The Influence of Activin on the Tumor Microenvironment in Colon Cancer Metastasis

BACKGROUND AND AIMS: TGFβ and activin A are key molecules regulating tumor growth, inflammatory responses in the TME, and have differential effects which are receptor and cell specific. Identification of the source of activin A/TGFβ production and site of signaling will be critical for directing therapeutic development. Here, we employ both mouse models and human tissue samples to 1) identify pathways in which SMAD4 mutations exacerbate tumor growth and metastasis, 2) recognize how TGFβ and activin A signaling are influencing immune responses in the TME, and 3) to use the cellular source and signaling of activin A, rather than the amount, to predict patient outcomes in CRC. We have previously shown elevated levels of circulating activin A in stage IV CRC patients and that activin signaling promotes epithelial to mesenchymal transition and migration. We now hypothesize that activin A signaling pathways have distinct effects on immune cells and tumor cells which regulate tumor development and metastasis of CRC. METHODS: Knockdown of activin A was assessed in a surgical colon cancer liver metastases mouse model. The influence of the SMAD4-dependent, canonical pathway was elucidated in a Ts4-Cre; cApcflox ;Smad4flox (TcAS) mouse model for colon cancer. Western blotting and migration assays were performed on human colon cancer cells with mutations in receptors for TGFꞵ and/or activin to determine functional interactions of the downstream pathways. We used a TMA of 1054 CRC patients with stage II and III colon cancer from the QUASAR2 cohort to correlate activin and TGFb expression with outcome. RESULTS: The non-canonical pathway pAKT is activated in ACVR2 restored HCT116+chr2 colon cancer cells, but not in ACVR2 mutated cells. Ablation of the the activin A canonical pathway target SMAD4 in mice leads to decreased survival in colon cancer mouse model and pERK is increased in SMAD4-/- knock out mice and correlates with dysplasia grade. Mice harboring colonic lesions with high stroma content were positive for activin A and CAF marker α-SMA and have shorter median survival. Activin knockdown in mouse metastasis model leads to decreased tumor growth. Patient data revealed that a high ratio of activin A relative to TGFꞵ in the tumor epithelium, but not in the stroma, is associated with an increased survival rate. Ongoing studies are being conducted to determine if activin A is being localized to immune cells interacting with the tumor epithelium which may be promoting anti-tumor activity in these patients. CONCLUSION: Recent clinical trials explored the feasibility of activin A and TGFβ blockade in solid tumors. Data presented here supports complex and intertwined roles for activin and TGFβ signaling in colorectal cancer, both in immune activation as well as metastatic spread of advanced cancers. © FASEB.