Serine/threonine kinase 11 (STK11) also known as liver kinase B1 (LKB1) or renal carcinoma antigen NY-REN-19 is a protein kinase that in humans is encoded by the STK11 gene. [5]
Serine/threonine kinase 11 (STK11) also known as liver kinase B1 (LKB1) or renal carcinoma antigen NY-REN-19 is a protein kinase that in humans is encoded by the STK11 gene. [5]
Testosterone and DHT treatment of murine 3T3-L1 or human SGBS adipocytes for 24 h significantly decreased the mRNA expression of LKB1 via the androgen receptor and consequently reduced the activation of AMPK by phosphorylation. In contrast, 17β-estradiol treatment increased LKB1 mRNA, an effect mediated by oestrogen receptor alpha. [6]
However, in ER-positive breast cancer cell line MCF-7, estradiol caused a dose-dependent decrease in LKB1 transcript and protein expression leading to a significant decrease in the phosphorylation of the LKB1 target AMPK. ERα binds to the STK11 promoter in a ligand-independent manner and this interaction is decreased in the presence of estradiol. Moreover, STK11 promoter activity is significantly decreased in the presence of estradiol. [7]
The STK11/LKB1 gene, which encodes a member of the serine/threonine kinase family, regulates cell polarity and functions as a tumour suppressor.
LKB1 is a primary upstream kinase of adenosine monophosphate-activated protein kinase (AMPK), a necessary element in cell metabolism that is required for maintaining energy homeostasis. It is now clear that LKB1 exerts its growth suppressing effects by activating a group of ~14 other kinases, comprising AMPK and AMPK-related kinases. Activation of AMPK by LKB1 suppresses growth and proliferation when energy and nutrient levels are scarce. Activation of AMPK-related kinases by LKB1 plays vital roles maintaining cell polarity thereby inhibiting inappropriate expansion of tumour cells. A picture from current research is emerging that loss of LKB1 leads to disorganization of cell polarity and facilitates tumour growth under energetically unfavorable conditions. [8] [9] A study in rats showed that LKB1 expression is upregulated in cardiomyocytes after birth and that LKB1 abundance negatively correlates with proliferation of neonatal rat cardiomyocytes. [10]
Loss of LKB1 activity is associated with highly aggressive HER2+ breast cancer. [11] HER2/neu mice were engineered for loss of mammary gland expression of Lkb1 resulting in reduced latency of tumorgenesis. These mice developed mammary tumors that were highly metabolic and hyperactive for MTOR. Pre-clinical studies that simultaneously targeted mTOR and metabolism with AZD8055 (inhibitor of mTORC1 and mTORC2) and 2-DG, respectively inhibited mammary tumors from forming. [12] Mitochondria function In control mice that did not have mammary tumors were not affected by AZD8055/2-DG treatments.
LKB1 catalytic deficient mutants found in Peutz–Jeghers syndrome activate the expression of cyclin D1 through recruitment to response elements within the promoter of the oncogene. LKB1 catalytically deficient mutants have oncogenic properties. [13]
At least 51 disease-causing mutations in this gene have been discovered. [14] Germline mutations in this gene have been associated with Peutz–Jeghers syndrome, an autosomal dominant disorder characterized by the growth of polyps in the gastrointestinal tract, pigmented macules on the skin and mouth, and other neoplasms. [15] [16] [17] However, the LKB1 gene was also found to be mutated in lung cancer of sporadic origin, predominantly adenocarcinomas. [18] Further, more recent studies have uncovered a large number of somatic mutations of the LKB1 gene that are present in cervical, breast, [11] intestinal, testicular, pancreatic and skin cancer. [19] [20]
LKB1 has been implicated as a potential target for inducing cardiac regeneration after injury as the regenerative potential of cardiomyocytes is limited in adult mammals. Knockdown of Lkb1 in rat cardiomyocytes suppressed phosphorylation of AMPK and activated Yes-associated protein, which subsequently promoted cardiomyocyte proliferation. [21]
LKB1 is activated allosterically by binding to the pseudokinase STRAD and the adaptor protein MO25. The LKB1-STRAD-MO25 heterotrimeric complex represents the biologically active unit, that is capable of phosphorylating and activating AMPK and at least 12 other kinases that belong to the AMPK-related kinase family. Several novel splice isoforms of STRADα that differentially affect LKB1 activity, complex assembly, subcellular localization of LKB1 and the activation of the LKB1-dependent AMPK pathway. [22]
The crystal structure of the LKB1-STRAD-MO25 complex was elucidated using X-ray crystallography, [23] and revealed the mechanism by which LKB1 is allosterically activated. LKB1 has a structure typical of other protein kinases, with two (small and large) lobes on either side of the ligand ATP-binding pocket. STRAD and MO25 together cooperate to promote LKB1 active conformation. The LKB1 activation loop, a critical element in the process of kinase activation, is held in place by MO25, thus explaining the huge increase in LKB1 activity in the presence of STRAD and MO25 .
