Submitted on December 2, 2005
Accepted on March 21, 2006
Tumor necrosis factor (TNF) inhibits growth hormone (GH)-mediated gene expression in hepatocytes
Tamer Ahmed1, Gladys Yumet1, Margaret L Shumate1, Charles H. Lang2, Peter C Rotwein3, and Robert N. Cooney4*
1 Surgery, The Pennsylvnia State University -College of Medicine, Hershey, Pennsylvania, United States
2 Dept. of Surgery; Dept. of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania, United States
3 Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon, United States
4 Surgery, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania, United States; Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania, United States
* To whom correspondence should be addressed. E-mail: rcooney{at}psu.edu.
GH stimulates STAT5 phosphorylation by JAK2 which activates IGF-I and Spi 2.1 transcription, while STAT5 dephosphorylation by protein tyrosine phosphatases (PTPs) terminates this signal. We hypothesized that the inhibitory effects of TNF on GH signaling and gene transcription were responsible for hepatic GH resistance. CWSV-1 hepatocytes were treated with TNF, pervanadate (a PTP inhibitor), or both prior to GH stimulation. Total and tyrosine-phosphorylated JAK2, STAT5, ERK1/2, SHP-1 and SHP-2, IGF-I and Spi 2.1 mRNA levels were measured. GH stimulated STAT5 and ERK1/2 phosphorylation, IGF-I and Spi 2.1 mRNA expression. TNF attenuated JAK2/STAT5 and ERK1/2 phosphorylation and IGF-I and Spi 2.1 mRNA expression following GH stimulation. SHP-1 and SHP-2 protein levels were unaltered by TNF or GH, and the GH-induced increase in SHP-1 PTP activity was not further increased by TNF. In TNF-treated cells, pervanadate restored STAT5 and ERK1/2 phosphorylation to control levels following GH stimulation, but did not restore IGF-I or Spi 2.1 mRNA induction. Cells transfected with a Spi 2.1 promoter-luciferase vector demonstrate a 50-fold induction in luciferase activity following GH stimulation or co-transfection with a constitutively active STAT5 vector. TNF prevented the induction of Spi 2.1 promoter activity by GH and the STAT5 construct. We conclude that TNF does not inhibit GH activity by inducing SHP-1 or -2 expression and that correction of GH signaling defects in TNF-treated cells by pervanadate does not restore GH-induced gene expression. The inhibitory effects of TNF on GH-mediated gene transcription appear independent of STAT5 activity and previously identified abnormalities in JAK2/STAT5 signaling.
