Abstract:
Regulation of insulin-mediated resumption of meiotic maturation in catfish oocytes was investigated.
Insulin stimulation of post-vitellogenic oocytes promotes the synthesis of cyclin B, histone H1 kinase
activation and a germinal vesicle breakdown (GVBD) response in a dose-dependent and durationdependent
manner. The PI3K inhibitor wortmannin abrogates recombinant human (rh)-insulin action
on histone H1 kinase activation and meiotic G2–M1 transition in denuded and follicle-enclosed oocytes
in vitro. While the translational inhibitor cycloheximide attenuates rh-insulin action, priming with
transcriptional blocker actinomycin D prevents insulin-stimulated maturational response appreciably,
albeit in low amounts. Compared with rh-insulin, human chorionic gonadotrophin (hCG) stimulation
of follicle-enclosed oocytes in vitro triggers a sharp increase in 17 ,20 -dihydroxy-4-pregnen-3-one
(17 ,20 -DHP) secreted in the incubation medium at 12 h. Interestingly, the insulin, but not the hCGinduced,
maturational response shows less susceptibility to steroidogenesis inhibitors, trilostane or DLaminoglutethimide.
In addition, priming with phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine
(IBMX) or cell-permeable dbcAMP or adenylyl cyclase activator forskolin reverses the action of insulin
on meiotic G2–M1 transition. Conversely, the adenylyl cyclase inhibitor, SQ 22536, or PKA inhibitor
H89 promotes the resumption of meiosis alone and further potentiates the GVBD response in the
presence of rh-insulin. Furthermore, insulin-mediated meiotic maturation involves the down-regulation
of endogenous protein kinase A (PKA) activity in a manner sensitive to PI3K activation, suggesting
potential involvement of a cross-talk between cAMP/PKA and insulin-mediated signalling cascade
in catfish oocytes in vitro. Taken together, these results suggest that rh-insulin regulation of the
maturational response in C. batrachus oocytes involves down-regulation of PKA, synthesis of cyclin
B, and histone H1 kinase activation and demonstrates reduced sensitivity to steroidogenesis and
transcriptional inhibition.
