Stimulation of phosphatidylinositol 3'-kinase (PI3K) and protein kinase B (
PKB) is implicated in the regulation of protein synthesis in various cells.
One mechanism involves PI3K/PKB-dependent phosphorylation of 4E-BP1, which
dissociates from eIF4E, allowing initiation of translation from the 7-meth
ylGTP cap of mRNAs, We examined the effects of insulin and H2O2 on this pat
hway in neonatal cardiac myocytes. Cardiac myocyte protein synthesis was in
creased by insulin, but was inhibited by H2O2. PI3K inhibitors attenuated b
asal levels of protein synthesis and inhibited the insulin-induced increase
in protein synthesis. Insulin or H2O2 increased the phosphorylation (activ
ation) of PKB through PI3K, but, whereas insulin induced a sustained respon
se, the response to H2O2 was transient, 4E-BP1 was phosphorylated in unstim
ulated cells, and 4E-BP1 phosphorylation was increased by insulin, H2O2 sti
mulated dephosphorylation of 4E-BP1 by increasing protein phosphatase (PP1/
PP2A) activity. This increased the association of 4E-BP1 with eIF4E, consis
tent with H2O2 inhibition of protein synthesis. The effects of H2O2 were su
fficient to override the stimulation of protein synthesis and 4E-BP1 phosph
orylation induced by insulin. These results indicate that PI3K and PKB are
important regulators of protein synthesis in cardiac myocytes, but other fa
ctors, including phosphatase activity, modulate the overall response.