receptor was also defective in activating p21 ras. 
These results provide an explanation for the pre- 
vious observation that PDGF receptors that have 
mutated PI 3-kinase-binding sites are unable to stim- 
ulate mitogenesis and suggest that PI 3-kinase regu- 
lates p21 ras activity as well as Raf-1 and MAP 
kinases. 
Although Dr. Williams's group showed previously 
that PI 3-kinase binds to activated PDGF receptors, 
it was not known whether the catalytic activity of PI 
3-kinase was increased by this interaction. This year, 
Dr. Jaime Escobedo showed that purified receptor 
stimulated a 10-fold increase in activity of purified 
PI 3-kinase in vitro. He also showed that the en- 
hancement of PI 3-kinase in vitro required tyrosine 
phosphorylation of the PI 3-kinase by the receptor. 
Both the 85 - and 1 1 0-kDa subunits were found to be 
tyrosine-phosphorylated in PDGF-stimulated cells. 
The conclusion was that PI 3-kinase is activated by 
tyrosine phosphorylation. 
Role of Phospholipase C-7 with Fibroblast 
Growth Factor Receptor Signaling 
In previously proposed models of signal transduc- 
tion by fibroblast growth factor (FGF) receptors, 
phospholipase C-7 (PLC-7) was thought to play 
a pivotal role. By testing candidate tyrosine- 
phosphorylated peptides for their ability to block 
binding of PLC-7 to the FGF receptor, Dr. Williams's 
group showed that tyrosine 766 is the binding site 
for PLC-7 on the FGF receptor. They then mutated 
this site and found that the mutated receptors lost 
the ability to stimulate PI hydrolysis and could not 
elicit an increase in cytosolic calcium. This showed 
that association of PLC-7 with receptors was essen- 
tial for growth factor-induced calcium elevations 
that were previously thought to be essential for mi- 
togenesis. However, the mutated FGF receptor elic- 
ited a normal mitogenic response and also activated 
Raf-1 and MAP kinases normally. It was concluded 
that neither calcium elevation nor PI turnover was 
required for the mitogenic effect of FGF or for FGF- 
stimulated MAP kinase and Raf-1 kinase activities. 
These conclusions refuted previously proposed 
models of FGF-stimulated signal transduction. 
Raf Kinase Mediates FGF-stimulated 
Mesoderm Induction 
Recent work from Dr. Marc Kirschner's group 
showed that FGF is important in the induction of 
mesoderm and in the formation of posterior struc- 
tures during Xenopus embryonic development. Dr. 
Williams's group investigated the role of Raf-1 ser- 
ine/threonine kinase in FGF-stimulated mesoderm 
induction, and Dr. Angus MacNicol constructed mu- 
tated Raf- 1 protein (Naf) that inhibited the activity 
of wild-type Raf-1. He expressed Naf in Xenopus 
embryos by injecting Naf mRNA into both cells of 
the two-cell-stage embryo. At stage 8 of develop- 
ment, explants of the animal hemisphere were ex- 
posed to FGF or activin and examined for the induc- 
tion of mesoderm in vitro. The mesoderm-inducing 
effect of FGF was blocked in embryos that expressed 
Naf. However, the induction of mesoderm by acti- 
vin, a factor that acts by different signaling path- 
ways, was not affected by Naf. The developmental 
consequences of allowing Naf-injected embryos to 
progress through later stages of embryogenesis were 
then examined. At the tadpole stage the Naf-injected 
embryos had normal eyes and cement glands (head 
structures), as well as normal hearts, but had severe 
truncations of their tails. This phenorype is remark- 
ably similar to the previously reported phenotype of 
embryos that had their FGF receptors blocked. This 
is the first demonstration of a role for Raf- 1 in verte- 
brate development and in a signaling pathway stimu- 
lated by FGF. 
Dr. Anthony Muslin tested the role of Raf-1 in an- 
other process, the maturation of Xenopus oocytes 
induced by progesterone. He found that progester- 
one stimulated a fourfold increase in Raf-1 kinase 
activity. Injection of the RNA encoding an onco- 
genic raf (v-ra/) caused oocytes to mature even in 
the absence of progesterone. When dominant nega- 
tive raf (Naf) RNA was injected, oocyte maturation 
induced by progesterone was blocked. The activa- 
tion of MAP kinase was also blocked. These findings 
suggest that Raf- 1 plays an important role in oocyte 
maturation and in the activation of MAP kinase. 
Cloning of the cDNA for a Receptor 
for Vascular Endothelial Growth Factor 
Vascular endothelial growth factor (VEGF) acts 
exclusively on endothelial cells to stimulate their 
proliferation and to increase endothelial permeabil- 
ity. Dr. Williams's group cloned a new cDNA that 
encodes the VEGF receptor and showed that it de- 
fined a new class of receptor tyrosine kinase. The 
mRNA for this receptor was expressed only in endo- 
thelial cells and was especially prominent in the 
primitive endothelial cells that line blood islands in 
the mouse yolk sac and in new blood vessels formed 
in healing wounds. This unique pattern of expres- 
sion suggests a role of VEGF receptor in angiogene- 
sis. (This work was supported by a grant from the 
National Hean, Lung and Blood Institute, National 
Institutes of Health.) 
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