Figure 4. Conditional pRB knockout mice are less sensitive to genotoxic stress.

(A) Analysis of the proximal small intestines of Rb^+/2lox;VillinCre⁺ (HET) and Rb^2lox/2lox;VillinCre⁺ (MUT) mice treated with vehicle control or doxo. Left panel: Immunostaining confirmed Cre-mediated loss of pRb in the proximal small intestine of MUT animals. Middle panel: Analysis of Ki67, a marker of proliferating cells, shows that pRb-loss caused proliferation throughout the villi. Ki67 levels were similar in the absence and presence of doxorubicin treatment. Right panel: Staining for cleaved caspase 3 showed high levels of apoptotic cells specifically in the base of the crypts in doxo treated, by not untreated, WT and MUT mice. Scale bar, 50μm for all panels. (B) The average number of apoptotic cells/intestinal crypt (± SE) in the proximal small intestines of doxo treated Rb^+/+;VillinCre⁺ (WT; n=5), Rb^+/2lox;VillinCre⁺ (HET; n=5), and Rb^2lox/2lox;VillinCre⁺ (MUT; n=6) mice was determined by counting cleaved caspase 3-positive cells in 21 crypts for each animal.