CDK8 Regulates Insulin Secretion and Mediates Postnatal and Stress-Induced Expression of Neuropeptides in Pancreatic β Cells

Cdk8/CDK19 promotes mitochondrial fission through Drp1 phosphorylation and can phenotypically suppress pink1 deficiency in Drosophila

Cdk8 in Drosophila is the orthologue of vertebrate CDK8 and CDK19. These proteins have been shown to modulate transcriptional control by RNA polymerase II. We found that neuronal loss of Cdk8 severely reduces fly lifespan and causes bang sensitivity. Remarkably, these defects can be rescued by expression of human CDK19, found in the cytoplasm of neurons, suggesting a non-nuclear function of CDK19/Cdk8. Here we show that Cdk8 plays a critical role in the cytoplasm, with its loss causing elongated mitochondria in both muscles and neurons. We find that endogenous GFP-tagged Cdk8 can be found in both the cytoplasm and nucleus. We show that Cdk8 promotes the phosphorylation of Drp1 at S616, a protein required for mitochondrial fission. Interestingly, Pink1, a mitochondrial kinase implicated in Parkinson's disease, also phosphorylates Drp1 at the same residue. Indeed, overexpression of Cdk8 significantly suppresses the phenotypes observed in flies with low levels of Pink1, including elevated levels of ROS, mitochondrial dysmorphology, and behavioral defects. In summary, we propose that Pink1 and Cdk8 perform similar functions to promote Drp1-mediated fission.

Pancreatic islet protection at the expense of secretory function involves serine-linked mitochondrial one-carbon metabolism

Type 2 diabetes is characterized by insulin hypersecretion followed by reduced glucose-stimulated insulin secretion (GSIS). Here we show that acute stimulation of pancreatic islets with the insulin secretagogue dextrorphan (DXO) or glibenclamide enhances GSIS, whereas chronic treatment with high concentrations of these drugs reduce GSIS but protect islets from cell death. Bulk RNA sequencing of islets shows increased expression of genes for serine-linked mitochondrial one-carbon metabolism (OCM) after chronic, but not acute, stimulation. In chronically stimulated islets, more glucose is metabolized to serine than to citrate, and the mitochondrial ATP/ADP ratio decreases, whereas the NADPH/NADP+ ratio increases. Activating transcription factor-4 (Atf4) is required and sufficient to activate serine-linked mitochondrial OCM genes in islets, with gain- and loss-of-function experiments showing that Atf4 reduces GSIS and is required, but not sufficient, for full DXO-mediated islet protection. In sum, we identify a reversible metabolic pathway that provides islet protection at the expense of secretory function.

Genetically Engineered Mice Unveil In Vivo Roles of the Mediator Complex

The Mediator complex is a multi-subunit protein complex which plays a significant role in the regulation of eukaryotic gene transcription. It provides a platform for the interaction of transcriptional factors and RNA polymerase II, thus coupling external and internal stimuli with transcriptional programs. Molecular mechanisms underlying Mediator functioning are intensively studied, although most often using simple models such as tumor cell lines and yeast. Transgenic mouse models are required to study the role of Mediator components in physiological processes, disease, and development. As constitutive knockouts of most of the Mediator protein coding genes are embryonically lethal, conditional knockouts and corresponding activator strains are needed for these studies. Recently, they have become more easily available with the development of modern genetic engineering techniques. Here, we review existing mouse models for studying the Mediator, and data obtained in corresponding experiments.

CDK8/19 inhibition plays an important role in pancreatic β-cell induction from human iPSCs

BACKGROUND

Transplantation of differentiated cells from human-induced pluripotent stem cells (hiPSCs) holds great promise for clinical treatments. Eliminating the risk factor of malignant cell transformation is essential for ensuring the safety of such cells. This study was aimed at assessing and mitigating mutagenicity that may arise during the cell culture process in the protocol of pancreatic islet cell (iPIC) differentiation from hiPSCs.

METHODS

We evaluated the mutagenicity of differentiation factors used for hiPSC-derived pancreatic islet-like cells (iPICs). We employed Ames mutagenicity assay, flow cytometry analysis, immunostaining, time-resolved fluorescence resonance energy transfer-based (TR-FRET) cell-free dose-response assays, single-cell RNA-sequencing and in vivo efficacy study.

