MYC Signal Research Articles

Dissociation of mitochondrial and ribosomal biogenesis during thallium administration in rat kidney.

Thallium (Tl) is a heavy metal with toxicity comparative to other heavy metals such as As, Cd, and Hg. Nevertheless, fewer studies have been reported concerning the molecular mechanism of Tl toxicity as compared to other heavy metals. To obtain insight into Tl toxicity in the kidney, rats were intraperitoneally administered Tl2SO4 (30 mg/kg), and the kidneys were removed 2 or 5 days later to examine the effects of Tl. Transcriptome analysis using DNA microarray of the rat kidney 2 and 5 days after Tl administration showed that cytoplasmic ribosomal proteins are the most upregulated category; many of the genes involved in ribosome biosynthesis were upregulated by Tl administration. This upregulation was associated with the activation of eukaryotic transcription initiation factor 2α (eIF2α), implying that increased ribosome biogenesis was linked to the subsequent activation of protein translation. In contrast, decreased mitochondrial biogenesis was revealed via proteomic analysis. Although we found an increase in Myc, a positive regulator of both ribosomal and mitochondrial biogenesis, decreased levels of NRF1 and TFAM, positive regulators of mitochondrial biogenesis whose gene expression is directory activated by Myc, were paradoxically observed. Taken together, differing responses of ribosomes and mitochondria to Tl toxicity were observed. Failure of transmission of the Myc signal to NRF1/TFAM might be involved in the observed disruption of coordinated responses in mitochondria and ribosomes during Tl administration in rat kidney.

Tumor cell-derived exosomes mediating hsa_circ_0001739/lncRNA AC159540.1 facilitate liver metastasis in colorectal cancer.

The current study probed into how tumor cell-derived exosomes (Exos) mediated hsa_circ_0001739/lncRNA AC159540.1 to manipulate microRNA (miR)-218-5p/FTO-N6-methyladenosine (m6A)/MYC signal axis in liver metastasis in colorectal cancer (CRC). hsa_circ_0001739 and lncRNA AC159540.1 were identified as the upstream regulator of miR-218-5p using ENCORI and LncBase databases. Expression patterns of miR-218-5p, hsa_circ_0001739, lncRNA AC159540.1, FTO, and MYC were detected, accompanied by loss-and-gain-of function assays to examine their effects on CRC cell biological functions. SW480 cells-derived Exos were purified, followed by in vitro studies to uncover the effect of hsa_circ_0001739/lncRNA AC159540. miR-218-5p was downregulated while hsa_circ_0001739/lncRNA AC159540.1 was upregulated in CRC tissues and cells. Silencing of hsa_circ_0001739/lncRNA AC159540.1 restrained the malignant phenotypes of CRC cells. Exos-mediated hsa_circ_0001739/lncRNA AC159540.1 competitively inhibited miR-218-5p to elevate FTO and MYC. The inducing role of Exos-mediated hsa_circ_0001739/lncRNA AC159540.1 in CRC was also validated in vivo. Conclusively, Exos-mediated circ_0001739/lncRNA AC159540.1 regulatory network is critical for CRC, offering a theoretical basis for CRC treatment.

17th International Conference on Malignant Lymphoma, Palazzo dei Congressi, Lugano, Switzerland, 13 - 17 June, 2023.

