Structure-Activity Relationship for Noncoplanar Polychlorinated Biphenyl Congeners toward the Ryanodine Receptor-Ca 2+ Channel Complex Type 1 (RyR1) Isaac N. Pessah,* ,² Larry G. Hansen, ‡ Timothy E. Albertson, § C. Edwin Garner, |,⊥ Tram Anh Ta, ² Zung Do, ² Kyung Ho Kim, ² and Patty W. Wong ²,# Department of VM:Molecular Biosciences and Center for Children’s EnVironmental Health, and Department of Internal Medicine, DiVision of Pulmonary and Critical Care Medicine, School of Medicine, UniVersity of California, DaVis, California 95616, Department of Veterinary Biosciences, College of Veterinary Medicine, UniVersity of Illinois, Urbana, Illinois 61802, and Laboratory of Pharmacology and Chemistry, EnVironmental Toxicology Program, National Institute of EnVironmental Health Sciences, Research Triangle Park, North Carolina 27709 ReceiVed July 15, 2005 Ryanodine receptor isoforms are expressed in both excitable and nonexcitable tissues where they form microsomal Ca 2+ release channels broadly involved in shaping cellular signaling. In this report, we provide a detailed structure-activity relationship (SAR) for polychlorinated biphenyl (PCB) congeners and metabolites necessary for enhancing ryanodine receptor type 1 (RyR1) activity using [ 3 H]ryanodine ([ 3 H]Ry) binding analysis. The 2,3,6-Cl PCB configuration is most important for optimal recognition by the RyR1 complex and/or critical for sensitizing its activation. Para substitution(s) diminishes the activity with para-chloro having a higher potency than the corresponding para-hydroxy derivative. The addition of a more bulky para-methyl-sulfonyl group eliminates the activity toward RyR1, supporting the importance of the para positions in binding RyR1. The requirement for an intact major T cell immunophilin FKBP12- RyR1 complex was observed with each of 12 active PCB congeners indicating a common mechanism requiring an immunophilin-regulated Ca 2+ release channel. An excellent correlation between the relative potencies for doubling [ 3 H]Ry binding and the corresponding initial rates of PCB-induced Ca 2+ efflux indicates that [ 3 H]Ry binding analysis provides a measure of dysregulation of microsomal Ca 2+ transport. The SAR for activating RyR1 is consistent with those previously reported in several in vivo and in vitro studies, suggesting that a common mechanism may contribute to the toxicity of noncoplanar PCBs. A practical application of the receptor-based screen developed here with RyR1 is that it provides a quantitative SAR that may be useful in predicting biological activity and risk of mixtures containing noncoplanar PCB congeners that have low or a lack of aryl hydrocarbon receptor activity. Introduction Polychlorinated biphenyls (PCBs) 1 along with polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) are halogenated aromatic hydrocarbons (HAHs). In the United States, PCBs were synthesized and marketed as Aroclor mixtures, which were widely used in several industries. To- gether, the improper disposal, the high lipophilicity, and the extreme chemical resistance of these chemicals led to worldwide contamination and accumulation in biota (1-3). Because of their diverse structures, differences in ecosystem partitioning, and variable metabolic degradations, complex mixtures of PCB congeners in biological samples with proportions unique to sources and species have been found (1). Attention has been directed toward ortho-poor PCB congeners (i.e., tetra- through octachlorobiphenyls with single or no ortho-chlorine atoms) because they can assume a coplanar configuration and interact with the aryl hydrocarbon receptor (AhR), mimicking the action of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (2-4). However, ortho-rich PCBs (congeners with 2-4 ortho-chlorine atoms) are more prevalent in former commercial mixtures, environmental reservoirs, and human samples (1, 5-7). The relative abundance of ortho-rich PCBs is further noted in air because of greater volatility (8-11). In fish from contaminated temperate waters, ortho-PCBs are enriched due to differential inducible metabo- lism (12). Indeed, there are elevated levels of ortho-substituted PCB congeners found in both the tissues of Great Lakes fish and the serum of humans consuming these fish (7, 13-15). * To whom correspondence should be addressed. Tel: 530-752-6696. Fax: 530-752-4698. E-mail: inpessah@ucdavis.edu. ² Department of VM:Molecular Biosciences and Center for Children’s Environmental Health, University of California. ‡ University of Illinois. § School of Medicine, University of California. | National Institute of Environmental Health Sciences. ⊥ Current address: Department of Drug Metabolism and Pharmacoki- netics, Schering-Plough Research Institute, P.O. Box 32, 144 Route 94, Lafayette, NJ 07848-0032. # Current address: Department of Toxic Substances Control, 1001 I Street, Sacramento, CA 95814. 1 Abbreviations: Ah, aryl hydrocarbon; Ah (H), aryl hydrocarbon hydroxylase; CICR, Ca 2+ -induced Ca 2+ release; DMSO, dimethyl sulfoxide; EC50, effective concentration that causes enhancement to 50% of the maximum activity; EROD, ethoxyresorufin o-deethylase; FKBP12/RyR, immunophilin FKBP12/ryanodine receptor complex; HAHs, halogenated aromatic hydrocarbons; [ 3 H]Ry, tritium-labeled ryanodine; IP3, inositol 1,4,5- trisphosphate; IP3R, inositol 1,4,5-trisphosphate receptor; PCBs, polychlo- rinated biphenyls; PCDFs, dibenzofurans; PCDDs, polychlorinated dibenzo- p-dioxins; PC12, rat pheochromocytoma cells; RyR1, ryanodine receptor type 1; RyR, all three isoforms of ryanodine receptor; SR/ER, sarcoplasmic or endoplasmic reticulum; SARs, structure-activity relationships; TCDD, 2,3,7,8-tetrachlorodibenzo-p-dibenzodioxin; TEFs, (toxic) TCDD equiva- lence factors; TEQ, (toxic) TCDD equivalence; TNP-LPS, trinitrophenyl- lipopolysaccharide. 92 Chem. Res. Toxicol. 2006, 19, 92-101 10.1021/tx050196m CCC: $33.50 © 2006 American Chemical Society Published on Web 11/22/2005