Stromal and epithelial TGF-beta signaling in cancer.
Authors: Harold L. Moses
Journal: Cancer Research (2006)

Proc Amer Assoc Cancer Res, Volume 47, 2006

SY15-02 

Although originally described as an activity that caused nontransformed cells to grow in an anchorage independent, there is now compelling evidence from transgenic mouse studies and analysis of mutations in human carcinomas indicating that the TGF-beta signal transduction pathway is tumor suppressive. A number of transgenic mouse studies demonstrated that overexpresssion of TGF-beta1 in epithelial cells suppressed tumor formation while expression of a dominant negative type II TGF-beta receptor enhanced carcinoma development. 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 I and type II TGF-beta receptor (TBRI and TBRII, respectively) (for review, see [[1][1]]). There is also evidence that TGF-beta can enhance the progression of tumors. This hypothesis has also 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 [[2][2]]. These mice show mammary lobuloalveolar hyperplasia. 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 support the hypothesis that TGF-beta signaling in mammary carcinoma cells can also have tumor suppressive actions even after carcinomas are well established. 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 knockout of the type II TGF-beta receptor gene in mammary stromal cells FSP1-Cre and Tgfbr2flox/flox mice were crossed to attain Tgfbr2fspKO mice [[3][3]]. 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 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 [4]. 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. A small molecule inhibitor of Met signaling inhibited tumor growth, invasion and metastasis in vivo. 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.

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