Rad, a novel Ras-related GTPase, interacts with skeletal muscle beta-tropomyosin

Identification and Functional Analysis of Ras-Related Associated with Diabetes Gene (rrad) in Edwardsiella piscicida-Resistant Individuals of Japanese Flounder (Paralichthys olivaceus)

Ras-related associated with diabetes (RRAD) is a member of the Ras GTPase superfamily that plays a role in several cellular functions, such as cell proliferation and differentiation. In particular, the superfamily acts as an NF-κB signaling pathway inhibitor and calcium regulator to participate in the immune response pathway. A recent transcriptome study revealed that rrad was expressed in the spleen of disease-resistant Japanese flounder (Paralichthys olivaceus) individuals compared with disease-susceptible individuals, and the results were also verified by qPCR. Thus, the present study aimed to explore how rrad regulates antimicrobial immunity via the NF-κB pathway. First, the coding sequence of P. olivaceus rrad was identified. The sequence was 1092 bp in length, encoding 364 amino acids. Based on phylogenetic and structural relationship analyses, P. olivaceus rrad appeared to be more closely related to teleosts. Next, rrad expression differences between disease-resistant and disease-susceptible individuals in immune-related tissues were evaluated, and the results revealed that rrad was expressed preferentially in the spleen of disease-resistant individuals. In response to Edwardsiella piscicida infection, rrad expression in the spleen changed. In vitro, co-culture was carried out to assess the hypo-methylated levels of the rrad promoter in the disease-resistant spleen, which was consistent with the high mRNA expression. The siRNA-mediated knockdown of rrad performed with the gill cell line of P. olivaceus affected many rrad-network-related genes, i.e., dcp1b, amagt, rus1, rapgef1, ralbp1, plce1, rasal1, nckipsd, prkab2, cytbc-1, sh3, and others, as well as some inflammation-related genes, such as bal2 and Il-1β. In addition, flow cytometry analysis showed that rrad overexpression was more likely to induce cell apoptosis, with establishing a link between rrad's function and its potential roles in regulating the NF-κB pathway. Thus,. the current study provided some clarity in terms of understanding the immune response about rrad gene differences between disease-resistant and disease-susceptible P. olivaceus individuals. This study provides a molecular basis for fish rrad gene functional analysis and may serve as a reference for in-depth of bacterial disease resistance of teleost.

Friend or Foe: Regulation, Downstream Effectors of RRAD in Cancer

Ras-related associated with diabetes (RRAD), a member of the Ras-related GTPase superfamily, is primarily a cytosolic protein that actives in the plasma membrane. RRAD is highly expressed in type 2 diabetes patients and as a biomarker of congestive heart failure. Mounting evidence showed that RRAD is important for the progression and metastasis of tumor cells, which play opposite roles as an oncogene or tumor suppressor gene depending on cancer and cell type. These findings are of great significance, especially given that relevant molecular mechanisms are being discovered. Being regulated in various pathways, RRAD plays wide spectrum cellular activity including tumor cell division, motility, apoptosis, and energy metabolism by modulating tumor-related gene expression and interacting with multiple downstream effectors. Additionally, RRAD in senescence may contribute to its role in cancer. Despite the twofold characters of RRAD, targeted therapies are becoming a potential therapeutic strategy to combat cancers. This review will discuss the dual identity of RRAD in specific cancer type, provides an overview of the regulation and downstream effectors of RRAD to offer valuable insights for readers, explore the intracellular role of RRAD in cancer, and give a reference for future mechanistic studies.

RRAD mutation causes electrical and cytoskeletal defects in cardiomyocytes derived from a familial case of Brugada syndrome

AIMS

The Brugada syndrome (BrS) is an inherited cardiac disorder predisposing to ventricular arrhythmias. Despite considerable efforts, its genetic basis and cellular mechanisms remain largely unknown. The objective of this study was to identify a new susceptibility gene for BrS through familial investigation.

METHODS AND RESULTS

Whole-exome sequencing performed in a three-generation pedigree with five affected members allowed the identification of one rare non-synonymous substitution (p.R211H) in RRAD, the gene encoding the RAD GTPase, carried by all affected members of the family. Three additional rare missense variants were found in 3/186 unrelated index cases. We detected higher levels of RRAD transcripts in subepicardium than in subendocardium in human heart, and in the right ventricle outflow tract compared to the other cardiac compartments in mice. The p.R211H variant was then subjected to electrophysiological and structural investigations in human cardiomyocytes derived from induced pluripotent stem cells (iPSC-CMs). Cardiomyocytes derived from induced pluripotent stem cells from two affected family members exhibited reduced action potential upstroke velocity, prolonged action potentials and increased incidence of early afterdepolarizations, with decreased Na+ peak current amplitude and increased Na+ persistent current amplitude, as well as abnormal distribution of actin and less focal adhesions, compared with intra-familial control iPSC-CMs Insertion of p.R211H-RRAD variant in control iPSCs by genome editing confirmed these results. In addition, iPSC-CMs from affected patients exhibited a decreased L-type Ca2+ current amplitude.

CONCLUSION

This study identified a potential new BrS-susceptibility gene, RRAD. Cardiomyocytes derived from induced pluripotent stem cells expressing RRAD variant recapitulated single-cell electrophysiological features of BrS, including altered Na+ current, as well as cytoskeleton disturbances.

Ras associated with diabetes may play a role in fracture nonunion development in rats

Background

Rad is the prototypic member of a subfamily of Ras-related small G-proteins and is highly expressed in the skeletal muscle of patients with type II diabetes. Our previous microarray analysis suggested that Rad may mediate fracture nonunion development. Thus, the present study used rat experimental models to investigate and compare the gene and protein expression patterns of both Rad and Rem1, another RGK subfamily member, in nonunions and standard healing fractures.

Methods

Standard healing fractures and nonunions (produced via periosteal cauterization at the fracture site) were created in the femurs of 3-month-old male Sprague-Dawley rats. At post-fracture days 7, 14, 21, and 28, the fracture callus and fibrous tissue from the standard healing fractures and nonunions, respectively, were harvested and screened (via real-time PCR) for Rad and Rem1 expression. The immunolocalization of both encoded proteins was analyzed at post-fracture days 14 and 21. At the same time points, hematoxylin and eosin staining was performed to identify the detailed tissue structures.

Results

Results of real-time PCR analysis showed that Rad expression increased significantly in the nonunions, compared to that in the standard healing fractures, at post-fracture days 14, 21, and 28. Conversely, immunohistochemical analysis revealed the immunolocalization of Rad to be similar to that of Rem1 in both fracture types at post-fracture days 14 and 21.

Conclusions

Rad may mediate nonunion development, and thus, may be a promising therapeutic target to treat these injuries.

