2. AMP-Activated Protein Kinase (AMPK)

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2.AMP-Activated Protein Kinase (AMPK)

 

The rise in cellular AMP:ATP or ADP:ATP ratios induces AMPK (a crucial controller of energy homeostasis). This activity usually takes place as there is a compromise in the level of ATP synthesis or when there is a rise in incomings of ATP (Carling D., 2017 as cited in Vara-Ciruelos D et al., 2020) (Lin S C and Hardie D G., 2017) (Ross F A et al., 2016b) (Vara-Ciruelos D et al., 2020). 

 

AMPK, which consists of catalytic α subunit and regulatory β and γ subunits, is a heterotrimeric complex (Vara-Ciruelos D et al., 2020).

 

As AMPK is phosphorylated at Thr172 (Threonine 172) in the activation loop of the kinase domain, then it becomes activated, the occurrence of which is opposed through ATP binding (Hawley S A et al., 1996) (Davies S P et al., 1995) (Gowans G J et al., 2013) (Ross F A et al., 2016a) (Vara-Ciruelos D et al., 2020).

 

There are three mechanisms of activation of AMPK as AMP attaches to the γ subunit (Vara-Ciruelos D et al., 2020):

 

1. Thr172 phosphorylation induction through upstream kinases

2. Protein phosphatase role in hindering the Thr172 dephosphorylation 

3. As AMPK is phosphorylated on Thr172 then its allosteric induction occurs

 

All these three influential mechanisms on induction of AMPK as a result of binding AMP to the γ subunit are reversed through binding of ATP to this subunit (Davies S P et al., 1995) (Gowans G J et al., 2013) (Ross F A et al., 2016) (Vara-Ciruelos D et al., 2020).

 

It is known that AMPK is the downstream target of STK11 (Serine/Threonine Kinase 11) or LKB1 kinase (Liver Kinase B1), a tumor suppressor and LKB1 plays role in phosphorylation of AMPK’s Thr172 (Hawley S A et al., 2003) (Shaw R J et al., 2004) (Woods A et al., 2003) (Alessi D R et al., 2006 as cited in Vara-Ciruelos D et al., 2020) (Vara-Ciruelos D et al., 2020). 

 

Also, there is a possibility of AMPK plays role in occurrence of cancer (Hardie D J and Alessi D R., 2013) (Vara-Ciruelos D et al., 2020).

 

AMP-Activated Protein Kinase (AMPK) References

 

1.        Alessi, D. R., Sakamoto, K. & Bayascas, J. R. LKB1-Dependent Signaling Pathways. Annu. Rev. Biochem. 75, 137–163 (2006).

2.        Carling, D. AMPK signalling in health and disease. Curr. Opin. Cell Biol. 45, 31–37 (2017).

3.        Davies, S. P., Helps, N. R., Cohen, P. T. & Hardie, D. G. 5’-AMP inhibits dephosphorylation, as well as promoting phosphorylation, of the AMP-activated protein kinase. Studies using bacterially expressed human protein phosphatase-2C alpha and native bovine protein phosphatase-2AC. FEBS Lett. 377, 421–5 (1995).

4.        Gowans, G. J., Hawley, S. A., Ross, F. A. & Hardie, D. G. AMP Is a True Physiological Regulator of AMP-Activated Protein Kinase by Both Allosteric Activation and Enhancing Net Phosphorylation. Cell Metab. 18, 556–566 (2013).

5.        Hardie, D. G. & Alessi, D. R. LKB1 and AMPK and the cancer-metabolism link - ten years after. BMC Biol. 11, 36 (2013).

6.        Hawley, S. A. et al. Complexes between the LKB1 tumor suppressor, STRAD alpha/beta and MO25 alpha/beta are upstream kinases in the AMP-activated protein kinase cascade. J. Biol. 2, 28 (2003).

7.        Hawley, S. A. et al. Characterization of the AMP-activated Protein Kinase Kinase from Rat Liver and Identification of Threonine 172 as the Major Site at Which It Phosphorylates AMP-activated Protein Kinase. J. Biol. Chem. 271, 27879–27887 (1996).

8.        Lin, S.-C. & Hardie, D. G. AMPK: Sensing Glucose as well as Cellular Energy Status. Cell Metab. 27, 299–313 (2018).