RESULTS

We observed a mutagenic effect of activin receptor-like kinase 5 inhibitor II (ALK5iII). ALK5iII is a widely used β-cell inducer but no other tested ALK5 inhibitors induced β-cells. We obtained kinase inhibition profiles and found that only ALK5iII inhibited cyclin-dependent kinases 8 and 19 (CDK8/19) among all ALK5 inhibitors tested. Consistently, CDK8/19 inhibitors efficiently induced β-cells in the absence of ALK5iII. A combination treatment with non-mutagenic ALK5 inhibitor SB431542 and CDK8/19 inhibitor senexin B afforded generation of iPICs with in vitro cellular composition and in vivo efficacy comparable to those observed with ALK5iII.

CONCLUSION

Our findings suggest a new risk mitigation approach for cell therapy and advance our understanding of the β-cell differentiation mechanism.

Neuropeptide Y1 receptor antagonism protects β-cells and improves glycemic control in type 2 diabetes

OBJECTIVES

Loss of functional β-cell mass is a key factor contributing to poor glycemic control in advanced type 2 diabetes (T2D). We have previously reported that the inhibition of the neuropeptide Y1 receptor improves the islet transplantation outcome in type 1 diabetes (T1D). The aim of this study was to identify the pathophysiological role of the neuropeptide Y (NPY) system in human T2D and further evaluate the therapeutic potential of using the Y1 receptor antagonist BIBO3304 to improve β-cell function and survival in T2D.

METHODS

The gene expression of the NPY system in human islets from nondiabetic subjects and subjects with T2D was determined and correlated with the stimulation index. The glucose-lowering and β-cell-protective effects of BIBO3304, a selective orally bioavailable Y1 receptor antagonist, in high-fat diet (HFD)/multiple low-dose streptozotocin (STZ)-induced and genetically obese (db/db) T2D mouse models were assessed.

RESULTS

In this study, we identified a more than 2-fold increase in NPY1R and its ligand, NPY mRNA expression in human islets from subjects with T2D, which was significantly associated with reduced insulin secretion. Consistently, the pharmacological inhibition of Y1 receptors by BIBO3304 significantly protected β cells from dysfunction and death under multiple diabetogenic conditions in islets. In a preclinical study, we demonstrated that the inhibition of Y1 receptors by BIBO3304 led to reduced adiposity and enhanced insulin action in the skeletal muscle. Importantly, the Y1 receptor antagonist BIBO3304 treatment also improved β-cell function and preserved functional β-cell mass, thereby resulting in better glycemic control in both HFD/multiple low-dose STZ-induced and db/db T2D mice.

CONCLUSIONS

Our results revealed a novel causal link between increased islet NPY-Y1 receptor gene expression and β-cell dysfunction and failure in human T2D, contributing to the understanding of the pathophysiology of T2D. Furthermore, our results demonstrate that the inhibition of the Y1 receptor by BIBO3304 represents a potential β-cell-protective therapy for improving functional β-cell mass and glycemic control in T2D.

Clinical Value and Potential Mechanisms of Oxysterol-Binding Protein Like 3 (OSBPL3) in Human Tumors

Cancer remains one of the top culprits causing disease-related deaths. A lack of effective multi-cancer therapeutic targets has limited the prolongation of cancer patients’ survival. Therefore, it is important to explore novel oncogenic genes or versatile targets and perform a comprehensive analysis to assess their roles in the process of tumorigenesis. OSBPL3 protein is an intracellular lipid receptor of the oxysterol-binding protein superfamily, which participates in some pathological and physiological processes in tumor progression. However, its clinical roles and potential mechanisms in cancers remain unknown. Thus, we aimed to systematic explore the potential oncogenic roles of OSBPL3 across thirty-three tumors using multiple web-based and publicly available tools, including the Cancer Genome Atlas, Gene Expression Omnibus, Genotype-Tissue Expression, cBioPortal, and Human Protein Atlas database. OSBPL3 is highly expressed in major subtypes of cancers, distinctly associated with the prognosis of tumor patients. We observed X676_splice/V676G alteration in the oxysterol domain and frequent mutations of OSBPL3 involve cell survival in skin cutaneous melanoma. We also first presented that the expression of OSBPL3 was associated with tumor mutational burden (TMB) in nine cancer types. Additionally, OSBPL3 shows an enhanced phosphorylation level at S426, S251, and S273 loci within the pleckstrin homology domain in multiple tumors, such as breast cancer or lung adenocarcinoma. And OSBPL3 expression was associated with active immune cells (CD8⁺ T cells) and cancer-associated fibroblasts in breast cancer, colon adenocarcinoma, and kidney renal clear cell carcinoma and immune checkpoint genes in more than 30 tumors, but weakly associated with immune suppressive cells (myeloid-derived suppressor cells, T regulatory cells). Moreover, protein processing and mRNA metabolic signaling pathways were involved in the functional mechanisms of OSBPL3. Our study first demonstrated that a novel agent OSBPL3 plays an important role in tumorigenesis from the perspective of publicly available databases and clinical tumor samples in various cancers, which comprehensively provide insights into its biological functions and may be helpful for further investigation.

