Map4k4 suppresses Srebp-1 and adipocyte lipogenesis independent of JNK signaling

Deciphering the role of alternative splicing as a potential regulator in fat-tail development of sheep: a comprehensive RNA-seq based study

Although research on alternative splicing (AS) has been widely conducted in mammals, no study has investigated the splicing profiles of genes involved in fat-tail formation in sheep. Here, for the first time, a comprehensive study was designed to investigate the profile of AS events and their involvement in fat-tail development of sheep. In total, 45 RNA-Seq samples related to seven different studies, which have compared the fat-tailed vs thin-tailed sheep breeds, were analyzed. Two independent tools, rMATS and Whippet, along with a set of stringent filters were applied to identify differential AS (DAS) events between the breeds per each study. Only DAS events that were detected by both tools as well as in at least three datasets with the same ΔPSI trend (percent spliced in), were considered as the final high-confidence set of DAS genes. Final results revealed 130 DAS skipped exon events (69 negative and 61 positive ΔPSI) belonged to 124 genes. Functional enrichment analysis highlighted the importance of the genes in the underlying molecular mechanisms of fat metabolism. Moreover, protein-protein interaction network analysis revealed that DAS genes are significantly connected. Of DAS genes, five transcription factors were found that were enriched in the biological process associated with lipid metabolism like "Fat Cell Differentiation". Further investigations of the findings along with a comprehensive literature review provided a reliable list of candidate genes that may potentially contribute to fat-tail formation including HSD11B1, SIRT2, STRN3 and TCF7L2. Based on the results, it can be stated that the AS patterns may have evolved, during the evolution of sheep breeds, as another layer of regulation to contribute to biological complexity by reprogramming the gene regulatory networks. This study provided the theoretical basis of the molecular mechanisms behind the sheep fat-tail development in terms of AS.

The molecular basis of the dichotomous functionality of MAP4K4 in proliferation and cell motility control in cancer

The finely tuned integration of intra- and extracellular cues by components of the mitogen-activated protein kinase (MAPK) signaling pathways controls the mutually exclusive phenotypic manifestations of uncontrolled growth and tumor cell dissemination. The Ser/Thr kinase MAP4K4 is an upstream integrator of extracellular cues involved in both proliferation and cell motility control. Initially identified as an activator of the c-Jun N-terminal kinase (JNK), the discovery of diverse functions and additional effectors of MAP4K4 beyond JNK signaling has considerably broadened our understanding of this complex kinase. The implication of MAP4K4 in the regulation of cytoskeleton dynamics and cell motility provided essential insights into its role as a pro-metastatic kinase in cancer. However, the more recently revealed role of MAP4K4 as an activator of the Hippo tumor suppressor pathway has complicated the understanding of MAP4K4 as an oncogenic driver kinase. To develop a better understanding of the diverse functions of MAP4K4 and their potential significance in oncogenesis and tumor progression, we have collected and assessed the current evidence of MAP4K4 implication in molecular mechanisms that control proliferation and promote cell motility. A better understanding of these mechanisms is particularly relevant in the brain, where MAP4K4 is highly expressed and under pathological conditions either drives neuronal cell death in neurodegenerative diseases or cell dissemination in malignant tumors. We review established effectors and present novel interactors of MAP4K4, which offer mechanistic insights into MAP4K4 function and may inspire novel intervention strategies. We discuss possible implications of novel interactors in tumor growth and dissemination and evaluate potential therapeutic strategies to selectively repress pro-oncogenic functions of MAP4K4.

