The effect of fasting on insulin-like growth factor-I nuclear transcript abundance in rat liver.
The abundance of insulin-like growth factor I (IGF-I) messenger RNA (mRNA) is decreased in the liver of fasting, protein-restricted, and energy-restricted rats. The extent to which this decrease in steady state mRNA abundance may be attributed to a decrease in IGF-I gene transcription remains unresolved. In the present study, we used an RNase protection assay to quantify IGF-I nuclear transcript (pre-mRNA) and mRNA abundance in whole cellular RNA isolated from liver of fasted and nonfasted male rats (4-6 weeks of age). The results of the RNase protection assay of IGF-I nuclear transcripts were strongly correlated with the results of nuclear transcription elongation (run-on) assays (r > 0.90; P < 0.001). In addition, the RNase protection assay allows for a greater capability for sensitively monitoring gene transcription in a large number of samples. In four different experiments, a consistent decrease in the quantity of IGF-I nuclear transcripts was observed in liver of animals fasted for 72 h, whereas IGF-I pre-mRNA abundance in animals fed ad libitum was highly variable (average intraassay coefficient of variation = 74% vs. 34% for nonfasted and fasted groups). When data from the four experiments were pooled, fasting reduced IGF-I pre-mRNA and mRNA levels by 78% and 70% (P < 0.001), respectively. Fasting also caused a significant decrease in mRNA and nuclear transcript abundance for another nutritionally sensitive gene, the gene encoding transthyretin (TTR). To determine whether the decrease in IGF-I and TTR nuclear transcripts was gene specific, levels of nuclear transcripts for serum albumin, H-ferritin, and ribosomal RNA were also quantified. The results indicated that serum albumin, H-ferritin, and ribosomal RNA nuclear transcripts were not decreased by fasting, demonstrating that the negative effect of fasting was specific for IGF-I and TTR. In summary, these results indicate that IGF-I and TTR nuclear transcripts are specifically decreased by fasting. The decrease in IGF-I mRNA is matched by a similar decrease in IGF-I nuclear transcripts, suggesting that fasting controls IGF-I gene expression primarily at the transcriptional level.
Hayden
J M
JM
Biomedical Sciences Division, University of California, Riverside 92521-0121.
Marten
N W
NW
Burke
E J
EJ
Straus
D S
DS
D000818:Animals; D001483:Base Sequence; D002467:Cell Nucleus / Q000378:metabolism*; D003001:Cloning, Molecular; D017931:DNA Primers; D005091:Exons; D005215:Fasting / Q000378:metabolism*; D005293:Ferritins / Q000096:biosynthesis / Q000235:genetics; D015870:Gene Expression; D007334:Insulin-Like Growth Factor I / Q000096:biosynthesis* / Q000235:genetics; D007438:Introns; D008099:Liver / Q000378:metabolism*; D008297:Male; D008969:Molecular Sequence Data; D016133:Polymerase Chain Reaction; D011228:Prealbumin / Q000096:biosynthesis / Q000235:genetics; D012322:RNA Precursors / Q000378:metabolism*; D012333:RNA, Messenger / Q000096:biosynthesis / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012709:Serum Albumin / Q000096:biosynthesis / Q000235:genetics; D013997:Time Factors; D014158:Transcription, Genetic
D017931:DNA Primers; D011228:Prealbumin; D012322:RNA Precursors; D012333:RNA, Messenger; D012709:Serum Albumin; D007334:Insulin-Like Growth Factor I; D005293:Ferritins