9.        Ross, F. A., Jensen, T. E. & Hardie, D. G. Differential regulation by AMP and ADP of AMPK complexes containing different γ subunit isoforms. Biochem. J. 473, 189–199 (2016).

10.     Ross, F. A., MacKintosh, C. & Hardie, D. G. AMP-activated protein kinase: a cellular energy sensor that comes in 12 flavours. FEBS J. 283, 2987–3001 (2016).

11.     Shaw, R. J. et al. The tumor suppressor LKB1 kinase directly activates AMP-activated kinase and regulates apoptosis in response to energy stress. Proc. Natl. Acad. Sci. 101, 3329–3335 (2004).

12.     Vara-Ciruelos, D., Dandapani, M. & Hardie, D. G. AMP-Activated Protein Kinase: Friend or Foe in Cancer? Annu. Rev. Cancer Biol. 4, 1–16 (2020).

13.     Woods, A. et al. LKB1 Is the Upstream Kinase in the AMP-Activated Protein Kinase Cascade. Curr. Biol. 13, 2004–2008 (2003).

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2.1.Noncanonical Activation of AMPK

 

In the noncanonical mechanism, Ca2+/calmodulin-dependent kinase (CaMKK2) is the second upstream kinase that induces AMP through phosphorylation of Thr172 (Threonine172) (Hawley S A et al., 2005) (Hurley R L et al., 2005) (Woods A et al., 2005) (Vara-Ciruelos D et al., 2020). This mechanism confirms that prompting the release of intracellular Ca2+, which is initiated by number of hormones that contribute in induction of AMPK exclusive of changing AMP:ATP ratios,, as an important factor in activation of noncanonical machinery (Stahmann N et al., 2006) (Stahmann N et al., 2010) (Vara-Ciruelos D et al., 2020).

 

In addition, when mammalian cells destitute from glucose then this results in induction of another noncanonical mechanism, which as a result of hindering glycolysis, causes a rise in cellular ADP:ATP and AMP:ATP ratios in some cells (Vara-Ciruelos D et al., 2020). However, in other cells and species such as mouse embryo fibroblasts, when other type of carbon basis elements like glutamine exists, then glucose destitute induces AMPK exclusive of noticeable alteration in ratios of nucleotide (Vara-Ciruelos D et al., 2020).  It is known that this noncanonical mechanism activated without the need for adenine nucleotides, is the consequence of AMPK induction in the absence of glucose. This mechanism engages the conscription to the LKB1 lysosome, which is presented in complex with AXIN adopter protein, where it intermingles with AMPK and is also dependent on resident lysosomal proteins (LAMTOR1) and the vacuoral ATPase (Vara-Ciruelos D et al., 2020).

 

The aforementioned mechanism may be important in occurrence of cancer as a number of tumor cells are dependent on high supply of glucose despite of having low blood source (Vara-Ciruelos D et al., 2020).

 

Noncanonical Activation of AMPK References

 

1.        Hawley, S. A. et al. Calmodulin-dependent protein kinase kinase-β is an alternative upstream kinase for AMP-activated protein kinase. Cell Metab. 2, 9–19 (2005).

2.        Hurley, R. L. et al. The Ca2+/Calmodulin-dependent Protein Kinase Kinases Are AMP-activated Protein Kinase Kinases. J. Biol. Chem. 280, 29060–29066 (2005).

3.        Stahmann, N., Woods, A., Carling, D. & Heller, R. Thrombin Activates AMP-Activated Protein Kinase in Endothelial Cells via a Pathway Involving Ca 2+ /Calmodulin-Dependent Protein Kinase Kinase β. Mol. Cell. Biol. 26, 5933–5945 (2006).

4.        Stahmann, N. et al. Activation of AMP-activated Protein Kinase by Vascular Endothelial Growth Factor Mediates Endothelial Angiogenesis Independently of Nitric-oxide Synthase. J. Biol. Chem. 285, 10638–10652 (2010).

5.        Vara-Ciruelos, D., Dandapani, M. & Hardie, D. G. AMP-Activated Protein Kinase: Friend or Foe in Cancer? Annu. Rev. Cancer Biol. 4, 1–16 (2020).

6.        Woods, A. et al. Ca2+/calmodulin-dependent protein kinase kinase-β acts upstream of AMP-activated protein kinase in mammalian cells. Cell Metab. 2, 21–33 (2005).