Increases in intracellular sodium activate transcription and gene expression via the salt-inducible kinase 1 network in an atrial myocyte cell line

Cardiac hypertrophy (CH) generally occurs as the result of the sustained mechanical stress caused by elevated systemic arterial blood pressure (BP). However, in animal models, elevated salt intake is associated with CH even in the absence of significant increases in BP. We hypothesize that CH is not exclusively the consequence of mechanical stress but also of other factors associated with elevated BP such as abnormal cell sodium homeostasis. We examined the effect of small increases in intracellular sodium concentration ([Na(+)](i)) on transcription factors and genes associated with CH in a cardiac cell line. Increases in [Na(+)](i) led to a time-dependent increase in the expression levels of mRNA for natriuretic peptide and myosin heavy chain genes and also increased myocyte enhancer factor (MEF)2/nuclear factor of activated T cell (NFAT) transcriptional activity. Increases in [Na(+)](i) are associated with activation of salt-inducible kinase 1 (snflk-1, SIK1), a kinase known to be critical for cardiac development. Moreover, increases in [Na(+)](i) resulted in increased SIK1 expression. Sodium did not increase MEF2/NFAT activity or gene expression in cells expressing a SIK1 that lacked kinase activity. The mechanism by which SIK1 activated MEF2 involved phosphorylation of HDAC5. Increases in [Na(+)](i) activate SIK1 and MEF2 via a parallel increase in intracellular calcium through the reverse mode of Na(+)/Ca(2+)-exchanger and activation of CaMK1. These data obtained in a cardiac cell line suggest that increases in intracellular sodium could influence myocardial growth by controlling transcriptional activation and gene expression throughout the activation of the SIK1 network.

SIK3 is essential for chondrocyte hypertrophy during skeletal development in mice

Chondrocyte hypertrophy is crucial for endochondral ossification, but the mechanism underlying this process is not fully understood. We report that salt-inducible kinase 3 (SIK3) deficiency causes severe inhibition of chondrocyte hypertrophy in mice. SIK3-deficient mice showed dwarfism as they aged, whereas body size was unaffected during embryogenesis. Anatomical and histological analyses revealed marked expansion of the growth plate and articular cartilage regions in the limbs, accumulation of chondrocytes in the sternum, ribs and spine, and impaired skull bone formation in SIK3-deficient mice. The primary phenotype in the skeletal tissue of SIK3-deficient mice was in the humerus at E14.5, where chondrocyte hypertrophy was markedly delayed. Chondrocyte hypertrophy was severely blocked until E18.5, and the proliferative chondrocytes occupied the inside of the humerus. Consistent with impaired chondrocyte hypertrophy in SIK3-deficient mice, native SIK3 expression was detected in the cytoplasm of prehypertrophic and hypertrophic chondrocytes in developing bones in embryos and in the growth plates in postnatal mice. HDAC4, a crucial repressor of chondrocyte hypertrophy, remained in the nuclei in SIK3-deficient chondrocytes, but was localized in the cytoplasm in wild-type hypertrophic chondrocytes. Molecular and cellular analyses demonstrated that SIK3 was required for anchoring HDAC4 in the cytoplasm, thereby releasing MEF2C, a crucial facilitator of chondrocyte hypertrophy, from suppression by HDAC4 in nuclei. Chondrocyte-specific overexpression of SIK3 induced closure of growth plates in adulthood, and the SIK3-deficient cartilage phenotype was rescued by transgenic SIK3 expression in the humerus. These results demonstrate an essential role for SIK3 in facilitating chondrocyte hypertrophy during skeletogenesis and growth plate maintenance.

Salt-inducible kinase is involved in the regulation of corticotropin-releasing hormone transcription in hypothalamic neurons in rats

