TGF-β is a potent regulator of tumor invasion, immune response, and inflammation, the processes that critically contribute to cancer progression. TGF-β functions as tumor suppressor but can also promote metastasis. This duality of TGF-β creates a major problem for targeting TGF-β signaling in cancer. The current study is aimed to the development of TGF-β therapeutics by identifying specific effectors of the TGF-β pathway contributing to alterations in tumor microenvironment and formation of breast cancer metastases. We show that three mitogen activated pro tein kinase (MAPK) signaling cascades mediated by JNK, p38 MAPK, and ERK contribute to TGF-β-enhanced tumor cell migration, invasion, and changes in tumor microenvironment. Constitutive signaling in breast cancer MDA-MB-231 cells by Alk5-T204D mutant of TGF-β type I receptor (TβRI/Alk5) significantly enhanced migration and invasion, and pulmonary metastases from orthotopic xenografts in SCID mice. Alk5-T204D increased tumor vascularization and reduced cell death as assessed by TUNEL staining but did not affect Ki67 index. TGF-β-mediated cell migration was blocked by inhibitors of JNK, p38, and ERK signaling indicating that all three kinases contribute to cell migration. TGF-β-mediated tumor invasion was associated with a specific up-regulation of matrix metalloproteinase 9 (MMP9) without an effect on MMP1 and MMP2. TGF-β increased secretion and matrix degradation at focal contacts. Studies with kinase inhibitors and dominant negative (dn) MKK6, an upstream kinase for p38 and JNK, showed that ERK and p38 but not JNK contribute to the MMP9 induction. Silencing of MMP9 by RNA interference (RNAi) significantly reduced tumor cell invasion but not migration. Importantly, siRNA to MMP9 reduced TGF-β-mediated tumor vascularization and lung metastases. Thus, MAPK signaling may represent a marker of breast cancer progression and an important therapeutic target against pro-metastatic function of TGF-β.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]