Inhibition of MAP4K4 Signaling Initiaties Metabolic Reprogramming to Protect Hepatocytes from Lipotoxic Damage

The primary hepatic consequence of obesity is non-alcoholic fatty liver disease (NAFLD), affecting about 25% of the global adult population. Non-alcoholic steatohepatitis (NASH) is a severe form of NAFLD characterized by liver lipid accumulation, inflammation, and hepatocyte ballooning, with a different degree of hepatic fibrosis. In the light of rapidly increasing prevalence of NAFLD and NASH, there is an urgent need for improved understanding of the molecular pathogenesis of these diseases. The aim of this study was to decipher the possible role of STE20-type kinase MAP4K4 in the regulation of hepatocellular lipotoxicity and susceptibility to NAFLD. We found that MAP4K4 mRNA expression in human liver biopsies was positively correlated with key hallmarks of NAFLD (i.e., liver steatosis, lobular inflammation, hepatocellular ballooning, and fibrosis). We also found that the silencing of MAP4K4 suppressed lipid deposition in human hepatocytes by stimulating β-oxidation and triacylglycerol secretion, while attenuating fatty acid influx and lipid synthesis. Furthermore, downregulation of MAP4K4 markedly reduced the glycolysis rate and lowered incidences of oxidative/endoplasmic reticulum stress. In parallel, we observed suppressed JNK and ERK and increased AKT phosphorylation in MAP4K4-deficient hepatocytes. Together, these results provide the first experimental evidence supporting the potential involvement of STE20-type kinase MAP4K4 as a component of the hepatocellular lipotoxic milieu promoting NAFLD susceptibility.

The Combined Partial Knockdown of CBS and MPST Genes Induces Inflammation, Impairs Adipocyte Function-Related Gene Expression and Disrupts Protein Persulfidation in Human Adipocytes

Recent studies in mice and humans demonstrated the relevance of H₂S synthesising enzymes, such as CTH, CBS, and MPST, in the physiology of adipose tissue and the differentiation of preadipocyte into adipocytes. Here, our objective was to investigate the combined role of CTH, CBS, and MPST in the preservation of adipocyte protein persulfidation and adipogenesis. Combined partial CTH, CBS, and MPST gene knockdown was achieved treating fully human adipocytes with siRNAs against these transcripts (siRNA_MIX). Adipocyte protein persulfidation was analyzed using label-free quantitative mass spectrometry coupled with a dimedone-switch method for protein labeling and purification. Proteomic analysis quantified 216 proteins with statistically different levels of persulfidation in KD cells compared to control adipocytes. In fully differentiated adipocytes, CBS and MPST mRNA and protein levels were abundant, while CTH expression was very low. It is noteworthy that siRNA_MIX administration resulted in a significant decrease in CBS and MPST expression, without impacting on CTH. The combined partial knockdown of the CBS and MPST genes resulted in reduced cellular sulfide levels in parallel to decreased expression of relevant genes for adipocyte biology, including adipogenesis, mitochondrial biogenesis, and lipogenesis, but increased proinflammatory- and senescence-related genes. It should be noted that the combined partial knockdown of CBS and MPST genes also led to a significant disruption in the persulfidation pattern of the adipocyte proteins. Although among the less persulfidated proteins, we identified several relevant proteins for adipocyte adipogenesis and function, among the most persulfidated, key mediators of adipocyte inflammation and dysfunction as well as some proteins that might play a positive role in adipogenesis were found. In conclusion, the current study indicates that the combined partial elimination of CBS and MPST (but not CTH) in adipocytes affects the expression of genes related to the maintenance of adipocyte function and promotes inflammation, possibly by altering the pattern of protein persulfidation in these cells, suggesting that these enzymes were required for the functional maintenance of adipocytes.

MAP4K4 expression in cardiomyocytes: multiple isoforms, multiple phosphorylations and interactions with striatins

