Fibrinogen Supports Colitis-Associated Adenoma Progression through a Mechanism Coupled to Engagement of the Leukocyte Integrin α_Mβ₂.

Abstract 334

A growing body of evidence points to a crucial role for fibrinogen in both tumor dissemination and the regulation of inflammation. Given this dual importance, we hypothesized that fibrinogen plays an important role in the progression of inflammation-driven cancers. To test this hypothesis, we induced colitis-associated cancer (CAC) in fibrinogen-deficient mice and controls using a combination of azoxymethane (AOM) and dextran sodium sulfate (DSS). Fibrinogen deficiency resulted in a dramatic diminution in the number of adenomas formed after AOM/DSS challenge in spite of the fact that overt gastrointestinal bleeding appeared more severe in Fib^- animals relative to controls. These results suggest that while fibrin deposition is crucial for the maintenance of vascular integrity and control of blood loss in colitis, fibrin(ogen) appears to drive colitis-associated adenoma development/outgrowth. Fibrin(ogen) could promote CAC through multiple mechanisms. One intriguing possibility is that fibrin(ogen) interactions with leukocytes through the integrin α_Mβ₂ are an important determinant of inflammation-induced tumor progression. To test this hypothesis, we explored CAC development in control mice and animals expressing a mutant form of fibrinogen (Fibγ^390-396A) that maintains full clotting function and supports thrombus formation normally in vivo, but does not support binding to α_Mβ₂. Consistent with our hypothesis, Fibγ^390-396A mice developed significantly fewer adenomas after AOM/DSS challenge. In fact, the majority of Fibγ^390-396A mice had no discernable adenomas while the phenotypic penetrance of adenoma development was 100% in control animals. Detailed analyses revealed that one mechanism coupling fibrin(ogen)-mediated α_Mβ₂ engagement to adenoma formation/outgrowth is by supporting inflammatory events during the antecedent colitis. Fibγ^390-396A mice manifested a dramatic diminution in inflammatory cell infiltrates, colonic edema, crypt loss and epithelial ulceration relative to control mice after chronic DSS exposure. Analyses of short-term DSS exposure revealed an ∼10-fold diminution in Fibγ^390-396A mice relative to controls in the elaboration of key cytokines known to promote CAC progression (i.e., IL-6, TNF-α, IL-1β, IFN-γ). Previous studies suggest that these cytokines promote CAC, at least in part, through changes in epithelial cell function. Consistent with this view, the number of colonic epithelial cells staining positive for established activation markers (i.e., phosphorylated cJun and RelA/p65) were significantly diminished in Fibγ^390-396A animals relative to control animals after short-term DSS exposure. Taken together, these data strongly suggest that one mechanism coupling fibrin(ogen) to adenoma progression is through α_Mβ₂-mediated proinflammatory events early in the disease process which lead to epithelial damage/turnover important in adenoma formation. However, the role of fibrin(ogen) in adenoma progression does not appear to be limited to these early inflammatory events. The adenomas harvested from Fibγ^390-396A mice were significantly smaller than those from control mice and less proliferative based on mitotic indices, suggesting an additional role for fibrin(ogen) in the growth of established adenomas. These studies demonstrate, for the first time, a unique link between fibrin(ogen) and the development of inflammation-driven malignancy. Given the importance of antecedent inflammation in the progression of numerous cancers, these studies suggest that therapies targeting fibrin(ogen)-α_Mβ₂ interactions may be useful in preventing and/or treating this important subset of malignancies.