Role of insulin receptor substrate-4 in IGF-I-stimulated HEPG2 proliferation.
OBJECTIVE
Insulin receptor substrate-4 (IRS-4) is a scaffold protein that mediates the actions of insulin-like growth factor-I (IGF-I). Its expression increases dramatically after partial hepatectomy (a liver regeneration model). Herein, we report IRS-4 expression in a human hepatoblastoma cell line (HepG2) and IGF-I-dependent IRS-4 tyrosine phosphorylation.


METHODS
The role of IRS-4 in HepG2 proliferation was established by RNA interference (siRNA). After 72h of transfection with IRS-4 siRNA, we observed a specific reduction in IRS-4 expression.


RESULTS
Depletion of IRS-4 levels decreased ERK phosphorylation, p70S6K phosphorylation and IGF-I-stimulated cell proliferation. Changes in ERK phosphorylation in IRS-4-depleted cells were independent of ras/raf/MEK1/2- and PI3K/Akt-cascades. IRS-4 down-regulation abolished IGF-I-, TPA- and IGF-I plus TPA-stimulated ERK and p70S6K activities. Our results suggest that PKC-epsilon mediates the effect of IRS-4 on ERK activity. Moreover, decreased IRS-4 levels diminished FBS- and IGF-I-stimulated HepG2 growth and cause stress fiber disruption in HepG2 cell line.


CONCLUSIONS
Collectively, our data suggest that IRS-4 plays an important role in HepG2 proliferation/differentiation and exerts its actions through ERK and p70S6K activation in a ras/raf/MEK1/2- and PI3Kinase/Akt-independent manner and in a PKC-dependent way.
Cuevas
Eva P
EP
Unidad de Toxicología Molecular Hepática, Departamento de Bioquímica y Biología Molecular, Universidad de Alcalá, E-28871 Alcalá de Henares, Spain.
Escribano
Oscar
O
Chiloeches
Antonio
A
Ramirez Rubio
Sara
S
Román
Irene Dolores
ID
Fernández-Moreno
María Dolores
MD
Guijarro
Luis G
LG
D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism*; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D005786:Gene Expression Regulation; D006498:Hepatectomy; D022781:Hepatocytes / Q000166:cytology; D006801:Humans; D055504:Insulin Receptor Substrate Proteins; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D008099:Liver / Q000473:pathology; D008858:Microscopy, Phase-Contrast; D008954:Models, Biological; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism; D013997:Time Factors; D014162:Transfection
D048868:Adaptor Proteins, Signal Transducing; C107686:IRS4 protein, human; D055504:Insulin Receptor Substrate Proteins; D007334:Insulin-Like Growth Factor I; D011493:Protein Kinase C