Sustained TGF beta exposure suppresses Smad and non-Smad signalling in mammary epithelial cells, leading to EMT and inhibition of growth arrest and apoptosis

• To better understand the dual, tumour-suppressive and tumour-promoting function of transforming growth factor-beta (TGFbeta), we analysed mammary epithelial NMuMG cells in response to short and long-term TGFbeta exposure. NMuMG cells became proliferation-arrested and apoptotic after exposure to TGFbeta for 2-5 days, whereas surviving cells underwent epithelial-mesenchymal transition (EMT). After chronic TGFbeta 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 TGFbeta-receptor-I kinase inhibitor or withdrawal of TGFbeta 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, TGFbeta-dependent downregulation of these signalling pathways in transdifferentiated (TD) cells was fully reversed upon TGFbeta 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 TGFbeta1 (CHO-TGFbeta1) allowed outgrowth of epithelioid cells in CHO-TGFbeta1 cell-induced tumours. These epithelial islands enhanced CHO-TGFbeta1 tumour cell proliferation, possibly due to chemokines (for example, JE/MCP-1) secreted by NMuMG/TD cells. We conclude that suppression of antiproliferative, proapoptotic TGFbeta signalling in TD cells may permit TGFbeta-dependent proliferation, survival and EMT-enhancing signalling pathways to act at low levels. Thus, TGFbeta may modulate its own signalling to facilitate switching from tumour suppression to tumour progression.

Nyckelord

epithelial–mesenchymal transition

metastasis

Smad

signal transduction

TGFb

tumour suppression

MEDICINE

MEDICIN

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