Increased hypothalamic expression of the p75 tumor necrosis factor receptor in New Zealand obese mice

Previous studies have demonstrated that tumor necrosis factor-alpha (TNF-alpha) production from adipose tissue is elevated in obese animal models and in obese humans. It plays an important role in the induction of insulin resistance in experimental animals. In this study, we examined hypothalamic tissue expression of TNF-alpha and its receptors and TNF-alpha expression of adipose tissue in lean C57BLKSJ+/+ and obese polygenic New Zealand obese (NZO) mice. Obese animals exhibited hyperglycemia, hyperinsulinemia, hypertriglyceridemia, and hypercholesterinemia. Using RT-PCR, we observed increased expression (2.4-fold) of TNF receptor 2 (p75) in the hypothalamus of obese mice. TNF-alpha expression in adipose tissue of obese mice was eight times higher than in controls. TNF-alpha and TNF receptor 1 (p55) expression in hypothalamic tissue was similar in obese and lean animals. These results suggest that the hypothalamic TNF receptor 2 (p75) might play a role in obesity by modulating the actions of TNF-alpha in conditions of leptin resistance.

Targeting of GLUT6 (formerly GLUT9) and GLUT8 in rat adipose cells

The subcellular targeting of the two recently cloned novel mammalian glucose transporters, GLUT6 [previously referred to as GLUT9 [Doege, Bocianski, Joost and Schürmann (2000) Biochem. J. 350, 771-776] and GLUT8, was analysed by expression of haemagglutinin (HA)-epitope-tagged GLUTs in transiently transfected primary rat adipose cells. Similar to HA-GLUT4, both transporters, HA-GLUT6 and HA-GLUT8, were retained in intracellular compartments in non-stimulated cells. In contrast, mutation of the N-terminal dileucine motifs in both constructs led to constitutive expression of the proteins on the plasma membrane. Likewise, when endocytosis was blocked by co-expression of a dominant-negative mutant of the dynamin GTPase, wild-type HA-GLUT6 and HA-GLUT8 accumulated on the cell surface. However, in contrast with HA-GLUT4, no translocation of HA-GLUT6 and HA-GLUT8 to the plasma membrane was observed when the cells were stimulated with insulin, phorbol ester or hyperosmolarity. Thus GLUT6 and GLUT8 appear to recycle in a dynamin-dependent manner between internal membranes and the plasma membrane in rat adipose cells, but are unresponsive to stimuli that induce translocation of GLUT4.

Phorbol ester-induced activation of mitogen-activated protein kinase/extracellular-signal-regulated kinase kinase and extracellular-signal-regulated protein kinase decreases glucose-6-phosphatase gene expression

Glucose-6-phosphatase (G6Pase) plays a central role in blood glucose homoeostasis, and insulin suppresses G6Pase gene expression by the activation of phosphoinositide 3-kinase (PI 3-kinase). Here, we show that the phorbol ester PMA decreases both basal and dexamethasone/cAMP-induced expression of a luciferase gene under the control of the G6Pase promoter in transiently transfected H4IIE hepatoma cells. This regulation was suppressed by the inhibitors of the mitogen-activated protein kinase/extracellular-signal-regulated kinase kinase (MEK), PD98059 and U0126, but not by the inhibitor of PI 3-kinase, LY294002. The co-expression of a constitutively active mutant of MEK mimicked the regulation of G6Pase promoter activity by PMA. The effect of PMA on both basal and induced G6Pase gene transcription was impaired by the overexpression of a dominant negative MEK construct, as well as by the expression of mitogen-activated protein kinase phosphatase-1. The mutation of the forkhead-binding sites within the insulin-response unit of the G6Pase promoter, which decreases the effect of insulin on G6Pase gene expression, did not alter the regulation of gene expression by PMA. The data show that PMA decreases G6Pase gene expression by the activation of MEK and extracellular-signal regulated protein kinase. With that, PMA mimics the effect of insulin on G6Pase gene expression by a different signalling pathway.

