Inhibition of membrane depolarisation-induced transcriptional activity of cyclic AMP response element binding protein (CREB) by the dual-leucine-zipper-bearing kinase in a pancreatic islet beta cell line

Based on systematic druggable genome-wide Mendelian randomization identifies therapeutic targets for diabetes

Purpose

Diabetes and its complications cause a heavy burden of disease worldwide. In recent years, Mendelian randomization (MR) has been widely used to discover the pathogenesis and epidemiology of diseases, as well as to discover new therapeutic targets. Therefore, based on systematic "druggable" genomics, we aim to identify new therapeutic targets for diabetes and analyze its pathophysiological mechanisms to promote its new therapeutic strategies.

Material and method

We used double sample MR to integrate the identified druggable genomics to evaluate the causal effect of quantitative trait loci (eQTLs) expressed by druggable genes in blood on type 1 and 2 diabetes (T1DM and T2DM). Repeat the study using different data sources on diabetes and its complications to verify the identified genes. Not only that, we also use Bayesian co-localization analysis to evaluate the posterior probabilities of different causal variations, shared causal variations, and co-localization probabilities to examine the possibility of genetic confounding. Finally, using diabetes markers with available genome-wide association studies data, we evaluated the causal relationship between established diabetes markers to explore possible mechanisms.

Result

Overall, a total of 4,477 unique druggable genes have been gathered. After filtering using methods such as Bonferroni significance (P<1.90e-05), the MR Steiger directionality test, Bayesian co-localization analysis, and validation with different datasets, Finally, 7 potential druggable genes that may affect the results of T1DM and 7 potential druggable genes that may affect the results of T2DM were identified. Reverse MR suggests that C4B may play a bidirectional role in the pathogenesis of T1DM, and none of the other 13 target genes have a reverse causal relationship. And the 7 target genes in T2DM may each affect the biomarkers of T2DM to mediate the pathogenesis of T2DM.

Conclusion

This study provides genetic evidence supporting the potential therapeutic benefits of targeting seven druggable genes, namely MAP3K13, KCNJ11, REG4, KIF11, CCNE2, PEAK1, and NRBP1, for T2DM treatment. Similarly, targeting seven druggable genes, namely ERBB3, C4B, CD69, PTPN22, IL27, ATP2A1, and LT-β, has The potential therapeutic benefits of T1DM treatment. This will provide new ideas for the treatment of diabetes and also help to determine the priority of drug development for diabetes.

Regulation of the Activity of the Dual Leucine Zipper Kinase by Distinct Mechanisms

The dual leucine zipper kinase (DLK) alias mitogen-activated protein 3 kinase 12 (MAP3K12) has gained much attention in recent years. DLK belongs to the mixed lineage kinases, characterized by homology to serine/threonine and tyrosine kinase, but exerts serine/threonine kinase activity. DLK has been implicated in many diseases, including several neurodegenerative diseases, glaucoma, and diabetes mellitus. As a MAP3K, it is generally assumed that DLK becomes phosphorylated and activated by upstream signals and phosphorylates and activates itself, the downstream serine/threonine MAP2K, and, ultimately, MAPK. In addition, other mechanisms such as protein-protein interactions, proteasomal degradation, dephosphorylation by various phosphatases, palmitoylation, and subcellular localization have been shown to be involved in the regulation of DLK activity or its fine-tuning. In the present review, the diverse mechanisms regulating DLK activity will be summarized to provide better insights into DLK action and, possibly, new targets to modulate DLK function.

Deciphering the multifunctional role of dual leucine zipper kinase (DLK) and its therapeutic potential in disease

Dual leucine zipper kinase (DLK, MAP3K12), a serine/threonine protein kinase, plays a key role in neuronal development, as it regulates axon regeneration and degeneration through its downstream kinase. Importantly, DLK is closely related to the pathogenesis of numerous neurodegenerative diseases and the induction of β-cell apoptosis that leads to diabetes. In this review, we summarize the current understanding of DLK function, and then discuss the role of DLK signaling in human diseases. Furthermore, various types of small molecule inhibitors of DLK that have been published so far are described in detail in this paper, providing some strategies for the design of DLK small molecule inhibitors in the future.

