S-Nitrosoglutathione Reductase Underlies the Dysfunctional Relaxation to Nitric Oxide in Preterm Labor

Abstract

Tocolytics show limited efficacy to prevent preterm delivery. In uterine smooth muscle cGMP accumulation following addition of nitric oxide (NO) has little effect on relaxation suggesting a role for protein S-nitrosation. In human myometrial tissues from women in labor at term (TL), or spontaneously in labor preterm (sPTL), direct stimulation of soluble guanylyl cyclase (sGC) fails to relax myometrium, while the same treatment relaxes vascular smooth muscle completely. Unlike term myometrium, effects of NO are not only blunted in sPTL, but global protein S-nitrosation is also diminished, suggesting a dysfunctional response to NO-mediated protein S-nitrosation. Examination of the enzymatic regulator of endogenous S-nitrosoglutathione availability, S-nitrosoglutathione reductase, reveals increased expression of the reductase in preterm myometrium associated with decreased total protein S-nitrosation. Blockade of S-nitrosoglutathione reductase relaxes sPTL tissue. Addition of NO donor to the actin motility assay attenuates force. Failure of sGC activation to mediate relaxation in sPTL tissues, together with the ability of NO to relax TL, but not sPTL myometrium, suggests a unique pathway for NO-mediated relaxation in myometrium. Our results suggest that examining the action of S-nitrosation on critical contraction associated proteins central to the regulation of uterine smooth muscle contraction can reveal new tocolytic targets.

Hiding in Plain Sight: Nebivolol Exhibits Compelling Tocolytic Properties

“…While nebivolol does not inhibit •NO metabolism as others have suggested (Figure B,C), the same functional end state is achieved through nebivolol‐mediated •NO generation. We found that nebivolol was able to decrease peak force and AUC in myometrial samples (Figure A) more effectively than N6022, a known GSNOR inhibitor, perhaps in part due to the limited bioavailability of N6022. The use of nebivolol as a tocolytic is further supported by the finding that β3AR is the predominant subtype in human myometrium, is upregulated during pregnancy, and direct stimulation of β3AR has been shown to promote myometrial relaxation …”

“…This resulting surge of nitric oxide increases total protein S‐nitrosations ( SNO s), promoting SNO ‐mediated myometrial smooth muscle relaxation . Inhibition of S‐nitrosoglutathione reductase ( GSNOR ) also increases nitric oxide availability, which relaxes myometrial tissue; however, nebivolol does not inhibit the SNO ‐metabolizing enzymes GSNOR or thioredoxin reductase (TrxR)…”

“…A conspicuous advantage to using nebivolol as a tocolytic agent is that it is already FDA‐approved. While other “borrowed” tocolytics have failed to delay PTB, nebivolol is unique in that it promotes relaxation by increasing the •NO pool, which leverages the problem of GSNOR upregulation in sPTL myometrium . That being said, β ‐blockers use during pregnancy would need to be weighed again potential adverse effects on the foetus, such as small‐for‐gestational‐age infants .…”

“…The myometrium is a unique subclass of smooth muscle that exhibits a signalling exception to the dogma that global cGMP accumulation drives nitric oxide (•NO)‐mediated relaxation. It is well‐established that term human myometrium relaxes to •NO in an cGMP‐independent fashion . •NO is a critical mediator of uterine quiescence; however, •NO's most pervasive role in the myometrium is through protein S‐nitrosation (SNO), rather than cGMP accumulation.…”

“…S‐nitrosoglutathione reductase (GSNOR) is an important modulator of •NO and S‐nitrosation . GSNOR is upregulated in human sPTL myometrium, and inhibition of GSNOR by N6022 can partially restore native myometrial function …”