Differential regulation of endogenous glucose-6-phosphatase and phosphoenolpyruvate carboxykinase gene expression by the forkhead transcription factor FKHR in H4IIE-hepatoma cells

The insulin responsive H4IIEC3 rat hepatoma cell line (H4 cells) was used in order to determine the role of the transcription factor FKHR in the regulation of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase). Both PEPCK and G6Pase contain putative FKHR binding sites in their promoter sequence. Using a retroviral expression system, we stably overexpressed FKHR in H4-cells. FKHR was phosphorylated in a PI 3-kinase- and Akt-dependent manner, and was translocated from the nucleus to the cytoplasm in response to insulin. Furthermore, overexpression of FKHR markedly increased the expression of the catalytic subunit of G6Pase (basal about 2.5-fold, dexamethasone/cAMP stimulated about fivefold, respectively). In contrast, both basal and dexamethasone/cAMP-induced levels of PEPCK mRNA were unaffected by FKHR-overexpression. These data suggest a specific function for FKHR in the regulation of hepatic gluconeogenesis at the level of G6Pase, but not PEPCK gene expression.

PSCA expression is regulated by phorbol ester and cell adhesion in the bladder carcinoma cell line RT112

The expression of the surface protein prostate stem cell antigen (PSCA) in prostate carcinoma increases in parallel with the progression of the tumor. In contrast, we have recently shown that PSCA expression is reduced or undetectable in other types of undifferentiated tumors. To elucidate the cellular mechanisms that underlie this complex pattern of expression, we studied regulatory parameters for PSCA expression in the bladder carcinoma cell line RT112 by Northern analysis. PSCA gene expression was stimulated by a culture dish surface that caused aggregation of cells, suggesting that its expression is regulated by mechanisms related to the adhesion of epithelial cells. Phorbol ester markedly stimulated PSCA gene expression in a cycloheximide- and actinomycin-inhibitable manner after a lag phase of 10 h, indicating that transcription of the PSCA gene is regulated by protein kinase C and a newly synthesized protein. In contrast, epidermal growth factor, platelet-derived growth factor (PDGF)-BB, tumor necrosis factor-alpha, interferon-gamma or a slightly lowered pH failed to increase PSCA mRNA levels. Consistent with the variable expression of PSCA in different tumors, our analysis in RT112 cells shows that its expression is controlled by a strongly inducible promoter that is specifically regulated by extracellular signals.

Quantitative trait loci for obesity and insulin resistance (Nob1, Nob2) and their interaction with the leptin receptor allele (LeprA720T/T1044I) in New Zealand obese mice

AIMS/HYPOTHESIS

To locate genes responsible for obesity and insulin resistance, a backcross model of New Zealand obese (NZO) mice with the lean Swiss/Jackson Laboratory (SJL) strain was stablished.

RESULTS

In female NZO x F1 backcross mice, two major quantitative trait loci for variables of obesity (body weight, body mass index, total body fat) and insulin resistance (hyperinsulinaemia) were identified on chromosomes 5 (Nob1) and 19 (Nob2) close to the markers D5Mit392 and D19Mit91. The aberrant alleles have presumably contributed by the NZO genome. Whereas Nob1 contributed mainly to higher body weight, Nob2 seemed to mainly aggravate insulin resistance independent of obesity. The leptin receptor variant of NZO (LeprA720T/T1044I) failed to alter any of the variables of obesity. It seemed, however, to enhance the effect of Nob1 on body weight and that of Nob2 on serum insulin concentration. When expressed in COS-7 cells, LeprA720T/T10441 produced a normal basal and maximum activation with a minor increase in the EC50 of leptin.

CONCLUSIONS/INTERPRETATION

The data identify two new quantitative trait loci that are responsible for a major part of obesity and hyperinsulinaemia as produced by recessive genes in NZO mice. LeprA720T/T1044I alone cannot produce obesity, but may enhance the effects of other obesity/insulin resistance genes in this mouse model.

