Signal Transduction for TNFα-Induced Type II SOCS Expression and Its Functional Implication in Growth Hormone Resistance in Carp Hepatocytes

In mammals, local production of tumor necrosis factor alpha (TNF alpha) inhibits growth hormone (GH)-induced IGF-I expression at tissue level and contributes to GH resistance caused by sepsis/endotoxemia and inflammation. Although the loss of GH responsiveness can be mediated by a parallel rise in SOCS expression, the signaling mechanisms for TNF alpha-induced SOCS expression at the hepatic level have not been characterized and the comparative aspects of the phenomenon, especially in lower vertebrates, are still unknown. Recently, type II SOCS, including SOCS1-3 and CISH, have been cloned in grass carp and shown to act as the feedback repressors for GH signaling via JAK(2)/STAT(5) pathway. To shed light on the mechanisms for TNF alpha-induced GH resistance in fish model, grass carp TNF alpha was cloned and confirmed to be a single-copy gene expressed in various tissues including the liver. In carp hepatocytes, incubation with the endotoxin LPS induced TNF alpha expression with parallel rises in SOCS1-3 and CISH mRNA levels. Similar to LPS, TNF alpha treatment could block GH-induced IGF-I/-II mRNA expression and elevate SOCS1, SOCS3, and CISH transcript levels. However, TNF alpha was not effective in altering SOCS2 expression. In parallel experiment, LPS blockade of IGF-I/-II signals caused by GH could be partially reverted by TNF alpha receptor antagonism. At hepatocyte level, TNF alpha induction also triggered rapid phosphorylation of I kappa B alpha, MEK1/2, ERK1/2, MKK3/6, P-38(MAPK), Akt, JAK(2), and STAT(1,3,5), and TNF alpha-induced SOCS1, SOCS3, and CISH mRNA expression could be negated by inhibiting the IKK/NF kappa B, MAPK, PI3K/Akt, and JAK/STAT cascades. Our findings, as a whole, suggest that local production of TNF alpha may interfere with IGF-I/-II induction by GH in the carp liver by up-regulation of SOCS1, SOCS3, and CISH via IKK/NF kappa B, MAPK, PI3K/Akt, and JAK/STAT-dependent mechanisms, which may contribute to GH resistance induced by endotoxin in carp species.