Leptin-induced IL-6 production is mediated by leptin receptor, insulin receptor substrate-1, phosphatidylinositol 3-kinase, Akt, NF-kappaB, and p300 pathway in microglia

Leptin, the adipocyte-secreted hormone that centrally regulates weight control, is known to function as an immunomodulatory regulator. We investigated the signaling pathway involved in IL-6 production caused by leptin in microglia. Microglia expressed the long (OBRl) and short (OBRs) isoforms of the leptin receptor. Leptin caused concentration- and time-dependent increases in IL-6 production. Leptin-mediated IL-6 production was attenuated by OBRl receptor antisense oligonucleotide, PI3K inhibitor (Ly294002 and wortmannin), Akt inhibitor (1L-6-hydroxymethyl-chiro-inositol-2-((R)-2-O-methyl-3-O-octadecylcarbonate)), NF-kappaB inhibitor (pyrrolidine dithiocarbamate), IkappaB protease inhibitor (L-1-tosylamido-2-phenylenylethyl chloromethyl ketone), IkappaBalpha phosphorylation inhibitor (Bay 117082), or NF-kappaB inhibitor peptide. Transfection with insulin receptor substrate (IRS)-1 small-interference RNA or the dominant-negative mutant of p85 and Akt also inhibited the potentiating action of leptin. Stimulation of microglia with leptin activated IkappaB kinase alpha/IkappaB kinase beta, IkappaBalpha phosphorylation, IkappaBalpha degradation, p65 phosphorylation at Ser(276), p65 and p50 translocation from the cytosol to the nucleus, and kappaB-luciferase activity. Leptin-mediated an increase of IkappaB kinase alpha/IkappaB kinase beta activity, kappaB-luciferase activity, and p65 and p50 binding to the NF-kappaB element was inhibited by wortmannin, Akt inhibitor, and IRS-1 small-interference RNA. The binding of p65 and p50 to the NF-kappaB elements, as well as the recruitment of p300 and the enhancement of histone H3 and H4 acetylation on the IL-6 promoter was enhanced by leptin. Our results suggest that leptin increased IL-6 production in microglia via the leptin receptor/IRS-1/PI3K/Akt/NF-kappaB and p300 signaling pathway.