FIG. 5.

Activation of Myc in intestinal epithelium of villin-MycER^TAM mice triggers the ARF/p53/p21^cip1 tumor suppressor pathway. (A) Real-time quantitative PCR analysis of ARF (p19^ARF mRNA) expression in intestinal epithelia of nontransgenic control mice treated with tamoxifen (tam) versus that in villin-MycER^TAM mice treated with the vehicle control (oil) or tamoxifen for 3 days (tam 3d). Myc activation triggers strong induction of ARF, which is lost upon MycER^TAM deactivation for 3 days (Off 3d). (B) Immunofluorescence staining for p53, p21^cip1, and phospho-H2.AX (in green, Hoechst nuclear counterstain in red) in intestinal epithelia of villin-MycER^TAM mice treated with either the vehicle control or tamoxifen for 3 days. Myc activation rapidly induced accumulation of both p53 and p21^cip1. In contrast, anti-phospho-H2.AX staining (a marker of DNA damage) showed only very occasional foci following Myc activation (yellow arrowheads) that may be attributable merely to normal mitosis (14). Hence, widespread p53 and p21^cip1 induction occurs without concomitant evidence of widespread DNA damage.