Abstract
Cyclic AMP (cAMP) is the archetypal smooth muscle relaxant, mediating the effects of many hormones and drugs. However, recently PGI2, acting via cAMP/PKA, was found to increase contraction-associated protein expression in myometrial cells and to promote oxytocin-driven myometrial contractility. Cyclo-oxygenase-2 (COX-2) is the rate-limiting enzyme in prostaglandin synthesis, which is critical to the onset and progression of human labour. We have investigated the impact of cAMP on myometrial COX-2 expression, synthesis and activity. Three cAMP agonists (8-bromo-cAMP, forskolin and rolipram) increased COX-2 mRNA expression and further studies confirmed that this was associated with COX-2 protein synthesis and activity (increased PGE2 and PGI2 in culture supernatant) in primary cultures of human myometrial cells. These effects were neither reproduced by specific agonists nor inhibited by specific inhibitors of known cAMP-effectors (PKA, EPAC and AMPK). We then used shRNA to knockdown the same effectors and another recently described cAMP-effector PDZ-GEF1-2, without changing the response to cAMP. We found that MAPK activation mediated the cAMP effects on COX-2 expression and that PGE2 acts through EP-2 to activate MAPK and increase COX-2. These data provide further evidence in support of a dual role for cAMP in the regulation of myometrial function.
Highlights
Preterm delivery is the prime cause of perinatal mortality and morbidity [1, 2]
We initially investigated whether Cyclic AMP (cAMP) increased COX-2 mRNA expression by exposing human myometrial cells to forskolin (100 M) and found increased COX-2 expression at 1 and 6 hrs (Fig. 1A)
We confirmed these findings with two other cAMP agonists 8bromo-cAMP (250 M) and rolipram (10 M), a specific type 4 phosphodiesterase inhibitor, and found that COX-2 mRNA expression was significantly increased at 6 hrs of 8bromocAMP treatment (Fig. 1B) and at 1 and 6 hrs of rolipram treatment (Fig. 1C)
Summary
Preterm delivery is the prime cause of perinatal mortality and morbidity [1, 2]. despite intense laboratory and clinical investigations, the frequency of preterm birth has increased by over 30% in the last 20 years [3]. The early onset of labour is the most frequent cause of preterm birth and is most commonly related to intrauterine inflammation, uterine overdistension and placental abruption [4]. These factors shift the balance from myometrial quiescence to contractility and promote the onset of labour. The second messenger, cAMP, influences a wide array of physiological and pathological events including smooth muscle contractility and inflammation. Both physiological (relaxin, CRH and CGRP) and pharmacological (2-agonists) agents act via cAMP to induce myometrial relaxation. Other mechanisms to increase myometrial cAMP levels have been explored and a recent publication showed that using the phosphodiesterase type 4 inhibitor, rolipram, successfully reduced rates of PTL in a mouse model [7]
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