Characterization and expression of a novel alternatively spliced human angiopoietin-2

Angiopoietin-2 (Ang2) is a naturally occurring antagonist of angiopoietin-1 (Ang1) that competes for binding to the Tie2 receptor and blocks Ang1-indu ced Tie2 autophosphorylation during vasculogenesis. Using the polymerase ch ain reaction, we isolated a cDNA encoding a novel shorter form of Ang2 from human umbilical vein endothelial cell cDNA and have designated it angiopoi etin-2(443) (Ang2(443)), because it contains 443 amino acids. Part of the c oiled-coil domain (amino acids 96-148) is absent in Ang2(443) because of al ternative splicing of the gene. Like Ang2, recombinant Ang2(443) expressed in COS-7 cells is secreted as a glycosylated homodimeric protein. Recombina nt Ang2, binds to the Tie2 receptor but does not induce Tie2 phosphorylatio n. Pre-occupation of Ang2(443) on Tie2 inhibits Ang1 or Ang2 binding and in hibits Ang1-induced phosphorylation, Expression of Ang2(443) mRNA is detect able in primary endothelial cells, several nonendothelial tumor cell lines, and primary tumor tissues. Interestingly, two cervical carcinoma cell line s express relatively moderate levels of Ang2(443) mRNA. and protein. Macrop hages express mainly Ang2 mRNA, but the expression of Ang2(443) mRNA is tem porarily up-regulated during macrophage differentiation. These results sugg est that Ang2(443) is a functional antagonist of Ang1 and could be an impor tant regulator of angiogenesis during some tumorigenic and inflammatory pro cesses.