The TGF-Beta/MUC4 Signaling Axis in Circulating Tumor Cells of Metastatic Breast Cancer

PROJECT SUMMARY Metastasis is responsible for the majority of breast cancer deaths, and standard-of-care treatments fail to effectively target metastasizing cells. Circulating tumor cells (CTCs) in the bloodstream rely on the physical protection and chemical signals of platelets to survive and seed metastatic lesions. One such chemical signal is transforming growth factor beta (TGF-β), which, after secretion by platelets, has been shown to modulate CTC gene expression and behavior. TGF-β has also been shown to upregulate expression of the cell-surface glycoprotein Mucin-4 (MUC4) in various cellular contexts. MUC4 has been implicated in tumor development and maintenance and was recently observed to contribute to platelet-CTC interactions. This raises the question of whether platelet-secreted TGF-β may be upregulating CTC-MUC4, enhancing platelet-CTC interaction and generating a positive feedback loop. The hypothesis driving the proposed studies is that platelet-TGF-β upregulates CTC-MUC4, reinforcing CTC-platelet binding and enhancing metastatic cell survival. Specific Aim 1 will determine the effects of platelet-derived TGF-β on tumor cell MUC4 expression using cellular, molecular, and biochemical techniques, and assess MUC4-dependent cellular aggressiveness in vitro. Specific Aim 2 will characterize the role of MUC4 in platelet-tumor cell interactions using in vitro binding assays. Specific Aim 3 will assess the effects of platelet-TGF-β and CTC-MUC4 crosstalk in vivo using tail vein and orthotopic xenograft tumor mouse models of metastasis. Successful completion of this research will reveal a novel form of platelet- CTC crosstalk, exposing an important means by which metastasizing cells survive and illuminating a potential new therapeutic target for breast cancer metastatic prevention.