MAINTENANCE OF MEIOTIC ARREST BY A PHOSPHORYLATED P34(CDC2) IS INDEPENDENT OF CYCLIC ADENOSINE-3',5'-MONOPHOSPHATE

The meiotic division in oocytes is arrested in the G(2) phase of the c ell cycle. Resumption of meiosis, also known as oocyte maturation, ent ails a G(2) to M transition and is associated with a drop in intraoocy te concentrations of cAMP. Recent studies imply that tyrosine dephosph orylation of p34(cdc2) is a prerequisite for entry into the M-phase of the cell cycle. Our study was designed to test the involvement of pro tein tyrosine phosphatase (PTPase)-regulated dephosphorylation of p34( cdc2) in resumption of meiosis in rat oocytes and to explore the possi ble control of this event by the intraoocyte concentrations of cAMP. I solated rat oocytes undergoing meiotic maturation spontaneously in vit ro served as our experimental model. We found that sodium metavanadate , an inhibitor of PTPase, reversibly blocked the spontaneous maturatio n in vitro of rat oocytes (ED(50) = 0.26 mM). We further demonstrated that the vanadate-sensitive event is completed by 2 h after reinitiati on of meiosis. Immunoblot analysis using specific antiphosphotyrosine antibodies revealed that vanadate caused accumulation of phosphotyrosi ne on a 34-kDa protein, also recognized by anti-p34(cdc2) antibodies. The phosphorylated form of p34(cdc2) was also detected in oocytes arre sted in the G(2) phase by the phosphodiesterase inhibitor isobutyl met hylxanthine (IBMX). Intraoocyte concentrations of cAMP in vanadate-inh ibited oocytes were similar to those in oocytes that resumed meiosis s pontaneously in vitro and lower than those in oocytes maintained in me iotic arrest by IBMX (0.073 +/- 0.08, 0.84 +/- 0.09, and 1.42 +/- 0.3 fmol/oocyte, respectively). We conclude that a PTPase that regulates t he phosphorylation state of p34(cdc2) participates in the control of m eiosis in rat oocytes. Furthermore, maintenance of meiotic arrest by a phosphorylated p34(cdc2) that is independent of cAMP suggests that th e above-mentioned PTPase activity occurs downstream to the oocyte matu ration-associate drop of intracellular concentrations of cAMP.