Molecular interactions of the plasma membrane calcium ATPase 2 at pre- and post-synaptic sites in rat cerebellum.

The plasma membrane calcium extrusion mechanism, PMCA (plasma membrane calcium ATPase) isoform 2 is richly expressed in the brain and particularly the cerebellum. Whilst PMCA2 is known to interact with a variety of proteins to participate in important signalling events [Strehler EE, Filoteo AG, Penniston JT, Caride AJ (2007) Plasma-membrane Ca(2+) pumps: structural diversity as the basis for functional versatility. Biochem Soc Trans 35 (Pt 5):919-922], its molecular interactions in brain synapse tissue are not well understood. An initial proteomics screen and a biochemical fractionation approach identified PMCA2 and potential partners at both pre- and post-synaptic sites in synapse-enriched brain tissue from rat. Reciprocal immunoprecipitation and GST pull-down approaches confirmed that PMCA2 interacts with the post-synaptic proteins PSD95 and the NMDA glutamate receptor subunits NR1 and NR2a, via its C-terminal PDZ (PSD95/Dlg/ZO-1) binding domain. Since PSD95 is a well-known partner for the NMDA receptor this raises the exciting possibility that all three interactions occur within the same post-synaptic signalling complex. At the pre-synapse, where PMCA2 was present in the pre-synapse web, reciprocal immunoprecipitation and GST pull-down approaches identified the pre-synaptic membrane protein syntaxin-1A, a member of the SNARE complex, as a potential partner for PMCA2. Both PSD95-PMCA2 and syntaxin-1A-PMCA2 interactions were also detected in the molecular and granule cell layers of rat cerebellar sagittal slices by immunohistochemistry. These specific molecular interactions at cerebellar synapses may allow PMCA2 to closely control local calcium dynamics as part of pre- and post-synaptic signalling complexes.

Calcium misregulation and the pathogenesis of muscular dystrophy.

Although the exact nature of the relationship between calcium and the pathogenesis of Duchenne muscular dystrophy (DMD) is not fully understood, this is an important issue which has been addressed in several recent reviews (Alderton and Steinhardt, 2000a, Gailly, 2002, Allen et al., 2005). A key question when trying to understand the cellular basis of DMD is how the absence or low level of expression of dystrophin, a cytoskeletal protein, results in the slow but progressive necrosis of muscle fibres. Although loss of cytoskeletal and sarcolemmal integrity which results from the absence of dystrophin clearly plays a key role in the pathogenesis associated with DMD, a number of lines of evidence also establish a role for misregulation of calcium ions in the DMD pathology, particularly in the cytoplasmic space just under the sarcolemma. A number of calcium-permeable channels have been identified which can exhibit greater activity in dystrophic muscle cells, and exIsting evidence suggests that these may represent different variants of the same channel type (perhaps the transient receptor potential channel, TRPC). In addition, a prominent role for calcium-activated proteases in the DMD pathology has been established, as well as modulation of other intracellular regulatory proteins and signaling pathways. Whether dystrophin and its associated proteins have a direct role in the regulation of calcium ions, calcium channels or intracellular calcium stores, or indirectly alters calcium regulation through enhancement of membrane tearing, remains unclear. Here we focus on areas of consensus or divergence amongst the existing literature, and propose areas where future research would be especially valuable.

Apoptosis mediated by activation of the G protein-coupled receptor for parathyroid hormone (PTH)/PTH-related protein (PTHrP).