IgM Antibodies Can Access Cryptic Antigens Denied to IgG: Hypothesis on Novel Binding Mechanism

Antibodies are well-known protein mediators of immunity. IgM is the primordial member and the neglected sibling of the later-evolved and more proficient IgG in regard to their therapeutic and diagnostic use. Serendipitously, however, we found a paradox: While murine IgM antibodies specific for guanosine triphosphate (GTP) were able to recognize native guanylyl antigens found in primate or rat muscle tissues by immunofluorescence assays (which mimicked the auto-antibodies from autoimmune patients to skeletal or smooth muscle), the murine and human IgG counterparts failed. The results were replicated in cell-free direct binding assays using small latex microspheres decorated densely with GTP. The IgG antibodies could bind, however, if GTP was presented more spaciously on larger particles or as a univalent hapten. Accordingly, oligomerization of GTP (30-mer) destroyed the binding of the IgG antibodies but enhanced that of the IgMs in inhibition ELISA. We reason that, contrary to current belief, IgM does not bind in a lock-and-key manner like IgG. We hypothesize that whereas the intact and rigid antigen-binding site of IgG hinders the antibody from docking with antigens that are obstructed, in IgM, the two component polypeptides of the antigen-binding site can dissociate from each other and navigate individually through obstacles like the ancestral single-polypeptide antibodies found in sharks and camelids, both components eventually re-grouping around the antigen. We further speculate that polyreactive IgMs, which enigmatically bind to more than one type of antigen, use the same modus operandi. These findings call for a re-look at the clinical potential of IgM antibodies particularly in specific areas of cancer therapy, tissue pathology and vaccine design, where IgG antibodies have failed due to target inaccessibility.

Ras-related associated with diabetes gene acts as a suppressor and inhibits Warburg effect in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is rapidly becoming one of the most prevalent cancers worldwide and is a prominent source of mortality. Ras-related associated with diabetes (RRAD), one of the first members of the 35–39 kDa class of novel Ras-related GTPases, is linked to several types of cancer, although its function in HCC remains unclear. In this study, we observed that RRAD was downregulated in HCC compared with adjacent normal tissues. This change was associated with a poor prognosis. Furthermore, knockdown of RRAD in SK-Hep-1 cells facilitated cell proliferation, accelerated the G1/S transition during the cell cycle, induced cell migration, and reduced apoptosis. In contrast, overexpression of RRAD in Huh7 cells had the opposite effects. Moreover, we demonstrated that RRAD induced cell proliferation through regulation of the cell cycle by downregulating cyclins and cyclin-dependent kinases. RRAD induced tumor cell apoptosis through the mitochondrial apoptosis pathway. In addition, we confirmed that knockdown of RRAD promoted aerobic glycolysis by upregulating glucose transporter 1, whereas overexpression of RRAD inhibited aerobic glycolysis. In conclusion, RRAD plays a pivotal role as a potential tumor suppressor in HCC. An improved understanding of the roles of RRAD in tumor metabolism may provide insights into its potential as a novel molecular target in HCC therapy.

RRAD inhibits the Warburg effect through negative regulation of the NF-κB signaling

Cancer cells preferentially use aerobic glycolysis to meet their increased energetic and biosynthetic demands, a phenomenon known as the Warburg effect. Its underlying mechanism is not fully understood. RRAD, a small GTPase, is a potential tumor suppressor in lung cancer. RRAD expression is frequently down-regulated in lung cancer, which is associated with tumor progression and poor prognosis. Recently, RRAD was reported to repress the Warburg effect, indicating that down-regulation of RRAD expression is an important mechanism contributing to the Warburg effect in lung cancer. However, the mechanism by which RRAD inhibits the Warburg effect remains unclear. Here, we found that RRAD negatively regulates the NF-κB signaling to inhibit the GLUT1 translocation and the Warburg effect in lung cancer cells. Mechanically, RRAD directly binds to the p65 subunit of the NF-κB complex and inhibits the nuclear translocation of p65, which in turn negatively regulates the NF-κB signaling to inhibit GLUT1 translocation and the Warburg effect. Blocking the NF-κB signaling largely abolishes the inhibitory effects of RRAD on the translocation of GLUT1 to the plasma membrane and the Warburg effect. Taken together, our results revealed a novel mechanism by which RRAD negatively regulates the Warburg effect in lung cancer cells.

RRAD inhibits aerobic glycolysis, invasion, and migration and is associated with poor prognosis in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most prevalent and lethal cancer worldwide. However, the mechanism underlying the HCC development remains unclear. Ras-related associated with diabetes (RRAD) is a small Ras-related GTPase which has been implicated in metabolic disease and several types of cancer, yet its functions in HCC remain unknown. A tissue microarray constructed by 90 paired HCC tissues and adjacent non-cancerous liver tissues was used to examine the protein levels of RRAD, and the messenger RNA (mRNA) expression of RRAD was also detected in a subset of this cohort. The prognostic significance of RRAD was estimated by the Kaplan-Meier analysis and Cox regression. The glucose utilization assay and lactate production assay were performed to measure the role of RRAD in HCC glycolysis. The effect of RRAD in HCC invasion and metastasis was analyzed by transwell assays. Our results suggested that the expression of RRAD was downregulated in HCC tissues compared to the adjacent non-tumorous liver tissues both in mRNA and protein levels and lower RRAD expression served as an independent prognostic indicator for the survival of HCC patients. Moreover, RRAD inhibited hepatoma cell aerobic glycolysis by negatively regulating the expression of glucose transporter 1 (GLUT1) and hexokinase II (HK-II). In addition, RRAD inhibition dramatically increased hepatoma cell invasion and metastasis. In conclusion, our study revealed that RRAD expression was decreased in HCC tumor tissues and predicted poor clinical outcome for HCC patients and played an important role in regulating aerobic glycolysis and cell invasion and metastasis and may represent potential targets for improving the treatment of HCC.

RRAD promotes EGFR-mediated STAT3 activation and induces temozolomide resistance of malignant glioblastoma

Glioblastoma multiforme (GBM) is an extremely aggressive brain cancer with a median survival of less than 2 years. GBM is characterized by abnormal activation of receptor tyrosine kinase and constitutively activated STAT3. Although EGFR phosphorylation and STAT3 activation are essential for the maintenance of GBM cancer stem cells, the molecular mechanism underlying endosome-mediated STAT3 activation is not fully understood. In the current study, we showed that GTP-binding protein RRAD (RAS associated with diabetes, RAD) physically associates with EGFR, and EEA1, enhancing the stability and endosome-associated nuclear translocation of EGFR. Functionally, RRAD contributes to the activation of STAT3 and expression of the stem cell factors OCT4, NANOG, and SOX2, thereby enhancing self-renewing ability, tumor sphere formation, EMT, and in vivo tumorigenesis. Most importantly, RRAD contributes to poor survival in patients with GBM. RRAD expression is correlated with temozolomide resistance, and, conversely, depletion of RRAD leads to sensitization of highly temozolomide-resistant GBM cells. Our data collectively support a novel function of RRAD in STAT3 activation and provide evidence that RRAD acts as a positive regulator in the EGFR signaling pathway. These results demonstrate a critical role for RRAD in GBM tumorigenesis and provide a rationale for the development of pharmacologic inhibitors of RRAD in GBM.

