Binding of the chemical carcinogen, p-dimethylaminoazobenzene, by human plasma low density lipoproteins

Itraconazole, a commonly used antifungal that inhibits Hedgehog pathway activity and cancer growth

In a screen of drugs previously tested in humans we identified itraconazole, a systemic antifungal, as a potent antagonist of the Hedgehog (Hh) signaling pathway that acts by a mechanism distinct from its inhibitory effect on fungal sterol biosynthesis. Systemically administered itraconazole, like other Hh pathway antagonists, can suppress Hh pathway activity and the growth of medulloblastoma in a mouse allograft model and does so at serum levels comparable to those in patients undergoing antifungal therapy. Mechanistically, itraconazole appears to act on the essential Hh pathway component Smoothened (SMO) by a mechanism distinct from that of cyclopamine and other known SMO antagonists, and prevents the ciliary accumulation of SMO normally caused by Hh stimulation.

Interception of reactive, DNA adduct-forming metabolites present in rodent serum following carcinogen exposure: implications for use of body fluids in biomonitoring

The detection of adduct-forming metabolites in the serum of carcinogen treated animals by 32P-postlabeling was evaluated as a novel approach to overcome the stringent requirement of obtaining DNA from tissues in human biomonitoring assessments. Benzo[a]pyrene (BP) was given i.p. to B6C3F1, C57B1/6, ICR, and DBA/2 mouse strains as well as Sprague-Dawley rats. Three adducts related to BP were detected in the liver and/or lung of Sprague-Dawley rats or B6C3F1, C57B1/6, and ICR mice; a single adduct was detected in the liver and lung of the DBA/2 mouse strain. Adducts chromatographically similar to those found in these tissues were also detected when salmon sperm DNA was incubated with the serum of BP-treated animals. Benzidine treatment induced the formation of one adduct in the liver of B6C3F1 mice, which was chromatographically similar to dG-C8-N'-acetylbenzidine. An identical adduct was detected in the salmon sperm DNA incubated with the serum of these mice. Cyclopenta[cd]pyrene treatment produced four major and three minor adducts in the liver or lung of B6C3F1 mice, all but two of which were detected in DNA incubated with serum of cyclopenta[cd]pyrene-treated animals. Large interstrain differences in the serum level of BP adduct-forming metabolites as well as tissue DNA adducts were found which correlated with previously observed strain-specific trends in sensitivity to PAH-mediated carcinogenesis. Thus, levels of BP adduct-forming metabolites were found in the following descending order: B6C3F1, C57B1/6, ICR, and DBA/2. BP-derived adduct-forming metabolites were detectable as late as 2 d and 5 d post-treatment in the serum of C57B1/6 mice or Sprague-Dawley rats, respectively, which seems to coincide well with the reported species-specific turnover of serum albumin; a protein know to be involved in the transport of reactive metabolites throughout the systemic circulation. The results obtained clearly indicate the presence of adduct-forming carcinogen metabolites in the serum of treated animals, which seemingly irrespective of their chemical nature, can be intercepted with exogenous DNA and detected by 32P-postlabeling. Successful application of a serum-based approach coupled with the use of the generally applicable, ultrasensitive 32P-postlabeling assay could evade the need for obtaining DNA from tissues, currently the major impediment in human biomonitoring studies.

Transport of organochlorine residues in the rat and human blood

Organochlorine residues (OCR)2 are poorly soluble in water and are transported in the organism bound by the blood components. The distribution among blood fractions (cells/plasma, lipoproteins/rest of plasma proteins) were variable depending on the residue (HCB, p p'-DDE, HCH, Aroclor 1260, PCP) and on the species (rat, man). Differences were not found between in vivo (after oral single dosing) and in vitro (blood incubation) experiments. Results indicated a high affinity of organochlorine residues for lipoproteins; however, binding to blood carriers was very weak as demonstrated by the rapid release of residues by elution through a reverse phase column. The effects of residue binding to blood components on the distribution kinetics to tissues are discussed.

