Stimulation of Insulin-Like Growth Factor Binding Protein-1 Synthesis by Interleukin-1 : Requirement of the Mitogen-Activated Protein Kinase Pathway

Abstract

Insulin-like growth factor (IGF) binding protein-1 (IGFBP-1) is a 28-kDa plasma protein that binds to IGF-I and IGF-II with high affinity. IGFBP-1 is elevated in the blood as a result of sepsis, AIDS, excessive alcohol consumption, and diabetes and may, in part, be responsible for the wasting observed during these pathophysiological conditions. The liver is the principal site of IGFBP-1 synthesis, and we have previously shown that proinflammatory cytokines can directly stimulate IGFBP-1 secretion in a human hepatoma cell line (HepG2). The purpose of the present study was to investigate the role of the MAP kinase pathway in regulating IGFBP-1 synthesis by IL-1β. We show that IL-1β stimulates the phosphorylation of ERK-1 and -2 in a time- and dose-dependent manner. In addition, the MAP kinase-kinase MEK-1 and the ribosomal S6-kinase RSK-1 are also phosphorylated in response to IL-1β. The transcription factor CREB, a potential substrate of both protein kinase A (PKA) and RSK-1, is phosphorylated in response to IL-1β and cAMP in HepG2 cells. The ability of IL-1β to stimulate the expression of IGFBP-1 and the phosphorylation of the above kinases was specifically inhibited by PD98059, a MEK-1 inhibitor. cAMP also stimulated IGFBP-1 synthesis, but PD98059 failed to block the cAMP effect. Conversely, a PKA inhibitor (H-89) inhibited the ability of cAMP, but not IL-1β to stimulate IGFBP-1 synthesis. The effect of IL-1β and cAMP on IGFBP-1 messenger RNA (mRNA) accumulation was additive. IL-1β, cAMP, PD98059, and H-89 had similar effects on the accumulation of IGFBP-1 protein and mRNA. IL-1β and cAMP did not change the half-life of IGFBP-1 mRNA, but PD98059 and SB202190, a p38 MAP kinase inhibitor, destabilized IGFBP-1 mRNA and blocked the phosphorylation of RSK-1 in response to IL-1β. Our data demonstrate that the MAP kinase signal transduction pathway plays an important role in the regulation of IGFBP-1 synthesis by IL-1β.