The ubiquitin-proteasome pathway mediates the regulated degradation of mammalian 3-hydroxy-3-methylglutaryl-coenzyme A reductase

3-Hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR), the key regulatory enzyme in the mevalonate (MVA) pathway, is rapidly degraded in mammalian cells supplemented with sterols or MVA. This accelerated turnover was blocked by N-acetyl-leucyl-leucyl-norleucinal (ALLN), MG-132, and lactacystin, and to a lesser extent by N-acetyl-leucyl-leucyl-methional (ALLM), indicating the involvement of the 26 S proteasome. Proteasome inhibition led to enhanced accumulation of high molecular weight polyubiquitin conjugates of HMGR and of HMGal, a chimera between the membrane domain of HMGR and beta-galactosidase. Importantly, increased amounts of polyubiquitinated HMGR and HMGal were observed upon treating cells with sterols or MVA. Cycloheximide inhibited the sterol-stimulated degradation of HMGR concomitantly with a marked reduction in polyubiquitination of the enzyme. Inhibition of squalene synthase with zaragozic acid blocked the MVA- but not sterol-stimulated ubiquitination and degradation of HMGR. Thus, similar to yeast, the ubiquitin-proteasome pathway is involved in the metabolically regulated turnover of mammalian HMGR. Yet, the data indicate divergence between yeast and mammals and suggest distinct roles for sterol and nonsterol metabolic signals in the regulated ubiquitination and degradation of mammalian HMGR.

X inactivation-specific transcript expression in mouse oocytes and zygotes

Expression of the X inactivation-specific transcript (XIST:) gene has previously been shown by reverse transcription-polymerase chain reaction (RT-PCR) to be present at the 4-cell stage of female mouse embryos. This early expression, which is followed by X inactivation in the extra-embryonic tissues, is maternally imprinted. By the blastocyst stage, as the embryonic lineages begin to form, the imprint is lost and expression becomes random. By applying in-situ RT-PCR, we showed that XIST: is expressed even earlier in development, in unfertilized mouse oocytes as well as in pronuclei stage zygotes. Our data demonstrate XIST: expression in oocytes and suggest that XIST: transcripts may occur in both XX and XY zygotes. A difference in the pattern of expression (rod-like or rounded punctate signal) is found among pronuclear-stage embryos. Early expression is in agreement with findings reported in human embryos.

Impaired regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase degradation in lovastatin-resistant cells

L-90 cells were selected to grow in the presence of serum lipoproteins and 90 microM lovastatin, an inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR). L-90 cells massively accumulate HMGR, a result of >10-fold amplification of the gene and 40-fold rise in mRNA, and also overexpress other enzymes of the mevalonate pathway. Western blot and promoter-luciferase analyses indicate that transcriptional regulation of sterol-responsive genes by 25-hydroxycholesterol or mevalonate is normal. Yet, none of these genes is regulated by lipoproteins, a result of severe impairment in the low density lipoprotein receptor pathway. Moreover, L-90 cells do not accelerate the degradation of HMGR or transfected HMGal chimera in response to 25-hydroxycholesterol or mevalonate. This aberrant phenotype persists when cells are grown without lovastatin for up to 37 days. The inability to regulate HMGR degradation is not due to its overproduction since in LP-90 cells, which were selected for lovastatin resistance in lipoprotein-deficient serum, HMGR is overexpressed, yet its turnover is regulated normally. Also, the rapid degradation of transfected alpha subunit of T cell receptor is markedly retarded in L-90 cells. These results show that in addition to gene amplification and overexpression of cholesterogenic enzymes, statin resistance can follow loss of regulated HMGR degradation.

