Cell cycle changes in the adenylate cyclase of C6 glioma cells

c-Myc-IMPDH1/2 axis promotes tumourigenesis by regulating GTP metabolic reprogramming

BACKGROUND

Metabolic reprogramming is a hallmark of cancer. Metabolic rate-limiting enzymes and oncogenic c-Myc (Myc) play critical roles in metabolic reprogramming to affect tumourigenesis. However, a systematic assessment of metabolic rate-limiting enzymes and their relationship with Myc in human cancers is lacking.

METHODS

Multiple Pan-cancer datasets were used to develop the transcriptome, genomic alterations, clinical outcomes and Myc correlation landscapes of 168 metabolic rate-limiting enzymes across 20 cancers. Real-time quantitative PCR and immunoblotting were, respectively, used to examine the mRNA and protein of inosine monophosphate dehydrogenase 1 (IMPDH1) in human colorectal cancer (CRC), azoxymethane/dextran sulphate sodium-induced mouse CRC and spontaneous intestinal tumours from APCMin/+ mice. Clone formation, CCK-8 and subcutaneous xenograft model were applied to investigate the possible mechanisms connecting IMPDH1 to CRC growth. Co-immunoprecipitation and protein half-life assay were used to explore the mechanisms underlying the regulation of IMPDH1.

RESULTS

We explored the global expression patterns, dysregulation profiles, genomic alterations and clinical relevance of 168 metabolic rate-limiting enzymes across human cancers. Importantly, a series of enzymes were associated with Myc, especially top three upregulated enzymes (TK1, RRM2 and IMPDH1) were positively correlated with Myc in multiple cancers. As a proof-of-concept exemplification, we demonstrated that IMPDH1, a rate-limiting enzyme in GTP biosynthesis, is highly upregulated in CRC and promotes CRC growth in vitro and in vivo. Mechanistically, IMPDH2 stabilizes IMPDH1 by decreasing the polyubiquitination levels of IMPDH1, and Myc promotes the de novo GTP biosynthesis by the transcriptional activation of IMPDH1/2. Finally, we confirmed that the Myc-IMPDH1/2 axis is dysregulated across human cancers.

CONCLUSIONS

Our study highlights the essential roles of metabolic rate-limiting enzymes in tumourigenesis and their crosstalk with Myc, and the Myc-IMPDH1/2 axis promotes tumourigenesis by altering GTP metabolic reprogramming. Our results propose the inhibition of IMPDH1 as a viable option for cancer treatment.

Insertion of ganglioside GM1 into rat glioma C6 cells renders them susceptible to growth inhibition by the B subunit of cholera toxin

The B subunit of cholera toxin does not affect the growth of rat glioma C6 cells which are deficient of its receptor, ganglioside GM1. Insertion of ganglioside GM1 into the plasma membrane of C6 cells renders them susceptible to inhibition of DNA synthesis by the B subunit. Exposure of C6 cells to butyrate induces an elevation of ganglioside GM1 as measured by an increase in binding of iodinated cholera toxin and also results in an inhibition of DNA synthesis by the B subunit. The extent of inhibition of DNA synthesis correlated with the binding of B subunit and was independent of adenylate cyclase activation or increases in intracellular cAMP levels.

Surface functions during mitosis in rat basophilic leukemia cells

At the entry into mitosis, cells abruptly lose membrane activities such as phagocytosis, pinocytosis, and capping. The present studies test if mitotic cells also resist functional responses to cell surface ligand-receptor interactions. The IgE receptors of RBL-2H3 rat basophilic leukemia cells were labeled with anti-dinitrophenol IgE (anti-DNP-IgE) and then cross-linked with multivalent ligands (DNP-bovine serum albumin [BSA]; DNP-B-phycoerythrin; DNP-BSA-gold). IgE-receptor cross-linking modulates cell surface organization and function and releases serotonin and other mediators of allergic and asthmatic reactions from interphase cells (Pfeiffer, J. R., JC. Seagrave, B. H. Davis, G. G. Deanin, and J. M. Oliver, 1985, J. Cell Biol., 101:2145-2155). It was found that anti-DNP-IgE-receptor complexes are preserved on the cell surface throughout mitosis; they continue to bind DNP-proteins, and the resulting antigen-IgE-receptor complexes can redistribute to coated pits on the cell surface. Furthermore, there is no loss of [3H]serotonin through mitosis. Nevertheless, antigen-stimulated [3H]-serotonin release is strongly impaired in mitotic-enriched as compared with mixed interphase or G1-enriched cell populations. In addition, antigen binding transforms the surface of interphase cells from a microvillous to a plicated topography and stimulates the uptake of fluorescein isothiocyanate-conjugated dextran by fluid pinocytosis. Mitotic cells maintain a microvillous surface topography after antigen treatment, and fluid pinocytosis virtually ceases from prometaphase to telophase. Phorbol myristate acetate, a tumor promoter that activates protein kinase C, restores surface ruffling activity to mitotic cells. Thus, the mitosis-specific freezing of membrane and secretory responses is most likely due to the failure of transmembrane signaling.

Beta adrenergic receptor repopulation of C6 glioma cells after irreversible blockade and down regulation

C6 glioma cells possess beta adrenergic receptors coupled with adenylate cyclase which can be irreversibly blocked by bromoacetylaminomethylpindolol (Br-AAM-pindolol), a beta adrenergic antagonist. With 1 microM Br-AAM-pindolol, more than 80% of beta adrenergic receptors, labeled by (3H)-dihydroalprenolol [3H)-DHA), were blocked. After this blockade, new beta adrenergic receptors were synthesized only during cell division. However, at cell confluency when the cell number was constant, turnover of beta adrenergic receptors was barely detectable. Cycloheximide (1 microgram/ml) inhibited cell growth as well as reappearance of beta adrenergic receptors. A 90% loss of beta adrenergic receptors in C6 glioma cells was obtained after down-regulation for 15 h with 10 microM isoproterenol, a beta adrenergic agonist. After removal of the agonist, recovery of beta-adrenergic-sensitive adenylate cyclase was complete within 2 to 3 days, whereas beta adrenergic receptors reached 90% of control value within 6 days. The half-life of the receptor recovery was 2 to 3 days. Pretreatment of C6 glioma cells by Br-AAM-pindolol and subsequent cell exposure to isoproterenol indicated that down regulation and recovery of unblocked beta adrenergic receptors did occur; however isoproterenol did not accelerate the biosynthesis of beta adrenergic receptors. The recovery of both biological response and beta adrenergic receptor occupancy was restored both in the presence or absence of cycloheximide (1 microgram/ml), a concentration which blocked 90% of protein synthesis. Our results suggest that reappearance of beta adrenergic receptors in C6 glioma cells, following isoproterenol-induced down regulation, was not due to synthesis of new receptors but to recycling of the beta adrenergic receptors.