Glucose deprivation induces Akt-dependent synthesis and incorporation of GLUT1, but not of GLUT4, into the plasma membrane of 3T3-L1 adipocytes

Reduction of the glucose concentration in the culture medium of 3T3-L1 adipose cells below 1.25 mM produces a 4-8-fold stimulation of 2-deoxyglucose uptake which starts after a lag phase of 2 h and is maximal after 10-16 h. In the present study, we employed the 'membrane sheet assay' in order to re-assess the contribution of the transporter isoforms GLUT1 and GLUT4 to this effect. Immunochemical assay of glucose transporters in membranes prepared with the 'sheet assay' revealed that the effect reflected a marked increase of GLUT1 in the plasma membrane with no effect on GLUT4. Glucose deprivation increased the total cellular GLUT1 protein in parallel with the transport activity, whereas GLUT4 was unaltered. The specific PI 3-kinase inhibitor wortmannin inhibited the effect of glucose deprivation on transport activity and also on GLUT1 synthesis. Glucose deprivation produced a moderate, biphasic increase in the activity of the protein kinase Akt/PKB that was inhibitable by wortmannin. When wortmannin was added after stimulation of cells in order to assess the internalization rate of transporters, the effect of insulin was reversed considerably faster (T1/2 = 18 min) than that of glucose deprivation (T1/2 > 60 min). These data are consistent with the conclusion that the effect of glucose deprivation reflects a specific, Akt-dependent de-novo synthesis of GLUT1, and not of GLUT4, and its insertion into a plasma membrane compartment which is distinct from that of the insulin-sensitive GLUT1.

Activity and genomic organization of human glucose transporter 9 (GLUT9), a novel member of the family of sugar-transport facilitators predominantly expressed in brain and leucocytes

The GLUT9 gene encodes a cDNA which exhibits significant sequence similarity with members of the glucose transporter (GLUT) family. The gene is located on chromosome 9q34 and consists of 10 exons separated by short introns. The amino acid sequence deduced from its cDNA predicts 12 putative membrane-spanning helices and all the motifs (sugar-transporter signatures) that have previously been shown to be essential for transport activity. A striking characteristic of GLUT9 is the presence of two arginines in the putative helices 7 and 8 at positions where the organic anion transporters harbour basic residues. The next relative of GLUT9 is the glucose transporter GLUT8/GLUTX1 (44.8% amino acid identity with GLUT9). A 2.6-kb transcript of GLUT9 was detected in spleen, peripheral leucocytes and brain. Transfection of COS-7 cells with GLUT9 produced expression of a 46-kDa membrane protein which exhibited reconstitutable glucose-transport activity and low-affinity cytochalasin-B binding. It is concluded that GLUT9 is a novel member of the family of sugar-transport facilitators with a tissue-specific function.

Reduced expression of PSCA, a member of the LY-6 family of cell surface antigens, in bladder, esophagus, and stomach tumors

Prostate stem cell antigen (PSCA) is a member of the LY-6 family of surface proteins that is overexpressed in prostate cancer. Using serial analysis of gene expression (SAGE), we identified PSCA as one of the most abundant transcripts in a differentiated urothelial tumor. As assessed by Northern blotting, PSCA is highly expressed in normal urothelium and noninvasive urothelial tumors. In contrast to the previously reported overexpression of PSCA in progressive and invasive forms of prostate cancer, we found a markedly reduced expression in undifferentiated bladder carcinoma. In addition, several aberrant splicing products derived from the PSCA gene were found in urothelial tumors. Furthermore, PSCA mRNA was highly abundant in normal esophagus and stomach, but was undetectable in esophageal or gastric tumors. The PSCA expression appeared to depend on cell contact, since mRNA levels were increased when RT112 bladder carcinoma cells were grown to confluence. Our data suggest that PSCA could serve as a potential marker for the early carcinogenesis in urothelial and gastric tissues and that its expression is specific for epithelial cells.

Type 2 diabetes-like hyperglycemia in a backcross model of NZO and SJL mice: characterization of a susceptibility locus on chromosome 4 and its relation with obesity

