Deletion of Lkb1 in adult mice results in body weight reduction and lethality

LKB1 biology: assessing the therapeutic relevancy of LKB1 inhibitors

Liver Kinase B1 (LKB1), encoded by Serine-Threonine Kinase 11 (STK11), is a master kinase that regulates cell migration, polarity, proliferation, and metabolism through downstream adenosine monophosphate-activated protein kinase (AMPK) and AMPK-related kinase signalling. Since genetic screens identified STK11 mutations in Peutz-Jeghers Syndrome, STK11 mutants have been implicated in tumourigenesis labelling it as a tumour suppressor. In support of this, several compounds reduce tumour burden through upregulating LKB1 signalling, and LKB1-AMPK agonists are cytotoxic to tumour cells. However, in certain contexts, its role in cancer is paradoxical as LKB1 promotes tumour cell survival by mediating resistance against metabolic and oxidative stressors. LKB1 deficiency has also enhanced the selectivity and cytotoxicity of several cancer therapies. Taken together, there is a need to develop LKB1-specific pharmacological compounds, but prior to developing LKB1 inhibitors, further work is needed to understand LKB1 activity and regulation. However, investigating LKB1 activity is strenuous as cell/tissue type, mutations to the LKB1 signalling pathway, STE-20-related kinase adaptor protein (STRAD) binding, Mouse protein 25-STRAD binding, splicing variants, nucleocytoplasmic shuttling, post-translational modifications, and kinase conformation impact the functional status of LKB1. For these reasons, guidelines to standardize experimental strategies to study LKB1 activity, associate proteins, spliced isoforms, post-translational modifications, and regulation are of upmost importance to the development of LKB1-specific therapies. Therefore, to assess the therapeutic relevancy of LKB1 inhibitors, this review summarizes the importance of LKB1 in cell physiology, highlights contributors to LKB1 activation, and outlines the benefits and risks associated with targeting LKB1.

Downregulation of a tumor suppressor gene LKB1 in lung transplantation as a biomarker for chronic murine lung allograft rejection

BACKGROUND

We recently demonstrated decreased tumor suppressor gene liver kinase B1 (LKB1) level in lung transplant recipients diagnosed with bronchiolitis obliterans syndrome. STE20-related adaptor alpha (STRADα) functions as a pseudokinase that binds and regulates LKB1 activity.

METHODS

A murine model of chronic lung allograft rejection in which a single lung from a B6D2F1 mouse was orthotopically transplanted into a DBA/2J mouse was employed. We examined the effect of LKB1 knockdown using CRISPR-CAS9 in vitro culture system.

RESULTS

Significant downregulation of LKB1 and STRADα expression was found in donor lung compared to recipient lung. STRADα knockdown significantly inhibited LKB1, pAMPK expression but induced phosphorylated mammalian target of rapamycin (mTOR), fibronectin, and Collagen-I, expression in BEAS-2B cells. LKB1 overexpression decreased fibronectin, Collagen-I, and phosphorylated mTOR expression in A549 cells.

CONCLUSIONS

We demonstrated that downregulation of LKB1-STRADα pathway accompanied with increased fibrosis, results in development of chronic rejection following murine lung transplantation.

Niche-Dependent Regulation of Lkb1 in the Proliferation of Lung Epithelial Progenitor Cells

Lung homeostasis and regeneration depend on lung epithelial progenitor cells. Lkb1 (Liver Kinase B1) has known roles in the differentiation of airway epithelial cells during embryonic development. However, the effects of Lkb1 in adult lung epithelial progenitor cell regeneration and its mechanisms of action have not been determined. In this study, we investigated the mechanism by which Lkb1 regulates lung epithelial progenitor cell regeneration. Organoid culture showed that loss of Lkb1 significantly reduced the proliferation of club cells and alveolar type 2 (AT2) cells in vitro. In the absence of Lkb1, there is a slower recovery rate of the damaged airway epithelium in naphthalene-induced airway epithelial injury and impaired expression of surfactant protein C during bleomycin-induced alveolar epithelial damage. Moreover, the expression of autophagy-related genes was reduced in club cells and increased in AT2 cells, but the expression of Claudin-18 was obviously reduced in AT2 cells after Lkb1 knockdown. On the whole, our findings indicated that Lkb1 may promote the proliferation of lung epithelial progenitor cells via a niche-dependent pathway and is required for the repair of the damaged lung epithelium.

