Angiogenesis of liver metastases

PURPOSE: Tumor-induced angiogenesis requires migration and remodeling of endothelialcells derived from pre-existing blood vessels. Vascular endothelial growth factoris the growth factor most closely implicated in the development of neovessels in colon cancer. However, vascular endothelial growth factor-specific receptors flt-1 and KDR mRNA expression are absent in normal sinusoidvessels surrounding vascular endothelial growth factor-producing secondary hepatic tumors. Thus, the potential role of sinusoidal endothelialcells in the mechanism of neovessel formation within liver metastatic carcinomasremains unclear. The purpose of this study was to determine whether sinusoidal endothelialcells are involved in tumor angiogenesis in a syngeneicmodel of liver metastases from colorectal cancer. METHODS: Sinusoidal endothelialcells were identified by fluorescence microscopy after uptake of acetylated low density lipoprotein labeled with a fluorescent probe (dioctadecylindocarbocyanine). One hundred microlitersof dioctadecylindocarbocyanine acetylated low density lipoprotein were injected intraportally at the start of experiment in BD IX rats. Two days later, intraportal injection of 107 DHD K12, a chemically induced colon carcinoma cell line, was performed in syngeneicBD IX rats. Animals were killed one week later and the livers were processed for routine histologic examination and immunohistochemistry using the rat endothelialcell antigen-1 monoclonal antibody. RESULTS: In normal parenchyma fluorescence was associated with sinusoidalcells but not with endothelium of large blood vessels. Thus, specific acetylated low density lipoprotein uptake allowed histological differentiation of sinusoidal endothelialcells from other large-vessel endothelialcells present in the hepatic parenchyma. In tumor-bearing liver a spatial gradient of fluorescence was generated. Labeled cells accumulated at the periphery of the metastases. When tumors grow beyond 200 µm, neovessel formation was observed; there was an invasion of fluorescent-labeled cells from the periphery, which were arranged in a tubular formation within neoplasia. CONCLUSION: In liver metastases tumor vessels are lined with sinusoidal endothelialcells. Identification of a specific cell type involved in the formation of the stromal compartment of tumors has important implications. Sinusoidal endothelialcells express well-characterized surface receptors and differ morphologically and metabolically from large-vessel endothelia. They should be considered as attractive targets for future and existing antiangiogenic strategies directed against the stromal compartment of liver metastases.