Heparan sulfate-dependent and low density lipoprotein receptor-related protein-dependent catabolic pathways for lipoprotein lipase in mouse embryonic fibroblasts

Heparan sulfate and low density lipoprotein receptor related protein (LRP) have been shown to participate in the uptake and degradation of the enzyme lipoprotein lipase (LPL). Yet, the contribution of each of these pathways to LPL metabolism and their possible dependence is unknown. In the present study we examined the metabolism of 125I-labeled LPL in untreated and heparinase-treated primary wild-type mouse embryonic fibroblasts (MEF) and in mouse fibroblasts that express single LRP allele (PEA-10) or are lacking the LRP (PEA-13). The degradation of LPL in PEA-13 cells was 30% lower than in MEF and PEA-10 cells. Heparinase treatment decreased the LPL degradation by 58%, 79% and 92%, whereas heparin reduced such degradation by 87%, 90% and 94% in MEF, PEA-10 and PEA-13 cultures, respectively. Assuming that a) heparinase treatment abolished the heparan-sulfate pathway, and that b) the degradation remaining in heparin-treated cultures represents nonspecific values, it appears that heparan sulfate contributes about 61%, 83% and 95% of total LPL degradation, whereas the LRP pathway contributes 39%, 17% and less than 5% of LPL degradation in MEF, PEA-10 and PEA-13 cells, respectively. In addition, the data indicate that LPL interaction with heparan sulfate and the LRP pathways is independent of each other. The study shows that these cells possess both a heparan sulfate-dependent pathway and an LRP-dependent pathway for LPL metabolism and that the two pathways are independent of each other.

RhD genotype determination by single sperm cell analysis

OBJECTIVE

An Rh-negative woman with preexisting anti-D antibodies may affect some or all subsequent fetuses, depending on the genotype of her Rh-positive partner. Currently, a reliable technique for an absolute determination of RhD genotype is not available. This study was initiated to develop an accurate method for RhD genotyping in men.

STUDY DESIGN

RhD genotype was determined by deoxyribonucleic acid amplification of a D-specific sequence in single sperm cell samples. Micromanipulation techniques were used for sampling of single sperm cells, which were further amplified by multiplex nested polymerase chain reaction at the RhD locus. A RhD sequence amplification product was expected in all of the successfully amplified samples from Rh-positive homozygotes, in some of the samples from heterozygotes, and in none of the samples form Rh-negative subjects.

RESULTS

RhD genotype was accurately determined in 10 of 10 donors. A total of 132 single sperm cells were analyzed (8 to 17 samples per donor), of which 96 were successfully amplified as assessed by an internal control. As expected, the specific region of the RhD gene was amplified in all, some, and none of the signal-positive sperm samples from Rh-positive homozygotes, heterozygotes, and Rh-negative subjects, respectively, allowing accurate determination of the genotype.

CONCLUSION

An accurate diagnosis of the RhD genotype can be attained from single sperm cell analysis by means of polymerase chain reaction and may have major clinical applications in the management of Rh isoimmunization.

Management of rhesus isoimmunization by preimplantation genetic diagnosis

A genetic assay by single blastomere analysis was developed for rhesus (RhD) blood group typing of early cleavage stage embryos. The method, which is based on the simultaneous amplification of an RhD-specific sequence and an internal control in single cells, was applied for the selective transfer of RhD-negative embryos in a family of an RhD sensitized woman and a heterozygote partner. The RhD status of two out of three biopsied embryos was determined. According to their amplified products, both were typed as RhD-negative and transferred to the uterus. Pregnancy was not achieved.

Binding to heparan sulfate is a major event during catabolism of lipoprotein lipase by HepG2 and other cell cultures

Lipoprotein lipase (LPL) is rapidly and efficiently cleared from the circulation by the liver through an as yet unclear mechanism. In the present study, we determined the nature of LPL interactions with the liver parenchimal cell line HepG2 as compared to other cells in culture. Binding, cell association and degradation of 125I-labelled bovine milk LPL by HepG2 cells, normal and low density lipoprotein (LDL) receptor-negative human fibroblasts and Chinese hamster ovary (CHO) cells show similar values irrespective of source and origin. LPL metabolism in HepG2 cells was characterized by a high capacity to degrade the enzyme, an extremely high sensitivity to heparin and was inhibited by 60%-70% after treatment of the cells with sodium chlorate and heparinase (but not chondroitinase). These findings suggested an important role for heparan sulfate in the process of cell interaction and metabolism of LPL. To further clarify the role of heparan sulfate in determining the LPL-cell interactions, we compared the metabolism of LPL in wild type and mutant heparan sulfate-deficient CHO cells. Heparan sulfate-deficient CHO cells show a low capacity to bind and degrade LPL, about 10%-20% that of the wild type cells. In another set of experiments, we sought to determine whether LPL interactions with HepG2 cells are affected by triglyceride-rich lipoproteins. The results clearly show that whereas unlabeled LPL dramatically enhanced the metabolism of radioiodinated very low density lipoprotein (VLDL), unlabeled VLDL had no effect on radioiodinated LPL metabolism in these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Preimplantation diagnosis of cystic fibrosis by simultaneous detection of the W1282X and delta F508 mutations

