RELATIVE POTENCIES OF INDIVIDUAL POLYCHLORINATED DIBENZO-p-DIOXIN, DIBENZOFURAN, AND BIPHENYL CONGENERS AND CONGENER MIXTURES BASED ON INDUCTION OF CYTOCHROME P4501A mRNA IN A RAINBOW TROUT GONADAL CELL LINE (RTG-2)

Cytochrome P450-catalyzed enzyme activity in cell culture was investigated as a bioassay for 2,3,7,8-tetrachlorodibenzop-dioxin (TCDD) equivalents in environmental mixtures of polychlorinated dibenzo-p-dioxin (PCDD), dibenzofuran (PCDF), and biphenyl (PCB) congeners. A problem with the use of enzyme induction is that certain congeners are unable to induce P4501A enzyme activity to the same maximal level as TCDD. We sought to eliminate this problem by measuring mRNA induction rather than enzyme activity. Rainbow trout gonadal cells (RTG-2) were exposed to PCDD, PCDF, and PCB congeners and congener mixtures, and induction of cytochrome P4501A mRNA was measured. A high level of induction in cells treated with only a medium change was seen and was due to a component of the fresh medium. 2,3,7,8-Substituted PCDD and PCDF congeners and four nonortho-substituted PCBs caused dose-related induction of P4501A mRNA. Di-ortho-substituted PCBs did not induce P450 mRNA, but three mono-ortho-substituted PCBs caused significant induction. Toxic equivalency factors determined in RTG-2 cells were generally higher than those in rainbow trout early life stages. Rainbow trout gonadal cell (RTG-2) bioassay TCDD equivalents (TEqs) for three environmental extracts were lower than those predicted by addition of individual congener TEqs, and the synthetic congener mixture acted additively. Keywords—2,3,7,8-Tetrachlorodibenzo-p-dioxin Polychlorinated dibenzofurans Polychlorinated biphenyls Toxic equivalency factors Cell culture