Up-regulation of Rnd1 during pregnancy serves as a negative-feedback control for Ca2+ sensitization of contractile elements in rat myometrium

Pathophysiological functions of Rnd proteins

Rnd proteins constitute a subfamily of Rho GTPases represented in mammals by Rnd1, Rnd2 and Rnd3. Despite their GTPase structure, their specific feature is the inability to hydrolyse GTP-bound nucleotide. This aspect makes them atypical among Rho GTPases. Rnds are regulated for their expression at the transcriptional or post-transcriptional levels and they are activated through post-translational modifications and interactions with other proteins. Rnd proteins are mainly involved in the regulation of the actin cytoskeleton and cell proliferation. Whereas Rnd3 is ubiquitously expressed, Rnd1 and 2 are tissue-specific. Increasing data has described their important role during development and diseases. Herein, we describe their involvement in physiological and pathological conditions with a focus on the neuronal and vascular systems, and summarize their implications in tumorigenesis.

The RND1 Small GTPase: Main Functions and Emerging Role in Oncogenesis

The Rho GTPase family can be classified into classic and atypical members. Classic members cycle between an inactive Guanosine DiPhosphate -bound state and an active Guanosine TriPhosphate-bound state. Atypical Rho GTPases, such as RND1, are predominantly in an active GTP-bound conformation. The role of classic members in oncogenesis has been the subject of numerous studies, while that of atypical members has been less explored. Besides the roles of RND1 in healthy tissues, recent data suggest that RND1 is involved in oncogenesis and response to cancer therapeutics. Here, we present the current knowledge on RND1 expression, subcellular localization, and functions in healthy tissues. Then, we review data showing that RND1 expression is dysregulated in tumors, the molecular mechanisms involved in this deregulation, and the role of RND1 in oncogenesis. For several aggressive tumors, RND1 presents the features of a tumor suppressor gene. In these tumors, low expression of RND1 is associated with a bad prognosis for the patients. Finally, we highlight that RND1 expression is induced by anticancer agents and modulates their response. Of note, RND1 mRNA levels in tumors could be used as a predictive marker of both patient prognosis and response to anticancer agents.

Rho GTPases, their post-translational modifications, disease-associated mutations and pharmacological inhibitors

The 20 members of the Rho GTPase family are key regulators of a wide-variety of biological activities. In response to activation, they signal via downstream effector proteins to induce dynamic alterations in the organization of the actomyosin cytoskeleton. In this review, post-translational modifications, mechanisms of dysregulation identified in human pathological conditions, and the ways that Rho GTPases might be targeted for chemotherapy will be discussed.

Molecular pathways regulating contractility in rat uterus through late gestation and parturition

OBJECTIVE

Endogenous uterine agonists can activate numerous signaling pathways to effect increased force. Our objective was to assess expression of key constituents of these pathways, in alliance with contractile function, through late gestation and during term and preterm labor.

STUDY DESIGN

Using myography, we measured the response to 3 agonists compared with depolarization alone (K(+), 124 mEq/L) and calculated agonist/depolarization ratio. We measured gene expression using quantitative reverse transcription-polymerase chain reaction.

RESULTS

Contractile responsiveness to depolarization alone, oxytocin, or endothelin-1 increased during pregnancy compared with nonpregnant animals. The agonist/depolarization ratio did not change during uterine activation or parturition. Inhibition of rhoA-associated kinase decreased responses to oxytocin in all tissues, but significantly more during uterine activation. Expression of rhoA and rhoA-associated kinase was increased significantly in active labor at term or preterm.

CONCLUSION

The rhoA/rhoA-associated kinase pathway is a key regulator of uterine activation during labor and may be a useful target for the prevention of spontaneous preterm birth.

