Transcription of the insulin-like growth factor-binding protein-2 gene is increased in neonatal and fasted adult rat liver.

The insulin-like growth factor-binding proteins (IGFBPs) are a family of proteins that specifically bind IGF-I and IGF-II, determine their bioavailability to tissues, and modulate their actions in target tissues. Levels of IGFBPs in plasma and IGFBP mRNAs in liver are highly regulated with developmental age and metabolic status. We now demonstrate that the increase in IGFBP-2 mRNA in fasted adult rat liver and in the liver of normal neonatal rats reflects an increased rate of transcription. When adult rats were fasted for 2-3 days, IGFBP-2 mRNA was increased in liver, but not in brain or kidney. The increase in hepatic IGFBP-2 mRNA was observed after only 1 day of fasting. Levels decreased by half after 6 h of refeeding and returned to their low starting values after 2 days of refeeding. Transcription-elongation experiments indicated that transcription of the IGFBP-2 gene was increased in fasted liver. The rate of transcription increased 9.2- +/- 3.5-fold for transcripts labeled in exon 1 and 6.6- +/- 2.4-fold for transcripts labeled in exons 2, 3, and 4, suggesting that fasting causes a uniform increase in the number of RNA polymerase II molecules along the length of the IGFBP-2 gene. We infer from these results that the regulation of IGFBP-2 gene transcription in fasting occurs at the level of initiation rather than elongation. IGFBP-2 gene transcription also was increased 3.8- +/- 1.2-fold (exon 1) and 2.9- +/- 0.9-fold (exons 2, 3, and 4) in nuclei from 2-day postnatal rat liver compared with adult rat liver, consistent with the greater abundance of IGFBP-2 mRNA in neonatal rat liver.