PMC

Expression of wild type Gab1 rescues EGF-induced PI-3 kinase and Akt activation in Gab1 deficient MEFs. A. The indicated cell lines were serum-starved for 24 hours and stimulated with 10 ng/ml EGF for five minutes at 37°C. Cell extracts were prepared and analyzed for Gab1 tyrosine phosphorylation and Gab1 expression. Both WT MEFs and cells expressing exogenous Gab1 show Gab1 tyrosine phosphorylation in response to EGF treatment. B. The indicated cell lines were serum-starved for 24 hours and stimulated with 100 ng/ml EGF for five minutes at 37°C. Cell extracts were prepared and phosphotyrosine immunoprecipitates were analyzed for PI-3 kinase activity. Gab1 -/- cells fail to show phosphotyrosine-associated PI-3 kinase activity, while cells expressing exogenous Gab1 display phosphotyrosine-associated PI-3 kinase activity that is augmented by EGF treatment. C. The indicated cell lines were serum-starved for 24 hours and stimulated with 1 ng/ml EGF for varying periods of time at 37°C. Cell extracts were prepared and analyzed for activation of Akt by using antibodies that specifically recognize the serine473 phosphorylated form of Akt. Membranes were subsequently stripped and immunoblotted for Akt to confirm equal loading. Ectopic expression of Gab1 in Gab1 -/- MEFs rescues the EGF-induced activation of Akt.

Expression of a Gab1 mutant protein deficient in p85 binding fails to rescue EGF-induced PI-3 kinase/Akt activation in Gab1 deficient MEFs. A. The indicated cell lines were serum-starved for 24 hours and stimulated with 10 ng/ml EGF for five minutes at 37°C. Cell extracts were prepared and analyzed for Gab1 tyrosine phosphorylation, Gab1 co-immunoprecipitation with p85, p85 expression, Gab1 co-immunoprecipitation with Shp2, and Gab1 expression. Both Gab1 and Gab1^F446/472/589 are tyrosine phosphorylated in response to EGF treatment, and both form a stable complex with Shp2. However, Gab1^F446/472/589 fails to associate with the p85 subunit of PI-3 kinase. Additionally total cell lysates from the indicated cell lines were immunoblotted with anti-Gab1 antibodies, providing independent evidence that Gab1 is expressed at approximately equal levels in all cell lines. B. The indicated cell lines were serum-starved for 24 hours and stimulated with 10 ng/ml EGF for varying periods of time at 37°C. Cell extracts were prepared and analyzed for EGFR tyrosine phosphorylation. All of the cell lines examined show similar kinetics of EGF-induced EGFR activation. Immunoblots were quantitated by densitometry, normalized for EGFR expression and represented linearly. Diamonds = Gab1 -/-, squares = Gab1, triangles = Gab1^F446/472/589. C. The indicated cell lines were serum-starved for 24 hours and stimulated with 100 ng/ml EGF for five minutes at 37°C. Cell extracts were prepared and Gab1 immunoprecipitates were analyzed for PI-3 kinase activity. Ectopic expression of wild type Gab1 restored EGF-induced PI-3 kinase activity, while expression of Gab1^F446/472/589 fails to rescue PI-3 kinase activity in response to EGF treatment. D. The indicated cell lines were serum-starved for 24 hours and stimulated with 1 ng/ml EGF for varying periods of time at 37°C. Cell extracts were prepared and analyzed for activation of Akt by using antibodies that specifically recognize the serine473-phosphorylated form of Akt. Membranes were subsequently stripped and immunoblotted for Akt to confirm equal loading. Ectopic expression of Gab1 in Gab1 -/- MEFs rescues activation of Akt in response to EGF treatment, while expression of Gab1^F446/472/589 fails to rescue the EGF-induced Akt activation.

