Uptake of persistent environmental chemicals by cultured human cells

Uptake of the persistent environmental chemicals 2,2',4,4',5,5'-hexachlorobiphenyl and 1,1,1-trichloro-2,2-di-(4-chlorophenyl)ethane (the insecticide DDT) by Chang liver cells, an established human cell line, has been investigated. Monolayer cells were incubated with culture medium to which the lipophilic model compounds had been added. The time course of uptake of either compound was biphasic, reaching equilibrium after about 5 hr of incubation. The ratio of DDT:hexachlorobiphenyl uptake was dependent on the presence of serum proteins. Increasing concentrations of serum proteins in the culture medium progressively inhibited uptake. Efflux from the cells was not entirely reversible: 10-20% of the chemicals were not released. Uptake was a linear function of the external concentration of the compounds. Absorptive binding to the outer cell plasma membrane could be determined by removing bound chemicals with fetal calf serum ("back exchange"). With this method, temperature-dependent translocation through the cell plasma membrane could directly be demonstrated. The effect of low temperature as well as the influence of metabolic inhibitors point out the contribution of energy-driven uptake pathways. Demonstration of LDL receptor-like binding protein on Chang liver cells facilitated estimation of the role of receptor-mediated uptake. This route of uptake proved to be of minor importance only, as was transport of the protein-bound chemicals via fluid pinocytosis. The results demonstrate that cellular endocytosis of plasma membrane-bound chemicals constitutes a major uptake pathway for lipophilic chemicals.