Differential amplification and disproportionate expression of five genes in three multidrug-resistant Chinese hamster lung cell lines.

At least five linked genes are amplified in the multidrug-resistant Chinese hamster ovary cell line CHRC5, selected with colchicine (A. M. Van der Bliek, T. Van der Velde-Koerts, V. Ling, and P. Borst, Mol. Cell. Biol. 6:1671-1678, 1986). We report here that only a subset of these, encoding the 170-kilodalton P-glycoprotein, are consistently amplified in three different multidrug-resistant Chinese hamster lung cell lines, selected with vincristine, daunorubicin, or actinomycin D. Within each cell line, genomic sequences homologous to the P-glycoprotein cDNA probe were amplified to different levels. The pattern of differential amplification was consistent with the presence of at least two and possibly three P-glycoprotein genes. In the actinomycin D-selected cell line, these genes were disproportionately overexpressed relative to the associated levels of amplification. These results underline a central role for P-glycoprotein in multidrug resistance. In the daunorubicin-selected cell line, another, as yet uncharacterized, gene was amplified but disproportionately underexpressed. Its amplification was therefore fortuitous. We present a tentative map of the region in the hamster genome that is amplified in the multidrug-resistant cell lines which were analyzed.     

In vitro site-directed mutagenesis with synthetic DNA oligonucleotides yields unexpected deletions and insertions at high frequency.

We have used in vitro site-directed mutagenesis with synthetic DNA oligonucleotides to introduce single nucleotide mutations in yeast mtDNA. In addition to the expected DNA alterations we also recovered with high frequency mutants with large deletions and insertions which arose through interaction with the synthetic DNA fragment. Characterization of a number of these by DNA sequence analysis has permitted reconstruction of the mutagenic events. In all cases, the DNA fragment had base paired with non-adjacent DNA sequences sometimes more than 1000 nucleotides apart from each other on the target strand. The products of such interactions cannot be avoided due to the non-stringent annealing conditions during complementary DNA strand synthesis. However, deliberate mispairing can be directed precisely, as shown by our ability to specifically delete the 1143-bp intron from the yeast mitochondrial gene coding for large ribosomal RNA with a synthetic DNA fragment consisting of the sequence of the exon borders flanking the intron.     

Niacin Reduces Atherosclerosis Development in APOE*3Leiden.CETP Mice Mainly by Reducing NonHDL-Cholesterol

Objective

Niacin potently lowers triglycerides, mildly decreases LDL-cholesterol, and largely increases HDL-cholesterol. Despite evidence for an atheroprotective effect of niacin from previous small clinical studies, the large outcome trials, AIM-HIGH and HPS2-THRIVE did not reveal additional beneficial effects of niacin (alone or in combination with laropiprant) on top of statin treatment. We aimed to address this apparent discrepancy by investigating the effects of niacin without and with simvastatin on atherosclerosis development and determine the underlying mechanisms, in APOE*3Leiden.CETP mice, a model for familial dysbetalipoproteinemia (FD).

Approach and Results

Mice were fed a western-type diet containing cholesterol without or with niacin (120 mg/kg/day), simvastatin (36 mg/kg/day) or their combination for 18 weeks. Similarly as in FD patients, niacin reduced total cholesterol by -39% and triglycerides by −50%, (both P<0.001). Simvastatin and the combination reduced total cholesterol (−30%; −55%, P<0.001) where the combination revealed a greater reduction compared to simvastatin (−36%, P<0.001). Niacin decreased total cholesterol and triglycerides primarily by increasing VLDL clearance. Niacin increased HDL-cholesterol (+28%, P<0.01) and mildly increased reverse cholesterol transport. All treatments reduced monocyte adhesion to the endothelium (−46%; −47%, P<0.01; −53%, P<0.001), atherosclerotic lesion area (−78%; −49%, P<0.01; −87%, P<0.001) and severity. Compared to simvastatin, the combination increased plaque stability index [(SMC+collagen)/macrophages] (3-fold, P<0.01). Niacin and the combination reduced T cells in the aortic root (−71%, P<0.01; −81%, P<0.001). Lesion area was strongly predicted by nonHDL-cholesterol (R² = 0.69, P<0.001) and to a much lesser extent by HDL-cholesterol (R² = 0.20, P<0.001).

Conclusion

Niacin decreases atherosclerosis development mainly by reducing nonHDL-cholesterol with modest HDL-cholesterol-raising and additional anti-inflammatory effects. The additive effect of niacin on top of simvastatin is mostly dependent on its nonHDL-cholesterol-lowering capacities. These data suggest that clinical beneficial effects of niacin are largely dependent on its ability to lower LDL-cholesterol on top of concomitant lipid-lowering therapy.     

A mitochondrial rhomboid protease

Rhomboid proteases are integral membrane proteins, typically associated with cleavage of peptide hormones along the secretory pathway. Recent publications demonstrate that yeast mitochondria contain a rhomboid protease required for the cleavage of two mitochondrial intermembrane space proteins, suggesting that rhomboid proteases play a regulatory role in mitochondria.     

