SIK2 regulates CRTCs, HDAC4 and glucose uptake in adipocytes

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

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

Regulation role of CRTC3 in skeletal muscle and adipose tissue

The cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA) signaling pathway plays important role in regulating energy homeostasis. Many of the effects of the cAMP-PKA signaling is mediated through the cAMP responsive element binding protein (CREB) and its coactivator CREB-regulated transcription coactivators (CRTCs). CRTC3 is a member of CRTCs family proteins and plays important roles in glucose and energy metabolism. Previous studies show that global knockout of CRTC3 enhances oxygen consumption and energy expenditure and subsequently protects the knockout animal against obesity. In skeletal muscle, CRTC3 affects lipid and glycogen metabolism and mitochondrial biogenesis. In white adipocytes, CRTC3 regulates GLUT4 expression and glucose uptake. More recently, the localization and function of CRTC3 in brown fat have been reported. In this review, we mainly discuss the regulatory role of CRTC3 in skeletal muscle and adipose tissues.

Deregulation of CRTCs in Aging and Age-Related Disease Risk

Advances in public health in the past century have seen a sharp increase in human life expectancy. With these changes have come an increased prevalence of age-related pathologies and health burdens in the elderly. Patient age is the biggest risk factor for multiple chronic conditions that often occur simultaneously within a single individual. An alternative to disease-centric therapeutic approaches is that of 'geroscience', which aims to define molecular mechanisms that link age to overall disease risk. One such mechanism is deregulation of CREB-regulated transcriptional coactivators (CRTCs). Initially identified for their role in modulating CREB transcription, the past 5 years has seen an expansion in knowledge of new cellular regulators and roles of CRTCs beyond CREB. CRTCs have been shown to modulate organismal aging in Caenorhabditis elegans and to impact on age-related diseases in humans. We discuss CRTC deregulation as a new driver of aging that integrates the link between age and disease risk.

Rosiglitazone drives cavin-2/SDPR expression in adipocytes in a CEBPα-dependent manner

Caveolae are abundant adipocyte surface domains involved in insulin signaling, membrane trafficking and lipid homeostasis. Transcriptional control mechanisms for caveolins and cavins, the building blocks of caveolae, are thus arguably important for adipocyte biology and studies in this area may give insight into insulin resistance and diabetes. Here we addressed the hypothesis that one of the less characterized caveolar components, cavin-2 (SDPR), is controlled by CCAAT/Enhancer Binding Protein (CEBPα) and Peroxisome Proliferator-Activated Receptor Gamma (PPARG). Using human mRNA expression data we found that SDPR correlated with PPARG in several tissues. This was also observed during differentiation of 3T3-L1 fibroblasts into adipocytes. Treatment of 3T3-L1-derived adipocytes with the PPARγ-activator Rosiglitazone increased SDPR and CEBPα expression at both the mRNA and protein levels. Silencing of CEBPα antagonized these effects. Further, adenoviral expression of PPARγ/CEBPα or Rosiglitazone-treatment increased SDPR expression in primary rat adipocytes. The myocardin family coactivator MKL1 was recently shown to regulate SDPR expression in human coronary artery smooth muscle cells. However, we found that actin depolymerization, known to inhibit MKL1 and MKL2, was without effect on SDPR mRNA levels in adipocytes, even though overexpression of MKL1 and MKL2 had the capacity to increase caveolins and cavins and to repress PPARγ/CEBPα. Altogether, this work demonstrates that CEBPα expression and PPARγ-activity promote SDPR transcription and further supports the emerging notion that PPARγ/CEBPα and MKL1/MKL2 are antagonistic in adipocytes.