Expression of a Gab1 mutant protein deficient in Shp2 binding enhances EGF-induced activation of the PI-3 kinase/Akt signaling pathway. A. The indicated cell lines were serum-starved for 24 hours and stimulated with 10 ng/ml EGF for five minutes at 37°C. Cell extracts were prepared and analyzed for Gab1 tyrosine phosphorylation, Gab1-Shp2 co-immunoprecipitation and Gab1 expression. Gab1^F627/659 becomes tyrosine phosphorylated in response to EGF treatment to levels approximately 1.5-fold higher than Gab1 as determined by densitometry, while Gab1^{F446/472/589/627/659} does not show EGF-induced tyrosine phosphorylation in this assay. Wild type Gab1 forms a stable complex with Shp2 in response to EGF treatment, while Gab1^F627/659 and Gab1^{F446/472/589/627/659} do not. B. The indicated cell lines were serum-starved for 24 hours and stimulated with 10 ng/ml EGF for varying periods of time at 37°C. Cell extracts were prepared and analyzed for EGFR tyrosine phosphorylation and EGFR expression. All of the cell lines examined show similar kinetics of EGF-induced EGFR activation. Immunoblots were quantitated by densitometry, normalized for EGFR expression, and represented linearly. Diamonds = Gab1 -/-, squares = Gab1, triangles = Gab1^F627/659, circles = Gab1^{F446/472/589/627/659}. C. The indicated cell lines were serum-starved for 24 hours and stimulated with 100 ng/ml EGF for five minutes at 37°C. Cell extracts were prepared and Gab1 immunoprecipitates were analyzed for PI-3 kinase activity. Cells expressing exogenous Gab1^F627/659 display enhanced PI-3 kinase activity relative to cells expressing wild type Gab1. Expression of exogenous Gab1^{F446/472/589/627/659} fails to rescue EGF-induced PI-3 kinase activity in Gab1 deficient MEFs. D. The indicated cell lines were serum-starved for 24 hours and stimulated with 1 ng/ml EGF for varying periods of time at 37°C. Cell extracts were prepared and analyzed for activation of Akt by using antibodies that specifically recognize the serine473-phosphorylated form of Akt. Membranes were subsequently stripped and immunoblotted for Akt to confirm equal loading. Cells expressing exogenous Gab1^F627/659 display enhanced activation of Akt with sustained kinetics relative to cells expressing wild type Gab1. Expression of exogenous Gab1^{F446/472/589/627/659} fails to rescue EGF-induced Akt activation in Gab1 deficient MEFs.

Expression of ErbB3 in Gab1-deficient MEFs partially rescues EGF-induced activation of the PI-3 kinase/Akt signaling pathway. A. The indicated cell lines were serum-starved for 24 hours and stimulated with 10 ng/ml EGF for five minutes at 37°C. Cell extracts were analyzed for EGFR tyrosine phosphorylation and EGFR expression, and for ErbB3 tyrosine phosphorylation and ErbB3 expression. Cells were additionally stimulated with 10 ng/ml NRG and cell extracts analyzed for ErbB3 tyrosine phosphorylation. The endogenous EGFR is tyrosine phosphorylated in response to EGF in all cell lines. ErbB3 exhibits weak constitutive tyrosine phosphorylation that is enhanced by NRG treatment, but is not significantly enhanced by treatment with EGF. Selected bands were quantitated by densitometry to determine relative increase in growth factor-induced tyrosine phosphorylation. B. The indicated cell lines were serum-starved for 24 hours and stimulated with 10 ng/ml EGF for five minutes at 37°C. Cell extracts were analyzed for Gab1 tyrosine phosphorylation and Gab1 expression. Cells rescued with wild type Gab1 exhibit Gab1 tyrosine phosphorylation in response to EGF treatment, while Gab1 -/- cells and Gab1-deficient cells expressing ErbB3 do not. C. The indicated cell lines were serum-starved for 24 hours and stimulated with 100 ng/ml EGF for five minutes at 37°C. Cell extracts were prepared and phosphotyrosine immunoprecipitates were analyzed for PI-3 kinase activity. Expression of wild type Gab1 in Gab1-deficient MEFs rescues the EGF-induced PI-3 kinase activity. Gab1 deficient MEFs exogenously expressing ErbB3 exhibit PI-3 kinase activity that is largely EGF-independent. D. The indicated cell lines were serum-starved for 24 hours and stimulated with 1 ng/ml EGF for varying periods of time at 37°. Cell extracts were analyzed for activation of Akt by using antibodies that specifically recognize the phosphorylated form of Akt. Membranes were subsequently stripped and immunoblotted for Akt to confirm equal loading. Ectopic expression of ErbB3 in Gab1 deficient cells results in a partial rescue of EGF-induced Akt activation relative to cells expressing wild type Gab1. E. The indicated cell lines were serum-starved for 24 hours and stimulated with 1 ng/ml NRG for varying periods of time at 37°C. Cell extracts were analyzed for activation of Akt by using antibodies that specifically recognize the phosphorylated form of Akt. Membranes were subsequently stripped and immunoblotted for Akt to confirm equal loading. Treatment of Gab1-deficient cells exogenously expressing ErbB3 with NRG results in a robust and sustained activation of Akt, while cells expressing exogenous Gab1 do not exhibit Akt activation in response to NRG treatment.