LKB1 acts as a critical brake for the glucagon-mediated fasting response

As important as the fasting response is for survival, an inability to shut it down once nutrients become available can lead to exacerbated disease and severe wasting. The liver is central to transitions between feeding and fasting states, with glucagon being a key initiator of the hepatic fasting response. However, the precise mechanisms controlling fasting are not well defined. One potential mediator of these transitions is liver kinase B1 (LKB1), given its role in nutrient sensing. Here, we show LKB1 knockout mice have a severe wasting and prolonged fasting phenotype despite increased food intake. By applying RNA sequencing and intravital microscopy, we show that loss of LKB1 leads to a dramatic reprogramming of the hepatic lobule through robust up-regulation of periportal genes and functions. This is likely mediated through the opposing effect that LKB1 has on glucagon pathways and gene expression. Conclusion: Our findings show that LKB1 acts as a brake to the glucagon-mediated fasting response, resulting in "periportalization" of the hepatic lobule and whole-body metabolic inefficiency. These findings reveal a mechanism by which hepatic metabolic compartmentalization is regulated by nutrient-sensing.

The sex-specific role of sensory neuron LKB1 on metabolic stress-induced mechanical hypersensitivity and mitochondrial respiration

Pain disorders induce metabolic stress in peripheral sensory neurons by reducing mitochondrial output, shifting cellular metabolism, and altering energy use. These processes implicate neuronal metabolism as an avenue for creating novel therapeutics. Liver kinase B1 (LKB1) mediates the cellular response to metabolic stress by inducing the AMPK pathway. The LKB1-AMPK pathway increases energy producing processes, including mitochondrial output. These processes inhibit pain by directly or indirectly restoring energetic balance within a cell. Although the LKB1-AMPK pathway has been linked to pain relief, it is not yet known which cell is responsible for this property, as well any direct ties to cellular metabolism. To elucidate this, we developed a genetic mouse model where LKB1 is selectively removed from Nav1.8-pain sensory neurons and metabolically stressed them by fasting for 24 hours. We found females, but not males, had neuron-specific, LKB1-dependent restoration of metabolic stress-induced mitochondrial metabolism. This was reflected in mechanical hypersensitivity, where the absence of LKB1 led to hypersensitivity in female, but not male, animals. This discrepancy suggests a sex- and cell-specific contribution to LKB1-depdendent fasting-induced mechanical hypersensitivity. While our data represent a potential role for LKB1 in anti-pain pathways in a metabolic-specific manner, more must be done to investigate these sex differences.

Disruption of the circadian clock component BMAL1 elicits an endocrine adaption impacting on insulin sensitivity and liver disease

SignificanceWhile increasing evidence associates the disruption of circadian rhythms with pathologic conditions, including obesity, type 2 diabetes, and nonalcoholic fatty liver diseases (NAFLD), the involved mechanisms are still poorly described. Here, we show that, in both humans and mice, the pathogenesis of NAFLD is associated with the disruption of the circadian clock combined with perturbations of the growth hormone and sex hormone pathways. However, while this condition protects mice from the development of fibrosis and insulin resistance, it correlates with increased fibrosis in humans. This suggests that the perturbation of the circadian clock and its associated disruption of the growth hormone and sex hormone pathways are critical for the pathogenesis of metabolic and liver diseases.

LKB1 Regulates Goat Intramuscular Adipogenesis Through Focal Adhesion Pathway

Intramuscular fat (IMF) deposition is one of the most important factors to affect meat quality in livestock and induce insulin resistance and adverse metabolic phenotypes for humans. However, the key regulators involved in this process remain largely unknown. Although liver kinase B1 (LKB1) was reported to participate in the development of skeletal muscles and classical adipose tissues. Due to the specific autonomic location of intramuscular adipocytes, deposited between or within muscle bundles, the exact roles of LKB1 in IMF deposition need further verified. Here, we cloned the goat LKB1 coding sequence with 1,317 bp, encoding a 438 amino acid peptide. LKB1 was extensively expressed in detected tissues and displayed a trend from decline to rise during intramuscular adipogenesis. Functionally, knockdown of LKB1 by two individual siRNAs enhanced the intramuscular preadipocytes differentiation, accompanied by promoting lipid accumulation and inducing adipogenic transcriptional factors and triglyceride synthesis-related genes expression. Conversely, overexpression of LKB1 restrained these biological signatures. To further explore the mechanisms, the RNA-seq technique was performed to compare the difference between siLKB1 and the control group. There were 1,043 differential expression genes (DEGs) were screened, i.e., 425 upregulated genes and 618 downregulated genes in the siLKB1 group. The Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis predicted that the DEGs were mainly enriched in the focal adhesion pathway and its classical downstream signal, the PI3K-Akt signaling pathway. Specifically, knockdown of LKB1 increased the mRNA level of focal adhesion kinase (FAK) and vice versa in LKB1-overexpressed cells, a key component of the activated focal adhesion pathway. Convincingly, blocking this pathway by a specific FAK inhibitor (PF573228) rescued the observed phenotypes in LKB1 knockdown adipocytes. In conclusion, LKB1 inhibited goat intramuscular adipogenesis through the focal adhesion pathway. This work expanded the genetic regulator networks of IMF deposition and provided theoretical support for improving human health and meat quality from the aspect of IMF deposition.

