Autocrine EGFR signaling in breast cancer cell invasion metastasis

Abstract

Metastasis is the spread of tumor cells from the primary tumor to a distant site. The ability of carcinoma cells to invade the surrounding stroma is a key step in this process. Previous work has shown that a paracrine interaction between tumor cells and macrophages in which imposed gradients of EGF or CSF-1 can induce invasion. An important question arising from this paracrine loop mechanism is whether the secretion of EGFR ligands by macrophages, which creates ligand gradients, enhances invasiveness through the oriented migration of tumor cells.;To address this question, retroviral vectors were used to overexpress HBEGF in MTLn3 rat mammary adenocarcinoma cells overexpressing EGFR/ErbB1 and MDA-MB 231 human breast cancer cells. We hypothesized that if invasion relies on cell migration directed by a spatial ligand gradient, then the autocrine expression of HBEGF by carcinoma cells would reduce chemotactic responses to ligand gradients of ligands generated by TAMs, causing decreased invasion, intravasation, and metastasis. However, we found that autocrine expression of HBEGF enhanced in vivo intravasation and metastasis. Interestingly, we showed that the in vivo invasion response was not inhibited by blocking the CSF-1R with a pharmacological inhibitor or macrophage function, indicating that macrophages are no longer required for invasion. Furthermore, we demonstrate in vitro that HBEGF expression enhanced invadopodium formation, providing a mechanism for cell autonomous invasion.;These results suggested the possibility that TAMs may no longer be recruited to the primary tumor. However, we discovered that tumors formed from HBEGF expressing lines enhanced macrophage density. To determine chemoattractants that could be acting to recruit TAMs, we conduced microarray studies on our cell lines and discovered VEGFA as a likely candidate. Preliminary phenotyping of the TAMs suggested that angiogeneic macrophages were being recruited. Also, we found that tumor angiogenesis was increased in the context of HBEGF expression.;In summary, these studies highlight the complexity of cancer cell invasion. To better understand how to inhibit this process, we must study not only the synergistic interactions between carcinoma cells and the tumor microenvironment but how these interactions can be altered upon changes in the intrinsic characteristics of the tumor cells.