Modulation of chemotactic and pro-inflammatory activities of endothelial progenitor cells by hepatocellular carcinoma

Endothelial progenitor cells (EPCs) participate in the neovascularization processes in the development of hepatocellular carcinoma (HCC). We investigated whether interactions between EPCs and HCC cells affect chemotactic and pro-inflammatory activities of EPCs. Two distinct phenotypes of circulating EPCs, i.e., myeloid-derived EPCs (colony forming unit-endothelial cells, CFU-ECs) and outgrowth EPCs (endothelial-colony forming cells, ECFCs), were co-cultured with Huh7 and Hep3B cells by using transwell chamber and IBIDI(TM) Culture-Inserts and μ-slide plates. Transwell and horizontal migration/invasion assays and time-lapse microscopy were used to monitor and analyze the migration and invasion of EPCs induced by these HCC cells. A human cytokine antibody array was used to compare protein expression profiles in EPCs and HCC cells. Flow cytometry and electromobility shift analysis were used to detect nuclear factor-κB (NF-κB)-DNA binding activity and pro-inflammatory adhesion molecule expression in EPCs. Ectopic full-length CC chemokine receptor 6 (CCR6) plasmid was used to transfect into ECFCs to investigate the role of CCR6 in HCC-induced EPC migration and invasion. The results show that co-culture with Huh7 and Hep3B cells induces the expression of endothelial cell (EC) markers KDR, Flt1, CD31 and VE-cadherin in CFU-ECs, but down-regulates the expressions of CD31 and VE-cadherin in ECFCs. These HCC cells induce migration and invasion of CFU-ECs, but not ECFCs, and do not affect the cell cycle distribution in these EPCs. Cytokine protein array identifies macrophage inflammatory protein-3α (MIP-3α) produced by HCC cells as a critical factor responsible for the HCC-induced chemotaxis of CFU-ECs, which highly express the specific MIP-3α counterreceptor CCR6. Overexpressing CCR6 in ECFCs significantly increases their chemotaxis in response to HCC cells. Co-culturing EPCs with HCC cells results in decreases in NF-κB binding activity and hence intracellular adhesion molecule-1, vascular cell adhesion molecule-1, and E-selectin expressions in EPCs. Our results indicate that HCC cells exert differential effects on CFU-ECs and ECFCs, with increased chemotaxis for CFU-ECs, but not ECFCs. This HCC-induced chemotaxis of CFU-ECs is mediated by MIP-3α produced by HCC cells, which targets to CCR6 on CFU-ECs. Tumors may provide a humoral microenvironment to attenuate the pro-inflammatory activity of EPCs, which might be associated with the tumor escape mechanism.