Activation of CRH transcription requires phosphorylation of cAMP response element-binding protein (CREB) and translocation of the CREB coactivator, transducer of regulated CREB activity (TORC) from cytoplasm to nucleus. In basal conditions, transcription is low because TORC remains in the cytoplasm, inactivated by phosphorylation through Ser/Thr protein kinases of the AMP-dependent protein kinases (AMPK) family, including salt-inducible kinase (SIK). To determine which kinase is responsible for TORC phosphorylation in CRH neurons, we measured SIK1 and SIK2 mRNA in the hypothalamic paraventricular nucleus of rats by in situ hybridization. In basal conditions, low mRNA levels of the two kinases were found in the dorsomedial paraventricular nucleus, consistent with location in CRH neurons. One hour of restraint stress increased SIK1 mRNA levels, whereas SIK2 mRNA showed only minor increases. In 4B hypothalamic neurons, or primary cultures, SIK1 mRNA (but not SIK2 mRNA) was inducible by the cAMP stimulator, forskolin. Overexpression of either SIK1 or SIK2 in 4B cells reduced nuclear TORC2 levels (Western blot) and inhibited forskolin-stimulated CRH transcription (luciferase assay). Conversely, the nonselective SIK inhibitor, staurosporine, increased nuclear TORC2 content and stimulated CRH transcription in 4Bcells and primary neuronal cultures (heteronuclear RNA). Unexpectedly, in 4B cells specific short hairpin RNA knockdown of endogenous SIK2 but not SIK1 induced nuclear translocation of TORC2 and CRH transcription, suggesting that SIK2 mediates TORC inactivation in basal conditions, whereas induction of SIK1 limits transcriptional activation. The study provides evidence that SIK represses CRH transcription by inactivating TORC, providing a potential mechanism for rapid on/off control of CRH transcription.

Cocaine induces the expression of MEF2C transcription factor in rat striatum through activation of SIK1 and phosphorylation of the histone deacetylase HDAC5

Distinct forms of MEF2 transcription factor act as positive or negative regulators of dendritic spine formation, with MEF2C playing a key regulator role in synapse plasticity. We report here that acute cocaine treatment of rats induced the expression of MEF2C in the striatum through a recently discovered transduction pathway. Repeated injections were found to induce MEF2C to a lesser extent. The mechanism by which MEF2C was induced involves the subsequent activation of the salt-inducible kinase SIK1 and the phosphorylation of HDAC5, a member of the class IIa of HDACs. Cocaine activated SIK1 by phosphorylation on Thr-182 residue, which was accompanied by the nuclear import of the kinase. In the nuclear compartment, SIK1 then phosphorylated HDAC5 causing the shuttling of its phospho-form from the nucleus to the cytoplasm of striatal cells. Activation of SIK1 by cocaine was further validated by the phosphorylation of TORC1/3, which was followed by the shuttling of TORC proteins from the nucleus to the cytoplasm. Activation of MEF2C was assessed by measuring the expression of the MEF2C gene itself, since the gene is known to be under the control of its own product. Since MEF2C plays a key role in memory/learning processes, activation of this pathway by cocaine is probably involved in plasticity mechanisms whereby the drug establishes its long-term effects such as drug dependence.

Carbohydrate metabolism is perturbed in peroxisome-deficient hepatocytes due to mitochondrial dysfunction, AMP-activated protein kinase (AMPK) activation, and peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) suppression

Hepatic peroxisomes are essential for lipid conversions that include the formation of mature conjugated bile acids, the degradation of branched chain fatty acids, and the synthesis of docosahexaenoic acid. Through unresolved mechanisms, deletion of functional peroxisomes from mouse hepatocytes (L-Pex5(-/-) mice) causes severe structural and functional abnormalities at the inner mitochondrial membrane. We now demonstrate that the peroxisomal and mitochondrial anomalies trigger energy deficits, as shown by increased AMP/ATP and decreased NAD(+)/NADH ratios. This causes suppression of gluconeogenesis and glycogen synthesis and up-regulation of glycolysis. As a consequence, L-Pex5(-/-) mice combust more carbohydrates resulting in lower body weights despite increased food intake. The perturbation of carbohydrate metabolism does not require a long term adaptation to the absence of functional peroxisomes as similar metabolic changes were also rapidly induced by acute elimination of Pex5 via adenoviral administration of Cre. Despite its marked activation, peroxisome proliferator-activated receptor α (PPARα) was not causally involved in these metabolic perturbations, because all abnormalities still manifested when peroxisomes were eliminated in a peroxisome proliferator-activated receptor α null background. Instead, AMP-activated kinase activation was responsible for the down-regulation of glycogen synthesis and induction of glycolysis. Remarkably, PGC-1α was suppressed despite AMP-activated kinase activation, a paradigm not previously reported, and they jointly contributed to impaired gluconeogenesis. In conclusion, lack of functional peroxisomes from hepatocytes results in marked disturbances of carbohydrate homeostasis, which are consistent with adaptations to an energy deficit. Because this is primarily due to impaired mitochondrial ATP production, these L-Pex5-deficient livers can also be considered as a model for secondary mitochondrial hepatopathies.

