Regulation of RNA metabolism in relation to insulin production and oxidative metabolism in mouse pancreatic islets in vitro

This study was undertaken to investigate the long-term effects of different substrates, in particular glucose, on the regulation of islet RNA metabolism and the relationship of this regulation to the metabolism and insulin production of the islet B-cell. For this purpose collagenase-isolated mouse islets were used either in the fresh state or after culture for 2 or 5 days in RPMI 1640 plus 10% calf serum supplemented with various test compounds. Islets cultured with 16.7 mM glucose contained more RNA than those cultured with 3.3 mM glucose. Culture of islets in glucose at low concentrations inhibited glucose-stimulated RNA synthesis and this inhibitory effect was reversed by prolonged exposure to high glucose concentrations. Culture with 10 mM leucine and 3.3 mM glucose or with 10 mM 2-ketoisocaproate and 3.3 mM glucose increased the total RNA content of islets as compared to that of islets cultured with 3.3 mM glucose alone. Islets cultured with 5 mM theophylline maintained a high RNA content in the presence of 3.3 mM glucose. Theophylline also increased the islet RNA content when added together with 16.7 mM glucose, as compared to 16.7 mM glucose alone. Theophylline probably exerted this effect by decreasing the rate of RNA degradation. Changes in islet RNA metabolism showed a close correlation to changes in islet total protein biosynthesis, whereas islet (pro)insulin biosynthesis and insulin release exhibited different glucose-dependency patterns. The response of islet oxygen uptake to glucose was similar to that of islet RNA and protein biosynthesis. It is concluded that the RNA content of the pancreatic islets is controlled at the levels of both synthesis and degradation. Glucose stimulates the RNA synthesis and inhibits its degradation. Moreover, the results suggest that regulation of RNA synthesis may be mediated through islet metabolic fluxes and the cAMP system.     

Plasma membrane oxidoreductase activity in cultured cells in relation to mitochondrial function and oxidative stress

Dichlorophenol indophenol (DCIP) reduction by intracellualr pyridine nucleotides was investigated in two different lines of cultured cells characterized by enhanced production of reacive oxygen species (ROS) with respect to suitable controls. The first line denominated XTC-UC1 was derived from a metastasis of an oxyphilic thyroid tumor characterized by mitochondrial hyperplasia and compared with a line (B-CPAP) derived from a papillary thyroid carcinoma with normal mitochondrial mass. The second line (170 MN) was a cybrid line derived from rho0 cells from an osteosarcoma line (143B) fused with platelets from a patient with a nucleotide 9957 mutation in mitochondrial DNA (encoding for cytochrome c oxidase subunit III) in comparison with the parent 143B line. The experimental lines had no major decreases of electron transfer activities with respect to the controls; both of them, however, exhibited an increased peroxide production. The XTC-UC1 cell line exhibited enhanced activity with respect to control of dicoumarol-sensitive DCIP reduction, identified with membrane bound DT-diaphorase, whereas dicoumarol insensitive DCIP reduction was not significantly changed. On the other hand the mtDNA mutated cybrids exhibited a strong increase of both dicoumarol sensitive and insensitive DCIP reduction. The results suggest that enhanced oxidative stress and not deficient respiratory activity per se is the stimulus triggering over-expression of plasma membrane oxidative enzymes.     

Glycolytic metabolism in cultured cells of the nervous system

The effects of thiamine deficiency and of the antithiamine drug pyrithiamine on the C-6 glioma and the C-1300 neuroblastoma cell lines have been studied. Thiamine deficiency increased the doubling time of the neuroblastoma cells without affecting that of the glioma cells. Pyrithiamine prevented both cell lines from doubling even once. (hiamine deficiency had only slight effects on intracellular pyruvate and lactate levels or on efflux rates for the acids, but pyrithiamine treatment resulted in large increases in both the intracellular levels and the efflux in both cell lines. For comparison, the pyruvate and lactate levels in mouse brain were measured. The levels from thiamine-deficient mouse brain were essentially unchanged from controls while pyrithiamine treatment caused a significant elevation only of the pyruvate concentration.     

Glycolytic metabolism in cultured cells of the nervous system

Pyruvate and lactate efflux from C-6 glioma cells has been found to be regulated by both the medium glucose concentration and the medium concentration of the two acids. Each moves down a concentration gradient until the extracellular level is in equilibrium with the intracellular. Long-term growth studies demonstrated that the cells preferentially utilize glucose but that once it is depleted, they will take up first pyruvate, followed by lactate, for further metabolism. Changes in the intracellular levels of the two metabolites correspond to those seen in the medium. The rate of glycogen breakdown parallels that of medium glucose ultilization. Preliminary results with the C-1300 neuroblastoma cells showed pyruvate and lactate efflux rates comparable to those of the glioma cells.     

Glycolytic metabolism in cultured cells of the nervous system

The transport and metabolism of glucose was examined in monolayers of C-6 glioma cells. 1) Glucose transport appeared to have both a low (Km = 7.74 mM) and a high (Km = 1.16 mM) affinity site in C-6cells; whereas 2-deoxyglucose had only one (Km = 3.7 mM). 2) A large portion of the accumulated glucose was rapidly metabolized to the two glycolytic end products, lactate and pyruvate, and then extruded into the medium. The temperature-dependent efflux of lactate and pyruvate was linear up to 2 hrs with 6 to 10 times more lactate being extruded into the medium than pyruvate. 3) The efflux of lactate and pyruvate increased with increasing extracellular (medium) pH. The presence of 5 percent CO2 not only inhibited the acid efflux but also inhibited the short-term uptake of glucose. The CO2 effect was attributed to a lowering of the medium pH since bicarbonate alone either increased or did not inhibit efflux. 4) Valinomycin increased the levels of cellular lactate but not those of pyruvate by almost three-fold. Lactate efflux was stimulated while that of pyruvate was inhibited. The addition of 5 percent CO2 increased the cellular levels of both lactate and pyruvate, but unlike valinomycin decreased the acid efflux. Idoacetate inhibited the acid efflux by 50 percent suggesting that glycolysis is necessary for efflux.     

The components of an α-glycerophosphate cycle and their relation to oxidative metabolism in the lens

1. The concentration of ATP in a lens brei is maintained when the brei is incubated in oxygen with alpha-glycerophosphate. Lack of alpha-glycerophosphate or incubation in nitrogen causes the concentration to decrease. alpha-Glycerophosphate has some effect under anaerobic conditions but this is not sufficient to account for the maintenance in oxygen. 2. Manometric experiments show that alpha-glycerophosphate enhances the respiration of lens preparations. This respiration can be further increased by the addition of ADP and is abolished by cyanide and antimycin. The inference from these experiments is that a mitochondrial system able to oxidize alpha-glycerophosphate is present, i.e. the particulate half of the alpha-glycerophosphate cycle. 3. More than the calculated proportion of NADH is used when limiting amounts of dihydroxyacetone phosphate are added to lens tissue in spectrophotometric experiments. Dihydroxyacetone phosphate is therefore regenerated and an alpha-glycerophosphate cycle is operative. 4. A preparation of a particulate alpha-glycerophosphate dehydrogenase that takes up oxygen with methylene blue as electron acceptor is described. 5. Methods for obtaining mitochondria from lens are compared, and a useful extraction medium is defined. 6. Mitochondria with activities of the same order of magnitude as those obtained from liver, with alpha-glycerophosphate and glutamate as substrates, are prepared from epithelium detached from the capsule; some respiratory control is observed.