Introduction: Multiple myeloma (MM) remains an incurable malignancy of plasma cells. The complex bone marrow microenvironment promotes resistance to both current anti-myeloma agents and emerging immunotherapies. Crosstalk between the NF-κB and p53 can play a pivotal role in various hematologic malignancies. DNp73, an inhibitor of the p53 tumor suppressor family, drives drug resistance and cancer progression in several solid malignancies. However, the biological functions and molecular mechanisms of DNp73 in MM remain unclear. In this study, we investigated the role of DNp73 in the drug resistance of MM, and disclosed the corresponding mechanism of how DNp73 promotes the immune escape of MM cells. Methods: We constructed DNp73 overexpression and sh-RNA interference plasmids to obtain stable cell lines. The effects of DNp73 on proliferation and drug sensitivity were determined by flow cytometry and xenotransplantation model. RNA-seq and CHIP-seq were performed to detect the mechanisms of drug resistance in MM cells. The DNA damage repair and invasion ability of MM cells were detected by immunofluorescence and transwell assay. To validate the role of DNp73 in immune escape, we performed phagocytosis assays. Results: Our previous study reported that miR-15a was downregulated in MM cells and correlated with the inferior outcome of MM patients. Further analysis demonstrated that DNp73 was a direct target of p65, loss of miR-15a led to the activation of NF-κB-p65 pathway in MM cells. In addition, the level of miR-15a was negatively correlated with the expression of DNp73 in MM cells of patients (r = −0.672, p < 0.05). After DNp73 was down-regulated, cell proliferation and drug resistance were effectively inhibited. Further in vivo study indicated the tumor volume in DNp73 knockdown group was reduced, and increased sensitivity to the carfilzomib, epirubicin and pomalidomide was observed (p < 0.01). Of note, RNA-seq analysis indicated that DNp73 expression was positively correlated with MYCN, MYC and CDK7 transcriptional programs in MM. GSEA analysis showed that MYC targets and DNA damage repair pathways were significantly enriched in DNp73-OE cells. Compared to control group, DNp73-OE cells were resistant to irradiation-induced cell death (p < 0.01). Meanwhile, DNp73 knockdown significantly reduced the migration and invasion of cells (p < 0.01). CHIP-seq data showed that DNp73 could bind to the promoter region of MYCN. DNp73 overexpression protects MM cells from phagocytosis, treated with anti-human CD47 antibody increased phagocytosis of macrophages (p < 0.01). Conclusions: We demonstrated that miR-15a downregulation activated NF-κB, which promoted DNp73 expression at transcription level. DNp73 promotes drug resistance and immune escape in multiple myeloma by modulating Myc and N-Myc signal pathway. The research was funded by: the CAMS Innovation Fund for Medical Sciences (2022-I2M-1-022), the Natural Science Foundation of China (82170194), the Non-profit Central Research Institute Fund of the Chinese Academy of Medical Sciences (2018RC320012), the Natural Science Foundation of China (81920108006), the Non-profit Central Research Institute Fund of the Chinese Academy of Medical Sciences (2018PT31006) Keywords: multiple myeloma, tumor biology and heterogeneity No conflicts of interests pertinent to the abstract.

Primary effusion lymphoma enhancer connectome links super-enhancers to dependency factors

Primary effusion lymphoma (PEL) has a very poor prognosis. To evaluate the contributions of enhancers/promoters interactions to PEL cell growth and survival, here we produce H3K27ac HiChIP datasets in PEL cells. This allows us to generate the PEL enhancer connectome, which links enhancers and promoters in PEL genome-wide. We identify more than 8000 genomic interactions in each PEL cell line. By incorporating HiChIP data with H3K27ac ChIP-seq data, we identify interactions between enhancers/enhancers, enhancers/promoters, and promoters/promoters. HiChIP further links PEL super-enhancers to PEL dependency factors MYC, IRF4, MCL1, CCND2, MDM2, and CFLAR. CRISPR knock out of MEF2C and IRF4 significantly reduces MYC and IRF4 super-enhancer H3K27ac signal. Knock out also reduces MYC and IRF4 expression. CRISPRi perturbation of these super-enhancers by tethering transcription repressors to enhancers significantly reduces target gene expression and reduces PEL cell growth. These data provide insights into PEL molecular pathogenesis.

Electroporated GLUT4-7myc-GFP detects in vivo glucose transporter 4 translocation in skeletal muscle without discernible changes in GFP patterns.