Salt-inducible kinase 1 is present in lung alveolar epithelial cells and regulates active sodium transport

Salt-inducible kinase 1 (SIK1) in epithelial cells mediates the increases in active sodium transport (Na(+), K(+)-ATPase-mediated) in response to elevations in the intracellular concentration of sodium. In lung alveolar epithelial cells increases in active sodium transport in response to β-adrenergic stimulation increases pulmonary edema clearance. Therefore, we sought to determine whether SIK1 is present in lung epithelial cells and to examine whether isoproterenol-dependent stimulation of Na(+), K(+)-ATPase is mediated via SIK1 activity. All three SIK isoforms were present in airway epithelial cells, and in alveolar epithelial cells type 1 and type 2 from rat and mouse lungs, as well as from human and mouse cell lines representative of lung alveolar epithelium. In mouse lung epithelial cells, SIK1 associated with the Na(+), K(+)-ATPase α-subunit, and isoproterenol increased SIK1 activity. Isoproterenol increased Na(+), K(+)-ATPase activity and the incorporation of Na(+), K(+)-ATPase molecules at the plasma membrane. Furthermore, those effects were abolished in cells depleted of SIK1 using shRNA, or in cells overexpressing a SIK1 kinase-deficient mutant. These results provide evidence that SIK1 is present in lung epithelial cells and that its function is relevant for the action of isoproterenol during regulation of active sodium transport. As such, SIK1 may constitute an important target for drug discovery aimed at improving the clearance of pulmonary edema.

cAMP stimulation of StAR expression and cholesterol metabolism is modulated by co-expression of labile suppressors of transcription and mRNA turnover

The steroidogenic acute regulatory (StAR) protein is generated in rodents from 1.6 kb and 3.5 kb mRNA formed by alternative polyadenylation. The zinc finger protein, TIS11B (also Znf36L1), is elevated by cAMP in adrenal cells in parallel with StAR mRNA. TIS11b selectively destabilizes the 3.5 kb mRNA through AU-rich sequences at the end of the 3'UTR. siRNA suppression shows that TIS11b surprisingly increases StAR protein and cholesterol metabolism. StAR transcription is directly activated by PKA phosphorylation. cAMP responsive element binding (CREB) protein 1 phosphorylation is a key step leading to recruitment of the co-activator, CREB binding protein (CBP). A second protein, CREB regulated transcription coactivator (TORC/CRTC), enhances this recruitment, but is inhibited by salt inducible kinase (SIK). Basal StAR transcription is constrained through this phosphorylation of TORC. PKA provides an alternative stimulation by phosphorylating SIK, which prevents TORC inactivation. PKA stimulation of StAR nuclear transcripts substantially precedes TORC recruitment to the StAR promoter, which may, therefore, mediate a later step in mRNA production. Inhibition of SIK by staurosporine elevates StAR transcription and TORC recruitment to maximum levels, but without CREB phosphorylation. TORC suppression by SIK evidently limits basal StAR transcription. Staurosporine and cAMP stimulate synergistically. SIK targets the phosphatase, PP2a (activation), and Type 2 histone de-acetylases (inhibition), which may each contribute to suppression. Staurosporine stimulation through SIK inhibition is repeated in cAMP stimulation of many steroidogenic genes regulated by steroidogenic factor 1 (SF-1) and CREB. TIS11b and SIK may combine to attenuate StAR expression when hormonal stimuli decline.

SIK2 is a key regulator for neuronal survival after ischemia via TORC1-CREB

The cAMP responsive element-binding protein (CREB) functions in a broad array of biological and pathophysiological processes. We found that salt-inducible kinase 2 (SIK2) was abundantly expressed in neurons and suppressed CREB-mediated gene expression after oxygen-glucose deprivation (OGD). OGD induced the degradation of SIK2 protein concomitantly with the dephosphorylation of the CREB-specific coactivator transducer of regulated CREB activity 1 (TORC1), resulting in the activation of CREB and its downstream gene targets. Ca(2+)/calmodulin-dependent protein kinase I/IV are capable of phosphorylating SIK2 at Thr484, resulting in SIK2 degradation in cortical neurons. Neuronal survival after OGD was significantly increased in neurons isolated from sik2(-/-) mice, and ischemic neuronal injury was significantly reduced in the brains of sik2(-)(/-) mice subjected to transient focal ischemia. These findings suggest that SIK2 plays critical roles in neuronal survival, is modulated by CaMK I/IV, and regulates CREB via TORC1.

