SIK2 regulates CRTCs, HDAC4 and glucose uptake in adipocytes

HDAC4: an emerging target in diabetes mellitus and diabetic complications

Diabetes mellitus (DM) is a metabolic disease with complex pathogenic mechanisms. Histone deacetylase 4 (HDAC4) is a member of an important family of epigenetic modifications. An increasing amount of research indicates that HDAC4 may control DM by modulating the epigenetic and post-translational expression of numerous transcription factors and taking part in different signaling cascades. In this review, HDAC4 was reported to control the differentiation, growth, and function of pancreatic β-cells. Furthermore, HDAC4 regulates glucose metabolism by targeting GLUT4 and FOXO1 and further modulates insulin signaling pathways through cytoplasmic-nuclear shuttling. Moreover, HDAC4 has also been implicated in the regulation of diabetic nephropathy, diabetic cardiomyopathy, diabetes osteoporosis, diabetic wounds, and diabetic encephalopathy. Therefore, HDAC4 is consider to be a viable therapeutic target for the treatment of DM and its complications. HDAC inhibitors and other targeted inhibitions of HDAC4 provide us with new ideas for developing novel intervention strategies. This article reviews the role of HDAC4 in diabetes mellitus and its complications.

Modulating the Secretome of Fat to Treat Heart Failure

Heart failure afflicts >6 million individuals in the United States alone and is associated with significant mortality (≈40% within 5 years of diagnosis) and cost (estimated to exceed $70 billion in the United States by 2030). Obesity is a major risk factor for the development of heart failure. The contribution of excess adipose tissue to heart failure pathogenesis is multifactorial. For example, adipose tissue-driven inflammation contributes to the development of other cardiometabolic comorbidities, such as hypertension, leading to left ventricular pressure overload and adverse remodeling of the heart. Adipose tissue also functions as an endocrine organ, and altered secretion of proteins, lipid mediators, metabolites, and small extracellular vesicles (collectively referred to as the secretome) from dysfunctional fat can lead to cardiac inflammation and oxidative stress, which drive changes in structure and function of the heart. In this review, we begin with an overview of current therapies for obesity and what is known about how they influence the heart. Then we focus on mechanisms by which fat communicates with the heart via secreted factors and highlight druggable nodes in this circuit that could be exploited to develop next-generation therapies for heart failure.

Salt-inducible Kinase Regulation of Adipose Tissue Metabolism

Salt-inducible kinases (SIKs) are a subfamily of the adenosine monophosphate-activated protein kinase-related kinase family. To be activated, SIKs require phosphorylation in the catalytic kinase domain by liver kinase B1. In response to extracellular stimulations, their activity can be further regulated through phosphorylation by protein kinase A (PKA), and Ca2+/calmodulin-dependent protein kinases. PKA-mediated SIK inhibition is a major link between G-protein coupled receptor activation and the target gene transcription program. All 3 SIK isoforms-SIK1, SIK2, and SIK3-are expressed in adipocytes, with SIK2 being the most abundant in both rodents and humans. SIKs play essential roles in maintaining adipose tissue homeostasis by regulating physiological processes involving insulin signaling, glucose uptake, lipogenesis, and thermogenesis. Each SIK isoform could play both redundant and unique roles in these physiological processes. Many of the substrates that mediate their physiological functions in adipocytes have been characterized, and downstream mechanisms of action have also been proposed. However, due to the functional redundancy of SIKs, a major challenge is to delineate their isoform-specific roles in adipose tissue in vivo using genetic mouse models. In addition, common genetic variants and rare mutations in the SIK genes have been identified to be associated with metabolic, cardiovascular, and developmental conditions, suggesting a translational implication for human disease that deserves investigation. Furthermore, small molecular SIK inhibitors have been developed and have shown therapeutic potential in multiple disease areas. Evaluation of their metabolic and cardiovascular effects will be required for future clinical development of SIK inhibitors.

Activation of SIK2 inhibits gluconeogenesis and alleviates lipogenesis-induced inflammatory response by SIK2-CRTC2-ACC1 in hepatocytes of large yellow croaker (Larimichthys crocea)

As an important kinase that maintains metabolic homeostasis, salt-induced kinase 2 (SIK2) inhibits adipogenesis in adipocytes and regulates mammalian glucose metabolism. Excessive lipid accumulation can lead to an inflammatory response, and SIK2 also controls the expression of inflammatory cytokines. However, the precise mechanisms and interconnections through which SIK2 modulates these processes remain incompletely understood. The present study explores the regulatory role of SIK2 in hepatic glucose and lipid metabolism and lipogenesis-induced inflammatory response of Larimichthys crocea. We found that palmitic acid or glucose reduces sik2 mRNA expression, while its expression increases in response to insulin. Further, SIK2 reduces glucose content by inhibiting gluconeogenesis and activating insulin signaling pathways. Meanwhile, SIK2 overexpression decreases lipogenesis-related gene expression and increases lipolysis-related gene expression, alleviating hepatocyte lipid accumulation. Due to sequence conservation, SIK2 mutants are constructed. Only the T176A mutant weakens the inhibition of wt-SIK2 on the acetyl-CoA carboxylase 1 (ACC1) promoter through cyclic AMP-responsive element-binding protein (CREB). It increases the expression of CREB-regulated transcription coactivator 2 (CRTC2) in the nucleus. Then, ACC1 inhibition attenuates the inflammatory response. SIK2 alleviates the inflammatory response by reducing pro-inflammatory cytokines expression and inactivating the mitogen-activated protein kinase (MAPK) pathway. Furthermore, similar results were obtained by inhibiting SIK2 in vivo. Overall, this study explores the role of SIK2 in hepatic glucose and lipid metabolism and lipogenesis-induced inflammatory response in fish for the first time, which provides insights into the functional protection of SIK2 in vertebrate evolution and may serve as a target for the treatment of nutritional and immune disorders.

Understanding the roles of salt-inducible kinases in cardiometabolic disease

Salt-inducible kinases (SIKs) are serine/threonine kinases of the adenosine monophosphate-activated protein kinase family. Acting as mediators of a broad array of neuronal and hormonal signaling pathways, SIKs play diverse roles in many physiological and pathological processes. Phosphorylation by the upstream kinase liver kinase B1 is required for SIK activation, while phosphorylation by protein kinase A induces the binding of 14-3-3 protein and leads to SIK inhibition. SIKs are subjected to auto-phosphorylation regulation and their activity can also be modulated by Ca2+/calmodulin-dependent protein kinase in response to cellular calcium influx. SIKs regulate the physiological processes through direct phosphorylation on various substrates, which include class IIa histone deacetylases, cAMP-regulated transcriptional coactivators, phosphatase methylesterase-1, among others. Accumulative body of studies have demonstrated that SIKs are important regulators of the cardiovascular system, including early works establishing their roles in sodium sensing and vascular homeostasis and recent progress in pulmonary arterial hypertension and pathological cardiac remodeling. SIKs also regulate inflammation, fibrosis, and metabolic homeostasis, which are essential pathological underpinnings of cardiovascular disease. The development of small molecule SIK inhibitors provides the translational opportunity to explore their potential as therapeutic targets for treating cardiometabolic disease in the future.

Salt-inducible kinases are required for glucose uptake and insulin signaling in human adipocytes

OBJECTIVE

Salt-inducible kinase 2 (SIK2) is abundantly expressed in adipocytes and downregulated in adipose tissue from individuals with obesity or insulin resistance. The main aims of this work were to investigate the involvement of SIKs in the regulation of glucose uptake in primary mature human adipocytes and to identify mechanisms underlying this regulation.

METHODS

Primary mature adipocytes were isolated from human, rat, or mouse adipose tissue and treated with pan-SIK inhibitors. Adipocytes isolated from wild type, ob/ob, and SIK2 knockout mice were also used. Glucose uptake was examined by glucose tracer assay. The insulin signaling pathway was monitored by Western blotting, co-immunoprecipitation, and total internal reflection fluorescence microscopy.

RESULTS

This study demonstrates that SIK2 is downregulated in obese ob/ob mice and that SIK activity is required for intact glucose uptake in primary human and mouse adipocytes. The underlying mechanism involves direct effects on the insulin signaling pathway, likely at the level of phosphatidylinositol (3,4,5)-trisphosphate (PIP3) generation or breakdown. Moreover, lack of SIK2 alone is sufficient to attenuate glucose uptake in mouse adipocytes.

CONCLUSIONS

SIK2 is required for insulin action in human adipocytes, and the mechanism includes direct effects on the insulin signaling pathway.