Endophilin is required for synaptic vesicle endocytosis by localizing synaptojanin

Endophilin is a membrane-associated protein required for endocytosis of synaptic vesicles. Two models have been proposed for endophilin: that it alters lipid composition in order to shape membranes during endocytosis, or that it binds the polyphosphoinositide phosphatase synaptojanin and recruits this phosphatase to membranes. In this study, we demonstrate that the unc-57 gene encodes the Caenorhabditis elegans ortholog of endophilin A. We demonstrate that endophilin is required in C. elegans for synaptic vesicle recycling. Furthermore, the defects observed in endophilin mutants closely resemble those observed in synaptojanin mutants. The electrophysiological phenotype of endophilin and synaptojanin double mutants are virtually identical to the single mutants, demonstrating that endophilin and synaptojanin function in the same pathway. Finally, endophilin is required to stabilize expression of synaptojanin at the synapse. These data suggest that endophilin is an adaptor protein required to localize and stabilize synaptojanin at membranes during synaptic vesicle recycling.     

Regulation of the mitochondrial dynamin-like protein Opa1 by proteolytic cleavage

The dynamin-related protein Opa1 is localized to the mitochondrial intermembrane space, where it facilitates fusion between mitochondria. Apoptosis causes Opa1 release into the cytosol and causes mitochondria to fragment. Loss of mitochondrial membrane potential also causes mitochondrial fragmentation but not Opa1 release into the cytosol. Both conditions induce the proteolytic cleavage of Opa1, suggesting that mitochondrial fragmentation is triggered by Opa1 inactivation. The opposite effect was observed with knockdown of the mitochondrial intermembrane space protease Yme1. Knockdown of Yme1 prevents the constitutive cleavage of a subset of Opa1 splice variants but does not affect carbonyl cyanide m-chlorophenyl hydrazone or apoptosis-induced cleavage. Knockdown of Yme1 also increases mitochondrial connectivity, but this effect is independent of Opa1 because it also occurs in Opa1 knockdown cells. We conclude that Yme1 constitutively regulates a subset of Opa1 isoforms and an unknown mitochondrial morphology protein, whereas the loss of membrane potential induces the further proteolysis of Opa1.     

The C. elegans Opa1 homologue EAT-3 is essential for resistance to free radicals

The C. elegans eat-3 gene encodes a mitochondrial dynamin family member homologous to Opa1 in humans and Mgm1 in yeast. We find that mutations in the C. elegans eat-3 locus cause mitochondria to fragment in agreement with the mutant phenotypes observed in yeast and mammalian cells. Electron microscopy shows that the matrices of fragmented mitochondria in eat-3 mutants are divided by inner membrane septae, suggestive of a specific defect in fusion of the mitochondrial inner membrane. In addition, we find that C. elegans eat-3 mutant animals are smaller, grow slower, and have smaller broodsizes than C. elegans mutants with defects in other mitochondrial fission and fusion proteins. Although mammalian Opa1 is antiapoptotic, mutations in the canonical C. elegans cell death genes ced-3 and ced-4 do not suppress the slow growth and small broodsize phenotypes of eat-3 mutants. Instead, the phenotypes of eat-3 mutants are consistent with defects in oxidative phosphorylation. Moreover, eat-3 mutants are hypersensitive to paraquat, which promotes damage by free radicals, and they are sensitive to loss of the mitochondrial superoxide dismutase sod-2. We conclude that free radicals contribute to the pathology of C. elegans eat-3 mutants.     

Prolonged niacin treatment leads to increased adipose tissue PUFA synthesis and anti-inflammatory lipid and oxylipin plasma profile

Prolonged niacin treatment elicits beneficial effects on the plasma lipid and lipoprotein profile that is associated with a protective CVD risk profile. Acute niacin treatment inhibits nonesterified fatty acid release from adipocytes and stimulates prostaglandin release from skin Langerhans cells, but the acute effects diminish upon prolonged treatment, while the beneficial effects remain. To gain insight in the prolonged effects of niacin on lipid metabolism in adipocytes, we used a mouse model with a human-like lipoprotein metabolism and drug response [female APOE*3-Leiden.CETP (apoE3 Leiden cholesteryl ester transfer protein) mice] treated with and without niacin for 15 weeks. The gene expression profile of gonadal white adipose tissue (gWAT) from niacin-treated mice showed an upregulation of the “biosynthesis of unsaturated fatty acids” pathway, which was corroborated by quantitative PCR and analysis of the FA ratios in gWAT. Also, adipocytes from niacin-treated mice secreted more of the PUFA DHA ex vivo. This resulted in an increased DHA/arachidonic acid (AA) ratio in the adipocyte FA secretion profile and in plasma of niacin-treated mice. Interestingly, the DHA metabolite 19,20-dihydroxy docosapentaenoic acid (19,20-diHDPA) was increased in plasma of niacin-treated mice. Both an increased DHA/AA ratio and increased 19,20-diHDPA are indicative for an anti-inflammatory profile and may indirectly contribute to the atheroprotective lipid and lipoprotein profile associated with prolonged niacin treatment.