Phosphorylation of the CREB-specific coactivator TORC2 at Ser(307) regulates its intracellular localization in COS-7 cells and in the mouse liver

The CREB-specific coactivator TORC2 (also known as CRTC2) upregulates gluconeogenic gene expression in the liver. Salt-inducible kinase (SIK) family enzymes inactivate TORC2 through phosphorylation and localize it in the cytoplasm. Ser(171) and Ser(275) were found to be phosphorylated in pancreatic beta-cells. Calcineurin (Cn) is proposed as the Ser(275) phosphatase, because its inhibitor cyclosporin A (CsA) stabilizes phospho-Ser(275) and retains TORC2 in the cytoplasm. Because the regulation of dephosphorylation at Ser(171) has not been fully clarified, we performed experiments with a range of doses of okadaic acid (OA), an inhibitor of PP2A/PP1, and with overexpression of various phosphatases and found that PP1 functions as an activator for TORC2, whereas PP2A acts as an inhibitor. In further studies using TORC2 mutants, we detected a disassociation between the intracellular distribution and the transcription activity of TORC2. Additional mutant analyses suggested the presence of a third phosphorylation site, Ser(307). The Ser(307)-disrupted TORC2 was constitutively localized in the nucleus, but its coactivator activity was normally suppressed by SIK1 in COS-7 cells. CsA, but not OA, stabilized the phosphogroup at Ser(307), suggesting that differential dephosphorylation at Ser(171) and Ser(307) cooperatively regulate TORC2 activity and that the nuclear localization of TORC2 is insufficient to function as a coactivator. Because the COS-7 cell line may not possess signaling cascades for gluconeogenic programs, we next examined the importance of Ser(307) and Ser(171) for TORC2's function in mouse liver. Levels of phosphorylation at Ser(171) and Ser(307) changed in response to fasting or fed conditions and insulin resistance of the mouse liver, which were modified by treatment with CsA/OA and by overexpression of PP1/PP2A/Cn. These results suggest that multiple phosphorylation sites and their phosphatases may play important roles in regulating TORC2/CREB-mediated gluconeogenic programs in the liver.

Downregulation of SIK2 expression promotes the melanogenic program in mice

cAMP response element-binding protein (CREB) promotes melanogenesis by inducing microphthalmia-associated transcription factor (Mitf ) gene expression. We report here that the CREB-specific coactivator TORC and its repressor, salt-inducible kinase 2 (SIK2), are fundamental determinants of the melanogenic program in mice. Exposure of B16 melanoma cells to ultraviolet (UV) light results in the immediate nuclear translocation of TORC1, which is inhibited by SIK2. Overexpression of dominant-negative TORC1 also inhibits UV-induced Mitf gene expression and melanogenesis. α-MSH signaling regulates hair pigmentation, and the decrease in α-MSH activity in hair follicle melanocytes switches the melanin synthesis from eumelanin (black) to pheomelanin (yellow). Mice with the lethal yellow allele of agouti (A(y)) have yellow hair because of impaired activation of the α-MSH receptor. To examine the involvement of SIK2 in the regulation of the melanogenesis switch in vivo, we prepared SIK2-knockout mice, and the Sik2(-/-) genotype was introduced into A(y)/a mice. The resultant Sik2(-/-); A(y)/a mice had brown hair, indicating that SIK2 represses eumelanogenesis in mice.

Involvement of SIK2/TORC2 signaling cascade in the regulation of insulin-induced PGC-1alpha and UCP-1 gene expression in brown adipocytes

