Functional interactions of peroxisome proliferator-activated receptor, retinoid-X receptor, and Sp1 in the transcriptional regulation of the acyl-coenzyme-A oxidase promoter.

Peroxisome proliferator-activated receptor (PPARs) are members of the nuclear receptor superfamily. For transcriptional activation of their target genes, PPARs heterodimerize with the retinoid-X receptor (RXR). The convergence of the PPAR and RXR signaling pathways has been shown to have an important function in lipid metabolism. The promoter of the gene encoding the acyl-coenzyme-A oxidase (ACO), the rate-limiting enzyme in peroxisomal beta-oxidation of fatty acids, is a target site of PPAR action. In this study, we examined the role and the contribution of both cis-and trans-acting factors in the transcriptional regulation of this gene using transient transfections in insect cells. We identified several functional cis-acting elements present in the promoter of the ACO gene and established that PPAR-dependent as well as PPAR-independent mechanisms can activate the ACO promoter in these cells. We show that the PPAR/RXR heterodimer exerts its effect through two response elements within the ACO promoter, in synergy with the transcription factor Sp1 via five Sp1-binding sites. Furthermore, this functional interaction also occurs when Sp1 is co-expressed with PPAR or RXR alone, indicating that activation can occur independently of PPAR/RXR heterodimers.

Activation of peroxisome proliferator-activated receptor-β/-δ (PPAR-β/-δ) ameliorates insulin signaling and reduces SOCS3 levels by inhibiting STAT3 in interleukin-6-stimulated adipocytes.

OBJECTIVE It has been suggested that interleukin (IL)-6 is one of the mediators linking obesity-derived chronic inflammation with insulin resistance through activation of STAT3, with subsequent upregulation of suppressor of cytokine signaling 3 (SOCS3). We evaluated whether peroxisome proliferator-activated receptor (PPAR)-β/-δ prevented activation of the IL-6-STAT3-SOCS3 pathway and insulin resistance in adipocytes. RESEARCH DESIGN AND METHODS First, we observed that the PPAR-β/-δ agonist GW501516 prevented both IL-6-dependent reduction in insulin-stimulated Akt phosphorylation and glucose uptake in adipocytes. In addition, this drug treatment abolished IL-6-induced SOCS3 expression in differentiated 3T3-L1 adipocytes. This effect was associated with the capacity of the drug to prevent IL-6-induced STAT3 phosphorylation on Tyr(705) and Ser(727) residues in vitro and in vivo. Moreover, GW501516 prevented IL-6-dependent induction of extracellular signal-related kinase (ERK)1/2, a serine-threonine-protein kinase involved in serine STAT3 phosphorylation. Furthermore, in white adipose tissue from PPAR-β/-δ-null mice, STAT3 phosphorylation (Tyr(705) and Ser(727)), STAT3 DNA-binding activity, and SOCS3 protein levels were higher than in wild-type mice. Several steps in STAT3 activation require its association with heat shock protein 90 (Hsp90), which was prevented by GW501516 as revealed in immunoprecipitation studies. Consistent with this finding, the STAT3-Hsp90 association was enhanced in white adipose tissue from PPAR-β/-δ-null mice compared with wild-type mice. CONCLUSIONS Collectively, our findings indicate that PPAR-β/-δ activation prevents IL-6-induced STAT3 activation by inhibiting ERK1/2 and preventing the STAT3-Hsp90 association, an effect that may contribute to the prevention of cytokine-induced insulin resistance in adipocytes.

Antigen Receptor Signaling to NF-kappa B via CARMA1, BCL10, and MALT1

The signaling pathway controlling antigen receptor-induced regulation of the transcription factor NF-kappa B plays a key role in lymphocyte activation and development and the generation of lymphomas. Work of the past decade has led to dramatic progress in the identification and characterization of new players in the pathway. Moreover, novel enzymatic activities relevant for this pathway have been discovered, which represent interesting drug targets for immuno-suppression or lymphoma treatment. Here, we summarize these findings and give an outlook on interesting open issues that need to be addressed in the future.

