Participation of MAPK, PKA and PP2A in the regulation of MPF activity in Bufo arenarum oocytes.
The objectives of the present paper were to study the involvement and possible interactions of both cAMP-PKA and protein phosphatases in Bufo arenarum oocyte maturation and to determine if these pathways are independent or not of the MAP kinase (MAPK) cascade. Our results indicated that the inhibition of PKA by treatment with H-89, an inhibitor of the catalytic subunit of PKA, was capable of inducing GVBD in a dose-dependent manner by a pathway in which Cdc25 phosphatase but not the MAPK cascade is involved. The injection of 50 nl of H-89 10 μM produced GVBD percentages similar to those obtained with treatment with progesterone. In addition, the assays with okadaic acid (OA), a PP2A inhibitor, significantly enhanced the percentage of oocytes that resumed meiosis by a signal transducing pathway in which the activation of the MEK-MAPK pathway is necessary, but in which Cdc25 phosphatase was not involved. Treatment with H-89, was able to overcome the inhibitory effect of PKA on GVBD; however, the inhibition of Cdc25 activity with NaVO3 was able to overcome the induction of GVBD by H-89. Although the connections between PKA and other signalling molecules that regulate oocytes maturation are still unclear, our results suggest that phosphatase Cdc25 may be the direct substrate of PKA. In Xenopus oocytes it was proposed that PP2A, a major Ser/Thr phosphatase present, is a negative regulator of Cdc2 activation. However, in Bufo arenarum oocytes, inhibition of Cdc25 with NaVO₃ did not inhibit OA-induced maturation, suggesting that the target of PP2A was not the Cdc25 phosphatase. MAPK activation has been reported to be essential in Xenopus oocytes GVBD. In B. arenarum oocytes we demonstrated that the inhibition of MAPK by PD 98059 prevented the activation of MPF induced by OA, suggesting that the activation of the MAPK cascade produced an inhibition of Myt1 and, in consequence, the activation of MPF without participation of the Cdc25 phosphatase. Our results suggest that in incompetent oocytes of B. arenarum two signal transduction pathways may be involved in the control of MPF activation: (1) the inhibition of phosphatase 2A that through the MEK-MAPK pathway regulates the activity of the Myt1; and (2) the inhibition of AMPc-PKA, which affects the activity of the Cdc25 phosphatase.
Toranzo
G Sánchez
GS
Departamento de Biología del Desarrollo (INSIBIO), Chacabuco 461, 4000-San Miguel de Tucumán, Argentina.
Bonilla
F
F
Bühler
M C Gramajo
MC
Bühler
M I
MI
D000818:Animals; D002022:Bufo arenarum; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D016200:Maturation-Promoting Factor / Q000378:metabolism*; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D009865:Oocytes / Q000201:enzymology*; D054648:Protein Phosphatase 2 / Q000378:metabolism*; D020687:cdc25 Phosphatases / Q000378:metabolism
D017868:Cyclic AMP-Dependent Protein Kinases; D016200:Maturation-Promoting Factor; D020928:Mitogen-Activated Protein Kinases; D054648:Protein Phosphatase 2; D020687:cdc25 Phosphatases