EFFECT OF FASTING ON INSULIN-LIKE-GROWTH-FACTOR (IGF)-IA AND IGF-IB MESSENGER RIBONUCLEIC-ACIDS AND PREHORMONES IN RAT-LIVER

The insulin-like growth factor I(IGF-I) gene generates by alternative splicing two IGF-I messenger RNAs (mRNAs) coding for IGF-I prehormones with different E domain sequences. In rats, these two mRNAs differ by the presence (IGF-IB) or absence (IGF-IA) of a 52-bp insert in the E domain coding region. The purpose of this study was to investigate the effect of nutritional perturbation on IGF-IA and -IB expression in ra t liver. Northern blot analysis of liver mRNA revealed that the 1.5-1. 9 kb and 0.9-1.2 kb IGF-I mRNA species were decreased in rats fasted f or 48 h compared with either fasted-refed (48 h of each, or control-fe d rats (each, P < 0.01), whereas the 7.5 kb IGF-I mRNA was decreased o nly when compared with the fasted-refed animals. Using semiquantitativ e RT-PCR, the IGF-IA transcript (114 bp amplicon) was not altered, whe reas the IGF-IB transcript (166 bp amplicon) was decreased in fasted r ats compared with the other two groups (both P < 0.01). We confirmed t he RT-PCR results by RNase protection assay (RPA), observing that the IGF-U (224 and 100 bases protected) was not decreased and that the IGF -IB transcript (376 bases protected), accounting for only 23% of the t otal IGF-I transcripts of control fed rats, was decreased by fasting. Because the results from RT-PCR and RPA do not necessarily predict ful l-length translatable mRNA, we subjected hepatic IGF-I transcripts to in vitro, translation, and we immunoprecipitated IGF-IA and -IB prehor mones. Both prehormones were translated principally from exon 1-contai ning mRNAs, with molecular weights of about 17K and 18K, representing 80% and 20% of the total IGF-I prehormones observed in control fed rat s, respectively. Both peptides were reduced in fasted rats compared wi th controls (P < 0.01), and refeeding restored both. By immunoblotting of the protein extract from liver of fasted rats, IGF-LA Ras decrease d by 77% compared with control-fed animals. Refeeding returned IGF-LA to normal. The lack of reduction of IGF-IA transcript at the alternati ve splice site suggests that posttranscriptional mechanisms are respon sible for the reduction in steady-state IGF-I mRNAs that occurs during fasting. Additionally, we present evidence that biosynthesis of IGF-I A and -IB prehormones by liver is impaired at a posttranscriptional le vel.