Identification of CARDIAK, a RIP-like kinase that associates with caspase-1.

Members of the tumor necrosis factor receptor (TNFR) superfamily have an important role in the induction of cellular signals resulting in cell growth, differentiation and death. TNFR-1 recruits and assembles a signaling complex containing a number of death domain (DD)-containing proteins, including the adaptor protein TRADD and the serine/threonine kinase RIP, which mediates TNF-induced NF-kappa B activation. RIP also recruits caspase-2 to the TNFR-1 signaling complex via the adaptor protein RAIDD, which contains a DD and a caspase-recruiting domain (CARD). Here, we have identified a RIP-like kinase, termed CARDIAK (for CARD-containing interleukin (IL)-1 beta converting enzyme (ICE) associated kinase), which contains a serine/threonine kinase domain and a carboxy-terminal CARD. Overexpression of CARDIAK induced the activation of both NF-kappa B and Jun N-terminal kinase (JNK). CARDIAK interacted with the TNFR-associated factors TRAF-1 and TRAF-2, and a dominant-negative form of TRAF-2 inhibited CARDIAK-induced NF-kappa B activation. Interestingly, CARDIAK specifically interacted with the CARD of caspase-1 (previously known as ICE), and this interaction correlated with the processing of pro-caspase-1 and the formation of the active p20 subunit of caspase-1. Together, these data suggest that CARDIAK may be involved in NF-kappa B/JNK signaling and in the generation of the proinflammatory cytokine IL-1 beta through activation of caspase-1.

The scaffold protein IB1/JIP-1 is a critical mediator of cytokine-induced apoptosis in pancreatic beta cells.

In insulin-secreting cells, cytokines activate the c-Jun N-terminal kinase (JNK), which contributes to a cell signaling towards apoptosis. The JNK activation requires the presence of the murine scaffold protein JNK-interacting protein 1 (JIP-1) or human Islet-brain 1(IB1), which organizes MLK3, MKK7 and JNK for proper signaling specificity. Here, we used adenovirus-mediated gene transfer to modulate IB1/JIP-1 cellular content in order to investigate the contribution of IB1/JIP-1 to beta-cell survival. Exposure of the insulin-producing cell line INS-1 or isolated rat pancreatic islets to cytokines (interferon-gamma, tumor necrosis factor-alpha and interleukin-1beta) induced a marked reduction of IB1/JIP-1 content and a concomitant increase in JNK activity and apoptosis rate. This JNK-induced pro-apoptotic program was prevented in INS-1 cells by overproducing IB1/JIP-1 and this effect was associated with inhibition of caspase-3 cleavage. Conversely, reducing IB1/JIP-1 content in INS-1 cells and isolated pancreatic islets induced a robust increase in basal and cytokine-stimulated apoptosis. In heterozygous mice carrying a selective disruption of the IB1/JIP-1 gene, the reduction in IB1/JIP-1 content in happloinsufficient isolated pancreatic islets was associated with an increased JNK activity and basal apoptosis. These data demonstrate that modulation of the IB1-JIP-1 content in beta cells is a crucial regulator of JNK signaling pathway and of cytokine-induced apoptosis.

Function of TRADD in tumor necrosis factor receptor 1 signaling and in TRIF-dependent inflammatory responses.

Tumor necrosis factor receptor 1 (TNFR1) and Toll-like receptors (TLRs) regulate immune and inflammatory responses. Here we show that the TNFR1-associated death domain protein (TRADD) is critical in TNFR1, TLR3 and TLR4 signaling. TRADD deficiency abrogated TNF-induced apoptosis, prevented recruitment of the ubiquitin ligase TRAF2 and ubiquitination of the adaptor RIP1 in the TNFR1 signaling complex, and considerably inhibited but did not completely abolish activation of the transcription factor NF-kappaB and mitogen-activated protein kinases 'downstream' of TNFR1. TRIF-dependent cytokine production induced by the synthetic double-stranded RNA poly(I:C) and lipopolysaccharide was lower in TRADD-deficient mice than in wild-type mice. Moreover, TRADD deficiency inhibited poly(I:C)-mediated RIP1 ubiquitination and activation of NF-kappaB and mitogen-activated protein kinase signaling in fibroblasts but not in bone marrow macrophages. Thus, TRADD is an essential component of TNFR1 signaling and has a critical but apparently cell type-specific function in TRIF-dependent TLR responses.