Salt-inducible kinase 2 (SIK2) is expressed abundantly in adipose tissues and represses cAMP-response element-binding protein (CREB)-mediated gene expression by phosphorylating the coactivator transducer of regulated CREB activity (TORC2). Phosphorylation at Ser(587) of SIK2 diminishes its TORC2 phosphorylation activity. In 3T3-L1 white adipocytes, SIK2 downregulates lipogenic gene in response to nutritional stresses. To investigate the impact of SIK2 on the function of brown adipose tissue (BAT), we used T37i brown adipocytes, mice with diet-induced obesity, and SIK2 mutant (S587A) transgenic mice. When T37i adipocytes were treated with insulin, the levels of peroxisome proliferator-activated receptor-coactivator-1alpha (PGC-1alpha) and uncoupling protein-1 (UCP-1) mRNA were increased, and the induction was inhibited by overexpression of SIK2 (S587A) mutant or dominant-negative CREB. Insulin enhanced SIK2 phosphorylation at Ser(587), which was accompanied by decrease in phospho-TORC2. Similarly, the decrease in the level of SIK2 phosphorylation at Ser(587) was observed in the BAT of mice with diet-induced obesity, which was negatively correlated with TORC2 phosphorylation. To confirm the negative correlation between SIK2 phosphorylation at Ser(587) and TORC2 phosphorylation in BAT, SIK2 mutant (S587A) was overexpressed in adipose tissues by using the adipocyte fatty acid-binding protein 2 promoter. The expression of recombinant SIK2 (S587A) was restricted to BAT, and the levels of phospho-TORC2 were elevated in BAT of transgenic mice. Male transgenic mice developed high-fat diet-induced obesity, and their BAT expressed low levels of PGC-1alpha and UCP-1 mRNA, suggesting that SIK2-TORC2 cascade may be important for the regulation of PGC-1alpha and UCP-1 gene expression in insulin signaling in BAT.

Expression and regulated nuclear transport of transducers of regulated CREB 1 in retinal ganglion cells

Calcium- and cAMP-dependent activation of CREB and transcription of cAMP-responsive element (CRE)-target genes play critical roles in various physiological and pathological conditions. TORCs (transducers of regulated CREB) represent a new family of conserved CREB coactivators that function as intracellular calcium- and cAMP-sensitive coincidence detectors, controlling the kinetics of CRE-mediated responses and long-term potentiation of synaptic transmission. Here we examined the expression and activity-dependent translocation of TORCs in adult retinal ganglion cells (RGCs), the primary target of acute retinal ischemic injury as well as chronic retinal degenerative diseases. We found that both mRNAs of TORC1 and TORC2, but not TORC3, were enriched in adult rat retina. Comparing with TORC2, TORC1 protein was highly and selectively expressed in RGCs. At resting condition, TORC1 protein was localized in the cytoplasm but not nucleus of RGCs. Activation of N-methyl-D-aspartate (NMDA) receptors by intravitreous injection of NMDA or increase of cAMP signaling by administration of forskolin triggered nuclear accumulation of TORC1. Furthermore, transient retinal ischemic injury resulted in peri-nuclear and nuclear accumulation of TORC1 as well as transcription of BDNF in RGCs. Our results demonstrate that TORC1 is enriched in RGCs and its subcellular location could be regulated by Ca(2+) and cAMP, suggesting that manipulation of TORC1 activity may promote survival of RGCs in some optic disease conditions.

Inactivation of HDAC5 by SIK1 in AICAR-treated C2C12 myoblasts

Salt inducible kinase (SIK) 1, a member of the AMP-activated kinase (AMPK) family, is activated by the AMPK-activator LKB1 which phosphorylates SIK1 at Thr182. The activated SIK1 then auto-phosphorylates its Ser186 located at the +4 position of Thr182. The phospho-Ser186 is essential for sustained activity of SIK1, which is maintained by sequential phosphorylation at Ser186-Thr182 by glycogen synthase kinase (GSK)-3beta. Meanwhile, SIK1 represses the transcription factor cAMP-response element binding protein (CREB) by phosphorylating its co-activator transducer of regulated CREB activity (TORC). Recently, histone deacetylase (HDAC) 5 was identified as a new substrate of SIK1. Inhibition of SIK1 or AMPK results in the stimulation of glyconeogensis in the liver by enhancing dephosphorylation of TORC2 followed by up-regulation of peroxisome proliferator-activated receptor coactivator (PGC)-1alpha gene expression. However, expression of the PGC-1alpha gene has been found to be repressed in LKB1-defective muscle cells. Our findings show that the AMPK agonist 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR)-dependent expression of PGC-1alpha is diminished by inhibitors of GSK-3beta or SIKs in C2C12 myoblasts. Treatment with AICAR or the overexpression of SIK1 induces nuclear export of HDAC5 followed by the activation of myogenic transcription factor (MEF)-2C. The levels of phosphorylation at Thr182 and Ser186 of SIK1 in AICAR-treated C2C12 cells are elevated, and GSK-3beta enzyme purified from AICAR-treated cells shows enhanced phosphorylation activity of SIK1 in vitro. These observations suggest that GSK-3 beta and SIK1 may play important roles in the regulation of PGC-1alpha gene expression by inactivating HDAC5 followed by activation of MEF2C.