Analysis of a cAMP regulated coactivator family reveals an alternative phosphorylation motif for AMPK family members

The second messenger cAMP stimulates cellular gene expression via the PKA-mediated phosphorylation of the transcription factor CREB and through dephosphorylation of the cAMP-responsive transcriptional coactivators (CRTCs). Under basal conditions, CRTCs are phosphorylated by members of the AMPK family of Ser/Thr kinases and sequestered in the cytoplasm via a phosphorylation-dependent association with 14-3-3 proteins. Increases in cAMP promote the dephosphorylation and nuclear translocation of CRTCs, where they bind to CREB and stimulate relevant target genes. Although they share considerable sequence homology, members of the CRTC family exert non-overlapping effects on cellular gene expression through as yet unidentified mechanisms. Here we show that the three CRTCs exhibit distinct patterns of 14-3-3 binding at three conserved sites corresponding to S70, S171, and S275 (in CRTC2). S171 functions as the gatekeeper site for 14-3-3 binding; it acts cooperatively with S275 in stabilizing this interaction following its phosphorylation by the cAMP-responsive SIK and the cAMP-nonresponsive MARK kinases. Although S171 contains a consensus recognition site for phosphorylation by AMPK family members, S70 and S275 carry variant motifs (MNTGGS₂₇₅LPDL), lacking basic residues that are otherwise critical for SIK/MARK recognition as well as 14-3-3 binding. Correspondingly, the activity of these motifs differs between CRTC family members. As the variant (SLPDL) motif is present and apparently phosphorylated in other mammalian proteins, our studies suggest that the regulation of cellular targets by AMPK family members is more extensive than previously appreciated.

Salt-inducible kinase 2 and -3 are downregulated in adipose tissue from obese or insulin-resistant individuals: implications for insulin signalling and glucose uptake in human adipocytes

AIMS/HYPOTHESIS

Salt-inducible kinases (SIKs) are related to the metabolic regulator AMP-activated protein kinase (AMPK). SIK2 is abundant in adipose tissue. The aims of this study were to investigate the expression of SIKs in relation to human obesity and insulin resistance, and to evaluate whether changes in the expression of SIKs might play a causal role in the development of disturbed glucose uptake in human adipocytes.

METHODS

SIK mRNA and protein was determined in human adipose tissue or adipocytes, and correlated to clinical variables. SIK2 and SIK3 expression and phosphorylation were analysed in adipocytes treated with TNF-α. Glucose uptake, GLUT protein levels and localisation, phosphorylation of protein kinase B (PKB/Akt) and the SIK substrate histone deacetylase 4 (HDAC4) were analysed after the SIKs had been silenced using small interfering RNA (siRNA) or inhibited using a pan-SIK-inhibitor (HG-9-91-01).

RESULTS

We demonstrate that SIK2 and SIK3 mRNA are downregulated in adipose tissue from obese individuals and that the expression is regulated by weight change. SIK2 is also negatively associated with in vivo insulin resistance (HOMA-IR), independently of BMI and age. Moreover, SIK2 protein levels and specific kinase activity display a negative correlation to BMI in human adipocytes. Furthermore, SIK2 and SIK3 are downregulated by TNF-α in adipocytes. Silencing or inhibiting SIK1-3 in adipocytes results in reduced phosphorylation of HDAC4 and PKB/Akt, less GLUT4 at the plasma membrane, and lower basal and insulin-stimulated glucose uptake in adipocytes.

CONCLUSION/INTERPRETATION

This is the first study to describe the expression and function of SIKs in human adipocytes. Our data suggest that SIKs might be protective in the development of obesity-induced insulin resistance, with implications for future treatment strategies.

SIK2 Restricts Autophagic Flux To Support Triple-Negative Breast Cancer Survival

Triple-negative breast cancer (TNBC) is a highly heterogeneous disease with multiple, distinct molecular subtypes that exhibit unique transcriptional programs and clinical progression trajectories. Despite knowledge of the molecular heterogeneity of the disease, most patients are limited to generic, indiscriminate treatment options: cytotoxic chemotherapy, surgery, and radiation. To identify new intervention targets in TNBC, we used large-scale, loss-of-function screening to identify molecular vulnerabilities among different oncogenomic backgrounds. This strategy returned salt inducible kinase 2 (SIK2) as essential for TNBC survival. Genetic or pharmacological inhibition of SIK2 leads to increased autophagic flux in both normal-immortalized and tumor-derived cell lines. However, this activity causes cell death selectively in breast cancer cells and is biased toward the claudin-low subtype. Depletion of ATG5, which is essential for autophagic vesicle formation, rescued the loss of viability following SIK2 inhibition. Importantly, we find that SIK2 is essential for TNBC tumor growth in vivo Taken together, these findings indicate that claudin-low tumor cells rely on SIK2 to restrain maladaptive autophagic activation. Inhibition of SIK2 therefore presents itself as an intervention opportunity to reactivate this tumor suppressor mechanism.

