Wang Y, Price S E, Jiang H
Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA.
Domest Anim Endocrinol. 2003 Nov;25(4):315-28. doi: 10.1016/j.domaniend.2003.06.001.
Insulin-like growth factor-I (IGF-I) is an important regulator of growth, development, and metabolism, and is the primary mediator of the growth-promoting activity of growth hormone (GH) in animals. In several species, the IGF-I polypeptide is generated from IGF-I mRNA containing either exon 1 (class 1 IGF-I mRNA) or exon 2 (class 2 IGF-I mRNA) as the leader exon. The objectives of this study were to identify class 1 and class 2 IGF-I mRNAs in cattle and to compare their expression in different tissues, their response to GH, and their translational efficiency. Three class 1 IGF-I cDNAs corresponding to three different transcription start sites in exon 1 and one class 2 IGF-I cDNA were identified from adult cattle liver using 5'-rapid amplification of cDNA ends (5'-RACE). Both classes of IGF-I mRNAs were expressed in a variety of tissues, with the highest level in liver; class 1 IGF-I mRNA was more abundant than class 2 IGF-I mRNA in all tissues. Six hours after a single intramuscular injection of 500 mg of recombinant bovine GH, class 1 and class 2 IGF-I mRNAs in steer liver were increased by 29% (P=0.07) and 62% (P<0.05), respectively. The luciferase reporter mRNA fused to a class 1 IGF-I 5'-untranslated region (5'-UTR) was translated four times more efficiently in vitro than the luciferase reporter mRNA fused to a class 2 IGF-I 5'-UTR (P<0.05). These results indicate that the IGF-I gene in cattle is transcribed as class 1 and class 2 IGF-I mRNAs and that the two classes of IGF-I mRNAs may be regulated differentially at both transcriptional and translational levels.
胰岛素样生长因子-I（IGF-I）是生长、发育和代谢的重要调节因子，是动物体内生长激素（GH）促生长活性的主要介导因子。在多个物种中，IGF-I多肽由含有外显子1（1类IGF-I mRNA）或外显子2（2类IGF-I mRNA）作为前导外显子的IGF-I mRNA产生。本研究的目的是鉴定牛的1类和2类IGF-I mRNA，并比较它们在不同组织中的表达、对GH的反应及其翻译效率。使用5'-cDNA末端快速扩增（5'-RACE）技术从成年牛肝脏中鉴定出3个与外显子1中3个不同转录起始位点相对应的1类IGF-I cDNA和1个2类IGF-I cDNA。两类IGF-I mRNA均在多种组织中表达，肝脏中表达水平最高；在所有组织中，1类IGF-I mRNA比2类IGF-I mRNA更丰富。单次肌内注射500 mg重组牛GH 6小时后，阉牛肝脏中1类和2类IGF-I mRNA分别增加了29%（P=0.07）和62%（P<0.05）。与2类IGF-I 5'-非翻译区（5'-UTR）融合的荧光素酶报告基因mRNA在体外的翻译效率比与1类IGF-I 5'-UTR融合的荧光素酶报告基因mRNA高4倍（P<0.05）。这些结果表明，牛的IGF-I基因转录为1类和2类IGF-I mRNA，且这两类IGF-I mRNA在转录和翻译水平上可能受到不同的调控。