The Ser/Thr kinase MAP4K4, like other GCKIV kinases, has N-terminal kinase and C-terminal citron homology (CNH) domains. MAP4K4 can activate c-Jun N-terminal kinases (JNKs), and studies in the heart suggest it links oxidative stress to JNKs and heart failure. In other systems, MAP4K4 is regulated in striatin-interacting phosphatase and kinase (STRIPAK) complexes, in which one of three striatins tethers PP2A adjacent to a kinase to keep it dephosphorylated and inactive. Our aim was to understand how MAP4K4 is regulated in cardiomyocytes. The rat MAP4K4 gene was not properly defined. We identified the first coding exon of the rat gene using 5′-RACE, we cloned the full-length sequence and confirmed alternative-splicing of MAP4K4 in rat cardiomyocytes. We identified an additional α-helix C-terminal to the kinase domain important for kinase activity. In further studies, FLAG-MAP4K4 was expressed in HEK293 cells or cardiomyocytes. The Ser/Thr protein phosphatase inhibitor calyculin A (CalA) induced MAP4K4 hyperphosphorylation, with phosphorylation of the activation loop and extensive phosphorylation of the linker between the kinase and CNH domains. This required kinase activity. MAP4K4 associated with myosin in untreated cardiomyocytes, and this was lost with CalA-treatment. FLAG-MAP4K4 associated with all three striatins in cardiomyocytes, indicative of regulation within STRIPAK complexes and consistent with activation by CalA. Computational analysis suggested the interaction was direct and mediated via coiled-coil domains. Surprisingly, FLAG-MAP4K4 inhibited JNK activation by H₂O₂ in cardiomyocytes and increased myofibrillar organisation. Our data identify MAP4K4 as a STRIPAK-regulated kinase in cardiomyocytes, and suggest it regulates the cytoskeleton rather than activates JNKs.

Control of Adipocyte Thermogenesis and Lipogenesis through β3-Adrenergic and Thyroid Hormone Signal Integration

Here, we show that β adrenergic signaling coordinately upregulates de novo lipogenesis (DNL) and thermogenesis in subcutaneous white adipose tissue (sWAT), and both effects are blocked in mice lacking the cAMP-generating G protein-coupled receptor Gs (Adipo-GsαKO) in adipocytes. However, UCP1 expression but not DNL activation requires rapamycin-sensitive mTORC1. Furthermore, β3-adrenergic agonist CL316243 readily upregulates thermogenic but not lipogenic genes in cultured adipocytes, indicating that additional regulators must operate on DNL in sWAT in vivo. We identify one such factor as thyroid hormone T3, which is elevated locally by adrenergic signaling. T3 administration to wild-type mice enhances both thermogenesis and DNL in sWAT. Mechanistically, T3 action on UCP1 expression in sWAT depends upon cAMP and is blocked in Adipo-GsαKO mice even as elevated DNL persists. Thus, T3 enhances sWAT thermogenesis by amplifying cAMP signaling, while its control of adipocyte DNL can be mediated independently of both cAMP and rapamycin-sensitive mTORC1.

MAP4K4 induces early blood-brain barrier damage in a murine subarachnoid hemorrhage model

Sterile-20-like mitogen-activated protein kinase kinase kinase kinase 4 (MAP4K4) is expressed in endothelial cells and activates inflammatory vascular damage. Endothelial cells are important components of the blood-brain barrier. To investigate whether MAP4K4 plays a role in the pathophysiology of subarachnoid hemorrhage, we evaluated the time-course expression of MAP4K4 after subarachnoid hemorrhage. A subarachnoid hemorrhage model was established using the intravascular perforation method. The model mice were assigned to four groups: MAP4K4 recombinant protein, scramble small interfering RNA, and MAP4K4 small interfering RNA were delivered by intracerebroventricular injection, while PF-06260933, a small-molecule inhibitor of MAP4K4, was administrated orally. Neurological score assessments, brain water assessments, Evans blue extravasation, immunofluorescence, western blot assay, and gelatin zymography were performed to analyze neurological outcomes and mechanisms of vascular damage. MAP4K4 expression was elevated in the cortex at 24 hours after subarachnoid hemorrhage, and colocalized with endothelial markers. MAP4K4 recombinant protein aggravated neurological impairment, brain edema, and blood-brain barrier damage; upregulated the expression of phosphorylated nuclear factor kappa B (p-p65) and matrix metalloproteinase 9 (MMP9); and degraded tight junction proteins (ZO-1 and claudin 5). Injection with MAP4K4 small interfering RNA reversed these effects. Furthermore, administration of the MAP4K4 inhibitor PF-06260933 reduced blood-brain barrier damage in mice, promoted the recovery of neurological function, and reduced p-p65 and MMP9 protein expression. Taken together, the results further illustrate that MAP4K4 causes early blood-brain barrier damage after subarachnoid hemorrhage. The mechanism can be confirmed by inhibiting the MAP4K4/NF-κB/MMP9 pathway. All experimental procedures and protocols were approved by the Experimental Animal Ethics Committee of General Hospital of Northern Theater Command (No. 2018002) on January 15, 2018.