Alternate transcriptional splice variants of this gene have been observed and characterized. There are two main splice isoforms denoted LKB1 long (LKB1L) and LKB1 short (LKB1S). [24] [25] The short LKB1 variant is predominantly found in testes.
STK11 has been shown to interact with:
p53, also known as Tumor protein P53, cellular tumor antigen p53, or transformation-related protein 53 (TRP53) is a regulatory protein that is often mutated in human cancers. The p53 proteins are crucial in vertebrates, where they prevent cancer formation. As such, p53 has been described as "the guardian of the genome" because of its role in conserving stability by preventing genome mutation. Hence TP53 is classified as a tumor suppressor gene.
5' AMP-activated protein kinase or AMPK or 5' adenosine monophosphate-activated protein kinase is an enzyme that plays a role in cellular energy homeostasis, largely to activate glucose and fatty acid uptake and oxidation when cellular energy is low. It belongs to a highly conserved eukaryotic protein family and its orthologues are SNF1 in yeast, and SnRK1 in plants. It consists of three proteins (subunits) that together make a functional enzyme, conserved from yeast to humans. It is expressed in a number of tissues, including the liver, brain, and skeletal muscle. In response to binding AMP and ADP, the net effect of AMPK activation is stimulation of hepatic fatty acid oxidation, ketogenesis, stimulation of skeletal muscle fatty acid oxidation and glucose uptake, inhibition of cholesterol synthesis, lipogenesis, and triglyceride synthesis, inhibition of adipocyte lipogenesis, inhibition of adipocyte lipolysis, and modulation of insulin secretion by pancreatic β-cells.
Peutz–Jeghers syndrome is an autosomal dominant genetic disorder characterized by the development of benign hamartomatous polyps in the gastrointestinal tract and hyperpigmented macules on the lips and oral mucosa (melanosis). This syndrome can be classed as one of various hereditary intestinal polyposis syndromes and one of various hamartomatous polyposis syndromes. It has an incidence of approximately 1 in 25,000 to 300,000 births.
Estrogen receptors (ERs) are a group of proteins found inside cells. They are receptors that are activated by the hormone estrogen (17β-estradiol). Two classes of ER exist: nuclear estrogen receptors, which are members of the nuclear receptor family of intracellular receptors, and membrane estrogen receptors (mERs), which are mostly G protein-coupled receptors. This article refers to the former (ER).
Phosphatase and tensin homolog (PTEN) is a phosphatase in humans and is encoded by the PTEN gene. Mutations of this gene are a step in the development of many cancers, specifically glioblastoma, lung cancer, breast cancer, and prostate cancer. Genes corresponding to PTEN (orthologs) have been identified in most mammals for which complete genome data are available.
Adenomatous polyposis coli (APC) also known as deleted in polyposis 2.5 (DP2.5) is a protein that in humans is encoded by the APC gene. The APC protein is a negative regulator that controls beta-catenin concentrations and interacts with E-cadherin, which are involved in cell adhesion. Mutations in the APC gene may result in colorectal cancer and desmoid tumors.
p16, is a protein that slows cell division by slowing the progression of the cell cycle from the G1 phase to the S phase, thereby acting as a tumor suppressor. It is encoded by the CDKN2A gene. A deletion in this gene can result in insufficient or non-functional p16, accelerating the cell cycle and resulting in many types of cancer.
