Dexamethasone Stimulates Transcription of the Insulin-Like Growth Factor-Binding Protein-1 Gene in H4-II-E Rat Hepatoma Cells

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Abstract
Binding proteins for insulin-like growth factors (IGFBP) are important modulators of the biological actions of IGF-I and IGF-II. Concentrations of one of these proteins, IGFBP-1, in human plasma and IGFBP-1 mRNA in rat liver are markedly altered in diabetes and fasting. We now examine the regulation of IGFBP-1 and IGFBP-I mRNA in H4-II-E cells, a rat cell line derived from the minimal deviation H35 Reuber hepatoma previously reported to synthesize IGFBP-1 as its predominant IGF-binding protein. Confluent H4-II-E cells in serum-free medium were incubated with different hormones for 48 h, and the conditioned medium was analyzed by ligand blotting. Dexamethasone (10-6 M) increased levels of 30-kDa IGFBP-1 approximately 10-fold; stimulation was half-maximal at 6 .times. 10-9 M dexamethasone. No stimulation was seen with progesterone, testosterone, IGF-I, or rat GH, whereas insulin gave a small inhibition. Immunoblot analysis using a monoclonal antibody to human IGFBP-1 confirmed that the 30-kDa IGFBP induced by dexamethasone was IGFBP-1. IGFBP-1 mRNA was increased to a similar extent (7-fold), as determined by Northern blot hybridization using human or rat IGFBP-1 cDNA probes. the stimulation of IGFBP-1 mRNA was observed within 3 h after the addition of dexamethasone; IGFBP-1 in the medium increased more slowly. After withdrawal of dexamethasone from stimulated cells, IGFBP-1 mRNA decreased by 80% after 48 h; IGFBP-1 decreased more slowly. The increased abundance of IGFBP-1 mRNA in dexamethasone-treated cells primarily reflected increased transcription rather than increased mRNA stability. The half-life of IGFBP-1 mRNA in unstimulated cells was 1.7 and increased 50% in dexamethasone-treated cells. This increase is too small to account for the 10-fold increase in steady state levels of IGFBP-1 mRNA. The H4-II-E cell line provides a useful system to define the molecular mechanisms of IGFBP-1 gene regulation and the biological role of IGFBP-I.