Abstract
BackgroundCellular energy failure in high metabolic rate organs is one of the underlying causes for many disorders such as neurodegenerative diseases, cardiomyopathies, liver and renal failures. In the past decade, numerous studies have discovered the cellular axis of LKB1/AMPK/mTOR as an essential modulator of cell homeostasis in response to energy stress. Through regulating adaptive mechanisms, this axis adjusts the energy availability to its demand by a systematized control on metabolism. Energy stress, however, could be sensed at different levels in various tissues, leading to applying different strategies in response to hypoxic insults.MethodsHere the immediate strategies of high metabolic rate organs to time-dependent short episodes of ischaemia were studied by using a rat model (n = 6/group) of cardiac arrest (CA) (15 and 30 s, 1, 2, 4 and 8 min CA). Using western blot analysis, we examined the responses of LKB1/AMPK/mTOR pathway in brain, heart, liver and kidney from 15 s up to 8 min of global ischaemia. The ratio of ADP/ATP was assessed in all ischemic and control groups, using ApoSENSOR bioluminescent assay kit.ResultsBrain, followed by kidney showed the early dephosphorylation response in AMPK (Thr172) and LKB1 (Ser431); in the absence of ATP decline (ADP/ATP elevation). Dephosphorylation of AMPK was followed by rephosphorylation and hyperphosphorylation, which was associated with a significant ATP decline. While heart’s activity of AMPK and LKB1 remained at the same level during short episodes of ischaemia, liver’s LKB1 was dephosphorylated after 2 min. AMPK response to ischaemia in liver was mainly based on an early alternative and a late constant hyperphosphorylation. No significant changes was observed in mTOR activity in all groups.ConclusionTogether our results suggest that early AMPK dephosphorylation followed by late hyperphosphorylation is the strategy of brain and kidney in response to ischaemia. While the liver seemed to get benefit of its AMPK system in early ischameia, possibly to stabilize ATP, the level of LKB1/AMPK activity in heart remained unchanged in short ischaemic episodes up to 8 min. Further researches must be conducted to elucidate the molecular mechanism underlying LKB1/AMPK response to oxygen supply.
Highlights
Cellular energy failure in high metabolic rate organs is one of the underlying causes for many disorders such as neurodegenerative diseases, cardiomyopathies, liver and renal failures
By using a reversible model of cardiac arrest in rat, which was developed in our laboratory [22], we investigated the time-dependent phosphorylation level of liver kinase b1 (LKB1) (Ser431), adenosine monophosphate protein kinase protein (AMPK) (Thr172) and mammalian target of rapamycin (mTOR) (Ser2448) along with their non-phosphorylated forms in brain, heart, liver and kidney
Effect of global ischemia on p-LKB1 (Ser431) /LKB1 in brain, heart, liver and kidney Previous studied showed the major role for LKB1 as an upstream kinase to facilitate AMPK phosphorylation in response to energy stress [6, 7]
Summary
Cellular energy failure in high metabolic rate organs is one of the underlying causes for many disorders such as neurodegenerative diseases, cardiomyopathies, liver and renal failures. Previous studies demonstrated that metabolic stress, which increased the ratio of ADP, and AMP to ATP, activated AMPK α-subunit to more than 100 folds through its phosphorylation at Threonine 172 (Thr172) [4, 5]. LKB1 is believed to be constitutively active, its higher phosphorylation at Serine 431 (Ser431) in response to some stimuli such as ischaemia, increases the activation of these kinase. This causes it to phosphorylate AMPK more rapidly if AMP binds to AMPK γ-subunit [8, 9]. AMPK activation is mediated by reactive oxygen spices (ROS) and calcium calmodulin in an independent way to ADP/ATP ratio [14, 15]; the rational of cell preference in choosing the mechanism of AMPK activation under different circumstances is still under investigation
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