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Growths in Your Colon Aren’t Fun, Get Yourself Some LKB1!
The Role of LKB1 in Cancerous Growths
Grafton High School Smart Team: Shabi Haider, Brendon Konon, Andrew Mosin, Yotaro Sueoka, and Hannah Weber
Advisors: Dan Goetz, Fran Grant, and Lisa Neeb
Mentor: Stephanie M Cossette PhD, Department of Pediatrics, Medical College of Wisconsin, Milwaukee, WI
Abstract
Billions of dollars are being poured into the research of cancer, the National Cancer Institute alone
spends $4.9 Billion every year. Liver Kinase B1 (LKB1) stands out as one important protein that regulates
cell metabolism, cell division, and therefore, cancerous growth. LKB1 is a key regulator of cell
metabolism and cell division by acting as a tumor suppressor by turning on other proteins that suppress
tumor growth. Human mutations in LKB1 causes the disease Peutz-Jeghers syndrome, which results in
benign tumor-like growth called polyps in the intestine and a 50% chance of developing cancer by the
age of 50. When cell energy, ATP, is low LKB1 will be activated. Active LKB1 regulates the activity of
adenosine monophosphate-activated protein kinase (AMPK). LKB1 directly activates AMPK by adding a
phosphate group to Thr-172. AMPK activity increases the production of ATP by activating glycolysis and
fatty acid oxidation. AMPK can also decrease the amount of energy needed by the cell by inhibiting
protein synthesis and cell growth. Both of these processes play a role in cancer development. Drugs like
Metformin, a successful diabetic drug, are thought to activate LKB1. Through the activation of AMPK to
cease cancerous growth, and with the whole cascade of proteins ceases cancerous growth.
Figure 4
LKB1 in Mouse Embryos
LKB1 is expressed in all cell types and appears to be important for early mouse development. When LKB1 is
knockout or deleted (-/-) there are serious defects in blood vessels as shown in Figure 2 (Ylikorkala et al.,
2001).
• A is an image of an early mouse embryo (embryonic day 8.5) that shows the development of the aorta in
both a LKB1 knockout mouse (-/-) and a wild type mouse, (+/+). The mutated aorta of the knockout mouse
is smaller in diameter.
• B is an image of a slightly older embryo (embryonic day 9.5), and the arrow points to where the aorta is
discontinuous in the knockout mouse (right), and the corresponding area in the wild type mouse, (+/+).
• C is an image of an embryonic day 9.5 head and shows the tissue of the embryo that will develop into the
brain. In the knockout mouse, (-/-), the mesenchyme layer is not fully developed, as displayed by the
asterisks.
Xie, Z., Dong, Y., Scholtz, R., Neumann, D., & Zou, M. (2008).
Phosporylation of lkb1 at serine 428 by protein kinase c is required for
metformin-enchanced activation of the amp-activated protein kinase
in endothelial cells. Circulation,117, 952-962
Figure 2
Behavior of LKB1
The model of how LKB1 and Metformin interacts is supported by evidence of the
location of LKB1 and phosphorylated LKB1.
The above image A-B shows that the addition of Metformin to cells called human
umbilical vein endothelial cells (HUVEC) results in LKB1 being shuttled from the nucleus
to the cytoplasm. D shows that phosphorylation of LKB1 at serine 428 is increased when
the cells are given Metformin (Xie et al., 2008).
LKB1 Mutations and Consequences
Peutz-Jeghers syndrome is characterized by polyps that form inside the intestine (Calva et al 2008). An
example of intestinal polyps in mice is shown in figure 5. Patients with Peutz-Jeghers syndrome are prone
to colon and rectal cancers as well as a wide variety of other cancers (Boardman et al., 1998). All cancers,
such as those listed, develop when cells rapidly divide in an area, and grow out of control. This growth is
called a tumor, and it can result in serious health issues. The process of rapid cell division requires a lot of
cellular energy in the form of ATP. When energy levels decrease, the cell activates a stress response
protein called Liver kinase B1 (LKB1) that will cause the cell to slow down the energy consuming processes
such as cell proliferation and protein production and will turn on energy producing processes such as
metabolism, by phosphorylating, and activating 5' adenosine monophosphate-activated protein kinase
(AMPK) (Zhao, Xu, 2014). When LKB1 is mutated or missing such as in Peutz-Jeghers syndrome, the
cancer cells have defective energy stress response and continue to proliferate uncontrollably (Xie et al.,
2008).
