Paracrine upregulation of VEGF receptor mRNA in endothelial cells by hypoxia-exposed Hep G2 cells

Although vascular endothelial growth factor (VEGF) plays a role in the grow th of hypervascular tumors, mechanisms for paracrine regulation of its rece ptor expression on vascular endothelial cells remain unknown. This study ai med to investigate whether VEGF released from hypoxia-exposed Hep G2 cells alters expression of the two distinct receptors, kinase insert domain-conta ining receptor (KDR) and fms-like tyrosine kinase I (flt-1), in human umbil ical venous endothelial cells (HUVEC). Hep G2 cells were cultured in 20% or 1% O-2 for 16 h to examine induction of VEGF mRNA and its protein expressi on. Conditioned medium from Hep G2 cells (CM) was applied to HUVEC under no rmoxic conditions, and expression of mRNA for the VEGF receptors was determ ined by RT-PCR. In response to the hypoxic challenge, Hep G2 cells upregula ted VEGF mRNA and the release of VEGF. Hypoxia-CM preferentially stimulated the mRNA expression of flt-1 but not that of KDR in HUVEC. When the VEGF r elease from hypoxia-exposed Hep G2 cells was blocked by its antisense oligo deoxynucleotide, the endothelial flt-1 mRNA upregulation elicited by the hy poxia-CM was still maintained. These results suggest that hypoxia-exposed H ep G2 cells not only produce VEGF but also evolve paracrine induction of fl t-1 through VEGF-independent mechanisms.