Ectopic TGFβ1 Expression in the Secretory Mammary Epithelium Induces Early Senescence of the Epithelial Stem Cell Population

Abstract

An important feature of the mammary gland is the regenerative capacity of its epithelium which is demonstrated upon successive cycles of lactation and involution. Pregnant mice expressing a whey-acidic protein (WAP) promoter-driven transforming growth factor-beta 1 (TGF beta 1) cDNA are unable either to generate a secretory mammary epithelium or to lactate. Here we investigate whether ectopic TGF beta 1 induces this phenotype by affecting the transgenic epithelium directly or in trans. Reciprocal transplantation of mammary tissue between normal and transgenic hosts resulted in the development of the respective phenotypes of the transplants within the same mammary fat pad. When isolated mammary epithelial cells from both were mixed before implantation so that transgenic and normal epithelium would develop together more proximately, both phenotypes were simultaneously observed in the resultant chimeric mammary outgrowths. Since no trans effect was detectable, we hypothesize that early expression of the transgene results in compromised lobular progenitor cells through an intracrine mechanism. Consistent with this posit, WAP promoter-driven protein expression was detected in individual cells of the subtending ducts of immature females at estrus. Transplantation of WAP-TGF beta 1 mammary gland into nonpregnant hosts revealed that transgenic implants, even those from young postpubertal virgin females, had a diminished ability to repopulate epithelium-free mammary fat pads. Accordingly, the ectopic expression of WAP-TGF beta 1 not only impairs lobular progenitors, but also promotes an early senescence of the regenerative capacity of the mammary ductal epithelium. This leads us to propose that mammary epithelial stem cells give rise to two functionally distinct progenitor cells in the mammary gland epithelium: one capable of producing daughters committed to ductal formation, the other capable only of producing daughters committed to lobular function.

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

“…5 The ratio of apoptotic nuclei (number of apoptotic cells/total number of tumor cells) was significantly higher in transgenic than in non-transgenic tumors: 1.74% vs. 0.93% (265/15,241 vs. 135/14,450; p Ͻ 0.0001, 2 test). This difference is very similar to what has been previously reported in WAP-TGF␤1 glands at multiple stages of development and differentiation; 5 and it is consistent with the conclusion that tumor cells do exhibit a secretory phenotype as suggested by the expression of milk proteins.…”

“…Subsequent transplantation studies demonstrated that this phenotype was intrinsic to the transgenic epithelium expressing WAPdriven TGF␤1 and that the effect was cis in nature, i.e., autocrine rather than paracrine. 5 Further analysis of the defect in lobular development in the WAP-TGF␤1 model revealed an increased rate of alveolar cell apoptosis in the transgenic glands. Alveolar cell mitosis, however, also continued at an increased rate over that observed in the normal control glands throughout pregnancy suggesting that autocrine TGF␤1 did not inhibit the proliferation of alveolar mammary epithelial cells.…”

“…Another consequence of the WAP-TGF␤1 transgene expression was premature senescence of the mammary specific epithelial stem cells manifested in the significant decreased capacity of WAP-TGF␤1 mammary transplants to grow sufficiently to repopulate a cleared normal fat pad. 5 In neither WAP nor MMTV driven TGF␤1 transgenic animals, mammary carcinomas have been reported. 4,6 In particular, no mammary tumors were observed in 20 WAP-TGF␤1 females maintained as breeders in our colony for over 2 years.…”

“…Alveolar cell mitosis, however, also continued at an increased rate over that observed in the normal control glands throughout pregnancy suggesting that autocrine TGF␤1 did not inhibit the proliferation of alveolar mammary epithelial cells. 5 Thus, overexpression of TGF␤1 in differentiating secretory mammary epithelial cells leads to premature programmed cell death. Another consequence of the WAP-TGF␤1 transgene expression was premature senescence of the mammary specific epithelial stem cells manifested in the significant decreased capacity of WAP-TGF␤1 mammary transplants to grow sufficiently to repopulate a cleared normal fat pad.…”

Overexpression of Bcl-2 inhibits alveolar cell apoptosis during involution and accelerates c-myc-induced tumorigenesis of the mammary gland in transgenic mice

“…SGP-2, for example, is not only induced during mammary gland involution but also during regression of the prostate upon androgenablation and in di erent tumor cell lines induced to undergo apoptosis (Wong et al, 1993(Wong et al, , 1994. Overexpression of TGF-b1 and p53, which are both induced during involution, led to apoptosis of mammary alveolar cells in transgenic mice (Kordon et al, 1995;Li et al, 1996a), pointing towards a functional role of these molecules in involution-associated apoptosis as well. In the mammary glands of WAP-bcl-2 transgenic mice the expression of these genes was not dramatically altered as compared to wildtype mice, indicating that Bcl-2 does not act via modulation of their expression.…”