What is the central question of this study? Resolving the mechanism(s) leading to glucose transporter 4 (GLUT4) translocation to the muscle surface membrane has great therapeutic potential. However, the measurement of GLUT4 translocation is technically challenging. Here, we asked whether electroporation of GLUT4-7myc-GFP into skeletal muscle could be used as a tool to study GLUT4 translocation in vivo. What is the main finding and its importance? By acutely inducing GLUT4-7myc-GFP expression in skeletal muscle, we verified that in vivo exercise and AICAR stimulation increased the GLUT4 presence in the sarcolemma measured as myc signal. Importantly, the increased myc signal in the sarcolemma was not accompanied by major visual changes in the distribution of the GFP signal. Insulin and exercise lead to translocation of the glucose transporter4 (GLUT4) to the surface membrane of skeletal muscle fibres. This process is pivotal for facilitating glucose uptake into skeletal muscle. To study this, a robust assay is needed to measure the translocation of GLUT4 in adult skeletal muscle directly. Here, we aimed to validate a simple GLUT4 translocation assay using a genetically encoded biosensor in mouse skeletal muscle. We transfected GLUT4-7myc-GFP into mouse muscle to study live GLUT4 movement and to evaluate GLUT4 insertion in the muscle surface membrane after in vivo running exercise and pharmacological activation of AMP-activated protein kinase (AMPK). Transfection led to expression of GLUT4-7myc-GFP that was dynamic in live flexor digitorum brevis fibres and which, upon insulin stimulation, exposed the myc epitope extracellularly. Running exercise, in addition to AMPK activation by 5-aminoimidazole-4-carboxamide ribonucleotide, induced ∼125 and ∼100% increase, respectively, in extracellularly exposure of GLUT4 in the surface membrane of tibialis anterior muscle. Interestingly, the clear increase in surface-exposed GLUT4 content induced by insulin, exercise or AMPK activation was not accompanied by any discernible reorganization of the GLUT4-GFP signal. In conclusion, we provide a detailed description of an easy-to-use translocation assay to study GLUT4 accumulation at the surface membrane induced by exercise and exercise-mimicking stimuli. Notably, our analyses revealed that increased GLUT4 surface membrane accumulation was not accompanied by a discernible change in the GLUT4 localization pattern.

Metabolic Reprogramming of Triple-Negative Breast Cancer: The Role of miRNAs

AbstractMicroRNAs (miRNAs) are well known to influence the expression of the genes that regulate critical cellular functions. Various reports have suggested that they play critical roles in breast cancer metabolism through the regulation of various metabolic pathways, including the metabolism of glucose, lipids, glycolysis and the mitochondrial tricarboxylic acid cycle (TCA). miRNAs regulate the metabolic process by targeting key molecules (enzymes, kinases transporters) or by modifying the expression of key transcription molecules. In addition, miRNAs can indirectly regulate mRNA translation by targeting chromatin-remodeling enzymes. Furthermore, miRNAs influence the expression of both oncogenes and tumor suppressors and have a major impact on PI3K/AKT, HIF, and MYC signal transduction, which contributes to the metabolic phenotype in human cancer. Although human epidermal growth factor and endocrine therapies have been effective in treating breast cancer, for locally advanced and distant metastases mortality remains high. Drug resistance and recurrence remain major hurdles for advanced breast cancer therapy. Given the critical influence of metabolic reprogramming in the progression of neoplasm, tumorigenesis and metastasis, research should focus on novel targets of metabolic enzymes to reverse drug resistance and improve overall survival rates. Blocking the miRNAs that contribute to metabolic reprogramming or the use of exogenous miRNAs as antisense oligonucleotides, may be an effective way to treat aggressive, chemo-resistant cancers. This review summarizes current knowledge on the mechanism of action of miRNAs in altering the metabolism of cancer cells and presents possible therapeutic approaches to treating breast cancers that are resistant to current drugs.

Therapeutic targeting of the MYC signal by inhibition of histone chaperone FACT in neuroblastoma.