Mechanism of TNFα-induced downregulation of salt-inducible kinase 2 in adipocytes

Salt-inducible kinase 2 (SIK2) is highly expressed in white adipocytes, but downregulated in individuals with obesity and insulin resistance. These conditions are often associated with a low-grade inflammation in adipose tissue. We and others have previously shown that SIK2 is downregulated by tumor necrosis factor α (TNFα), however, involvement of other pro-inflammatory cytokines, or the mechanisms underlying TNFα-induced SIK2 downregulation, remain to be elucidated. In this study we have shown that TNFα downregulates SIK2 protein expression not only in 3T3L1- but also in human in vitro differentiated adipocytes. Furthermore, monocyte chemoattractant protein-1 and interleukin (IL)-1β, but not IL-6, might also contribute to SIK2 downregulation during inflammation. We observed that TNFα-induced SIK2 downregulation occurred also in the presence of pharmacological inhibitors against several kinases involved in inflammation, namely c-Jun N-terminal kinase, mitogen activated protein kinase kinase 1, p38 mitogen activated protein kinase or inhibitor of nuclear factor kappa-B kinase (IKK). However, IKK may be involved in SIK2 regulation as we detected an increase of SIK2 when inhibiting IKK in the absence of TNFα. Increased knowledge about inflammation-induced downregulation of SIK2 could ultimately be used to develop strategies for the reinstalment of SIK2 expression in insulin resistance.

The role of liver kinase B1 in tumor progression through regulation of lipid metabolism

The somatic mutation of liver kinase B1 (LKB1) has been implicated in various tumors, which is reflected in the survival, proliferation, and metastasis of tumor cells. However, the regulation of LKB1 in lipid metabolism, a process that is involved in tumor progression is not completely clear. We conclude that LKB1 deficiency results in abnormal expression and activation of multiple molecules related to lipid metabolism which locate downstream of AMP-activated protein kinase (AMPK) or salt-induced kinase (SIK). Abnormal lipid metabolism induced by LKB1 deficiency contributes to the proliferation and metastasis of tumor cells through energy regulation.

Designing of potent anti-diabetic molecules by targeting SIK2 using computational approaches

Diabetes mellitus (DM) is one of the major health problems worldwide. WHO have estimated that 439 million people may have DM by the year 2030. Several classes of drugs such as sulfonylureas, meglitinides, thiazolidinediones etc. are available to manage this disease, however, there is no cure for this disease. Salt inducible kinase 2 (SIK2) is expressed several folds in adipose tissue than in normal tissues and thus SIK2 is one of the attractive targets for DM treatment. SIK2 inhibition improves glucose homeostasis. Several analogues have been reported and experimentally proven against SIK for DM treatment. But, identifying potential SIK2 inhibitors with improved efficacy and good pharmacokinetic profiles will be helpful for the effective treatment of DM. The objective of the present study is to identify selective SIK2 inhibitors with good pharmacokinetic profiles. Due to the unavailability of SIK2 structure, the modeled structure of SIK2 will be an important to understand the atomic level of SIK2 inhibitors in the binding site pocket. In this study, different molecular modeling studies such as Homology Modeling, Molecular Docking, Pharmacophore-based virtual screening, MD simulations, Density Functional Theory calculations and WaterMap analysis were performed to identify potential SIK2 inhibitors. Five molecules from different databases such as Binding_4067, TosLab_837067, NCI_349155, Life chemicals_ F2565-0113, Enamine_7623111186 molecules were identified as possible SIK2 inhibitors.

Salt-inducible kinase 3 protects tumor cells from cytotoxic T-cell attack by promoting TNF-induced NF-κB activation

BACKGROUND

Cancer immunotherapeutic strategies showed unprecedented results in the clinic. However, many patients do not respond to immuno-oncological treatments due to the occurrence of a plethora of immunological obstacles, including tumor intrinsic mechanisms of resistance to cytotoxic T-cell (TC) attack. Thus, a deeper understanding of these mechanisms is needed to develop successful immunotherapies.

METHODS

To identify novel genes that protect tumor cells from effective TC-mediated cytotoxicity, we performed a genetic screening in pancreatic cancer cells challenged with tumor-infiltrating lymphocytes and antigen-specific TCs.

RESULTS

The screening revealed 108 potential genes that protected tumor cells from TC attack. Among them, salt-inducible kinase 3 (SIK3) was one of the strongest hits identified in the screening. Both genetic and pharmacological inhibitions of SIK3 in tumor cells dramatically increased TC-mediated cytotoxicity in several in vitro coculture models, using different sources of tumor and TCs. Consistently, adoptive TC transfer of TILs led to tumor growth inhibition of SIK3-depleted cancer cells in vivo. Mechanistic analysis revealed that SIK3 rendered tumor cells susceptible to tumor necrosis factor (TNF) secreted by tumor-activated TCs. SIK3 promoted nuclear factor kappa B (NF-κB) nuclear translocation and inhibited caspase-8 and caspase-9 after TNF stimulation. Chromatin accessibility and transcriptome analyses showed that SIK3 knockdown profoundly impaired the expression of prosurvival genes under the TNF-NF-κB axis. TNF stimulation led to SIK3-dependent phosphorylation of the NF-κB upstream regulators inhibitory-κB kinase and NF-kappa-B inhibitor alpha on the one side, and to inhibition of histone deacetylase 4 on the other side, thus sustaining NF-κB activation and nuclear stabilization. A SIK3-dependent gene signature of TNF-mediated NF-κB activation was found in a majority of pancreatic cancers where it correlated with increased cytotoxic TC activity and poor prognosis.

CONCLUSION

Our data reveal an abundant molecular mechanism that protects tumor cells from cytotoxic TC attack and demonstrate that pharmacological inhibition of this pathway is feasible.

PP2A and cancer epigenetics: a therapeutic opportunity waiting to happen

The epigenetic state of chromatin is altered by regulators which influence gene expression in response to environmental stimuli. While several post-translational modifications contribute to chromatin accessibility and transcriptional programs, our understanding of the role that specific phosphorylation sites play is limited. In cancer, kinases and phosphatases are commonly deregulated resulting in increased oncogenic signaling and loss of epigenetic regulation. Aberrant epigenetic states are known to promote cellular plasticity and the development of therapeutic resistance in many cancer types, highlighting the importance of these mechanisms to cancer cell phenotypes. Protein Phosphatase 2A (PP2A) is a heterotrimeric holoenzyme that targets a diverse array of cellular proteins. The composition of the PP2A complex influences its cellular targets and activity. For this reason, PP2A can be tumor suppressive or oncogenic depending on cellular context. Understanding the nuances of PP2A regulation and its effect on epigenetic alterations can lead to new therapeutic avenues that afford more specificity and contribute to the growth of personalized medicine in the oncology field. In this review, we summarize the known PP2A-regulated substrates and potential phosphorylation sites that contribute to cancer cell epigenetics and possible strategies to therapeutically leverage this phosphatase to suppress tumor growth.

Activation of the adipocyte CREB/CRTC pathway in obesity

Obesity is a major risk factor for the development of type II diabetes. Increases in adipose tissue mass trigger insulin resistance via the release of pro-inflammatory cytokines from adipocytes and macrophages. CREB and the CRTC coactivators have been found to promote insulin resistance in obesity, although the mechanism is unclear. Here we show that high fat diet feeding activates the CREB/CRTC pathway in adipocytes by decreasing the expression of SIK2, a Ser/Thr kinase that phosphorylates and inhibits CRTCs. SIK2 levels are regulated by the adipogenic factor C/EBPα, whose expression is reduced in obesity. Exposure to PPARγ agonist rescues C/EBPα expression and restores SIK2 levels. CRTC2/3 promote insulin resistance via induction of the chemokines CXCL1/2. Knockout of CRTC2/3 in adipocytes reduces CXCL1/2 expression and improves insulin sensitivity. As administration of CXCL1/2 reverses salutary effects of CRTC2/3 depletion, our results demonstrate the importance of the CREB/CRTC pathway in modulating adipose tissue function.

