Session: FP01-Cell Specific GH & IGF-1 Signaling
Room 134 (Moscone Center)
Poster Board SAT-91
Adaptation under fasting conditions is critical for survival in animals. SIRT1 a protein deacetylase, plays an essential role for adaptive metabolic and endocrine responses under fasting conditions by modifying acetylating status of various proteins. Fasting induces growth hormone (GH) resistance in the liver and thus decreases serum insulin-like growth factor-I (IGF-I) levels as an endocrine adaptation for malnutrition; however, the underlying mechanisms are yet to be fully elucidated. In the present study, we aim to clarify the involvement of SIRT1 in GH induced IGF-I production under the fasting state in the liver.
We analyzed the effect of SIRT1 activators and inhibitors on IGF-I mRNA induction and STAT5 phosphorylation by GH in human hepatocellular carcinoma cell line HepG2 cells and rat primary hepatocytes in vitro and in mouse liver in vivo. In addition, the effects of overexpression of SIRT1, dominant-negative (DN) SIRT1, and knockdown (KD) of SIRT1 on IGF-I production and STAT5 phosphorylation both in vitro and vivo were examined. Furthermore, the interaction between STAT5 and SIRT1, and deacetylation of STAT5 by SIRT1 were analyzed.
Knockdown of SIRT1 in the liver resulted in a restoration of fasting-induced decrease in serum IGF-I levels and enhanced GH-dependent increase in IGF-I levels, indicating that SIRT1 negatively regulates GH-dependent IGF-I production in vivo. In vitro analysis, when cells were treated with SIRT1 activator or SIRT1 overexpression, GH-induced IGF-I mRNA production was suppressed concomitant with a decrease in tyrosine phosphorylation of STAT5. Conversely, when the activity was suppressed by SIRT1 inhibitors, overexpression of DN SIRT1, or KD of SIRT1, the IGF-I production by GH was enhanced concomitant with a decrease in tyrosine phosphorylation of STAT5. GST pull-down assay exhibited that STAT5 directly interacts with SIRT1. When the lysine residues in STAT5 neighboring to the SH2 domain were mutated, the acetylation of STAT5 protein was decreased accompanied by a reduction in the GH-induced tyrosine phosphorylation and its transcriptional activity. Knockdown of SIRT1 in the liver enhanced the acetylation of STAT5, GH-induced tyrosine phosphorylation of STAT5, and the GH-induced interaction between GH receptor and STAT5 in vivo, indicating that SIRT1 deacetylates STAT5 and thus inhibits the interaction between GH receptor and STAT5.
SIRT1 directly interacts with STAT5 and negatively regulates GH-induced STAT5 phosphorylation and IGF-I production via deactylation of STAT5 in the liver. These results explain at least part of the underlying mechanisms of GH resistance under fasting conditions.
Nothing to Disclose: MY, GI, HF, KS, HB, MT, HN, YT
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