Increase of c-FOS promoter transcriptional activity by the dual leucine zipper kinase

The dual leucine zipper kinase (DLK) and the ubiquitously expressed transcription factor c-FOS have important roles in beta-cell proliferation and function. Some studies in neuronal cells suggest that DLK can influence c-FOS expression. Given that c-FOS is mainly regulated at the transcriptional level, the effect of DLK on c-FOS promoter activity was investigated in the beta-cell line HIT. The methods used in this study are the following: Luciferase reporter gene assays, immunoblot analysis, CRISPR-Cas9-mediated genome editing, and real-time quantitative PCR. In the beta-cell line HIT, overexpressed DLK increased c-FOS promoter activity twofold. Using 5'-,3'-promoter deletions, the promoter regions from - 348 to - 339 base pairs (bp) and from a - 284 to - 53 bp conferred basal activity, whereas the promoter region from - 711 to - 348 bp and from - 53 to + 48 bp mediated DLK responsiveness. Mutation of the cAMP response element within the promoter prevented the stimulatory effect of DLK. Treatment of HIT cells with KCl and the adenylate cyclase activator forskolin increased c-FOS promoter transcriptional activity ninefold. Since the transcriptional activity of those promoter fragments activated by KCl and forskolin was decreased by DLK, DLK might interfere with KCl/forskolin-induced signaling. In a newly generated, genome-edited HIT cell line lacking catalytically active DLK, c-Fos mRNA levels were reduced by 80% compared to the wild-type cell line. DLK increased c-FOS promoter activity but decreased stimulated transcriptional activity, suggesting that DLK fine-tunes c-FOS promoter-dependent gene transcription. Moreover, at least in HIT cells, DLK is required for FOS mRNA expression.

Regulation of dual leucine zipper kinase activity through its interaction with calcineurin

Hyperglycemia enhancing the intracellular levels of reactive oxygen species (ROS) contributes to dysfunction and progressive loss of beta cells and thereby to diabetes mellitus. The oxidation sensitive calcium/calmodulin dependent phosphatase calcineurin promotes pancreatic beta cell function and survival whereas the dual leucine zipper kinase (DLK) induces apoptosis. Therefore, it was studied whether calcineurin interferes with DLK action. In a beta cell line similar concentrations of H₂O₂ decreased calcineurin activity and activated DLK. DLK interacted via its φLxVP motif (aa 362-365) with the interface of the calcineurin subunits A and B. Mutation of the Val prevented this protein protein interaction, hinting at a distinct φLxVP motif. Indeed, mutational analysis revealed an ordered structure of DLK's φLxVP motif whereby Val mediates the interaction with calcineurin and Leu maintains an enzymatically active conformation. Overexpression of DLK wild-type but not the DLK mutant unable to bind calcineurin diminished calcineurin-induced nuclear localisation of the nuclear factor of activated T-cells (NFAT), suggesting that both, DLK and NFAT compete for the substrate binding site of calcineurin. The calcineurin binding-deficient DLK mutant exhibited increased DLK activity measured as phosphorylation of the downstream c-Jun N-terminal kinase, inhibition of CRE-dependent gene transcription and induction of apoptosis. These findings show that calcineurin interacts with DLK; and inhibition of calcineurin increases DLK activity. Hence, this study demonstrates a novel mechanism regulating DLK action. These findings suggest that ROS through inhibition of calcineurin enhance DLK activity and thereby lead to beta cell dysfunction and loss and ultimately diabetes mellitus.

