Chinese hamster ovary cells deficient in peroxisomes lack the nonspecific lipid transfer protein (sterol carrier protein 2)

Chinese hamster ovary cells deficient in intact peroxisomes were compared with wild type cells for the presence of the nonspecific lipid transfer protein (nsL-TP; sterol carrier protein 2). With the immunoblotting technique using the affinity purified antibody against rat liver nsL-TP, this protein was shown to be present in the homogenates from wild type cells, but could not be detected in mutant cells. In agreement with a previous study using livers from Zellweger patients it appears that there is a positive, as yet unknown, correlation between peroxisomes and the occurrence of nsL-TP in the cell. As a control using the affinity-purified antibody against the phosphatidylinositol transfer protein from bovine brain, levels of this protein were found to be normal in mutant cells. By use of metrizamide density gradients, nsL-TP was shown to cosediment with a membrane fraction different from peroxisomes. A protein of 58,000 daltons cross-reactive with the antibody against nsL-TP did cosediment with the peroxisomes in wild type cells and possibly with a "peroxisomal remnant" in the mutant cells. Incubation of wild type and mutant cells with L-[3-14C]serine showed that the biosynthesis of phosphatidylserine and the subsequent conversion into phosphatidylethanolamine was comparable in both cell types. This indicates that nsL-TP is not involved in the translocation of phosphatidylserine from the endoplasmic reticulum to the mitochondria as the site of decarboxylation.     

Sterol-responsive element-binding protein (SREBP) 2 down-regulates ATP-binding cassette transporter A1 in vascular endothelial cells: a novel role of SREBP in regulating cholesterol metabolism

ATP-binding cassette transporter A1 (ABCA1) is a pivotal regulator of cholesterol efflux from cells to apolipoproteins, whereas sterol-responsive element-binding protein 2 (SREBP2) is the key protein regulating cholesterol synthesis and uptake. We investigated the regulation of ABCA1 by SREBP2 in vascular endothelial cells (ECs). Our results showed that sterol depletion activated SREBP2 and increased its target, low density lipoprotein receptor mRNA, with a concurrent decrease in the ABCA1 mRNA. Transient transfection analysis revealed that sterol depletion decreased the ABCA1 promoter activity by 50%, but low density lipoprotein receptor promoter- and the sterol-responsive element-driven luciferase activities were increased. Overexpression of the N terminus of SREBP2 (SREBP2(N)), an active form of SREBP2, also inhibited the ABCA1 promoter activity. Functionally adenovirus-mediated SREBP2(N) expression increased cholesterol accumulation and decreased apoA-I-mediated cholesterol efflux. The conserved E-box motif was responsible for the SREBP2(N)-mediated inhibition since mutation of the E-box increased the basal activity of the ABCA1 promoter and abolished the inhibitory effect of SREBP2(N). Furthermore sterol depletion and SREBP2(N) overexpression induced the binding of SREBP2(N) to both consensus and ABCA1-specific E-box. Chromatin immunoprecipitation assay demonstrated that serum starvation enhanced the association of SREBP2 and the ABCA1 promoter in ECs. To correlate this mechanism pathophysiologically, we found that oscillatory flow caused the activation of SREBP2 and therefore attenuated ABCA1 promoter activity in ECs. Thus, this SREBP-regulated mechanism may control the efflux of cholesterol, which is a newly defined function of SREBP2 in ECs in addition to its role in cholesterol uptake and biosynthesis.     

Lanosterol metabolism and sterol regulatory element binding protein (SREBP) expression in male germ cell maturation

Expression of genes involved in cholesterol biosynthesis in male germ cells is insensitive to the negative cholesterol feedback regulation, in contrast to cholesterol level-sensitive/sterol regulatory element binding protein (SREBP)-dependent gene regulation in somatic cells. The role of sterol regulatory element binding proteins in spermatogenic cells was an enigma until recently, when a soluble, 55 kDa cholesterol-insensitive form of SREBP2 (SREBP2gc) was discovered [Mol. Cell. Endocrinol. 22 (2002) 8478], being translated from a germ cell-specific SREBP2 mRNA. Our RT-PCR results also show that SREBP2 as well as SREBP1c mRNAs are detectable in prepubertal and postpubertal male germ cells while SREBP1a is not detected. Surprisingly, three SREBP2 immunoreactive proteins (72, 63 and 55 kDa), that are not present in mouse liver nuclei, reside in testis nuclei of prepubertal and adult mice. The 55 kDa protein is likely SREBP2gc, the other two isoforms are novel. HPLC measurements in liver and testes of fasted prepubertal and postpubertal mice showed no significant difference in cholesterol level. However, FF-MAS and lanosterol/testis-meiosis activating sterol (T-MAS) intermediates that are detectable mainly in testes, increase in fasted postpubertal mice which coincides well with the elevated level of 68 kDa SREBP2. Similar to SREBP2gc, the two novel SREBP2 immunoreactive proteins seem to be insensitive to the level of cholesterol.     