Distribution of toxaphene, DDT, and PCB among lipoprotein fractions in rat and human plasma

The distribution of 14C-toxaphene, 14C-DDT, and 14C-PCB among lipoprotein fractions was studied in vitro and in vivo using rat and human plasma. The association of these substances with rat plasma fractions was similar in both in vitro and in vivo experiments. Thirty-seven to fifty-two per cent of the total radioactivity was associated with the cholesterol-rich high density lipoproteins (HDL2, d = 1.075-1.21 g/ml) and 18-52% was recovered in the albumin-rich bottom fraction (BF, d greater than 1.21 g/ml). A time-dependent redistribution of the radioactivity from the lipoprotein fractions to the BF was also observed in the in vivo studies. In human plasma, the distribution of the three compounds was different and uncorrelated to the cholesterol level of the individual lipoprotein fractions. Toxaphene was almost equally distributed between BF (d greater than 1.21 ml), HDL (d = 1.063-1.21 g/ml) and low density lipoproteins (LDL, d = 1.006-1.063 g/ml) (26%, 27% and 29%, respectively), while only 18% appeared in the very low density lipoprotein (VLDL, d less than 1.006) fraction. In contrast, a large proportion of DDT and PCB radioactivity was recovered in the BF (52% and 62%, respectively) while only 38-48% was present in lipoprotein fractions. The complex nature of the interaction between xenobiotics and plasma lipoproteins is discussed.

Comparative uptake, vascular transport, and cellular internalization of aflatoxin-B1 and benzo(a)pyrene

Studies of the uptake of benzo(a)pyrene (BaP) and aflatoxin-B1 (AFB1) after gastric instillation showed that BaP was absorbed via the intestinal lymphatic drainage and transported to the vascular circulation sequestered within lipoproteins in thoracic duct lymph, while AFB1 was absorbed with water soluble compounds into the gastrointestinal venous drainage and was not transported in association with lipoproteins. BaP was taken up into plasma lipoproteins over a broad concentration range, while AFB1 was not sequestered within lipoproteins over the same concentration range. Low density lipoproteins (LDL) facilitated BaP uptake into fibroblasts and impeded BaP uptake into hepatocytes. High density lipoproteins (HDL) facilitated BaP uptake into hepatocytes and impeded BaP uptake into fibroblasts. The uptake of AFB1 into either fibroblasts or hepatocytes was not affected by lipoproteins.

Intracellular lipoproteins as carriers for 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo(a)pyrene in rat and mouse liver

The possible role of hepatic lipoproteins as intracellular carriers in the transport of 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo(a)pyrene was assessed by in vitro and in vivo studies. Following administration of [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin or unlabelled 2,3,7,8-tetrachlorodibenzofuran to C57 BL/6 mice or Sprague-Dawley rats these compounds were bound to lipoproteins which subsequently underwent rapid and pronounced degradative processing, possibly catalysed by lipoprotein lipase, to heavier entities. At the highest doses of xenobiotics administered, an almost complete disappearance of lipoprotein particles was observed. The in vitro incubation of [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin-lipoprotein and [3H]benzo(a)pyrene-lipoprotein complexes with separated Ah receptor and 4S protein, respectively, demonstrated that a passive transfer occurred; the latter was likely dependent on both the relative affinities of the ligands towards the different cellular binding components as well as on their quantitative binding capacity. Taken together, these findings support the idea of a carrier-role for lipoproteins in the intracellular transport of hydrophobic xenobiotics and it may be asked whether the widespread modulators of lipoprotein level such as fibrates or others affect drug transfer or action.