Amplification of the MYCN oncogene predicts treatment resistance in childhood neuroblastoma. We used a MYC target gene signature that predicts poor neuroblastoma prognosis to identify the histone chaperone FACT (facilitates chromatin transcription) as a crucial mediator of the MYC signal and a therapeutic target in the disease. FACT and MYCN expression created a forward feedback loop in neuroblastoma cells that was essential for maintaining mutual high expression. FACT inhibition by the small-molecule curaxin compound CBL0137 markedly reduced tumor initiation and progression in vivo. CBL0137 exhibited strong synergy with standard chemotherapy by blocking repair of DNA damage caused by genotoxic drugs, thus creating a synthetic lethal environment in MYCN-amplified neuroblastoma cells and suggesting a treatment strategy for MYCN-driven neuroblastoma.

Chromosome 8 Abnormalities (8p Losses and 8q Gains) in Patients with Chronic Lymphocytic Leukemia (CLL) and Del(17p)

Basis.Abnormalities in chromosome 8 (8p-/8q+) are observed in 2-5% of CLL patients. Microarray studies have revealed up to 30-40% of 8 alterations in del(17p) patients and an independent association with poor outcome. Large series assessing CLL patients with 8p-/8q+ are scarce.Aims.1. To describe the frequency of 8q gains (8q+) and 8p losses (8p-) in CLL patients with del(17p); 2. To compare cytogenetic and clinical characteristics between patients with 8p-/8q+ (Alt-chr8) and those with normal chromosome 8 (N-chr8); 3. To assess their prognostic value.Patients and methods.From 2,249 patients included in the Spanish CLL database, 75 del(17p) cases were selected. Gains of MYC (8q24) and losses of LPL (8p22) were studied by FISH. Clinical and cytogenetic data of Alt-chr8 and N-chr8 were compared.Results.8p- and/or 8q+ were found in 21/75 patients (28%). In the Alt-chr8 group, 8q+ was more frequent than 8p- (71% vs. 52%) and 29% showed concomitance of both abnormalities, suggesting the presence of i(8q). Six different FISH patterns were identified, some of them coexisting in the same patient (Table 1). Conventional cytogenetics data were available in 47 cases (15 Alt-chr8 and 32 N-chr8). Alt-chr8 group showed a higher median number of alterations and frequency of complex karyotypes (P=0.048 and P=0.013). In the Alt-chr8 group, the karyotype revealed 8p-/8q+ in 3 patients and in 9 cases with abnormal karyotype, the presence of marker chromosomes, added material and/or cryptic alterations would explain the FISH results (Table 1). From 66 cases, routine FISH data (13q, 12 and 11q) were available and no significant differences were detected among Alt-chr8 and N-chr8, as with other clinical and analytical parameters at diagnosis. Of note, shorter Overall Survival (OS) was observed for Alt-chr8, although differences were only significant for patients with 8p- (P=0.012, Figure 1). Interestingly, for 3 patients of Alt-chr8 group, previous non-del(17p) samples already presented 8p-/8q+.Conclusions.1. In CLL patients with del(17p), detection of 8p- and/or 8q+ is associated with an increased karyotypic complexity and a worse outcome; 2. 8p-/8q+ could act as a primary event that trigger del(17p). More cases are required to confirm this hypothesis.Acknowledgments.PI11/01621; RD12/0036/0044, RD12/0036/0069; 2014/SGR585; Fundació La Caixa.Table 1Karyotypes and FISH results of patients with del(17p) and Alt-chr8.Conventional CytogeneticsFISHIDKaryotype% del(17p)Chromosome 8 alteration%Pattern*146,XX,del(8)(p21),add(10)(q26),add(17)(p13),+2ac[5]/47,XX,+12,add(17)(p13),del(18)(q21),add(22)(q13)[3]80201O2G8p-2-9575346,XX,add(6)(q24),add(14)(q32,3),i(17)(q10)[6]/46,XX[8]95754-7650545,XY,-5,-9,-15,add(17)(p13),+18,-21,+2mar[13]/46,XY[37]7017646,X,der(X),add(8)(p23),del(13)(q12q22),add(17)(p13)[11]/46,XX[13]10321O3G8p-and8q+7-9564845,XY,add(3)(q29),del(4)(q26q35),der(7)(1p36-1p32::7p22-7q32::15q22-15q26), -8,der(9),del(13)(q21q34),-15,-17,-18,+19,add(19)(p13),+2mar,+ac[17]/46,XY[3]7834/211O3G/2O3G944,X,-X,-6,der(13;15)(q10;q10),add(17)(p13),-20,+mar[13]/46,XX[7]9510/231O3G/1O2G10-9566/311145,XY,add(6)(q22),del(11)(q11q22),-17[15]/44,XY,add(6)(q22),del(11)(q11q22),-17,-20,-22,+mar[2]8740/241246,XX,del(13)(q14q21)[2]/45,X,-X,del(13)(q14q21)[3]/45,XX,add(3)(q27), t(9;10)(q21;q22),+12,der(12)t(12;17)(q11;p11),del(13)(q14q21),-14,-17[7]/46,XX[8]70622O3G 8q+1346,XY[30]14881446,XY[13]80821547,XY,+12[8]/46,XY,add(1)(p34),add(2)(q34),t(11;22)(p14;q11),+12,-22[15]/46,XY[11]75181643,X,-X,del(2)(p15),+4,-7,add(11)(q21),-12,-13,add(14)(q32),add(17)(p11)[6]/46,XX[9]19141745,XY,del(6)(q?),-9,add(14)(q32),-22,+mar[9]/ 46,XY,del(6)(q?),add(17)(p13),add(19)(q13)[21]55231845,XY,add(6)(p11),-22[13]/46,XY,i(17)(q10)[5]/46,XY[16]165719-70662O4G20-90812OnG2146,XY[11]43604O4GTetraploid*O: LPL signal in orange, G: MYC signal in green. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Mahogunin Ring Finger-1 (MGRN1) E3 Ubiquitin Ligase Inhibits Signaling from Melanocortin Receptor by Competition with Gαs

