Increased [3H]phorbol ester binding in rat cerebellar granule cells and inhibition of 45Ca2+ sequestration in rat cerebellum by polychlorinated diphenyl ether congeners and analogs: structure-activity relationships

Abstract

Our previous reports indicate that ortho-substituted non-coplanar polychlorinated biphenyl (PCB) congeners perturbed neuronal Ca2+-homeostasis in vitro, altered agonist-stimulated inositol phosphate accumulation, and caused protein kinase C (PKC) translocation. The structure-activity relationship (SAR) with 24 PCB congeners was consistent with a chlorination pattern that favored non-coplanarity while those with chlorination that favored coplanarity were less active. To test the hypothesis that coplanarity (or lack thereof) is a significant factor in the activity of PCBs, studies with related classes of chemicals such as the polychlorinated diphenyl ethers (PCDEs), in which coplanarity is more difficult to achieve regardless of degree and pattern of chlorination, were initiated. The selected PCDEs and their analogs are predicted to be active, since they are non-coplanar in nature. The effects of these chemicals were studied using the same measures for which PCBs had differential effects based on structural configuration. These measures include PKC translocation as determined by [3H]-phorbol ester ([3H]PDBu) binding in cerebellar granule cells and 45Ca2+ sequestration as determined by 45Ca2+ uptake by microsomes and mitochondria isolated from adult rat cerebellum. All the PCDE congeners studied, increased [3H]PDBu binding in a concentration-dependent manner. The order of potency was 2,4,4'-trichlorodiphenyl ether > 4,4'-dichlorodiphenyl ether > diphenyl ether, 3,3',4,4'-tetrachlorodiphenyl ether and, 2,2',4,4',5- and 2,3',4,4',5-pentachlorodiphenyl ethers. The structurally related diphenyl ether nitrofen and diphenyl ethanes o,p'-1,1,1-trichloro-2,2-bis[p-chlorophenyl]ethane (DDT) and p,p'-DDT increased [3H]PDBu binding to a similar extent (28-35% stimulation at 100 microM). All PCDE congeners and their analogs inhibited 45Ca2+ sequestration by microsomes and mitochondria. Of all the chemicals, unchlorinated diphenyl ether was the least active. These results are in agreement with previous SAR findings in which non-coplanar PCBs are active and support our hypothesis that the extent of coplanarity determined by a pattern of chlorination on certain aromatic hydrocarbons can weaken their potency in vitro, although the extent of chlorination is also important.