Multitasking: Dual Leucine Zipper-Bearing Kinases in Neuronal Development and Stress Management

The dual leucine zipper-bearing kinase (DLK) and leucine zipper-bearing kinase (LZK) are evolutionarily conserved MAPKKKs of the mixed-lineage kinase family. Acting upstream of stress-responsive JNK and p38 MAP kinases, DLK and LZK have emerged as central players in neuronal responses to a variety of acute and traumatic injuries. Recent studies also implicate their function in astrocytes, microglia, and other nonneuronal cells, reflecting their expanding roles in the multicellular response to injury and in disease. Of particular note is the potential link of these kinases to neurodegenerative diseases and cancer. It is thus critical to understand the physiological contexts under which these kinases are activated, as well as the signal transduction mechanisms that mediate specific functional outcomes. In this review we first provide a historical overview of the biochemical and functional dissection of these kinases. We then discuss recent findings on regulating their activity to enhance cellular protection following injury and in disease, focusing on but not limited to the nervous system.

Role of alpha-lipoic acid in ameliorating Cyclosporine A-induced pancreatic injury in albino rats: A structural, ultrastructural, and morphometric study

The aim of this study was to investigate the effectiveness of alpha-lipoic acid (α-LA) against cyclosporine A (CsA)-induced pancreatic toxicity in rats. Thirty-two male albino rats were divided into four equal groups. Group I treated orally (per os, p.o.) with vehicles and served as control; Group II received α-LA (100 mg/kg b.w. /day, p.o.) for 21 days; Group III received CsA (25 mg/kg b.w./day, p.o.) for 21 days; and Group IV received α-LA 1 hr before oral treatment by CsA for 21 days. Histological examination of the pancreas of CsA-treated rats showed marked changes represented by wide interlobular septae that contained congested blood vessels, cytoplasmic vacuolation of some acinar cells, and distortion of the other cells. Most of the islets of Langerhans showed vacuolation, degenerative changes, and loss of uniform cellular distribution. Some of the islets appeared shrunken with few cells. In the CsA group, the immunohistochemical and morphometric study demonstrated a decrease in the number of insulin-secreting β-cells and also a reduction in islet diameters, with statistically significant difference (p < 0.001 and p = 0.004), respectively, compared with the control group. Ultrastructure of the exocrine and endocrine pancreatic cells of the CsA-treated group confirmed the light microscopic observation and showed dilated rough endoplasmic reticulum, decreased zymogen and secretory granules, damaged mitochondria, and abnormal nuclei. However, α-LA administration simultaneously with CsA resulted in some sort of regression of the previously mentioned effects.

CONCLUSION

α-LA attenuated CsA-induced structural and ultrastructural changes in pancreatic cells, which were nearly reverted to their normal structure.

Regulation of Beta-Cell Function and Mass by the Dual Leucine Zipper Kinase

Diabetes mellitus is one of the most rapidly increasing diseases worldwide, whereby approximately 90-95% of patients suffer from type 2 diabetes. Considering its micro- and macrovascular complications like blindness and myocardial infarction, a reliable anti-diabetic treatment is needed. Maintaining the function and the mass of the insulin producing beta-cells despite elevated levels of beta-cell-toxic prediabetic signals represents a desirable mechanism of action of anti-diabetic drugs. The dual leucine zipper kinase (DLK) inhibits the action of two transcription factors within the beta-cell, thereby interfering with insulin secretion and production and the conservation of beta-cell mass. Furthermore, DLK action is regulated by prediabetic signals. Hence, the inhibition of this kinase might protect beta-cells against beta-cell-toxic prediabetic signals and prevent the development of diabetes. DLK might thus present a novel drug target for the treatment of diabetes mellitus type 2.

Distinct functions of the dual leucine zipper kinase depending on its subcellular localization

