Effectsof Arginineon The Expression And Secretion Of Hepatic IGF-1 And Its Underlying Mechanism

Insulin-like growth factor 1(IGF-1)is an important endocrine hormone regulating the growth and development of animals.C irculating IGF-1 is mainly produced and secreted by the liver.In addition to endocrine hormones such as growth hormone(GH),nutritional levels,including proteins and amino acids,but also an important limiting factor affecting the level of IGF-1.Arginine(Arg)is a conditionally essential amino acid which closely related to the growth,development,metabolism,immunity and a series of physiological functions of animals.Toinvestigate the effect of Arg on the expression and secretion of hepatic IGF-1,the C57BL/6J mice(in vivo)andhuman hepatoma cell line HepG2cells(in vitro)were selected as the experiment model in this study.The results areasfollows: theconcentrationofserumIGF-1inmicewasremarkably decreased afterfasting,but sharply reboundedbyinjectionof Arg(P<0.05).After mice were intraperitoneallyinjected withArg,the expression and secretion levels of hepatic IGF-1 were markedly elevated(P<0.05).Under normal feeding conditions,long-term drinking water added withArg had no significant effect on body weight in mice(P>0.05).However,under 40% dietary restriction conditions,adding Arg in drinking water can significantly relieve the dietary restriction induced weight loss inmice(P<0.05).Moreover,theexpressionlevelofIGF-1wasobservably improvedafter Argadministrationin HepG2 cells,sodoesthelevelofintracellular IGF-1(P<0.05).To furtherexplore the regulatory mechanism of Arg on the expression and secretion of hepatic IGF-1,alterations of IGF-1 related signaling pathways in Hep G2 cells and C57BL/6J mice after Arg treatment were detected in this paper.The results areasfollows:Arg can significantly promote the phosphorylation level of hepatic mTOR(P<0.05),but noobvious alteration were be found on the expression of IGF-1 mRNA and protein after treatment with mTOR inhibitor rapamycin(P>0.05).In addition,Arg can evidentlyenhance the hepaticJAK2/STAT5 signaling pathway phosphorylation(P<0.05).The activity of JAK2/STAT5 signaling pathway aswellashepatic IGF-1 expression and secretion,were observablydecreased after Arg and JAK2 inhibitor AZD1480 cotreatment(P<0.05).Compared with GH single treatment group,the expression level of JAK2/STAT5/IGF-1 pathway were further enhanced in Arg and GH cotreatment group in HepG2 cells(P<0.05).These results indicated that the JAK2/STAT5 signaling pathway is involved in mediating the effects of Arg,whic h is independent of GH stimulation.Moreover,the mRNA expression levelsof hepatic proinflammatory cytokines(IL-1β,IL-6 and TNFα),the content of serum IL-1β,the expression of inflammatory pathway and the combinationbetween PTP1 Bor SOCS3 with JAK2 weredistinctly reduced after Argadministration(P<0.05).The activity of JAK2/STAT5 signaling pathway and the expression and secretion of hepatic IGF-1 were visibly depressed by Arg and LPS cotreatment(P<0.05).These results illustrated that Arg could diminish the negative regulation of inflammatory pathway on JAK2/STAT5 signaling pathway activity.The biotin-Argpull down test displayed that Arg can directly bind with TLR4,a membrane receptor ofinflammatory pathway.The expression of hepatic inflammatory pathway was increased whilethe expression of JAK2/STAT5/IGF-1 pathway as well as IGF-1 secretion weredecreased after TLR4 overexpression in mice(P<0.05),these alterations were reversedby TLR4 siRNA treatment in HepG2 cells(P<0.05).Similar to the TLR4 interference experiment,the effects of Arg were all disappearedafter liver-specific knockdown of TLR4 inmice(P>0.05).In summary,Argcanpromote the expression and secretion of IGF-1 in liver.Arginhibitsthe inflammatory pathway mediated by TLR4,thereby weakening the inhibitory effect of PTP1 B and SOCS3 on the activity of JAK2/STAT5 signaling pathway,then promote the activation of JAK2/STAT5 signaling pathway and ultimatelyleadtothe increased IGF-1 expression and secretion.O urstudy provide a experimentalbasis for further understanding the molecular nutritional mechanism of Arg inregulatinghepatic IGF-1 expression and secretion,and provide a theoretical basis for promoting the growth and development of animals and improving the efficiency of animal husbandry.