The present studies were carried out to evaluate the mechanisms by which PTH/PTHrP receptor (PTHR) activation influences cell viability. In 293 cells expressing recombinant PTHRs, PTH treatment markedly reduced the number of viable cells. This effect was associated with a marked apoptotic response including DNA fragmentation and the appearance of apoptotic nuclei. Similar effects were evidenced in response to serum withdrawal or to the addition of tumor necrosis factor (TNFalpha). Addition of caspase inhibitors or overexpression of bcl-2 partially abrogated apoptosis induced by serum withdrawal. Caspase inhibitors also protected cells from PTH-induced apoptosis, but overexpression of bcl-2 did not. The effects of PTH on cell number and apoptosis were neither mimicked by activators of the cAMP pathway (forskolin, isoproterenol) nor blocked by an inhibitor (H-89). However, elevation of Ca(i)2+ by addition of thapsigargin induced rapid apoptosis, and suppression of Ca(i)2+ by overexpression of the calcium- binding protein, calbindin D28k, inhibited PTH-induced apoptosis. The protein kinase C inhibitor GF 109203X partially inhibited PTH-induced apoptosis. Regulator of G protein signaling 4 (RGS4) (an inhibitor of the activity of the alpha-subunit of Gq) suppressed apoptotic signaling by the PTHR, whereas the C-terminal fragment of GRK2 (an inhibitor of the activity of the beta(gamma)-subunits of G proteins) was without effect. Chemical mutagenesis allowed selection of a series of 293 cell lines resistant to the apoptotic actions of PTH; a subset of these were also resistant to TNFalpha. These results suggest that 1) apoptosis produced by PTHR and TNF receptor signaling involve converging pathways; and 2) Gq-mediated phospholipase C/Ca2+ signaling, rather than Gs-mediated cAMP signaling, is required for the apoptotic effects of PTHR activation.

Role of DNA minor groove alkylation and DNA cross-linking in the cytotoxicity of polybenzamide mustards.

Interstrand DNA cross-links have been considered essential to the activity of current clinical DNA-alkylating antitumour drugs, which generally alkylate in the major groove. However, the relationship between cross-linking adducts located in the minor groove of DNA with cytotoxicity and antitumour activity has not been extensively investigated. Previous studies have shown that cross-linking ability is not correlated with cytotoxicity in a novel series of polybenzamide-linked nitrogen mustard compounds which alkylate DNA at adenines in the minor groove. In the present study the nature of these cross-linking adducts was explored for a related pair of compounds which are both highly effective cross-linkers but which differ in antitumour potential. Both of these drugs effectively interact with adenines in the minor groove, although their sequence specificity differs. However, the cross-linking event was not inhibited by pre-treatment with Hoechst 33258, although this pre-treatment effectively prevented adenine alkylation. The primary cross-links detected may thus represent guanine N7 alkylations in the major groove. Whether minor groove cross-linking adducts can be formed is uncertain, since the effect of background guanine N7 alkylation may complicate analysis. The cytotoxicity of the polybenzamides may therefore be related to other factors such as their interaction with cellular repair systems.

The genome as a drug target: sequence specific minor groove binding ligands.

The ability to target defined sequences on the DNA molecule would be of enormous benefit to the treatment of human disease. Towards this goal much research has been invested in examining the DNA binding and biological mechanisms of action of sequence selective minor groove binding ligands. These compounds act in a variety of ways to inhibit gene expression and DNA replication and also alter nuclear architecture. Concomitant with this, minor groove adducts formed by certain compounds are inefficiently removed by cellular DNA repair systems and are extremely cytotoxic. Additionally compounds targeting A.T rich DNA sequences have found clinical use in the treatment of particular parasitic infections.

Polybenzamide mustards: structure-activity relationships for DNA sequence-specific alkylation.

A series of cytotoxic polybenzamide mustards targeted to the minor groove of DNA were used to define structure-activity relationships for sequence-specific DNA alkylation. Compounds with an annular structure closely matched to the minor groove of DNA, and with concave-facing, potentially H-bonding NH groups, had a strong preference for alkylating adenines in sequences possessing four or more consecutive adenines. Two compounds whose annular structure matched that of the minor groove better when at least one carboxamide NH group faced outwards showed a high specificity for the consensus sequence (A/T)A(G/C) (A/T)N. Several compounds also alkylated specific guanines, presumably at the N3 position. Modelling studies suggest the most important contribution to sequence-specific alkylation is the H-bonds formed between these compounds and DNA, with factors such as the degree and positioning of cationic charge being less influential.