A splicing variant of NME1 negatively regulates NF-κB signaling and inhibits cancer metastasis by interacting with IKKβ

IKKβ functions as a principal upstream activator of the canonical NF-κB pathway by phosphorylating IκB, leading to its proteasomal degradation. Because IKKβ is considered a therapeutic target, understanding its regulation may facilitate the design of efficient regulators of this molecule. Here, we report a novel IKKβ-interacting molecule, NME1L, a splicing variant of the NME1 protein. NME1 has attracted attention in cancer research because of its antimetastatic activity and reduced expression in multiple aggressive types of cancer. However, the effect was just moderate but not dramatic in anti-cancer activities. We found that only NME1L interacts with IKKβ. Exogenous expression of NME1L resulted in a potent decrease in TNFα-stimulated NF-κB activation, whereas knockdown of NME1/NME1L with shRNA enhanced activity of NF-κB. NME1L down-regulates IKKβ signaling by blocking IKKβ-mediated IκB degradation. When NME1L was introduced into highly metastatic HT1080 cells, the mobility was efficiently inhibited. Furthermore, in a metastasis assay, NME1L-expressing cells did not colonize the lung. Based on these results, NME1L is a potent antimetastatic protein and may be a useful weapon in the fight against cancers.

Establishment of a cell-type-specific genetic network by the mediator complex component Med1

The intense physiologic demand to generate vast numbers of red blood cells requires the establishment of a complex genetic network by the master regulatory transcription factor GATA-1 and its coregulators. This network dictates the genesis of enucleated erythrocytes by orchestrating the survival, proliferation, and differentiation of progenitor cells. In addition to the crucial GATA-1 coregulator Friend of GATA-1 (FOG-1), a component of the Mediator complex, Med1, facilitates GATA-1-dependent transcription at select target genes and controls erythropoiesis. It is not known to what extent Med1 contributes to GATA-1 function or whether Med1 controls a large or restricted cohort of genes that are not regulated by GATA-1. Using a genetic complementation assay in GATA-1-null erythroid cells, we demonstrate that Med1 and another Mediator component, Med25, regulate a restricted cohort of genes that are predominantly not controlled by GATA-1. Most of these genes were not regulated by Med1 in fibroblasts. Loss-of-function analyses with GATA-1-independent Med1 target genes indicate that Rrad, which encodes a small GTPase induced during human erythropoiesis, conferred erythroid cell survival. Thus, while Med1 is a context-dependent GATA-1 coregulator, it also exerts specialized functions in erythroid cells to control GATA-1-independent, cell-type-specific genes, which include candidate regulators of erythroid cell development and function.

The GTPase Gem and its partner Kif9 are required for chromosome alignment, spindle length control, and mitotic progression

Within the Ras superfamily, Gem is a small GTP-binding protein that plays a role in regulating Ca(2+) channels and cytoskeletal remodeling in interphase cells. Here, we report for the first time that Gem is a spindle-associated protein and is required for proper mitotic progression. Functionally, loss of Gem leads to misaligned chromosomes and prometaphase delay. On the basis of different experimental approaches, we demonstrate that loss of Gem by RNA interference induces spindle elongation, while its enforced expression results in spindle shortening. The spindle length phenotype is generated through deregulation of spindle dynamics on Gem depletion and requires the expression of its downstream effector, the kinesin Kif9. Loss of Kif9 induces spindle abnormalities similar to those observed when Gem expression is repressed by siRNA. We further identify Kif9 as a new regulator of spindle dynamics. Kif9 depletion increases the steady-state levels of spindle α-tubulin by increasing the rate of microtubule polymerization. Overall, this study demonstrates a novel mechanism by which Gem contributes to the mitotic progression by maintaining correct spindle length through the kinesin Kif9.

Cyclin-dependent kinase-associated protein Cks2 is associated with bladder cancer progression

In this observational retrospective study, expression of possible cancer-related genes was measured in patients with a pathological diagnosis of superficial bladder cancer. Further measurements were made in those who subsequently developed muscle-invasive cancer. Seven of the 45 patients with superficial bladder cancer progressed to muscle-invasive cancer. Expression of fatty acid binding protein 5 (FABP5), poly(A) binding protein cytoplasmic 1 (PABPC1), DEAD box polypeptide 5 (DDX5), splicing factor 3b subunit 1 (SF3B1), murine mammary tumour integration site 6 (EIF3S6), tropomyosin 2β (TPM2), transgelin (TAGLN) and cyclin-dependent kinase-associated protein (Cks2) genes was measured in bladder samples using real-time reverse transcription-polymerase chain reaction. FABP5, PABPC1, DDX5, SF3B1, EIF3S6 and Cks2 expression levels were significantly increased, and TPM2 and TAGLN were significantly decreased, in superficial bladder cancer compared with normal bladder tissue. In patients who developed muscle-invasive cancer, the Cks2 gene showed significantly increased expression after, compared with before, invasion. The Cks2 gene may have potential as a biomarker for predicting superficial bladder cancer progression to muscle-invasive cancer.

Platelet-derived growth factor induces Rad expression through Egr-1 in vascular smooth muscle cells

BACKGROUND

Ras associated with diabetes (Rad) inhibits vascular lesion formation by reducing the attachment and migration of vascular smooth muscle cells (VSMCs). However, the transcriptional regulation of Rad in VSMCs is unclear.

METHODOLOGY AND PRINCIPAL FINDINGS

We found that Platelet-Derived Growth Factor (PDGF)induced Rad expression in a time- and dose-dependent manner in rat aortic smooth muscle cells (RASMCs) using quantitative real-time PCR. By serial deletion analysis of the Rad promoter, we identified that two GC-rich early growth response-1 (Egr-1) binding sites are essential for PDGF-induced Rad promoter activation. Overexpression of Egr-1 in RASMCs strongly stimulated Rad expression while the Egr-1 corepressor, NGFI-A binding protein 2 (NAB2), repressed PDGF-induced Rad up-regulation in a dose-dependent manner. Direct binding of Egr-1 to the Rad promoter region was further confirmed by chromatin immunoprecipitation assays.

CONCLUSIONS

Our results demonstrate that Rad is regulated by PDGF through the transcriptional factor Egr-1 in RASMCs.

Tropomyosin-binding properties of the CHASM protein are dependent upon its calponin homology domain

The calponin homology-associated smooth muscle protein (CHASM) can modulate muscle contractility, and its biological action may involve an interaction with the contractile filament. In this study, we demonstrate an interaction between CHASM and tropomyosin. Deletion constructs of CHASM were generated, and pull-down assays revealed a minimal deletion construct that could bind tropomyosin. Removal of the calponin homology (CH) domain or expression of the CH domain alone did not enable binding. The interaction was characterized by microcalorimetry with a dissociation constant of 2.0x10(-6) M. Confocal fluorescence microscopy also showed green fluorescent protein (GFP)-CHASM localization to filamentous structures within smooth muscle cells, and this targeting was dependent upon the CH domain.

