Regulatory mechanisms of gene expression of hepatic enzyme by insulin and fructose

研究了胰岛素和果糖对糖尿病大鼠肝脏l型丙酮酸激酶基因表达的调节作用。胰岛素刺激丙酮酸激酶L基因的转录。环己亚胺抑制胰岛素引起的诱导，提示胰岛素可能通过刺激某种未知蛋白的合成来刺激丙酮酸激酶L基因的转录。另一方面，饲喂果糖对丙酮酸激酶L基因的转录没有影响。果糖喂养后，假定的丙酮酸激酶L核RNA前体物种的水平增加，在胞浆丙酮酸激酶L mRNA水平变化之前，果糖可能通过稳定核RNA物种来增加丙酮酸激酶L mRNA的水平。果糖对丙酮酸激酶L的影响不是由胰高血糖素、糖皮质激素和甲状腺激素等激素介导的，因此似乎直接作用于肝脏。大鼠r型同工酶mRNA的核苷酸序列与l型同工酶的核苷酸序列比较表明，r型同工酶mRNA与l型同工酶的核苷酸序列除5′端区（包括编码序列）外，与l型同工酶的核苷酸序列完全相同：l型第5个编码残基上游的序列被r型同工酶的98个核苷酸编码序列加上一个5′未翻译区所取代。通过对LPK30基因组克隆的序列分析，鉴定出12个外显子，其中第1外显子编码R特异序列，第2外显子编码l特异序列。其余的外显子呈现两种同工酶共有的下游编码氨基酸。因此，我们认为这两种同工酶mrna是由不同的启动子产生的。

Regulation of the expression of the hepatic L-type pyruvate kinase gene by insulin and fructose was studied in diabetic rats. Insulin stimulated transcription of the pyruvate kinase L gene. Cycloheximide inhibited the induction caused by insulin, suggesting that insulin may stimulate transcription of the pyruvate kinase L gene by stimulating synthesis of some unknown protein. On the other hand, feeding fructose had no effect on transcription of the pyruvate kinase L gene. Since increases in the levels of putative nuclear RNA precursor species of the pyruvate kinase L after fructose feeding preceded changes in the levels of cytosolic pyruvate kinase L mRNA, fructose may increase the levels of pyruvate kinase L mRNA by stabilizing nuclear RNA species. The effect of fructose on pyruvate kinase L was not mediated by hormones such as glucagon, glucocorticoid and thyroid hormones, and thus seems to be directly on the liver.Comparison of the nucleotide sequence of rat R-type isozyme mRNA with that of the L-type isozyme showed that the R-type isozyme mRNA had an identical nucleotide sequence to that of the L-type except in the 5' terminal region including the coding sequence: the sequence upstream of the 5th coding residue of the L-type was replaced by a 98-nucleotide coding sequence plus a 5' untranslated region in the R-type isozyme. By sequence analysis of a genomic clone of LPK30 twelve exons were identified, of which the first and second exons coded the R- and L-specific sequences, respectively. The remaining exons present downstream coded amino acids common to the two isozymes. Thus, we suggest that the two isozyme mRNAs are generated by use of different promoters.