Molecular characterization of signalling complexes involved in G-protein coupled receptor-induced cardiac hypertrophy

In response to pathological stresses, the heart undergoes a remodelling process associated with cardiac hypertrophy. Since sustained hypertrophy can progress to heart failure, there is an intense investigation about the intracellular signalling pathways that control cardiomyocyte growth. Accumulating evidence has demonstrated that most stimuli known to initiate pathological changes associated with the development of cardiac hypertrophy activate G protein-coupled receptors (GPCRs) including the αl-adrenergic- (αl-AR), Angiotensin II- (AT-R) and endothelin-1- (ET-R) receptors. In this context, we have previously identified a cardiac scaffolding protein, called AKAP-Lbc (Α-kinase anchoring protein), with an intrinsic Rho specific guanine nucleotide exchange factor activity, that plays a key role in integrating and transducing hypertrophic signals initiated by these GPCRs (Appert-Collin, Cotecchia et al. 2007). Activated RhoA controls the transcriptional activation of genes involved in cardiomyocyte hypertrophy through signalling pathways that remain to be characterized. Here, we identified the nuclear factor-Kappa Β (NF-κΒ) activating kinase ΙΚΚβ as a novel AKAP-Lbc interacting protein. This raises the hypothesis that AKAP-Lbc might promote cardiomyocyte growth by maintaining a signalling complex that promotes the activation of the pro-hypertrophic transcription factor NF-κΒ. In fact, the activation of NF- κΒ-dependent transcription has been detected in numerous disease contexts, including hypertrophy, ischemia/reperfusion injury, myocardial infarction, allograft rejection, myocarditis, apoptosis, and more (Hall, Hasday et al. 2006). While it is known by more than a decade that NF-κΒ is a critical mediator of cardiac hypertrophy, it is currently poorly understood how pro-hypertrophic signals controlling NF-κΒ transcriptional activity are integrated and coordinated within cardiomyocytes. In this study, we show that AKAP-Lbc and ΙΚΚβ form a transduction complex in cardiomyocytes that couples activation of αl-ARs to NF-κB-mediated transcriptional reprogramming events associated with cardiomyocyte hypertrophy. In particular, we can show that activation of ΙΚΚβ within the AKAP-Lbc complex promotes NF-κB-dependent production of interleukine-6 (IL-6), which, in turn, enhances foetal gene expression. These findings indicate that the AKAP-Lbc/ΙΚΚβ complex is critical for selectively directing catecholamine signals to the induction of cardiomyocyte hypertrophy.

Tau hyperphosphorylation and increased BACE1 and RAGE levels in the cortex of PPARβ/δ-null mice.

The role of peroxisome proliferator activator receptor (PPAR)β/δ in the pathogenesis of Alzheimer's disease has only recently been explored through the use of PPARβ/δ agonists. Here we evaluated the effects of PPARβ/δ deficiency on the amyloidogenic pathway and tau hyperphosphorylation. PPARβ/δ-null mice showed cognitive impairment in the object recognition task, accompanied by enhanced DNA-binding activity of NF-κB in the cortex and increased expression of IL-6. In addition, two NF-κB-target genes involved in β-amyloid (Aβ) synthesis and deposition, the β site APP cleaving enzyme 1 (Bace1) and the receptor for advanced glycation endproducts (Rage), respectively, increased in PPARβ/δ-null mice compared to wild type animals. The protein levels of glial fibrillary acidic protein (GFAP) increased in the cortex of PPARβ/δ-null mice, which would suggest the presence of astrogliosis. Finally, tau hyperphosphorylation at Ser199 and enhanced levels of PHF-tau were associated with increased levels of the tau kinases CDK5 and phospho-ERK1/2 in the cortex of PPARβ/δ(-/-) mice. Collectively, our findings indicate that PPARβ/δ deficiency results in cognitive impairment associated with enhanced inflammation, astrogliosis and tau hyperphosphorylation in the cortex.

