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EGF receptor‐mediated, c‐Src‐dependent, activation of Stat5b is downregulated in mitogenically responsive hepatocytes
Author(s) -
Guren Tormod K.,
Ødegård John,
Abrahamsen Hilde,
Thoresen G. Hege,
Susa Mira,
Andersson Yvonne,
Østby Eva,
Christoffersen Thoralf
Publication year - 2003
Publication title -
journal of cellular physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.529
H-Index - 174
eISSN - 1097-4652
pISSN - 0021-9541
DOI - 10.1002/jcp.10282
Subject(s) - epidermal growth factor , tyrosine phosphorylation , phosphorylation , biology , dna synthesis , heparin binding egf like growth factor , receptor , signal transduction , microbiology and biotechnology , tyrosine kinase , proto oncogene tyrosine protein kinase src , medicine , mapk/erk pathway , endocrinology , biochemistry , dna
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. © 2003 Wiley‐Liss, Inc.