A gonadotropin-releasing hormone-responsive phosphatase hydrolyses lysophosphatidic acid within time plasma membrane of ovarian cancer cells

Lysophosphatidic acid (LPA) mediates pleomorphic effects on multiple cell l ineages, including an increased proliferative response of ovarian cancer ce lls both in vitro and in vivo, at least in part through the novel expressio n of LPA receptors. Thus, LPA hydrolysis is necessary to limit the duration of LPA's action on multiple cell types, including ovarian cancer cells. We determined the principal mechanism of LPA hydrolysis by ovarian cancer cel ls and its regulation by GnRH, which is known to have antiproliferative act ions on ovarian carcinomas. LPA-hydrolyzing activity in cell membranes of o varian cancer specimens was assessed by measuring the conversion of exogeno us [H-3-oleoyl]LPA to [H-3]oleic acid or mono[H-3-oleoyl] glycerol. Approxi mately 98% of LPA hydrolysis could be accounted for by the dephosphorylatio n of LPA to yield monoglyceride, with the deacylation reaction accounting f or less than 1% of LPA hydrolysis. The phosphatase activity in the plasma m embrane ovarian cancer cells was approximately 2.5- and 8-fold higher than those in microsome and homogenate fractions, respectively. The membrane pho sphatase was Mg2+ independent and insensitive to inhibition by N-ethylmalei mide, characteristics suggestive of phosphatidic acid phosphatase activity. Incubation of membranes from GnRH receptor-positive ovarian cancer specime ns with the GnRH agonist, buserelin, induced a dose-dependent increase in L PA phosphatase activity, with a half-maximal effect occurring with 30 nmol/ L buserelin. The stimulatory action of buserelin could be neutralized by di splacement of GnRH from its receptor by the GnRH antagonist, antide. The pl asma membranes from GnRH receptor-negative ovarian cancer specimens did not respond to GnRH stimulation. LPA phosphatase activity was also increased w hen the ovarian cancer cell line Caov-3 was exposed to GnRH agonist in inta ct cells before assay of cell membranes. These data demonstrate that LPA is hydrolyzed in the plasma membrane of ovarian cancer cells by the action of LPA phosphatase and provide initial evidence for functional coupling of LP A phosphatase to GnRH receptor occupancy.