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.
Gal
A
A
Ludwig Institute for Cancer Research, Uppsala, Sweden.
Sjöblom
T
T
Fedorova
L
L
Imreh
S
S
Beug
H
H
Moustakas
A
A
D000818:Animals; D017209:Apoptosis / Q000235:genetics; D016466:CHO Cells; D018929:Cell Culture Techniques; D002460:Cell Line; D002471:Cell Transformation, Neoplastic / Q000378:metabolism / Q000473:pathology*; D006224:Cricetinae; D003412:Cricetulus; D004847:Epithelial Cells / Q000166:cytology / Q000378:metabolism / Q000473:pathology*; D005260:Female; D016147:Genes, Tumor Suppressor / Q000502:physiology; D008321:Mammary Glands, Animal / Q000378:metabolism / Q000473:pathology*; D008648:Mesoderm / Q000166:cytology / Q000378:metabolism / Q000473:pathology*; D051379:Mice; D015398:Signal Transduction / Q000502:physiology*; D051785:Smad Proteins / Q000037:antagonists & inhibitors* / Q000502:physiology; D016212:Transforming Growth Factor beta / Q000235:genetics / Q000502:physiology*
D051785:Smad Proteins; D016212:Transforming Growth Factor beta