Analysis of the Role of the AMP-Activated Protein Kinase in the Response to Cellular Stress

The AMP-activated protein kinase (AMPK) is the central component of a protein kinase cascade that is activated by cellular stresses causing ATP depletion and has been referred to as a “fuel gauge” or “metabolic sensor” of the eukaryotic cell (1 ,2 ). The kinase is activated by phosphorylation by an upstream protein kinase termed AMP-activated protein kinase kinase (AMPKK) (3 ). Elevation of 5’-AMP activates the cascade by a complex mechanism involving binding of the nucleotide to both the upstream kinase (AMP-activated protein kinase kinase, AMPKK) and the downstream kinase, AMPK (See Subheading 1.2. ). These effects of AMP are also antagonized by high concentrations (mM ) of ATP. The AMP: ATP ratio in the cell varies approximately as the square of the ADP:ATP ratio, due to the action of adenylate kinase which maintains its reaction (2ADP ’ ATP + AMP) close to equilibrium at all times. Therefore, any cellular stress that affects the ability of the cell to maintain a high ATP: ADP ratio (normally approx 10:1 in an unstressed cell) leads to activation of the AMPK cascade. Cellular stresses can do this either by inhibiting ATP production or by increasing ATP consumption, and stresses shown to cause AMPK activation include heat shock (4 ), various mitochondrial inhibitors such as arsenite, antimycin A, dinitrophenol, and azide (4 ,5 ), ischemia/hypoxia in heart muscle (6 ), and exercise in skeletal muscle (7 ). ATP can also be depleted, and AMPK activated, by incubation of cells with high concentrations of certain sugars which trap cellular phosphate, such as fructose (8 ) and 2-deoxyglucose (9 ). Detachment of cultured cells from their substrate by trypsinization has also been reported to increase cellular AMP:ATP and to inhibit lipid synthesis, consistent with the activation of AMPK (10 ). Downstream targets for the system include biosynthetic pathways that are inhibited, thus conserving ATP, and catabolic pathways tht are activated, thus generating more ATP (1 ,2 ). Although most of the currently known targets for the system are metabolic enzymes, the yeast homolog of AMPK (i.e., the SNF1 complex) regulates gene expression (2 ). At least one isoform of AMPK is partly localized to the nucleus (see Subheading 1.1. ), and it seems very likely that the mammalian system will also turn out to regulate gene expression.