Importance of autophosphorylation at Ser186 in the A-loop of salt inducible kinase 1 for its sustained kinase activity

Autophosphorylation is an important mechanism by which protein kinases regulate their own biological activities. Salt inducible kinase 1 (SIK1) is a regulator in the feedback cascades of cAMP-mediated gene expression, while its kinase domain also features autophosphorylation activity. We provide evidence that Ser186 in the activation loop is the site of autophosphorylation and essential for the kinase activity. Ser186 is located at the +4 position of the critical Thr residue Thr182, which is phosphorylated by upstream kinases such as LKB1. The relationship between phosphorylation at Ser186 and at Thr182 in COS-7 cells indicates that the former is a prerequisite for the latter. Glycogen synthase kinase-3beta (GSK-3beta) phosphorylates Ser/Thr residues located at the fourth position ahead of the pre-phosphorylated Ser/Thr residues, and inhibitors of GSK-3beta reduce the phosphorylation at Thr182. The results of an in vitro reconstitution assay also indicate that GSK-3beta could be the SIK1 kinase. However, overexpression and knockdown of GSK-3beta in LKB1-defective HeLa cells suggests that GSK-3beta alone may not be able to phosphorylate or activate SIK1, indicating that LKB1 may play a crucial role by phosphorylating SIK1 at Thr182, possibly as an initiator of the autophosphorylation cascade, and GSK-3beta may phosphorylate SIK1 at Thr182 by recognizing the priming-autophosphorylation at Ser186 in cultured cells. This may also be the case for the other isoform SIK2, but not for SIK3.

SIK2 can be activated by deprivation of nutrition and it inhibits expression of lipogenic genes in adipocytes

OBJECTIVE

Determine the biological function of SIK2 in 3T3-L1 adipocytes.

METHODS AND PROCEDURES

3T3-L1 adipocytes were treated with serum-free, serum/glucose-free, or complete Dulbecco's Modified Eagle's Medium (DMEM) for 15 min, or 10 microg/ml oligomycin in serum-free DMEM for 15 min, or different doses of 5-amino-4-imidazolecarboxamide (AICAR) for different periods of time in serum-free DMEM. After treatment, SIK2 kinase activity was measured by examining phosphorylation of a peptide substrate. SIK2 was overexpressed and knocked down in 3T3-L1 CAR adipocytes using adenovirus-mediated gene transfer for the assessment of lipogenic gene expression and triglyceride content. Electroporation was used to transiently transfect 3T3-L1 adipocytes with fatty acid synthase (FAS) promoter-driven luciferase construct to evaluate the effect of SIK2 on FAS transcription.

RESULTS

In 3T3-L1 adipocytes, SIK2 can be activated by nutrient deprivation, inhibition of ATP synthesis, and treatment of AICAR. Overexpression of SIK2 repressed the expression of lipogenic genes, including FAS, acetyl CoA carboxylase 2 (ACC2), and stearoyl CoA desaturase 1 (SCD1), and reduced triglyceride content. Reduction of endogenous SIK2 expression through RNA interference increased the expression of FAS, ACC2, and SCD1. This effect is independent of adenosine monophosphate-activated protein kinase (AMPK) since neither phosphorylation state nor protein level of AMPKalpha1 and AMPKalpha2 was affected by SIK2 overexpression. SIK2 inhibits the expression of FAS-promoter driven luciferase reporter gene, and this effect can be reversed by overexpression of constitutively active sterol regulatory element binding protein-1 (SREBP-1). Furthermore, SIK2 also reduces the nuclear translocation of endogenous SREBP-1.

DISCUSSION

Our results support a role for SIK2 in adipocyte energy metabolism. SIK2 may function similarly to AMPK for turning off lipogenesis in low-energy state.

