FIG. 4.
Growth factor-induced pp70S6K activation is not sufficient to stimulate CREB Ser-133 phosphorylation. HepG2 cell lines stably expressing wild-type and mutant versions of PDGFR were serum starved for 2 days and either left untreated (lanes 1, 3, 5, 7, and 9) or treated for 10 min with PDGF (20 ng/ml) (lanes 2, 4, 6, 8, and 10). Cell lysates were separated by SDS-PAGE and immunoblotted with anti-PCREB (top) or anti-CREB (bottom) antibodies. The HepG2 lines were stably transfected with the empty pLXSN vector (N), the wild-type PDGFR (wt PDGFR), a mutant PDGFR in which Y740, Y751, Y771, Y1009, and Y1021 were replaced by phenylalanine (F5), the F5 mutant in which F740 and F751 were changed back to tyrosines (Y740/Y751), and a mutant PDGFR in which Y740 and Y751 were changed to phenylalanine (F740/F751).