Inhibition of histone acetyltransferase by naringenin and hesperetin suppresses Txnip expression and protects pancreatic β cells in diabetic mice

BACKGROUND

The damage of pancreatic β cells is a major pathogenesis of the development and progression of type 2 diabetes and there is still no effective therapy to protect pancreatic β cells clinically. In our previous study, we found that Quzhou Fructus Aurantii (QFA), which is rich in flavanones, had the protective effect of pancreatic β cells in diabetic mice. However, the underlying mechanism is still unclear.

PURPOSE

In the current study, we administered naringenin and hesperetin, two major active components of QFA, to protect pancreatic β cells and to investigate the underlying molecular mechanism focusing on the epigenetic modifications.

METHODS

We used diabetic db/db mouse and INS-1 pancreatic β cell line as in vivo and in vitro models to investigate the protective effect of naringenin and hesperetin on pancreatic β cells under high glucose environment and the related mechanism. The phenotypic changes were evaluatedby immunostaining and the measurement of biochemical indexes. The molecular mechanism was explored by biological techniques such as western blotting, qPCR, ChIP-seq and ChIP-qPCR, flow cytometry and lentivirus infection.

RESULTS

We found that naringenin and hesperetin had an inhibitory effect on histone acetylation. We showed that naringenin and hesperetin protected pancreatic β cells in vivo and in vitro, and this effect was independent of their direct antioxidant capacity. The further study found that the inhibition of thioredoxin-interacting protein (Txnip) expression regulated by histone acetylation was critical for the protective role of naringenin and hesperetin. Mechanistically, the histone acetylation inhibition by naringenin and hesperetin was achieved through regulating AMPK-mediated p300 inactivation.

CONCLUSION

These findings highlight flavanones and the phytomedicine rich in flavanones as important dietary supplements in protecting pancreatic β cells in advanced diabetes. In addition, targeting histone acetylation by phytomedicine is a potential strategy to delay the development and progression of diabetes.

Oxytocin signal contributes to the adaptative growth of islets during gestation

BACKGROUND

Increased insulin production and secretion by pancreatic β-cells are important for ensuring the high insulin demand during gestation. However, the underlying mechanism of β-cell adaptation during gestation or gestational diabetes mellitus (GDM) remains unclear. Oxytocin is an important physiological hormone in gestation and delivery, and it also contributes to the maintenance of β-cell function. The aim of this study was to investigate the role of oxytocin in β-cell adaptation during pregnancy.

METHODS

The relationship between the blood oxytocin level and pancreatic β-cell function in patients with GDM and healthy pregnant women was investigated. Gestating and non-gestating mice were used to evaluate the in vivo effect of oxytocin signal on β-cells during pregnancy. In vitro experiments were performed on INS-1 insulinoma cells.

RESULTS

The blood oxytocin levels were lower in patients with GDM than in healthy pregnant women and were associated with impaired pancreatic β-cell function. Acute administration of oxytocin increased insulin secretion in both gestating and non-gestating mice. A 3-week oxytocin treatment promoted the proliferation of pancreatic β-cells and increased the β-cell mass in gestating but not non-gestating mice. Antagonism of oxytocin receptors by atosiban impaired insulin secretion and induced GDM in gestating but not non-gestating mice. Oxytocin enhanced glucose-stimulated insulin secretion, activated the mitogen-activated protein kinase pathway, and promoted cell proliferation in INS-1 cells.

CONCLUSIONS

These findings provide strong evidence that oxytocin is needed for β-cell adaptation during pregnancy to maintain β-cell function, and the lack of oxytocin could be associated with the risk of GDM.

