Green R P, Birkenmeier E H, Beamer W G, Maltais L J, Gordon J I
Department of Biological Chemistry, Washington University School of Medicine, Saint Louis, MO 63110.
Proc Natl Acad Sci U S A. 1988 Aug;85(15):5592-6. doi: 10.1073/pnas.85.15.5592.
Thyroid hormone has been implicated as an important factor in rodent development. We have used a strain of mice with a recessive mutation producing congenital primary hypothyroidism (C.RF/J-hyt/+) to study the effects of thyroid hormone on developmental changes in the expression of genes encoding a number of proteins involved in lipid metabolism and transport. Total cellular RNA was prepared from the small intestine and liver of hyt/hyt mice and their unaffected littermates (+/?) at various times during postnatal development. RNA blots were probed with apolipoprotein A-I, A-II, A-IV, B, and E cDNAs plus cDNAs encoding the low density lipoprotein receptor, 3-hydroxy-3-methylglutaryl coenzyme A reductase, and three cytoplasmic hydrophobic ligand-binding proteins (two fatty acid-binding proteins and a protein that binds all-trans-retinol). Hypothyroidism results in small changes (1.5- to 5-fold) in the concentration of many of these mRNAs in liver and small intestine between postnatal days 15 and 50. A much greater tissue-specific effect was noted on apolipoprotein B (apoB) gene expression. In euthyroid +/? animals, apoB mRNA levels fall by a factor of 30 in liver between days 20 and 35 without a comparable decrease in the small intestine. This liver-specific decrease does not occur in hyt/hyt animals. The normal decrease in hepatic apoB mRNA levels is accompanied by a decrease in plasma apoB-100 but not apoB-48. No reduction in either form of plasma apoB was noted in hyt/hyt animals. Mutant hyt/hyt mice given thyroxine from birth to 35 days had liver apoB mRNA levels comparable to those in +/? littermates. In contrast, hepatic apoB mRNA concentrations did not fall to normal levels in hyt/hyt mice given thyroxine from postnatal days 15 to 35. All treatment groups have comparable levels of plasma corticosteroids. These data suggest that (i) there is a critical period or a required response time during postnatal development for thyroid hormone action on apoB gene expression, (ii) thyroid hormone's effect on apoB is tissue specific, and (iii) the hyt/hyt mouse represents a useful system to evaluate the developmental effects of thyroid hormone on specific gene expression.
甲状腺激素被认为是啮齿动物发育中的一个重要因素。我们使用了一种具有隐性突变、会导致先天性原发性甲状腺功能减退的小鼠品系（C.RF/J-hyt/+），来研究甲状腺激素对一些参与脂质代谢和转运的蛋白质编码基因表达的发育变化的影响。在出生后的不同发育阶段，从小肠和肝脏中提取hyt/hyt小鼠及其未受影响的同窝小鼠（+/？）的总细胞RNA。用载脂蛋白A-I、A-II、A-IV、B和E的cDNA，以及编码低密度脂蛋白受体、3-羟基-3-甲基戊二酰辅酶A还原酶和三种细胞质疏水配体结合蛋白（两种脂肪酸结合蛋白和一种结合全反式视黄醇的蛋白）的cDNA对RNA印迹进行杂交检测。甲状腺功能减退导致出生后第15天到50天期间，肝脏和小肠中许多这些mRNA的浓度发生小幅度变化（1.5至5倍）。在载脂蛋白B（apoB）基因表达上观察到了更大的组织特异性效应。在甲状腺功能正常的+/？动物中，肝脏中apoB mRNA水平在第20天到35天之间下降了30倍，而小肠中没有类似的下降。这种肝脏特异性的下降在hyt/hyt动物中并未发生。肝脏中apoB mRNA水平的正常下降伴随着血浆中apoB-100的减少，但apoB-48没有减少。在hyt/hyt动物中，两种形式的血浆apoB均未减少。从出生到35天给予甲状腺素的突变hyt/hyt小鼠，其肝脏apoB mRNA水平与+/？同窝小鼠相当。相比之下，从出生后第15天到35天给予甲状腺素的hyt/hyt小鼠，其肝脏apoB mRNA浓度并未降至正常水平。所有治疗组的血浆皮质类固醇水平相当。这些数据表明：（i）在出生后的发育过程中，甲状腺激素对apoB基因表达的作用存在关键时期或所需的反应时间；（ii）甲状腺激素对apoB的影响具有组织特异性；（iii）hyt/hyt小鼠是评估甲状腺激素对特定基因表达的发育影响的有用系统。
