Cycloheximide stabilizes insulin-like growth factor-binding protein-1 (IGFBP-1) mRNA and inhibits IGFBP-1 transcription in H4-II-E rat hepatoma cells.
The insulin-like growth factor-binding proteins (IGFBPs) are a family of six proteins that modulate the biological activity of IGF-I and IGF-II and determine their bioavailability to tissues. One of the IGFBPs, IGFBP-1, is distinctive in the dynamic response of its levels in human plasma to metabolic changes. Parallel changes occur in IGFBP-1 mRNA and IGFBP-1 transcription in rat liver. Using the well differentiated H4-II-E rat hepatoma cell line as a model system, we demonstrated previously that IGFBP-1 transcription is positively regulated by dexamethasone and negatively regulated by insulin. We now examine the effect of the protein synthesis inhibitor, cycloheximide, on the hormonal regulation of IGFBP-1 gene expression. Preincubation of H4-II-E cells with 10.7 microM cycloheximide for 1.5 h did not prevent the induction of IGFBP-1 mRNA and IGFBP-1 transcription (determined in nuclear run-on assays) by dexamethasone. By contrast, cycloheximide treatment abolished the decrease in IGFBP-1 mRNA induced by insulin. Insulin rapidly decreased IGFBP-1 transcription in the absence of cycloheximide (> 50% inhibition in 20 min) and caused a similar decrease in cells pretreated with cycloheximide. Cycloheximide alone also decreased IGFBP-1 transcription. Similar results were observed with a second protein synthesis inhibitor, anisomycin, which also prevented the insulin-induced decrease in IGFBP-1 mRNA without abolishing the insulin-induced inhibition of IGFBP-1 transcription. These results suggest that although insulin decreases IGFBP-1 gene transcription in the presence of protein synthesis inhibitors, IGFBP-1 mRNA levels are maintained because of stabilization of the mRNA. Stabilization was demonstrated directly in actinomycin D-treated cells, where the t1/2 of IGFBP-1 mRNA increased from approximately 2 to approximately 20 h in the presence of cycloheximide; insulin did not affect IGFBP-1 mRNA turnover. Thus, cycloheximide-sensitive labile proteins contribute to the maintenance of basal IGFBP-1 promoter activity and the rapid turnover of IGFBP-1 mRNA, which determine the dynamic regulation of IGFBP-1 gene expression.
Ooi
G T
GT
Growth and Development Section, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892.
Brown
D R
DR
Suh
D S
DS
Tseng
L Y
LY
Rechler
M M
MM
D000818:Animals; D000841:Anisomycin / Q000494:pharmacology; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism; D003513:Cycloheximide / Q000494:pharmacology*; D003907:Dexamethasone / Q000494:pharmacology; D005786:Gene Expression Regulation / Q000187:drug effects; D006207:Half-Life; D007328:Insulin / Q000502:physiology; D018970:Insulin-Like Growth Factor Binding Protein 1; D008114:Liver Neoplasms, Experimental; D010729:Phosphoenolpyruvate Carboxykinase (GTP) / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism*; D051381:Rats; D013552:Swine; D014158:Transcription, Genetic / Q000187:drug effects*; D014407:Tumor Cells, Cultured
D002352:Carrier Proteins; D007328:Insulin; D018970:Insulin-Like Growth Factor Binding Protein 1; D012333:RNA, Messenger; D000841:Anisomycin; D003907:Dexamethasone; D003513:Cycloheximide; D010729:Phosphoenolpyruvate Carboxykinase (GTP)