Costeas P A, Chinsky J M
Department of Pediatrics, University of Maryland School of Medicine, Baltimore 21201, USA.
Biochem J. 1996 Aug 15;318 ( Pt 1)(Pt 1):85-92. doi: 10.1042/bj3180085.
Alterations in dietary intake, especially of protein, may produce changes in the hepatic levels of the branched-chain alpha-keto acid dehydrogenase (BCKAD) complex. The possible role of insulin in the regulation of these observed changes in hepatic capacity for BCKAD expression was therefore examined. Steady-state RNA levels encoding three of the subunits, E1 alpha, E1 beta and E2, increased by 2-4-fold in the livers of mice starved for 3 days, a known hypoinsulinaemic state. In contrast, the levels of E1 beta and E2, but not E1 alpha, RNA were decreased when mice were fed 0% protein diets compared with the levels observed in mice fed standard (23%) or higher protein isocaloric diets. BCKAD subunit protein levels under these conditions changed co-ordinately even though the changes in RNA were not co-ordinate. The effects of hormonal changes that might be associated with these dietary changes were examined, using the rodent hepatoma cell line H4IIEC3. In these cells, the levels of E1 alpha protein and mRNA were significantly depressed in the presence of insulin. In contrast, the levels of E1 beta and E2 RNAs were not decreased by insulin. The half-lives of the E1 alpha and E2 RNAs were determined to be quite long, from 13 to 18 h, with insulin having no dramatic overall effect on the half-lives determined over 24 h. Therefore, it is likely that insulin directly affects the transcription of the E1 alpha gene rather than RNA stability in exerting its negative regulatory effect. This effect is specific to the E1 alpha subunit. The differences in BCKAD subunit RNA levels observed under various nutritional and developmental conditions may therefore be the result of the differential effects of insulin and other hormones on the multiple regulatory mechanisms modulating BCKAD subunit expression.
饮食摄入的改变，尤其是蛋白质摄入的改变，可能会引起肝脏中支链α-酮酸脱氢酶（BCKAD）复合体水平的变化。因此，研究了胰岛素在调节肝脏BCKAD表达能力的这些变化中可能发挥的作用。编码三个亚基E1α、E1β和E2的稳态RNA水平在饥饿3天的小鼠肝脏中增加了2至4倍，这是一种已知的低胰岛素血症状态。相反，与喂食标准（23%）或更高蛋白质等热量饮食的小鼠相比，喂食0%蛋白质饮食的小鼠中，E1β和E2的RNA水平降低，但E1α的RNA水平未降低。尽管RNA的变化并不协调，但在这些条件下BCKAD亚基蛋白水平是协同变化的。使用啮齿动物肝癌细胞系H4IIEC3研究了可能与这些饮食变化相关的激素变化的影响。在这些细胞中，胰岛素存在时E1α蛋白和mRNA水平显著降低。相反，胰岛素并未降低E1β和E2的RNA水平。E1α和E2 RNA的半衰期被确定为相当长，从13到18小时，胰岛素在24小时内对所测定的半衰期没有显著的总体影响。因此，胰岛素在发挥其负调节作用时，可能直接影响E1α基因的转录而不是RNA稳定性。这种作用对E1α亚基具有特异性。因此，在各种营养和发育条件下观察到的BCKAD亚基RNA水平差异可能是胰岛素和其他激素对调节BCKAD亚基表达的多种调节机制产生不同影响的结果。
