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LY 294002, an Inhibitor of Phosphatidylinositol 3-Kinase, Inhibits GH-Mediated Expression of the IGF-I Gene in Rat Hepatocytes

Department of Medicine, Veterans Affairs Chicago Healthcare System, Lakeside Division, and Northwestern University Medical School, Chicago, Illinois 60611

Address all correspondence and requests for reprints to: William L. Lowe, Jr., M.D., Center for Endocrinology, Metabolism, and Molecular Medicine, Tarry 15-703, Northwestern University Medical School, 303 East Chicago Avenue, Chicago, Illinois 60611. E-mail: wlowe{at}northwestern.edu

The signal transduction pathways that mediate GH-dependent regulation of IGF-I gene expression remain poorly defined. To establish a GH-responsive in vitro model system to study the effect of GH on the expression of the endogenous IGF-I gene, primary hepatocytes from adult male rats were prepared. These cells expressed both the GH receptor and the IGF-I gene, as demonstrated using a ribonuclease protection assay. Western blot analyses using antibodies directed against the phosphorylated forms of the ERKs, signal transducer and activator of transcription-5, and Akt/protein kinase B, a protein kinase that is downstream of PI3K, demonstrated GH-dependent phosphorylation of these signaling molecules. These signaling molecules are components of the major signal transduction pathways that are activated by GH. To determine whether GH-responsive IGF-I gene expression could be demonstrated in these cells, hepatocytes were treated without or with 50 ng/ml GH for 3–48 h. IGF-I mRNA levels increased within 3 h, and a maximal 2.2-fold increase was observed after 24 h of GH treatment. To determine whether ERK activation contributes to GH-induced IGF-I gene expression, hepatocytes were treated for 12 h without or with 50 ng/ml GH and 50 µM PD98059, an inhibitor of MAPK kinase-1 and -2. Treatment with PD98059 did not have a significant effect on GH-induced IGF-I gene expression. Similar studies were performed using 50 µM LY 294002, an inhibitor of PI3K. These studies demonstrated that treatment with LY 294002 completely abrogated GH-induced IGF-I gene expression. In contrast, PI3K-specific doses of another inhibitor of PI3K, wortmannin, failed to inhibit the GH-induced increase in IGF-I mRNA levels. In summary, rat hepatocytes in primary culture provide a good model system to study GH-induced IGF-I gene expression, and the results of these studies suggest that a signaling pathway inhibited by LY 294002, possibly a PI3K-dependent pathway, is important for GH-stimulated IGF-I gene expression in hepatocytes.

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