Sensing of nutrients and energy by the AMP-activated protein kinase
Event details
| Date | 01.05.2017 |
| Hour | 13:30 › 14:30 |
| Speaker | Grahame HARDIE University of Dundee, UK |
| Location | |
| Category | Conferences - Seminars |
SEMINAR SERIES : Trends in Physiology and Metabolism (Bio-682)
Abstract:
AMP-activated protein kinase (AMPK) occurs as heterotrimeric complexes containing catalytic a subunits and regulatory b and g subunits. Genes encoding these subunits are found in the genomes of almost all eukaryotes, suggesting that this is an ancient signalling pathway that arose during early eukaryotic evolution. In mammalian cells, increased ADP:ATP ratios (signifying a compromised cellular energy state) are amplified by adenylate kinase into even larger increases in AMP:ATP ratio. Binding of AMP or ADP to one or more of three sites on the AMPK-g subunit promotes activation of AMPK by enhancing phosphorylation of Thr172 (located within the activation loop of the a subunit kinase domain) by the upstream kinase LKB1, as well as by inhibiting Thr172 dephosphorylation. Binding of AMP, but not ADP, also causes a further allosteric activation of phosphorylated AMPK. All three activating effects of AMP or ADP are antagonized by ATP binding, so that AMPK is activated in a sensitive manner by falling cellular energy status. Once activated, AMPK acts to restore energy homeostasis by switching on catabolic pathways producing ATP, while switching off ATP-consuming processes, including cell growth and proliferation.
This energy-sensing role of AMPK is well established, and is now referred to as the canonical pathway. However, it is becoming increasingly clear that there are also non-canonical, AMP/ADP-independent pathways by which this kinase can be activated. One example is the phosphorylation of Thr172 by the Ca2+-activated kinase CaMKK2, which is the mechanism by which some hormones and cytokines activate AMPK. Another that has recently emerged is the ability of AMPK to sense glucose availability. It has been known for many years that AMPK is activated by acute starvation of cells for glucose, but it had been assumed that this occurs because the cells are dependent on glucose for catabolic production of ATP. However, we have recently shown that, as long as alternate carbon sources such as glutamine are present, removing glucose from the medium of mammalian cells causes rapid AMPK activation, yet does not always increase cellular AMP:ATP or ADP:ATP ratios. Unlike the canonical mechanism, this effect requires N-myristoylation of the AMPK-b subunits, and is associated with translocation of AMPK to lysosomal membranes. Working with the group of Shengcai Lin at Xiamen University, we have shown that AMPK, and a complex between the adaptor protein AXIN and LKB1, are recruited to the lysosome by binding to LAMTOR1. The latter, a resident protein of the lysosomal membrane that associates with the vacuolar proton pump (v-ATPase), is a component of the Ragulator complex also involved in the regulation of the mechanistic target-of-rapamycin complex-1 (mTORC1). Lin’s group has shown that inactivation of AMPK by glucose requires its metabolism as far as fructose-1,6-bisphosphate (FBP), the substrate of FBP aldolase and the first intermediate committed to glycolysis. Their results suggest that a conformational change in aldolase in the absence of FBP causes it to dissociate from the v-ATPase, perhaps allowing AXIN:LKB1 complex and AMPK to bind instead.
The AMPK ortholog in budding yeast (the SNF1 complex) is activated by glucose starvation and is required for most responses to glucose starvation, but it has always been puzzling that it is not regulated by AMP in the same manner as mammalian AMPK. Intriguingly, however, yeast aldolase has been shown to associate with the v-ATPase located on the yeast vacuole (equivalent to the lysosome), and this interaction only occurs when glucose is present in the medium [Lu et al (2004) J Biol Chem 279:8732]. This suggests that some elements of the novel glucose-sensing mechanism that we have discovered in mammalian cells may be conserved in budding yeast. Glucose sensing may in fact represent the ancestral role of AMPK, with its energy-sensing role emerging later.