Comparative mutational spectra of the nitrogen mustard chlorambucil and its half-mustard analogue in Chinese hamster AS52 cells.

Nitrogen mustards play an important role in current cancer chemotherapy. The most effective antitumour agents are those carrying two alkylating functions, probably through their ability to form interstrand cross-links in DNA. Such lesions appear to create more of a block in DNA replication and are more difficult to repair than are most monoadducts. Although there were early reports that monofunctional drugs were more mutagenic than the bifunctional drugs, this has not been formally proved using structurally related drugs in a mutagenicity assay capable of detecting a range of different events. We have studied both the mutagenic potency and spectrum of events caused by treatment with the clinical agent, chlorambucil, compared with its half-mustard analogue, in Chinese hamster ovary (CHO)-AS52 cells. Although both drugs caused comparable increases in mutation frequency at doses killing 90% of cells (from around 9x10-6 to around 9x10-5 mutant cells), the nature of events differed significantly between the drugs. By far the majority of mutations caused by the half-mustard were transversion mutations, and almost all of these could be interpreted in relation to the DNA adducts that are known to be formed. In contrast, the majority of chlorambucil-induced mutations were major deletions, and point mutations were only identified from a few clones. Parallel micronucleus assays verified that chlorambucil has a stronger ability to break chromosomes than the half-mustard. These two drugs are thought to form similar monoadducts, but only the full mustard can form interstrand cross-links. The data suggest that DNA cross-links, although only a minor fraction of the total lesions, dominate the mutagenic spectrum and lead to gross changes at the chromosome level that can not be readily associated with individual lesions produced by the drug.

Transmembrane residues together with the amino terminus limit the response of the parathyroid hormone (PTH) 2 receptor to PTH-related peptide.

The mechanisms of ligand binding and receptor activation for G-protein-coupled receptors in the secretin/parathyroid hormone (PTH) receptor subfamily are not understood. The PTH1 receptor (PTH1R) signals in response to both PTH and parathyroid hormone-related peptide (PTHrP), whereas the PTH2 receptor (PTH2R) responds only to PTH, not to PTHrP. To locate PTHrP discriminatory domains in the PTH2R, we generated PTH1R/PTH2R chimeras in which the extracellular amino-terminal domains were exchanged. Production of cAMP in response to 1 microM PTHrP or PTH was identical in cells expressing the PTH1R with the PTH2R amino terminus and in cells expressing the PTH2R with the PTH1R amino terminus. The ability of the chimeric receptor with the PTH2R amino terminus to respond fully to PTHrP showed that the body of the PTH2R must contain sites that limit the response to PTHrP. Mutations to PTH1R sequence were therefore made in each of the seven transmembrane domains of the PTH2R. Mutations in transmembrane domains 3 and 7 resulted in receptors able to respond to PTHrP. Thus, residues in more than one domain form a barrier or filter, allowing the receptor to discriminate between different ligands.

Binding of polybenzamides to DNA: studies by DNase I and chlorambucil interference footprinting and comparison with Hoechst 33258.

The DNA sequence-specific binding ability of polybenzamide minor groove binding ligands was investigated. These ligands were compared with the known minor groove binder Hoechst 33258, using both DNase I footprinting and chlorambucil interference footprinting. The monocationic derivative showed some sequence specific binding to A/T-rich sequences, as shown by DNase I footprinting, but results for the biscationic polybenzamide were inconclusive. A general non-specific inhibition of cleavage at high drug concentrations was observed, suggesting these compounds had a low DNA binding affinity compared to Hoechst 33258. Using a complementary technique, chlorambucil interference footprinting, the biscationic derivative displayed a clear preference for sites containing at least three consecutive adenines and in contrast with the monocationic analogue, a lesser affinity for mixed A/T sequences.

A variation in the apolipoprotein C-III gene is associated with an increased number of circulating VLDL and IDL particles in familial combined hyperlipidemia.

