Role of calcium-independent phospholipase A2 in PCB-induced stimulation of late gestation rat uterus and amnion

Polychlorinated biphenyls (PCBs) are persistent environmental toxicants that have been associated with decreased gestation length. Phospholipase A 2 (PLA2)-mediated mobilization of arachidonic acid from glycerophospholipids in gestational tissues is thought to be an essential step in the onset of parturition. However, how PLA2 expression patterns change throughout pregnancy and how PLA2 availability influences uterine sensitivity to toxicant-induced stimulation are not known. Because PCB 50 activates PLA 2 to release arachidonic acid from membrane phospholipids, this ortho-substituted PCB congener was used as a model compound to examine the hypothesis that stimulation of late gestation rat uterine and amniotic phospholipase A2 enzyme(s) may pose a threat to pregnancy maintenance by stimulating uterine contractility through the release of arachidonic acid and prostaglandins. Western blot analysis revealed a gestational age-related increase in uterine expression of calcium-dependent secretory (s)PLA 2-IIA and a previously unreported 50 kDa calcium-independent (i)PLA 2-related protein. This 50 kDa iPLA2-related protein was also detected in late gestation amnion tissue, where over ninety percent of PLA2 activity was calcium-independent. Increases in uterine PLA 2 expression were associated with a gestational age-related increase in both calcium-dependent and calcium-independent PLA2 activity. PCB 50 stimulated the force and frequency of uterine contractions in late, but not mid, gestation rat uterine strips in vitro. Furthermore, pharmacologic inhibition of iPLA2 significantly reduced PCB 50-induced stimulation of uterine strips, suggesting that sensitivity to PCB stimulation is dependent on the gestational age-related increase in the uterine 50 kDa iPLA2-related protein. PCB 50 also stimulated the release of arachidonic acid and prostaglandins from late gestation rat uterine myocytes. Arachidonic acid release was attenuated with inhibition of calcium-dependent and calcium-independent PLA2 inhibitors. Additionally, PCB 50 stimulated the release of arachidonic acid and prostaglandins from late gestation rat amnion cells in vitro, an effect that was associated with a significant increase in 85 kDa iPLA2 expression. These data provide evidence of a previously uncharacterized contribution of calcium-independent PLA2 to total rat uterine and amniotic phospholipase activity and support the hypothesis that late gestation rat uterine and amniotic PLA2 mediate PCB-induced stimulation of uterine contractions via arachidonic acid and prostaglandin release.