Hemoxygenase and Nitric Oxide Synthase Do Not Maintain Human Uterine Quiescence during Pregnancy

Abstract

The nitric oxide (NO)-cGMP pathway has been proposed as a mechanism for relaxation of myometrium during pregnancy and as a modulator of labor. Carbon monoxide (CO) , produced by hemeoxygenases (HO-1 and HO-2) , also activates soluble guanylate cyclase to increase cGMP. A recent study reported a large increase in HO-1 and HO-2 proteins during pregnancy , suggesting that the HO-CO pathway may be important in the maintenance of uterine quiescence during pregnancy. In this study we used Western blotting , reverse transcription-polymerase chain reaction , and immunohistochemistry to determine HO-1 and HO-2 expression in nonpregnant , pregnant, and laboring myometrium. Immunolocalization of HO was also compared with endothelial and inducible nitric oxide synthases (eNOS and iNOS). In contrast to HO-1 protein , which was not detected in myometrium , HO-2 protein and mRNA were constitutively expressed , although there were no differences in expression between the groups. eNOS was expressed in endothelial cells but not in myometrial smooth muscle. iNOS protein was not detected in myometrium. These data do not support an up-regulation of HO-1 and HO-2 during pregnancy and are not consistent with a role for NO or a major role for CO in human myometrial quiescence. Our results are also in keeping with HO-2 being an noninducible protein. Although spontaneous preterm birth (delivery before 37 weeks) accounts for 5 to 10% of all births, it is associated with up to 70% of neonatal deaths. It is also responsible for up to 75% of neonatal morbidity with considerable long-term morbidity in the child. 1-3 Drugs used to treat preterm labor, such as prostaglandin synthesis inhibitors, calcium channel antagonists and ␤-adrenoceptor agonists, have only short-lived effects or have potential side effects for mother and fetus. 4,5 Agents that have been used more recently, such as oxytocin receptor antagonists 6 and nitric oxide (NO) donors, 7 are currently the focus of intensive research, but early reports suggest that they will not confer major benefits. One reason there is no effective method of treating preterm labor is our limited knowledge of the mechanisms that control myometrial quiescence during pregnancy and its contractile state in normal and preterm labor. 8,9 NO, produced by the enzyme nitric oxide synthase (NOS), binds to the heme prosthetic moiety of the soluble guanylate cyclase, leading to increased cGMP production and smooth muscle relaxation. Carbon monoxide (CO) also binds to heme and activates soluble guanylate cyclase. 10 Discrepancies in localization of NOS and guanylate cyclase in the brain indicate that a substantial portion of guanylate cyclase may not serve as a target for NO. 11 CO is produced by heme oxygenase (HO), a microsomal enzyme that oxidatively cleaves heme, a prooxidant, to produce biliverdin and CO in the presence of NADPH-cytochrome P 450 reductase and NADPH. 12 HO consists of two homologous isoenzymes: HO-1, which is inducible, and HO-2, which is constitutive. 13,14 HO-1 is expressed at high concent...

Relaxation of Myometrium by Calcitonin Gene-Related Peptide Is Independent of Nitric Oxide Synthase Activity in Mouse Uterus1

“…This suggests that either iNOS is absent or is present in such low quantities, expressed only by certain cell types in the uterus, that it was below the detection threshold by Western analysis. This is in agreement with a previous report regarding lack of expression of iNOS protein in the human myometrium, in both the pregnant and in the nonpregnant state [38], where the authors conclude that uterine quiescence during pregnancy is not maintained by NOS activity. Similarly, the myometrium from pregnant rats is not sensitive to relaxation by cGMP, probably due to reduced expression of cGMP-dependent protein kinase [39].…”

Heme oxygenase–carbon monoxide (HO–CO) system in rat uterus: Effect of sexual steroids and prostaglandins

“…Acevedo and Ahmed [18] postulated the HO-CO system as an endogenous tocolytic agent in human myometrium during pregnancy, but Barber et al [39] did not observed an increment of HO expression in human pregnancy, suggesting that the CO pathway is not involved in uterine quiescence or contractility in labor. Our results suggest that CO via cGMP could be implicated in uterine quiescence since two specific HO inhibitors significantly decreased uterine cGMP accumulation.…”

Carbon Monoxide: Endogenous Production, Physiological Functions, and Pharmacological Applications

“…In addition, both the spontaneous and oxytocin-induced contractions of myometrial strips were inhibited by hemin injection (Acevedo and Ahmed, 1998;Longo et al, 1999). Contradictory reports showed that HO-1 and HO-2 were not up-regulated during pregnancy in myometrium at all (Barber et al, 1999). Therefore, role of the HO/CO system in maintaining uterine quiescence during pregnancy is still not settled.…”