If you like music, think of the orchestra conductor. If you like football, think of a central midfielder, such as Kevin De Bruyne, or Andrés Iniesta. This is the role of AMPK within the cell: to sense the cellular energy status and balance the energy consumption with its production. This makes AMPK a central node of energy metabolism.
As we talked in the previous blogpost introducing Exercise Physiology and Sport Nutrition, the body responds and adapts to the homeostatic disruption that occurs when it is exposed to physical activity, among other potentially stressful situations. When these stressful situations affect the energy status of the cell, the responses and adaptations are mainly led by one of the master molecules of the cell’s metabolism. I am talking about the AMP-activated protein kinase (AMPK), which is the main sensor of cellular energy status. In this blogpost we will address the main and most understood mechanism of AMPK activation, as well as briefly comment the most important consequences of AMPK activation.
Let’s begin this!
The role of AMPK within the cell is to balance energy consumption with its production. Activation of AMPK when low energy state is sensed switches on catabolic processes that generate ATP while switching off anabolic (biosynthetic) pathways that consume ATP.
This fascinating molecule senses energy deficit through increases in both AMP:ATP and ADP:ATP ratios. This occurs when the cell is exposed to situations in which either ATP is consumed significantly faster than in normal conditions or its production is impaired. Such situations may be muscle contraction, starvation, ischemia, and the presence of mitochondrial inhibitors, among many others. Moreover, the activity of AMPK is extensively regulated by multiple upstream signals, which makes it a central node for the regulation of metabolism with specific energy demands.
Nucleotide-independent regulation of AMPK
The regulation of AMPK as a sensor of changes in intracellular levels of AMP, ADP, and ATP, such as in the case of energy stress, is well established as the canonical regulation of AMPK.
AMPK contains 3 adenosine nucleotide-binding sites to which AMP, ADP, and ATP bind in a mutually exclusive manner, competing for the binding sites. In consequence, depending on the AMP:ATP and ADP:ATP ratios either AMP, ADP, or ATP will be bound to AMPK.
Then, AMPK becomes fully activated through a three-pronged mechanism:
- Binding of AMP or ADP to AMPK promotes phosphorylation in its catalytic domain by upstream kinases. The main upstream kinase responsible for such phosphorylation is LKB1 (liver-kinase-B1) and it represents the principal event required for full activation of AMPK.
- Binding of AMP or ADP to AMPK induces a conformational change on it that protects it against dephosphorylation of the catalytic domain by protein phosphatases, thus keeping it active and avoiding its inhibition.
- Binding of AMP, but not ADP, results in a conformational change that accounts for up to 10-fold allosteric activation of AMPK.
Of note, ATP inhibits all three mechanisms. Therefore, when the body is at rest and normal conditions, the AMP:ATP and ADP:ATP ratios are low enough for the ATP to displace AMP and ADP from the binding site, thus inactivating AMPK.
The fact that AMP activates AMPK by 3 different mechanisms, rather than one, means that the system is exquisitely sensitive to small changes in cellular AMP.
Nucleotide-independent regulation of AMPK
In addition to changes in adenine nucleotide levels, it has become increasingly clear that there are other important alternative, non-canonical modes of AMPK regulation, which can be classified as nucleotide-independent regulation.
The best characterized nucleotide-independent regulation of AMPK is phosphorylation of the catalytic domain by CAMKK2 (calcium/calmodulin-dependent kinase kinase 2, also known as CAMKKβ), the other major upstream kinase of AMPK. CAMKK2 is activated by increases in intracellular Ca2+ levels, which is released into the cytoplasm during muscle contraction as well as in response to metabolically relevant hormones.
Besides phosphorylation of the catalytic domain, phosphorylation of a Serine/Threonine-rich domain, called ST-loop, has recently emerged as another point of regulation and inhibition of AMPK by other kinases, as Hardie has recently reviewed (click here to see article). Kinases such as PKA, AKT, S6K, GSK3, PKD1, and PKC have been reported to phosphorylate the ST-loop. However, since this newly discovered site of regulation is not well understood yet, we will focus on the other two main regulation mechanisms mentioned above.
In the same way, other regulatory mechanisms of AMPK activity have not long ago been reported, such as co-localization of AMPK and LKB1 to the cell membranes, including lysosomes and Golgi apparatus membranes.
Metabolic consequences of AMPK activation
Accordingly, both glucose uptake and flux through glycolysis is stimulated, together with fatty acid oxidation. Besides, as an attempt to optimize the just mentioned processes, mitochondrial biogenesis, mitophagy and turnover of macromolecules by autophagy are also stimulated.
As for avoiding further ATP consumption, both glucose synthesis and storage in the form of glycogen is inhibited. Also, fatty acid and sterol synthesis as well as protein synthesis are downregulated by AMPK. Finally, cell growth is stopped so that the cell can focus ATP on finding back the energy equilibrium.
Altogether, AMPK is activated by different stimulus that tell the body that it is undergoing a negative energy balance, or that something is going to happen that requires energy. That means that the body must make several metabolic adjustments to fight that situation and reach a new energy equilibrium. These adjustments are basically inhibiting any anabolic process that consumes ATP and stimulating catabolic processes that provide the body with more ATP.
- Garcia, Daniel, and Reuben J. Shaw. 2017. “AMPK: Mechanisms of Cellular Energy Sensing and Restoration of Metabolic Balance.” Molecular Cell 66 (6): 789–800.