Dissection of two routes to naïve pluripotency using different kinase inhibitors

Embryonic stem cells (ESCs) can be maintained in the naïve state through inhibition of Mek1/2 and Gsk3 (2i). A relevant effect of 2i is the inhibition of Cdk8/19, which are negative regulators of the Mediator complex, responsible for the activity of enhancers. Inhibition of Cdk8/19 (Cdk8/19i) stimulates enhancers and, similar to 2i, stabilizes ESCs in the naïve state. Here, we use mass spectrometry to describe the molecular events (phosphoproteome, proteome, and metabolome) triggered by 2i and Cdk8/19i on ESCs. Our data reveal widespread commonalities between these two treatments, suggesting overlapping processes. We find that post-transcriptional de-repression by both 2i and Cdk8/19i might support the mitochondrial capacity of naive cells. However, proteome reprogramming in each treatment is achieved by different mechanisms. Cdk8/19i acts directly on the transcriptional machinery, activating key identity genes to promote the naïve program. In contrast, 2i stabilizes the naïve circuitry through, in part, de-phosphorylation of downstream transcriptional effectors.

TCF7 is not essential for glucose homeostasis in mice

Objective

Glucose-dependent insulinotropic polypeptide (GIP) and Glucagon-like peptide-1 (GLP-1) are incretin hormones that exert overlapping yet distinct actions on islet β-cells. We recently observed that GIP, but not GLP-1, upregulated islet expression of Transcription Factor 7 (TCF7), a gene expressed in immune cells and associated with the risk of developing type 1 diabetes. TCF7 has also been associated with glucose homeostasis control in the liver. Herein we studied the relative metabolic importance of TCF7 expression in hepatocytes vs. islet β-cells in mice.

Methods

Tcf7 expression was selectively inactivated in adult mouse hepatocytes using adenoviral Cre expression and targeted in β-cells using two different lines of insulin promoter-Cre mice. Glucose homeostasis, plasma insulin and triglyceride responses, islet histology, hepatic and islet gene expression, and body weight gain were evaluated in mice fed regular chow or high fat diets. Tcf7 expression within pancreatic islets and immune cells was evaluated using published single cell RNA-seq (scRNA-seq) data, and in islet RNA from immunodeficient Rag2^−/−Il2rg^−/− mice.

Results

Reduction of hepatocyte Tcf7 expression did not impair glucose homeostasis, lipid tolerance or hepatic gene expression profiles linked to control of metabolic or immune pathways. Similarly, oral and intraperitoneal glucose tolerance, plasma insulin responses, islet histology, body weight gain, and insulin tolerance were not different in mice with targeted recombination of Tcf7 in insulin-positive β-cells. Surprisingly, islet Tcf7 mRNA transcripts were not reduced in total islet RNA containing endocrine and associated non-endocrine cell types from Tcf7^{βcell−/−} mice, despite Cre-mediated recombination of islet genomic DNA. Furthermore, glucose tolerance was normal in whole body Tcf7^−/− mice. Analysis of scRNA-seq datasets localized pancreatic Tcf7 expression to islet progenitors during development, and immune cells, but not within differentiated islet β-cells or endocrine lineages within mature islets. Moreover, the expression of Tcf7 was extremely low in islet RNA from Rag2^−/−Il2rg^−/− mice and, consistent with expression within immune cells, Tcf7 was highly correlated with levels of Cd3g mRNA transcripts in RNA from wild type mouse islets.

Conclusions

These findings demonstrate that Tcf7 expression is not a critical determinant of glucose homeostasis in mice. Moreover, the detection of Tcf7 expression within islet mRNA is attributable to the expression of Tcf7 RNA in islet-associated murine immune cells, and not in islet β-cells.

•Reduction of hepatocyte Tcf7 does not impair glucose homeostasis.

•Targeting beta cell Tcf7 using insulin-promoter-Cre does not reduce islet Tcf7 expression.

•RNA-seq localizes pancreatic Tcf7 to islet progenitors and lymphocytes.

•Tcf7 expression is markedly reduced in islet RNA from Rag2^−/−Il2rg^−/− mice.