A backcross model of New Zealand obese mice (NZO) with the lean, atherosclerosis-resistant SJL strain was established to locate genes responsible for obesity, insulin resistance, and type 2 diabetes-like hyperglycemia. In male NZO x F1 backcross mice, a major susceptibility locus for the development of hyperglycemia and hypoinsulinemia (Nidd/SJL) was identified on chromosome 4 between the markers D4Mit278 and D4Mit232, 10-28 cM distal of the previously described Nidd1 locus. The diabetogenic allele has presumably been contributed by the SJL genome, and it appeared to be responsible for approximately 60% of the total prevalence of hyperglycemia. The presence of Nidd/SJL did not alter body weight or weight gain by week 12. Thereafter, it was associated with reduced weight gain or weight loss, presumably as a consequence of decompensated hyperglycemia. In all male backcross mice, the prevalence of hyperglycemia at week 22 increased with the body weight at week 12, suggesting that the development of hyperglycemia was dependent on the degree of obesity. In the absence of Nidd/SJL, mice weighing <50 g at week 12 did not develop hyperglycemia by week 22. In contrast, in animals carrying the diabetogenic allele, the prevalence of hyperglycemia was 20 and 64% when the 12-week weight was <45 and 45-50 g, respectively. These data are consistent with the conclusion that Nidd/SJL represents a diabetes gene that lowers the obesity threshold for the development of hyperglycemia and hypoinsulinemia.

Regulation of subcellular distribution of GLUT4 in cardiomyocytes: Rab4A reduces basal glucose transport and augments insulin responsiveness

Members of the Rab subfamily of small-GTP binding proteins have been suggested to be involved in insulin-regulated translocation of the glucose transporter GLUT4. To directly study this process in muscle tissue, we have established an insulin-sensitive cardiac cell line (H9K6) stably overexpressing GLUT4, which was derived from H9c2 cardiac myoblasts. H9K6-cells were transiently transfected with rab4A and rab3C with an efficiency of 65% and glucose uptake and the cellular distribution and expression of the transporter isoforms GLUT1 and GLUT4 was subsequently determined. Rab3C-overexpression caused no significant change in both basal and insulin-stimulated 2-deoxyglucose uptake compared to control cells transfected with the blank vector. Rab4A was barely detectable in membranes of H9K6 cells. However, after transient transfection this protein was expressed at a level comparable to adult cardiomyocytes. This resulted in a reduction of basal glucose uptake by 31% compared to control cells. Under these conditions insulin was able to stimulate 2-deoxyglucose uptake by 120%. Total expression of GLUT1 and GLUT4 was not affected by Rab4-overexpression. Cell surface biotinylation was used to quantify the abundance of GLUT1 and GLUT4 in the plasma membrane. A decrease of cell surface GLUT4 by about 40% compared to control cells was found in Rab4-overexpressing cells Insulin treatment increased cell surface-GLUT4 by 100% compared to only 26% in control cells. Distribution of GLUT1 was not affected under these conditions. Our data show that Rab4A but not Rab3C is able to reduce basal glucose uptake and cell surface content of GLUT4 in cardiac muscle cells. This results in an increased stimulation of glucose uptake by insulin which can be fully explained by enhanced translocation of GLUT4. We suggest that Rab4A participates in the redistribution of GLUT4 to intracellular pools and represents an essential determinant of the insulin responsiveness of GLUT4 translocation in cardiac muscle cells.

GLUT8, a novel member of the sugar transport facilitator family with glucose transport activity

GLUT8 is a novel glucose transporter-like protein that exhibits significant sequence similarity with the members of the sugar transport facilitator family (29.4% of amino acids identical with GLUT1). Human and mouse sequence (86.2% identical amino acids) comprise 12 putative membrane-spanning helices and several conserved motifs (sugar transporter signatures), which have previously been shown to be essential for transport activity, e.g. GRK in loop 2, PETPR in loop 6, QQLSGVN in helix 7, DRAGRR in loop 8, GWGPIPW in helix 10, and PETKG in the C-terminal tail. An expressed sequence tag (STS A005N15) corresponding with the 3'-untranslated region of GLUT8 has previously been mapped to human chromosome 9. COS-7 cells transfected with GLUT8 cDNA expressed a 42-kDa protein exhibiting specific, glucose-inhibitable cytochalasin B binding (K(D) = 56.6 +/- 18 nm) and reconstitutable glucose transport activity (8.1 +/- 1. 4 nmol/(mg protein x 10 s) versus 1.1 +/- 0.1 in control transfections). In human tissues, a 2.4-kilobase pair transcript was predominantly found in testis, but not in testicular carcinoma. Lower amounts of the mRNA were detected in most other tissues including skeletal muscle, heart, small intestine, and brain. GLUT8 mRNA was found in testis from adult, but not from prepubertal rats; its expression in human testis was suppressed by estrogen treatment. It is concluded that GLUT8 is a sugar transport facilitator with glucose transport activity and a hormonally regulated testicular function.