Mahogunin ring finger-1 (MGRN1) is a RING domain-containing ubiquitin ligase mutated in mahoganoid, a mouse mutation causing coat color darkening, congenital heart defects, high embryonic lethality, and spongiform neurodegeneration. The melanocortin hormones regulate pigmentation, cortisol production, food intake, and body weight by signaling through five G protein-coupled receptors positively coupled to the cAMP pathway (MC1R-MC5R). Genetic analysis has shown that mouse Mgrn1 is an accessory protein for melanocortin signaling that may inhibit MC1R and MC4R by unknown mechanisms. These melanocortin receptors (MCRs) regulate pigmentation and body weight, respectively. We show that human melanoma cells express 4 MGRN1 isoforms differing in the C-terminal exon 17 and in usage of exon 12. This exon contains nuclear localization signals. MGRN1 isoforms decreased MC1R and MC4R signaling to cAMP, without effect on beta(2)-adrenergic receptor. Inhibition was independent on receptor plasma membrane expression, ubiquitylation, internalization, or stability and occurred upstream of Galpha(s) binding to/activation of adenylyl cyclase. MGRN1 co-immunoprecipitated with MCRs, suggesting a physical interaction of the proteins. Significantly, overexpression of Galpha(s) abolished the inhibitory effect of MGRN1 and decreased co-immunoprecipitation with MCRs, suggesting competition between MGRN1 and Galpha(s) for binding to MCRs. Although all MGRN1s were located in the cytosol in the absence of MCRs, exon 12-containing isoforms accumulated in the nuclei upon co-expression with the receptors. Therefore, MGRN1 inhibits MCR signaling by a new mechanism involving displacement of Galpha(s), thus accounting for key features of the mahoganoid phenotype. Moreover, MGRN1 might provide a novel pathway for melanocortin signaling from the cell surface to the nucleus.

Burkitt Transformation of Mantle Cell Lymphoma.