The orphan receptor CRF2-4 is an essential subunit of the interleukin 10 receptor.

The orphan receptor CRF2-4 is a member of the class II cytokine receptor family (CRF2), which includes the interferon receptors, the interleukin (IL) 10 receptor, and tissue factor. CRFB4, the gene encoding CRF2-4, is located within a gene cluster on human chromosome 21 that comprises three interferon receptor subunits. To elucidate the role of CRF2-4, we disrupted the CRFB4 gene in mice by means of homologous recombination. Mice lacking CRF2-4 show no overt abnormalities, grow normally, and are fertile. CRF2-4 deficient cells are normally responsive to type I and type II interferons, but lack responsiveness to IL-10. By approximately 12 wk of age, the majority of mutant mice raised in a conventional facility developed a chronic colitis and splenomegaly. Thus, CRFB4 mutant mice recapitulate the phenotype of IL-10-deficient mice. These findings suggest that CRF2-4 is essential for IL-10-mediated effects and is a subunit of the IL-10 receptor.

Protein kinase A-dependent phosphorylation of GLUT2 in pancreatic beta cells.

In pancreatic beta cells, cyclic AMP-dependent protein kinase regulates many cellular processes including the potentiation of insulin secretion. The substrates for this kinase, however, have not been biochemically characterized. Here we demonstrate that the glucose transporter GLUT2 is rapidly phosphorylated by protein kinase A following activation of adenylyl cyclase by forskolin or the incretin hormone glucagon-like peptide-1. We show that serines 489 and 501/503 and threonine 510 in the carboxyl-terminal tail of the transporter are the in vitro and in vivo sites of phosphorylation. Stimulation of GLUT2 phosphorylation in beta cells reduces the initial rate of 3-O-methyl glucose uptake by approximately 48% but does not change the Michaelis constant. Similar differences in transport kinetics are observed when comparing the transport activity of GLUT2 mutants stably expressed in insulinoma cell lines and containing glutamates or alanines at the phosphorylation sites. These data indicate that phosphorylation of GLUT2 carboxyl-terminal tail modifies the rate of transport. This lends further support for an important role of the transporter cytoplasmic tail in the modulation of catalytic activity. Finally, because activation of protein kinase A stimulates glucose-induced insulin secretion, we discuss the possible involvement of GLUT2 phosphorylation in the amplification of the glucose signaling process.

Control of the peroxisomal beta-oxidation pathway by a novel family of nuclear hormone receptors.

Three novel members of the Xenopus nuclear hormone receptor superfamily have been cloned. They are related to each other and similar to the group of receptors that includes those for thyroid hormones, retinoids, and vitamin D3. Their transcriptional activity is regulated by agents causing peroxisome proliferation and carcinogenesis in rodent liver. All three Xenopus receptors activate the promoter of the acyl coenzyme A oxidase gene, which encodes the key enzyme of peroxisomal fatty acid beta-oxidation, via a cognate response element that has been identified. Therefore, peroxisome proliferators may exert their hypolipidemic effects through these receptors, which stimulate the peroxisomal degradation of fatty acids. Finally, the multiplicity of these receptors suggests the existence of hitherto unknown cellular signaling pathways for xenobiotics and putative endogenous ligands.

Ins1 (Cre) knock-in mice for beta cell-specific gene recombination.