Human lipoprotein influence on the partition of 2,2',4,4',5,5'-hexabromobiphenyl between 3T3L1 adipocytes and culture medium

A previous study established that 2,2',4,4',5,5'-hexabromobiphenyl (HBB) entered 3T3L1 adipocytes in culture by passive diffusion from the surrounding medium. The extent to which HBB accumulated within the cell was mediated by the level of triglyceride in the cell. The present study was concerned with the conditions that would facilitate HBB removal from adipocytes as part of a continuing effort to establish an effective and safe technology for reducing body burdens of lipophilic xenobiotics. Addition of human lipoprotein to the culture medium increased HBB removal from preloaded adipocytes 18 to 80 times more than did the addition of other blood proteins. Lipoproteins also decreased equilibrium deposition of HBB in the cells. The order of effect was low-density lipoprotein (LDL) much greater than high-density lipoproteins (HLD) greater than very-low-density lipoproteins (VLDL). These results are consistent with the hypothesis that lipoproteins act as a depot by binding HBB to immobilize the xenobiotic in the medium. The rate of removal of HBB was correlated with concentrations of lipoprotein cholesterol, cholesterol ester, and phospholipid in the culture medium (r greater than .95). Total lipoprotein fractions from individuals with high levels of serum cholesterol significantly increased HBB removal from preloaded adipocytes when compared with lipoproteins from normal human serum. Decreased removal was observed with lipoproteins from individuals with low serum cholesterol or triglyceride. These results suggest that cholesterol and/or cholesterol esters in the blood play an important role in both delivery and removal of HBB from the adipose tissue. Evidence has been presented that supports the hypothesis that HBB moves freely across the adipocyte membrane and is sequestered in either the cell or pseudoblood according to its relative solubility in these compartments.

Vitamins E and K induce aryl hydrocarbon hydroxylase activity in human cell cultures

Two fat soluble vitamins, Vitamins E and K, when added into culture medium, were found to increase aryl hydrocarbon hydroxylase activity in human cultured cells. The extent of induction in a hepatoma-derived cell line (Hep G2) by these vitamins is of similar magnitude to those cells receiving benz[a]anthracene; whereas in a mammary tumor-derived cell line (MCF-7), benz[a]anthracene is the best inducer for the hydroxylase activity. The increase of the hydroxylase activity is associated with increased levels of a specific mRNA coding for polynuclear aromatic hydrocarbons-induced form of cytochrome P-450 with Vitamins E and K treatment. The size of the induced mRNA is 3.3 kilobase which is the same as that of benz[a]anthracene treatment.

High-density lipoproteins decrease both DNA binding and mutagenicity of r-7,t-8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene in V79 Chinese hamster cells

The effects of separate lipoproteins or of serum with high or low lipoprotein concentrations on formation of lipophilic carcinogen adducts with DNA and on mutagenicity of the carcinogen was investigated using V79 Chinese hamster lung cells. Binding of r-7,t-8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) to DNA and BPDE induction of 6-thioguanine (6-TG)-resistant mutants in V79 cells was significantly lower after 1 or 4 h when the medium was supplemented with purified HDL, and was lower after 1 h but not 4 h when the medium was supplemented with serum containing a high concentration of mixed lipoproteins (LP). Cells grown in medium without serum or LP supplementation exhibited the highest levels of both BPDE-DNA adduct formation and mutagenesis after 1 h. At 1 h, cells exposed to BPDE in LDL-supplemented medium showed decreased adduct formation and mutagenesis when compared to cells treated with BPDE in PBS-supplemented medium. After 4 h, cells treated with BPDE in LDL-supplemented medium gave the highest levels of adduct formation and the highest mutation frequency. These results suggest that both LDL and HDL effectively decrease the concentration of BPDE available to V79 cells exposed to the mutagen for short periods of time, resulting in decreased interaction of BPDE with DNA and decreased BPDE-associated mutagenesis, but that both BPDE-DNA adduct formation and mutagenesis increased as a function of increased exposure time in the presence of LDL. The results suggest that LDL, but not HDL, uptake by adsorptive endocytosis may be associated with potentiated entry of BPDE into V79 cells as a function of time.

