Impairment of mammary lobular development induced by expression of TGF?1 under the control of WAP promoter does not suppress tumorigenesis in MMTV-infected transgenic mice

Abstract

It has previously been shown that transgenic female mice expressing TGF␤1 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-TGF␤1 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 TGF␤1 expression in the secretory mammary epithelium does not suppress mammary tumorigenesis in MMTV infected mice. Although MMTV infected WAP-TGF␤1 transgenic females displayed a strong impairment of lobule-alveolar development, carcinogenesis induced by any of the four MMTV variants used herein proceeded unabated. WAP-TGF␤1 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 TGF␤1. 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. © 2001 Wiley-Liss, Inc. Key words: mammary tumor; MMTV; TGF␤1; stem cellsTransforming growth factor-␤1 is a member of a large superfamily of polypeptides and seems to affect many key events in normal growth and differentiation. 1,2 There is good evidence that members of the TGF␤1 gene family may indeed contribute essential signals for the development and secretory maturation of the mammary epithelium. An approach to TGF␤ function and mammary gland development has been the use of transgenic mice. TGF␤1 has been targeted to the mammary epithelium from different transcriptional promoters in two transgenic mouse models. In one, the mouse mammary tumor virus-long terminal repeat (MMTV-LTR) was used to promote expression of TGF␤1 in mammary epithelium. 3 In this biological model, a transient but distinct inhibition of mammary ductal extension and side branching was observed between 3 and 6 weeks of age; however subsequent development during pregnancy and lactation proceeded apparently unperturbed. In the second transgenic mouse model, TGF␤1 was expressed under the control of the whey acidic protein (WAP) promoter. This promoter is mammary epithelium-specific and most active during late pregnancy and lactation. In this model no inhibition of mammary ductal growth at puberty was observed. 4 With the onset of pregnancy, lobule-alveolar growth and development were impaired to the extent that lactation was...

Reducing mammary cancer risk through premature stem cell senescence

“…Moreover, WAP promoter expression is restricted to individual luminal cells of the subtending ducts in cycling virgin mice as is never found in the terminal end bud, the site of glandular growth and extension. Our results and those of Buggiano et al (2001) show that MMTV can infect and also transform B + mammary epithelium. The distinction between our respective results is the frequency of transformation and progression to malignancy that occurs in B + females infected with MMTV later in mammary development as compared to those infected at birth.…”

“…In an earlier set of experiments, the consequence of impaired survival of di erentiating lobulo-alveolar cells in WAP-TGF-b1 females and susceptibilty to MMTVinduced mammary tumorigenesis was examined (Buggiano et al, 2001). In these studies, MMTV was introduced into WT and B + littermates at birth by foster-nursing on MMTV-infected mothers.…”

“…In all cases, MMTV-induced mammary tumorigenesis proceeded unabated in WAP-TGF-b1 mice (Buggiano et al, 2001). This result suggested that MMTV remained e ective as a tumorigenic agent, despite the severe reduction in alveolar development and increased alveolar cell apoptosis in pregnant WAP-TGF-b1 females (Kordon et al, 1995).…”

“…Tumorigenesis by MMTV is associated with the development of premalignant alveolar lesions (HAN) which subsequently develop into focal mammary tumors (DeOme et al, 1959). Buggiano et al (2001) introduced several di erent strains of MMTV into B + females to assess the tumorigenic response of their poorly developed mammary epithelium. There was no di erence in the susceptibility to infection with MMTV, tumor induction or ®nal tumor incidence in WAP-TGF-b1 (B + ) females compared to their wild type (WT) sisters.…”

The pleiotropic roles of transforming growth factor beta in homeostasis and carcinogenesis of endocrine organs

“…A number of studies have been conducted to determine the effects of TGF-b on mammary epithelial stem cell function (Robinson et al 1991, Kordon et al 1995, Boulanger & Smith 2001, Buggiano et al 2001. Consistent with its general growth inhibitory function, ectopic expression of constitutively active TGF-b1 under the WAP promoter was shown to lead to premature senescence of the mammary stem cell population as shown by a decrease in serial transplantation capacity and failure of the gland to transform into a lactating phenotype (Boulanger & Smith 2001).…”