Inhibition of sarco-endoplasmic reticulum calcium ATPases (SERCA) by polycyclic aromatic hydrocarbons: lack of evidence for direct effects on cloned rat enzymes.

Previous studies have demonstrated that immunosuppressive polycyclic aromatic hydrocarbons (PAHs) disrupt Ca2+ homeostasis leading to inhibition of the Ca(2+)-dependent pathways of T cell and B cell activation. The sustained Ca(2+)-elevation produced by immunosuppressive PAHs may result from the inhibition of Ca(2+)-ATPases in the endoplasmic reticulum (SERCA). The purpose of the present study was to determine whether PAHs directly inhibit cloned SERCA enzymes, and whether there is any selectivity for certain isoforms. PAHs were examined for their effects on purified cloned rat SERCA enzymes, including SERCA1, SERCA2a and SERCA3, transiently expressed in human embryonic kidney (HEK) cells. Results showed that known SERCA inhibitors, thapsigargin (100 nM) and 2,5-di(t-butyl)-1,4-benzohydroquinone (10 mumol), completely inhibited all rat SERCA isoforms, whereas 7,12-dimethylbenz(a)anthracene, benzo(a)pyrene, benzo(e)pyrene, anthracene, 3-methyl-cholanthrene, 9,10-dimethylanthracene and benz(a)anthracene at concentrations as high as 10 mumol appeared to have little inhibitory effect on any of the SERCA. The results demonstrating that PAHs do not inhibit cloned SERCA enzymes suggest that metabolism may be required for PAH-induced inhibition, or that other cellular elements, not present in the HEK transfection model, may be required for activity.