Stromal and epithelial TGF-β signaling in carcinoma initiation and progression

Proc Amer Assoc Cancer Res, Volume 46, 2005 SY03-2 There is compelling evidence from transgenic mouse studies and analysis of mutations in human carcinomas indicating that the TGF-beta signal transduction pathway is tumor suppressive. We have shown that over expression of TGF-beta in mammary epithelial cells suppresses the development of carcinomas and that expression of a dominant negative type II TGF-beta receptor (DNIIR) in mammary epithelial cells under control of the MMTV promoter/enhancer increases the incidence of mammary carcinomas. Studies of human tumors have demonstrated inactivating mutations in human tumors of genes encoding proteins involved in TGF-beta signal transduction, including DPC4/Smad4, Smad 2, and the type II TGF-beta receptor (TBRII). There is also evidence that TGF-beta can enhance the progression of tumors. This hypothesis is being tested in genetically modified mice. To attain complete loss of TBRII, we have generated mice with loxP sites flanking exon 2 of Tgfbr2 and crossed them with mice expressing Cre recombinase under control of the MMTV promoter/enhancer to obtain Tgfbr2mgKO mice. These mice show mammary lobuloalveolar hyperplasia. Mice are being followed for mammary tumor development. Tgfbr2mgKO mice that also express polyoma virus middle T antigen under control of the MMTV promoter (MMTV-PyVmT) develop mammary tumors with a significantly shorter latency than MMTV-PyVmT mice and show a marked increase in pulmonary metastases. Our data do not support the hypothesis that TGF-beta signaling in mammary carcinoma cells is important for invasion and metastasis, at least in this model system. The importance of stromal-epithelial interactions in mammary gland development and tumorigenesis is well established. These interactions likely involve autocrine and paracrine action of multiple growth factors, including members of the TGF-beta family, which are expressed in both stroma and epithelium. Again, to accomplish complete knock-out of the type II TGF-beta receptor gene in mammary stromal cells FSP1-Cre and Tgfbr2flox/flox mice were crossed to attain Tgfbr2fspKO mice. The loss of TGF-beta responsiveness in fibroblasts resulted in intraepithelial neoplasia in prostate and invasive squamous cell carcinoma of the forestomach with high penetrance by six weeks of age. Both epithelial lesions were associated with an increased abundance of stromal cells. Activation of paracrine hepatocyte growth factor (HGF) signaling was identified as one possible mechanism for stimulation of epithelial proliferation. Tgfbr2fspKO mice also exhibit defective mammary ductal development, characterized in part by increased ductal epithelial cell turnover associated with an increase in stromal fibroblast abundance. Tgfbr2fspKO mammary fibroblasts transplanted with mammary carcinoma cells promote growth and invasion, which is associated with increased activating phosphorylation of the receptors: erbB1, erbB2, RON, and c-Met. Furthermore, the increased receptor phosphorylation correlates with increased secretion of the cognate ligands by Tgfbr2fspKO fibroblasts. Treatment of tumor cells with fibroblast conditioned medium leads to increased tumor cell proliferation and motility, which are blocked by addition of pharmacologic inhibitors of TGF-alpha signaling or neutralizing antibodies to MSP, HGF, or c-Met. These studies demonstrate a significant role for stromal TGF-beta signaling in mammary tissue homeostasis and mammary tumor progression via regulation of TGF-alpha MSP, and HGF signaling pathways. Thus, TGF-beta signaling in fibroblasts modulates the growth and oncogenic potential of adjacent epithelia in selected tissues. The Tgfbr2fspKO mouse model illustrates that a signaling pathway known to suppress cell-cycle progression when activated in epithelial cells can also have an indirect inhibitory effect on epithelial proliferation when activated in adjacent stromal fibroblasts in vivo. Loss of this inhibitory effect can result in increased epithelial proliferation and may even progress to invasive carcinoma in some tissues.