Monocyte/macrophage recruitment, activation and differentiation modulate interleukin-8 production: a paracrine role of tumor-associated macrophages in tumor angiogenesis

Tumor-associated macrophages (TAM) have been shown to play an important role in tumor angiogenesis. The purpose of this study was to determine whether monocyte recruitment, activation and differentiation mediated by monocyte chemotactic protein-1 (MCP-1) and macrophage colony stimulating factor (M-CSF) modulate the expression of the angiogenic factor, Interleukin (IL)-8. Isolated human peripheral blood monocytes secreted low basal levels of IL-8. Incubation of monocytes with M-CSF or MCP-1 resulted in an up-regulation of IL-8 mRNA and protein expression. The differential expression of IL-8 by monocytes following MCP-1 and M-CSF treatments involved activation of the NFkB transcription factor. Further activation with lipopolysaccharide (LPS) caused an increase in IL-8 secretion in monocytes but not in monocyte-derived macrophages (MDM). MDM-conditioned media significantly up-regulated IL-8 expression in human malignant melanoma cells in vitro. In summary, we demonstrated that MCP-1 and M-CSF, critical for monocyte recruitment, activation and differentiation, differentially regulate IL-8 expression and may play an important role in monocyte/macrophage-mediated tumor angiogenesis.