MYC rearrangements are not included as a genetic change in the blastoid variants of mantle cell lymphoma (Jaffe, et al (2001) WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues. Lyon: IARC Press.). We present two cases both with CCND1/IGH and MYC rearrangements. Case 1. An 82-year-old male with no known history of lymphoma presented with thrombocytopenia, loss of appetite and “abdominal fullness.” Imaging studies showed enlarged retroperitoneal lymph nodes. The peripheral blood smear had 18,000 WBC with approximately 30% circulating atypical lymphocytes. Flow cytometric studies of the bone marrow revealed a surface kappa light chain restricted CD10+ B-cell population. A bone marrow biopsy showed >90% of marrow cellularity comprised of neoplastic lymphocytes. The neoplastic lymphocytes were small to intermediate in size with minimal amounts of dark blue cytoplasm and several cytoplasmic vacuoles (Burkitt like morphology). By immunohistochemical stains, the neoplastic cells were positive for CD20, CD43, CD10, BCL-6, and cyclin D1, weakly and focally positive for BCL-2, and negative for CD23. The Ki-67 proliferation fraction was ∼100%. An immunohistochemical stain for CD5 was predominantly negative with a possible very faint blush on a subset of neoplastic B-cells. The FISH tests on bone marrow interphases were positive for a CCND1/IGH, a variant MYC/IGH, a variant MYC-BA rearrangements and negative for BCL6-BA and BCL2/IGH rearrangements. The variant MYC/IGH pattern was 3xMYC, 3xIGH, 1xFusion signals and MYC-BA pattern was 2x5′MYCcon3′MYC, 1x3′MYC. rearrangements. The karyotype was 44∼45,XY,del(2)(q11.2q21),der(3;17)(p10;q10), der(5)t(3;5)(q12;q15), t(11;14) (q13;q32) [cp6]/46,XY[14]. Since the karyotype had a t(11;14) and two normal 8 chromosomes, a metaphase FISH was analyzed to localize the signals for the MYC/IGH probe. The MYC signal were on both normal 8 chromosomes, a fusion signal was on a F-G sized chromosome. While the IGH signals were on the normal 14, der(14) and der(11). This was consistent with a cryptic MYC/IGH fusion in a three way rearrangement between chromosomes 8, 11 and 14. Case 2. A 69-year-old male having had a kidney transplant in 2001 was on immunosuppressive therapy. He presented with severe leukocytosis, anemia and thrombocytopenia and weight loss of about 12 pounds over several months. A peripheral blood smear showed 74,000 WBC with approximately 30% blasts. Bone marrow biopsies revealed normocellular bone marrow (50% cellularity). Interspersed large neoplastic lymphoid cells were shown by immunohistochemical stains to be positive for CD20, BCL-1, weak positive for BCL-2 and a Ki-67 staining > 90%. Flow cytometry indicated that the neoplastic cells were positive for kappa and CD5 but negative for CD11c and CD23. Interphases FISH on peripheral blood was positive for a CCND1/IGH rearrangement. The karyotype was 42∼44,X,-Y,add(1)(p13), t(2;8)(p12;q24), der(2)t(2;15)(p25;q11.2),+3,del(9)(p22p24),+del(9)(p22p24), − 10, del(11)(q21q23), t(11;14)(q13;q32) , − 13, − 15, − 17,add(17)(p11.2)[cp7]/46,XY[17]. FISH confirmed a MYC rearrangement. Therefore, this case had both CCND1/IGH and MYC/IGK rearrangement. Concomitant occurrence of a CCND1/IGH and a MYC rearrangement is rare in lymphomas. In Mitelman database of chromosome aberrations in cancer 2007, Four cases had both a t(11;14) and a t(8;14) translocation and two cases had both a t(11;14) and a t(2;8) translocation. This study expands the repertoire of abnormalities seen in blastoid transformation of mantle cell lymphoma. Being cognizant of a possible MYC involvement in the transformation of mantle cell lymphoma and its exploration would influence therapy.