Serine/threonine-protein kinase ATR, also known as ataxia telangiectasia and Rad3-related protein (ATR) or FRAP-related protein 1 (FRP1), is an enzyme that, in humans, is encoded by the ATR gene. It is a large kinase of about 301.66 kDa. ATR belongs to the phosphatidylinositol 3-kinase-related kinase protein family. ATR is activated in response to single strand breaks, and works with ATM to ensure genome integrity.
Cell division protein kinase 6 (CDK6) is an enzyme encoded by the CDK6 gene. It is regulated by cyclins, more specifically by Cyclin D proteins and Cyclin-dependent kinase inhibitor proteins. The protein encoded by this gene is a member of the cyclin-dependent kinase, (CDK) family, which includes CDK4. CDK family members are highly similar to the gene products of Saccharomyces cerevisiae cdc28, and Schizosaccharomyces pombe cdc2, and are known to be important regulators of cell cycle progression in the point of regulation named R or restriction point.
The miR-15 microRNA precursor family is made up of small non-coding RNA genes that regulate gene expression. The family includes the related mir-15a and mir-15b sequences, as well as miR-16-1, miR-16-2, miR-195 and miR-497. These six highly conserved miRNAs are clustered on three separate chromosomes. In humans miR-15a and miR-16 are clustered within 0.5 kilobases at chromosome position 13q14. This region has been found to be the most commonly affected in chronic lymphocytic leukaemia (CLL), with deletions of the entire region in more than half of cases. Both miR-15a and miR-16 are thus frequently deleted or down-regulated in CLL samples with 13q14 deletions; occurring in more than two thirds of CLL cases. The expression of miR-15a is associated with survival in triple negative breast cancer.
The tumor suppressor gene FLCN encodes the protein folliculin, also known as Birt–Hogg–Dubé syndrome protein, which functions as an inhibitor of Lactate Dehydrogenase-A and a regulator of the Warburg effect. Folliculin (FLCN) is also associated with Birt–Hogg–Dubé syndrome, which is an autosomal dominant inherited cancer syndrome in which affected individuals are at risk for the development of benign cutaneous tumors (folliculomas), pulmonary cysts, and kidney tumors.
Tuberous sclerosis complex 2 (TSC2), also known as tuberin, is a protein that in humans is encoded by the TSC2 gene.
Cyclin-dependent kinase 4 inhibitor B also known as multiple tumor suppressor 2 (MTS-2) or p15INK4b is a protein that is encoded by the CDKN2B gene in humans.
5'-AMP-activated protein kinase catalytic subunit alpha-1 is an enzyme that in humans is encoded by the PRKAA1 gene.
Ribosomal protein S6 kinase alpha-5 is an enzyme that in humans is encoded by the RPS6KA5 gene. This kinase, together with RPS6KA4, are thought to mediate the phosphorylation of histone H3, linked to the expression of immediate early genes.
NUAK family SNF1-like kinase 1 also known as AMPK-related protein kinase 5 (ARK5) is an enzyme that in humans is encoded by the NUAK1 gene.
Calcium-binding protein 39 is a protein that in humans is encoded by the CAB39 gene.
Protein kinase LYK5, also known as LYK5 or STRADα, is a human protein and also denotes the gene encoding it.
The Hippo signaling pathway, also known as the Salvador-Warts-Hippo (SWH) pathway, is a signaling pathway that controls organ size in animals through the regulation of cell proliferation and apoptosis. The pathway takes its name from one of its key signaling components—the protein kinase Hippo (Hpo). Mutations in this gene lead to tissue overgrowth, or a "hippopotamus"-like phenotype.
Reuben Shaw is an American cancer researcher. He is a professor and director of the National Cancer Institute-Designated Cancer Center at the Salk Institute for Biological Studies, one of the NCI's seven basic laboratory cancer centers in the United States. He researches signaling pathways that control tumor metabolism, with a specific focus on an ancient energy-sensing pathway that controls biological response to starvation.
This article incorporates text from the United States National Library of Medicine, which is in the public domain.