Regulation of CREB-mediated gene expression by salt inducible kinase

Salt inducible kinase (SIK) was identified as a molecule induced in the adrenal glands of rats fed with a high-salt diet. A major downstream of SIK is regulation of camp-responsive element (CRE)-dependent gene expression. SIK represses the activity of CRE-binding protein (CREB) by phosphorylating a CREB-specific co-activator transducer of regulated CREB activity (TORC). When TORC is dephosphorylated it activates CREB in a CREB-phosphorylation independent manner. The importance of the dephosphorylation of TORC has been suggested by the fact that a kinase inhibitor staurosporine induces dephosphorylation of TORC and upregulates the gene expression of CYP11A, CYP11B1, CYP11B2 and StAR in adrenocortical cells. The identification of SIK caused a stir in the field of CREB studies and led to disclosure of cascades hidden behind the classical mechanism for CREB activity.

TORC-SIK cascade regulates CREB activity through the basic leucine zipper domain

The transcription factor cAMP response element-binding protein (CREB) plays important roles in gene expression induced by cAMP signaling and is believed to be activated when its Ser133 is phosphorylated. However, the discovery of Ser133-independent activation by the activation of transducer of regulated CREB activity coactivators (TORC) and repression by salt inducible kinase cascades suggests that Ser133-independent regulation of CREB is also important. The activation and repression are mediated by the basic leucine zipper domain of CREB. In this review, we focus on the basic leucine zipper domain in the regulation of transcriptional activity of CREB and describe the functions of TORC and salt inducible kinase.

SIK1 is a class II HDAC kinase that promotes survival of skeletal myocytes

During physical exercise, increases in motor neuron activity stimulate the expression of muscle-specific genes through the myocyte enhancer factor 2 (MEF2) family of transcription factors. Elevations in intracellular calcium increase MEF2 activity via the phosphorylation-dependent inactivation of class II histone deacetylases (HDACs). In studies to determine the role of the cAMP responsive element binding protein (CREB) in skeletal muscle, we found that mice expressing a dominant-negative CREB transgene (M-ACREB mice) exhibited a dystrophic phenotype along with reduced MEF2 activity. Class II HDAC phosphorylation was decreased in M-ACREB myofibers due to a reduction in amounts of Snf1lk (encoding salt inducible kinase, SIK1), a CREB target gene that functions as a class II HDAC kinase. Inhibiting class II HDAC activity either by viral expression of Snf1lk or by the administration of a small molecule antagonist improved the dystrophic phenotype in M-ACREB mice, pointing to an important role for the SIK1-HDAC pathway in regulating muscle function.

Dephosphorylation of TORC initiates expression of the StAR gene

Cyclic AMP responsive element (CRE) binding protein (CREB) is known to activate transcription when its Ser133 is phosphorylated. However, transducer of regulated CREB activity (TORC), a CREB specific co-activator, upregulates CREB activity in a phospho-Ser133-independent manner. Interestingly, TORC is also regulated by phosphorylation; the phospho-form is inactive, and the dephospho-form active. When PKA phosphorylates CREB, it inhibits TORC kinases simultaneously and accelerates dephosphorylation of TORC. We show in this report that staurosporine, a kinase inhibitor, induces the expression of the StAR gene in Y1 adrenocortical cells, possibly a result of an increase in the population of dephospho-TORC. The expression of the StAR gene is known to be regulated by SF-1 and CREB, and the co-activators CBP/p300 may mediate the actions of both factors. Our experiments using KG501, a disruptor of the interaction between phospho-CREB and CBP/p300, also support the importance of TORC in the regulation of StAR gene expression.

Requirement of TORC1 for late-phase long-term potentiation in the hippocampus

Late-phase long-term potentiation (L-LTP) and long-term memory depend on the transcription of mRNA of CRE-driven genes and synthesis of proteins. However, how synaptic signals propagate to the nucleus is unclear. Here we report that the CREB coactivator TORC1 (transducer of regulated CREB activity 1) undergoes neuronal activity-induced translocation from the cytoplasm to the nucleus, a process required for CRE-dependent gene expression and L-LTP. Overexpressing a dominant-negative form of TORC1 or down-regulating TORC1 expression prevented activity-dependent transcription of CREB target genes in cultured hippocampal neurons, while overexpressing a wild-type form of TORC1 facilitated basal and activity-induced transcription of CREB target genes. Furthermore, overexpressing the dominant-negative form of TORC1 suppressed the maintenance of L-LTP without affecting early-phase LTP, while overexpressing the wild-type form of TORC1 facilitated the induction of L-LTP in hippocampal slices. Our results indicate that TORC1 is essential for CRE-driven gene expression and maintenance of long-term synaptic potentiation.