AIMS/HYPOTHESIS: Pancreatic beta cells play a central role in the control of glucose homeostasis by secreting insulin to stimulate glucose uptake by peripheral tissues. Understanding the molecular mechanisms that control beta cell function and plasticity has critical implications for the pathophysiology and therapy of major forms of diabetes. Selective gene inactivation in pancreatic beta cells, using the Cre-lox system, is a powerful approach to assess the role of particular genes in beta cells and their impact on whole body glucose homeostasis. Several Cre recombinase (Cre) deleter mice have been established to allow inactivation of genes in beta cells, but many show non-specific recombination in other cell types, often in the brain. METHODS: We describe the generation of Ins1 (Cre) and Ins1 (CreERT2) mice in which the Cre or Cre-oestrogen receptor fusion protein (CreERT2) recombinases have been introduced at the initiation codon of the Ins1 gene. RESULTS: We show that Ins1 (Cre) mice induce efficient and selective recombination of floxed genes in beta cells from the time of birth, with no recombination in the central nervous system. These mice have normal body weight and glucose homeostasis. Furthermore, we show that tamoxifen treatment of adult Ins1 (CreERT2) mice crossed with Rosa26-tdTomato mice induces efficient recombination in beta cells. CONCLUSIONS/INTERPRETATION: These two strains of deleter mice are useful new resources to investigate the molecular physiology of pancreatic beta cells.

Covalent assembly of a soluble T cell receptor-peptide-major histocompatibility class I complex.

We used stepwise photochemical cross-linking for specifically assembling soluble and covalent complexes made of a T-cell antigen receptor (TCR) and a class I molecule of the major histocompatibility complex (MHC) bound to an antigenic peptide. For that purpose, we have produced in myeloma cells a single-chain Fv construct of a TCR specific for a photoreactive H-2Kd-peptide complex. Photochemical cross-linking of this TCR single-chain Fv with a soluble form of the photoreactive H-2Kd-peptide ligand resulted in the formation of a ternary covalent complex. We have characterized the soluble ternary complex and showed that it reacted with antibodies specific for epitopes located either on the native TCR or on the Kd molecules. By preventing the fast dissociation kinetics observed with most T cell receptors, this approach provides a means of preparing soluble TCR-peptide-MHC complexes on large-scale levels.

Differential T cell receptor photoaffinity labeling among H-2Kd restricted cytotoxic T lymphocyte clones specific for a photoreactive peptide derivative. Labeling of the alpha-chain correlates with J alpha segment usage.

Using a direct binding assay based on photoaffinity labeling, we studied the interaction of T cell receptor (TCR) with a Kd-bound photoreactive peptide derivative on living cells. The Kd-restricted Plasmodium berghei circumsporozoite (PbCS) peptide 253-260 (YIPSAEKI) was reacted NH2-terminally with biotin and at the TCR contact residue Lys259 with photoreactive iodo, 4-azido salicylic acid (IASA) to make biotin-YIPSAEK(IASA)I. Cytotoxic T lymphocyte (CTL) clones derived from mice immunized with this derivative recognized this conjugate, but not a related one lacking the IASA group nor the parental PbCS peptide. The clones were Kd restricted. Recognition experiments with variant conjugates, lacking substituents from IASA, revealed a diverse fine specificity pattern and indicated that this group interacted directly with the TCR. The TCR of four clones could be photoaffinity labeled by biotin-YIPSAEK(125IASA)I. This labeling was dependent on the conjugates binding to the Kd molecule and was selective for the TCR alpha (2 clones) or beta chain (1 clone), or was common for both chains (1 clone). TCR sequence analysis showed a preferential usage of J alpha TA28 containing alpha chains that were paired with V beta 1 expressing beta chains. The TCR that were photoaffinity labeled at the alpha chain expressed these J alpha and V beta segments. The tryptophan encoded by the J alpha TA28 segment is rarely found in other J alpha segments. Moreover, we show that the IASA group interacts preferentially with tryptophan in aqueous solution. We thus propose that for these CTL clones, labeling of the alpha chain occurs via the J alpha-encoded tryptophan residue.