Selective binding of sterol regulatory element-binding protein isoforms and co-regulatory proteins to promoters for lipid metabolic genes in liver

Mice were subjected to different dietary manipulations to selectively alter expression of hepatic sterol regulatory element-binding protein 1 (SREBP-1) or SREBP-2. mRNA levels for key target genes were measured and compared with the direct binding of SREBP-1 and -2 to the associated promoters using isoform specific antibodies in chromatin immunoprecipitation studies. A diet supplemented with Zetia (ezetimibe) and lovastatin increased and decreased nuclear SREBP-2 and SREBP-1, respectively, whereas a fasting/refeeding protocol dramatically altered SREBP-1 but had modest effects on SREBP-2 levels. Binding of both SREBP-1 and -2 increased on promoters for 3-hydroxy-3-methylglutaryl-CoA reductase, fatty-acid synthase, and squalene synthase in livers of Zetia/lovastatin-treated mice despite the decline in total SREBP-1 protein. In contrast, only SREBP-2 binding was increased for the low density lipoprotein receptor promoter. Decreased SREBP-1 binding during fasting and a dramatic increase upon refeeding indicates that the lipogenic "overshoot" for fatty-acid synthase gene expression known to occur during high carbohydrate refeeding can be attributed to a similar overshoot in SREBP-1 binding. SREBP co-regulatory protein recruitment was also increased/decreased in parallel with associated changes in SREBP binding, and there were clear distinctions for different promoters in response to the dietary manipulations. Taken together, these studies reveal that there are alternative molecular mechanisms for activating SREBP target genes in response to the different dietary challenges of Zetia/lovastatin versus fasting/refeeding. This underscores the mechanistic flexibility that has evolved at the individual gene/promoter level to maintain metabolic homeostasis in response to shifting nutritional states and environmental fluctuations.     

Mutants of Chinese hamster ovary (CHO) cells with altered colcemid-binding affinity.

Clones of CHO cells stably resistant to colcemid have been isolated in the presence of the nonionic detergent Tween 80 after mutagen treatment. Successive single-step selections for increasing resistance were performed resulting in lines after three selection steps about 10 fold more resistant to colcemid than the parental cells. Three observations indicate that these colcemid-resistant (CMR) mutants are different from the colchicine-resistant permeability mutants isolated previously. First, their relative resistance to colcemid was not diminished in the presence of detergent which promoted increased drug permeability. Second, the CMR clones displayed limited cross-resistances only to tubulin-binding compounds. Third, the binding affinity of labeled colcemid by cytoplasmic extracts from CMR clones was reduced, and the reduction was greater in the more resistant clones. No reduction in binding of labeled colcemid was observed in the membrane-altered colchicine-resistant mutants. All these observations are consistent with the CMR clones being tubulin-altered mutants. In further support of this conclusion, we observed that tubulin purified from a CMR mutant still possessed reduced colcemid-binding affinity compared with that from parental cells.     

Esterified cholesterol and triglyceride are present in plasma membranes of Chinese hamster ovary cells.

The chemical composition of highly purified plasma membrane preparations from a series of malignant Chinese hamster ovary (CHO) cell lines were undertaken to ascertain if neutral lipid, including cholesteryl ester and triacylglycerol, were present. Triacylglycerols (33-41 nmol/mg total lipid) and cholesteryl ester (226-271 nmol/mg) were measured in the plasma membranes and differences in the chemical composition of these membranes recorded. The most significant difference was a gradual decrease in the level of free cholesterol from wild type (312 +/- 7 nmol/mg total plasma membrane lipid), Pod RII-6 (268 +/- 64 nmol/mg total plasma membrane lipid), Col R-22 (243 +/- 39 nmol/mg total plasma membrane lipid) to EOT (204 +/- 20 nmol/mg total plasma membrane lipid), with a concomitant increase in the degree of saturation of the cholesteryl ester fatty acids, particularly palmitic acid. No statistically significant differences were apparent in the chemical composition of the whole cells in this series. The one-dimensional (1D) 1H-NMR spectra of the four malignant cell lines showed a gradation in intensity of lipid resonances, in the order of wild type, Pod RII-6, Col R-22 and EOT, with EOT having the strongest lipid spectrum. Interestingly, the increase in acyl-chain signal intensities in the 1H-NMR spectra of this series of CHO cells and emergence of signals from cholesterol and/or cholesteryl ester, coincide with alterations in the amount of free cholesterol and the degree of saturation of the fatty-acyl chain of the esterified cholesterol in the plasma membranes. It is our hypothesis that, together, cholesteryl ester and triacylglycerol form domains in the plasma membrane and that when the cholesteryl ester has a largely saturated fatty acid content, the lipids are in isotropic liquid phase and hence visible by NMR.