PLATELET-DERIVED GROWTH FACTOR-DEPENDENT ACTIVATION OF PHOSPHATIDYLINOSITOL 3-KINASE IS REGULATED BY RECEPTOR-BINDING OF SH2-DOMAIN-CONTAINING PROTEINS WHICH INFLUENCE RAS ACTIVITY
Ra. Klinghoffer et al., PLATELET-DERIVED GROWTH FACTOR-DEPENDENT ACTIVATION OF PHOSPHATIDYLINOSITOL 3-KINASE IS REGULATED BY RECEPTOR-BINDING OF SH2-DOMAIN-CONTAINING PROTEINS WHICH INFLUENCE RAS ACTIVITY, Molecular and cellular biology, 16(10), 1996, pp. 5905-5914
Upon binding of platelet-derived growth factor (PDGF), the PDGF beta r
eceptor (PDGFR) undergoes autophosphorylation on distinct tyrosine res
idues and binds several SH2-domain-containing signal relay enzymes, in
cluding phosphatidylinositol 3-kinase (PI3K), phospholipase C gamma (P
LC gamma), the GTPase-activating protein of Ras (RasGAP), and the tyro
sine phosphatase SHP-2, In this study, we have investigated whether PD
GF-dependent PI3K activation is affected by the other proteins that as
sociate with the PDGFR, We constructed and characterized a series of P
DGFR mutants which contain binding sites for PI3K as well as one addit
ional protein, either RasGAP, SHP-2, or PLC gamma, While all of the re
ceptors had wild-type levels of PDGF-stimulated tyrosine kinase activi
ty and associated with comparable amounts of PI3K activity, their abil
ities to trigger accumulation of PI3K products in vivo differed dramat
ically, The wild-type receptor, as well as receptors that recruited PI
3K or PI3K and SHP-2, were all capable of fully activating PI3K. In co
ntrast, receptors that associated with PI3K and RasGAP or PI3K and PLC
gamma displayed a greatly reduced ability to stimulate production of
PI3K products, When this series of receptors was tested for their abil
ity to activate Ras, we observed a strong positive correlation between
Ras activation and PI3K activation, Further investigation of the rela
tionship between Ras and PI3K indicated that Ras was upstream of PI3K.
Thus, activation of PI3K requires not only binding of PI3K to the tyr
osine-phosphorylated PDGFR but accumulation of GTP-bound Ras as well,
Furthermore, PLC gamma and RasGAP negatively modulate PDGF-dependent P
I3K activation, Finally, PDGF-stimulated signal relay can be regulated
by altering the ratio of SH2-domain-containing enzymes that are recru
ited to the PDGFR.