Attenuated transforming growth factor beta signaling promotes metastasis in a model of HER2 mammary carcinogenesis

Introduction: Transforming growth factor beta (TGF beta) plays a major role in the regulation of tumor initiation, progression, and metastasis. It is depended on the type II TGF beta receptor (T beta RII) for signaling. Previously, we have shown that deletion of T beta RII in mammary epithelial of MMTV-PyMT mice results in shortened tumor latency and increased lung metastases. However, active TGF beta signaling increased the number of circulating tumor cells and metastases in MMTV-Neu mice. In the current study, we describe a newly discovered connection between attenuated TGF beta signaling and human epidermal growth factor receptor 2 (HER2) signaling in mammary tumor progression. Methods: All studies were performed on MMTV-Neu mice with and without dominant-negative T beta RII (DNIIR) in mammary epithelium. Mammary tumors were analyzed by flow cytometry, immunohistochemistry, and immunofluorescence staining. The levels of secreted proteins were measured by enzyme-linked immunosorbent assay. Whole-lung mount staining was used to quantitate lung metastasis. The Cancer Genome Atlas (TCGA) datasets were used to determine the relevance of our findings to human breast cancer. Results: Attenuated TGF beta signaling led to a delay tumor onset, but increased the number of metastases in MMTVNeu/DNIIR mice. The DNIIR tumors were characterized by increased vasculogenesis, vessel leakage, and increased expression of vascular endothelial growth factor (VEGF). During DNIIR tumor progression, both the levels of CXCL1/5 and the number of CD11b+Gr1+ cells and T cells decreased. Analysis of TCGA datasets demonstrated a significant negative correlation between TGFBR2 and VEGF genes expression. Higher VEGFA expression correlated with shorter distant metastasis-free survival only in HER2+ patients with no differences in HER2-, estrogen receptor +/- or progesterone receptor +/- breast cancer patients. Conclusion: Our studies provide insights into a novel mechanism by which epithelial TGF beta signaling modulates the tumor microenvironment, and by which it is involved in lung metastasis in HER2+ breast cancer patients. The effects of pharmacological targeting of the TGF beta pathway in vivo during tumor progression remain controversial. The targeting of TGF beta signaling should be a viable option, but because VEGF has a protumorigenic effect on HER2+ tumors, the targeting of this protein could be considered when it is associated with attenuated TGF beta signaling.