Cloning and regulation of rat and Xenopus laevis hepatic 3-hydroxy-3-methylglutaryl Coenzyme A reductase
Chen, Hangjiong
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Permalink
https://hdl.handle.net/2142/20680
Description
Title
Cloning and regulation of rat and Xenopus laevis hepatic 3-hydroxy-3-methylglutaryl Coenzyme A reductase
Author(s)
Chen, Hangjiong
Issue Date
1990
Doctoral Committee Chair(s)
Shapiro, David J.
Department of Study
Biochemistry
Discipline
Biochemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Molecular
Chemistry, Biochemistry
Language
eng
Abstract
The cDNAs coding for rat and Xenopus laevis 3-hydroxy-3-methylglutaryl Coenzyme A (HMG-CoA) reductase have been cloned and sequenced. The rat liver HMG-CoA reductase cDNA clone contains one open reading frame coding for 887 amino acids. Its 106 nucleotide long 5$\sp\prime$-noncoding region contains no additional start codons. It does not contain a poly(A) tract at the 3$\sp\prime$ end. The Xenopus liver HMG-CoA reductase cDNA encodes 883 amino acids, and contains 114 nucleotides at the 5$\sp\prime$-untranslated region. Primer extension analysis and RNA sequencing indicate that HMG-CoA reductase mRNAs in both animals are transcribed from multiple initiation sites. Rat reductase RNA precursor, like the human precursor, is processed at a single donor site to excise the first intron, producing only one class of mRNA equivalent to hamster class I reductase mRNA. Comparison of the amino acid sequences of the mammalian and amphibian HMG-CoA reductases reveals a high degree of amino acid sequence conservation. The seven membrane spanning domains exhibit the highest amino acid sequence similarity, while the linker region displays the lowest level of amino acid homology.
The estrogen induction of HMG-CoA reductase activity in Xenopus laevis livers has also been examined using Xenopus HMG-CoA reductase cDNA probes. Previous experiments showed that HMG-CoA reductase activity and the capacity for cholesterol biosynthesis increase in parallel upon estrogen administration. Northern blot analysis demonstrates that estradiol-17$\beta$ induces hepatic HMG-CoA reductase mRNA in male Xenopus 10-fold within 24 hours. Reductase mRNA levels reach a plateau 23-fold higher than the basal level between 5 and 8 days after initial estrogen administration. These data are in good agreement with earlier observations that a maximum increase of 40-fold in HMG-CoA reductase activity occurs 6 days after estrogen injection. In contrast to the vitellogenin and retinol binding protein genes, whose transcription is induced in Xenopus liver, the rate of HMG-CoA reductase gene transcription was not changed by estrogen. These data suggest that estrogen induction of HMG-CoA reductase mRNA is achieved through posttranscriptional mechanisms. The estrogen induction of HMG-CoA reductase activity by estrogen occurs before maximum vitellogenin secretion. Therefore, the increased capacity of cholesterol production is a preparative step for proliferation of the endoplasmic reticulum membrane and Golgi apparatus required for synthesis and secretion of large quantities of vitellogenin.
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