Adamo M, Lowe W L, LeRoith D, Roberts C T
Section of Molecular and Cellular Physiology, Diabetes Branch, National Institute of Diabetes, Bethesda, Maryland 20892.
Endocrinology. 1989 Jun;124(6):2737-44. doi: 10.1210/endo-124-6-2737.
Solution hybridization/RNase protection assays were used to study the developmental expression of insulin-like growth factor I (IGF-I) mRNA levels in rats. In liver, heart, and kidney, total IGF-I mRNA levels were low at birth and increased during the 50-day postnatal period, with liver levels increasing by over 100-fold. In contrast, stomach, muscle, and testicular IGF-I mRNA levels were highest at the earliest stages examined (late fetal or early neonatal) and declined thereafter to the levels observed in 50-day-old rats. In brain, IGF-I mRNA levels rose 2-fold during the first week of life and declined over the next 6-7 weeks. Lung IGF-I mRNA levels were highest in 20-day-old fetuses and exhibited some fluctuation during the postnatal period. Alternative splicing in the 5'-untranslated region of the primary rat IGF-I transcript gives rise to three transcripts, classes A, B, and C, which have divergent 5'-untranslated region sequences associated with a common region that encodes the mature IGF-I peptide. These sequences contain upstream in-frame translation initiation codons and may, therefore, encode alternate IGF-I prepropeptides. The class C variant was the predominant mRNA species at all stages of development studied and was the only IGF-I transcript in brain, heart, and muscle. In tissues where multiple 5'-untranslated region splicing variants occurred, therefore, changes in total IGF-I mRNA primarily reflected changes in this splicing variant. However, the class C and class A (as well as class B in liver) transcripts exhibited temporally divergent changes over some developmental intervals. Class A transcripts in the liver, stomach, testes, and lung as well as class B transcripts in liver, exhibited sustained increases from 15 or 22 postnatal days to maximal levels at 50 postnatal days. In kidney, class A transcripts also increased steadily, but beginning at an earlier stage, i.e. at 8-15 days of postnatal life. These results demonstrate that the temporal expression of total IGF-I mRNA in the developing rat occurs in a tissue-specific manner, and additionally, that IGF-I mRNA variants are differentially expressed during development.
采用溶液杂交/核糖核酸酶保护分析方法研究大鼠胰岛素样生长因子I（IGF-I）mRNA水平的发育性表达。在肝脏、心脏和肾脏中，出生时总的IGF-I mRNA水平较低，在出生后50天内升高，肝脏中的水平升高超过100倍。相比之下，胃、肌肉和睾丸中的IGF-I mRNA水平在最早检测阶段（胎儿晚期或新生儿早期）最高，此后下降至50日龄大鼠中观察到的水平。在大脑中，IGF-I mRNA水平在出生后第一周上升2倍，在接下来的6 - 7周内下降。肺中IGF-I mRNA水平在20日龄胎儿中最高，在出生后时期有一些波动。大鼠IGF-I初级转录本5'-非翻译区的可变剪接产生三种转录本，A类、B类和C类，它们具有与编码成熟IGF-I肽的共同区域相关的不同5'-非翻译区序列。这些序列包含上游框内翻译起始密码子，因此可能编码不同的IGF-I前原肽。C类变体是所研究的所有发育阶段中主要的mRNA种类，并且是大脑、心脏和肌肉中唯一的IGF-I转录本。因此，在发生多种5'-非翻译区剪接变体的组织中，总的IGF-I mRNA的变化主要反映了这种剪接变体的变化。然而，C类和A类（以及肝脏中的B类）转录本在一些发育间隔中表现出时间上不同的变化。肝脏、胃、睾丸和肺中的A类转录本以及肝脏中的B类转录本从出生后15或22天持续增加至出生后50天的最高水平。在肾脏中，A类转录本也稳步增加，但从更早阶段开始，即出生后8 - 15天。这些结果表明，发育中大鼠总的IGF-I mRNA的时间表达以组织特异性方式发生，此外，IGF-I mRNA变体在发育过程中差异表达。
