CREB activity is required for epidermal growth factor-induced mouse cumulus expansion.

The release of a fertilizable oocyte from the ovary is dependent upon the expansion of the cumulus cells. The expansion requires cooperation between epidermal growth factor (EGF) family peptide-activated mitogen-activated protein kinase (MAPK)3/1 and oocyte paracrine factor-activated-Sma- and Mad-related protein (SMAD)2/3 signaling in cumulus cells. However, the mechanism underlying (MAPK)3/1 signaling is unclear. In the present study, the EGF-activation of EGF receptor (EGFR) induced cyclic adenosine 3',5'-monophosphate (cAMP) response element-binding protein (CREB) phosphorylation in cumulus cells, and the interruption of CREB functional complex formation by naphthol AS-E phosphate (KG-501) completely blocked the EGF-stimulated expansion-related gene expression. EGF-stimulated phosphorylation of CREB was completely inhibited by MAPK3/1 inhibitor U0126, suggesting that EGF-activated MAPK3/1 results in the activation of CREB for cumulus expansion. Also, the role of EGF-stimulated calcium signaling was studied. Calcium-elevating reagents ionomycin and sphingosine-1-phosphate mimicked, but calcium chelators bis-(o'aminophenoxy)-ethane-N,N,N,N-tetraacetic acid, tetra(acetoxymethyl)-ester, and 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate abolished the activity of EGF on CREB phosphorylation, cumulus expansion, and expansion-related gene expression. Furthermore, EGF-induced cumulus expansion was inhibited by calmodulin (CaM)-dependent protein kinase II (CaMKII) inhibitors, KN-93 and autocamtide-2-related inhibitory peptide. However, the inhibition of SMAD2/3 activity by removal of oocyte from cumulus-oocyte complexes did not affect the EGF-induced CREB phosphorylation, indicating that EGF-activated CREB is independent of oocyte-activated SMAD2/3 signaling. Therefore, EGF-induced CREB activity by MAPK3/1 and Ca²⁺ /CaMKII signaling pathways promotes the expansion-related gene expression and consequent cumulus expansion.

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