Fig. 6.

Notch signaling is active in mouse and human tumors. (A) Real-time qPCR was used to quantify the expression levels of Hes1 mRNA in mouse samples. Hes1 expression is elevated in adenomas derived from the small intestine of Apc^+/− mice to levels comparable with those found in N/Apc mice (compare the bar of Apc^+/− adenomas with the bars corresponding to N/Apc normal intestine and adenomas). Expression of Hes1 in the normal intestine of Apc^+/− mice is restricted to the crypt proliferative compartment, as illustrated in Fig. 5A. The standard deviation represents the variability between at least 5 different mice. Each sample was normalized to β₂-microglobulin. (B–E) Real-time qPCR reveals that the Notch targets Hes1 (B) and HeyL (C) and the Notch ligand Jagged2 (E) are significantly up-regulated in human adenomas (n = 7) compared with human adenocarcinomas (n = 9). The levels of Notch1 mRNA (D) are also elevated in adenomas versus carcinomas, albeit to a lesser extent. The black horizontal lines represent the median value for each group of individual tumors. Each sample was normalized to β-actin (black bars), hypoxyanthine-guanine phosphoribosyltransferase (dark gray bars), and glyceraldehyde-3-phosphate dehydrogenase (light gray bars). (F) Schematic diagram illustrating the synergy of Notch and Wnt signals during intestinal tumor initiation. Activation of Notch signaling may occur before (Model 1), after (Model 2), or concomitantly (Model 3) with the occurrence of Apc mutations leading to constitutive activation of the Wnt cascade. Our results, however, favor Model 1 because in mice where Notch activation precedes LOH for Apc (N/Apc mice) we observe ≈20-fold more adenomas than in Apc^+/− mice.