SIKs control osteocyte responses to parathyroid hormone

Parathyroid hormone (PTH) activates receptors on osteocytes to orchestrate bone formation and resorption. Here we show that PTH inhibition of SOST (sclerostin), a WNT antagonist, requires HDAC4 and HDAC5, whereas PTH stimulation of RANKL, a stimulator of bone resorption, requires CRTC2. Salt inducible kinases (SIKs) control subcellular localization of HDAC4/5 and CRTC2. PTH regulates both HDAC4/5 and CRTC2 localization via phosphorylation and inhibition of SIK2. Like PTH, new small molecule SIK inhibitors cause decreased phosphorylation and increased nuclear translocation of HDAC4/5 and CRTC2. SIK inhibition mimics many of the effects of PTH in osteocytes as assessed by RNA-seq in cultured osteocytes and following in vivo administration. Once daily treatment with the small molecule SIK inhibitor YKL-05-099 increases bone formation and bone mass. Therefore, a major arm of PTH signalling in osteocytes involves SIK inhibition, and small molecule SIK inhibitors may be applied therapeutically to mimic skeletal effects of PTH.

Parathyroid hormone (PTH) is an endogenous hormone and osteoporosis therapeutic that suppresses sclerostin activity. Here the authors develop SIK inhibitors as potential therapeutic tools and use them to show that PTH-cAMP signalling in osteocytes inhibits SIK2 from driving Hdac4/5 nuclear shuttling to suppress sclerostin.

MiR-155 Enhances Insulin Sensitivity by Coordinated Regulation of Multiple Genes in Mice

miR-155 plays critical roles in numerous physiological and pathological processes, however, its function in the regulation of blood glucose homeostasis and insulin sensitivity and underlying mechanisms remain unknown. Here, we reveal that miR-155 levels are downregulated in serum from type 2 diabetes (T2D) patients, suggesting that miR-155 might be involved in blood glucose control and diabetes. Gain-of-function and loss-of-function studies in mice demonstrate that miR-155 has no effects on the pancreatic β-cell proliferation and function. Global transgenic overexpression of miR-155 in mice leads to hypoglycaemia, improved glucose tolerance and insulin sensitivity. Conversely, miR-155 deficiency in mice causes hyperglycemia, impaired glucose tolerance and insulin resistance. In addition, consistent with a positive regulatory role of miR-155 in glucose metabolism, miR-155 positively modulates glucose uptake in all cell types examined, while mice overexpressing miR-155 transgene show enhanced glycolysis, and insulin-stimulated AKT and IRS-1 phosphorylation in liver, adipose tissue or skeletal muscle. Furthermore, we reveal these aforementioned phenomena occur, at least partially, through miR-155-mediated repression of important negative regulators (i.e. C/EBPβ, HDAC4 and SOCS1) of insulin signaling. Taken together, these findings demonstrate, for the first time, that miR-155 is a positive regulator of insulin sensitivity with potential applications for diabetes treatment.

Development of Chemical Probes for Investigation of Salt-Inducible Kinase Function in Vivo

Salt-inducible kinases (SIKs) are promising therapeutic targets for modulating cytokine responses during innate immune activation. The study of SIK inhibition in animal models of disease has been limited by the lack of selective small-molecule probes suitable for modulating SIK function in vivo. We used the pan-SIK inhibitor HG-9-91-01 as a starting point to develop improved analogs, yielding a novel probe 5 (YKL-05-099) that displays increased selectivity for SIKs versus other kinases and enhanced pharmacokinetic properties. Well-tolerated doses of YKL-05-099 achieve free serum concentrations above its IC50 for SIK2 inhibition for >16 h and reduce phosphorylation of a known SIK substrate in vivo. While in vivo active doses of YKL-05-099 recapitulate the effects of SIK inhibition on inflammatory cytokine responses, they did not induce metabolic abnormalities observed in Sik2 knockout mice. These results identify YKL-05-099 as a useful probe to investigate SIK function in vivo and further support the development of SIK inhibitors for treatment of inflammatory disorders.

