A role for retinoids in human oocyte fertilization: regulation of connexin 43 by retinoic acid in cumulus granulosa cells

Retinoids are essential for ovarian steroid production and oocyte maturation in mammals. Oocyte competency is known to positively correlate with efficient gap junction intercellular communication (GJIC) among granulosa cells in the cumulus-oocyte complex. Connexin 43 (Cx43) is the main subunit of gap junction channels in human cumulus granulosa cells (CGC) and is regulated by all-trans retinoic acid (ATRA) in other hormone responsive cell types. The objectives of this study were to quantify retinoid levels in human CGC obtained during IVF oocyte retrievals, to investigate the potential relationship between CGC ATRA levels and successful oocyte fertilization, and to determine the effects of ATRA on Cx43 protein expression in CGC. Results showed that CGC cultures actively metabolize retinol to produce ATRA. Grouped according to fertilization rate tertiles, mean ATRA levels were 2-fold higher in pooled CGC from women in the highest versus the lowest tertile (P < 0.05). ATRA induced a rapid dephosphorylation of Cx43 in CGC and granulosa cell line (KGN) cultures resulting in a > 2-fold increase in the expression of the functional non-phosphorylated (P0) species (P < 0.02). Similar enhancement of P0 by ATRA was shown in CGC and KGN cultures co-treated with LH or hCG which, by themselves, enhanced the protein levels of Cx43 without altering its phosphorylation profile. Correspondingly, the combination of ATRA+hCG treatment of KGN caused a significant increase in GJIC compared with single agent treatments (P < 0.025) and a doubling of GJIC from that seen in untreated cells (P < 0.01). These findings indicate that CGC are a primary site of retinoid uptake and ATRA biosynthesis. Regulation of Cx43 by ATRA may serve an important role in folliculogenesis, development of oocyte competency, and successful fertilization by increasing GJIC in CGC.