Smith J D, Liu A Y
Department of Pharmacology, Harvard Medical School, Boston, MA 02115.
Biochem J. 1988 Apr 1;251(1):261-7. doi: 10.1042/bj2510261.
Addition of 1-3 mM-8-bromo-cyclic AMP to monolayer cultures of H-4 rat hepatoma cells resulted in a rapid but short-lived increase in tyrosine aminotransferase (EC 2.6.1.5) activity. The transient nature of this induction is due to desensitization to 8-bromo-cyclic AMP. Throughout this time course of induction and desensitization, removal of 8-bromo-cyclic AMP resulted in a rapid and significant decrease in tyrosine aminotransferase activity, a process referred to as 'de-induction' in this study. We showed that the changes in tyrosine aminotransferase activity in its induction, desensitization and de-induction by 8-bromo-cyclic AMP were directly attributable to changes in the synthesis rate of the protein, and the amount of translatable and hybridizable mRNA encoding for tyrosine aminotransferase (mRNATAT). We further showed that this desensitization was specific to cyclic AMP. First, only active analogues of cyclic AMP and agents which increased cellular concentrations of cyclic AMP elicited this desensitization. Second, the desensitized cells were refractory only to the effects of 8-bromo-cyclic AMP; dexamethasone and insulin induced the tyrosine aminotransferase activity in the 8-bromo-cyclic AMP-desensitized cells in a manner similar to that of the controls. Studies on the metabolism of 8-bromo-cyclic AMP suggest that neither its degradation nor the accumulation of its primary metabolite, 8-bromoadenosine, played a significant role in modulating the expression of tyrosine aminotransferase during the time course of action of 8-bromo-cyclic AMP. These results provide evidence for a specific pretranslational mode of action of cyclic AMP in the control of tyrosine aminotransferase expression in its desensitization and de-induction, in addition to the early phase of induction.
向H-4大鼠肝癌细胞单层培养物中添加1-3 mM的8-溴环磷酸腺苷（8-bromo-cyclic AMP）会导致酪氨酸转氨酶（EC 2.6.1.5）活性迅速但短暂地增加。这种诱导的短暂性是由于对8-溴环磷酸腺苷脱敏所致。在整个诱导和脱敏过程中，去除8-溴环磷酸腺苷会导致酪氨酸转氨酶活性迅速且显著下降，本研究中将此过程称为“去诱导”。我们表明，8-溴环磷酸腺苷对酪氨酸转氨酶活性的诱导、脱敏和去诱导过程中的变化直接归因于蛋白质合成速率的变化，以及编码酪氨酸转氨酶（mRNATAT）的可翻译和可杂交mRNA的量的变化。我们进一步表明，这种脱敏作用对环磷酸腺苷具有特异性。首先，只有环磷酸腺苷的活性类似物和增加细胞内环磷酸腺苷浓度的试剂会引发这种脱敏作用。其次，脱敏细胞仅对8-溴环磷酸腺苷的作用无反应；地塞米松和胰岛素以与对照相似的方式诱导8-溴环磷酸腺苷脱敏细胞中的酪氨酸转氨酶活性。对8-溴环磷酸腺苷代谢的研究表明，在8-溴环磷酸腺苷作用的时间过程中，其降解及其主要代谢产物8-溴腺苷的积累在调节酪氨酸转氨酶表达方面均未发挥重要作用。这些结果为环磷酸腺苷在酪氨酸转氨酶表达的控制中，除了早期诱导阶段外，在其脱敏和去诱导过程中存在特定的翻译前作用模式提供了证据。