Abstract:
AMP-activated protein kinase (AMPK) occurs as heterotrimeric complexes containing catalytic a subunits and regulatory b and g subunits. Genes encoding these subunits are found in the genomes of almost all eukaryotes, suggesting that this is an ancient signalling pathway that arose during early eukaryotic evolution. In mammalian cells, increased ADP:ATP ratios (signifying a compromised cellular energy state) are amplified by adenylate kinase into even larger increases in AMP:ATP ratio. Binding of AMP or ADP to one or more of three sites on the AMPK-g subunit promotes activation of AMPK by enhancing phosphorylation of Thr172 (located within the activation loop of the a subunit kinase domain) by the upstream kinase LKB1, as well as by inhibiting Thr172 dephosphorylation. Binding of AMP, but not ADP, also causes a further allosteric activation of phosphorylated AMPK. All three activating effects of AMP or ADP are antagonized by ATP binding, so that AMPK is activated in a sensitive manner by falling cellular energy status. Once activated, AMPK acts to restore energy homeostasis by switching on catabolic pathways producing ATP, while switching off ATP-consuming processes, including cell growth and proliferation.
This energy-sensing role of AMPK is well established, and is now referred to as the canonical pathway. However, it is becoming increasingly clear that there are also non-canonical, AMP/ADP-independent pathways by which this kinase can be activated. One example is the phosphorylation of Thr172 by the Ca2+-activated kinase CaMKK2, which is the mechanism by which some hormones and cytokines activate AMPK. Another that has recently emerged is the ability of AMPK to sense glucose availability. It has been known for many years that AMPK is activated by acute starvation of cells for glucose, but it had been assumed that this occurs because the cells are dependent on glucose for catabolic production of ATP. However, we have recently shown that, as long as alternate carbon sources such as glutamine are present, removing glucose from the medium of mammalian cells causes rapid AMPK activation, yet does not always increase cellular AMP:ATP or ADP:ATP ratios. Unlike the canonical mechanism, this effect requires N-myristoylation of the AMPK-b subunits, and is associated with translocation of AMPK to lysosomal membranes. Working with the group of Shengcai Lin at Xiamen University, we have shown that AMPK, and a complex between the adaptor protein AXIN and LKB1, are recruited to the lysosome by binding to LAMTOR1. The latter, a resident protein of the lysosomal membrane that associates with the vacuolar proton pump (v-ATPase), is a component of the Ragulator complex also involved in the regulation of the mechanistic target-of-rapamycin complex-1 (mTORC1). Lin’s group has shown that inactivation of AMPK by glucose requires its metabolism as far as fructose-1,6-bisphosphate (FBP), the substrate of FBP aldolase and the first intermediate committed to glycolysis. Their results suggest that a conformational change in aldolase in the absence of FBP causes it to dissociate from the v-ATPase, perhaps allowing AXIN:LKB1 complex and AMPK to bind instead.
The AMPK ortholog in budding yeast (the SNF1 complex) is activated by glucose starvation and is required for most responses to glucose starvation, but it has always been puzzling that it is not regulated by AMP in the same manner as mammalian AMPK. Intriguingly, however, yeast aldolase has been shown to associate with the v-ATPase located on the yeast vacuole (equivalent to the lysosome), and this interaction only occurs when glucose is present in the medium [Lu et al (2004) J Biol Chem 279:8732]. This suggests that some elements of the novel glucose-sensing mechanism that we have discovered in mammalian cells may be conserved in budding yeast. Glucose sensing may in fact represent the ancestral role of AMPK, with its energy-sensing role emerging later.
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Practical information
- Informed public
- Free
Organizer
- Prof. Lluis Fajas (UNIL), Prof. Kei Sakamoto (NIHS) and Prof. Kristina Schoonjans (EPFL)
Contact
- Prof. Kei Sakamoto (NIHS)- [email protected]