Independent action of serine/threonine protein phosphatase in ovarian cancer plasma membrane and cytosol during gonadotropin-releasing hormone stimulation

Reversible serine/threonine protein phosphorylation catalyzed by kinases and phosphatases 2A (PP2A) plays a crucial role in cellular growth and differentiation. We attempted to determine the subcellular location of PP2A in ovarian cancer cells and its regulation by gonadotropin-releasing hormone (GnRH), which is known to have anti-proliferative actions on ovarian cancers. Surgically removed ovarian cancers were screened for GnRH receptor expression prior to subcellular fractionations. PP2A activity was assessed by measuring the dephosphorylation of phosphopeptide highly selective for the PP2A in cytosol and membranes fractionated on a continuous sucrose density gradient. To assess GnRH effects, cloned cell lines were pretreated with or without a GnRH agonist. There were three peaks of PP2A activity, corresponding by marker enzyme analysis to the cytoplasm, plasma membrane and endoplasmic reticulum fractions. The kinetic analysis showed a different activity in cytosol and membrane; Km values for substrate of 185 µM and Vmax of 555 pmol/mg protein/30 min for cytosol, and 28 µM and 83 pmol/mg protein/30 min for plasma membrane, respectively. PP2A-specific inhibitor okadaic acid inhibited the cytosolic and membrane-associated activity by 50% when added at 2 nM and 50 nM (p<0.001). A 50% inhibitory effect of NaF was obtained at 0.5-1 mM for cytosol and 5 mM for membranes (p<0.001). In Caov-3 cells exposed GnRH, PP2A activity of plasma membrane increased by 1.3-fold (p<0.001) but that of cytosol was not affected. PP2A activity in the plasma membrane of ovarian cancer cells might be distinct from that present in the cytosol. The plasma membrane PP2A may be responsible for a portion of an increased ser/thr protein phosphorylation-dephosphorylation turnover that occurs during cell exposure to GnRH.