Plasma lipoproteins as drug carriers: pharmacological activity and disposition of the complex of beta-sitosteryl-beta-D-glucopyranoside with plasma lipoproteins

The ability of plasma lipoproteins to act as carriers in site-specific drug delivery systems was evaluated by determining the disposition and pharmacological effects of beta-sitosteryl-beta-D-glucopyranoside (SG, 3). In the disposition studies, [3H]SG was absorbed from the intestinal tract by the formation of chylomicrons and was specifically associated with lipoproteins in vivo. [3H]SG was incorporated into various rat plasma lipoproteins in vitro. [3H]SG complexed with the lower density lipoproteins (d less than 1.063 g/mL), especially with the intermediate density lipoproteins (1.006 less than or equal to d less than 1.019 g/mL) which following intravenous administration to rats. In pharmacological studies, the hemostatic effect of SG in mice and the inhibitory effect of SG on vascular permeability in rats were only observed after intravenous administration of the complexes of SG with the lower density lipoproteins. The same results were obtained after the intravenous administration of the complexes of SG with human and mouse lipoproteins.

Cellular uptake and intracellular localization of benzo(a)pyrene by digital fluorescence imaging microscopy

Uptake of benzo(a)pyrene by living cultured cells has been visualized in real time using digital fluorescence-imaging microscopy. Benzo(a)pyrene was noncovalently associated with lipoproteins, as a physiologic mode of presentation of the carcinogen to cells. When incubated with either human fibroblasts or murine P388D1 macrophages, benzo(a)pyrene uptake occurred in the absence of endocytosis, with a halftime of approximately 2 min, irrespective of the identity of the delivery vehicles, which were high density lipoproteins, low density lipoproteins, very low density lipoproteins, and 1-palmitoyl-2-oleoylphosphatidylcholine single-walled vesicles. Thus, cellular uptake of benzo(a)pyrene from these hydrophobic donors occurs by spontaneous transfer through the aqueous phase. Moreover, the rate constant for uptake, the extent of uptake, and the intracellular localization of benzo(a)pyrene were identical for both living and fixed cells. Similar rate constants for benzo(a)pyrene efflux from cells to extracellular lipoproteins suggests the involvement of the plasma membrane in the rate-limiting step. The intracellular location of benzo(a)pyrene at equilibrium was coincident with a fluorescent cholesterol analog, N-(7-nitrobenz-2-oxa-1,3-diazole)-23,24-dinor-5-cholen-22-amine-3 beta-ol. Benzo(a)pyrene did not accumulate in acidic compartments, based on acridine orange fluorescence, or in mitochondria, based on rhodamine-123 fluorescence. When the intracellular lipid volume of isolated mouse peritoneal macrophages was increased by prior incubation of these cells with either acetylated low density lipoproteins or with very low density lipoproteins from a hypertriglyceridemic individual, cellular accumulation of benzo(a)pyrene increased proportionately with increased [1-14C]oleate incorporation into cellular triglycerides and cholesteryl esters. Thus, benzo(a)pyrene uptake by cells is a simple partitioning phenomenon, controlled by the relative lipid volumes of extracellular donor lipoproteins and of cells, and does not involve lipoprotein endocytosis as an obligatory step.

Human lymphocytes treated with r-7,t-8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene require low-density lipoproteins for DNA excision repair

Human lymphocytes which were non-mitogen-stimulated, and which were depleted of lipoproteins, were found to be deficient in DNA excision repair typically initiated in these cells in response to treatment with a direct-acting polynuclear aromatic hydrocarbon carcinogen. Lymphocytes either depleted of lipoproteins or supplemented with human low-density lipoproteins formed DNA-carcinogen adducts which were not chromatographically distinguishable. The state of lipoprotein depletion did not alter lymphocyte uptake of thymidine from the medium. Lymphocytes which were depleted of lipoproteins, treated with carcinogen, and subsequently supplemented with low-density lipoproteins, regained the ability to engage in DNA excision repair. The data suggest that either low-density lipoprotein(s), or a component(s) of low-density lipoprotein(s), is required by human lymphocytes in order to initiate excision repair of carcinogen-damaged DNA.