Pregnancy reduces RhoA/Rho kinase and protein kinase C signaling pathways downstream of thromboxane receptor activation in the rat uterine artery

During pregnancy, reduced vascular responses to constrictors contribute to decreased uterine and total vascular resistance. Thromboxane A(2) (TxA(2)) is a potent vasoconstrictor that exerts its actions via diverse signaling pathways, and its biosynthesis increases in preeclampsia. In this study, we hypothesized that maternal vascular responses to TxA(2) will be attenuated via Rho kinase, PKC, p38 MAPK, and ERK1/2 signaling pathways. Isolated ring segments of uterine and small mesenteric arteries from late pregnant (19-21 days) and virgin rats were suspended in a myograph, and isometric force was measured. Pregnancy did not affect uterine and mesenteric artery responses to the TxA(2) analog U-46619 (10(-9)-10(-5) M), but transduction signals associated with these contractions were different between pregnant and nonpregnant rats. Inhibition of Rho kinase (10(-6) M Y-27632) reduced sensitivity to U-46619 in virgin uterine vessels but did not inhibit these contractions in pregnant uterine arteries and had no effect on mesenteric vessels. Treatment of arterial segments with a PKC inhibitor (10(-6) M bisindolylmaleimide I) reduced U-46619-induced contractions in virgin uterine and mesenteric arteries and in pregnant mesenteric arteries. Pregnant uterine arteries, however, were unresponsive to PKC inhibition. Inhibition of ERK1/2 (10(-5) M PD-98059) and p38 MAPK (10(-5) M SB-203580) reduced U46619-induced contractions in nonpregnant vessels and in pregnant uterine and mesenteric vessels. These data suggest that normal pregnancy does not affect uterine and mesenteric contractile responses to TxA(2) but reduces the contribution of Rho kinase and PKC signaling pathways to these contractions in the uterine vasculature. In contrast, the role of ERK1/2 and p38 MAPK in U-46619-induced uterine contractions remains unchanged with pregnancy. TxA(2)-associated transduction signals and its regulators might present potential targets for the development of new treatments for preeclampsia and other pregnancy-associated vascular diseases.

RHO protein regulation of contraction in the human uterus

The state of contraction in smooth muscle cells of the human uterus is dependent on the interaction of activated forms of actin and myosin. Ras homology (RHO) proteins are small monomeric GTP-binding proteins that regulate actin polymerisation and myosin phosphorylation in smooth muscle cells. Their action is determined by their level of expression, GTP-bound state, intracellular localisation and phosphorylated status. Agonist activated RHO proteins bind to effector kinases such as RHO kinase (ROCK) and diaphanous proteins (DIAPH) to regulate smooth muscle contraction by two mechanisms: ROCK activates smooth muscle myosin either by direct phosphorylation at Ser19/Thr18 or through inhibition of myosin phosphatase which is a trimeric protein regulated by ROCK and by other protein kinases. Actin-polymerising proteins such as DIAPH homolog 1 increase filamentous actin assembly to enhance acto-myosin cross bridge formation and contraction. This review explores recent advances in RHO protein signalling in human myometrium and proposes areas of further research to investigate the involvement of these proteins in the regulation of uterine contractility in pregnancy and labour.

Endothelin-1 Induces Contraction via a Syk-Mediated p38 Mitogen-Activated Protein Kinase Pathway in Rat Aortic Smooth Muscle

Although spleen tyrosine kinase (Syk) has crucial roles in various cells, its function on vascular smooth muscle contraction has not been determined. In the present study, we performed experiments to determine if Syk contributes to the endothelin-1 (ET-1)-mediated contraction in rat aortic smooth muscle. ET-1-induced contraction of aortic strips was inhibited by piceatannol, PD98059, and SB203580, inhibitors of Syk, extracellular signal-regulated kinase 1/2 (ERK1/2), and p38 mitogen-activated protein kinase (MAPK), respectively. Piceatannol also attenuated high K(+)-induced contraction. ET-1 dose-dependently enhanced the activity of Syk and this was inhibited by piceatannol in both rat aortic strip and rat aortic smooth muscle cells. The phosphorylation of p38 MAPK and heat shock protein 27 (HSP27), but not that of ERK1/2, in response to ET-1 was inhibited by both piceatannol and SB203580. These results suggest that Syk may play an important role in the regulation of aortic smooth muscle contraction induced by ET-1, which may be mediated by the p38 MAPK/HSP27 signaling pathway.