The dual leucine zipper kinase DLK induces β-cell apoptosis by inhibiting the transcriptional activity conferred by the β-cell protective transcription factor cAMP response element binding protein CREB. This action might contribute to β-cell loss and ultimately diabetes. Within its kinase domain DLK shares high homology with the mixed lineage kinase (MLK) 3, which is activated by tumor necrosis factor (TNF) α and interleukin (IL)-1β, known prediabetic signals. In the present study, the regulation of DLK in β-cells by these cytokines was investigated. Both, TNFα and IL-1β induced the nuclear translocation of DLK. Mutations within a putative nuclear localization signal (NLS) prevented basal and cytokine-induced nuclear localization of DLK and binding to the importin receptor importin α, thereby demonstrating a functional NLS within DLK. DLK NLS mutants were catalytically active as they phosphorylated their down-stream kinase c-Jun N-terminal kinase to the same extent as DLK wild-type but did neither inhibit CREB-dependent gene transcription nor transcription conferred by the promoter of the anti-apoptotic protein BCL-xL. In addition, the β-cell apoptosis-inducing effect of DLK was severely diminished by mutation of its NLS. In a murine model of prediabetes, enhanced nuclear DLK was found. These data demonstrate that DLK exerts distinct functions, depending on its subcellular localization and thus provide a novel level of regulating DLK action. Furthermore, the prevention of the nuclear localization of DLK as induced by prediabetic signals with consecutive suppression of β-cell apoptosis might constitute a novel target in the therapy of diabetes mellitus.

Inhibition of human insulin gene transcription and MafA transcriptional activity by the dual leucine zipper kinase

Insulin biosynthesis is an essential β-cell function and inappropriate insulin secretion and biosynthesis contribute to the pathogenesis of diabetes mellitus type 2. Previous studies showed that the dual leucine zipper kinase (DLK) induces β-cell apoptosis. Since β-cell dysfunction precedes β-cell loss, in the present study the effect of DLK on insulin gene transcription was investigated in the HIT-T15 β-cell line. Downregulation of endogenous DLK increased whereas overexpression of DLK decreased human insulin gene transcription. 5'- and 3'-deletion human insulin promoter analyses resulted in the identification of a DLK responsive element that mapped to the DNA binding-site for the β-cell specific transcription factor MafA. Overexpression of DLK wild-type but not its kinase-dead mutant inhibited MafA transcriptional activity conferred by its transactivation domain. Furthermore, in the non-β-cell line JEG DLK inhibited MafA overexpression-induced human insulin promoter activity. Overexpression of MafA and DLK or its kinase-dead mutant into JEG cells revealed that DLK but not its mutant reduced MafA protein content. Inhibition of the down-stream DLK kinase c-Jun N-terminal kinase (JNK) by SP600125 attenuated DLK-induced MafA loss. Furthermore, mutation of the serine 65 to alanine, shown to confer MafA protein stability, increased MafA-dependent insulin gene transcription and prevented DLK-induced MafA loss in JEG cells. These data suggest that DLK by activating JNK triggers the phosphorylation and degradation of MafA thereby attenuating insulin gene transcription. Given the importance of MafA for β-cell function, the inhibition of DLK might preserve β-cell function and ultimately retard the development of diabetes mellitus type 2.

Microtubule depolymerization in Caenorhabditis elegans touch receptor neurons reduces gene expression through a p38 MAPK pathway

Microtubules are integral to neuronal development and function. They endow cells with polarity, shape, and structure, and their extensive surface area provides substrates for intracellular trafficking and scaffolds for signaling molecules. Consequently, microtubule polymerization dynamics affect not only structural features of the cell but also the subcellular localization of proteins that can trigger intracellular signaling events. In the nematode Caenorhabditis elegans, the processes of touch receptor neurons are filled with a bundle of specialized large-diameter microtubules. We find that conditions that disrupt these microtubules (loss of either the MEC-7 β-tubulin or MEC-12 α-tubulin or growth in 1 mM colchicine) cause a general reduction in touch receptor neuron (TRN) protein levels. This reduction requires a p38 MAPK pathway (DLK-1, MKK-4, and PMK-3) and the transcription factor CEBP-1. Cells may use this feedback pathway that couples microtubule state and MAPK activation to regulate cellular functions.

Calcineurin/NFAT signaling in the beta-cell: From diabetes to new therapeutics

Pancreatic beta-cells in the islet of Langerhans produce the hormone insulin, which maintains blood glucose homeostasis. Perturbations in beta-cell function may lead to impairment of insulin production and secretion and the onset of diabetes mellitus. Several essential beta-cell factors have been identified that are required for normal beta-cell function, including six genes that when mutated give rise to inherited forms of diabetes known as Maturity Onset Diabetes of the Young (MODY). However, the intracellular signaling pathways that control expression of MODY and other factors continue to be revealed. Post-transplant diabetes mellitus in patients taking the calcineurin inhibitors tacrolimus (FK506) or cyclosporin A indicates that calcineurin and its substrate the Nuclear Factor of Activated T-cells (NFAT) may be required for beta-cell function. Here recent advances in our understanding of calcineurin and NFAT signaling in the beta-cell are reviewed. Novel therapeutic approaches for the treatment of diabetes are also discussed.