Salt-Inducible Kinase 3 Promotes Vascular Smooth Muscle Cell Proliferation and Arterial Restenosis by Regulating AKT and PKA-CREB Signaling

Objective

Arterial restenosis is the pathological narrowing of arteries after endovascular procedures, and it is an adverse event that causes patients to experience recurrent occlusive symptoms. Following angioplasty, vascular smooth muscle cells (SMCs) change their phenotype, migrate, and proliferate, resulting in neointima formation, a hallmark of arterial restenosis. SIKs (salt-inducible kinases) are a subfamily of the AMP-activated protein kinase family that play a critical role in metabolic diseases including hepatic lipogenesis and glucose metabolism. Their role in vascular pathological remodeling, however, has not been explored. In this study, we aimed to understand the role and regulation of SIK3 in vascular SMC migration, proliferation, and neointima formation.

Approach and Results

We observed that SIK3 expression was low in contractile aortic SMCs but high in proliferating SMCs. It was also highly induced by growth medium in vitro and in neointimal lesions in vivo. Inactivation of SIKs significantly attenuated vascular SMC proliferation and up-regulated p21CIP1 and p27KIP1. SIK inhibition also suppressed SMC migration and modulated actin polymerization. Importantly, we found that inhibition of SIKs reduced neointima formation and vascular inflammation in a femoral artery wire injury model. In mechanistic studies, we demonstrated that inactivation of SIKs mainly suppressed SMC proliferation by down-regulating AKT (protein kinase B) and PKA (protein kinase A)-CREB (cAMP response element-binding protein) signaling. CRTC3 (CREB-regulated transcriptional coactivator 3) signaling likely contributed to SIK inactivation-mediated antiproliferative effects.

Conclusions

These findings suggest that SIK3 may play a critical role in regulating SMC proliferation, migration, and arterial restenosis. This study provides insights into SIK inhibition as a potential therapeutic strategy for treating restenosis in patients with peripheral arterial disease.

A-769662 inhibits adipocyte glucose uptake in an AMPK-independent manner

Activation of AMP-activated protein kinase (AMPK) is considered a valid strategy for the treatment of type 2 diabetes. However, despite the importance of adipose tissue for whole-body energy homeostasis, the effect of AMPK activation in adipocytes has only been studied to a limited extent and mainly with the AMP-mimetic 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR), which has limited specificity. The aim of this study was to evaluate the effect of the allosteric AMPK activators A-769662 and 991 on glucose uptake in adipocytes. For this purpose, primary rat or human adipocytes, and cultured 3T3-L1 adipocytes, were treated with either of the allosteric activators, or AICAR, and basal and insulin-stimulated glucose uptake was assessed. Additionally, the effect of AMPK activators on insulin-stimulated phosphorylation of Akt and Akt substrate of 160 kDa was assessed. Furthermore, primary adipocytes from ADaM site binding drug-resistant AMPKβ1 S108A knock-in mice were employed to investigate the specificity of the drugs for the observed effects. Our results show that insulin-stimulated adipocyte glucose uptake was significantly reduced by A-769662 but not 991, yet neither activator had any clear effects on basal or insulin-stimulated Akt/AS160 signaling. The use of AMPKβ1 S108A mutant-expressing adipocytes revealed that the observed inhibition of glucose uptake by A-769662 is most likely AMPK-independent, a finding which is supported by the rapid inhibitory effect A-769662 exerts on glucose uptake in 3T3-L1 adipocytes. These data suggest that AMPK activation per se does not inhibit glucose uptake in adipocytes and that the effects of AICAR and A-769662 are AMPK-independent.

SIK2 kinase synthetic lethality is driven by spindle assembly defects in FANCA-deficient cells

The Fanconi anemia (FA) pathway safeguards genomic stability through cell cycle regulation and DNA damage repair. The canonical tumor suppressive role of FA proteins in the repair of DNA damage during interphase is well established, but their function in mitosis is incompletely understood. Here, we performed a kinome-wide synthetic lethality screen in FANCA^-/- fibroblasts, which revealed multiple mitotic kinases as necessary for survival of FANCA-deficient cells. Among these kinases, we identified the depletion of the centrosome kinase SIK2 as synthetic lethal upon loss of FANCA. We found that FANCA colocalizes with SIK2 at multiple mitotic structures and regulates the activity of SIK2 at centrosomes. Furthermore, we found that loss of FANCA exacerbates cell cycle defects induced by pharmacological inhibition of SIK2, including impaired G2-M transition, delayed mitotic progression, and cytokinesis failure. In addition, we showed that inhibition of SIK2 abrogates nocodazole-induced prometaphase arrest, suggesting a novel role for SIK2 in the spindle assembly checkpoint. Together, these findings demonstrate that FANCA-deficient cells are dependent upon SIK2 for survival, supporting a preclinical rationale for targeting of SIK2 in FA-disrupted cancers.

Comparable Initial Engagement of Intracellular Signaling Pathways by Parathyroid Hormone Receptor Ligands Teriparatide, Abaloparatide, and Long-Acting PTH

Multiple analogs of parathyroid hormone, all of which bind to the PTH/PTHrP receptor PTH1R, are used for patients with osteoporosis and hypoparathyroidism. Although ligands such as abaloparatide, teriparatide (hPTH 1-34 [TPTD]), and long-acting PTH (LA-PTH) show distinct biologic effects with respect to skeletal and mineral metabolism endpoints, the mechanistic basis for these clinically-important differences remains incompletely understood. Previous work has revealed that differential signaling kinetics and receptor conformation engagement between different PTH1R peptide ligands. However, whether such acute membrane proximal differences translate into differences in downstream signaling output remains to be determined. Here, we directly compared short-term effects of hPTH (1-34), abaloparatide, and LA-PTH in multiple cell-based PTH1R signaling assays. At the time points and ligand concentrations utilized, no significant differences were observed between these three ligands at the level of receptor internalization, β-arrestin recruitment, intracellular calcium stimulation, and cAMP generation. However, abaloparatide showed significantly quicker PTH1R recycling in washout studies. Downstream of PTH1R-stimulated cAMP generation, protein kinase A regulates gene expression via effects on salt inducible kinases (SIKs) and their substrates. Consistent with no differences between these ligands on cAMP generation, we observed that hPTH (1-34), abaloparatide, and LA-PTH showed comparable effects on SIK2 phosphorylation, SIK substrate dephosphorylation, and downstream gene expression changes. Taken together, these results indicate that these PTH1R peptide agonists engage downstream intracellular signaling pathways to a comparable degree. It is possible that differences observed in vivo in preclinical and clinical models may be related to pharmacokinetic factors. It is also possible that our current in vitro systems are insufficient to perfectly match the complexities of PTH1R signaling in bona fide target cells in bone in vivo. © 2020 American Society for Bone and Mineral Research © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Nuts and bolts of the salt-inducible kinases (SIKs)

The salt-inducible kinases, SIK1, SIK2 and SIK3, most closely resemble the AMP-activated protein kinase (AMPK) and other AMPK-related kinases, and like these family members they require phosphorylation by LKB1 to be catalytically active. However, unlike other AMPK-related kinases they are phosphorylated by cyclic AMP-dependent protein kinase (PKA), which promotes their binding to 14-3-3 proteins and inactivation. The most well-established substrates of the SIKs are the CREB-regulated transcriptional co-activators (CRTCs), and the Class 2a histone deacetylases (HDAC4/5/7/9). Phosphorylation by SIKs promotes the translocation of CRTCs and Class 2a HDACs to the cytoplasm and their binding to 14-3-3s, preventing them from regulating their nuclear binding partners, the transcription factors CREB and MEF2. This process is reversed by PKA-dependent inactivation of the SIKs leading to dephosphorylation of CRTCs and Class 2a HDACs and their re-entry into the nucleus. Through the reversible regulation of these substrates and others that have not yet been identified, the SIKs regulate many physiological processes ranging from innate immunity, circadian rhythms and bone formation, to skin pigmentation and metabolism. This review summarises current knowledge of the SIKs and the evidence underpinning these findings, and discusses the therapeutic potential of SIK inhibitors for the treatment of disease.

Comprehensive evaluation of the metabolic effects of porcine CRTC3 overexpression on subcutaneous adipocytes with metabolomic and transcriptomic analyses

BACKGROUND

Meat quality is largely driven by fat deposition, which is regulated by several genes and signaling pathways. The cyclic adenosine monophosphate (cAMP) -regulated transcriptional coactivator 3 (CRTC3) is a coactivator of cAMP response element binding protein (CREB) that mediates the function of protein kinase A (PKA) signaling pathway and is involved in various biological processes including lipid and energy metabolism. However, the effects of CRTC3 on the metabolome and transcriptome of porcine subcutaneous adipocytes have not been studied yet. Here, we tested whether porcine CRTC3 expression would be related to fat deposition in Heigai pigs (a local fatty breed in China) and Duroc×Landrace×Yorkshire (DLY, a lean breed) pigs in vivo. The effects of adenovirus-induced CRTC3 overexpression on the metabolomic and transcriptomic profiles of subcutaneous adipocytes were also determined in vitro by performing mass spectrometry-based metabolomics combined with RNA sequencing (RNA-seq).