Detailed plasma lipoprotein analyses were conducted on 16 familial combined hyperlipidemic (FCHL) probands, all their available family members (n = 106) together with 12 normolipidemic control families (n = 68), and the results were assessed in relation to a C1100-T polymorphism in exon 3 of the apoC-III gene. The frequency of the T1100 genotype (CT+TT) was significantly elevated in the probands relative to control subjects (0.64 vs. 0.36; P < 0.01) and was associated with elevated concentrations of plasma triglyceride (P < 0.02) and apoC-III (P < 0.03), VLDL cholesterol (P < 0.005), VLDL triglyceride (P < 0.009), IDL cholesterol (P < 0.01). and IDL triglyceride (P < 0.007). The T1100 genotype was also associated with elevations in VLDL-apoB (P < 0.005) and IDL-apoB (P < 0.04) indicating a relationship between this variation and an increased number of triglyceride-rich particles. These findings were confined to the hyperlipidemic members of the FCHL families and showed a strong genotype-status interaction (P < 0.001). It is considerable clinical relevance that the apoC-III gene may be acting as a modifier gene that is only expressed in the presence of other factors (e.g., increased VLDL flux, low LPL activity) and therefore may predispose those members of FCHL families carrying the T1100 allele to express the FCHL phenotype.

A critical evaluation of resting intracellular free calcium regulation in dystrophic mdx muscle.

There are conflicting reports regarding whether resting free calcium levels ([Ca2+]i) are elevated in dystrophic mouse (mdx) myotubes and adult myofibers. We reinvestigated this question and found several lines of evidence supporting the hypothesis that increased calcium influx via leak channels leads to increases in resting [Ca2+]i. 1) Step calibration of fura 2/free acid in myofibers with use of microinjected Ca(2+)-ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid buffers revealed greater [Ca2+]i in dystrophic cells. Careful calibration of fura PE3-AM, a compartmentalization-resistant derivative of fura 2, also showed elevated [Ca2+]i in mdx myotubes. 2) Chronic, but not acute, application of tetrodotoxin reduced resting [Ca2+]i in dystrophic myotubes, suggesting that elevated resting [Ca2+]i is a consequence of previous long-term contractile activity. 3) Rates of manganese quenching of fura 2 fluorescence, an indirect indicator of calcium influx, were significantly higher in mdx myotubes and were increased by nifedipine, a calcium leak channel agonist. 4) Calcium leak channel activity, measured using patch clamping, was greater in the sarcolemma of adult non-enzyme-treated mdx myofibers.

The mutagenic properties of DNA minor-groove binding ligands.

This review summarises mutagenesis-related research on the major classes of DNA minor groove binding ligands. These compounds can bind to DNA covalently or non-covalently, and span a range of DNA sequence selectivities. Many of the non-covalent binders show effects on topoisomerase enzymes in mammalian cells, with the bisbenzimidazoles being the most active. Mutagenic effects consistent with topoisomerase inhibition are observed in vitro. Many of these compounds induce aneuploidy and polyploidy, properties which may also contribute to carcinogenic processes. Similarly, uvrA trapping by some minor groove binders may alter mutagenetic processes by inhibiting efficient repair. Distamycin has been shown to enhance the mutagenicity of ethidium bromide in bacteria by an undetermined mechanism. However, the inhibitory effects of minor groove binders on human DNA repair systems have not yet been reported. Hoechst 33258 and distamycin cause chromosome decondensation in both mouse and human cells particularly at heterochromatic regions which are rich in AT content. Various minor groove binders have been shown to induce fragile sites in cultured lymphocytes from susceptible individuals, which may have a propensity to develop particular cancers. Investigation of the relationship between fragile site inducing drugs and chromosomal rearrangements in fragile site carriers has not been investigated but may yield interesting results. Some DNA alkylating minor groove binders can generate lesions extremely toxic to mammalian cells (e.g., CC-1065 and analogues), and induce a range of DNA sequence changes in vivo, both at the site of covalent bonding as well as at surrounding sequences. This may be typical of alkylating minor groove binders which have a binding site size of several base pairs, and which stabilise helical structure. Minor groove binders have effects on gene expression in vitro by inhibiting the sequence selective binding of various transcription factors to DNA. These effects may result in expression or repression of downstream genes also. This class of ligand thus offers the possibility of mutations targeted to specific genes or genomic regions. It will be interesting to determine whether such examples of targeted mutagenesis, as has already been observed with CC-1065 and adozelesin, will result in an enhanced or in a lowered capacity to promote neoplastic disease. However it should be noted that pentamidine, a minor groove binder used in the treatment of AIDS-related PCP, has thus far shown no mutagenic effects in nuclear DNA and only a weak effect in mitochondrial DNA of yeast. These results suggest that minor groove binding does not necessarily lead to mutagenesis.