Nuclear CoRepressors, NCOR1 and SMRT, are required for maintaining systemic metabolic homeostasis

Objective

The nuclear receptor corepressor 1 (NCOR1) and the silencing mediator of retinoic acid and thyroid hormone (SMRT, also known as NCOR2) play critical and specific roles in nuclear receptor action. NCOR1, both in vitro and in vivo specifically regulates thyroid hormone (TH) action in the context of individual organs such as the liver, and systemically in the context of the hypothalamic-pituitary-thyroid (HPT) axis. In contrast, selective deletion of SMRT in the liver or globally has shown that it plays very little role in TH signaling. However, both NCOR1 and SMRT have some overlapping roles in hepatic metabolism and lipogenesis. Here, we determine the roles of NCOR1 and SMRT in global physiologic function and find if SMRT could play a compensatory role in the regulation of TH action, globally.

Methods

We used a postnatal deletion strategy to disrupt both NCOR1 and SMRT together in all tissues at 8–9 weeks of age in male and female mice. This was performed using a tamoxifen-inducible Cre recombinase (UBC-Cre-ERT2) to KO (knockout) NCOR1, SMRT, or NCOR1 and SMRT together. We used the same strategy to KO HDAC3 in male and female mice of the same age. Metabolic parameters, gene expression, and thyroid function tests were analyzed.

Results

Surprisingly, adult mice that acquired NCOR1 and SMRT deletion rapidly became hypoglycemic and hypothermic and perished within ten days of deletion of both corepressors. Postnatal deletion of either NCOR1 or SMRT had no impact on mortality. NCOR1/SMRT KO mice rapidly developed hepatosteatosis and mild elevations in liver function tests. Additionally, alterations in lipogenesis, beta oxidation, along with hepatic triglyceride and glycogen levels suggested defects in hepatic metabolism. The intestinal function was intact in the NCOR1/SMRT knockout (KO) mice. The KO of HDAC3 resulted in a distinct phenotype from the NCOR1/SMRT KO mice, whereas none of the HDAC3 KO mice succumbed after tamoxifen injection.

Conclusions

The KO of NCOR1 and SMRT rapidly leads to significant metabolic abnormalities that do not survive – including hypoglycemia, hypothermia, and weight loss. Hepatosteatosis rapidly developed along with alterations in hepatic metabolism suggesting a contribution to the dramatic phenotype from liver injury. Glucose production and absorption were intact in NCOR1/SMRT KO mice, demonstrating a multifactorial process leading to their demise. HDAC3 KO mice have a distinct phenotype from the NCOR1/SMRT KO mice—which implies that NCOR1/SMRT together regulate a critical pathway that is required for survival in adulthood and is separate from HDAC3.

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The knockout of corepressors NCoR1 and SMRT is rapidly lethal.

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Metabolic abnormalities observed include hypoglycemia and hypothermia.

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Hepatic glucose production and intestinal absorption is intact despite hypoglycemia.

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The lethal action of NCoR1/SMRT deletion is independent of HDAC3.

Cloning and prokaryotic expression of the chicken liver kinase B1 (LKB1) and its localization in liver, heart and hypothalamus

Liver kinase B1 (LKB1) is a member of the serine/threonine kinase family, which plays an indispensable role in the organism of animals. In the current study, the chicken LKB1 protein gene was amplified by PCR based on the primers and cDNA templates. Then, the cloning vector was constructed and the target gene was cloned. After that, the target gene was inserted into the expression vector to construct the recombinant plasmid. The recombinant plasmid was transformed into BL21 (DE3) host cells and the LKB1 recombinant proteins were successfully expressed by using Isopropyl-β-D-thiogalactopyranoside (IPTG). Finally, purified LKB1 proteins were used as antigen and the rabbit-derived antiserums were collected. The antiserum titer determined by ELISA was not less than 1:128000. The results of Western blot suggested that the polyclonal antibody is highly specific to chicken LKB1 protein. Immunofluorescence indicated that the LKB1 protein is mainly expressed in the cytoplasm of liver, heart and hypothalamus cells of chicken. Our study showed that the LKB1 polyclonal antibodies produced by this method are effective and can be used to further study the role of LKB1 in the pathogenesis of chicken disease.

