Conclusions
The key findings here are that: TGF-beta and Wnt5a regulate Wnt/beta-catenin activity; and loss of TGF-beta and Wnt5a redirect the phenotype of tumours so that they resemble tumours induced by activation of Wnt/beta-catenin. The findings suggest a new mechanism for the tumour-suppressive effects of TGF-beta.
Methods
Wnt/beta-catenin signalling was measured in tumours with altered TGF-beta (dominant-negative TGF-beta type II receptor, DNIIR) or Wnt5a (Wnt5a-/-) signalling as the accumulation of nuclear beta-catenin using both confocal microscopy and cell fractionation. RT-PCR was used to measure the expression of Wnt/beta-catenin target genes. Sca1 expression was determined by western blot and keratin (K) 6- and K14-positive populations were determined by immunohistochemistry.
Results
Loss of TGF-beta or Wnt5a signalling resulted in stabilisation of nuclear beta-catenin and expression of Wnt/beta-catenin target genes suggesting that TGF-beta and Wnt5a act to inhibit Wnt/beta-catenin signalling in mammary epithelium. Increased expression of Sca-1 was observed in developing DNIIR and Wnt5a-/- mammary glands. DNIIR and Wnt5a-/- tumours demonstrated an expanded population of K6- and K14-expressing cells typically seen in Wnt/beta-catenin-induced tumours. Conclusions: The key findings here are that: TGF-beta and Wnt5a regulate Wnt/beta-catenin activity; and loss of TGF-beta and Wnt5a redirect the phenotype of tumours so that they resemble tumours induced by activation of Wnt/beta-catenin. The findings suggest a new mechanism for the tumour-suppressive effects of TGF-beta.