Updated Role of Neuropeptide Y in Nicotine-Induced Endothelial Dysfunction and Atherosclerosis

Cardiovascular disease is the leading cause of death worldwide. Endothelial dysfunction of the arterial vasculature plays a pivotal role in cardiovascular pathogenesis. Nicotine-induced endothelial dysfunction substantially contributes to the development of arteriosclerotic cardiovascular disease. Nicotine promotes oxidative inflammation, thrombosis, pathological angiogenesis, and vasoconstriction, and induces insulin resistance. However, the exact mechanism through which nicotine induces endothelial dysfunction remains unclear. Neuropeptide Y (NPY) is widely distributed in the central nervous system and peripheral tissues, and it participates in the pathogenesis of atherosclerosis by regulating vasoconstriction, energy metabolism, local plaque inflammatory response, activation and aggregation of platelets, and stress and anxiety-related emotion. Nicotine can increase the expression of NPY, suggesting that NPY is involved in nicotine-induced endothelial dysfunction. Herein, we present an updated review of the possible mechanisms of nicotine-induced atherosclerosis, with a focus on endothelial cell dysfunction associated with nicotine and NPY.

CDK2 limits the highly energetic secretory program of mature β cells by restricting PEP cycle-dependent KATP channel closure

Hallmarks of mature β cells are restricted proliferation and a highly energetic secretory state. Paradoxically, cyclin-dependent kinase 2 (CDK2) is synthesized throughout adulthood, its cytosolic localization raising the likelihood of cell cycle-independent functions. In the absence of any changes in β cell mass, maturity, or proliferation, genetic deletion of Cdk2 in adult β cells enhanced insulin secretion from isolated islets and improved glucose tolerance in vivo. At the single β cell level, CDK2 restricts insulin secretion by increasing K_ATP conductance, raising the set point for membrane depolarization in response to activation of the phosphoenolpyruvate (PEP) cycle with mitochondrial fuels. In parallel with reduced β cell recruitment, CDK2 restricts oxidative glucose metabolism while promoting glucose-dependent amplification of insulin secretion. This study provides evidence of essential, non-canonical functions of CDK2 in the secretory pathways of quiescent β cells.

Despite loss of proliferative capacity with age, mature β cells continually synthesize CDK2. Sdao et al. demonstrate that CDK2 depletion in adult β cells improves glucose tolerance in vivo. By augmenting PEP cycle-dependent K_ATP channel closure, CDK2 inactivation lowers the set point for membrane depolarization, augmenting oxidative metabolism and insulin secretion.

Angel or Devil ? - CDK8 as the new drug target

Cyclin-dependent kinase 8 (CDK8) plays an momentous role in transcription regulation by forming kinase module or transcription factor phosphorylation. A large number of evidences have identified CDK8 as an important factor in cancer occurrence and development. In addition, CDK8 also participates in the regulation of cancer cell stress response to radiotherapy and chemotherapy, assists tumor cell invasion, metastasis, and drug resistance. Therefore, CDK8 is regarded as a promising target for cancer therapy. Most studies in recent years supported the role of CDK8 as a carcinogen, however, under certain conditions, CDK8 exists as a tumor suppressor. The functional diversity of CDK8 and its exceptional role in different types of cancer have aroused great interest from scientists but even more controversy during the discovery of CDK8 inhibitors. In addition, CDK8 appears to be an effective target for inflammation diseases and immune system disorders. Therefore, we summarized the research results of CDK8, involving physiological/pathogenic mechanisms and the development status of compounds targeting CDK8, provide a reference for the feasibility evaluation of CDK8 as a therapeutic target, and guidance for researchers who are involved in this field for the first time.

The Role of CDKs and CDKIs in Murine Development

Cyclin-dependent kinases (CDKs) and their inhibitors (CDKIs) play pivotal roles in the regulation of the cell cycle. As a result of these functions, it may be extrapolated that they are essential for appropriate embryonic development. The twenty known mouse CDKs and eight CDKIs have been studied to varying degrees in the developing mouse, but only a handful of CDKs and a single CDKI have been shown to be absolutely required for murine embryonic development. What has become apparent, as more studies have shone light on these family members, is that in addition to their primary functional role in regulating the cell cycle, many of these genes are also controlling specific cell fates by directing differentiation in various tissues. Here we review the extensive mouse models that have been generated to study the functions of CDKs and CDKIs, and discuss their varying roles in murine embryonic development, with a particular focus on the brain, pancreas and fertility.