Initiation of oocyte maturation by multiple protein kinase pathways
Romani, Michael E.
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Within the mature mammalian ovarian follicle, the oocyte is arrested in prophase of meiosis I until LH causes the resumption of meiosis. LH stimulates protein kinase A and C. Both kinases have been linked to oocyte maturation. Other protein kinases will also initiate maturation. Activation of tyrosine kinase by epidermal growth factor will also stimulate maturation. This study is designed to examine various metabolic pathways capable of initiating maturation in follicle-enclosed hamster oocytes. A 4 hr. incubation with LH, TPA and EGF significantly increased maturation. Forskolin stimulated maturation by 30 min. Follicle-enclosed oocytes were committed to undergo meiosis by 2 hr. when incubated with either LH, TPA or EGF; however, forskolin caused commitment after a 5 min. exposure. Incubations with LH and various protein kinase inhibitors showed that inhibition of both PKA and PKC reduces maturation rates to control levels, while inhibiting either PKA or PKC alone results in maturational levels which are approximately half as high as with LH alone. When tyrosine kinase is inhibited, EGF-stimulated maturation is inhibited, no effect is seen on the initiation of maturation by LH. LH (1 ug/ml)increased total and bound cAMP concentrations in the granulosa cells after a 5 min. incubation period; however, LH did not change total or bound cAMP levels in the cumulus oocyte complex (COC). r 32 1 Incorporation of P orthophosphate into proteins was analyzed by 2D-PAGE. Experimental treatments stimulated phosphorylation of a 39 kilodalton group of proteins in the granulosa cells; however, no effect was seen on the COCs in the controls or LH-treated follicles. The results indicate that the signal which initiates the resumption of maturation in follicle-enclosed oocytes originates in the granulosa cells and is then transferred to the oocyte. This signal may involve both protein kinase A and protein kinase C. The results also suggest that various protein kinases may phosphorylate a common substrate, leading to the initiation of maturation.