A novel senescence-evasion mechanism involving Grap2 and Cyclin D interacting protein inactivation by Ras associated with diabetes in cancer cells under doxorubicin treatment

Ras associated with diabetes (Rad) is a Ras-related GTPase that promotes cell growth by accelerating cell cycle transitions. Rad knockdown induced cell cycle arrest and premature senescence without additional cellular stress in multiple cancer cell lines, indicating that Rad expression might be critical for the cell cycle in these cells. To investigate the precise function of Rad in this process, we used human Rad as bait in a yeast two-hybrid screening system and sought Rad-interacting proteins. We identified the Grap2 and cyclin D interacting protein (GCIP)/DIP1/CCNDBP1/HHM, a cell cycle-inhibitory molecule, as a binding partner of Rad. Further analyses revealed that Rad binds directly to GCIP in vitro and coimmunoprecipitates with GCIP from cell lysates. Rad translocates GCIP from the nucleus to the cytoplasm, thereby inhibiting the tumor suppressor activity of GCIP, which occurs in the nucleus. Furthermore, in the presence of Rad, GCIP loses its ability to reduce retinoblastoma phosphorylation and inhibit cyclin D1 activity. The function of Rad in transformation is also evidenced by increased telomerase activity and colony formation according to Rad expression level. In vivo tumorigenesis analyses revealed that tumors derived from Rad knockdown cells were significantly smaller than those from control cells (P = 0.0131) and the preestablished tumors are reduced in size after the injection of siRad (P = 0.0064). Therefore, we propose for the first time that Rad may promote carcinogenesis at least in part by inhibiting GCIP-mediated tumor suppression.

Rem, a member of the RGK GTPases, inhibits recombinant CaV1.2 channels using multiple mechanisms that require distinct conformations of the GTPase

Rad/Rem/Gem/Kir (RGK) GTPases potently inhibit Ca(V)1 and Ca(V)2 (Ca(V)1-2) channels, a paradigm of ion channel regulation by monomeric G-proteins with significant physiological ramifications and potential biotechnology applications. The mechanism(s) underlying how RGK proteins inhibit I(Ca) is unknown, and it is unclear how key structural and regulatory properties of these GTPases (such as the role of GTP binding to the nucleotide binding domain (NBD), and the C-terminus which contains a membrane-targeting motif) feature in this effect. Here, we show that Rem inhibits Ca(V)1.2 channels by three independent mechanisms that rely on distinct configurations of the GTPase: (1) a reduction in surface density of channels is accomplished by enhancing dynamin-dependent endocytosis, (2) a diminution of channel open probability (P(o)) that occurs without impacting on voltage sensor movement, and (3) an immobilization of Ca(V) channel voltage sensors. The presence of both the Rem NBD and C-terminus (whether membrane-targeted or not) in one molecule is sufficient to reconstitute all three mechanisms. However, membrane localization of the NBD by a generic membrane-targeting module reconstitutes only the decreased P(o) function (mechanism 2). A point mutation that prevents GTP binding to the NBD selectively eliminates the capacity to immobilize voltage sensors (mechanism 3). The results reveal an uncommon multiplicity in the mechanisms Rem uses to inhibit I(Ca), predict new physiological dimensions of the RGK GTPase-Ca(V) channel crosstalk, and suggest original approaches for developing novel Ca(V) channel blockers.

The RGK family of GTP-binding proteins: regulators of voltage-dependent calcium channels and cytoskeleton remodeling

RGK proteins constitute a novel subfamily of small Ras-related proteins that function as potent inhibitors of voltage-dependent (VDCC) Ca(2+) channels and regulators of actin cytoskeletal dynamics. Within the larger Ras superfamily, RGK proteins have distinct regulatory and structural characteristics, including nonconservative amino acid substitutions within regions known to participate in nucleotide binding and hydrolysis and a C-terminal extension that contains conserved regulatory sites which control both subcellular localization and function. RGK GTPases interact with the VDCC beta-subunit (Ca(V)beta) and inhibit Rho/Rho kinase signaling to regulate VDCC activity and the cytoskeleton respectively. Binding of both calmodulin and 14-3-3 to RGK proteins, and regulation by phosphorylation controls cellular trafficking and the downstream signaling of RGK proteins, suggesting that a complex interplay between interacting protein factors and trafficking contribute to their regulation.

The effect of insulin on expression of genes and biochemical pathways in human skeletal muscle

To study the insulin effects on gene expression in skeletal muscle, muscle biopsies were obtained from 20 insulin sensitive individuals before and after euglycemic hyperinsulinemic clamps. Using microarray analysis, we identified 779 insulin-responsive genes. Particularly noteworthy were effects on 70 transcription factors, and an extensive influence on genes involved in both protein synthesis and degradation. The genetic program in skeletal muscle also included effects on signal transduction, vesicular traffic and cytoskeletal function, and fuel metabolic pathways. Unexpected observations were the pervasive effects of insulin on genes involved in interacting pathways for polyamine and S-adenoslymethionine metabolism and genes involved in muscle development. We further confirmed that four insulin-responsive genes, RRAD, IGFBP5, INSIG1, and NGFI-B (NR4A1), were significantly up-regulated by insulin in cultured L6 skeletal muscle cells. Interestingly, insulin caused an accumulation of NGFI-B (NR4A1) protein in the nucleus where it functions as a transcription factor, without translocation to the cytoplasm to promote apoptosis. The role of NGFI-B (NR4A1) as a new potential mediator of insulin action highlights the need for greater understanding of nuclear transcription factors in insulin action.

Overexpression of Rad in muscle worsens diet-induced insulin resistance and glucose intolerance and lowers plasma triglyceride level

Rad is a low molecular weight GTPase that is overexpressed in skeletal muscle of some patients with type 2 diabetes mellitus and/or obesity. Overexpression of Rad in adipocytes and muscle cells in culture results in diminished insulin-stimulated glucose uptake. To further elucidate the potential role of Rad in vivo, we have generated transgenic (tg) mice that overexpress Rad in muscle using the muscle creatine kinase (MCK) promoter-enhancer. Rad tg mice have a 6- to 12-fold increase in Rad expression in muscle as compared to wild-type littermates. Rad tg mice grow normally and have normal glucose tolerance and insulin sensitivity, but have reduced plasma triglyceride levels. On a high-fat diet, Rad tg mice develop more severe glucose intolerance than the wild-type mice; this is due to increased insulin resistance in muscle, as exemplified by a rightward shift in the dose-response curve for insulin stimulated 2-deoxyglucose uptake. There is also a unexpected further reduction of the plasma triglyceride levels that is associated with increased levels of lipoprotein lipase in the Rad tg mice. These results demonstrate a potential synergistic interaction between increased expression of Rad and high-fat diet in creation of insulin resistance and altered lipid metabolism present in type 2 diabetes.

Nuclear Sequestration of β-Subunits by Rad and Rem is Controlled by 14-3-3 and Calmodulin and Reveals a Novel Mechanism for Ca2+ Channel Regulation

Voltage-gated Ca2+ channels (VDCCs) are heteromultimeric proteins that mediate Ca2+ influx into cells upon membrane depolarization. These channels are involved in various cellular events, including gene expression, regulation of hormone secretion and synaptic transmission. Kir/Gem, Rad, Rem, and Rem2 belong to the RGK family of Ras-related small G proteins. RGK proteins interact with the beta-subunits and downregulate VDCC activity. Kir/Gem was proposed to prevent surface expression of functional Ca2+ channels, while for Rem2 the mechanism remains controversial. Here, we have analyzed the mechanism by which Rad and Rem regulate VDCC activity. We show that, similar to Kir/Gem and Rem2, 14-3-3 and CaM binding regulate the subcellular distribution of Rad and Rem, which both inhibit Ca2+ channel activity by preventing its expression on the cell surface. This function is regulated by calmodulin and 14-3-3 binding only for Rad and not for Rem. Interestingly, nuclear targeting of Rad and Rem can relocalize and sequester the beta-subunit to the nucleus, thus providing a novel mechanism for Ca2+ channel downregulation.

