Abstract:
Participation and relative importance of phosphatidylinositol-3 kinase (PI3K) and mitogen-activated
protein kinase (MAPK) signalling, either alone or in combination, have been investigated during
17 ,20 -dihydroxy-4-pregnen-3-one (DHP)-induced meiotic G2−M1 transition in denuded zebrafish
oocyte. Results demonstrate that concomitant with rapid phosphorylation (activation) of Akt (Ser473)
and MAPK (ERK1/2) at as early as 15 min of incubation, DHP stimulation promotes enhanced an GVBD
response and histone H1 kinase activation between 1 and 5 h in full-grown oocytes in vitro.While p-Akt
reaches its peak at 60 to 90 min and undergoes downregulation to the basal level by 240 min, ERK1/2
phosphorylation (activation) increases gradually until 120 min and remains high thereafter. Although,
priming with MEK1/2 inhibitor U0126 is without effect, PI3K inhibitors, wortmannin or LY294002,
delay the GVBD response significantly (P < 0.001) until 3 h but not at 5 h of incubation. Interestingly,
blocking PI3K and MEK function together could abrogate steroid-induced oocyte maturation at all time
points tested. While DHP stimulation promotes phospho-PKA catalytic (p-PKAc) dephosphorylation
(inactivation) between 30–120 min of incubation, simultaneous inhibition of PI3K and MEK1/2 kinases
abrogates DHP action. Conversely, elevated intra-oocyte cAMP, through priming with either adenylyl
cyclase (AC) activator forskolin (FK) or dibutyryl cAMP (db-cAMP), abrogates steroid-induced Akt
and ERK1/2 phosphorylation. Taken together, these results suggest that DHP-induced Akt and ERK
activation precedes the onset of meiosis (GVBD response) in a cAMP-sensitive manner and PI3K/Akt
and MEK/MAPK pathways together have a pivotal influence in the downregulation of PKA and
resumption of meiotic maturation in zebrafish oocytes in vitro.
