To determine whether a functional type II receptor of transforming growth factor ␤ (TGF-␤) is required to mediate the growth inhibitory effect of TGF-␤ on the skin in vivo, we have generated transgenic mice that overexpress a dominant negative-type II TGF-␤ receptor (⌬␤RII) in the epidermis. The ⌬␤RII mice exhibited a thickened and wrinkled skin, and histologically the epidermis was markedly hyperplastic and hyperkeratotic. In vivo labeling with BrdUrd showed a 2.5-fold increase in the labeling index over controls, with labeled nuclei occurring in both basal and suprabasal cells of transgenic epidermis. In heterozygotes, this skin phenotype gradually diminished, and by 10-14 days after birth the transgenic mice were indistinguishable from their normal siblings. However, when F 1 mice were mated to homozygosity, perinatal lethality occurred due to the severe hyperkeratotic phenotype, which restricted movement. Cultured primary keratinocytes from ⌬␤RII mice also exhibited an increased rate of growth in comparison with nontransgenic controls, and were resistant to TGF-␤-induced growth inhibition. These data document the role of the type II TGF-␤ receptor in mediating TGF-␤-induced growth inhibition of the epidermis in vivo and in maintenance of epidermal homeostasis.

To develop an in vivo model for studying the role of the p53 tumor suppressor in skin carcinogenesis, a murine p53 172H mutant (equivalent to human p53 175H ) was expressed in the epidermis of transgenic mice, utilizing a targeting vector based on the human keratin 1 gene (HK1.p53 m ). HK1.p53 m mice developed normally and did not exhibit an obvious epidermal phenotype or develop spontaneous tumors. However, these mice demonstrated an increased susceptibility to a two-stage chemical carcinogenesis protocol, with the rate of formation and number of papillomas being dramatically increased as compared to non-transgenic controls. The majority of papillomas in control mice regressed after termination of 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment, whereas p53 m papillomas progressed to carcinomas and metastases. In addition, more advanced malignancy, i.e., undierentiated spindle cell carcinomas, were exclusively observed in p53 m mice. Increased bromodeoxyuridine (BrdU) labeling, accompanied by decreased expression of p21, was observed in HK1.p53 m papillomas. In situ examination of centrosomes in HK1.p53 m papillomas also revealed marked abnormalities, with 75% of the cells containing 53 centrosomes/cell, whereas centrosome numbers in papillomas from control animals remained normal. These data suggest that the accelerated tumorigenesis observed in chemically-treated p53 m mice is most likely due to increased genomic instability resulting from an inhibition of G1 arrest and abnormal ampli®cation of centrosomes.

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.