MAP4K Interactome Reveals STRN4 as a Key STRIPAK Complex Component in Hippo Pathway Regulation

Mitogen-activated protein kinase kinase kinase kinases (MAP4Ks) constitute a mammalian STE20-like serine/threonine kinase subfamily. Recent studies provide substantial evidence for MAP4K family kinases in the Hippo pathway regulation, suggesting a broad role of MAP4Ks in human physiology and diseases. However, a comprehensive analysis of the regulators and effectors for this key kinase family has not been fully achieved. Using a proteomic approach, we define the protein-protein interaction network for human MAP4K family kinases and reveal diverse cellular signaling events involving this important kinase family. Through it, we identify a STRIPAK complex component, STRN4, as a generic binding partner for MAP4Ks and a key regulator of the Hippo pathway in endometrial cancer development. Taken together, the results of our study not only generate a rich resource for further characterizing human MAP4K family kinases in numerous biological processes but also dissect the STRIPAK-mediated regulation of MAP4Ks in the Hippo pathway.

The gluconeogenic enzyme PCK1 phosphorylates INSIG1/2 for lipogenesis

Cancer cells increase lipogenesis for their proliferation and the activation of sterol regulatory element-binding proteins (SREBPs) has a central role in this process. SREBPs are inhibited by a complex composed of INSIG proteins, SREBP cleavage-activating protein (SCAP) and sterols in the endoplasmic reticulum. Regulation of the interaction between INSIG proteins and SCAP by sterol levels is critical for the dissociation of the SCAP-SREBP complex from the endoplasmic reticulum and the activation of SREBPs^1,2. However, whether this protein interaction is regulated by a mechanism other than the abundance of sterol-and in particular, whether oncogenic signalling has a role-is unclear. Here we show that activated AKT in human hepatocellular carcinoma (HCC) cells phosphorylates cytosolic phosphoenolpyruvate carboxykinase 1 (PCK1), the rate-limiting enzyme in gluconeogenesis, at Ser90. Phosphorylated PCK1 translocates to the endoplasmic reticulum, where it uses GTP as a phosphate donor to phosphorylate INSIG1 at Ser207 and INSIG2 at Ser151. This phosphorylation reduces the binding of sterols to INSIG1 and INSIG2 and disrupts the interaction between INSIG proteins and SCAP, leading to the translocation of the SCAP-SREBP complex to the Golgi apparatus, the activation of SREBP proteins (SREBP1 or SREBP2) and the transcription of downstream lipogenesis-related genes, proliferation of tumour cells, and tumorigenesis in mice. In addition, phosphorylation of PCK1 at Ser90, INSIG1 at Ser207 and INSIG2 at Ser151 is not only positively correlated with the nuclear accumulation of SREBP1 in samples from patients with HCC, but also associated with poor HCC prognosis. Our findings highlight the importance of the protein kinase activity of PCK1 in the activation of SREBPs, lipogenesis and the development of HCC.

The interplay of signaling pathway in endothelial cells-matrix stiffness dependency with targeted-therapeutic drugs

Cardiovascular diseases (CVDs) have been one of the major causes of human deaths in the world. The study of CVDs has focused on cell chemotaxis for decades. With the advances in mechanobiology, accumulating evidence has demonstrated the influence of mechanical stimuli on arterial pathophysiology and endothelial dysfunction that is a hallmark of atherosclerosis development. An increasing number of drugs have been exploited to decrease the stiffness of vascular tissue for CVDs therapy. However, the underlying mechanisms have yet to be explored. This review aims to summarize how matrix stiffness mediates atherogenesis through various important signaling pathways in endothelial cells and cellular mechanophenotype, including RhoA/Rho-associated protein kinase (ROCK), mitogen-activated protein kinase (MAPK), and Hippo pathways. We also highlight the roles of putative mechanosensitive non-coding RNAs in matrix stiffness-mediated atherogenesis. Finally, we describe the usage of tunable hydrogel and its future strategy to improve our knowledge underlying matrix stiffness-mediated CVDs mechanism.