RESULTS

Porcine CRTC3 expression is associated with fat deposition in vivo. In addition, CRTC3 overexpression increased lipid accumulation and the expression of mature adipocyte-related genes in cultured porcine subcutaneous adipocytes. According to the metabolomic analysis, CRTC3 overexpression induced significant changes in adipocyte lipid, amino acid and nucleotide metabolites in vitro. The RNA-seq analysis suggested that CRTC3 overexpression alters the expression of genes and pathways involved in adipogenesis, fatty acid metabolism and glycerophospholipid metabolism in vitro.

CONCLUSIONS

We identified significant alterations in the metabolite composition and the expression of genes and pathways involved in lipid metabolism in CRTC3-overexpressing adipocytes. Our results suggest that CRTC3 might play an important regulatory role in lipid metabolism and thus affects lipid accumulation in porcine subcutaneous adipocytes.

Phosphoproteomic identification of vasopressin-regulated protein kinases in collecting duct cells

BACKGROUND AND PURPOSE

The peptide hormone vasopressin regulates water transport in the renal collecting duct largely via the V2 receptor, which triggers a cAMP-mediated activation of a PKA-dependent signalling network. The protein kinases downstream from PKA have not been fully identified or mapped to regulated phosphoproteins.

EXPERIMENTAL APPROACH

We carried out systems-level analysis of large-scale phosphoproteomic data quantifying vasopressin-induced changes in phosphorylation in aquaporin-2-expressing cultured collecting duct (mpkCCD) cells. Quantification was done using stable isotope labelling (SILAC method).

KEY RESULTS

Six hundred forty phosphopeptides were quantified. Stringent statistical analysis identified significant changes in response to vasopressin in 429 of these phosphopeptides. The corresponding phosphoproteins were mapped to known vasopressin-regulated cellular processes. The vasopressin-regulated sites were classified according to the sequences surrounding the phosphorylated amino acids giving 11 groups. Among the vasopressin-regulated phosphoproteins were 25 distinct protein kinases. Among these, six plus PKA appeared to account for phosphorylation of about 81% of the 313 vasopressin-regulated phosphorylation sites. The six downstream kinases were salt-inducible kinase 2 (Sik2), cyclin-dependent kinase 18 (Cdk18), calmodulin-dependent kinase kinase 2 (Camkk2), protein kinase D2 (Prkd2), mitogen-activated kinase 3 (Mapk3) and myosin light chain kinase (Mylk).

CONCLUSION AND IMPLICATIONS

In V2 receptor-mediated signalling, PKA is at the head of a complex network that includes at least six downstream vasopressin-regulated protein kinases that are prime targets for future study. The extensive phosphoproteomic data reported in this study are provided as a web-based data resource for future studies of GPCRs.

JUP/plakoglobin is regulated by salt-inducible kinase 2, and is required for insulin-induced signalling and glucose uptake in adipocytes

BACKGROUND

Salt-inducible kinase 2 (SIK2) is abundant in adipocytes, but downregulated in adipose tissue from individuals with obesity and insulin resistance. Moreover, SIK isoforms are required for normal insulin signalling and glucose uptake in adipocytes, but the underlying molecular mechanisms are currently not known. The adherens junction protein JUP, also termed plakoglobin or γ-catenin, has recently been reported to promote insulin signalling in muscle cells.

OBJECTIVE

The objective of this study was to analyse if JUP is required for insulin signalling in adipocytes and the underlying molecular mechanisms of this regulation.

METHODS

Co-expression of SIK2 and JUP mRNA levels in adipose tissue from a human cohort was analysed. siRNA silencing and/or pharmacological inhibition of SIK2, JUP, class IIa HDACs and CRTC2 was employed in 3T3-L1- and primary rat adipocytes. JUP protein expression was analysed by western blot and mRNA levels by qPCR. Insulin signalling was evaluated by western blot as levels of phosphorylated PKB/Akt and AS160, and by monitoring the uptake of ³H-2-deoxyglucose.

RESULTS

mRNA expression of SIK2 correlated with that of JUP in human adipose tissue. SIK2 inhibition or silencing resulted in downregulation of JUP mRNA and protein expression in 3T3-L1- and in primary rat adipocytes. Moreover, JUP silencing reduced the expression of PKB and the downstream substrate AS160, and consequently attenuated activity in the insulin signalling pathway, including insulin-induced glucose uptake. The known SIK2 substrates CRTC2 and class IIa HDACs were found to play a role in the SIK-mediated regulation of JUP expression.

CONCLUSIONS

These findings identify JUP as a novel player in the regulation of insulin sensitivity in adipocytes, and suggest that changes in JUP expression could contribute to the effect of SIK2 on insulin signalling in these cells.

Inhibition of AMPK activity in response to insulin in adipocytes: involvement of AMPK pS485, PDEs, and cellular energy levels

Insulin resistance in obesity and type 2 diabetes has been shown to be associated with decreased de novo fatty acid (FA) synthesis in adipose tissue. It is known that insulin can acutely stimulate FA synthesis in adipocytes; however, the mechanisms underlying this effect are unclear. The rate-limiting step in FA synthesis is catalyzed by acetyl-CoA carboxylase (ACC), known to be regulated through inhibitory phosphorylation at S79 by the AMP-activated protein kinase (AMPK). Previous results from our laboratory showed an inhibition of AMPK activity by insulin, which was accompanied by PKB-dependent phosphorylation of AMPK at S485. However, whether the S485 phosphorylation is required for insulin-induced inhibition of AMPK or other mechanisms underlie the reduced kinase activity is not known. To investigate this, primary rat adipocytes were transduced with a recombinant adenovirus encoding AMPK-WT or a nonphosphorylatable AMPK S485A mutant. AMPK activity measurements by Western blot analysis and in vitro kinase assay revealed that WT and S485A AMPK were inhibited to a similar degree by insulin, indicating that AMPK S485 phosphorylation is not required for insulin-induced AMPK inhibition. Further analysis suggested an involvement of decreased AMP-to-ATP ratios in the insulin-induced inhibition of AMPK activity, whereas a possible contribution of phosphodiesterases was excluded. Furthermore, we show that insulin-induced AMPK S485 phosphorylation also occurs in human adipocytes, suggesting it to be of an importance yet to be revealed. Altogether, this study increases our understanding of how insulin regulates AMPK activity, and with that, FA synthesis, in adipose tissue.

The potent roles of salt-inducible kinases (SIKs) in metabolic homeostasis and tumorigenesis

Salt-inducible kinases (SIKs) belong to AMP-activated protein kinase (AMPK) family, and functions mainly involve in regulating energy response-related physiological processes, such as gluconeogenesis and lipid metabolism. However, compared with another well-established energy-response kinase AMPK, SIK roles in human diseases, especially in diabetes and tumorigenesis, are rarely investigated. Recently, the pilot roles of SIKs in tumorigenesis have begun to attract more attention due to the finding that the tumor suppressor role of LKB1 in non-small-cell lung cancers (NSCLCs) is unexpectedly mediated by the SIK but not AMPK kinases. Thus, here we tend to comprehensively summarize the emerging upstream regulators, downstream substrates, mouse models, clinical relevance, and candidate inhibitors for SIKs, and shed light on SIKs as the potential therapeutic targets for cancer therapies.

Salt-inducible Kinases Are Critical Determinants of Female Fertility

Follicle development is the most crucial step toward female fertility and is controlled mainly by follicle-stimulating hormone (FSH). In ovarian granulosa cells (GCs), FSH activates protein kinase A by increasing 3',5'-cyclic adenosine 5'-monophosphate (cAMP). Since cAMP signaling is impinged in part by salt-inducible kinases (SIKs), we examined the role of SIKs on the regulation of FSH actions. Here, we report that SIKs are essential for normal ovarian function and female fertility. All SIK isoforms are expressed in human and rodent GCs at different levels (SIK3>SIK2>SIK1). Pharmacological inhibition of SIK activity potentiated the stimulatory effect of FSH on markers of GC differentiation in mouse, rat, and human GCs and estradiol production in rat GCs. In humans, SIK inhibition strongly enhanced FSH actions in GCs of patients with normal or abnormal ovarian function. The knockdown of SIK2, but not SIK1 or SIK3, synergized with FSH on the induction of markers of GC differentiation. SIK inhibition boosted gonadotropin-induced GC differentiation in vivo, while the genomic knockout of SIK2 led to a significant increase in the number of ovulated oocytes. Conversely, SIK3 knockout females were infertile, FSH insensitive, and had abnormal folliculogenesis. These findings reveal novel roles for SIKs in the regulation of GC differentiation and female fertility, and contribute to our understanding of the mechanisms regulated by FSH. Furthermore, these data suggest that specific pharmacological modulation of SIK2 activity could be of benefit to treat ovulatory defects in humans and to increase the propagation of endangered species and farm mammals.

