AMP-activated protein kinase: the current landscape for drug development

Until recently, most envisaged atherosclerosis as a bland arterial collection of cholesterol, complicated by smooth muscle cell accumulation. According to that concept, endothelial denuding injury led to platelet aggregation and release of platelet factors which would trigger the proliferation of smooth muscle cells in the arterial intima. These cells would then elaborate an extracellular matrix that would entrap lipoproteins, forming the nidus of the atherosclerotic plaque. Beyond the vascular smooth muscle cells long recognized in atherosclerotic lesions, subsequent investigations identified immune cells and mediators at work in atheromata, implicating inflammation in this disease. Multiple independent pathways of evidence now pinpoint inflammation as a key regulatory process that links multiple risk factors for atherosclerosis and its complications with altered arterial biology. Knowledge has burgeoned regarding the operation of both innate and adaptive arms of immunity in atherogenesis, their interplay, and the balance of stimulatory and inhibitory pathways that regulate their participation in atheroma formation and complication. This revolution in our thinking about the pathophysiology of atherosclerosis has now begun to provide clinical insight and practical tools that may aid patient management. This review provides an update of the role of inflammation in atherogenesis and highlights how translation of these advances in basic science promises to change clinical practice.

Replicative cell division is an energetically demanding process that can be executed only if cells have sufficient metabolic resources to support a doubling of cell mass. Here we show that proliferating mammalian cells have a cell-cycle checkpoint that responds to glucose availability. The glucose-dependent checkpoint occurs at the G(1)/S boundary and is regulated by AMP-activated protein kinase (AMPK). This cell-cycle arrest occurs despite continued amino acid availability and active mTOR. AMPK activation induces phosphorylation of p53 on serine 15, and this phosphorylation is required to initiate AMPK-dependent cell-cycle arrest. AMPK-induced p53 activation promotes cellular survival in response to glucose deprivation, and cells that have undergone a p53-dependent metabolic arrest can rapidly reenter the cell cycle upon glucose restoration. However, persistent activation of AMPK leads to accelerated p53-dependent cellular senescence. Thus, AMPK is a cell-intrinsic regulator of the cell cycle that coordinates cellular proliferation with carbon source availability.

The AMP-activated protein kinase (AMPK) is a critical regulator of energy balance at both the cellular and whole-body levels. Two upstream kinases have been reported to activate AMPK in cell-free assays, i.e., the tumor suppressor LKB1 and calmodulin-dependent protein kinase kinase. However, evidence that this is physiologically relevant currently only exists for LKB1. We now report that there is a significant basal activity and phosphorylation of AMPK in LKB1-deficient cells that can be stimulated by Ca2+ ionophores, and studies using the CaMKK inhibitor STO-609 and isoform-specific siRNAs show that CaMKKbeta is required for this effect. CaMKKbeta also activates AMPK much more rapidly than CaMKKalpha in cell-free assays. K(+)-induced depolarization in rat cerebrocortical slices, which increases intracellular Ca2+ without disturbing cellular adenine nucleotide levels, activates AMPK, and this is blocked by STO-609. Our results suggest a potential Ca(2+)-dependent neuroprotective pathway involving phosphorylation and activation of AMPK by CaMKKbeta.