Lysine Enhances the Stimulation of Fatty Acids on Milk Fat Synthesis via the GPRC6A-PI3K-FABP5 Signaling in Bovine Mammary Epithelial Cells

Amino acids can enhance milk fat synthesis in bovine mammary epithelial cells (BMECs), but the molecular mechanism is not well-known. In this study, we explored the regulatory role and molecular mechanism of lysine (Lys) on milk fat synthesis induced by fatty acids (FAs). We show that Lys dose-dependently affects number of cells and milk fat synthesis, and has more stimulatory effects in the presence of FAs. Lys enhances FA-induced sterol regulatory element binding protein 1c (SREBP-1c) expression and maturation in a fatty-acid-binding protein 5 (FABP5)-dependent manner. We further show that the Lys stimulates FABP5 expression via the GPRC6A (GPCR, class C, group 6, subtype A)-PI3K (phosphatidylinositol 3-kinase) signaling. Lys dose-dependently affects GPRC6A expression and localization at the plasma membrane. In summary, our data reveals that Lys enhances FAs-stimulated SREBP-1c expression and maturation leading to milk fat synthesis via the GPRC6A-PI3K-FABP5 signaling in BMECs.

Delayed Feeding Alters Transcriptional and Post-Transcriptional Regulation of Hepatic Metabolic Pathways in Peri-Hatch Broiler Chicks

Hepatic fatty acid oxidation of yolk lipoproteins provides the main energy source for chick embryos. Post-hatching these yolk lipids are rapidly exhausted and metabolism switches to a carbohydrate-based energy source. We recently demonstrated that many microRNAs (miRNAs) are key regulators of hepatic metabolic pathways during this metabolic switching. MiRNAs are small non-coding RNAs that post-transcriptionally regulate gene expression in most eukaryotes. To further elucidate the roles of miRNAs in the metabolic switch, we used delayed feeding for 48 h to impede the hepatic metabolic switch. We found that hepatic expression of several miRNAs including miR-33, miR-20b, miR-34a, and miR-454 was affected by delaying feed consumption for 48 h. For example, we found that delayed feeding resulted in increased miR-20b expression and conversely reduced expression of its target FADS1, an enzyme involved in fatty acid synthesis. Interestingly, the expression of a previously identified miR-20b regulator FOXO3 was also higher in delayed fed chicks. FOXO3 also functions in protection of cells from oxidative stress. Delayed fed chicks also had much higher levels of plasma ketone bodies than their normal fed counterparts. This suggests that delayed fed chicks rely almost exclusively on lipid oxidation for energy production and are likely under higher oxidative stress. Thus, it is possible that FOXO3 may function to both limit lipogenesis as well as to help protect against oxidative stress in peri-hatch chicks until the initiation of feed consumption. This is further supported by evidence that the FOXO3-regulated histone deacetylase (HDAC2) was found to recognize the FASN (involved in fatty acid synthesis) chicken promoter in a yeast one-hybrid assay. Expression of FASN mRNA was lower in delayed fed chicks until feed consumption. The present study demonstrated that many transcriptional and post-transcriptional mechanisms, including miRNA, form a complex interconnected regulatory network that is involved in controlling lipid and glucose molecular pathways during the metabolic transition in peri-hatch chicks.

RBM4a-SRSF3-MAP4K4 Splicing Cascade Constitutes a Molecular Mechanism for Regulating Brown Adipogenesis