A putative selectivity filter in the G-protein-coupled receptors for parathyroid hormone and secretion.

The seven transmembrane segments (TMs) of many G-protein-coupled receptors (GPCRs) are thought to form a cavity into which cognate ligands insert, leading to receptor activation. Residues lining the cavity are often essential for optimal ligand binding and/or signal transduction. The present studies evaluated whether residues lining the cavity also contribute to specificity, using GPCRs for the polypeptides parathyroid hormone (PTH) and secretin as models. These ligands display no sequence homology with one another, and neither ligand cross-reacts with the other's receptor. However, mutation of a single amino acid in the second TM of the secretin receptor to the corresponding residue in the PTH receptor (N192I) resulted in a receptor that binds and signals in response to PTH. The reciprocal mutation in the PTH receptor (I234N) likewise unmasked responsiveness to secretin. Neither mutation significantly altered the response of the receptors to their own ligands. The results suggest a model of specificity wherein TM residues near the extracellular surface of the receptor function as a selectivity filter that restricts access of inappropriate ligands to an activation site in the transmembrane cavity.

Mutations of neighboring polar residues on the second transmembrane helix disrupt signaling by the parathyroid hormone receptor.

Site-directed mutagenesis was used to assess the role of transmembrane (TM)-charged amino acids in the expression and function of the G protein-coupled receptor for PTH and PTH-related protein (PTHrP). Charged residues that are conserved in the TM regions of most or all members of the PTH/secretin receptor subfamily were targeted. Four mutants (E296A, R337A, H414A, and E459K) displayed properties similar to the wild type PTH/PTHrP receptor with respect to agonist binding and stimulation of adenylyl cyclase when expressed in COS-7 cells. Several mutations, all in TM II, produced receptors that signaled extremely poorly. Mutation of three residues (227S, 230R, and 233S), predicted to be aligned on one helical face of TM II, displayed a similar phenotype: markedly blunted adenylyl cyclase activity in response to PTH (20-30% of the wild type response) and a lower binding affinity for agonist, with no reduction in cell surface receptor expression. These results suggest that TM II contains a polar face that is involved in TM signaling by the PTH/PTHrP receptor. Two of these mutations were made at the corresponding sites in the secretin receptor, and a similar reduction in secretin-stimulated adenylyl cyclase activity was observed. Thus this region of TM II may participate in a mechanism of TM signal transduction that is shared by the PTH/secretin sub-family of G protein-coupled receptors.

DNA polymorphisms of the apoprotein B gene are associated with altered plasma lipoprotein concentrations but not with perceived risk of cardiovascular disease: European Atherosclerosis Research Study.

