Fibrinogen Activates FAK to Promote the Colorectal Adenocarcinoma Growth

Along with other proteins in the coagulation cascade, including tissue factor and thrombin, fibrinogen has been shown to promote tumor metastasis across numerous cancer types. However, the role of fibrin(ogen) in primary tumor growth is context dependent and not universally important. One cancer strongly dependent on fibrin(ogen) for primary tumor growth is colorectal cancer (CRC), but the mechanisms by which fibrinogen supports colon cancer growth are not well understood. To delineate the mechanism of fibrin(ogen)-supported tumor growth, we implanted C57BL/6-derived murine colon cancer cells (MC38) into the dorsal subcutis of syngeneic mice with specific deletions/alteration in fibrinogen or FXIII-deficiency. Complete fibrinogen deficiency significantly limited the growth of colon cancer cells. However, the imposition of FXIII-deficiency or specific mutations in fibrinogen that 1) prevent the formation of fibrin polymer (Fib^AEK), limit fibrin(ogen) binding to the integrin α_Mβ₂ (Fibɣ^390-396A), or remove the ɣ chain binding motif for the platelet integrin α_IIbβ₃ (Fibɣ^Δ5) had no impact on colon cancer growth. To explore the mechanisms coupling fibrin(ogen) to colon cancer growth using a nonbiased approach, we performed RNA-Seq analyses of murine CRC tumors harvested from Fib+ and Fib- mice. We detected significant differences in the expression of 214 genes (128 downregulated and 86 upregulated) involved in cellular proliferation, survival, migration and metabolism. Notably, Stratifin (SFN), which encodes 14-3-3-σ, was one of the genes found to be highly upregulated in tumors from Fib- mice relative to Fib+ mice. 14-3-3-σ is a potent cell cycle regulator and it is also known to stabilize p53, which ultimately inhibits tumor growth. In a separate validation cohort, we observed significantly increased protein expression of 14-3-3-σ and its upstream and downstream targets (i.e., p53, p21 and p27) in tumors harvested from Fib- mice relative to controls. We also compared FAK activation, a key negative regulator of p53, in tumors from Fib+ and Fib- mice. FAK was inactive in tumors from Fib- mice, as indicated by lack of phosphorylation at tyrosine 397 (p-FAK Tyr397). Also, MDM2 was less phosphorylated at Ser166 in Fib- tumors, suggesting that p53 is not degraded by MDM2 in the absence of fibrinogen. Taken together, these data suggest that fibrin(ogen)-mediated downregulation of p53 and other targets via FAK activation and downregulation of 14-3-3-σ results in senescence of CRC cells. Consistent with this view, Ki67 positive nuclei were significantly less in tumor from Fib- mice relative to controls We also observed senescence-associated-β-galactosidase (SA-β-gal) activity in the tumors from Fib- mice, but not those from Fib+ animals. Furthermore, NMR-based metabolomics analyses demonstrated significantly less NAD⁺ and lactate levels in tumors from Fib- mice, which further confirms that fibrinogen deficiency hampers proliferation by inhibiting major metabolic pathways. In order to determine if fibrinogen-mediated support of CRC growth is tumor cell intrinsic, we compared MC38 growth in the 3D bioprinted matrices consisting of fibrin or albumin. MC38 cells showed a higher proliferation rate in the fibrinogen printed 3D environment compared to the albumin environment. These findings suggest that fibrin(ogen)-mediated engements of tumor cells activates FAK which inhibits p53 and its downstream targets including 14-3-3-σ and p21 which promote cellular proliferation and prevent senescence. Overall, these studies suggest that fibrin(ogen) is the important component of the CRC microenvironment and could be exploited for targeting and treating the CRC.