Phosphoinositide hydrolysis is important in mediating the actions of oxytocin and prostaglandin (PG) F2 alpha on uterine contractions during labour. We have measured the effect of oxytocin, PGF2 alpha and other agents on the formation of inositol phosphates (IPs) in cultured human myometrial cells labelled with [3H]inositol and on changes in intracellular free Ca2+ concentration ([Ca2+]i) in cells loaded with Fura-2. Oxytocin induced the formation of [3H]IPs in a concentration-dependent manner with an EC50 (concentration of agonist producing 50% of the maximal response) of 1.4 +/- 0.5 nmol/l (mean +/- S.E.M.). The maximal response was obtained with 1 mumol oxytocin/l and represented a stimulation of 670% over basal. PGF2 alpha also stimulated the formation of [3H]IPs and the response at 1 mumol/l was a 204% stimulation over basal. The effects of PGF2 alpha were independent of extracellular Ca2+ but the effect of oxytocin was reduced with low extracellular Ca2+. Cyclic AMP formation, induced by forskolin or PGE2, had no effect on the stimulated levels of [3H]IPs. Pertussis toxin (PT) reduced the oxytocin-stimulated formation of [3H]IPs in a concentration-dependent manner. The maximal effect of PT resulted in an 80% reduction in the formation of [3H]IPs. However, PGF2 alpha stimulation was not affected by PT treatment. To analyse the action of PT further, we studied its effect on oxytocin-induced changes in [Ca2+]i. The basal [Ca2+]i was 112 +/- 4 nmol/l (n = 225 cells) and was not affected by PT treatment (109 +/- 3 nmol/l; n = 200 cells). In the absence of PT, 1 mumol oxytocin/l increased [Ca2+]i to a peak of 522 +/- 26 nmol/l, and in PT-treated cells, the [Ca2+]i peak was reduced to 348 +/- 16 nmol/l. Similar inhibitory effects of PT were obtained at oxytocin concentrations ranging from 1 to 100 nmol/l. Our data suggest that in human myometrial cells, the oxytocin-induced production of [3H]IPs and increase in [Ca2+]i are mediated by a PT-sensitive G-protein. However, a significant fraction of the oxytocin response appears to be mediated by a PT-insensitive G-protein, possibly a member of the Gq family.

Phosphorylation of heat shock protein 20 (Hsp20) by protein kinase A (PKA) is now recognized as an important regulatory mechanism modulating contractile activity in the human myometrium. Thus agonists that stimulate cyclic AMP production may cause relaxation with resultant beneficial effects on pathologies that affect this tissue such as the onset of premature contractions prior to term. Here we describe for the first time that acetylation of Hsp20 is also a potent post-translational modification that can affect human myometrial activity. We show that histone deacetylase 8 (HDAC8) is a non-nuclear lysine deacetylase (KDAC) that can interact with Hsp20 to affect its acetylation. Importantly, use of a selective linkerless hydroxamic acid HDAC8 inhibitor increases Hsp20 acetylation with no elevation of nuclear-resident histone acetylation nor marked global gene expression changes. These effects are associated with significant inhibition of spontaneous and oxytocin-augmented contractions of ex vivo human myometrial tissue strips. A potential molecular mechanism by which Hsp20 acetylation can affect myometrial activity by liberating cofilin is described and further high-lights the use of specific effectors of KDACs as therapeutic agents in regulating contractility in this smooth muscle.In developed countries premature birth prior to 37 weeks gestation accounts for nearly 75% of newborn deaths and is related to a high risk for survivors of long-term physical or mental disability (1). There are still no effective and safe therapeutic treatments for decreasing the incidence of premature deliveries. Accordingly, an increased understanding of the molecular mechanisms underlying myometrial activity is required to aid in the development of new strategies for the treatment of premature labor. To this end, recent evidence indicates a novel role for post-translational modification by acetylation in regulating myometrial activity. This was first highlighted by the class I/II histone deacetylase inhibitors (HDACIs) 3 TSA, VPA, and SBHA effecting relaxations of ϳ60% to spontaneously or oxytocin-mediated contracting human myometrial tissues ex vivo (2). Potential epigenetic events resulting from increased histone acetylation seem to be excluded due to the comparatively short time period observed for contractile inhibition of 20 -60 min. This pointed to the above HDACIs having non-epigenetic effects involving increased acetylation of lysine residues of protein components of the myometrial contractile machinery. This possibility is supported by recent proteomic findings of Kim et al. (3) and Choudhary et al. (4). They, respectively, showed that administration of the HDACIs TSA/Sirtinol to HeLa or SAHA/MS-275 to leukemia MV4 -11 cell cultures resulted in acetylation of a range of non-nuclear proteins. Importantly, proteins involved in regulating the cytoskeletal/filamentous architecture of cells were observed to be acetylated. These included actin, cofilin, 14-3-3 as well as the heat shock family protein member Hsp27.

We have recently provided evidence for the desensitization of oxytocin receptors in human myometrial cells. In the present study, we have investigated the possible mechanisms by which oxytocin (OT) might regulate OT receptor density. The steady state level of OT binding in cultured myometrial cells was 220 x 10(3) binding sites/ cell, but this was time-dependently reduced to 27 x 10(3) sites/cell by exposure to OT for up to 20 h. Similarly, OT exposure decreased the binding of OT to cell membranes. In contrast, Western blotting data showed that the total amount of OT receptor protein was not affected by OT treatment for up to 48 h. Flow cytometry experiments demonstrated that OT receptors are not internalized during prolonged exposure of the cells to OT. However, RNase protection assays and Northern analysis showed that OT receptor mRNA was reduced by OT treatment to reach a new low steady state level with a time course similar to that of the disappearance of cell surface OT binding sites. Possible mechanisms involved in mRNA down-regulation include transcriptional suppression and destabilization of mRNA by RNA binding proteins.

We have previously reported that G alpha s is expressed at considerably higher levels in myometrium taken from pregnant than from nonpregnant women. In the present study we have determined adenylyl cyclase activity in myometrial membranes by measuring the conversion of [alpha-32P]ATP to [32P]cAMP and have measured guanosine triphosphate-binding protein expression by immunoblotting with specific antibodies. Here we report that the increase in G alpha s expression in pregnant myometrium is associated with a significant increase in G alpha s-coupled adenylyl cyclase activity, as estimated by incubating myometrial membranes in the presence of 5'-guanylyl-imidodiphosphate with and without prostaglandin E2. Moreover, in myometrium from women in spontaneous labor G alpha s levels and G alpha s-coupled adenylyl cyclase activity are reduced to the levels observed in nonpregnant tissue. There was no apparent change in forskolin-stimulated adenylyl cyclase activity in nonpregnant, pregnant, and laboring tissue. The increase in G alpha s expression in pregnant myometrium may facilitate agonist-induced cAMP formation, resulting in prolonged relaxation of the uterus during gestation. Down-regulation of G alpha s would decrease the relaxing effect exerted by cAMP and may be a triggering mechanism for the initiation of labor.