Phorbol ester-induced contraction through p38 mitogen-activated protein kinase is diminished in aortas from DOCA-salt hypertensive rats

The role of mitogen-activated protein kinase (MAPK) in the decreased contractile response to phorbol ester in aortic smooth muscle strips from deoxycorticosterone acetate (DOCA)-salt hypertensive rats was examined. Norepinephrine (NE) evoked greater contractility in aortic strips from DOCA rats than in those of sham-operated rats. 12-Deoxyphorbol 13-isobutyrate (DPB) induced contraction in Ca2+-free medium, which was diminished in strips from DOCA rats compared to sham-operated rats. Vasoconstrictions induced by these stimulants were inhibited by SB203580 and PD098059, inhibitors of p38 MAPK and extracellular signal-regulated kinase (ERK) 1/2, respectively, in both strips. The phosphorylation of p38 MAPK and ERK1/2 induced by NE was greater in strips from DOCA rats compared to those from sham-operated rats, and this phosphorylation was inhibited by the kinase inhibitors. DPB increased the phosphorylation of p38 MAPK and ERK1/2 in strips from both animals, and the increment of p38 MAPK phosphorylation by the stimulant was diminished in strips from DOCA rats compared to sham-operated rats. These findings suggest that the Ca2+-independent contraction evoked by DPB results from the activation of MAPKs in rat aortic smooth muscle and that the attenuated contractility by DPB in DOCA rat appears to be associated with diminished p38 MAPK activity.

Expression of RND proteins in human myometrium

RHO GTPases are key regulators of the actin cytoskeleton and stress fiber formation. In the human uterus, activated RHOA forms a complex with RHO-associated protein kinase (ROCK) which inhibits myosin light chain phosphatase (PPP1R12A), causing a calcium-independent increase in myosin light chain phosphorylation and tension (Ca2+ sensitization). Recently discovered small GTP binding RND proteins can inhibit RHOA and ROCK interaction to reduce calcium sensitization. Very little is known about the expression of RND proteins in the human uterus. We tested the hypothesis that the uterine quiescence observed during gestation is mediated by an increase in RND protein expression inhibiting RHOA-ROCK-mediated PPP1R12A phosphorylation. Immunohistochemistry and immunoblotting were used to determine RHOA and RND protein expression and localization in nonpregnant, pregnant nonlaboring, and laboring patients at term and patients in spontaneous preterm labor. Changes in protein expression estimated by densitometry between different patient groups were measured. A significant increase of RND2 and RND3 protein expression was observed in pregnant relative to nonpregnant myometrium associated with a loss of PPP1R12A phosphorylation. RND transfected myometrial cells demonstrated a dramatic loss of stress fiber formation and a "rounding" phenotype. RND upregulation in pregnancy may inhibit RHOA-ROCK-mediated increase in calcium sensitization to facilitate the uterine quiescence observed during gestation.

Function and regulation of Rnd proteins

The Rnd proteins, which form a distinct sub-group of the Rho family of small GTP-binding proteins, have been shown to regulate the organization of the actin cytoskeleton in several tissues. In the brain, they participate in neurite extension, whereas in smooth muscle, they modulate contractility. Recent evidence has shown that Rnd3 (RhoE) is also involved in the regulation of cell-cycle progression and transformation, indicating that these proteins might have other, as yet unexplored roles.

Function and regulation of RhoE

The three Rnd proteins, Rnd1, Rnd2 and RhoE/Rnd3, are a subset of Rho family proteins that are unusual in that they bind but do not hydrolyse GTP, and are therefore not regulated by the classical GTP/GDP conformational switch of small GTPases. Increased expression of each Rnd protein induces loss of stress fibres in cultured fibroblasts and epithelial cells, acting antagonistically to RhoA, which stimulates stress fibre formation. RhoE is farnesylated and localizes partly on membranes, including the Golgi and plasma membrane, and in the cytosol. RhoE inhibits RhoA signalling in part by binding to the RhoA-activated serine/threonine kinase ROCK I (Rho-associated kinase I), thereby preventing it from phosphorylating its targets. RhoE activity is itself regulated by phosphorylation by ROCK I on multiple sites. RhoE phosphorylation enhances its stability, leading to an increase in RhoE levels. In addition, phosphorylation reduces its association with membranes and correlates with its ability to induce loss of stress fibres. RhoE also acts independently of ROCK to inhibit cell cycle progression, in part by preventing translation of cyclin D1, and to inhibit transformation of fibroblasts by oncogenic H-Ras. RhoE is therefore a multifunctional protein whose localization and actions are regulated by phosphorylation.

