Romanus J A, Yang Y W, Adams S O, Sofair A N, Tseng L Y, Nissley S P, Rechler M M
Growth and Development Section, National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda, Maryland 20892.
Endocrinology. 1988 Feb;122(2):709-16. doi: 10.1210/endo-122-2-709.
The single insulin-like growth factor II (IGF-II) gene is transcribed into multiple RNA species in most fetal and neonatal rat tissues. For IGF-II to serve as a local growth factor in fetal tissues, IGF-II RNA must be translated into pre-pro-rat (r) IGF-II, and the biosynthetic precursor processed to smaller biologically active forms. IGF-II RNA extracted from fetal rat liver, muscle, intestine, lung, and stomach, from rat placenta, and from fetal or neonatal mouse liver and lung directed the synthesis of 22,000 mol wt pre-pro-IGF-II in a reticulocyte lysate cell-free translation system. A biosynthetic precursor of this size had been observed previously in translation of RNA from BRL-3A rat liver cells and is predicted by the nucleotide sequence of cDNA clones encoding rIGF-II. Consistent with the developmental pattern of expression of IGF-II RNA observed in hybridization studies, RNA from adult rat liver, muscle, and intestine did not direct the synthesis of pre-pro-rIGF-II. To determine whether the IGF-II biosynthetic precursor was processed to smaller biologically active IGF-II, term fetal rat tissues were extracted with acid-ethanol, the extracts were fractionated by acid gel filtration, and the IGF pools were examined in a RIA specific for IGF-II. Levels of 1-2 micrograms/g were observed in liver, limb, lung, intestine, and brain; lower levels were observed in heart and kidney. In general, the levels of immunoreactive IGF-II corresponded to the levels of IGF-II mRNA. These results suggest that IGF-II mRNA is translated, and pre-pro-IGF-II processed to mature IGF-II in different fetal rat tissues. In contrast to IGF-I, in which alternative RNA splicing generates possible precursor molecules containing different COOH-terminal propeptide segments, we find no evidence for an IGF-II precursor in rat tissues other than 22,000 mol wt pre-pro-rIGF-II.
在大多数胎鼠和新生大鼠组织中，单一的胰岛素样生长因子II（IGF-II）基因转录成多种RNA。为使IGF-II在胎儿组织中作为局部生长因子发挥作用，IGF-II RNA必须翻译成前胰岛素原-大鼠（r）IGF-II，且生物合成前体要加工成更小的具有生物活性的形式。从胎鼠肝脏、肌肉、肠道、肺和胃、大鼠胎盘以及胎鼠或新生小鼠肝脏和肺中提取的IGF-II RNA，在网织红细胞裂解物无细胞翻译系统中指导合成了分子量为22,000道尔顿的前胰岛素原-IGF-II。此前在翻译来自BRL-3A大鼠肝细胞的RNA时曾观察到这种大小的生物合成前体，并且由编码rIGF-II的cDNA克隆的核苷酸序列预测得到。与杂交研究中观察到的IGF-II RNA表达的发育模式一致，成年大鼠肝脏、肌肉和肠道的RNA不能指导前胰岛素原-rIGF-II的合成。为确定IGF-II生物合成前体是否加工成更小的具有生物活性的IGF-II，用酸乙醇提取足月胎鼠组织，提取物经酸性凝胶过滤分级分离，并用对IGF-II特异的放射免疫分析（RIA）检测IGF库。在肝脏、肢体、肺、肠道和大脑中观察到的水平为1 - 2微克/克；在心脏和肾脏中观察到的水平较低。一般来说，免疫反应性IGF-II的水平与IGF-II mRNA的水平相对应。这些结果表明，IGF-II mRNA在不同的胎鼠组织中被翻译，且前胰岛素原-IGF-II被加工成成熟的IGF-II。与IGF-I不同，IGF-I通过可变RNA剪接产生可能含有不同COOH末端前肽段的前体分子，我们在大鼠组织中未发现除分子量为22,000道尔顿的前胰岛素原-rIGF-II之外的IGF-II前体的证据。