An increase in mitogen-activated protein kinase kinase kinase kinase 4 (MAP4K4) reportedly attenuates insulin-mediated signaling which participates in the development of brown adipose tissues (BATs). Nevertheless, the effect of MAP4K4 on brown adipogenesis remains largely uncharacterized. In this study, results of a transcriptome analysis (also referred as RNA-sequencing) showed differential expressions of MAP4K4 or SRSF3 transcripts isolated from distinct stages of embryonic BATs. The discriminative splicing profiles of MAP4K4 or SRSF3 were noted as well in brown adipocytes (BAs) with RNA-binding motif protein 4-knockout (RBM4-/-) compared to the wild-type counterparts. Moreover, the relatively high expressions of authentic SRSF3 transcripts encoding the splicing factor functioned as a novel regulator toward MAP4K4 splicing during brown adipogenesis. The presence of alternatively spliced MAP4K4 variants exerted differential effects on the phosphorylation of c-Jun N-terminal protein kinase (JNK) which was correlated with the differentiation or metabolic signature of BAs. Collectively, the RBM4-SRSF3-MAP4K4 splicing cascade constitutes a novel molecular mechanism in manipulating the development of BAs through related signaling pathways.

Identification of microRNAs controlling hepatic mRNA levels for metabolic genes during the metabolic transition from embryonic to posthatch development in the chicken

Background

The transition from embryonic to posthatch development in the chicken represents a massive metabolic switch from primarily lipolytic to primarily lipogenic metabolism. This metabolic switch is essential for the chick to successfully transition from the metabolism of stored egg yolk to the utilization of carbohydrate-based feed. However, regulation of this metabolic switch is not well understood. We hypothesized that microRNAs (miRNAs) play an important role in the metabolic switch that is essential to efficient growth of chickens. We used high-throughput RNA sequencing to characterize expression profiles of mRNA and miRNA in liver during late embryonic and early posthatch development of the chicken. This extensive data set was used to define the contributions of microRNAs to the metabolic switch during development that is critical to growth and nutrient utilization in chickens.

Results

We found that expression of over 800 mRNAs and 30 miRNAs was altered in the embryonic liver between embryonic day 18 and posthatch day 3, and many of these differentially expressed mRNAs and miRNAs are associated with metabolic processes. We confirmed the regulation of some of these mRNAs by miRNAs expressed in a reciprocal pattern using luciferase reporter assays. Finally, through the use of yeast one-hybrid screens, we identified several proteins that likely regulate expression of one of these important miRNAs.

Conclusions

Integration of the upstream regulatory mechanisms governing miRNA expression along with monitoring the downstream effects of this expression will ultimately allow for the construction of complete miRNA regulatory networks associated with the hepatic metabolic switch in chickens. Our findings support a key role for miRNAs in controlling the metabolic switch that occurs between embryonic and posthatch development in the chicken.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4096-5) contains supplementary material, which is available to authorized users.

MAP4K4: an emerging therapeutic target in cancer

The serine/threonine kinase MAP4K4 is a member of the Ste20p (sterile 20 protein) family. MAP4K4 was initially discovered in 1995 as a key kinase in the mating pathway in Saccharomyces cerevisiae and was later found to be involved in many aspects of cell functions and many biological and pathological processes. The role of MAP4K4 in immunity, inflammation, metabolic and cardiovascular disease has been recognized. Information regarding MAP4K4 in cancers is extremely limited, but increasing evidence suggests that MAP4K4 also plays an important role in cancer and MAP4K4 may represent a novel actionable cancer therapeutic target. This review summarizes our current understanding of MAP4K4 regulation and MAP4K4 in cancer. MAP4K4-specific inhibitors have been recently developed. We hope that this review article would advocate more basic and preclinical research on MAP4K4 in cancer, which could ultimately provide biological and mechanistic justifications for preclinical and clinical test of MAP4K4 inhibitor in cancer patients.

Map4k4 Signaling Nodes in Metabolic and Cardiovascular Diseases

Mitogen-activated kinase kinase kinase kinase 4 (Map4k4), originally identified in small interfering (si)RNA screens and characterized by tissue-specific gene deletions, is emerging as a regulator of glucose homeostasis and cardiovascular health. Recent studies have shown that Map4k4 gene ablation or inhibition of its kinase activity attenuates hyperglycemia and plaque formation in mouse models of insulin resistance and atherosclerosis, and suggest roles for Map4k4 in multiple signaling systems, including NFκB activation, small GTPase regulation, the Hippo cascade, and regulation of cell dynamics by FERM domain proteins. This new and promising area of inquiry raises key questions that need to be addressed, such as defining which of the above or other effectors mediate Map4k4 control of metabolic and vascular functions, and identifying upstream activators of Map4k4.