Salt inducible kinases as novel Notch interactors in the developing Drosophila retina

Developmental processes require strict regulation of proliferation, differentiation and patterning for the generation of final organ size. Aberrations in these fundamental events are critically important in tumorigenesis and cancer progression. Salt inducible kinases (Siks) are evolutionarily conserved genes involved in diverse biological processes, including salt sensing, metabolism, muscle, cartilage and bone formation, but their role in development remains largely unknown. Recent findings implicate Siks in mitotic control, and in both tumor suppression and progression. Using a tumor model in the Drosophila eye, we show that perturbation of Sik function exacerbates tumor-like tissue overgrowth and metastasis. Furthermore, we show that both Drosophila Sik genes, Sik2 and Sik3, function in eye development processes. We propose that an important target of Siks may be the Notch signaling pathway, as we demonstrate genetic interaction between Siks and Notch pathway members. Finally, we investigate Sik expression in the developing retina and show that Sik2 is expressed in all photoreceptors, basal to cell junctions, while Sik3 appears to be expressed specifically in R3/R4 cells in the developing eye. Combined, our data suggest that Sik genes are important for eye tissue specification and growth, and that their dysregulation may contribute to tumor formation.

Breed difference and regulatory role of CRTC3 in porcine intramuscular adipocyte

The cAMP responsive element binding protein (CREB)-regulated transcription coactivator 3 (CRTC3) is a member of the CRTC protein family and plays an important role in energy metabolism. The aim of this study was to determine if the expression of porcine CRTC3 is related to intramuscular fat (IMF) deposition and meat quality in Heigai pigs (a local fatty breed in China) and Duroc × Landrace × Yorkshire (DLY) pigs (a lean crossbred pig widely cultured in China). In addition, the effect of ectopic expression of CRTC3 on gene expression in porcine IMF adipocytes was also examined. Our results showed that Heigai pigs had lower lean percentage, thicker back fat thickness and smaller loin muscle area than DLY pigs. Compared with DLY pigs, Heigai pigs had higher marbling scores, better meat color and higher IMF contents and triglyceride concentrations. Higher levels of oxidative metabolic enzyme and expression of the slow oxidative muscle fiber-related genes were observed in longissimus dorsi muscle and psoas major muscle (P < 0.05) from Heigai pigs. Notably, CRTC3 and adipocyte-specific marker genes were highly expressed in muscle tissues of Heigai pigs. The expression of lipolysis-related genes ATGL and HSL were lower in Heigai muscles. Moreover, forced expression of CRTC3 promoted lipid accumulation and increased the expression of PPARγ, C/EBPα, leptin and FABP4 (P < 0.05), whereas it decreased the expression of ATGL and HSL in IMF adipocytes. These results suggest that CRTC3 expression is associated with lipid accumulation and IMF deposition in pigs.

Site-Dependent Lineage Preference of Adipose Stem Cells

Adult stem cells have unique properties in both proliferation and differentiation preference. In this study, we hypothesized that adipose stem cells have a depot-dependent lineage preference. Four rabbits were used to provide donor-matched adipose stem cells from either subcutaneous adipose tissue (ScAT) or infrapatellar fat pad (IPFP). Proliferation and multi-lineage differentiation were evaluated in adipose stem cells from donor-matched ScAT and IPFP. RNA sequencing (RNA-seq) and proteomics were conducted to uncover potential molecular discrepancy in adipose stem cells and their corresponding matrix microenvironments. We found that stem cells from ScAT exhibited significantly higher proliferation and adipogenic capacity compared to those from donor-matched IPFP while stem cells from IPFP displayed significantly higher chondrogenic potential compared to those from donor-matched ScAT. Our findings are strongly endorsed by supportive data from transcriptome and proteomics analyses, indicating a site-dependent lineage preference of adipose stem cells.

Cold Atmospheric Plasma and Silymarin Nanoemulsion Activate Autophagy in Human Melanoma Cells

Background: Autophagy is reported as a survival or death-promoting pathway that is highly debatable in different kinds of cancer. Here, we examined the co-effect of cold atmospheric plasma (CAP) and silymarin nanoemulsion (SN) treatment on G-361 human melanoma cells via autophagy induction. Methods: The temperature and pH of the media, along with the cell number, were evaluated. The intracellular glucose level and PI3K/mTOR and EGFR downstream pathways were assessed. Autophagy-related genes, related transcriptional factors, and autophagy induction were estimated using confocal microscopy, flow cytometry, and ELISA. Results: CAP treatment increased the temperature and pH of the media, while its combination with SN resulted in a decrease in intracellular ATP with the downregulation of PI3K/AKT/mTOR survival and RAS/MEK transcriptional pathways. Co-treatment blocked downstream paths of survival pathways and reduced PI3K (2 times), mTOR (10 times), EGFR (5 times), HRAS (5 times), and MEK (10 times). CAP and SN co-treated treatment modulates transcriptional factor expressions (ZKSCAN3, TFEB, FOXO1, CRTC2, and CREBBP) and specific genes (BECN-1, AMBRA-1, MAP1LC3A, and SQSTM) related to autophagy induction. Conclusion: CAP and SN together activate autophagy in G-361 cells by activating PI3K/mTOR and EGFR pathways, expressing autophagy-related transcription factors and genes.

High-Throughput Implementation of the NanoBRET Target Engagement Intracellular Kinase Assay to Reveal Differential Compound Engagement by SIK2/3 Isoforms

The real-time quantification of target engagement (TE) by small-molecule ligands in living cells remains technically challenging. Systematic quantification of such interactions in a high-throughput setting holds promise for identification of target-specific, potent small molecules within a pathophysiological and biologically relevant cellular context. The salt-inducible kinases (SIKs) belong to a subfamily of the AMP-activated protein kinase (AMPK) family and are composed of three isoforms in humans (SIK1, SIK2, and SIK3). They modulate the production of pro- and anti-inflammatory cytokines in immune cells. Although pan-SIK inhibitors are sufficient to reverse SIK-dependent inflammatory responses, the apparent toxicity associated with SIK3 inhibition suggests that isoform-specific inhibition is required to realize therapeutic benefit with acceptable safety margins. Here, we used the NanoBRET TE intracellular kinase assay, a sensitive energy transfer technique, to directly measure molecular proximity and quantify TE in HEK293T cells overexpressing SIK2 or SIK3. Our 384-well high-throughput screening of 530 compounds demonstrates that the NanoBRET TE intracellular kinase assay was sensitive and robust enough to reveal differential engagement of candidate compounds with the two SIK isoforms and further highlights the feasibility of high-throughput implementation of NanoBRET TE intracellular kinase assays for target-driven small-molecule screening.

Salt-inducible kinases dictate parathyroid hormone 1 receptor action in bone development and remodeling

The parathyroid hormone 1 receptor (PTH1R) mediates the biologic actions of parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP). Here, we showed that salt-inducible kinases (SIKs) are key kinases that control the skeletal actions downstream of PTH1R and that this GPCR, when activated, inhibited cellular SIK activity. Sik gene deletion led to phenotypic changes that were remarkably similar to models of increased PTH1R signaling. In growth plate chondrocytes, PTHrP inhibited SIK3, and ablation of this kinase in proliferating chondrocytes rescued perinatal lethality of PTHrP-null mice. Combined deletion of Sik2 and Sik3 in osteoblasts and osteocytes led to a dramatic increase in bone mass that closely resembled the skeletal and molecular phenotypes observed when these bone cells express a constitutively active PTH1R that causes Jansen's metaphyseal chondrodysplasia. Finally, genetic evidence demonstrated that class IIa histone deacetylases were key PTH1R-regulated SIK substrates in both chondrocytes and osteocytes. Taken together, our findings establish that SIK inhibition is central to PTH1R action in bone development and remodeling. Furthermore, this work highlights the key role of cAMP-regulated SIKs downstream of GPCR action.