Three polymorphisms of the apoprotein B gene (XbaI, signal peptide insertion/deletion and the 3'-variable number of tandem repeats) selected on the basis of previously published reports as likely to be the most informative, were investigated in a cross-cultural study in Europe. Students from 14 universities, grouped for analyses into five regions, were recruited as cases (n = 682) if they had a paternal history of premature myocardial infarction. For comparison, twice the number of age- and sex-matched controls (n = 1312) were recruited from the same student populations. There were significant regional differences in allele frequencies of the XbaI and VNTR polymorphisms but not of the signal peptide. There were no significant differences in allele frequencies between cases and controls. Adjusted for age, gender and region, the lipoprotein concentrations differed significantly with genotype. The XbaI polymorphism was associated with differences in plasma cholesterol (P = 0.007), triglyceride (P = 0.050), apo B (P = 0.001) and LDL cholesterol (P = 0.01). An interaction between XbaI genotype and body mass index was observed on plasma triglyceride (P = 0.015) and apo B (P = 0.005) concentrations. The signal peptide deletion allele was associated with increased plasma cholesterol (P = 0.03), apo B (P = 0.04) and LDL cholesterol (P = 0.02). The VNTR was not significantly associated with any of these variables although there was a significant genotype/status interaction in relation to HDL cholesterol (P = 0.001) and apo AI (P = 0.001) concentrations. We conclude that, although they are associated with significant differences in lipoprotein concentrations within- and between-populations, the apo B DNA polymorphisms studied are of less value as indicators of cardiovascular risk-factor status in the offspring of individuals affected by the disease.

Synthesis, DNA interactions and biological activity of DNA minor groove targeted polybenzamide-linked nitrogen mustards.

A series of polybenzamide DNA minor groove binding ligands bearing either one or two monofunctional mustards have been synthesised, and their cytotoxicities and interactions with DNA have been studied. Analogues with two alkylating functions (e.g. compounds 7 and 14) are the most cytotoxic, with 7 being 1000-fold more potent than the clinical mustard chlorambucil against P388 leukemia in culture, as well as being more potent in vivo. Monofunctional analogues were also significantly more cytotoxic than chlorambucil, despite bearing much less reactive mustard species. These results support the concept that targeting nitrogen mustard alkylating agents to DNA by attachment to DNA-affinic carriers can greatly enhance cytotoxicity due to alkylation, and that even for such DNA-targeted mustards, crosslinking is a more toxic event than monoalkylation. Close analogues of 7 differing only in their radius of curvature, appear to alkylate and crosslink DNA in similar fashion, yet have widely differing cytotoxicities. The most cytotoxic compound (7) possesses a geometry most complementary to that of duplex DNA, suggesting that the most toxic lesions are those which result in least DNA distortion, thus being less easily recognised by DNA repair systems.

Plasma lipoprotein alterations in patients with chronic hepatocellular liver disease resulting from alcohol abuse: effects of alcohol intake cessation.

Cholesterol and triglyceride in plasma and lipoprotein fractions and serum apoprotein concentrations were measured in 51 chronic alcoholic subjects; 23 had minimal or mild hepatic changes (steatosis and/or fibrosis) and 28 had cirrhosis. Of the latter, 16 had stopped alcohol consumption at least 3 months before the study, while the other 12 and all the mildly affected patients had continued drinking. None of the patients presented with cholestasis or alcoholic hepatitis. The control group was composed of 15 healthy, non-drinking volunteers selected from the hospital staff with an age- and sex-distribution similar to that of the alcoholic group. Patients with minimal hepatic changes had plasma total cholesterol concentrations within the ranges of the normal population but with increased high density lipoprotein and decreased low density lipoprotein fractions. Total plasma triglyceride values were not significantly elevated but the distributions in the low density lipoprotein and high density lipoprotein fractions were significantly increased in patients compared to controls. This alteration was accompanied by a consistent increase in serum apolipoprotein C-III concentration. Conversely, in patients with cirrhosis, serum concentrations of apolipoproteins A-I and B were significantly lower and were reflected in the cholesterol concentrations in the lipoprotein fractions. Comparisons between abstainers and non-abstainers within the group with cirrhosis indicated that cessation of alcohol intake was not sufficient to rectify lipoprotein dysfunction following damage from cirrhosis.

Limitations of the Friedewald formula for estimating low-density lipoprotein cholesterol in alcoholics with liver disease.