Activation of the dual-leucine-zipper-bearing kinase and induction of beta-cell apoptosis by the immunosuppressive drug cyclosporin A

Post-transplant diabetes is an untoward effect often observed under immunosuppressive therapy with cyclosporin A. Besides the development of peripheral insulin resistance and a decrease in insulin gene transcription, a beta-cell toxic effect has been described. However, its molecular mechanism remains unknown. In the present study, the effect of cyclosporin A and the dual leucine-zipper-bearing kinase (DLK) on beta-cell survival was investigated. Cyclosporin A decreased the viability of the insulin-producing pancreatic islet cell line HIT in a time- and concentration-dependent manner. Upon exposure to the immunosuppressant fragmentation of DNA, the activation of the effector caspase-3 and a decrease of full-length caspase-3 and Bcl(XL) were observed in HIT cells and in primary mature murine islets, respectively. Cyclosporin A and tacrolimus, both potent inhibitors of the calcium/calmodulin-dependent phosphatase calcineurin, stimulated the enzymatic activity of cellular DLK in an in vitro kinase assay. Immunocytochemistry revealed that the overexpression of DLK but not its kinase-dead mutant induced apoptosis and enhanced cyclosporin A-induced apoptosis to a higher extent than the drug alone. Moreover, in the presence of DLK, the effective concentration for cyclosporin A-caused apoptosis was similar to its known IC(50) value for the inhibition of calcineurin activity in beta cells. These data suggest that cyclosporin A through inhibition of calcineurin activates DLK, thereby leading to beta-cell apoptosis. This action may thus be a novel mechanism through which cyclosporin A precipitates post-transplant diabetes.

Enhancement by lithium of cAMP-induced CRE/CREB-directed gene transcription conferred by TORC on the CREB basic leucine zipper domain

The molecular mechanism of the action of lithium salts in the treatment of bipolar disorder is not well understood. As their therapeutic action requires chronic treatment, adaptive neuronal processes are suggested to be involved. The molecular basis of this are changes in gene expression regulated by transcription factors such as CREB (cAMP-response-element-binding protein). CREB contains a transactivation domain, in which Ser119 is phosphorylated upon activation, and a bZip (basic leucine zipper domain). The bZip is involved in CREB dimerization and DNA-binding, but also contributes to CREB transactivation by recruiting the coactivator TORC (transducer of regulated CREB). In the present study, the effect of lithium on CRE (cAMP response element)/CREB-directed gene transcription was investigated. Electrically excitable cells were transfected with CRE/CREB-driven luciferase reporter genes. LiCl (6 mM or higher) induced an up to 4.7-fold increase in 8-bromo-cAMP-stimulated CRE/CREB-directed transcription. This increase was not due to enhanced Ser119 phosphorylation or DNA-binding of CREB. Also, the known targets inositol monophosphatase and GSK3beta (glycogen-synthase-kinase 3beta) were not involved as specific GSK3beta inhibitors and inositol replenishment did not mimic and abolish respectively the effect of lithium. However, lithium no longer enhanced CREB activity when the CREB-bZip was deleted or the TORC-binding site inside the CREB-bZip was specifically mutated (CREB-R300A). Otherwise, TORC overexpression conferred lithium responsiveness on CREB-bZip or the CRE-containing truncated rat somatostatin promoter. This indicates that lithium enhances cAMP-induced CRE/CREB-directed transcription, conferred by TORC on the CREB-bZip. We thus support the hypothesis that lithium salts modulate CRE/CREB-dependent gene transcription and suggest the CREB coactivator TORC as a new molecular target of lithium.