Glucose transporter 4 can be inserted in the membrane without exposing its catalytic site for photolabeling from the medium

Insulin stimulates the production of PI(3,4,5)P(3) in muscle cells, and this is required to stimulate GLUT4 fusion with the plasma membrane. Introduction of exogenous PI(3,4,5)P(3) to muscle cells recapitulates insulin's effects on GLUT4 fusion with the plasma membrane, but not glucose uptake. This study aims to explore the mechanism behind this difference. In L6-GLUT4myc muscle cells, the availability of the GLUT4 intracellular C-terminus and extracellular myc epitopes for immunoreactivity on plasma membrane lawns was detected with the corresponding antibody. The availability of the active site of GLUT4 from extracellular medium was assessed by affinity photolabeling with the cell impermeant compound Bio-LC-ATB-BMPA. 100 nmol/L insulin and 10 mumol/L PI(3,4,5)P(3) caused myc signal gain on the plasma membrane lawns by 1.64-fold and 1.58-fold over basal, respectively. Insulin, but not PI(3,4,5)P(3), increased photolabeling of GLUT4 and immunolabeling with C-terminus antibody by 2.47-fold and 2.04-fold over basal, respectively. Upon insulin stimulation, the C-terminus signal gain was greater than myc signal gain (2.04-fold vs. 1.64-fold over basal, respectively) in plasma membrane lawns. These results indicate that (i) PI(3,4,5)P(3) does not make the active site of GLUT4 available from the extracellular surface despite causing GLUT4 fusion with the plasma membrane; (ii) the availability of the active site of GLUT4 from the extracellular medium and availability of the C-terminus from the cytosolic site are correlated; (iii) in addition to stimulating GLUT4 translocation, insulin stimulation displaces a protein which masks the GLUT4 C-terminus. We propose that a protein which masks the C-terminus also prevents the active site from being available for photolabelling and possibly glucose uptake after treatment with PI(3,4,5)P(3).

Insulin Regulates the Membrane Arrival, Fusion, and C-terminal Unmasking of Glucose Transporter-4 via Distinct Phosphoinositides

Insulin increases glucose uptake into muscle via glucose transporter-4 (GLUT4) translocation to the cell membrane, but the regulated events in GLUT4 traffic are unknown. Here we focus on the role of class IA phosphatidylinositol (PI) 3-kinase and specific phosphoinositides in the steps of GLUT4 arrival and fusion with the membrane, using L6 muscle cells expressing GLUT4myc. To this end, we detected the availability of the myc epitope at the cell surface or intravesicular spaces and of the cytosol-facing C-terminal epitope, in cells and membrane lawns derived from them. We observed the following: (a) Wortmannin and LY294002 at concentrations that inhibit class IA PI 3-kinase reduced but did not abate the C terminus gain, yet the myc epitope was unavailable for detection unless lawns or cells were permeabilized, suggesting the presence of GLUT4myc in docked, unfused vesicles. Accordingly, GLUT4myc-containing vesicles were detected by immunoelectron microscopy of membranes from cells pretreated with wortmannin and insulin, but not insulin or wortmannin alone. (b) Insulin caused greater immunological availability of the C terminus than myc epitopes, suggesting that C terminus unmasking had occurred. Delivering phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)) to intact cells significantly increased lawn-associated myc signal without C terminus gain. Conversely, phosphatidylinositol 3-phosphate (PI3P) increased the detection of C terminus epitope without any myc gain. We propose that insulin regulates GLUT4 membrane arrival, fusion, and C terminus unmasking, through distinct phosphoinositides. PI(3,4,5)P(3) causes arrival and fusion without unmasking, whereas PI3P causes arrival and unmasking without fusion.

Isoform-Specific Differences in the Size of Desmosomal Cadherin/Catenin Complexes

Isoform-Specific Differences in the Size of Desmosomal Cadherin/Catenin Complexes

In situ study of c-myc protein expression during avian development.