The posttranslational modification of HDAC4 in cell biology: Mechanisms and potential targets

Histone deacetylase 4 (HDAC4) is a member of the HDACs family, its expression is closely related to the cell development. The cell is an independent living entity that undergoes proliferation, differentiation, senescence, apoptosis, and pathology, and each process has a strict and complex regulatory system. With deepening of its research, the expression of HDAC4 is critical in the life process. This review focuses on the posttranslational modification of HDAC4 in cell biology, providing an important target for future disease treatment.

Sirtuin 3-mediated pyruvate dehydrogenase activity determines brown adipocytes phenotype under high-salt conditions

Previous study indicated that Sirtuin 3 (SIRT3) is a central regulator of adaptive thermogenesis in brown adipose tissue (BAT). Here we investigate the role of SIRT3 in the modulation of cellular phenotype in BAT under high salt intake (HS). HS downregulated SIRT3 level in BAT, accompanied by decreased oxygen consumption rate, and caused a severe loss of BAT characteristics. Mechanically, SIRT3 interacted with pyruvate dehydrogenase E1α (PDHA1) and deacetylated Lys-83 both in vitro and in vivo under HS. In parallel, HS suppressed salt-induced kinase (Sik) 2 phosphorylation. Silencing Sik2 further diminished SIRT3 activity and enhanced acetylation of PDHA1 K83 level. Reconstruction of SIRT3 restored PDH activity and thermogenic markers expression in differentiated brown adipocytes from SIRT3 knockout (KO) mice. In addition, loss of SIRT3 induced selective remodelling of phospholipids and glycerolipids in BAT exposure to HS. These data indicate that SIRT3 is an essential enzymatic switch that controls brown adipose cell phenotype.

The AMPK-Related Kinases SIK1 and SIK3 Mediate Key Tumor-Suppressive Effects of LKB1 in NSCLC

Mutations in the LKB1 (also known as STK11) tumor suppressor are the third most frequent genetic alteration in non-small cell lung cancer (NSCLC). LKB1 encodes a serine/threonine kinase that directly phosphorylates and activates 14 AMPK family kinases ("AMPKRs"). The function of many of the AMPKRs remains obscure, and which are most critical to the tumor-suppressive function of LKB1 remains unknown. Here, we combine CRISPR and genetic analysis of the AMPKR family in NSCLC cell lines and mouse models, revealing a surprising critical role for the SIK subfamily. Conditional genetic loss of Sik1 revealed increased tumor growth in mouse models of Kras-dependent lung cancer, which was further enhanced by loss of the related kinase Sik3. As most known substrates of the SIKs control transcription, gene-expression analysis was performed, revealing upregulation of AP1 and IL6 signaling in common between LKB1- and SIK1/3-deficient tumors. The SIK substrate CRTC2 was required for this effect, as well as for proliferation benefits from SIK loss. SIGNIFICANCE: The tumor suppressor LKB1/STK11 encodes a serine/threonine kinase frequently inactivated in NSCLC. LKB1 activates 14 downstream kinases in the AMPK family controlling growth and metabolism, although which kinases are critical for LKB1 tumor-suppressor function has remained an enigma. Here we unexpectedly found that two understudied kinases, SIK1 and SIK3, are critical targets in lung cancer.This article is highlighted in the In This Issue feature, p. 1469.

Effects of histone deacetylase inhibitor Scriptaid and parathyroid hormone on osteocyte functions and metabolism

Bone is a highly metabolic organ that undergoes continuous remodeling to maintain its structural integrity. During development, bones, in particular osteoblasts, rely on glucose uptake. However, the role of glucose metabolism in osteocytes is unknown. Osteocytes are terminally differentiated osteoblasts orchestrating bone modeling and remodeling. In these cells, parathyroid hormone (PTH) suppresses Sost/sclerostin expression (a potent inhibitor of bone formation) by promoting nuclear translocation of class IIa histone deacetylase (HDAC) 4 and 5 and the repression of myocyte enhancer factor 2 (MEF2) type C. Recently, Scriptaid, an HDAC complex co-repressor inhibitor, has been shown to induce MEF2 activation and exercise-like adaptation in mice. In muscles, Scriptaid disrupts the HDAC4/5 co-repressor complex, increases MEF2C function, and promotes cell respiration. We hypothesized that Scriptaid, by affecting HDAC4/5 localization and MEF2C activation, might affect osteocyte functions. Treatment of the osteocytic Ocy454-12H cells with Scriptaid increased metabolic gene expression, cell respiration, and glucose uptake. Similar effects were also seen upon treatment with PTH, suggesting that both Scriptaid and PTH can promote osteocyte metabolism. Similar to PTH, Scriptaid potently suppressed Sost expression. Silencing of HDAC5 in Ocy454-12H cells abolished Sost suppression but not glucose transporter type 4 (Glut4) up-regulation induced by Scriptaid. These results demonstrate that Scriptaid increases osteocyte respiration and glucose uptake by mechanisms independent of HDAC complex inhibition. In osteocytes, Scriptaid, similar to PTH, increases binding of HDAC5 to Mef2c with suppression of Sost but only partially increases receptor activator of NF-κB ligand (Rankl) expression, suggesting a potential bone anabolic effect.

HDAC4 mutations cause diabetes and induce β-cell FoxO1 nuclear exclusion

Background

Studying patients with rare Mendelian diabetes has uncovered molecular mechanisms regulating β‐cell pathophysiology. Previous studies have shown that Class IIa histone deacetylases (HDAC4, 5, 7, and 9) modulate mammalian pancreatic endocrine cell function and glucose homeostasis.

Methods

We performed exome sequencing in one adolescent nonautoimmune diabetic patient and detected one de novo predicted disease‐causing HDAC4 variant (p.His227Arg). We screened our pediatric diabetes cohort with unknown etiology using Sanger sequencing. In mouse pancreatic β‐cell lines (Min6 and SJ cells), we performed insulin secretion assay and quantitative RT‐PCR to measure the β‐cell function transfected with the detected HDAC4 variants and wild type. We carried out immunostaining and Western blot to investigate if the detected HDAC4 variants affect the cellular translocation and acetylation status of Forkhead box protein O1 (FoxO1) in the pancreatic β‐cells.

Results

We discovered three HDAC4 mutations (p.His227Arg, p.Asp234Asn, and p.Glu374Lys) in unrelated individuals who had nonautoimmune diabetes with various degrees of β‐cell loss. In mouse pancreatic β‐cell lines, we found that these three HDAC4 mutations decrease insulin secretion, down‐regulate β‐cell‐specific transcriptional factors, and cause nuclear exclusion of acetylated FoxO1.

Conclusion

Mutations in HDAC4 disrupt the deacetylation of FoxO1, subsequently decrease the β‐cell function including insulin secretion, resulting in diabetes.

Mitogenic Signals Stimulate the CREB Coactivator CRTC3 through PP2A Recruitment

The second messenger 3',5'-cyclic adenosine monophosphate (cAMP) stimulates gene expression via the cAMP-regulated transcriptional coactivator (CRTC) family of cAMP response element-binding protein coactivators. In the basal state, CRTCs are phosphorylated by salt-inducible kinases (SIKs) and sequestered in the cytoplasm by 14-3-3 proteins. cAMP signaling inhibits the SIKs, leading to CRTC dephosphorylation and nuclear translocation. Here we show that although all CRTCs are regulated by SIKs, their interactions with Ser/Thr-specific protein phosphatases are distinct. CRTC1 and CRTC2 associate selectively with the calcium-dependent phosphatase calcineurin, whereas CRTC3 interacts with B55 PP2A holoenzymes via a conserved PP2A-binding region (amino acids 380-401). CRTC3-PP2A complex formation was induced by phosphorylation of CRTC3 at S391, facilitating the subsequent activation of CRTC3 by dephosphorylation at 14-3-3 binding sites. As stimulation of mitogenic pathways promoted S391 phosphorylation via the activation of ERKs and CDKs, our results demonstrate how a ubiquitous phosphatase enables cross talk between growth factor and cAMP signaling pathways at the level of a transcriptional coactivator.

