Insulin Is a Prominent Modulator of the Cytokine-Stimulated Expression of Acute-Phase Plasma Protein Genes

Abstract

Several endocrine hormones which influence liver metabolism are known to increase in activity during the acute phase of injury or inflammation. We determined whether these hormones have the potential to influence acute-phase protein production in human and rat hepatoma cells. Catecholamines, glucagon, growth hormone, triiodothyronine, and cyclic nucleotides individually or in combination did not modulate the basal or the interleukin-1 (IL-1)-, IL-6-, and dexamethasone-stimulated levels of acute-phase plasma proteins. Insulin, however, was found to be a rapid, nonspecific, and dose-dependent inhibitor of the cytokine and glucocorticoid stimulation of acute-phase protein gene expression and to exert its effect at the transcriptional level. The insulin inhibition applied to all cytokines tested but to various degrees, depending upon the particular acute-phase gene. Insulin resulted in an early and prominent increase in the transcription of genes encoding the AP-1 components of JunA, JunB, and c-Fos, as has been observed for other growth factors. However, the effect of insulin on C/EBPβ was unexpected and paradoxical: while insulin completely inhibited the transcriptional activation of the C/EBPβ gene in cytokine- and dexamethasone-treated cells, the level of cytoplasmic C/EBPβ RNA was elevated. Quantitation of C/EBPβ mRNA by Northern (RNA) blot analysis and of C/EBPβ DNA binding activity by Southwestern (DNA-protein) blot analysis showed that insulin, when combined with cytokines and dexamethasone, stimulated both the mRNA and DNA binding activity by a factor of 1.6 compared with that of cells treated with cytokines and dexamethasone alone. Transient transfection of H-35 and HepG2 cells with a chloramphenicol acetyltransferase (CAT) gene expression vector containing the C/EBPβ response element also resulted in a 1.5-fold increase of C/EBPβ-mediated transcription in insulin-treated cells. Transfection of CAT gene constructs containing increasing lengths of haptoglobin gene 5′ flanking sequences indicated that insulin inhibition of IL-6 stimulation required the presence of the region from —4100 to —1030. These results suggest that insulin has the potential to control the transcription of acute-phase genes by at least two separate mechanisms.