The SV40 small t-antigen prevents mammary gland differentiation and induces breast cancer formation in transgenic mice; truncated large T-antigen molecules harboring the intact p53 and pRb binding region do not have this effect

Abstract

We report here for the ®rst time, that the SV40 small tantigen inhibits mammary gland dierentiation during mid-pregnancy and that about 10% of multiparous WAP-SVt transgenic animals develop breast tumors with latencies ranging from 10 ± 17 months. Cyclin D1 is deregulated and over expressed in the small t-antigen positive mammary gland epithelial cells (ME-cells) and in the breast tumor cells. SV40 small t-antigen immortalized ME-cells (t-ME-cells) exhibit a strong intranuclear cyclin D1 staining, also in the absence of external growth factors and the cells continue to divide for several days without serum. In addition, the expression rate of cyclin E and p21Waf1 but not of p53 is increased. Coimmunoprecipitation experiments revealed that p21Waf1 is mainly associated with the cyclin D/CDK4 but not with the cyclin E/CDK2 complex. WAP-SVT transgenic animals exhibit an almost regular mammary gland development until late pregnancy but the majority of the ME-cells are eliminated by apoptosis during the early lactation period. Tumor formation is delayed and less ecient than in T/tantigen positive animals. Sequestration of p53 and pRb by the N-terminal truncated T-antigen molecules (T1-antigen and T2-antigen) does not aect mammary gland dierentiation and the transgenic animals (WAP-SVBstBam) do not develop breast tumors. Oncogene (2001) 20, 2325 ± 2332.

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

“…However, the cells do not respond to differentiation signals and development of the regular lobulo-alveolar network of the mammary gland is inhibited. These animals show low levels of breast cancer (Goetz et al, 2001). In this investigation, we demonstrate that WAP-HBX transgenic animals synthesize HBX in the ME-cells.…”

“…This is in contrast to the SV40 T-antigen, which induces premature mammary gland involution during late pregnancy by increased apoptosis ( Figure 3A, c). The SV40 t-antigen inhibits mammary gland differentiation during early pregnancy (Goetz et al, 2001). However, as in the case of the HBX, it does not increase the rate of ME-cell death (data not shown).…”

“…The SV40 t-antigen also causes cyclin D1 upregulation in a growth factor-independent fashion in t-MEcells, as reported elsewhere (Goetz et al, 2001). To test whether cyclin D1 upregulation in these cells is also mediated by the stress-related MAP kinases, the inhibition experiments described above were repeated with the t-ME-cells.…”

“…Furthermore, WAP-HBX1 animals were crossed with p53À/À animals (Donehower et al, 1992) and a WAP-HBXl p53 þ /À mouse line was established. Construction of the WAP-SVt and WAP-SVT animals has been described elsewhere (Goetz et al, 2001). The accuracy of all constructs was confirmed by DNA sequencing with the Applied Biosystems Model 373 A utilizing dye terminators (Perkin-Elmer).…”

“…Thin sections of these segments were then subjected to histological examination. Furthermore, three kinds of control animals were investigated, namely nontransgenic NMRI mice, WAP-SVT and WAP-SVt transgenic animals, which selectively synthesize either the SV40 large T-antigen or the SV40 small t-antigen in ME-cells (Goetz et al, 2001). These experiments revealed that the WAP-HBX transgenic animals develop a well-defined lobulo-alveolar morphology with single layers of epithelial cells as seen in the NMRI control mice during pregnancy and early lactation ( Figure 3A, a, b).…”