Abstract
BACKGROUND: Delta like 4 (Dll4)/Notch signaling is a key regulator of tumor angiogenesis. Additionally, the role of Dll4 has been studied on tumor stem cells. However, as these cells are implicated in tumor angiogenesis, it is conceivable that the effect of Dll4 on these cells may be a consequence of its angiogenic function. Our aim was to evaluate the expression and dissect the functions of Dll4 in the Apc (Min/+) model of colorectal cancer. METHODS: We evaluated the protein expression pattern of Dll4 and other Notch members in the Apc (Min/+) tumors relatively to the normal gut and compared endothelial-specific with ubiquitous Dll4 knockout mice on an Apc (Min/+) background. RESULTS: All Notch pathway members were present in the normal small and large intestine and in the adenomas of the same regions. Dll4, all Notch receptors and Hes1 expression seemed upregulated in the tumors, with some regional differences. The same members and Hes5, instead of Hes1, presented ectopic expression in the tumor parenchyma. Dll4 expression was most pronounced in the tumor cells but it was also present in the tumor blood vessels and in other stromal cells. Ubiquitous and endothelial-specific Dll4 deletion led to an equivalent reduction of tumor growth because of a similarly marked tumoral angiogenic phenotype promoting non-productive vasculature and consequently hypoxia and apoptosis. The ubiquitous Dll4 inhibition led to a stronger decrease of tumor multiplicity than the endothelial-specific deletion by further reducing tumor proliferation and tumor stem cell density through upregulation of the cyclin-dependent kinase inhibitors 1C and 1B and downregulation of Myc, Cyclin D1 and D2 independently of β-catenin activation. This phenotype was associated to the observed increased epithelial differentiation deviated towards the secretory lineages by Atoh1 and Klf4 upregulation only in the ubiquitous Dll4 mutants. CONCLUSIONS: Dll4 seems to promote Apc (Min/+) tumorigenesis through both angiogenic and non-angiogenic related mechanisms.