Hypoxic and highly angiogenic non-tumor tissues surrounding hepatocellular carcinoma: the 'niche' of endothelial progenitor cells.

Our previous investigations showed that mobilized endothelial progenitor cells (EPCs) are enriched in non-tumor tissues (NT) surrounding hepatocellular carcinoma (HCC), compared to in tumor tissues (TT). This particular recruitment of EPCs is worth investigating further. The mobilization, recruitment, homing, and incorporation of EPCs into tumors require the participation of multiple factors, including angiogenic factors, adherent molecules, endothelial cells, hypoxic environment, etc. Therefore, we hypothesized that NT might be a hypoxic and highly angiogenic area, into which many more EPCs are recruited and homed. In the last three years, we evaluated the hypoxic condition, angiogenic factors and angiogenic index using frozen tissues or tissue microarrays from 105 patients who had undergone hepatectomy for HCC, and here we review our results and the studies of others. All results showed the expression of Hypoxiainducible factor-1α was higher in NT than in TT. The expression of VEGFA, bFGF, TGF-β, MCP-1, MMP-9, TIMP-2, and endostatin in NT was significantly higher than in normal liver and TT. Meanwhile, the expression of CD105-the surface marker of activated endothelial cells-was also higher in NT than in TT at the protein and mRNA levels. These investigations showed that NT is a hypoxic and highly angiogenic area, which may be the 'niche' of EPCs. The particular background in HCC may be related to liver cirrhosis. Therefore, non-tumor tissues surrounding HCC may be the 'niche' of endothelial progenitor cells.