Phospholipid and glyceride biosynthesis in 2,4,5,2',4',5'-hexachlorobiphenyl-treated human skin fibroblasts

Accumulation of polychlorinated biphenyls in adipocytes: selective targeting to lipid droplets and role of caveolin-1

Background

Polychlorinated biphenyls (PCBs) are persistent environmental pollutants that preferentially accumulate in lipid-rich tissues of contaminated organisms. Although the adipose tissue constitutes a major intern reservoir of PCBs and recent epidemiological studies associate PCBs to the development of obesity and its related disorders, little is known about the mechanisms involved in their uptake by the adipose tissue and their intracellular localization in fat cells.

Methodology/Principal Findings

We have examined the intracellular distribution of PCBs in mouse cultured adipocytes and tested the potential involvement of caveolin-1, an abundant adipocyte membrane protein, in the uptake of these compounds by fat cells. We show that 2,4,4′-trichlorobiphenyl (PCB-28), 2,3′,4,4′,5-pentachlorobiphenyl (PCB-118) and 2,2′,4,4′,5,5′-hexachlorobiphenyl (PCB-153) congeners rapidly and extensively accumulate in 3T3-L1 or mouse embryonic fibroblast (MEF) derived cultured adipocytes. The dynamics of accumulation differed between the 3 congeners tested. By subcellular fractionation of primary adipocytes, we demonstrate that these pollutants were almost exclusively recovered within the lipid droplet fraction and practically not associated to cell membranes. The absence of caveolin-1 expression in primary adipocytes from cav-1 deficient mice did not modify lipid droplet selective targeting of PCBs. In cav-1 KO MEF differentiated adipocytes, PCB accumulation was decreased, which correlated with reduced cell triglyceride content. Conversely, adenoviral mediated cav-1 overexpressing in 3T3-L1 cells, which had no impact on total cell lipid content, did not change PCB accumulation.

Conclusion/Significance

Our data indicate that caveolin-1 per se is not required for selective PCB accumulation, but rather point out a primary dependence on adipocyte triglyceride content. If the crucial role of lipid droplets in energy homeostasis is considered, the almost exclusive accumulation of PCBs in these organelles warrants future attention as the impairment of their function could be linked to the worldwide obesity epidemic.

The use of 2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) as an unmetabolizable lipophilic model compound

2,4,5,2',4'5'-Hexachlorobiphenyl (6-CB)--a polychlorinated biphenyl (PCB) congener resistant to metabolism in most species--has become a major residue in the biosphere including human adipose tissue. Its use as a model of unmetabolizable lipophilic compounds and as a tool in toxicokinetics in the last two decades is reviewed. This extremely water-insoluble compound is transported in plasma by albumin and lipoproteins. Binding to these plasma proteins appears to be important for uptake and release processes in different tissues. The redistribution kinetics of 6-CB as well as its pronounced adipose tissue storage and a very slow excretion with the faeces has been established in long-term animal studies. Excretion is strongly influenced by an increasing or diminishing adipose storage compartment size. Other minor pathways of elimination, e.g., via hair, become also important in the absence of metabolism and renal excretion. 6-CB has revealed the possibility of an almost quantitative transfer of the maternal body burden to the offspring via milk. The use of 6-CB in studies with tissue preparations in vitro is providing insight into transport mechanisms of uptake and release.

Photo-induced formation of DNA adducts of 2,2',4,4',5,5'-hexachlorobiphenyl in cultured human cells

Association of the PCB congener 2,2',4,4',5,5'-hexachlorobiphenyl (6-CB) with cell nuclei has been studied in cultured monolayer human Chang liver cells. Photo-induced formation of covalent bonds determined 6-CB binding to protein of cell nuclei and to DNA. Nuclear binding of 6-CB approached equilibrium after approximately 30 min of incubation. Photo-induced binding in vitro to purified Chang liver cell DNA substantiated direct interaction of the PCB congener with DNA. In monolayer cells, low levels of photo-induced 6-CB DNA adducts could be detected using the very sensitive 32P-postlabeling method. Adduct formation was dependent on 6-CB concentration as well as on incubation time. Highest adduct levels were in the range of 2 X 10(-8). Model reactions in vitro showed photo-induced binding of 6-CB to individual purine deoxyribonucleotide-3'-phosphates. The results demonstrate rapid intracellular movement of the PCB congener into the cell nucleus. The vast majority is associated with nuclear protein, minute amounts of 6-CB are found proximate to the DNA helix as evidenced by photo-induced adducts of purine nucleotides.