[Insulin resistance and metabolic syndrome]

The metabolic syndrome represents a complex combination of the symptoms obesity, insulin resistance, dyslipoproteinemia, hypertension, and type 2 diabetes. These components have a heterogeneous genetic basis and appear to be closely linked. Obesity is determined by a polygenic constellation and produces insulin resistance, hypertension and dyslipidemia. In addition, defects in the signal transduction of insulin appear to aggravate the insulin resistance independent of obesity. Type 2 diabetes is produced by a third genetic predisposition and is precipitated by the failure of pancreatic beta-cell to compensate insulin resistance. Because prevalence and course of the diabetes markedly depend on the extent of obesity and insulin resistance, these symptoms of the metabolic syndrome represent crucial targets for preventive and therapeutic strategies.

[Minibrain/DYRK1A gene: candidate gene for mental retardation in Down's syndrome?]

DYRK1A is the first member of a novel subfamily of protein kinases with dual specificity. The human gene for DYRK1A is located in the "Down syndrome critical region" (21q22.2). Due to its relationship to the Drosophila gene minibrain (Mnb), whose mutation results in specific defects in neurogenesis, and based on functional experiments on transgenic mice, DYRK1A is discussed as a candidate gene for mental retardation in Down syndrome. The kinase is characterized by its ability to catalyze tyrosine-directed autophosphorylation as well as phosphorylation of serine/threonine residues in substrates. Its exact cellular function is yet unknown. DYRK1A is, however, known to be translocated into the nucleus and supposed to be involved in the control of cell growth and development. The pathogenetic impact of DYRK1A on Down syndrome needs further elucidation.

A metabolic syndrome of hypertension, hyperinsulinaemia and hypercholesterolaemia in the New Zealand obese mouse

BACKGROUND

New Zealand obese (NZO) mice exhibit a polygenic obesity associated with hyperinsulinaemia and hyperglycaemia. Here we show that the strain presents additional features of a metabolic syndrome, i.e. elevated blood pressure, serum cholesterol and serum triglyceride levels.

MATERIALS AND METHODS

A back-cross model of NZO mice with the lean Swiss Jackson Laboratory (SJL) strain was established in order to investigate further the correlation between hypertension, obesity, serum insulin and hyperglycaemia.

RESULTS

Systolic blood pressure was significantly elevated at 6 weeks of age and appeared to parallel the weight gain of the animals. Serum insulin levels, presumably reflecting insulin resistance, and systolic blood pressure values were significantly correlated with the body mass index (r2 = 0.707 and 0.486, respectively) in the back-cross mice. In contrast, blood pressure was only weakly correlated with serum insulin (r2 = 0.288) in non-diabetic mice, and was independent of serum insulin levels in diabetic animals.

CONCLUSION

The data are consistent with the concept that hypertension and insulin resistance are a characteristic consequence of the genetic constellation leading to obesity in the NZO strain, and that these traits reflect related mechanisms. It appears unlikely, however, that hypertension is a direct consequence of hyperinsulinaemia.

Association of impaired phosphatidylinositol 3-kinase activity in GLUT1-containing vesicles with malinsertion of glucose transporters into the plasma membrane of fibroblasts from a patient with severe insulin resistance and clinical features of Werner syndrome

