1. ¹Ludwig Institute for Cancer Research, Uppsala, Sweden
2. ²Research Institute of Molecular Pathology, Vienna, Austria
3. ³Microbiology and Tumour Biology Center, Karolinska Institute, Stockholm, Sweden

Correspondence: Dr H Beug, Research Institute of Molecular Pathology, Dr Bohr-Gasse 7, A-1030 Vienna, Austria. E-mail: beug@imp.univie.ac.at; Dr A Moustakas, TGF signaling group, Ludwig Institute for Cancer Research, Biomedical Center, Husargartan 3, Box 595, SE-75124 Uppsala, Uppland, Sweden. E-mail: aris.moustakas@LICR.uu.se

⁴Current address: Department of Genetics and Pathology, Uppsala University Rudbeck Laboratory, SE-75185 Uppsala, Sweden.

⁵These authors contributed equally to this work.

Received 7 November 2006; Revised 11 July 2007; Accepted 24 July 2007; Published online 27 August 2007.

Abstract

To better understand the dual, tumour-suppressive and tumour-promoting function of transforming growth factor- (TGF), we analysed mammary epithelial NMuMG cells in response to short and long-term TGF exposure. NMuMG cells became proliferation-arrested and apoptotic after exposure to TGF for 2–5 days, whereas surviving cells underwent epithelial–mesenchymal transition (EMT). After chronic TGF exposure (2–3 weeks), however, NMuMG cells became resistant to proliferation arrest and apoptosis, showing sustained EMT instead (TD cells). EMT was fully reversed by a pharmacologic TGF-receptor-I kinase inhibitor or withdrawal of TGF for 6–12 days. Interestingly, both cell cycle arresting/proapoptotic (Smads, p38 kinase) and antiapoptotic, proliferation and EMT-promoting signalling pathways (PI3K–PKB/Akt, ERK) were co-suppressed to low, but significant levels. Except for PI3K-Akt, TGF-dependent downregulation of these signalling pathways in transdifferentiated (TD) cells was fully reversed upon TGF withdrawal, together with partial re-induction of proliferation arrest and apoptosis. Co-injection of non-tumorigenic NMuMG cells with tumour-forming CHO cells oversecreting exogenous TGF1 (CHO-TGF1) allowed outgrowth of epithelioid cells in CHO-TGF1 cell-induced tumours. These epithelial islands enhanced CHO-TGF1 tumour cell proliferation, possibly due to chemokines (for example, JE/MCP-1) secreted by NMuMG/TD cells. We conclude that suppression of antiproliferative, proapoptotic TGF signalling in TD cells may permit TGF-dependent proliferation, survival and EMT-enhancing signalling pathways to act at low levels. Thus, TGF may modulate its own signalling to facilitate switching from tumour suppression to tumour progression.