Triglyceride accumulation in long-term cultures of adult rat hepatocytes by chronic exposure to Aroclor 1254

The effect of chronic exposure to micromolar concentrations of Aroclor 1254 (Aro) on the hepatic lipid metabolism was studied in long-term cultures of adult rat hepatocytes. Hepatocytes were cocultivated with mitomycin C-treated 3T3 cells and exposed for 2 wk to Aroclor 1254 concentrations ranging from 0.01 to 20 micrograms/ml. The Aro-exposed cultures showed intracytoplasmic lipid droplets and a maximum increase of 55% in the triglyceride (TG) content and of 4.4-fold in the cytochrome P-450 content. Labeling studies with [14C]acetic and [14C]oleic acid showed no changes in the uptake of fatty acid and TG precursors by the Aro-treated cultures; the synthesis of cellular lipids from [14C]acetic acid was slightly inhibited by Aroclor 1254, but that from [14C]oleic acid was increased, specially for TG (37%). The secretion of total lipids and TG was 2.1- and 2.7-fold lower, respectively, in the cultures treated with 20 micrograms/ml of Aroclor 1254, resulting in an increase of 1.9-fold in the intracellular content of TG. The synthesis of cellular proteins labeled with [3H]leucine was unchanged in the Aro-treated cultures, but the secretion of exportable proteins was 1.7-fold lower in the cultures treated with 20 micrograms/ml of Aroclor 1254. Our results showed that long-term exposure to in vivo relevant concentrations of Aroclor 1254 produced morphological and biochemical changes in cultured hepatocytes, like those described in vivo, and intracellular TG accumulation due mostly to impaired secretion of TG by the hepatocytes. Our results also suggest that this culture system could be useful for the screening of toxic agents producing fatty liver and the study of the involved mechanism(s).

Photo-induced binding of 2,2',4,4',5,5'-hexachlorobiphenyl to cultured human cells

The polychlorinated biphenyl congener 2,2',4,4',5,5'-hexachlorobiphenyl can be photoactivated by brief high-intensity ultraviolet irradiation. Photoactivated intermediates are bound to neighboring biological macromolecules. Properties and stability of hexachlorobiphenyl photobinding were examined with bovine serum albumin, a protein known to strongly bind lipophilic compounds. Photobinding to cultured human Chang liver cells was a function of ligand and cell protein concentration as well as of irradiation time. Binding increased with incubation time, in support of the time course of uptake previously measured in the same system by alternative methods. Separation of cell proteins by gel electrophoresis showed that the distribution pattern of photobinding changed at different rates for different proteins. Photobinding to major cell lipid groups and to individual phospholipids likewise reflected uptake of the compound. Notably, photobinding to phosphatidyl choline was elevated relative to phosphatidyl ethanolamine. Thus, the presented method is suitable to follow up transport and intracellular equilibrium distribution of photoactivatable ligands. As a particular advantage, artefactual redistribution of persistent lipophilic compounds during cell fractionation can be avoided.

2,4,5,2',4',5'-Hexachlorobiphenyl-lipoprotein (LDL, HDL, VLDL) interaction and induced lipidosis in cultured skin fibroblasts

2,4,5,2',4',5'-Hexachlorobiphenyl (HCB) induced cytoplasmic inclusions and lipidosis in normal ( AG1437 ) and hypercholesterolemic ( GM488 ) human skin fibroblasts. Quantitative and qualitative microsocopic fluorescence analysis showed that the cytoplasmic inclusions are formed as early as 3 hr after treatment with HCB. The inclusions contain lipids but no detectable nonesterified cholesterol or cholesteryl ester. High density lipoproteins (HDL), low density lipoproteins (LDL), and very low density lipoproteins (VLDL) facilitate the apparent uptake of HCB by skin fibroblasts. HDL and LDL appeared to reverse the induction of cytoplasmic inclusions and lipidosis when cells were pretreated with HCB, the HCB was removed from media, and the cells were incubated with LDL or HDL. The results suggest that lipoproteins participate in the uptake and egress of HCB from skin fibroblasts.