EGF receptor-mediated, c-Src-dependent, activation of Stat5b is downregulated in mitogenically responsive hepatocytes.
Signal transducer and activator of transcription (STAT) proteins may be activated by epidermal growth factor (EGF), but their role in EGF receptor-mediated mitogenic signaling is not clear. We previously showed that Stat5b was activated by EGF in rat hepatocytes in primary monolayer culture. In the present study, we found that EGF induced tyrosine phosphorylation of Stat5b both on Tyr-699, which correlated with specific DNA binding activity, and also on other tyrosine residues. The Src tyrosine kinase inhibitor CGP77675 blocked the EGF-induced activation of Stat5b, but did not affect the Stat5b activation by growth hormone (GH) or prolactin (PRL). The Stat5b response to EGF was most pronounced soon (3 h) after plating (early G(1)) and at high cell density (50,000 hepatocytes per cm(2)). However, at this cell density EGF did not stimulate DNA synthesis. In hepatocytes at 24 h of culturing (mid/late G(1)) with 20,000 cells per cm(2), EGF induced strong phosphorylation of the EGF receptor, as well as Shc and ERK, and stimulated DNA synthesis, but did not activate Stat5b, although the Stat5b response to GH or PRL was retained. A strong GH-induced Stat5b activation neither influenced the DNA synthesis alone nor enhanced the mitogenic effect of EGF. The results show that EGF induces tyrosine phosphorylation and DNA-binding activity of Stat5b in a manner different from GH and PRL, apparently by a Src-dependent mechanism. The data also provide further evidence that Stat5b is not required for mitogenic signaling from the EGF receptor.
Guren
Tormod K
TK
Department of Pharmacology, Faculty of Medicine, University of Oslo, Oslo, Norway. t.k.guren@labmed.uio.no
Ødegård
John
J
Abrahamsen
Hilde
H
Thoresen
G Hege
GH
Susa
Mira
M
Andersson
Yvonne
Y
Østby
Eva
E
Christoffersen
Thoralf
T
D000818:Animals; D002455:Cell Division / Q000187:drug effects; D002478:Cells, Cultured; D004247:DNA / Q000096:biosynthesis / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D015536:Down-Regulation / Q000187:drug effects; D004815:Epidermal Growth Factor / Q000037:antagonists & inhibitors / Q000494:pharmacology; D066246:ErbB Receptors / Q000378:metabolism*; D013006:Growth Hormone / Q000494:pharmacology; D022781:Hepatocytes / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008894:Milk Proteins; D008934:Mitogens / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects; D010768:Phosphoserine / Q000378:metabolism; D019000:Phosphotyrosine / Q000378:metabolism; D011388:Prolactin / Q000494:pharmacology; D011485:Protein Binding / Q000187:drug effects; D016392:Proto-Oncogene Proteins pp60(c-src) / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011743:Pyrimidines / Q000494:pharmacology; D011758:Pyrroles / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D050799:STAT5 Transcription Factor; D015534:Trans-Activators / Q000037:antagonists & inhibitors / Q000378:metabolism*; D016211:Transforming Growth Factor alpha / Q000494:pharmacology
D004268:DNA-Binding Proteins; D008894:Milk Proteins; D008934:Mitogens; D011743:Pyrimidines; D011758:Pyrroles; D050799:STAT5 Transcription Factor; C494097:Stat5b protein, rat; D015534:Trans-Activators; D016211:Transforming Growth Factor alpha; D010768:Phosphoserine; D019000:Phosphotyrosine; D004815:Epidermal Growth Factor; D011388:Prolactin; D013006:Growth Hormone; D004247:DNA; D066246:ErbB Receptors; D016392:Proto-Oncogene Proteins pp60(c-src); C119957:CGP 77675