Using Mouse and Drosophila Models to Investigate the Mechanistic Links between Diet, Obesity, Type II Diabetes, and Cancer

Many of the links between diet and cancer are controversial and over simplified. To date, human epidemiological studies consistently reveal that patients who suffer diet-related obesity and/or type II diabetes have an increased risk of cancer, suffer more aggressive cancers, and respond poorly to current therapies. However, the underlying molecular mechanisms that increase cancer risk and decrease the response to cancer therapies in these patients remain largely unknown. Here, we review studies in mouse cancer models in which either dietary or genetic manipulation has been used to model obesity and/or type II diabetes. These studies demonstrate an emerging role for the conserved insulin and insulin-like growth factor signaling pathways as links between diet and cancer progression. However, these models are time consuming to develop and expensive to maintain. As the world faces an epidemic of obesity and type II diabetes we argue that the development of novel animal models is urgently required. We make the case for Drosophila as providing an unparalleled opportunity to combine dietary manipulation with models of human metabolic disease and cancer. Thus, combining diet and cancer models in Drosophila can rapidly and significantly advance our understanding of the conserved molecular mechanisms that link diet and diet-related metabolic disorders to poor cancer patient prognosis.

Cold exposure induces nuclear translocation of CRTC3 in brown adipose tissue

In mammals, cold stress activates the cAMP-protein kinase A (PKA) signaling pathway, increases brown adipose tissue (BAT) activity, and induces thermogenesis to maintain body temperature. The cAMP responsive element binding protein (CREB)-regulated transcription coactivator 3 (CRTC3) plays important role in adipose development and energy metabolism. However, the effect of cold exposure on the intracellular localization of CRTC3 in BAT is unclear. Here, we report that cold-treated mice have higher expression of uncoupling protein 1 (UCP1) in adipose tissues and lower body weights and fat masses. Notably, cold exposure results in the nuclear translocation of CRTC3 in BAT. Moreover, forskolin (FSK), the activator of PKA pathway, induces the nuclear translocation of CRTC3 in brown adipocytes. At the molecular level, cold exposure and FSK treatment decrease liver kinase B1 (Lkb1) expression in brown adipocytes, which is related to the nuclear localization of CRTC3. These results demonstrate that the localization of CRTC3 involves in regulating cold-induced upregulation of UCP1 in BAT and provide useful information for understanding the molecular regulation of BAT thermogenesis induced by a cold environment.

The Salt-Inducible Kinases: Emerging Metabolic Regulators

The discovery of liver kinase B1 (LKB1) as an upstream kinase for AMP-activated protein kinase (AMPK) led to the identification of several related kinases that also rely on LKB1 for their catalytic activity. Among these, the salt-inducible kinases (SIKs) have emerged as key regulators of metabolism. Unlike AMPK, SIKs do not respond to nucleotides, but their function is regulated by extracellular signals, such as hormones, through complex LKB1-independent mechanisms. While AMPK acts on multiple targets, including metabolic enzymes, to maintain cellular ATP levels, SIKs primarily regulate gene expression, by acting on transcriptional regulators, such as the cAMP response element-binding protein-regulated transcription coactivators and class IIa histone deacetylases. This review describes the development of research on SIKs, from their discovery to the most recent findings on metabolic regulation.

Rapid Communication: Porcine CRTC3 gene clone, expression pattern, and its regulatory role in intestinal epithelial cells

In the current study, we aimed to clone the full-length cDNA of porcine CRTC3 (pCRTC3) gene and examine its expression pattern and function in intestinal epithelial cells. The full-length cDNA sequence of pCRTC3 was 2,173 bp (GenBank accession no. MF964215), with a 1,860-bp open reading frame encoding a 620-AA protein. Comparison of the deduced AA sequence with different species including human, mouse, rat, Papio, cattle, and rabbit showed 89% to 91.9% similarity. The pCRTC3 was highly expressed in small intestine and spleen, to a lesser degree in lung, liver, and adipose tissue, and was expressed at a low but detectable level in skeletal muscle, kidney, and heart. In addition, high protein levels of pCRTC3 were found in IPEC-J2 cells, in which pCRTC3 was mainly localized in cytoplasm. Furthermore, we demonstrated that knockdown of pCRTC3 significantly decreased the expression of the porcine tight junction-related genes including zonula occludens-1 (ZO-1), ZO-2, occludin, and claudin-1 by 57.88% (P < 0.01), 40.19% (P < 0.01), 51.59% (P < 0.01), and 35.70% (P < 0.05), respectively. Taken together, we first cloned the full-length sequence of pCRTC3 and revealed the tissue-specific expression pattern, localization, and function of pCRTC3 in regulating the expression of intestinal tight junction-related genes. This study could provide some useful information for understanding the function of CRTC3 in pigs.

AMPK activation by A-769662 and 991 does not affect catecholamine-induced lipolysis in human adipocytes

Activation of AMP-activated protein kinase (AMPK) is considered an attractive strategy for the treatment of type 2 diabetes. Favorable metabolic effects of AMPK activation are mainly observed in skeletal muscle and liver tissue, whereas the effects in human adipose tissue are only poorly understood. Previous studies, which largely employed the AMPK activator 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR), suggest an antilipolytic role of AMPK in adipocytes. The aim of this work was to reinvestigate the role of AMPK in the regulation of lipolysis, using the novel allosteric small-molecule AMPK activators A-769662 and 991, with a focus on human adipocytes. For this purpose, human primary subcutaneous adipocytes were treated with A-769662, 991, or AICAR, as a control, before being stimulated with isoproterenol. AMPK activity status, glycerol release, and the phosphorylation of hormone-sensitive lipase (HSL), a key regulator of lipolysis, were then monitored. Our results show that both A-769662 and 991 activated AMPK to a level that was similar to, or greater than, that induced by AICAR. In contrast to AICAR, which as expected was antilipolytic, neither A-769662 nor 991 affected lipolysis in human adipocytes, although 991 treatment led to altered HSL phosphorylation. Furthermore, we suggest that HSL Ser660 is an important regulator of lipolytic activity in human adipocytes. These data suggest that the antilipolytic effect observed with AICAR in previous studies is, at least to some extent, AMPK independent.

Parathyroid hormone(1-34) and its analogs differentially modulate osteoblastic Rankl expression via PKA/SIK2/SIK3 and PP1/PP2A-CRTC3 signaling

Osteoporosis can result from the loss of sex hormones and/or aging. Abaloparatide (ABL), an analog of parathyroid hormone-related protein (PTHrP(1-36)), is the second osteoanabolic therapy approved by the United States Food and Drug Administration after teriparatide (PTH(1-34)). All three peptides bind PTH/PTHrP receptor type 1 (PTHR1), but the effects of PTHrP(1-36) or ABL in the osteoblast remain unclear. We show that, in primary calvarial osteoblasts, PTH(1-34) promotes a more robust cAMP response than PTHrP(1-36) and ABL and causes a greater activation of protein kinase A (PKA) and cAMP response element-binding protein (CREB). All three peptides similarly inhibited sclerostin (Sost). Interestingly, the three peptides differentially modulated two other PKA target genes, c-Fos and receptor activator of NF-κB ligand (Rankl), and the latter both in vitro and in vivo Knockdown of salt-inducible kinases (SIKs) 2 and 3 and CREB-regulated transcription coactivator 3 (CRTC3), indicated that all three are part of the pathway that regulates osteoblastic Rankl expression. We also show that the peptides differentially regulate the nuclear localization of CRTC2 and CRTC3, and that this correlates with PKA activation. Moreover, inhibition of protein phosphatases 1 and 2A (PP1/PP2A) activity revealed that they play a major role in both PTH-induced Rankl expression and the effects of PTH(1-34) on CRTC3 localization. In summary, in the osteoblast, the effects of PTH(1-34), PTHrP(1-36), and ABL on Rankl are mediated by differential stimulation of cAMP/PKA signaling and by their downstream effects on SIK2 and -3, PP1/PP2A, and CRTC3.

Adipose mTORC1 Suppresses Prostaglandin Signaling and Beige Adipogenesis via the CRTC2-COX-2 Pathway

Beige adipocytes are present in white adipose tissue (WAT) and have thermogenic capacity to orchestrate substantial energy metabolism and counteract obesity. However, adipocyte-derived signals that act on progenitor cells to control beige adipogenesis remain poorly defined. Here, we show that adipose-specific depletion of Raptor, a key component of mTORC1, promoted beige adipogenesis through prostaglandins (PGs) synthesized by cyclooxygenase-2 (COX-2). Moreover, Raptor-deficient mice were resistant to diet-induced obesity and COX-2 downregulation. Mechanistically, mTORC1 suppressed COX-2 by phosphorylation of CREB-regulated transcription coactivator 2 (CRTC2) and subsequent dissociation of CREB to cox-2 promoter in adipocytes. PG treatment stimulated PKA and promoted differentiation of progenitor cells to beige adipocytes in culture. Ultimately, we show that pharmacological inhibition or suppression of COX-2 attenuated mTORC1 inhibition-induced thermogenic gene expression in inguinal WAT in vivo and in vitro. Our study identifies adipocyte-derived PGs as key regulators of white adipocyte browning, which occurs through mTORC1 and CRTC2.

