Proliferation of Estrogen Receptor-α-Positive Mammary Epithelial Cells Is Restrained by Transforming Growth Factor-β1 in Adult Mice

Abstract

Transforming growth factor (TGF)-beta1 is a potent inhibitor of mammary epithelial proliferation. In human breast, estrogen receptor (ER)-alpha cells rarely co-localize with markers of proliferation, but their increased frequency correlates with breast cancer risk. To determine whether TGF-beta1 is necessary for the quiescence of ER-alpha-positive populations, we examined mouse mammary epithelial glands at estrus. Approximately 35% of epithelial cells showed TGF-beta1 activation, which co-localized with nuclear receptor-phosphorylated Smad 2/3, indicating that TGF-beta signaling is autocrine. Nuclear Smad co-localized with nuclear ER-alpha. To test whether TGF-beta inhibits proliferation, we examined genetically engineered mice with different levels of TGF-beta1. ER-alpha co-localization with markers of proliferation (ie, Ki-67 or bromodeoxyuridine) at estrus was significantly increased in the mammary glands of Tgf beta1 C57/bl/129SV heterozygote mice. This relationship was maintained after pregnancy but was absent at puberty. Conversely, mammary epithelial expression of constitutively active TGF-beta1 via the MMTV promoter suppressed proliferation of ER-alpha-positive cells. Thus, TGF-beta1 activation functionally restrains ER-alpha-positive cells from proliferating in adult mammary gland. Accordingly, we propose that TGF-beta1 dysregulation may promote proliferation of ER-alpha-positive cells associated with breast cancer risk in humans.

CITED1 homozygous null mice display aberrant pubertal mammary ductal morphogenesis

“…The evidence of Ewan et al (2002) is suggestive of a regulatory circuit where estrogen regulates the activation of TGFb, which in turn functionally restrains the proliferative response to hormonal signalling. This is supported by the evidence that steroid receptors (both ER and progesterone receptors) can be frequently colocalized in cells with active TGFb and that this population of cells is generally not proliferative (Zeps et al, 1998;Ewan et al, 2002Ewan et al, , 2005. Thus, we can suggest that estrogen controls cell proliferation by an indirect mechanism where ER positive cells, when exposed to hormonal stimuli, produce growth factors that stimulate the proliferation of adjacent (mainly ER negative) cells (Russo et al, 1999).…”

“…It has been proposed that estrogen and TGFb act in a regulatory circuit where estrogen modulates the activation of TGFb which in turn regulates the proliferative response to estrogen (Ewan et al, 2002(Ewan et al, , 2005. Following our identification of CITED1 among a group of genes whose expression was specifically upregulated in the pubertal mouse mammary gland, we hypothesized that CITED1 acted to mediate signals from both estrogen and TGFb in accordance with its previously elucidated mechanism of action.…”

“…Thus, we can suggest that estrogen controls cell proliferation by an indirect mechanism where ER positive cells, when exposed to hormonal stimuli, produce growth factors that stimulate the proliferation of adjacent (mainly ER negative) cells (Russo et al, 1999). The ER positive cells are proposed to not be proliferative as a result of the action of TGFb (Ewan et al, 2002(Ewan et al, , 2005. Exactly how TGFb may negatively influence the growth of ER positive cells is unknown but one possible mechanism is that of an inhibitory interaction between SMAD4 and the ER as it has recently been shown that SMAD4 acts as a transcriptional corepressor for ER (Wu et al, 2003).…”

RETRACTED ARTICLE: Regulation of estrogen receptor signaling in breast carcinogenesis and breast cancer therapy

“…In order to explore the relationship between proliferation and both ERα and TGF-β signaling, ewan and colleagues sought to determine their expression in mouse mammary glands at estrus, and discovered co-localization with phosphorylated Smads and nuclear ERα. This indicated a co-regulation of the pathways and suggested that TGF-β could act to restrict ERα-mediated proliferation [230]. Furthermore, the proliferation rate of mammary gland cells was significantly increased in a mouse model of heterozygous TGF-β1 expression and proliferation marker Ki67 expressing cells frequently coexpressed ERα.…”

Progesterone signalling in breast cancer: a neglected hormone coming into the limelight

“…This dissociation of proliferation and HR expression was subsequently demonstrated in vivo, both in mice and in rats 50,51 , and this led to the suggestion that HR + cells function as organizers that may instruct their neighbours. The low propensity of HR + cells to proliferate has been linked to the presence in the subjacent extracellular matrix (ECM) of processed transforming growth factor-β (TGFβ) that activates TGFβ receptor (TGFβR) signalling, leading to the accumulation of phosphorylated SMAD2 and SMAD3 in the nucleus 52 .…”