We previously developed a transgenic mouse model that expresses in the epidermis a murine p53172R-->H mutant (p53m) under the control of a human keratin-1-based vector (HK1.p53m). In contrast to mice with wild-type p53 and p53-knockout mice, HK1.p53m mice exhibit increased susceptibility to chemical
carcinogenesis, with greatly accelerated benign
papilloma formation, malignant conversion, and
metastasis. In the study presented here, we examined the expression pattern of several
differentiation markers and observed that p53m
tumors exhibited a less differentiated phenotype than
tumors elicited in non-transgenic mice.
Metastasis in p53m
tumors was also associated with a poorly differentiated phenotype. To determine whether
genomic instability was associated with a putative gain-of-function role for this p53m, in situ examination of centrosomes was performed in HK1.p53m and equivalent p53-null
papillomas. In contrast to HK1.p53m
papillomas, which had centrosome abnormalities at high frequencies (75% of cells contained more than three centrosomes/cell), p53-null
tumors exhibited few abnormal centrosomes (4% of cells contained more than three centrosomes/cell). To determine whether angiogenesis played a role in the rapid progression of p53m
tumors, the expression of
vascular endothelial growth factor, a promoter of angiogenesis, and thrombospondin-1, an inhibitor of angiogenesis, was examined in
tumors derived from either p53m or p53-knockout mice. Regardless of their p53 status (wild type, p53m, p53-/-), all of the
papillomas exhibited similar levels of
vascular endothelial growth factor expression and decreased expression of thrombospondin-1 as did normal epidermis. In addition,
tumors from different p53 genotypes showed a similar density of blood vessels. Because p53 status did not appear to play an overt role in angiogenesis, these data suggest that p53m accelerates
tumorigenesis primarily by exerting a gain of function associated with
genomic instability.