Functions of LKB1
LKB1 works as a complex of three parts: LKB1, the pseudokinase STRAD, and MO25 (Zhao, Xu, 2014).
When LKB1 is activated, it in turn activates AMPK by phosphorylating the Threonine 172 amino acid
residue (Zhao, Xu, 2014) AMPK also is activated by low blood oxygen, as well as low blood glucose (Zhao,
Xu, 2014). Once activated, AMPK can turn on catabolic pathways that produce ATP such as glucose
metabolism and inhibit anabolic pathways for instance cell growth and protein synthesis. Since defects in
glucose metabolism can result in diabetes, it is not surprising that LKB1 dysfunction has been linked to
diabetes (Zhao, Xu, 2014). There are drugs like Metformin that activate LKB1 indirectly in order to treat
those with diabetes. LKB1 and AMPK are also involved in regulating cell polarity and cell growth (Zhao,
Xu, 2014). If either of these processes is not functioning properly, uncontrolled cell growth (cancer) can
occur.
Figure 5
Ylikorkala, A., Rossi, D. J., Korsisaali, N., Luukko, K.,Alitalo, K., Henkemeyer, M., & Makela, T. P. (2001). Vascular abnormalities and deregulation
of vegf in lkb1-deficient mice. Sciencemag, 293, 1323-1326.
Metformin Activating ATM
Metformin activates Ataxia Telangiectasia Mutated (ATM) which is a serine/threonine protein kinase responsible for
the phosphorylation LKB1 (Shaw, 2011).
• The activation of ATM and the phosphorylation of LKB1 guides the LKB1 to move out of the nucleus and into the
cytosol
• AMPK is abundant in the cytosol and that is where LKB1 is able to phosphorylate AMPK and continue the cycle of
the activation of the metabolism which will eventually lower the blood glucose.
Figure 3
Figure 1
Miyoshi H, Nakau M, Ishikawa T., Seldin M., Oshima M., and Taketo MM. (2002) Grastrointestinal hamartomatous polyposis in Lkb1 heterozygous knockout mice. Cancer Research
62; 2261-2266
Conclusion
Liver Kinase B1 has vital functions inside the cell. It is clear that the proper activation
of LKB1 results in the control over the proliferation, cellular metabolism, and cellular
integrity of the cell. As a kinase its sole purpose is to phosphorylate other proteins
making it a stepping stone inside of an important cellular stress response. LKB1 acts as
a check point or sensor for the cell. Without this checkpoint the cell would be left
helpless in managing its most important components such as energy. This would
ultimately disrupt the overall homeostasis of the cell and most importantly, if not
targeted by apoptosis, have the ability to turn cancerous.
SMART Teams are supported by the National Center for Advancing
Translational Sciences, National Institutes of Health, through Grant Number
8UL1TR000055. Its contents are solely the responsibility of the authors and do
not necessarily represent the official views of the NIH.
Citations
Zhao, R., & Xu, Z. (2014). The biological functions of lkb1. Targeting the LKB1 Tumor supressor, 32-52
Shaw, R. (2011). The cancer connection. Drugs, diabetes and cancer, 470(338)
Boudeau, J., Scott, J. W., & Resta, R., et al. (2004). Analysis of the lkb1-strad-mo25 complex. Journal of Cell
Science,117, 6365-6375.
Shaw, R. (2011). The cancer connection. Drugs, diabetes and cancer, 470(338).
Xie, Z., Dong, Y., Scholtz, R., Neumann, D., & Zou, M. (2008). Phosporylation of lkb1 at serine 428 by protein
kinase c is required for metformin-enchanced activation of the amp-activated protein kinase in
endothelial cells. Circulation,117, 952-962.
Ylikorkala, A., Rossi, D. J., Korsisaali, N., Luukko, K., Alitalo, K., Henkemeyer, M., & Makela, T. P. (2001).
Vascular abnormalities and
deregulation of vegf in lkb1-deficient mice. Sciencemag, 293, 1323-1326.
Zhao, R., & Xu, Z. (2014). The biological functions of lkb1.Targeting the LKB1 Tumor supressor, 32-52.