Autophagy compensates for Lkb1 loss to maintain adult mice homeostasis and survival

Liver kinase B1 (LKB1), also known as serine/threonine kinase 11 (STK11) is the major energy sensor for cells to respond to metabolic stress. Autophagy degrades and recycles proteins, macromolecules, and organelles for cells to survive starvation. To assess the role and cross-talk between autophagy and Lkb1 in normal tissue homeostasis, we generated genetically engineered mouse models where we can conditionally delete Stk11 and autophagy essential gene, Atg7, respectively or simultaneously, throughout the adult mice. We found that Lkb1 was essential for the survival of adult mice, and autophagy activation could temporarily compensate for the acute loss of Lkb1 and extend mouse life span. We further found that acute deletion of Lkb1 in adult mice led to impaired intestinal barrier function, hypoglycemia, and abnormal serum metabolism, which was partly rescued by the Lkb1 loss-induced autophagy upregulation via inhibiting p53 induction. Taken together, we demonstrated that autophagy and Lkb1 work synergistically to maintain adult mouse homeostasis and survival.

Liver kinase B1 suppresses growth of lung cancer cells through sonic hedgehog signaling pathway

Lung cancer is one of life-threatening cancers in the worldwide. Liver kinase B1 (LKB1) has been reported to be closely related to cancers; however, the underlying mechanism of LKB1 in lung cancer remains unclear. In our study, a LKB1 specific shRNA was employed to down-regulate LKB1 levels and a LKB1 over-expression plasmid was constructed to up-regulate LKB1 levels. Thereafter, growth of lung cancer cells was assessed by MTT assay and flow cytometry. Effects of LKB1 on the activation of sonic hedgehog (Shh) signaling pathway were detected by Western blot. Effects of LKB1 on lung cancer growth and Shh signaling pathway activation were also assessed in vivo. Our results showed that LKB1 inhibited proliferation of lung cancer cells and induced their apoptosis. Moreover, LKB1 inhibited Shh signaling pathway activation. Our in vivo study also showed that LKB1 inhibited lung cancer growth in vivo and modulated Shh signaling pathway. Treatment with cyclopamine, a Shh signaling pathway inhibitor, reversed the effects of LKB1 silencing and enhanced the effects of LKB1 over-expression. Results of our study demonstrate that LKB1 inhibits lung cancer growth in vitro and in vivo through Shh signaling pathway.

Microtubule affinity-regulating kinases are potential druggable targets for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that affects normal functions of the brain. Currently, AD is one of the leading causes of death in developed countries and the only one of the top ten diseases without a means to prevent, cure, or significantly slow down its progression. Therefore, newer therapeutic concepts are urgently needed to improve survival and the quality of life of AD patients. Microtubule affinity-regulating kinases (MARKs) regulate tau-microtubule binding and play a crucial role in neurons. However, their role in hyperphosphorylation of tau makes them potential druggable target for AD therapy. Despite the relevance of MARKs in AD pathogenesis, only a few small molecules are known to have anti-MARK activity and not much has been done to progress these compounds into therapeutic candidates. But given the diverse role of MARKs, the specificity of novel inhibitors is imperative for their successful translation from bench to bedside. In this regard, a recent co-crystal structure of MARK4 in association with a pyrazolopyrimidine-based inhibitor offers a potential scaffold for the development of more specific MARK inhibitors. In this manuscript, we review the biological role of MARKs in health and disease, and draw attention to the largely unexplored area of MARK inhibitors for AD.

Nischarin inhibition alters energy metabolism by activating AMP-activated protein kinase

Nischarin (Nisch) is a key protein functioning as a molecular scaffold and thereby hosting interactions with several protein partners. To explore the physiological importance of Nisch, here we generated Nisch loss-of-function mutant mice and analyzed their metabolic phenotype. Nisch-mutant embryos exhibited delayed development, characterized by small size and attenuated weight gain. We uncovered the reason for this phenotype by showing that Nisch binds to and inhibits the activity of AMP-activated protein kinase (AMPK), which regulates energy homeostasis by suppressing anabolic and activating catabolic processes. The Nisch mutations enhanced AMPK activation and inhibited mechanistic target of rapamycin signaling in mouse embryonic fibroblasts as well as in muscle and liver tissues of mutant mice. Nisch-mutant mice also exhibited increased rates of glucose oxidation with increased energy expenditure, despite reduced overall food intake. Moreover, the Nisch-mutant mice had reduced expression of liver markers of gluconeogenesis associated with increased glucose tolerance. As a result, these mice displayed decreased growth and body weight. Taken together, our results indicate that Nisch is an important AMPK inhibitor and a critical regulator of energy homeostasis, including lipid and glucose metabolism.