Role of interleukin-6 in the control of DNA synthesis of hepatocytes: involvement of PKC, p44/42 MAPKs, and PPARdelta.

Interleukin-6 (IL-6) is a pleiotropic cytokine with a pivotal role in normal hepatic growth and liver regeneration. Therefore, in the present study, we examined the effect of IL-6 on cell proliferation and the related signaling pathways in primary cultured chicken hepatocytes. IL-6 increased the level of [(3)H]thymidine incorporation in a time (>or= 6 hr)- and a dose (>or= 0.1 ng/ml)-dependent manner. Indeed, IL-6 increased the number of BrdU-positive cells and the total number of cells. IL-6 (10 ng/ml) increased the level of IL-6Ralpha and glycoprotein (gp) 130 (IL-6Rbeta) protein expression, Janus Kinase (JAK) 2, signal transducer and activator of transcription (STAT) 3, PKC, p44/42 MAPKs phosphorylation, and PPARdelta protein expression. Inhibition of each pathways blocked IL-6-induced [(3)H]thymidine incorporation increase. IL-6 increased c-fos, c-jun, and c-myc proto-oncogene mRNA levels and the percentage of cells in the S phase according to fluorescence-activated cell sorter (FACS) analysis. IL-6-induced G1/S phase progression was inhibited by AG 490 (2x10(-5) M, JAK2 inhibitor), a STAT3 inhibitor peptide (10(-5) M), bisindolylmaleimide I (10(-6) M, PKC inhibitor), PD 98059 (10(-5) M, p44/42 MAPKs blocker), or PPARdelta-specific small interfering RNAs (siRNAs). In conclusion, IL-6 stimulates the proliferation of primary cultured chicken hepatocytes through PKC, p44/42 MAPKs, and PPARdelta pathways.