Gem Protein Signaling and Regulation

Gem is a member of the RGK family of GTP-binding proteins within the Ras superfamily possessing a ras-like core and terminal extensions. We have used a variety of cell-based assays to investigate the physiological role of Gem and combined these assays with site-directed mutagenesis of Gem protein to identify the sites responsible for regulation of Gem activity. One function of Gem that has been explained is the inhibition of Rho kinase (ROK)-mediated cytoskeletal rearrangement. Transient expression of Gem in endothelial cells and stable transfection of fibroblasts resulted in decreased stress fiber formation and focal adhesion assembly. A neurite extension model using N1E-115 murine neuroblastoma showed that Gem inhibits actinomyosin-related contractility by specifically opposing ROKbeta activity. Phospho-specific antibodies were used in Western blot analysis to show that Gem prevents phosphorylation of the regulatory subunit of myosin light chain and myosin phosphatase by ROKbeta. On the contrary, LIMK, another substrate of ROKbeta, was unaffected by Gem expression as demonstrated by an in vitro kinase assay, suggesting that Gem exerts its effect by changing the substrate specificity of ROKbeta rather than by blocking its catalytic activity. Point mutations of Gem at serines 261 and 289 in the carboxyl-terminus inhibited Gem function, indicating that posttranslational phosphorylation of these serines regulates Gem's effect on cytoskeletal reorganization. Another biological role of Gem is inhibition of voltage-gated calcium channel activity. By use of a PC12 cell model combined with site-directed mutagenesis, we demonstrated that Gem inhibits growth hormone secretion stimulated by calcium influx through L-type calcium channels and that this function is dependent on GTP and calmodulin binding to Gem. The theory and method for the assays discussed previously are reviewed here.

Expression, purification, crystallization and preliminary crystallographic analysis of human Rad GTPase

Human Rad is a new member of the Ras GTPase superfamily and is overexpressed in human skeletal muscle of individuals with type II diabetes. The GTPase core domain was overexpressed in Escherichia coli and purified for crystallization. Crystals were obtained at 293 K by vapour diffusion using a crystallization robot. The crystals were found to belong to space group P2(1), with unit-cell parameters a = 52.2, b = 58.6, c = 53.4 A, beta = 97.9 degrees , and contained two Rad molecules in the crystallographic asymmetric unit. A diffraction data set was collected to a resolution of 1.8 A using synchrotron radiation at SPring-8.

Rad is temporally regulated within myogenic progenitor cells during skeletal muscle regeneration

The successful use of myogenic progenitor cells for therapeutic applications requires an understanding of the intrinsic and extrinsic cues involved in their regulation. Herein we demonstrate the expression pattern and transcriptional regulation of Rad, a prototypical member of a family of novel Ras-related GTPases, during mammalian development and skeletal muscle regeneration. Rad was identified using microarray analysis, which revealed robust upregulation of its expression during skeletal muscle regeneration. Our current findings demonstrate negligible Rad expression with resting adult skeletal muscle; however, after muscle injury, Rad is expressed within the myogenic progenitor cell population. Rad expression is significantly increased and localized to the myogenic progenitor cell population during the early phases of regeneration and within the newly regenerated myofibers during the later phases of regeneration. Immunohistochemical analysis demonstrated that Rad and MyoD are coexpressed within the myogenic progenitor cell population of regenerating skeletal muscle. This expression profile of Rad during skeletal muscle regeneration is consistent with the proposed roles for Rad in the inhibition of L-type Ca(2+) channel activity and the inhibition of Rho/RhoA kinase activity. We also have demonstrated that known myogenic transcription factors (MEF2, MyoD, and Myf-5) can increase the transcriptional activity of the Rad promoter and that this ability is significantly enhanced by the presence of the Ca(2+)-dependent phosphatase calcineurin. Furthermore, this enhanced transcriptional activity appears to be dependent on the presence of a conserved NFAT binding motif within the Rad promoter. Taken together, these data define Rad as a novel factor within the myogenic progenitor cells of skeletal muscle and identify key regulators of its transcriptional activity.

Rad GTPase attenuates vascular lesion formation by inhibition of vascular smooth muscle cell migration

BACKGROUND

Rad (Ras associated with diabetes) GTPase is a prototypic member of a new subfamily of Ras-related GTPases with unique structural features, although its physiological role remains largely unknown. In the present study, we characterized the Rad function in vascular smooth muscle cells (VSMCs) and the influence of adenovirus-mediated Rad (Ad-Rad) gene delivery on vascular remodeling after experimental angioplasty.

METHODS AND RESULTS

We documented for the first time that neointimal formation using balloon-injured rat carotid arteries was associated with a significant increase in Rad expression as determined by immunohistochemistry and quantitative real-time reverse-transcriptase polymerase chain reaction. The levels of Rad expression in VSMCs were highly induced by platelet-derived growth factor and tumor necrosis factor-alpha. Morphometric analyses 14 days after injury revealed significantly diminished neointimal formation in the Ad-Rad-treated carotid arteries compared with Ad-GFP or PBS controls, whereas the mutated form of Rad GTPase, which can bind GDP but not GTP, increased neointimal formation. Overexpression of Rad significantly inhibited the attachment and migration of VSMCs. In addition, Rad expression dramatically reduced the formation of focal contacts and stress fibers in VSMCs by blocking the Rho/ROK signaling pathway.

CONCLUSIONS

Our data clearly identified Rad GTPase as a novel and critical mediator that inhibits vascular lesion formation. Manipulation of the Rad signaling pathway may provide new therapeutic approaches that will limit vascular pathological remodeling.

Phosphorylation of critical serine residues in Gem separates cytoskeletal reorganization from down-regulation of calcium channel activity

Gem is a small GTP-binding protein that has a ras-like core and extended chains at each terminus. The primary structure of Gem and other RGK family members (Rad, Rem, and Rem2) predicts a GTPase deficiency, leading to the question of how Gem functional activity is regulated. Two functions for Gem have been demonstrated, including inhibition of voltage-gated calcium channel activity and inhibition of Rho kinase-mediated cytoskeletal reorganization, such as stress fiber formation and neurite retraction. These functions for Gem have been ascribed to its interaction with the calcium channel beta subunit and Rho kinase beta, respectively. We show here that these functions are separable and regulated by distinct structural modifications to Gem. Phosphorylation of serines 261 and 289, located in the C-terminal extension, is required for Gem-mediated cytoskeletal reorganization, while GTP and possibly calmodulin binding are required for calcium channel inhibition. In addition to regulating cytoskeletal reorganization, phosphorylation of serine 289 in conjunction with serine 23 results in bidentate 14-3-3 binding, leading to increased Gem protein half-life. Evidence presented shows that phosphorylation of serine 261 is mediated via a cdc42/protein kinase Czeta-dependent pathway. These data demonstrate that phosphorylation of serines 261 and 289, outside the GTP-binding region of Gem, controls its inhibition of Rho kinase beta and associated changes in the cytoskeleton.