Transformation by different oncogenes relies on specific metabolic adaptations

Metabolic adaptations are emerging as common traits of cancer cells and tumor progression. In vitro transformation of NIH 3T3 cells allows the analysis of the metabolic changes triggered by a single oncogene. In this work, we have compared the metabolic changes induced by H-RAS and by the nuclear resident mutant of histone deacetylase 4 (HDAC4). RAS-transformed cells exhibit a dominant aerobic glycolytic phenotype characterized by up-regulation of glycolytic enzymes, reduced oxygen consumption and a defect in complex I activity. In this model of transformation, glycolysis is strictly required for sustaining the ATP levels and the robust cellular proliferation. By contrast, in HDAC4/TM transformed cells, glycolysis is only modestly up-regulated, lactate secretion is not augmented and, instead, mitochondrial oxygen consumption is increased. Our results demonstrate that cellular transformation can be accomplished through different metabolic adaptations and HDAC4/TM cells can represent a useful model to investigate oncogene-driven metabolic changes besides the Warburg effect.

Activation of salt-inducible kinase 2 promotes the viability of peritoneal mesothelial cells exposed to stress of peritoneal dialysis

Maintaining mesothelial cell viability is critical to long-term successful peritoneal dialysis (PD) treatment. To clarify the viability mechanism of peritoneal mesothelial cells under PD solutions exposure, we examined the mechanisms of cellular response to this stress conditions. Here we report that the proteasome activity is inhibited when treated with PD solutions. Proteasome inhibition-mediated activation of salt-inducible kinase 2 (SIK2), an endoplasmic reticulum-resident protein, is important for mesothelial cell viability. SIK2 is mobilized to promote autophagy and protect the cells from apoptosis under PD solution or MG132 treatment. Immunofluorescence staining showed that SIK2 is colocalized with LC3B in the autophagosomes of mesothelial cells treated with PD solution or derived from patients undergoing PD treatment. SIK2 activation is likely via a two-step mechanism, upstream kinases relieving the autoinhibitory conformation of SIK2 molecule followed by autophosphorylation of Thr175 and activation of kinase activity. These results suggest that activation of SIK2 is required for the cell viability when proteasome activity is inhibited by PD solutions. Maintaining or boosting the activity of SIK2 may promote peritoneal mesothelial cell viability and evolve as a potential therapeutic target for maintaining or restoring peritoneal membrane integrity in PD therapy.

Transcriptional regulation of the miR-212/miR-132 cluster in insulin-secreting β-cells by cAMP-regulated transcriptional co-activator 1 and salt-inducible kinases

MicroRNAs are central players in the control of insulin secretion, but their transcriptional regulation is poorly understood. Our aim was to investigate cAMP-mediated transcriptional regulation of the miR-212/miR-132 cluster and involvement of further upstream proteins in insulin secreting β-cells. cAMP induced by forskolin+IBMX or GLP-1 caused increased expression of miR-212/miR-132, and elevated phosphorylation of cAMP-response-element-binding-protein (CREB)/Activating-transcription-factor-1 (ATF1) and Salt-Inducible-Kinases (SIKs). CyclicAMP-Regulated Transcriptional Co-activator-1 (CRTC1) was concomitantly dephosphorylated and translocated to the nucleus. Silencing of miR-212/miR-132 reduced, and overexpression of miR-212 increased, glucose-stimulated insulin secretion. Silencing of CRTC1 expression resulted in decreased insulin secretion and miR-212/miR-132 expression, while silencing or inhibition of SIKs was associated with increased expression of the microRNAs and dephosphorylation of CRTC1. CRTC1 protein levels were reduced after silencing of miR-132, suggesting feed-back regulation. Our data propose cAMP-dependent co-regulation of miR-212/miR-132, in part mediated through SIK-regulated CRTC1, as an important factor for fine-tuned regulation of insulin secretion.

Regulation of class IIa HDAC activities: it is not only matter of subcellular localization

In response to environmental cues, enzymes that influence the functions of proteins, through reversible post-translational modifications supervise the coordination of cell behavior like orchestral conductors. Class IIa histone deacetylases (HDACs) belong to this category. Even though in vertebrates these deacetylases have discarded the core enzymatic activity, class IIa HDACs can assemble into multiprotein complexes devoted to transcriptional reprogramming, including but not limited to epigenetic changes. Class IIa HDACs are subjected to variegated and interconnected layers of regulation, which reflect the wide range of biological responses under the scrutiny of this gene family. Here, we discuss about the key mechanisms that fine tune class IIa HDACs activities.

