A bioactive metabolite of benzo[a]pyrene, benzo[a]pyrene-7,8- dione, selectively alters microsomal Ca²⁺ transport and ryanodine receptor function

Polycyclic aromatic hydrocarbons are environmental pollutants known to be carcinogenic and immunotoxic. In intact cell assays, benzo[a]pyrene (B[a]P) disrupts Ca²⁺ homeostasis in both immune and nonimmune cells, but the molecular mechanism is undefined. In this study, B[a]P and five metabolites are examined for their ability to alter Ca²⁺ transport across microsomal membranes. Using a well-defined model system, junctional SR vesicles from skeletal muscle, we show that a single o-quinone metabolite of B[a]P, B[a]P-7,8-dione, can account for altered Ca²⁺ transport across microsomal membranes. B[a]P-7,8-dione induces net Ca²⁺ release from actively loaded vesicles in a dose-, time-, and Ca²⁺-dependent manner. In the presence of 5 μM extravesicular Ca²⁺, B[a]P-7,8-dione exhibited threshold and EC₅₀ values of 0.4 and 2 μM, respectively, and a maximal release rate of 2 μmol of Ca²⁺ min^-1 mg^-1. The mechanism by which B[a]P-7,8-dione enhanced Ca²⁺ efflux was further investigated by measuring macroscopic fluxes and single RyR1 channels reconstituted in bilayer lipid membranes and direct measurements of SERCA catalytic activity. B[a]P-7,8-dione (≤ 20 μM) had no measurable effect on initial rates of Ca²⁺ accumulation in the presence of ruthenium red to block ryanodine receptor (RyR1), nor did it alter Ca²⁺-dependent (thapsigarginsensitive) ATPase activity. B[a]P-7,8-dione selectively altered the function of RyR1 in a time-dependent diphasic manner, first activating then inhibiting channel activity. Considering that RyR1 and its two alternate isoforms are broadly expressed in mammalian cells and their important role in Ca²⁺-signaling, the present results reveal a mechanism by which metabolic bioactivation of B[a]P may mediate RyR dysfunction of pathophysiological significance.