The supernatant of apoptotic cells causes transcriptional activation of hypoxia-inducible factor-1α in macrophages via sphingosine-1-phosphate and transforming growth factor-β

Abstract

Macrophages infiltrating solid tumors exhibit a tumor-supporting phenotype and are critical for tumor development. Little is known which tumor-derived signal provokes this phenotype shift and how these signals are interpreted in macrophages to support tumor growth. We used the super-natant of apoptotic cells and noticed transcriptional, nuclear factor of activated T cells-dependent up-regulation of hypoxia-inducible factor (HIF)-1α mRNA, subsequent protein expression, and HIF-1 activity. Blocking calcineurin with cyclosporine A attenuated nuclear factor of activated T cells binding during electrophoretic mobility shift assay analysis and circumvented the HIF-1α mRNA increase. Knockdown experiments, receptor analysis, and antibody neutralization pointed to sphingosine-1-phosphate and transforming growth factor-β as the initiators of the HIF-1 response. The use of macrophages from conditional HIF-1α knockout mice revealed that macrophages, under the impact of apoptotic cell supernatants, use HIF-1 to produce factors that induce CD31 expression in murine embryonic stem cells. Our study supports the notion that soluble factors produced from apoptotic tumor cells activate the HIF-1 system under normoxia in macrophages to enhance their tumor-promoting capacity by, for example, releasing vascular endothelial growth factor. This shows the importance of HIF-1-elicited responses in regulatory macrophages under normoxia. © 2009 by The American Society of Hematology.