High-density lipoproteins decrease both binding of a polynuclear aromatic hydrocarbon carcinogen to DNA and carcinogen-initiated cell transformation

Lipophilic carcinogens partition into the three major classes of lipoproteins potentially present in serum used as a medium supplement for cell culture. Different serum lots sequester differing quantities of the procarcinogen benzo[a]pyrene, dependent on the serum lipoprotein concentrations. In the presence of high-density lipoproteins a mutagenic benzo[a]pyrene metabolite was bound to cellular DNA at decreased levels when compared to cells exposed to the mutagen in the absence of high-density lipoproteins. Fetal calf serum with low levels of lipoproteins, specifically, high-density lipoproteins, is associated with efficient methylcholanthrene-initiated transformation of C3H/10T1/2 cells, while calf serum with a significant concentration of high-density lipoproteins requires up to a 500% increase in methylcholanthrene concentration to achieve similar levels of transformation in this mouse embryo cell line. When concentrated serum lipoproteins or purified HDL were added to fetal calf serum containing MCA at 1 microgram/ml, the C3H/10T1/2 transformation frequency was decreased compared to the transformation frequency achieved in the presence of 1 microgram/ml of MCA in fetal calf serum without supplementation. The results suggest that high-density lipoprotein partitioning of lipophilic polynuclear aromatic hydrocarbon mutagens from the cell culture medium may effectively reduce the concentration of carcinogen available for interaction with cellular DNA in vitro, which, in turn, may be associated with decreased carcinogen-induced transformation of cells.

Characteristics of 2,4,5,2',4',5'-hexachlorobiphenyl distribution among lipoproteins in vitro

The uptake, distribution, and transfer of 2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) were examined in vitro with human and rat whole blood, plasma, and lipoprotein fractions. 6-CB distribution between plasma and erythrocytes as well as among lipoproteins was determined following sedimentation of erythrocytes and ultracentrifugal fractionation of plasma. In both rat and human whole blood, 70 to 75% of 6-CB partitioned into plasma and 25 to 30% into erythrocytes. The uptake of 6-CB into plasma was extremely rapid and the rate of uptake was found to be dependent upon temperature. The distribution of 6-CB among lipoproteins was relatively homogeneous with 20 to 30% being distributed in very low-density lipoproteins (VLDL, d = 0.95-1.006 g/ml), 15 to 20% in low-density lipoproteins (LDL, d = 1.006-1.063 g/ml), and 15 to 25% in high-density lipoproteins (HDL, d = 1.063-1.21 g/ml). Over 25% of 6-CB was found in the remaining bottom fraction. In addition, each isolated fraction when incubated alone with 6-CB was shown capable of uptake. The relative proportion of 6-CB among the lipoproteins was independent of the level taken up by plasma. 6-CB was also found to transfer among lipoproteins. This exchange of 6-CB proved to be dependent upon the concentrations of both protein and triacylglycerol in the incubations. Two proteins in the bottom fraction (Bf), albumin and a steroid binding globulin, were capable of competing with the lipoproteins for 6-CB uptake.

Fate of intravenously administered high-density lipoprotein labeled with radioiodinated cholesteryl oleate in normal and hypolipidemic rats

Radioiodinated cholesteryl oleate (125I-CO) was found to associate rapidly with plasma lipoproteins following intravenous administration to rats. The high-density lipoprotein (HDL) fraction was observed to contain the highest amount of radioiodinated ester. Isolation and purification of this HDL fraction (125I-CO-HDL) and subsequent administration to rats demonstrated a plasma clearance similar to that previously observed for HDL labeled by direct iodination. Moreover, the concentration of radioactivity appearing in the adrenal cortex and ovary 0.5 h after intravenous administration of 125I-CO-HDL was greater than that observed after administration of 125I-CO, and the uptake of radioactivity by these tissues was considerably greater in hypolipidemic rats. These findings are consistent with existing knowledge relating to the metabolic fate of HDL and radioiodinated cholesterol derivatives in the rat, and suggest that radioiodinated cholesteryl esters may become useful probes for labeling lipoproteins.