The purpose of this study was to examine the molecular mechanism responsible for the defective insulin-stimulated glucose transport in cultured fibroblasts from a patient (VH) with clinical features of Werner syndrome and severe insulin resistance. Thus, in cells derived from VH, the subcellular distribution, structure, functional activity, as well as plasma membrane insertion of GLUT1 glucose transporters were analyzed. Furthermore, the insulin signal transduction pathway leading to activation of phosphatidylinositol (PI) 3-kinase as well as components of GLUT1-containing membrane vesicles were characterized. In fibroblasts derived from VH, GLUT1 glucose transporters were overexpressed by 8-fold in plasma membranes (PM) and by 5-fold in high density microsomes, respectively. Exofacial photolabeling revealed that only 14% of the overexpressed PM-GLUT1 transporters were properly inserted into the plasma membrane. The complementary DNA structure of the patient's insulin receptor and the GLUT1 glucose transporter, the intrinsic activity of plasma membrane glucose transporters, the tyrosine phosphorylation, as well as the protein expression of insulin receptor substrate-1/2 and p85 alpha/beta- and p110 alpha/beta-subunits of PI 3-kinase were normal. However, insulin-stimulated association of the p85 subunit of PI 3-kinase was defective in fibroblasts derived from VH compared to those from controls, and this defect was associated with a reduced IRS-1-dependent activation of PI 3-kinase by 50.2% and 63.6% after incubation for 5 and 10 min with 100 nmol/L insulin, respectively. Furthermore, immunodetection of small GTP-binding Rab proteins in subcellular membrane fractions indicated a decreased expression of Rab4 in total cellular homogenates as well as in high density microsomes by 70% and 58%, respectively. After preparation of GLUT1-containing vesicles, Rab4 was not detected to be a component of these vesicles. Analysis of the PI 3-kinase in GLUT1-containing membrane vesicles revealed insulin-dependent targeting of the p85 subunit to the vesicles immunoadsorbed from VH and control fibroblasts. Importantly, the association of the p85 subunit as well as the p85-immunoprecipitable PI 3-kinase activity were markedly reduced in GLUT1-vesicles derived from the patient. In conclusion, impaired PI 3-kinase activity in GLUT1-containing membrane vesicles derived from fibroblasts of VH is associated with a defective docking and/or fusion process of glucose transporters with the plasma membrane and thus might contribute to the molecular defect causing insulin resistance in this patient.

Specificity determinants of substrate recognition by the protein kinase DYRK1A

DYRK1A is a dual-specificity protein kinase that is thought to be involved in brain development. We identified a single phosphorylated amino acid residue in the DYRK substrate histone H3 (threonine 45) by mass spectrometry, phosphoamino acid analysis, and protein sequencing. Exchange of threonine 45 for alanine abolished phosphorylation of histone H3 by DYRK1A and by the related kinases DYRK1B, DYRK2, and DYRK3 but not by CLK3. In order to define the consensus sequence for the substrate specificity of DYRK1A, a library of 300 peptides was designed in variation of the H3 phosphorylation site. Evaluation of the phosphate incorporation into these peptides identified DYRK1A as a proline-directed kinase with a phosphorylation consensus sequence (RPX(S/T)P) similar to that of ERK2 (PX(S/T)P). A peptide designed after the optimal substrate sequence (DYRKtide) was efficiently phosphorylated by DYRK1A (K(m) = 35 microM) but not by ERK2. Both ERK2 and DYRK1A phosphorylated myelin basic protein, whereas only ERK2, but not DYRK1A, phosphorylated the mitogen-activated protein kinase substrate ELK-1. This marked difference in substrate specificity between DYRK1A and ERK2 can be explained by the requirement for an arginine at the P -3 site of DYRK substrates and its presumed interaction with aspartate 247 conserved in all DYRKs.

ADP-ribosylation factor (ARF)-like 4, 6, and 7 represent a subgroup of the ARF family characterization by rapid nucleotide exchange and a nuclear localization signal

The novel ARF-like GTPase ARL7 is a close relative of ARL4 and ARL6 (71% and 59%) identical amino acids). A striking characteristic of these GTPases is their basic C-terminus which, when fused to the C-terminus of green fluorescent protein (GFP), targets the constructs to the nucleus of transfected COS-7 cells. Full length ARL4 was detected in both nuclear and extranuclear compartments, whereas a construct of ARL4 lacking its C-terminus was excluded from the nucleus. Nucleotide exchange rates of recombinant ARL4, ARL6 and ARL7 were similar and appeared considerably higher than those of other members of the ARF family (ARF1, ARP). It is concluded that ARL4, ARL6 and ARL7 form a subgroup within the ARF family with similar, possibly nuclear, function.

