Accumulation of CD11b+Gr1 myeloid cells in liver metastases stimulates tumor growth and angiogenesis

Host immune cells in the tumor microenvironment play a vital role in tumor development and metastasis. We previously reported that accumulation of CD11b+ Gr1mid- and Gr1low-expressing myeloid cells is important for metastatic progression and growth in a mouse model of liver metastasis. However, little is known about the mechanisms whereby these cells support metastatic development. To characterize the molecular mechanisms by which CD11b+ Gr1mid and Gr1low cells contribute to liver metastasis, MC38 tumor cells were isolated after CD11b+ cell depletion and analyzed using whole-genome expression profiling. CD11b+ Gr1mid- and Gr1low cells accumulate in liver metastases generated by splenic injection of MC38 colon carcinoma and Lewis lung carcinoma cells. The Gr1mid population is recruited to liver metastases where they differentiated into Gr1low cells. This differentiation is stimulated by tumor cells in tissue culture. CD11b+ cells were depleted by DT administration in CD11b-DTR mice, resulting in markedly smaller metastases, decreased proliferation of the tumor cells and greatly reduced metastasis-associated blood vessels. However, depletion had only a negligible effect on tumor cell apoptosis and necrosis. CD11b+ Gr1mid and Gr1low cells enhanced MC38 and LLC tumor cell proliferation, migration and invasion in in vitro co-culture systems. These responses were dependent on ERK1/2 activation and its inhibition in tumor cells minimized the pro-tumorigenic effects of Gr1mid and Gr1low cells. Stimulation of ERK1/2 by Gr1mid and Gr1low cells may explain why depletion of CD11b+ cells diminished proliferation of liver metastases in the murine model. Gene expression analysis of MC38 cells in liver metastases after CD11b+ cell depletion revealed upregulation of ANGPTL7, an angiopoietin-like protein. To test the functional significance of this upregulation, we overexpressed ANGPTL7 in MC38 tumor cells. Overexpression of ANGPTL7 resulted in a striking reduction in tumor burden and tumor-associated vessels. Our findings show that CD11b+ Gr1mid and Gr1low myeloid cells augment proliferation and the migratory and invasive potential of tumor cells in liver metastases by stimulating ERK1/2 signaling. They further suggest that these myeloid cells mediate angiogenesis through upregulation of ANGPTL7 in tumor cells. These studies reinforce the importance of host myeloid cells in sustaining liver metastasis and identify potential targets for therapeutic manipulation.