Endothelial protein kinase MAP4K4 promotes vascular inflammation and atherosclerosis

Signalling pathways that control endothelial cell (EC) permeability, leukocyte adhesion and inflammation are pivotal for atherosclerosis initiation and progression. Here we demonstrate that the Sterile-20-like mitogen-activated protein kinase kinase kinase kinase 4 (MAP4K4), which has been implicated in inflammation, is abundantly expressed in ECs and in atherosclerotic plaques from mice and humans. On the basis of endothelial-specific MAP4K4 gene silencing and gene ablation experiments in Apoe(-/-) mice, we show that MAP4K4 in ECs markedly promotes Western diet-induced aortic macrophage accumulation and atherosclerotic plaque development. Treatment of Apoe(-/-) and Ldlr(-/-) mice with a selective small-molecule MAP4K4 inhibitor also markedly reduces atherosclerotic lesion area. MAP4K4 silencing in cultured ECs attenuates cell surface adhesion molecule expression while reducing nuclear localization and activity of NFκB, which is critical for promoting EC activation and atherosclerosis. Taken together, these results reveal that MAP4K4 is a key signalling node that promotes immune cell recruitment in atherosclerosis.

Recent Advances in Adipose mTOR Signaling and Function: Therapeutic Prospects

The increasing epidemic of obesity and its comorbidities has spurred research interest in adipose biology and its regulatory functions. Recent studies have revealed that the mechanistic target of rapamycin (mTOR) signaling pathway has a critical role in the regulation of adipose tissue function, including adipogenesis, lipid metabolism, thermogenesis, and adipokine synthesis and/or secretion. Given the importance of mTOR signaling in controlling energy homeostasis, it is not unexpected that deregulated mTOR signaling is associated with obesity and related metabolic disorders. In this review, we highlight current advances in understanding the roles of the mTOR signaling pathway in adipose tissue. We also provide a more nuanced view of how the mTOR signaling pathway regulates adipose tissue biology and function. Finally, we describe approaches to modulate the activity and tissue-specific function of mTOR that may pave the way towards counteracting obesity and related metabolic diseases.

Inducible Deletion of Protein Kinase Map4k4 in Obese Mice Improves Insulin Sensitivity in Liver and Adipose Tissues

Studies in vitro suggest that mitogen-activated protein kinase kinase kinase kinase 4 (Map4k4) attenuates insulin signaling, but confirmation in vivo is lacking since Map4k4 knockout is lethal during embryogenesis. We thus generated mice with floxed Map4k4 alleles and a tamoxifen-inducible Cre/ERT2 recombinase under the control of the ubiquitin C promoter to induce whole-body Map4k4 deletion after these animals reached maturity. Tamoxifen administration to these mice induced Map4k4 deletion in all tissues examined, causing decreased fasting blood glucose concentrations and enhanced insulin signaling to AKT in adipose tissue and liver but not in skeletal muscle. Surprisingly, however, mice generated with a conditional Map4k4 deletion in adiponectin-positive adipocytes or in albumin-positive hepatocytes displayed no detectable metabolic phenotypes. Instead, mice with Map4k4 deleted in Myf5-positive tissues, including all skeletal muscles tested, were protected from obesity-induced glucose intolerance and insulin resistance. Remarkably, these mice also showed increased insulin sensitivity in adipose tissue but not skeletal muscle, similar to the metabolic phenotypes observed in inducible whole-body knockout mice. Taken together, these results indicate that (i) Map4k4 controls a pathway in Myf5-positive cells that suppresses whole-body insulin sensitivity and (ii) Map4k4 is a potential therapeutic target for improving glucose tolerance and insulin sensitivity in type 2 diabetes.

