely careful prescribing program, the long-term utility of this agent as a maintenance treatment strategy for chronic inflammation remains limited by a side effect profile that includes significant neuropathy. Many other agents with antiangiogenic potential, including cyclo-oxygenase-2 antagonists,34Reuter B.K. Asfaha S. Buret A. et al.Exacerbation of inflammation-associated colonic injury in rat through inhibition of cyclooxygenase-2.J Clin Invest. 1996; 98: 2076-2085Google Scholar interferon-α,35Heidemann J. Oqawa H. Otterson M.F. et al.Antiangiogenic treatment of mesenteric desmoid tumors with toremifene and interferon alfa-2b: report of two cases.Dis Colon Rectum. 2004; 47: 118-122Google Scholar and radiation therapy,36Willett C.G. Ooi C.J. Sietman A.L. et al.Acute and late toxicity of patients with inflammatory bowel disease undergoing irradiation for abdominal and pelvic neoplasms.Int J Radiat Oncol Biol Phys. 2000; 46: 995-998Google Scholar are potentially deleterious to IBD patients or pose significant side effects, which have limited exploration of antiangiogenic strategies for chronic inflammation at the present time.37Heidemann J. Binion D.G. Domschke W. et al.Antiangiogenic therapy in human gastrointestinal malignancies.Gut. 2006; 55: 1497-1511Google Scholar Over the past decade, antiangiogenic therapy has become a reality in the treatment of metastatic tumors. The emergence of antiangiogenic therapy in cancer treatment is a direct legacy of the pioneering investigation of Judah Folkman, where anti-VEGF therapy with the compound bevacizumab, received FDA approval for the treatment of metastatic colorectal adenocarcinoma.38Hurwitz H. Fehrenbacher L. Novotny W. et al.Bevacizumab plus irinotecan, fluorouracil, and leucovorin for metastatic colorectal cancer.N Engl J Med. 2004; 350: 2335-2342Google Scholar Novel compounds targeting blood vessel growth are currently undergoing investigation. The natural compound curcumin, a component Ayurvedic medicine for centuries and a component of the spice turmeric, has antiangiogenic potential, demonstrating significant ability to inhibit the growth of HIMEC in vitro.39Binion D.G. Otterson M.F. Rafiee P. Curcumin inhibits VEGF-mediated angiogenesis in human intestinal microvascular endothelial cells through COX-2 and MAPK inhibition.Gut. 2008; 57: 1509-1517Google Scholar This mechanistic insight into curcumin's therapeutic potential is intriguing; this compound has demonstrated efficacy in maintenance of UC remission in a multicenter, randomized, controlled trial.40Hanai H. Iida T. Takeuchi K. et al.Curcumin maintenance therapy for ulcerative colitis: randomized, multicenter, double-blind, placebo-controlled trial.Clin Gastroenterol Hepatol. 2006; 4: 1502-1506Google Scholar One of the important issues that needs to be considered regarding the evaluation of antiangiogenic therapy in the treatment of Crohn's disease and ulcerative colitis is the design of appropriate clinical trials. If tissue remodeling and blood vessel growth are slow and gradual processes, then inhibiting blood vessel growth and reversing pathologic angiogenesis will likewise represent a slow process. Trial endpoints evaluating long-term outcome and reversal of structural remodeling of the chronically inflamed bowel may need to emerge to assess these agents and their potential efficacy. In summary, our understanding of the contribution of the gut microvasculature to the process of chronic inflammation in IBD continues to expand. The gut microvascular endothelium, once considered an inert tube in which leukocytes would travel into the intestine, is now gaining importance as a critical cellular component that serves to initiate and perpetuate inflammation. Leukocyte recruitment through endothelial interaction has emerged as a therapeutic reality in the treatment of Crohn's disease and is undergoing evaluation for the treatment of UC. The 2 papers by Chidlow et al30Chidlow J.H. Greer J.J.M. Anthoni C. et al.Endothelial caveolin-1 regulates pathologic angiogenesis in a mouse model of colitis.Gastroenterology. 2009; 135: 575-584Google Scholar and Scaldaferri et al26Scaldaferri F. Vetrano S. Sans M. et al.VEGF-A links angiogenesis and inflammatory bowel disease pathogenesis.Gastroenterology. 2009; 135: 585-595Google Scholar in this issue of Gastroenterology highlight an additional component of the vascular biology of IBD, angiogenesis. These studies demonstrating the therapeutic potential of antiangiogenic strategies provide further evidence for new treatment strategies that target neovascularization and tissue remodeling in hopes of maximizing treatment responses in our patients with refractory IBD. Endothelial Caveolin-1 Regulates Pathologic Angiogenesis in a Mouse Model of ColitisGastroenterologyVol. 136Issue 2PreviewIncreased vascular density has been associated with progression of human inflammatory bowel diseases (IBDs) and animal models of colitis. Pathologic angiogenesis in chronically inflamed tissues is mediated by several factors that are regulated at specialized lipid rafts known as caveolae. Caveolin-1 (Cav-1), the major structural protein of caveolae in endothelial cells, is involved in the regulation of angiogenesis, so we investigated its role in experimental colitis. Full-Text PDF VEGF-A Links Angiogenesis and Inflammation in Inflammatory Bowel Disease PathogenesisGastroenterologyVol. 136Issue 2PreviewVascular endothelial growth factor A (VEGF-A) mediates angiogenesis and might also have a role in inflammation and immunity. We examined whether VEGF-A signaling has a role in inflammatory bowel disease (IBD). Full-Text PDF