p5325,26 efficiently suppresses tumorigenesis in transplanted transformed MEF fibrosarcomas through the induction of apoptosis. (A) p5325,26 restricts tumor growth. In this and subsequent panels of this figure, MEFs expressed E1A, H-RasV12, and no p53 (p53LSL-25,26/LSL-25,26 + Ad-empty), p53wt (p53+/+ + Ad-empty or p53+/+ + Ad-Cre), or p5325,26 (p53LSL-25,26/LSL-25,26 + Ad-Cre). E1A-Ras MEFs of various p53 genotypes were injected into the flanks of Scid mice, and average tumor volume was monitored for 16 d. The number of tumors analyzed is indicated. Error bars show the SD. Photograph shows a representative example of tumors of each genotype at day 16. (B) A selective pressure exists for cells lacking p5325,26 expression during E1A-Ras tumor growth in vivo. (Left) Representative images showing that p5325,26 is expressed in greater than 96% of E1A-Ras MEFs before injection into mice, with DAPI as a nuclear stain. (Right) Representative example of p53 immunofluorescence staining of E1A-Ras p5325,26 tumors. DAPI stain shows that the observed field is uniformly filled with tumor cells. (C) TUNEL staining of histological sections from tumors. (Left) Representative example showing TUNEL+ cells staining brown, with hematoxylin as a counterstain. (Right) Quantification of TUNEL staining. The number of TUNEL+ cells in three 40× fields for each of three to five tumors of each genotype was averaged and graphed ±SEM. *P < 0.03 (Student's t test). (D) p5325,26 does not affect proliferation in vivo. E1A-Ras MEF tumors expressing no p53, p53wt, or p5325,26 were stained for Ki67, with DAPI as a nuclear stain. Shown are representative pictures from multiple experiments. (E) p5325,26 does not inhibit proliferation in vitro. E1A-Ras MEFs expressing no p53, p53wt, or p5325,26 were pulsed for 4 h with BrdU and immunostained for p53 and BrdU. Graph shows the average BrdU-labeling index in three independent experiments, ±SD. Efficient Cre-mediated recombination of the LSL-25,26 allele was confirmed by immunofluorescence staining for p53, with over 94% of the cells expressing p5325,26. For C and D, due to the outgrowth of p53-deficient cells in the p5325,26 tumors, we costained tumors for p53 and either TUNEL or Ki67 and analyzed only regions with homogenous p5325,26 expression.