The ADP-ribosylation factor (ARF)-related GTPase ARF-related protein binds to the ARF-specific guanine nucleotide exchange factor cytohesin and inhibits the ARF-dependent activation of phospholipase D

ADP-ribosylation factor-related protein (ARP) is a membrane-associated GTPase with remote similarity to the family of ADP-ribosylation factors (ARF). In a yeast two-hybrid screen designed to identify proteins interacting with ARP, we isolated a partial cDNA of the ARF-specific guanine nucleotide exchange factor mSec7-1/cytohesin encoding its N terminus and most of the Sec7 domain (codons 1-200). ARP and ARP-Q79L (GTPase-negative ARP) exhibited a higher affinity to mSec7-1-(1-200) than ARP-T31N (nucleotide exchange-defective ARP) in the two-hybrid assay. Similarly, full-length [35S]mSec7-1/cytohesin was specifically adsorbed to glutathione-Sepharose loaded with glutathione S-transferase (GST)-ARP-Q79L, GST-ARP, or GST-ARP-T31N, the latter exhibiting the lowest binding affinity. Overexpression of ARP-Q79L, but not of ARP-T31N, in COS-7 cells reduced the fluorescence from co-expressed green fluorescent protein fused with mSec7-1/cytohesin or mSec7-2/ARNO in plasma membranes as detected by deconvolution microscopy. Recombinant ARP and ARP-Q79L, but not ARP-T31N, inhibited the phospholipase D (PLD) activity stimulated by mSec7-2/ARNO and ARF in a system of isolated membranes. Furthermore, transfection of HEK-293 cells with ARP or ARP-Q79L, but not ARP-T31N, inhibited the muscarinic acetylcholine receptor-3 induced PLD stimulation and translocation of ARF from cytosol to membranes. These data suggest that the GTP-bound form of ARP specifically binds mSec7-1/cytohesin, and that ARP may be involved in a pathway inhibiting the ARF-controlled activity of PLD.

Cloning and characterization of DYRK1B, a novel member of the DYRK family of protein kinases

The DYRK1A gene on human chromosome 21 encodes a protein kinase presumed to be involved in the pathogenesis of mental retardation in Down's syndrome. Here we describe a highly similar homolog, DYRK1B, which is, in contrast to DYRK1A, predominately expressed in muscle and testis. The human DYRK1B gene was mapped to chromosome 19 (19q12-13.11) by radiation hybrid analysis. The amino acid sequences of DYRK1A and DYRK1B are 84% identical in the N-terminus and the catalytic domain but show no extended sequence similarity in the C-terminal region. DYRK1B contains all motifs characteristic for the DYRK family of protein kinases. In addition, the sequence comprises a bipartite nuclear localization motif. A green fluorescent protein (GFP) fusion protein of DYRK1B was found mainly in the nucleus of transfected COS-7 cells. These data suggest that DYRK1B is a muscle- and testis-specific isoform of DYRK1A and is involved in the regulation of nuclear functions.

Structural and functional characteristics of Dyrk, a novel subfamily of protein kinases with dual specificity

Dyrk-related kinases represent a novel subfamily of protein kinases with unique structural and enzymatic features. Its members have been identified in distantly related organisms. The yeast kinase, Yak1, has been characterized as a negative regulator of growth. Mnb from Drosophila is encoded by the minibrain gene, whose mutation results in specific defects in neurogenesis. Its mammalian homolog, Dyrk1A, is activated by tyrosine phosphorylation in the activation loop between subdomains VII and VIII of the catalytic domain. The human gene for Dyrk1A is located in the "Down syndrome critical region" of chromosome 21 and is therefore a candidate gene for mental retardation in Down syndrome. More recently, six additional mammalian Dyrk-related kinases have been identified (Dyrk1B, Dyrk1C, Dyrk2, Dyrk3, Dyrk4A, and Dyrk4B). All members of the Dyrk family contain in the activation loop the tyrosines that are essential for the full activity of Dyrk1A. Outside their catalytic domains, Dyrk kinases exhibit little sequence similarity except for a small segment immediately preceding the catalytic domain (DH-box, Dyrk homology box). An unusual enzymatic property of Dyrk-related kinases is their ability to catalyze tyrosine-directed autophosphorylation as well as phosphorylation of serine/threonine residues in exogenous substrates. The exact cellular function of the Dyrk kinases is yet unknown. However, it appears reasonable to assume that they are involved in the regulation of cellular growth and/or development.