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 c arcinogenic and immunotoxic. In intact cell assays, benzo[a]pyrene (B[a]P) disrupts Ca2+ homeostasis in both immune and nonimmune cells, but the molec ular mechanism is undefined. In this study, B[a]P and five metabolites are examined for their ability to alter Ca2+ transport across microsomal membra nes. Using a well-defined model system, junctional SR vesicles from skeleta l muscle, we show that a single o-quinone metabolite of B[a]P, B[a]P-7,8-di one, can account for altered Ca2+ transport across microsomal membranes. B[ a]P-7,8-di-one induces net Ca2+ release from actively loaded vesicles in a dose-, time-, and Ca2+-dependent manner. In the presence of 5 muM extravesi cular Ca2+, B[a]P-7,8-dione exhibited threshold and EC50 values of 0.4 and 2 muM, respectively, and a maximal release rate of 2 mmol of Ca2+ min(-1) m g(-1). The mechanism by which B[a]P-7,8-dione enhanced Ca2+ efflux was furt her investigated by measuring macroscopic fluxes and single RyR1 channels r econstituted in bilayer lipid membranes and direct measurements of SERCA ca talytic activity. B[a]P-7,8-dione (less than or equal to 20 muM) had no mea surable effect on initial rates of Ca2+ accumulation in the presence of rut henium red to block ryanodine receptor (RyR1), nor did it alter Ca2+-depend ent (thapsigargin-sensitive) ATPase activity. B[a]P-7,8-dione selectively a ltered the function of RyR1 in a time-dependent diphasic manner, first acti vating then inhibiting channel activity. Considering that RyR1 and its two alternate isoforms are broadly expressed in mammalian cells and their impor tant role in Ca2+-signaling, the present results reveal a mechanism by whic h metabolic bioactivation of B[a]P may mediate RyR dysfunction of pathophys iological significance.