Regulation of myometrial connexin 43 expression during late pregnancy

The onset of labour is a complex process involving the stimulation of uterine contractions and the loss of uterine quiescence. At the end of pregnancy there is a transition in the contractile characteristics of the myometrium from an unresponsive, poorly co-ordinated and inexcitable state to a highly co-ordinated, responsive and contractile state. This transition has been shown to involve an increase in the expression of a set of contraction associated proteins (CAPs) including connexin 43 (Cx43) and the oxytocin receptor. I characterised the expression of activator protein-1 (AP-1) transcription factors in pregnant rat myometrium and hypothesised that these proteins regulate the expression of Cx43 in the myometrium. An increase in the levels of c-fos, fra-1, fra-2, fosB and junB mRNA was detected with the onset labour, blocked by progesterone and only occurred in the gravid horn of pregnant rats suggesting that the expression of these genes is regulated by both endocrine and mechanical factors. An in vitro stretch system revealed that several AP-1 genes are positively regulated by mechanical forces and negatively regulated by progesterone in isolated smooth muscle cells. I have used an in vitro transfection system to characterise the contribution of each AP-1 dimer to the transcriptional activity of a Cx43 promoter construct. Fos/Jun dimers were stimulatory whereas Jun/Jun dimers had little effect on transcriptional activity. AP-1 proteins displayed variable activities depending on the particular Fos and Jun partner, with Fra-2/JunB dimers conferring the greatest activity. A two base pair mutation of the proximal AP-1 consensus sequence reduced the response of the Cx43 promoter region to transfection of Fos/Jun dimers indicating this site is required for AP-1 induction of Cx43 expression. These data suggest that the increased expression of members of the Fos family prior to the onset of labour contribute to the enhanced expression of Cx43. In addition the myometrial relaxant parathyroid hormone related protein (PTHrP) was identified as a novel inhibitor of CAP gene expression. Loss of this inhibition coincides with the onset of labour suggesting it is of critical importance in the transition of the myometrium from the pregnant to the labour state.