Vascular endothelial growth factor, which was iden-heart cDNA library, respectively, by using a serendipitously found partial mouse cDNA clone as a probe tified almost 10 years ago, has so far been considered as the only growth factor relatively specific for endothelial (20,. Independently, another group found the same gene when attempting to identify candidate genes for cells. VEGF is an important regulator of endothelial cell proliferation, migration, and permeability during multiple endocrine neoplasia type 1 (MEN1). The product of this alternatively spliced gene was designated as embryonic vasculogenesis and in physiological and pathological angiogenesis [reviewed in (1 -3)]. The piv-VRF (21).
The two currently known isoforms of VEGF-B are otal role of VEGF in embryogenesis is emphasized by the unprecedented result that the inactivation of even a generated by alternative splicing of mRNA from the VEGF-B gene, spanning about 4 kb of DNA. The human single VEGF allele results in embryonic lethality (4,5). VEGF acts through its two known high-affinity recep-and murine VEGF-B genes are composed of 7 exons, and their exon -intron organization resembles that of tors Flt1, vascular endothelial growth factor receptor 1 (VEGFR-1) and KDR/Flk1, or VEGFR-2 (6 -10). A VEGF and PlGF genes (21,25). The mature VEGF-B proteins (devoid of signal sequence) have 167 (VEGF-third receptor tyrosine kinase homologous with VEGFR-1 and VEGFR-2, designated Flt4, was cloned B 167 ) and 186 (VEGF-B 186 ) amino acid residues, respectively. VEGF-B 186 is generated by using an alternative as an orphan receptor by two research groups and was shown not to bind VEGF (11 -14). Additional VEGF splice acceptor site in exon 6, resulting in an insertion of 101 bp between nucleotides 410 and 411 in the coding receptors of unknown nature also exist on endothelial and tumor cells (15,16).
sequence of VEGF-B167. This insertion introduces a frame shift and a stop codon at the position correspond-The second member of the VEGF family of growth factors, placenta growth factor (PlGF), is 53% identical ing to nucleotides 521 -523 of the coding region of VEGF-B 167 cDNA (Fig. ). Thus, the two VEGF-B iso-with VEGF within its platelet-derived growth factorlike region and binds only . Both forms have an identical NH2-terminal domain of 115 aa and different COOH-terminal domains. Although VEGF and PlGF are dimeric glycoproteins related in structure to the platelet-derived growth factors A and the C-terminus of VEGF-B 167 is highly basic, that of VEGF-B 186 is rich in alanine, proline, serine, and threo-B (PDGF-A and PDGF-B). This relation is based on the presence of several conserved amino acid residues nine amino acid residues and has no significant similarity with amino acid sequences of known proteins including 8 equally spaced cysteines. Compared with VEGF, the mitogenic or permeability-enhancing activi- (21,. Unlike other growth factors of the VEGF-family, both isoforms of human and mouse VEGF-B lack ties of PlGF are weak; however, PlGF is able to potentiate the action of VEGF in vivo and in vitro (19). PlGF -the consensus sequence for N-linked glycosylation (NXT/S); instead, VEGF 186 is O-glycosylated (24). VEGF heterodimers occur in vivo and have intermediate potency in mitogenic stimulation of endothelial VEGF-B 167 remains cell associated with secretion, but it is released into the culture medium with treat-cells (35).
Two novel endothelial cell-specific growth factors, ment of the producing cells with heparin or high salt.
The cell (or matrix) association of VEGF-B 167 likely oc-structurally related to VEGF and PlGF, were recently discovered. These factors, designated as vascular endo-curs via its basic region, as observed for the highly basic splice variants of VEGF. This notion is supported thelial growth factor B (VEGF-B) or VEGF-related factor (20,21) and vascular endothelial growth factor C by the fact that VEGF-B 186 , lacking the highly basic region, is freely secreted from cells and is not bound to (VEGF-C) or VEGF-related protein (22,23) expand the known VEGF family and demonstrate the complexity cell-surface or pericellular heparan sulfate proteoglycans (20,24). of regulation of endothelial functions. This review summarizes the initial studies on VEGF-B and VEGF-C.
The apparent molecular masses of the secreted VEGF-B 167 and VEGF-B 186 polypeptides are 21 kDa
VEGF-B/VRF
The human and mouse VEGF-B cDNA clones were *Correspondence to: Kari Alitalo, Molecular/Cancer Biology Labo- isolated from human fibrosarcoma and erythroleukeratory, Haartman Institute, PL21 (Haartmaninkatu 3),