Angiogenesis is a hallmark of many diseases, including
cancer,
ischemic heart disease,
inflammation, and others. It is well known that
vascular endothelial growth factor (
VEGF) is the most important
angiogenic factor. Recently, we demonstrated that
orphan nuclear receptor TR3 (mouse Nur77 and rat NGFI-B) plays critical roles in
tumor growth and angiogenesis induced by
VEGF-A in vitro and in vivo. However, the signaling pathways that mediate the expression of TR3 induced by
VEGF are still not completely understood. Here we reported that 3 TR3 transcript variants (TR3-TVs) are expressed at differential levels, and regulated differentially in endothelial cells. While the expression of TR3-TV1 is relatively low, the expression of TR3-TV2 is up-regulated markedly, and the expression of TR3-TV3 is up-regulated moderately in endothelial cells induced by
VEGF-A. The kinetics of the induction of these TR3-TVs is different. We also found that several signaling pathways, including calcium-PLC-PKC-PKD1 pathway, NF-κB pathway, and
MAP kinase (ERK, p38, and JNK) pathways are important for
VEGF-A-induced TR3-TV2 and TR3-TV3
mRNA induction. More important, we found that
VEGF-A or
VEGF-E, but not
VEGF-B, nor
placenta growth factor (PlGF), induces the phosphorylation of
insulin-like growth factor-1 receptor (IGF-1R) and the interaction of
VEGF receptor 2/
kinase insert domain receptor (VEGFR2/KDR) with IGF-1R, which mediates the expression of TR3-TV2, but not TR3-TV3. Taking together, we demonstrate that TR3-TVs are differentially regulated by
VEGF-A and identify a novel signaling pathway by which
VEGF-A and
VEGF-E, but neither
VEGF-B, nor PlGF, induce the interaction of VEGFR2/KDR with IGF-1R, resulting in IGF-1R transactivation to induce the high level expression of TR3-TV2. Our data not only elucidate the signaling pathways by which TR3-TVs are regulated, but extend the molecular mechanism, by which
VEGF-A-induced angiogenesis. These studies should permit the development of screening assays for compounds that inhibit
VEGF signaling.