Corton J M, Gillespie J G, Hardie D G
Biochemistry Department, The University, Dundee, UK.
Curr Biol. 1994 Apr 1;4(4):315-24. doi: 10.1016/s0960-9822(00)00070-1.
AMP-activated protein kinase is the central component of a protein kinase cascade that phosphorylates and inactivates key regulatory enzymes of several biosynthetic pathways. Elevation of cellular AMP levels activates this kinase, both by allosteric activation, which causes more than 5-fold activation, and by phosphorylation by an upstream kinase kinase, leading to more than 20-fold activation; the result is a greater than 100-fold activation overall. As AMP is usually elevated when cellular ATP is depleted, we have assessed the possibility that the AMP-activated kinase is involved in the cellular response to stress, which is known to lead to ATP depletion.
We report that AMP is elevated, and ATP depleted, when isolated rat hepatocytes are subjected to treatments that activate the cellular stress response, namely heat shock or treatment with arsenite. Several events are correlated with these changes in nucleotide levels: first, a large activation of the AMP-activated protein kinase, which can be reversed by treatment with a protein phosphatase; second, phosphorylation and inactivation of one of the known substrates of the AMP-activated kinase, HMG-CoA reductase; and third, inhibition of two of the biosynthetic pathways known to be affected by the AMP-activated kinase, namely sterol and fatty-acid synthesis.
Our results suggest that a major function of the AMP-activated protein kinase is to act protectively, switching off biosynthetic pathways when the cell is subjected to stress that causes ATP depletion, the key signal being a rise in AMP level. By this mechanism, ATP is preserved for processes that may be more essential in the short term, such as the maintenance of ion gradients. This function of the kinase represents a novel role for protein phosphorylation.
AMP 活化蛋白激酶是蛋白激酶级联反应的核心成分，该级联反应可使几种生物合成途径的关键调节酶磷酸化并使其失活。细胞内 AMP 水平的升高可通过变构激活（导致激活超过 5 倍）和上游激酶激酶的磷酸化作用（导致激活超过 20 倍）来激活此激酶；总体结果是激活超过 100 倍。由于细胞内 ATP 耗尽时 AMP 通常会升高，我们评估了 AMP 活化激酶参与细胞应激反应的可能性，已知应激会导致 ATP 耗尽。
我们报告，当分离的大鼠肝细胞受到激活细胞应激反应的处理，即热休克或亚砷酸盐处理时，AMP 升高而 ATP 耗尽。有几个事件与这些核苷酸水平的变化相关：第一，AMP 活化蛋白激酶的大量激活，可通过蛋白磷酸酶处理逆转；第二，AMP 活化激酶的已知底物之一 HMG-CoA 还原酶的磷酸化和失活；第三，则是对已知受 AMP 活化激酶影响的两条生物合成途径，即甾醇和脂肪酸合成的抑制。
我们的结果表明，AMP 活化蛋白激酶的主要功能是起到保护作用，当细胞受到导致 ATP 耗尽的应激时，关闭生物合成途径，关键信号是 AMP 水平的升高。通过这种机制，ATP 被保留用于短期内可能更重要的过程，例如维持离子梯度。该激酶的这一功能代表了蛋白磷酸化的一种新作用。