Genome-wide linkage and positional association analyses identify associations of novel AFF3 and NTM genes with triglycerides: the GenSalt study

We conducted a genome-wide linkage scan and positional association study to identify genes and variants influencing blood lipid levels among participants of the Genetic Epidemiology Network of Salt-Sensitivity (GenSalt) study. The GenSalt study was conducted among 1906 participants from 633 Han Chinese families. Lipids were measured from overnight fasting blood samples using standard methods. Multipoint quantitative trait genome-wide linkage scans were performed on the high-density lipoprotein, low-density lipoprotein, and log-transformed triglyceride phenotypes. Using dense panels of single nucleotide polymorphisms (SNPs), single-marker and gene-based association analyses were conducted to follow-up on promising linkage signals. Additive associations between each SNP and lipid phenotypes were tested using mixed linear regression models. Gene-based analyses were performed by combining P-values from single-marker analyses within each gene using the truncated product method (TPM). Significant associations were assessed for replication among 777 Asian participants of the Multi-ethnic Study of Atherosclerosis (MESA). Bonferroni correction was used to adjust for multiple testing. In the GenSalt study, suggestive linkage signals were identified at 2p11.2‒2q12.1 [maximum multipoint LOD score (MML) = 2.18 at 2q11.2] and 11q24.3‒11q25 (MML = 2.29 at 11q25) for the log-transformed triglyceride phenotype. Follow-up analyses of these two regions revealed gene-based associations of charged multivesicular body protein 3 (CHMP3), ring finger protein 103 (RNF103), AF4/FMR2 family, member 3 (AFF3), and neurotrimin (NTM) with triglycerides (P = 4 × 10(-4), 1.00 × 10(-5), 2.00 × 10(-5), and 1.00 × 10(-7), respectively). Both the AFF3 and NTM triglyceride associations were replicated among MESA study participants (P = 1.00 × 10(-7) and 8.00 × 10(-5), respectively). Furthermore, NTM explained the linkage signal on chromosome 11. In conclusion, we identified novel genes associated with lipid phenotypes in linkage regions on chromosomes 2 and 11.

The conserved misshapen-warts-Yorkie pathway acts in enteroblasts to regulate intestinal stem cells in Drosophila

Similar to the mammalian intestine, the Drosophila adult midgut has resident stem cells that support growth and regeneration. How the niche regulates intestinal stem cell activity in both mammals and flies is not well understood. Here, we show that the conserved germinal center protein kinase Misshapen restricts intestinal stem cell division by repressing the expression of the JAK-STAT pathway ligand Upd3 in differentiating enteroblasts. Misshapen, a distant relative to the prototypic Warts activating kinase Hippo, interacts with and activates Warts to negatively regulate the activity of Yorkie and the expression of Upd3. The mammalian Misshapen homolog MAP4K4 similarly interacts with LATS (Warts homolog) and promotes inhibition of YAP (Yorkie homolog). Together, this work reveals that the Misshapen-Warts-Yorkie pathway acts in enteroblasts to control niche signaling to intestinal stem cells. These findings also provide a model in which to study requirements for MAP4K4-related kinases in MST1/2-independent regulation of LATS and YAP.

HGK/MAP4K4 deficiency induces TRAF2 stabilization and Th17 differentiation leading to insulin resistance

Proinflammatory cytokines play important roles in insulin resistance. Here we report that mice with a T-cell-specific conditional knockout of HGK (T-HGK cKO) develop systemic inflammation and insulin resistance. This condition is ameliorated by either IL-6 or IL-17 neutralization. HGK directly phosphorylates TRAF2, leading to its lysosomal degradation and subsequent inhibition of IL-6 production. IL-6-overproducing HGK-deficient T cells accumulate in adipose tissue and further differentiate into IL-6/IL-17 double-positive cells. Moreover, CCL20 neutralization or CCR6 deficiency reduces the Th17 population or insulin resistance in T-HGK cKO mice. In addition, leptin receptor deficiency in T cells inhibits Th17 differentiation and improves the insulin sensitivity in T-HGK cKO mice, which suggests that leptin cooperates with IL-6 to promote Th17 differentiation. Thus, HGK deficiency induces TRAF2/IL-6 upregulation, leading to IL-6/leptin-induced Th17 differentiation in adipose tissue and subsequent insulin resistance. These findings provide insight into the reciprocal regulation between the immune system and the metabolism.