HDAC4 mediates IFN-γ induced disruption of energy expenditure-related gene expression by repressing SIRT1 transcription in skeletal muscle cells

Metabolic homeostasis is achieved through balanced energy storage and output. Impairment of energy expenditure is a hallmark event in patients with obesity and type 2 diabetes. Previously we have shown that the pro-inflammatory cytokine interferon gamma (IFN-γ) disrupts energy expenditure in skeletal muscle cells via hypermethylated in cancer 1 (HIC1)-class II transactivator (CIITA) dependent repression of SIRT1 transcription. Here we report that repression of SIRT1 transcription by IFN-γ paralleled loss of histone acetylation on the SIRT1 promoter region with simultaneous recruitment of histone deacetylase 4 (HDAC4). IFN-γ activated HDAC4 in vitro and in vivo by up-regulating its expression and stimulating its nuclear accumulation. HIC1 and CIITA recruited HDAC4 to the SIRT1 promoter and cooperated with HDAC4 to repress SIRT1 transcription. HDAC4 depletion by small interfering RNA or pharmaceutical inhibition normalized histone acetylation on the SIRT1 promoter and restored SIRT1 expression in the presence of IFN-γ. Over-expression of HDAC4 suppressed the transcription of genes involved in energy expenditure in a SIRT1-dependent manner. In contrast, HDAC4 knockdown/inhibition neutralized the effect of IFN-γ on cellular metabolism by normalizing SIRT1 expression. Therefore, our data reveal a role for HDAC4 in regulating cellular energy output and as such provide insights into rationalized design of novel anti-diabetic therapeutics.

Skeletal muscle salt inducible kinase 1 promotes insulin resistance in obesity

OBJECTIVE

Insulin resistance causes type 2 diabetes mellitus and hyperglycemia due to excessive hepatic glucose production and inadequate peripheral glucose uptake. Our objectives were to test the hypothesis that the proposed CREB/CRTC2 inhibitor salt inducible kinase 1 (SIK1) contributes to whole body glucose homeostasis in vivo by regulating hepatic transcription of gluconeogenic genes and also to identify novel SIK1 actions on glucose metabolism.

METHODS

We created conditional (floxed) SIK1-knockout mice and studied glucose metabolism in animals with global, liver, adipose or skeletal muscle Sik1 deletion. We examined cAMP-dependent regulation of SIK1 and the consequences of SIK1 depletion on primary mouse hepatocytes. We probed metabolic phenotypes in tissue-specific SIK1 knockout mice fed high fat diet through hyperinsulinemic-euglycemic clamps and biochemical analysis of insulin signaling.

RESULTS

SIK1 knockout mice are viable and largely normoglycemic on chow diet. On high fat diet, global SIK1 knockout animals are strikingly protected from glucose intolerance, with both increased plasma insulin and enhanced peripheral insulin sensitivity. Surprisingly, liver SIK1 is not required for regulation of CRTC2 and gluconeogenesis, despite contributions of SIK1 to hepatocyte CRTC2 and gluconeogenesis regulation ex vivo. Sik1 mRNA accumulates in skeletal muscle of obese high fat diet-fed mice, and knockout of SIK1 in skeletal muscle, but not liver or adipose tissue, improves insulin sensitivity and muscle glucose uptake on high fat diet.

CONCLUSIONS

SIK1 is dispensable for glycemic control on chow diet. SIK1 promotes insulin resistance on high fat diet by a cell-autonomous mechanism in skeletal muscle. Our study establishes SIK1 as a promising therapeutic target to improve skeletal muscle insulin sensitivity in obese individuals without deleterious effects on hepatic glucose production.

Beyond energy homeostasis: the expanding role of AMP-activated protein kinase in regulating metabolism

The recent exciting advances in our understanding of the regulation of the energy sensor AMP-activated protein kinase (AMPK), together with renewed appreciation of its importance in maintaining cellular function, brought together leading scientists at a recent FASEB-sponsored meeting in September 2014. Here, we report some of the highlights of this conference.