Effect of acute heat shock on gene expression by human peripheral blood mononuclear cells

We studied the effect of heat shock on gene expression by normal human cells. Peripheral blood mononuclear cells (PBMCs) were obtained from healthy adults. Paired samples from each subject were subjected to either 20 min of heat shock (43 degrees C) or control (37 degrees C) conditions and then returned to 37 degrees C. RNA was isolated 160 min later, and five representative samples were analyzed on Affymetrix gene chip arrays containing approximately 12,600 probes. A biologically meaningful effect was defined as a statistically significant, twofold or greater difference in expression of sequences that were detected in all five experiments under control (downregulated sequences) or heat shock (upregulated sequences) conditions. Changes occurred in 395 sequences (227 increased by heat shock, 168 decreased), representing 353 Unigene numbers, in every functional category previously implicated in the heat shock response. By RT-PCR, we confirmed the findings for one upregulated sequence (Rad, a G protein) and one downregulated sequence (osteopontin, a cytokine). We conclude that heat shock causes extensive gene expression changes in PBMCs, affecting all functional categories of the heat shock response.

The GTP binding proteins Gem and Rad are negative regulators of the Rho-Rho kinase pathway

The cytoskeletal changes that alter cellular morphogenesis and motility depend upon a complex interplay among molecules that regulate actin, myosin, and other cytoskeletal components. The Rho family of GTP binding proteins are important upstream mediators of cytoskeletal organization. Gem and Rad are members of another family of small GTP binding proteins (the Rad, Gem, and Kir family) for which biochemical functions have been mostly unknown. Here we show that Gem and Rad interface with the Rho pathway through association with the Rho effectors, Rho kinase (ROK) alpha and beta. Gem binds ROKbeta independently of RhoA in the ROKbeta coiled-coil region adjacent to the Rho binding domain. Expression of Gem inhibited ROKbeta-mediated phosphorylation of myosin light chain and myosin phosphatase, but not LIM kinase, suggesting that Gem acts by modifying the substrate specificity of ROKbeta. Gem or Rad expression led to cell flattening and neurite extension in N1E-115 neuroblastoma cells. In interference assays, Gem opposed ROKbeta- and Rad opposed ROKalpha-mediated cell rounding and neurite retraction. Gem did not oppose cell rounding initiated by ROKbeta containing a deletion of the Gem binding region, demonstrating that Gem binding to ROKbeta is required for the effects observed. In epithelial or fibroblastic cells, Gem or Rad expression resulted in stress fiber and focal adhesion disassembly. In addition, Gem reverted the anchorage-independent growth and invasiveness of Dbl-transformed fibroblasts. These results identify physiological roles for Gem and Rad in cytoskeletal regulation mediated by ROK.

The Ras-like GTPase Gem is involved in cell shape remodelling and interacts with the novel kinesin-like protein KIF9

Gem belongs to the Rad/Gem/Kir (RGK) subfamily of Ras-related GTPases, which also comprises Rem, Rem2 and Ges. The RGK family members Ges and Rem have been shown to produce endothelial cell sprouting and reorganization of the actin cytoskeleton upon overexpression. Here we show that high intracellular Gem levels promote profound changes in cell morphology and we investigate how this phenotype arises dynamically. We also show that this effect requires intact microtubules and microfilaments, and that Gem is associated with both cytoskeletal components. In order to investigate the mechanisms of Gem recruitment to the cytoskeleton, we performed a yeast two-hybrid screen and identified a novel kinesin-like protein, termed KIF9, as a new Gem interacting partner. We further show that Gem and KIF9 interact by co-immunoprecipitation. Furthermore, Gem and KIF9 display identical patterns of gene expression in different tissues and developmental stages. The Gem- KIF9 interaction reported here is the first molecular link between RGK family members and the microtubule cytoskeleton.

Plasmodium protein phosphatase 2C dephosphorylates translation elongation factor 1beta and inhibits its PKC-mediated nucleotide exchange activity in vitro

The elongation step of protein synthesis involves binding of aminoacyl-tRNA to the ribosomal A site, formation of a peptide bond and translocation of the newly formed peptidyl-tRNA to the P site. The nucleotide exchange factor EF-1beta plays a major role in the regulation of this process by regenerating a GTP-bound EF-1alpha necessary for each elongation cycle. EF-1beta has been shown to be phosphorylated and its phosphorylation is critical for optimal activity. We have previously identified a serine/threonine protein phosphatase 2C (PP2C) from the human malaria parasite Plasmodium falciparum. In the current work, we performed Far-Western analysis to identify PfPP2C substrates. Several components of the translation and transcription machinery were identified, including translation elongation factor 1-beta (PfEF-1beta). PfEF-1beta is efficiently phosphorylated by protein kinase C and this phosphorylation results in a 400% increase in its nucleotide exchange activity. PKC-phosphorylated PfEF-1beta is readily and selectively dephosphorylated by recombinant and native PfPP2C, which downregulates the nucleotide exchange activity to its basal level. The identification of a translation elongation component as substrate for PP2C suggests an important regulatory function for this enzyme and suggests that it may be a good target for drug design in the fight against malaria.

Domain 2 of gelsolin binds directly to tropomyosin

Gelsolin is an actin filament severing protein composed of six similar structured domains that differ with respect to actin, calcium and polyphospho-inositide binding. Previous work has established that gelsolin binds tropomyosin [Koepf, E.K. and Burtnick, L.D. (1992) FEBS Lett. 309, 56-58]. We have produced various specific gelsolin domains in Escherichia coli in order to establish which of the six domains binds tropomyosin. Gelsolin domains 1-3 (G1-3), G1-2 and G2 all bind tropomyosin in a pH and calcium insensitive manner whereas binding of G4-6 to tropomyosin was barely detectable under the conditions tested. We conclude that gelsolin binds tropomyosin via domain 2 (G2).

Interaction of the Ras-related protein associated with diabetes rad and the putative tumor metastasis suppressor NM23 provides a novel mechanism of GTPase regulation

Rad is the prototypic member of a new class of Ras-related GTPases. Purification of the GTPase-activating protein (GAP) for Rad revealed nm23, a putative tumor metastasis suppressor and a development gene in Drosophila. Antibodies against nm23 depleted Rad-GAP activity from human skeletal muscle cytosol, and bacterially expressed nm23 reconstituted the activity. The GAP activity of nm23 was specific for Rad, was absent with the S105N putative dominant negative mutant of Rad, and was reduced with mutations of nm23. In the presence of ATP, GDP.Rad was also reconverted to GTP.Rad by the nucleoside diphosphate (NDP) kinase activity of nm23. Simultaneously, Rad regulated nm23 by enhancing its NDP kinase activity and decreasing its autophosphorylation. Melanoma cells transfected with wild-type Rad, but not the S105N-Rad, showed enhanced DNA synthesis in response to serum; this effect was lost with coexpression of nm23. Thus, the interaction of nm23 and Rad provides a potential novel mechanism for bidirectional, bimolecular regulation in which nm23 stimulates both GTP hydrolysis and GTP loading of Rad whereas Rad regulates activity of nm23. This interaction may play important roles in the effects of Rad on glucose metabolism and the effects of nm23 on tumor metastasis and developmental regulation.