Differences in the gestational pattern of mRNA expression of the Rnd family in rat and human myometria

Uterine myometrial contractility remains a poorly characterized area of research in reproductive physiology. Rnd1, a novel member of the GTP-binding Rho protein family, inhibits Ca(2+)-sensitization by specifically interfering with a RhoA/Rho-activated kinases-dependent mechanism in smooth muscle. In addition to Rnd1, there are two other members, Rnd2 and Rnd3, in the Rnd family of Rho proteins. In the present comparative study of myometrial contractility in rats and humans, we found that all three Rnd mRNAs were expressed in nonpregnant rat myometrium and in nonpregnant human myometrial tissues. Although all three mRNA levels increased significantly after gestation in rat myometria, only Rnd1 expression was significantly greater after gestation in human samples. In the ovariectomized rat, administration of estrogen and/or progesterone increased the expression of all Rnd mRNAs. These results suggest that universal Rnd family up-regulation during pregnancy in rats may have an important role for negative-feedback control of uterine contraction during gestation by inhibiting RhoA-mediated increase in Ca(2+) sensitivity of contractile elements. Such increases in Rnd levels may be due to augmented levels of reproductive steroids in rats. Our data also point to gestational differences between rats and humans in Rnd isoform patterns.

Rho A/Rho kinase mRNA and protein levels in human myometrium during pregnancy and labor

OBJECTIVE

To quantify mRNA levels of Rho A, and rho-associated coil-forming kinases I and II (ROCK I and ROCK II) in human pregnant (before and after labor onset) and nonpregnant myometrium, and to investigate protein expression of Rho A, ROCK I, and ROCK II in these three tissue types.

METHODS

Real-time fluorescence reverse transcriptase-polymerase chain reaction (RT-PCR) using primers for Rho A, ROCK I, and ROCK II was performed on total RNA isolated from the three tissue types under investigation. Western blot analysis using antibodies specific to Rho A, ROCK I, and ROCK II was performed on protein isolated from the three tissue types.

RESULTS

Real-time fluorescence RT-PCR, using primers for Rho A, ROCK I, and ROCK II, revealed Rho A mRNA expression was significantly greater in human pregnant myometrium after labor onset, in comparison to pregnant myometrium before labor onset (P < .05), or nonpregnant myometrium (P < .01). ROCK I and ROCK II mRNA expression levels were similar in the three tissue types (P > .05). With Western blot analysis, using antibodies specific to Rho A, ROCK I, and ROCK II, Rho A protein levels were significantly lower in pregnant (before and after labor onset) in comparison to nonpregnant myometrium (P < .01). ROCK I protein levels were similar in the three tissue types (P > .05). No signal for ROCK II was detected in myometrial tissue.

CONCLUSIONS

These results outline the presence of the Rho A/Rho kinase system in modulating contractility of human myometrium. Total Rho A protein expression is down-regulated in the third trimester of pregnancy while up-regulation of Rho A mRNA occurs with labor onset.

Ligusticum wallichi-induced vasorelaxation mediated by mitogen-activated protein kinase in rat aortic smooth muscle

Traditional herbal medicines have been widely used for the treatment of cardiovascular disorders in oriental countries. To determine the effects of Ch1LW, a chloroform extract of Ligusticum wallichi, on the vascular system, we studied changes in rat aortic smooth muscle in terms of magnitude of contraction and the activity of mitogen-activated protein kinases (MAPKs). Ch1LW inhibited the muscle contraction induced by norepinephrine (NE) in aortic strips. Ch1LW also abolished Ca2+-independent contraction evoked by 12-deoxyphorbol 13-isobutyrate in Ca2+-free medium containing 1 mM EGTA. Furthermore, western blotting analysis using phosphorylated MAPK antibodies showed that NE increased the activity of both extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 MAPK, which were inhibited by PD98059 and SB203580, blockers of ERK1/2 and p38 MAPK, respectively. Furthermore, treatment with Ch1LW significantly abolished NE-mediated activation of ERK1/2, whereas the activity of p38 MAPK was not affected by the extract. These results suggest that Ch1LW induces vasorelaxation in rat aortic smooth muscle, which may be mediated by the inhibition of ERK1/2 pathway, but not p38 MAPK.