[Lipoproteins associated with lipoprotein B in human serum low density lipoproteins]

Small amounts of lipoprotein C and lipoprotein D could be observed in low density lipoproteins (1.030-1.055 g/ml), using electroimmunomigration and polyacrylamide gel electrophoresis. Lipoprotein structures containing several apolipoproteins such as lipoprotein (B+C) or (B+D) were not detected in these low density lipoproteins. Lipoproteins C and D could not be separated from lipoprotein B by using gel filtration and affinity chromatography on heparin-Sepharose. Apolipoproteins C-III and D measured by electroimmunoassay are 3.2 +/- 1.2% and 1.15 +/- 0.6%, respectively, of the proteins found in the density range 1.030-1.055 g/ml, so there is, therefore, about 1 mol of apolipoprotein C-III and 0.1 mol of apolipoprotein D per mol of apolipoprotein B.

Preferential binding of chlordecone to the protein and high density lipoprotein fractions of plasma from humans and other species

The preferential distribution of the relatively nonpolar pesticide chlordecone (CD) to liver rather than to fat tissues in humans suggests that it may be transported in plasma differently from other organochlorine pesticides. The plasma binding of [14C] CD was investigated in vitro in human, rat, and pig plasma and in vivo in rat plasma. Protein and lipoprotein fractions were separated by serial ultracentrifugation. Heparin-manganese precipitation and agarose gel electrophoresis were also carried out to determine whether separation techniques altered CD binding to plasma components. In human plasma, the distribution of [14C] CD among proteins and high density, low density, and very low density lipoproteins (HDL, LDL, and VLDL) was 46, 30, 20, and 6%, respectively. The distribution of cholesterol in the same plasma fractions was 4, 20, 63, and 7%, respectively. In the pig and rat the order of binding was similar to that in humans, with protein greater than or equal to HDL greater than LDL greater than or equal to VLDL. Separation by heparin-Mn precipitation confirmed the results obtained by ultracentrifugation. The distribution of [14C] CD in rat lipoprotein was similar whether the CD was administered in vivo or incubated with plasma in vitro, with approximately 80% bound to HDL, 11% to LDL, and 9% to VLDL in either case. Agarose gel electrophoresis of plasma-bound [14C] CD indicated that albumin was the major component of the protein fraction responsible for CD binding. Preferential binding of CD by albumin and HDL may explain its unusual tissue distribution compared to other organochlorine pesticides such as aldrin and dieldrin, which bind preferentially to VLDL and LDL and distribute preferentially to fat tissues.

Removal of benzo[a]pyrene from cells by various components of medium

Benzo[a]pyrene is removed from cells in culture by various additions to the medium. During post-treatment incubation, WI-38 fibroblasts were incubated with a low density, very low density and high density lipoproteins, delipidated or complete serum or plasma, or serum albumin. The time course of removal was followed. Increasing concentrations of lipoproteins resulted in increasing percentages of removal of benzo[a]pyrene from cell membranes. The most efficient addition was 10% complete human plasma. These results indicate that benzo[a]pyrene remains at or close to the plasma membrane for at least several hours and readily redistributes to medium components.

Uptake of lipophilic carcinogens by plasma lipoproteins. Structure-activity studies

This report describes the interaction between plasma lipoproteins and two hydroxylated metabolites of benzo[a]pyrene, 3-hydroxybenzo[a]pyrene and benzo[a]pyrene-7,8-dihydrodiol, which differ significantly in lipophilicity. When incubated with plasma, the metabolites of benzo[a]pyrene exhibit a decreasing distribution into the ultracentrifugal lipoprotein fraction (d less than or equal to 1.20) and an increasing distribution into the albumin-rich fraction (d greater than 1.20) as the degree of hydroxylation of the metabolite increases. At saturation, uptake of benzo[a]pyrene by VLDL, LDL and HDL correlates with lipoprotein and total-lipid volume. Uptake of hydroxylated derivatives per lipoprotein total-lipid volume, in general, decreases with increasing hydroxylation. Contrary to this trend, HDL uptake of 3-hydroxybenzo[a]pyrene at saturation is significantly higher than its uptake of benzo[a]pyrene. Uptake of benzo[a]pyrene-7,8-dihydrodiol per total-lipid volume by all of the lipoprotein classes at saturation is considerably lower than their uptake of 3-hydroxybenzo[a]pyrene. Factors in addition to lipid solubility substantially alter lipoprotein uptake of the metabolites.