The accuracy of the Friedewald formula in estimating low-density lipoprotein (LDL) cholesterol was investigated in 47 alcoholic patients with liver disease (21 minimal-change, 26 cirrhotic) by comparing the results with those obtained by sequential preparative ultracentrifugation. In 14% of subjects with minimal-change disease, the error in the estimated LDL cholesterol was 50% +/- 9% (mean +/- SD; range 40-59%) and was related to the degree of attendant hypertriglyceridemia (r = 0.98; P < 0.001). A similar degree of error was observed in patients with cirrhosis, despite the absence of hypertriglyceridemia; an abnormal VLDL cholesterol: triglyceride ratio was the contributory factor in the discrepancy. We conclude that, as is the case in other clinical pathologies in which abnormalities of lipoprotein composition have been described (e.g., diabetes), the Friedewald formula to estimate LDL cholesterol may be inappropriate in chronic alcoholics, particularly those in whom a degree of hepatic dysfunction may be suspected.

In vitro oxidised HDL is recognized by the scavenger receptor of macrophages: implications for its protective role in vivo.

To assess the effects of oxidative modification, human HDL was oxidised in vitro for 12 h (Ox-HDL12) and 24 h (Ox-HDL24) under similar conditions to those commonly used for LDL. The procedure resulted in: an increase in thiobarbituric acid reactive substances but with marginal change in electronegativity; protein denaturation accounting for 16% and 45% loss of immunoreactive apoprotein A-I in the Ox-HDL12 and Ox-HDL24 respectively relative to the non-oxidised, native HDL (Nat-HDL); a decrease in the polyunsaturated fatty acids of the triglyceride, cholesterol ester and phospholipid components of the lipoprotein; an increase in the proportion of short chain saturated fatty acids while the monounsaturated fatty acids remained relatively unchanged. Studies with human macrophages demonstrated: a decrease of 16% and 30% in the capacity of the Ox-HDL12 and Ox-HDL24 respectively to efflux intracellular free cholesterol; 125I-Ox-HDL24 uptake and degradation was directly comparable with that of 125I-Ac-LDL; the addition of excess unlabelled Ox-HDL24, Ac-LDL, Ox-LDL24 and Nat-HDL resulted in 74%, 67%, 69% and 19% displacement of the 125I-Ox-HDL24 respectively; fucoidin and dextran sulphate displaced 125I-Ox-HDL by 20% and 40% respectively; intracellular free and esterified cholesterol was increased 2.5-fold and 4-fold respectively relative to Nat-HDL on incubation with Ox-HDL24. These findings suggest that HDL is susceptible to oxidative modification leading to recognition by the scavenger receptor of macrophages and subsequent intracellular cholesterol accumulation. As such, the in vivo protective role of HDL in cardiovascular disease can be reversed in those circumstances in which HDL, like LDL, undergoes oxidative modification.

Low-density lipoprotein metabolism in rats treated with cyclosporine.

Metabolic mechanisms underlying the observations of elevated cholesterol concentration of low-density lipoprotein (LDL) in organ-transplanted patients on long-term immunosuppressant cyclosporine therapy were explored using cyclosporine-treated rats as an experimental model. As in patients, treatment with cyclosporine induced a significant elevation of plasma cholesterol level, mainly in LDL cholesterol, with a decrease in high-density lipoprotein (HDL) cholesterol level. In an in vivo cross-over study design, differentially radioiodinated homologous LDL from donor cyclosporine-treated rats (Cyc-LDL) and excipient-only-treated control rats (Exc-LDL) were injected into recipient cyclosporine-treated rats (Cyc-rats), excipient-only--treated control rats (Exc-rats), and untreated rats (Unt-rats). From the isotope disappearance curves, the fractional catabolic rate (FCR) and production rate were calculated. The results showed that FCR and production rate were significantly reduced in Cyc-rats compared with control Exc-rats and Unt-rats. The decrease was independent of the donor LDL source. In vitro LDL ligand-receptor assays indicated a twofold higher degradation of Cyc-LDL by cultured rat fibroblasts, and hence could not account for the decreased clearance observed in vivo. These results suggest that the elevated concentrations of LDL cholesterol associated with cyclosporine treatment result not from a cyclosporine-induced modification of the LDL molecule, which could diminish its receptor-mediated clearance/catabolism, but possibly from an in vivo pharmacological property of cyclosporine such as an induced hepatic dysfunction.