The distribution of the c-myc protein was studied in the developing embryo from the two-somite stage to embryonic day 17 (E17). A triple labelling method was used, with a polyclonal serum recognizing the human and avian c-myc proteins as the first marker followed by Hoechst 33258 for nuclear staining and the monoclonal antibody 13F4 which reveals the avian myogenic lineage. In situ hybridization was carried out at three selected stages (E3, E6 and E8), in order to compare the distribution of myc mRNA and myc protein. The c-myc protein signal was barely detectable in blastodisc nuclei during the period of somite formation, after which it became ubiquitous in the embryonic body until E4. Myotomal cell nuclei displayed a strong signal until their organization into premuscular masses. On day 4, the level of c-myc protein decreased in all embryonic tissues. By doubling the antibody titre and amplifying the signal by means of the streptavidin-biotin method, c-myc could still be detected in nuclei of defined groups of cells. Such was the case in some mesenchyme-derived tissues at critical periods of organogenesis, for instance in prechondrogenic condensations or hemopoietic cell foci at E6, the latter becoming negative at E9. The heart ventricle displayed a patch-work of positive and negative nuclei from E6 to E10. A myc signal restricted to the quail species was found in the wall of the carotid arteries. Cell nuclei in the nervous system displayed a detectable signal which became restricted to postmitotic neurones. In the ectoderm, the c-myc protein was generally not present after E4, except in presumptive feather buds at the time of epitheliomesenchymal interactions. Endodermal cells (such as hepatocytes, oesophageal and tracheal epithelia) did not express detectable levels of c-myc at any time. Our results reveal a time- and tissue-specific expression of c-myc during avian development. It is noteworthy that the expression of the c-myc protein often appears dissociated from cell proliferation as shown by the absence of the signal in endodermal cells at E3-E13 as well as its presence in postmitotic neurones. Finally, although RNA and protein are simultaneously detected in some structures such as presumptive feather buds, their expression is dissociated in endodermal tissues, notably hepatocytes, where in situ hybridization detects a large number of RNA copies with no detectable protein signal.

Oncogene Expression in Human Leukemia

AbstractThe transforming genes of retroviruses (v-onc) are derived from normal cellular genes referred to as proto-oncogenes. These cellular genes have the capacity for conversion to oncogenes (c-onc) that are capable of inducing or maintaining the transformed state when they are overly expressed or altered by mutation or rearrangement. To study the possible involvement of these genes in human leukemia, we have analyzed their expression in a variety of fresh samples. We found that a number of oncogenes are expressed in different leukemic types and that although the transcript size did not vary for each gene, the copy number did. The myc gene (2.4 kb transcript) and the rasHa gene (1.5 kb transcript) were universally expressed. But in contrast to rasHa, the myc signal intensity varied. Myb (4.5 kb transcript) was expressed in all samples except B cell diseases. We detected low levels of abl expression (multiple mRNA species) in all leukemic types analyzed. Sis gene (4.2 kb transcript) expression was restricted to one patient sample with chronic myelogenous leukemia in blast transformation.

Oncogene expression in human leukemia

The transforming genes of retroviruses (v-onc) are derived from normal cellular genes referred to as proto-oncogenes. These cellular genes have the capacity for conversion to oncogenes (c-onc) that are capable of inducing or maintaining the transformed state when they are overly expressed or altered by mutation or rearrangement. To study the possible involvement of these genes in human leukemia, we have analyzed their expression in a variety of fresh samples. We found that a number of oncogenes are expressed in different leukemic types and that although the transcript size did not vary for each gene, the copy number did. The myc gene (2.4 kb transcript) and the rasHa gene (1.5 kb transcript) were universally expressed. But in contrast to rasHa, the myc signal intensity varied. Myb (4.5 kb transcript) was expressed in all samples except B cell diseases. We detected low levels of abl expression (multiple mRNA species) in all leukemic types analyzed. Sis gene (4.2 kb transcript) expression was restricted to one patient sample with chronic myelogenous leukemia in blast transformation.