Uptake of 2,3,7,8-tetrachlorodibenzo-p-dioxin from plasma lipoproteins by cultured human fibroblasts

Cellular uptake of lipoproteins and persistent organic compounds--an update and new data

There are a number of interactions related to the transport of lipophilic xenobiotic compounds in the blood stream of mammals. This paper will focus on the interactions between lipoproteins and persistent organic pollutants (POPs) and how these particles are taken up by cells. A number of POPs including the pesticide p,p'-dichlorodiphenyltrichloroethane (DDT), and especially its metabolite p,p'-dichlorodiphenyldichloroethene (DDE), interacts with nuclear hormone receptors causing these to malfunction, which in turn results in a range of deleterious health effects in humans. The aim of the present study was to determine the role of lipoprotein receptors in mouse embryonic fibroblast (MEF) cells in conjunction with uptake of DDT-lipoprotein complexes from supplemented media in vitro. Uptake of DDT by MEF cells was investigated using MEF1 cells carrying the receptors low-density lipoprotein receptor-related protein (LRP) and low-density lipoprotein receptor (LDLR) present and MEF4 cells with no LRP and LDLR expression. Cells were incubated together with the complex of low-density lipoproteins (LDL) and [(14)C]DDT. The receptor function was further evaluated by adding the 40kDa receptor-associated protein (RAP) which blocks receptor activity. The results showed that [(14)C]DDT uptake was decreasing when the LDL concentration was increasing. There was no strong evidence for a receptor-mediated uptake of the [(14)C]DDT-lipoprotein complex. To conclude, DDT travels in the blood stream and can cross cell membranes while being transported as a DDT-lipoprotein complex. The lipoproteins do not need receptors to cross cell membranes since passive diffusion constitutes a major passageway.

Plasma lipoproteins as targeting carriers to tumour tissues after administration of a lipophilic agent to mice

We synthesized 14C-warfarin hexadecyl ether (14C-WHE) by addition of a palmityl moiety to the hydroxyl group at the 4-position of 14C-warfarin, a compound known to bind to serum albumin. 14C-WHE preferentially bound to the lipoproteins, low-density lipoprotein (LDL) and high-density lipoprotein (HDL), in mouse plasma both in vitro and in vivo. 14C-Warfarin mainly concentrated in the liver immediately after intravenous administration to mice bearing M5076 sarcoma, and was found at only low concentrations in other tissues including the tumour. 14C-WHE highly distributed to the tumour, adrenal, and spleen, as well as the liver. These tissues coincided with those in which human 125I-LDL was vigorously incorporated. The results indicate that chemical modification of an agent, giving it high lipophilicity, will enable it to bind to lipoproteins after intravenous administration. These modifications raise the possibility of lipoproteins as endogenous targeting carriers into tumour cells, which have high LDL-receptor activity.

Contribution of serum lipoproteins as carriers of antitumour agent RS-1541 (palmitoyl rhizoxin) in mice

The tumour uptake as well as the anti-tumour activity of RS-1541 (palmitoyl rhizoxin), a potent antineoplastic agent, were investigated in mice bearing M5076 sarcoma. After intravenous administration, 14C-RS-1541 preferentially bound to the lipoproteins, to which 14C-rhizoxin did not bind. 14C-RS-1541 showed persisting high concentrations of radioactivity in the plasma (T 1/2 alpha, 4.9 h). The uptake of radioactivity by the tumour was second to those by the liver and spleen, and several times greater than those by the other tissues. Selective and sustained uptake by the tumour was also demonstrated by whole-body autoradiography. A considerable amount of rhizoxin was detected only in the tumour after administration of 14C-RS-1541, and the area under the tissue-concentration-time curve (AUCt) and the mean residence time (MRT) of rhizoxin in the tumour were much higher than those after administration of 14C-rhizoxin itself. The rhizoxin formation in the tumour was significantly reduced by chloroquine, a lysosomal enzyme inhibitor. RS-1541 showed a higher therapeutic activity than rhizoxin. At a 4 mg kg-1 dose, the maximum growth inhibition was 92% for RS-1541 and 41% for rhizoxin. These results indicate that RS-1541, but not rhizoxin, is taken up by the tumour via endocytosis, most likely via the low-density-lipoprotein receptor, after binding to lipoproteins. Thus, RS-1541 was considered to exhibit sustained high concentration in tumours and potent anti-tumour activity.

The toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and their relevance for toxicity

This article reviews the present state of the art regarding the toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs). The absorption, body distribution, and metabolism can vary greatly between species and also may depend on the congener and dose. In biota, the 2,3,7,8-substituted PCDDs and PCDFs are almost exclusively retained in all tissue types, preferably liver and fat. This selective tissue retention and bioaccumulation are caused by a reduced rate of biotransformation and subsequent elimination of congeners with chlorine substitution at the 2,3,7, and 8 positions. 2,3,7,8-Substituted PCDDs and PCDFs also have the greatest toxic and biological activity and affinity for the cytosolic arylhydrocarbon (Ah)-receptor protein. The parent compound is the causal agent for Ah-receptor-mediated toxic and biological effects, with metabolism and subsequent elimination of 2,3,7,8- substituted congeners representing a detoxification process. Congener-specific affinity of PCDDs and PCDFs for the Ah-receptor, the genetic events following receptor binding, and toxicokinetics are factors that contribute to the relative in vivo potency of an individual PCDD or PCDF in a given species. Limited human data indicate that marked species differences exist in the toxicokinetics of these compounds. Thus, human risk assessment for PCDDs and PCDFs needs to consider species-, congener-, and dose-specific toxicokinetic data. In addition, exposure to complex mixtures, including PCBs, has the potential to alter the toxicokinetics of individual compounds. These alterations in toxicokinetics may be involved in some of the nonadditive toxic or biological effects that are observed after exposure to mixtures of PCDDs or PCDFs with PCBs.

Assessment of biological activities of mixtures of polychlorinated dibenzo-p-dioxins (PCDDs) and their constituents in human HepG2 cells

Dose-response curves of the induction of P4501A1-dependent 7-ethoxyresorufin O-deethylase (EROD) were analyzed in human hepatoma HepG2 cells treated with defined mixtures of polychlorinated dibenzo-p-dioxins (PCDDs) and their 2,3,7,8-substituted constituents, similar to previous studies with rat hepatocytes and H4IIE cells (Schrenk et al. 1991). PCDDs appear to act less potent in human HepG2 cells in comparison with rat cells. For example, EC50 values of 2,3,7,8-Cl4DD were 8-fold and 19-fold higher than in rat H4IIE cells and hepatocytes, respectively. EC50 values of PCDDs were compared with that of 2,3,7,8-Cl4DD and expressed as 2,3,7,8-Cl4DD equivalents (TEs). Although the rank order of PCDD potencies was similar, TEs for some PCDDs (1,2,3,7,8-Cl5DD; TE = 0.75 and 1,2,3,7,8-Cl6DD; TE = 0.61) were found to be higher than in the rat system. In contrast to rat cells no significant induction of EROD could be detected with Cl8DD in HepG2 cells up to its limit of solubility. Experimentally determined TEs of PCDD mixtures containing 49 constituents were found to be largely due to additive effects of their 2,3,7,8-substituted constituents.

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.