SV40 T/t-antigen induces premature mammary gland involution by apoptosis and selects for p53 missense mutation in mammary tumors

Abstract

We recently established transgenic animals (WAP-SV-T/ t) carrying the early coding region of Simian Virus 40 (SV40) under the transcriptional control of the whey acidic milk protein promoter (WAP), which restricts the expression of the transgene to mammary gland epithelial cells (ME-cells). SV40 T/t-antigen synthesis causes premature mammary gland involution during late pregnancy by inducing apoptosis and leads to development of mammary tumors after the ®rst lactation period in both p53 positive (WAP-SV-T/t) and p53 negative double transgenic animals (WAP-SV-T/t.p537/7). The high apoptotic rate persists in all of the T/t-antigen positive breast tumor cells, as well as in established MEtissue culture cell lines. ME-cells which spontaneously switch o the expression of the WAP-SV-T/t transgene do not undergo apoptosis. However, these cells again exhibit an extensive DNA fragmentation when SV40 T/ t-antigen synthesis is reintroduced, which indicates that it is the expression of T/t antigen which is the critical factor for induction of apoptosis. In addition, we isolated several ME-cell lines from di erent breast tumors which have spontaneously lost the T/t-antigen yet remain maximally transformed. Strikingly, these cells contain a missense mutation of the p53 gene at codon 242 (p53 242 ), which substitutes alanine for glycine. This mutation increases p53 stability and it reduces the transactivating function of p53, albeit without a ecting the ability of the protein to interact with the DNA. This indicates that p53 missense mutations are selected for in breast tumors initially expressing T/t-antigen. Therefore, the p53 242 mutation is su cient to maintain the transformed state after the ME-cells have switched o the WAP-SV-T/t transgene. Interestingly, the p53 minus state per se is not su cient to induce ME-cell transformation since homozygous null mice for the p53 gene (p537/7) fail to develop breast cancer.

HBX causes cyclin D1 overexpression and development of breast cancer in transgenic animals that are heterozygous for p53

“…Mouse ME-cells from normal and p53À/À animals have a very limited lifespan in vitro and it has not been possible to establish stable cell lines under standard tissue culture conditions (Dulbecco's modified Eagle's medium, DMEM, 10% fetal calf serum, FCS) to date (Tzeng et al, 1998). However, we demonstrate here that Breast tumor ME-cell apoptosis ME-cell immortalization…”

“…This confirmed that HBX does not increase the ME-cell death rate during pregnancy and lactation. Furthermore, DNA fragmentation, a hallmark of ME-cell apoptosis (Tzeng et al, 1998), could not be shown by TUNEL analysis (data not shown) or agarose gel electrophoresis, but was pronounced in the mammary glands of control WAP-SVT transgenic animals ( Figure 3B). Taken together, these experiments demonstrate that HBX, as the SV40 t-antigen, does not exert a spontaneous proapoptotic effect on ME-cells in monoor multiparous animals.…”

“…estrogen, progesterone, prolactin, insulin, hydrocortisone) and different tissue factors. After weaning the majority of ME-cells, which account for up to 25% of the animal body weight, are eliminated by apoptosis within a few days (Tzeng et al, 1998).…”

A transgenic mouse model for the ductal carcinoma in situ (DCIS) of the mammary gland

“…The common architectural forms of human DCIS, cribriform, papillary, and clinging type, were also frequently found in mouse DCIS, with and without comedo-type necrosis, whereas forms displaying a solid architecture appeared to be underrepresented in the mouse model. A reason for this observation could be the induction of apoptosis in malignant cells by T-Ag (Tzeng et al, 1998), preventing the development of duct ®lling solid tumor growth. Although the correlation between the morphology of a given in situ carcinoma presented by an individual transgenic mouse line and the prognosis for this line remains to be investigated, there are several hints that the lesions presented by the transgenic line WAP-T-NP8 are highly malignant.…”

New developments about the association of SV40 with human mesothelioma

“…There are multiple lines of evidence, in transgenic mice (Ewald et al, 1996;Tzeng et al, 1998), in rat glioblastoma (Salewski et al, 1999) and in human cells in vitro (Moorwood et al, 1996), indicating that SV40 can transform with a 'hit and run' type of mechanism.…”