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 cell cycle. Resumption of meiosis, also known as oocyte maturation, entails a G(2) to M transition and is associated with a drop in intraoocyte concentrations of cAMP. Recent studies imply that tyrosine dephosphorylation 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 protein tyrosine phosphatase (PTPase)-regulated dephosphorylation of p34(cdc2) in resumption of meiosis in rat oocytes and to explore the possible control of this event by the intraoocyte concentrations of cAMP. Isolated rat oocytes undergoing meiotic maturation spontaneously in vitro served as our experimental model. We found that sodium metavanadate, an inhibitor of PTPase, reversibly blocked the spontaneous maturation in vitro of rat oocytes (ED(50) = 0.26 mM). We further demonstrated that the vanadate-sensitive event is completed by 2 h after reinitiation of meiosis. Immunoblot analysis using specific antiphosphotyrosine antibodies revealed that vanadate caused accumulation of phosphotyrosine on a 34-kDa protein, also recognized by anti-p34(cdc2) antibodies. The phosphorylated form of p34(cdc2) was also detected in oocytes arrested in the G(2) phase by the phosphodiesterase inhibitor isobutyl methylxanthine (IBMX). Intraoocyte concentrations of cAMP in vanadate-inhibited oocytes were similar to those in oocytes that resumed meiosis spontaneously in vitro and lower than those in oocytes maintained in meiotic 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 the phosphorylation state of p34(cdc2) participates in the control of meiosis in rat oocytes. Furthermore, maintenance of meiotic arrest by a phosphorylated p34(cdc2) that is independent of cAMP suggests that the above-mentioned PTPase activity occurs downstream to the oocyte maturation-associate drop of intracellular concentrations of cAMP.