The balance of cell surface and soluble type III TGF-ß receptor regulates BMP signaling in normal and cancerous mammary epithelial cells.

Bone morphogenetic proteins (BMPs) are members of the TGF-ß superfamily that are over-expressed in breast cancer, with context dependent effects on breast cancer pathogenesis. The type III TGF-ß receptor (TßRIII) mediates BMP signaling. While TßRIII expression is lost during breast cancer progression, the role of TßRIII in regulating BMP signaling in normal mammary epithelium and breast cancer cells has not been examined. Restoring TßRIII expression in a 4T1 murine syngeneic model of breast cancer suppressed Smad1/5/8 phosphorylation and inhibited the expression of the BMP transcriptional targets, Id1 and Smad6, in vivo. Similarly, restoring TßRIII expression in human breast cancer cell lines or treatment with sTßRIII inhibited BMP-induced Smad1/5/8 phosphorylation and BMP-stimulated migration and invasion. In normal mammary epithelial cells, shRNA-mediated silencing of TßRIII, TßRIII over-expression, or treatment with sTßRIII inhibited BMP-mediated phosphorylation of Smad1/5/8 and BMP induced migration. Inhibition of TßRIII shedding through treatment with TAPI-2 or expression of a non-shedding TßRIII mutant rescued TßRIII mediated inhibition of BMP induced Smad1/5/8 phosphorylation and BMP induced migration and/or invasion in both in normal mammary epithelial cells and breast cancer cells. Conversely, expression of a TßRIII mutant, which exhibited increased shedding, significantly reduced BMP-mediated Smad1/5/8 phosphorylation, migration, and invasion. These data demonstrate that TßRIII regulates BMP-mediated signaling and biological effects, primarily through the ligand sequestration effects of sTßRIII in normal and cancerous mammary epithelial cells and suggest that the ratio of membrane bound versus sTßRIII plays an important role in mediating these effects.

Ectodomain shedding of TßRIII is required for TßRIII-mediated suppression of TGF-ß signaling and breast cancer migration and invasion.

The type III transforming growth factor ß (TGF-ß) receptor (TßRIII), also known as betaglycan, is the most abundantly expressed TGF-ß receptor. TßRIII suppresses breast cancer progression by inhibiting migration, invasion, metastasis, and angiogenesis. TßRIII binds TGF-ß ligands, with membrane-bound TßRIII presenting ligand to enhance TGF-ß signaling. However, TßRIII can also undergo ectodomain shedding, releasing soluble TßRIII, which binds and sequesters ligand to inhibit downstream signaling. To investigate the relative contributions of soluble and membrane-bound TßRIII on TGF-ß signaling and breast cancer biology, we defined TßRIII mutants with impaired (ΔShed-TßRIII) or enhanced ectodomain shedding (SS-TßRIII). Inhibiting ectodomain shedding of TßRIII increased TGF-ß responsiveness and abrogated TßRIII's ability to inhibit breast cancer cell migration and invasion. Conversely, expressing SS-TßRIII, which increased soluble TßRIII production, decreased TGF-ß signaling and increased TßRIII-mediated inhibition of breast cancer cell migration and invasion. Of importance, SS-TßRIII-mediated increases in soluble TßRIII production also reduced breast cancer metastasis in vivo. Taken together, these studies suggest that the ratio of soluble TßRIII to membrane-bound TßRIII is an important determinant for regulation of TßRIII- and TGF-ß-mediated signaling and biology.