Gabrielsson B G, Carmignac D F, Flavell D M, Robinson I C
Division of Neurophysiology and Neuropharmacology, National Institute for Medical Research, London, United Kingdom.
Endocrinology. 1995 Jan;136(1):209-17. doi: 10.1210/endo.136.1.7828533.
In the rat, the GH receptor (GHR) and the GH-binding protein (GHBP), which arise from alternative splicing of the same gene, show a sexually dimorphic and GH-dependent expression pattern. Multiple alternative 5'-untranslated regions (UTRs) are present in GHR and GHBP transcripts in the rat, one of which, GHR1, has recently been shown to be liver specific and found at higher levels in females. We have measured the hepatic GHR1, GHR, and GHBP transcript levels, by RNase protection and solution hybridization assay, in animals with differing hormonal status, in which hepatic GHR binding and plasma GHBP have been previously assayed. Estradiol (E2) induced GHR1 in males, whereas ovariectomy or the antiestrogen tamoxifen reduced GHR1 expression in females. The induction of GHR1 by E2 was GH dependent, being lower in GH-deficient dwarf rats and absent in hypophysectomized rats, paralleling previous measurements of plasma GHBP and hepatic GHR binding in these animals. Significant changes in GHR1 could explain the trends seen in the same extracts when coding region probes were used. Short-term adrenalectomy had no effect on GHR and GHBP expression, but dexamethasone markedly reduced both protein and messenger RNA (mRNA) levels. Corticosterone treatment had no effect alone but reduced the E2-induced increase in GHR1 levels, whereas methylprednisolone administered orally reduced hepatic GH binding, plasma GHBP, and GHR1 mRNA levels. Thus, 5'-UTRs, encoded by different first exons, are involved in the regulation of hepatic GHR and GHBP expression and need to be considered when comparing effects of hormonal manipulation on the mRNA transcripts and protein products of the GHR gene. Previous studies have found discrepancies between levels of protein expression and mRNA transcripts measured only with coding region probes. Our results suggest that posttranscriptional differences related to 5'-UTR heterogeneity in the GHR gene explain some of these discrepancies.
在大鼠中，生长激素受体（GHR）和生长激素结合蛋白（GHBP）由同一基因的可变剪接产生，呈现出性别二态性且依赖于生长激素的表达模式。大鼠的GHR和GHBP转录本中存在多个可变的5'非翻译区（UTR），其中之一GHR1最近被证明具有肝脏特异性，且在雌性大鼠中水平更高。我们通过核糖核酸酶保护和溶液杂交试验，测量了处于不同激素状态动物的肝脏GHR1、GHR和GHBP转录本水平，此前已对这些动物的肝脏GHR结合和血浆GHBP进行了检测。雌二醇（E2）在雄性大鼠中诱导GHR1表达，而卵巢切除术或抗雌激素他莫昔芬则降低雌性大鼠的GHR1表达。E2对GHR1的诱导作用依赖于生长激素，在生长激素缺乏的侏儒大鼠中较低，在垂体切除的大鼠中则不存在，这与之前对这些动物血浆GHBP和肝脏GHR结合的测量结果一致。当使用编码区探针时，GHR1的显著变化可以解释在相同提取物中观察到的趋势。短期肾上腺切除术对GHR和GHBP表达没有影响，但地塞米松显著降低了蛋白质和信使核糖核酸（mRNA）水平。单独使用皮质酮治疗没有效果，但降低了E2诱导的GHR1水平升高，而口服甲基泼尼松龙则降低了肝脏GH结合、血浆GHBP和GHR1 mRNA水平。因此，由不同第一外显子编码的5'UTR参与了肝脏GHR和GHBP表达的调节，在比较激素操作对GHR基因的mRNA转录本和蛋白质产物的影响时需要考虑这一点。先前的研究发现，仅用编码区探针测量的蛋白质表达水平和mRNA转录本水平之间存在差异。我们的结果表明，与GHR基因中5'UTR异质性相关的转录后差异解释了其中一些差异。