The role of G proteins in insulin signalling

Insulin modulates many intracellular processes including cellular metabolism, cell proliferation and cell differentiation. Some of these processes involve significant changes in the traffic of intracellular vesicles or in the structural organization of the cell. These phenomena have been linked to the activity of regulatory GTP-binding proteins. Most, if not all functions, of the insulin receptor are associated with its tyrosine kinase activity. Thus, over the past few years, a significant effort has been dedicated to elucidate the cross-talk between the tyrosine kinase activity of the receptor and the regulation of G protein-mediated pathways. Recent progress indicates that G proteins may mediate the control of several of insulin's intracellular functions. These include the regulation of the MAP kinase pathway, the activation of phospholipase D and the regulation of glucose uptake. This article discusses some recent advances in this area.

Analysis of trinucleotide-repeat combination polymorphism at the rad gene in patients with type 2 diabetes mellitus

A combined (GTT)n (ATT)n trinucleotide-repeat polymorphism designated as RAD1 has been identified at intron 2 of the rad gene on chromosome 16q. An association between the total length of the RAD1 locus and type 2 diabetes has been shown in white American subjects, but not in Finns. We genotyped 115 Japanese patients with type 2 diabetes and 114 nondiabetic control subjects at the RAD1 locus by the direct sequencing method, and found 16 RAD1 alleles composed of various combinations of GTTs and ATTs. Allele 14 consisting of four GTTs and seven ATTs accounted for the majority in both control subjects and diabetic patients, suggesting that RAD1 polymorphism is not a major genetic component for susceptibility to common forms of diabetes in the Japanese. There was no significant association between total repeat length and diabetes. However, the frequency of minor alleles containing five GTTs or three GTTs was significantly higher in diabetic patients versus nondiabetic subjects (4.8% v 0.9%, P = .012). Thus, genetic variability at the rad gene in linkage disequilibrium with RAD1 could be associated with a predisposition to type 2 diabetes in the Japanese population.

Early steps in insulin action

The biological effects of insulin are initiated by the binding of insulin to the insulin receptor. Insulin binds to the extracellular domain of the insulin receptor and induces conformational changes in the receptor, leading to autophosphorylation of the receptor on intracellular tyrosine residues. These phosphorylated tyrosine residues act as binding sites for proteins which subsequently may be phosphorylated by the insulin receptor. As a result, yet other proteins can be recruited to form larger complexes and, in the case of enzymes, changes in their activity may take place. By a combination of these processes, the activated insulin receptor initiates cascades of biochemical events which are regulated mainly by specific phosphorylation or dephosphorylation reactions. Intermediates which are involved in the normal insulin signalling pathway are subjects of expanding research.

The ras-related protein rad associates with the cytoskeleton in a non-lipid-dependent manner

Rad is the prototypic member of a new family of Ras-related proteins (Rad, Gem, and Kir) which lack typical C-terminal amino acid motifs for isoprenylation. In mouse C2C12 muscle cell lines about 50% of Rad protein resides in the cytosol and behaves as a hydrophilic protein partitioning away from TX-114. The remainder of Rad is associated with plasma and internal membranes. The association of Rad with the membrane does not occur through the lipid bilayer, but instead depends on the interaction of Rad with the cytoskeleton or membrane skeleton. In contrast to Ras, biosynthetic labeling of cellular proteins in C2Cl2 cells with [3H]palmitic acid demonstrates that Rad is not modified with this fatty acid, and inhibition of isoprenylation with lovastatin treatment has no effect on Rad subcellular distribution. Furthermore, removal of the C-terminal 11 amino acids that are precisely conserved in all three Rad family members has no effect on Rad subcellular distribution. Addition of the 9 amino acids from the C-terminus of H-Ras to the truncated Rad protein results in a redistribution of Rad from the cytosol to the membrane skeleton without the presence of any detectable lipid modification of the chimeric protein. These data suggest that Rad possesses unique cellular localization signals which, in contrast to other Ras-related family members, do not depend on the lipid modification of the C-terminus.

Effects of phosphorylation on function of the Rad GTPase

Rad, Gem and Kir possess unique structural features in comparison with other Ras-like GTPases, including a C-terminal 31-residue extension that lacks typical prenylation motifs. We have recently shown that Rad and Gem bind calmodulin in a Ca2+-dependent manner via this C-terminal extension, involving residues 278-297 in human Rad. This domain also contains several consensus sites for serine phosphorylation, and Rad is complexed with calmodulin-dependent protein kinase II (CaMKII) in C2C12 cells. Here we show that Rad serves as a substrate for phosphorylation by CaMKII, cAMP-dependent protein kinase (PKA), protein kinase C (PKC) and casein kinase II (CKII) with stoichiometries in vitro of 0.2-1.3 mol of phosphate/mol of Rad. By deletion and point mutation analysis we show that phosphorylation by CaMKII and PKA occurs on a single serine residue at position 273, whereas PKC and CKII phosphorylate multiple C-terminal serine residues, including Ser214, Ser257, Ser273, Ser290 and Ser299. Incubation of Rad with PKA decreases GTP binding by 60-70%, but this effect seems to be independent of phosphorylation, as it is observed with the Ser273-->Ala mutant of Rad containing a mutation at the site of PKA phosphorylation. The remainder of the serine kinases have no effect on Rad GTP binding, intrinsic GTP hydrolysis or GTP hydrolysis stimulated by the putative tumour metastasis suppressor nm23. However, phosphorylation of Rad by PKC and CKII abolishes the interaction of Rad with calmodulin. These findings suggest that the binding of Rad to calmodulin, as well as its ability to bind GTP, might be regulated by the activation of several serine kinases.

PED/PEA-15 gene controls glucose transport and is overexpressed in type 2 diabetes mellitus

We have used differential display to identify genes whose expression is altered in type 2 diabetes thus contributing to its pathogenesis. One mRNA is overexpressed in fibroblasts from type 2 diabetics compared with non-diabetic individuals, as well as in skeletal muscle and adipose tissues, two major sites of insulin resistance in type 2 diabetes. The levels of the protein encoded by this mRNA are also elevated in type 2 diabetic tissues; thus, we named it PED for phosphoprotein enriched in diabetes. PED cloning shows that it encodes a 15 kDa phosphoprotein identical to the protein kinase C (PKC) substrate PEA-15. The PED gene maps on human chromosome 1q21-22. Transfection of PED/PEA-15 in differentiating L6 skeletal muscle cells increases the content of Glut1 transporters on the plasma membrane and inhibits insulin-stimulated glucose transport and cell-surface recruitment of Glut4, the major insulin-sensitive glucose transporter. These effects of PED overexpression are reversed by blocking PKC activity. Overexpression of the PED/PEA-15 gene may contribute to insulin resistance in glucose uptake in type 2 diabetes.

