Transcriptional control of aldolase B gene by food intake in the liver

醛缩酶B是糖酵解途径的肝脏特异性酶，其基因在肝脏中的表达受碳水化合物摄入的显著诱导。有趣的是，当醛缩酶B的mRNA水平大大升高时，也在肝脏中特异性合成的血清白蛋白的mRNA水平并没有升高。我们通过连续试验证实了这种对醛缩酶B基因表达的诱导是在转录水平上的诱导。通过测量在通过碳水化合物食物摄入诱导期间血液中几种激素的浓度，我们发现，与诱导平行，胰岛素水平升高，而胰高血糖素水平降低。皮质酮水平变化不大。这些结果表明，摄入碳酸盐可增加大鼠胰岛素分泌，抑制胰高血糖素分泌，这两种激素在血液中浓度的变化是醛缩酶基因诱导的原因，为了验证上述观点，我们采用原代培养的肝细胞，观察了上述激素对分离的肝细胞的直接影响。结果表明，胰岛素和地塞米松以浓度依赖的方式促进醛缩酶B基因的转录，而胰高血糖素则抑制该基因的转录，且胰高血糖素对醛缩酶B基因转录的抑制作用不受胰岛素和地塞米松的影响。为了了解胰高血糖素对醛缩酶B基因转录的影响，我们在原代培养的肝细胞中加入毛喉素或二丁酰环腺苷酸。二者均呈剂量依赖性抑制醛缩酶B基因转录。蛋白激酶A的特异性抑制剂H-7也呈剂量依赖性地抵消胰高血糖素的作用。因此，胰高血糖素通过蛋白激酶A的活化而对吲哚酶B基因转录发挥作用是肯定的。

Aldolase B is a liver-specific enzyme of glycolysis pathway, and its gene expression in the liver was significantly induced by the intake of carbohydrate. Interestingly, the mRNA level of serum albumin which is also specifically synthesized in the liver did not elevated when that of aldolase B was much elevated. We confirmed by run-on assay that this induction of aldolase B gene expression is the induction at the transcriptional level. By measuring the concentrations of several hormones in the blood during the induction by carbohydrate food intake, we found that insulin level increased, while glucagon level decreased reciprocally, in parallel to the induction. Corticosterone level did not change as much as other formones. These results suggested that the carbordhyate intake increases insulin secretion and suppresses glucagon secretio in the rat and the change of both hormone concentration in the blood is the cause of aldolase gene induction.To verify above idea, we employed primary cultured hepatocytes and assessed direct dffects of above hormones on the isolated hepatocytes. The result showed that insulin and dexamethazone stimulated aldolase B gene transcription by a concentration-dependent manner and glucagon suppressed it. The suppressive effect of glucagon on aldolase B gene transcription did not change in the presence or absence of insulin and dexamethazone. This, we realized that glucagon is the most important factor to aldolase B gene transcription.In order to understand how glucagon exerts its effect, we added forskolin or dibutyryl-cAMP to primary cultured hepatocytes. Both of them suppressed aldolase B gene transcription dose-dependently. H-7, a specific inhibitor of protein-kinase A,canceled the effect of glucagon also dose-dependently. Thus, it is certain that glucagon exerts its effect on sldolase B gene transcription by the activation of protein-kinase A.