Impairment of mammary lobular development induced by expression of TGFbeta1 under the control of WAP promoter does not suppress tumorigenesis in MMTV-infected transgenic mice.

It has previously been shown that transgenic female mice expressing TGFbeta1 under control of regulatory elements of the whey-acidic protein (WAP) gene were unable to lactate. This was due to the increased apoptosis of the cells committed to the lobular-lactogenic phenotype. Our goal was to determine whether the expression of WAP-TGFbeta1 transgene could inhibit MMTV (mouse mammary tumor virus) tumorigenic activity in the mammary gland. It is well known that the infection with this virus produces focal hyperplastic secretory nodules (HANs) and, some variants can also induce ductal pregnancy-dependent lesions (plaques). In either case, MMTV infection leads ultimately to the appearance of malignant mammary tumors. The results shown herein demonstrate that TGFbeta1 expression in the secretory mammary epithelium does not suppress mammary tumorigenesis in MMTV infected mice. Although MMTV infected WAP-TGFbeta1 transgenic females displayed a strong impairment of lobule-alveolar development, carcinogenesis induced by any of the four MMTV variants used herein proceeded unabated. WAP-TGFbeta1 tumors that showed a strong expression at the WAP promoter, appeared later and grew more slowly than their wild-type counterparts. Transgenic females also had a lower incidence of HANs and plaques. Our study suggests that the epithelial target cells for tumorigenic mutations are probably progenitor cells that are not susceptible to the apoptotic effect of TGFbeta1. Alternatively, their daughters cells that display the secretory phenotype and could be more involved in the formation of premalignant lesions continue to die due to the expression of the transgene.