Rem is a new member of the Rad- and Gem/Kir Ras-related GTP-binding protein family repressed by lipopolysaccharide stimulation

We report the cDNA cloning and characterization of a novel GTP-binding protein, termed Rem (for Rad and Gem-related), that was identified as a product of polymerase chain reaction amplification using oligonucleotide primers derived from conserved regions of the Rad, Gem, and Kir Ras subfamily. Alignment of the full-length open reading frame of mouse Rem revealed the encoded protein to be 47% identical to the Rad, Gem, and Kir proteins. The distinct structural features of the Rad, Gem, and Kir subfamily are maintained including a series of nonconservative amino acid substitutions at positions important for GTPase activity and a unique sequence motif thought to direct membrane association. Recombinant Rem binds GTP in a specific and saturable manner. Ribonuclease protection analysis found Rem to be expressed at comparatively high levels in cardiac muscle and at moderate levels in lung, skeletal muscle, and kidney. The administration of lipopolysaccharide to mice, a potent activator of the inflammatory and immune systems, results in the general repression of Rem mRNA levels in a dose- and time-dependent manner. Thus, Rem is the first Ras-related gene whose mRNA levels have been shown to be regulated by repression.

Rad and Rad-related GTPases interact with calmodulin and calmodulin-dependent protein kinase II

Members of the Rad family of GTPases (including Rad, Gem, and Kir) possess several unique features of unknown function in comparison to other Ras-like proteins, with major N-terminal and C-terminal extensions, a lack of typical prenylation motifs, and several non-conservative changes in the sequence of the GTP binding domain. Here we show that Rad and Gem bind to calmodulin (CaM)-Sepharose in vitro in a calcium-dependent manner and that Rad can be co-immunoprecipitated with CaM in C2C12 cells. The interaction is influenced by the guanine nucleotide binding state of Rad with the GDP-bound form exhibiting 5-fold better binding to CaM than the GTP-bound protein. In addition, the dominant negative mutant of Rad (S105N) which binds GDP, but not GTP, exhibits enhanced binding to CaM in vivo when expressed in C2C12 cells. Peptide competition studies and expression of deletion mutants of Rad localize the binding site for CaM to residues 278-297 at the C terminus of Rad. This domain contains a motif characteristic of a calmodulin-binding region, consisting of numerous basic and hydrophobic residues. In addition, we have identified a second potential regulatory domain in the extended N terminus of Rad which, when removed, decreases Rad protein expression but increases the binding of Rad to CaM. The ability of Rad mutants to bind CaM correlates with their localization in cytoskeletal fractions of C2C12 cells. Immunoprecipitates of calmodulin-dependent protein kinase II, the cellular effector of Ca2+-calmodulin, also contain Rad, and in vitro both Rad and Gem can serve as substrates for this kinase. Thus, the Rad family of GTP-binding proteins possess unique characteristics of binding CaM and calmodulin-dependent protein kinase II, suggesting a role for Rad-like GTPases in calcium activation of serine/threonine kinase cascades.

Calmodulin binds to and inhibits GTP binding of the ras-like GTPase Kir/Gem

Recently, a new subfamily of Ras-related GTP-binding proteins consisting of Rad (Ras associated with diabetes), Gem (immediate early gene expressed in mitogen-stimulated T-cells), and Kir (tyrosine kinase-inducible Ras-like) was discovered. The C terminus of these proteins contains an extension of approximately 30 amino acids not present in other members of the Ras family and which exhibits all the hallmarks typical for calmodulin (CaM)-binding domains. A peptide corresponding to the putative CaM-binding domain of the Kir/Gem protein was synthesized, and its affinity for CaM was determined by fluorescence spectrometry. Titration of dansyl-CaM with the Kir/Gem peptide gave an affinity constant of 1 nM. Furthermore, a single point mutation of the peptide, W269G, abolished this high affinity interaction. Gel-shift analysis showed that the complex formation between CaM and the Kir/Gem peptide is strictly calcium-dependent. We also demonstrate with a newly developed [32P]CaM overlay technique that full-length Kir/Gem and Rad proteins bind CaM in a Ca2+-dependent fashion. The binding of CaM to glutathione S-transferase-Kir and GST-Gem inhibited the binding of GTP to Kir/Gem significantly. These results suggest the existence of a direct link between Ca2+/CaM and growth factor signal transduction pathways at the level of small Ras-like GTPases.

Overexpression of Rad inhibits glucose uptake in cultured muscle and fat cells

Rad is a Ras-like GTPase that was isolated by subtraction cloning of human muscle and shown to have increased expression in some individuals with Type II diabetes. To ascertain the potential role of Rad in insulin-mediated signaling, we have overexpressed Rad in myocyte and adipocyte cell lines. Expression of Rad resulted in a 50-90% reduction in insulin-stimulated 2-deoxyglucose glucose uptake in C2C12 murine myotubes, L6 rat myotubes, and 3T3-L1 adipocytes and a 25% reduction in 3-O-methylglucose uptake in 3T3-L1 adipocytes. This occurred despite unaltered levels of glucose transporter expression, with no detectable change in Glut4 translocation and with no alteration in insulin receptor or substrate phosphorylation or phosphatidylinositol 3-kinase activity. These data indicate that Rad is a negative regulator of glucose uptake and that this effect may be due to a decrease in the intrinsic activity of the transporter molecules, rather than an effect on the translocation of Glut4.

Insulin receptor substrate 1 binds two novel splice variants of the regulatory subunit of phosphatidylinositol 3-kinase in muscle and brain

We have identified two novel alternatively spliced forms of the p85alpha regulatory subunit of phosphatidylinositol (PI) 3-kinase by expression screening of a human skeletal muscle library with phosphorylated baculovirus- produced human insulin receptor substrate 1. One form is identical to p85alpha throughout the region which encodes both Src homology 2 (SH2) domains and the inter-SH2 domain/p110 binding region but diverges in sequence from p85alpha on the 5' side of nucleotide 953, where the entire break point cluster gene and SH3 regions are replaced by a unique 34-amino-acid N terminus. This form has an estimated molecular mass of approximately 53 kDa and has been termed p85/AS53. The second form is identical to p85 and p85/AS53 except for a 24-nucleotide insert between the SH2 domains that results in a replacement of aspartic acid 605 with nine amino acids, adding two potential serine phosphorylation sites in the vicinity of the known serine autophosphorylation site (Ser-608). Northern (RNA) analyses reveal a wide tissue distribution of p85alpha, whereas p85/AS53 is dominant in skeletal muscle and brain, and the insert isoforms are restricted to cardiac muscle and skeletal muscle. Western blot (immunoblot) analyses using an anti-p85 polyclonal antibody and a specific anti-p85/AS53 antibody confirmed the tissue distribution of p85/AS53 protein and indicate a approximately 7-fold higher expression of p85/AS53 protein than of p85 in skeletal muscle. Both p85 and p85/AS53 bind to p110 in coprecipitation experiments, but p85alpha itself appears to have preferential binding to insulin receptor substrate 1 following insulin stimulation. These data indicate that the gene for the p85alpha regulatory subunit of PI 3-kinase can undergo tissue-specific alternative splicing. Two novel splice variants of the regulatory subunit of PI 3-kinase are present in skeletal muscle, cardiac muscle, and brain; these variants may have important functional differences in activity and may play a role in tissue-specific signals such as insulin-stimulated glucose transport or control of neurotransmitter secretion or action.