W1282X (W) and delta F508 (delta) are the two most common mutations of the cystic fibrosis Israeli population. Patients who are homozygotes (WW and delta delta) as well as compound heterozygotes (W delta) present a severe phenotype of the disease. In the present study, we have developed a polymerase chain reaction (PCR)-based method for the detection of both mutations simultaneously in a single blastomere. Unfertilized human oocytes and single polyspermic blastomeres were subjected to a two-round PCR amplification: a first round of multiplex PCR followed by a second round of nested PCR, done separately at each locus. Clear signals at both loci were obtained in 51% (47/65) of oocytes and 69% (24/35) of blastomeres. The genotype of the single cell analysed was determined by endonuclease digestion of the W products and by heteroduplex formation of the delta F products. This diagnostic system will allow the identification of affected embryos (WW, delta delta, W delta) as well as phenotypically normal carriers (W+, +delta), and therefore may be used for cystic fibrosis preimplantation diagnosis in families who carry either or both mutations.

Enhanced cellular metabolism of very low density lipoprotein by simvastatin. A novel mechanism of action of HMG-CoA reductase inhibitors

To test the possibility that HMG-CoA reductase inhibitors reduce LDL mass by an increased VLDL catabolism, we determined the effect of simvastatin therapy on cellular metabolism of VLDL in 18 patients with primary hypercholesterolaemia. Six months of simvastatin therapy was followed by 26%, 31% and 21% reduction of plasma total cholesterol, LDL-cholesterol and plasma triglyceride levels, respectively. Before therapy, patients' VLDL metabolism in cultured human normal skin fibroblasts was similar to control VLDL. Six months after therapy was initiated, a remarkable 2-5-fold increase in VLDL cell metabolism was found. These effects were even more marked when the VLDL was enriched with exogenous recombinant apo E-3. A comparison of the metabolism of the patients' VLDL to control VLDL and LDL, revealed that simvastatin increased metabolic ratios of 60-70% and 45-95%, respectively. Simvastatin therapy was associated with a decrease of VLDL cholesteryl ester content of 19% and increase of the phospholipid content of 13%. The data strongly indicate that simvastatin therapy stimulates VLDL: cell interactions and catabolism, possibly reflecting alterations of the physico-chemical properties of the particle. It is proposed that in addition to other previously described pathways, HMG-CoA reductase inhibitors decrease LDL mass through a novel mechanism of enhanced VLDL catabolism prior to the conversion to LDL.

The synthesis of alpha-amylase by rough and in vitro reconstituted rough endoplasmic reticulum derived from rat parotid gland

The isolation of rough and smooth endoplasmic reticulum from rat parotid salivary gland is described. The rough membrane was stripped of its bound ribosomes using the KCl-puromycin method. Rough endoplasmic reticulum was reconstituted from stripped-rough membrane and polyribosomes. The reconstituted rough membrane resembled the native rough membrane in the following aspects: RNA/protein ratio, buoyant density in a continuous sucrose gradient and amino acid incorporation capacity. The in vitro synthesis of alpha-amylase by both rough and in vitro reconstituted rough membrane was demonstrated using SDS polyacrylamide gel electrophoresis. The reconstituted rough membrane could be restripped by KCl-puromycin. The in vitro synthesized alpha-amylase remained associated with the rough or the in vitro reconstituted rough membrane, even after these membranes were stripped of their bound ribosomes.