Suppression of gluconeogenic gene transcription by SIK1-induced ubiquitination and degradation of CRTC1

CRTCs are a group of three transcriptional coactivators required for CREB-dependent transcription. CREB and CRTCs are critically involved in the regulation of various biological processes such as cell proliferation, metabolism, learning and memory. However, whether CRTC1 efficiently induces gluconeogenic gene expression and how CRTC1 is regulated by upstream kinase SIK1 remain to be understood. In this work, we demonstrated SIK1-induced phosphorylation, ubiquitination and degradation of CRTC1 in the context of the regulation of gluconeogenesis. CRTC1 protein was destabilized by SIK1 but not SIK2 or SIK3. This effect was likely mediated by phosphorylation at S155, S167, S188 and S346 residues of CRTC1 followed by K48-linked polyubiquitination and proteasomal degradation. Expression of gluconeogenic genes such as that coding for phosphoenolpyruvate carboxykinase was stimulated by CRTC1, but suppressed by SIK1. Depletion of CRTC1 protein also blocked forskolin-induced gluconeogenic gene expression, knockdown or pharmaceutical inhibition of SIK1 had the opposite effect. Finally, SIK1-induced ubiquitination of CRTC1 was mediated by RFWD2 ubiquitin ligase at a site not equivalent to K628 in CRTC2. Taken together, our work reveals a regulatory circuit in which SIK1 suppresses gluconeogenic gene transcription by inducing ubiquitination and degradation of CRTC1. Our findings have implications in the development of new antihyperglycemic agents.

14-3-3 proteins mediate inhibitory effects of cAMP on salt-inducible kinases (SIKs)

The salt-inducible kinase (SIK) family regulates cellular gene expression via the phosphorylation of cAMP-regulated transcriptional coactivators (CRTCs) and class IIA histone deacetylases, which are sequestered in the cytoplasm by phosphorylation-dependent 14-3-3 interactions. SIK activity toward these substrates is inhibited by increases in cAMP signaling, although the underlying mechanism is unclear. Here, we show that the protein kinase A (PKA)-dependent phosphorylation of SIKs inhibits their catalytic activity by inducing 14-3-3 protein binding. SIK1 and SIK3 contain two functional PKA/14-3-3 sites, while SIK2 has four. In keeping with the dimeric nature of 14-3-3s, the presence of multiple binding sites within target proteins dramatically increases binding affinity. As a result, loss of a single 14-3-3-binding site in SIK1 and SIK3 abolished 14-3-3 association and rendered them insensitive to cAMP. In contrast, mutation of three sites in SIK2 was necessary to fully block cAMP regulation. Superimposed on the effects of PKA phosphorylation and 14-3-3 association, an evolutionary conserved domain in SIK1 and SIK2 (the so called RK-rich region; 595-624 in hSIK2) is also required for the inhibition of SIK2 activity. Collectively, these results point to a dual role for 14-3-3 proteins in repressing a family of Ser/Thr kinases as well as their substrates.

BMPs as new insulin sensitizers: enhanced glucose uptake in mature 3T3-L1 adipocytes via PPARγ and GLUT4 upregulation

Insulin-resistance is the main cause of type 2 diabetes. Here we describe the identification and characterization of BMP2 and BMP6 as new insulin-sensitizing growth factors in mature adipocytes. We show that BMP2 and BMP6 lead to enhanced insulin-mediated glucose uptake in both insulin-sensitive and -insensitive adipocytes. We exclude a direct effect of BMP2 or BMP6 on translocation of GLUT4 to the plasma membrane and demonstrate that these BMPs increase GLUT4 protein levels equipotent to Rosiglitazone. BMPs induce expression of PPARγ as the crucial mediator for the insulin-sensitizing effect. A comprehensive RNA-Seq analysis in mature adipocytes revealed regulation of both BMP/Smad and PPARγ target genes. The effects of BMP2 and BMP6 are not completely redundant and include regulation of genes involved in glucose and fatty acid metabolism and adipokine expression. Collectively, these findings suggest the BMP2 and BMP6 pathway(s) as promising new drug targets to treat insulin resistance.

Bosutinib Attenuates Inflammation via Inhibiting Salt-Inducible Kinases in Experimental Model of Intracerebral Hemorrhage on Mice

BACKGROUND AND PURPOSE

Intracerebral hemorrhage (ICH) is a subtype of stroke with highest mortality and morbidity. Pronounced inflammation plays a significant role in the development of the secondary brain injury after ICH. Recently, SIK-2 (salt-inducible kinase-2) was identified as an important component controlling inflammatory response. Here we sought to investigate the role of SIK-2 in post-ICH inflammation and potential protective effects of SIK-2 inhibition after ICH.

METHODS

Two hundred and ninety-three male CD-1 mice were used. ICH was induced via injection of 30 μL of autologous blood. Recombinant SIK-2 was administrated 1 hour after ICH intracerebroventricularly. SIK-2 small interfering RNA was injected intracerebroventricularly 24 hours before ICH. Bosutinib, a clinically approved tyrosine kinase inhibitor with affinity to SIK-2, was given intranasally 1 hour or 6 hours after ICH. Effects of treatments were evaluated by neurological tests and brain water content calculation. Molecular pathways were investigated by Western blots and immunofluorescence studies.

RESULTS

Endogenous SIK-2 was expressed in microglia and neurons. SIK-2 expression was reduced after ICH. Exogenous SIK-2 aggravated post-ICH inflammation, leading to brain edema and the neurobehavioral deficits. SIK-2 inhibition attenuated post-ICH inflammation, reducing brain edema and ameliorating neurological dysfunctions. Bosutinib inhibited SIK-2-attenuating ICH-induced brain damage. Protective effects of Bosutinib were mediated, at least partly, by CRTC3 (cyclic amp-response element binding protein-regulated transcription coactivator 3)/cyclic amp-response element binding protein/NF-κB (nuclear factor-κB) pathway.

CONCLUSIONS

SIK-2 participates in inflammation induction after ICH. SIK-2 inhibition via Bosutinib or small interfering RNA decreased inflammation, attenuating brain injury. SIK-2 effects are, at least partly, mediated by CRTC3-cyclic amp-response element binding protein-NF-κB signaling pathway.

Systems-level identification of PKA-dependent signaling in epithelial cells

G protein stimulatory α-subunit (G_αs)-coupled heptahelical receptors regulate cell processes largely through activation of protein kinase A (PKA). To identify signaling processes downstream of PKA, we deleted both PKA catalytic subunits using CRISPR-Cas9, followed by a "multiomic" analysis in mouse kidney epithelial cells expressing the G_αs-coupled V2 vasopressin receptor. RNA-seq (sequencing)-based transcriptomics and SILAC (stable isotope labeling of amino acids in cell culture)-based quantitative proteomics revealed a complete loss of expression of the water-channel gene Aqp2 in PKA knockout cells. SILAC-based quantitative phosphoproteomics identified 229 PKA phosphorylation sites. Most of these PKA targets are thus far unannotated in public databases. Surprisingly, 1,915 phosphorylation sites with the motif x-(S/T)-P showed increased phosphooccupancy, pointing to increased activity of one or more MAP kinases in PKA knockout cells. Indeed, phosphorylation changes associated with activation of ERK2 were seen in PKA knockout cells. The ERK2 site is downstream of a direct PKA site in the Rap1GAP, Sipa1l1, that indirectly inhibits Raf1. In addition, a direct PKA site that inhibits the MAP kinase kinase kinase Map3k5 (ASK1) is upstream of JNK1 activation. The datasets were integrated to identify a causal network describing PKA signaling that explains vasopressin-mediated regulation of membrane trafficking and gene transcription. The model predicts that, through PKA activation, vasopressin stimulates AQP2 exocytosis by inhibiting MAP kinase signaling. The model also predicts that, through PKA activation, vasopressin stimulates Aqp2 transcription through induction of nuclear translocation of the acetyltransferase EP300, which increases histone H3K27 acetylation of vasopressin-responsive genes (confirmed by ChIP-seq).