Contribution of phospholipase D in endothelin-1-mediated extracellular signal-regulated kinase activation and proliferation in rat uterine leiomyoma cells

Uterine fibroids: pathogenesis and interactions with endometrium and endomyometrial junction

Uterine leiomyomas (fibroids or myomas) are benign tumors of uterus and clinically apparent in a large part of reproductive aged women. Clinically, they present with a variety of symptoms: excessive menstrual bleeding, dysmenorrhoea and intermenstrual bleeding, chronic pelvic pain, and pressure symptoms such as a sensation of bloatedness, increased urinary frequency, and bowel disturbance. In addition, they may compromise reproductive functions, possibly contributing to subfertility, early pregnancy loss, and later pregnancy complications. Despite the prevalence of this condition, myoma research is underfunded compared to other nonmalignant diseases. To date, several pathogenetic factors such as genetics, microRNA, steroids, growth factors, cytokines, chemokines, and extracellular matrix components have been implicated in the development and growth of leiomyoma. This paper summarizes the available literature regarding the ultimate relative knowledge on pathogenesis of uterine fibroids and their interactions with endometrium and subendometrial myometrium.

Inhibition of lipid phosphate phosphatase activity by VPC32183 suppresses the ability of diacylglycerol pyrophosphate to activate ERK(1/2) MAP kinases

The lipidic metabolite, diacylglycerol pyrophosphate (DGPP), in its dioctanoyl form (DGPP 8:0), has been described as an antagonist for mammalian lysophosphatidic acid (LPA) receptors LPA1 and LPA3. In this study we show that DGPP 8:0 does not antagonize LPA dependent activation of ERK(1/2) MAP kinases but strongly stimulated them in various mammalian cell lines. LPA and DGPP 8:0 stimulation of ERK(1/2) occurred through different pathways. The DGPP 8:0 effect appeared to be dependent on PKC, Raf and MEK but was insensitive to pertussis toxin and did not involve G protein activation. Finally we showed that DGPP 8:0 effect on ERK(1/2) was dependent on its dephosphorylation by a phosphatase activity sharing lipid phosphate phosphatase properties. The inhibition of this phosphatase activity by VPC32183, a previously characterized LPA receptor antagonist, blocked the DGPP 8:0 effect on ERK(1/2) activation. Moreover, down-regulation of lipid phosphate phosphatase 1 (LPP1) expression by RNA interference technique also reduced DGPP 8:0-induced ERK(1/2) activation. Consistently, over expression of LPP1 in HEK293 cells increases DGPP 8:0 hydrolysis and this increased activity was inhibited by VPC32183. In conclusion, DGPP 8:0 does not exert its effect by acting on a G protein coupled receptor, but through its dephosphorylation by LPP1, generating dioctanoyl phosphatidic acid which in turn activates PKC. These results suggest that LPP1 could have a positive regulatory function on cellular signaling processes such as ERK(1/2) activation.

The endothelin axis in uterine leiomyomas: new insights

The endothelin axis, comprising endothelin-1 (ET-1) and its receptors (ETA and ETB), is involved in the pathophysiology of different human tumors. Here we review conventional approaches and gene expression profiling indicating the association of ET-1 and its cognate receptors with human and rat leiomyomas, the most common benign tumors of myometrium. Specifically, ET-1/ETA interactions affect human and rat leiomyoma cell proliferation through protein kinase C and mitogen-activated protein kinase-dependent signaling pathways. Recent experiments demonstrate that the ET-1 axis exerts a potent antiapoptotic effect involving sphingolipid metabolism and prostaglandin-endoperoxide synthase 2/prostaglandin system in the rat Eker leiomyoma tumor-derived ELT3 cell line. Evidence supports that steroid hormones, growth factors, and extracellular matrix are key regulators of the leiomyoma growth. Interestingly, the ET-1 axis is under steroid hormones and can cooperate with these growth factors. Therefore, ET-1 alone or in association with these factors could contribute to the complex regulation of uterine tumor growth, such as proliferation, survival, and extracellular matrix production. This review summarizes current knowledge and emerging data on ET-1 in uterine leiomyoma pathology.

ATP-binding cassette ABCC1 is involved in the release of sphingosine 1-phosphate from rat uterine leiomyoma ELT3 cells and late pregnant rat myometrium

Sphingosine 1-phosphate (S1P), a bioactive lipid generated by sphingosine kinases (SphK1/2), initiates different signalling pathways involved in physiological and pathological processes. We previously demonstrated that in rat myometrium at late (day 19) gestation, SphK1 increases the expression of COX2 via S1P generation and release. In rat uterine leiomyoma cells (ELT3), SphK1/S1P axis controls survival and proliferation. In the present study we demonstrate that PDBu activates SphK1 but not SphK2. SphK1 activation requires PKC and MAPK ERK1/2. S1P produced by PDBu is released in the medium. PDBu-induced S1P export is abolished by Ro-318220 and BIM (PKC inhibitors), by U0126 and PD98059 (MEK inhibitors), SKI-II (SphKI/2 inhibitor) and SphK1-siRNA, suggesting the involvement of PKC, ERK and SphK1 respectively. The release of S1P is insensitive to inhibitors of ATP Binding Cassette (ABC)A1 and ABCB1 transporters, but is abolished when ABCC1 transporters are inhibited by MK571 or down-regulated by ABCC1-siRNA. PDBu increases COX2 expression that is blocked by the inhibition of PKC, ERK1/2, SphK1, and when cells are treated with MK571 or transfected with ABCC1-siRNA. The induction of COX2 by the S1P release due to PDBu or by exogenous S1P involves S1P2 receptors coupled to Gi. In myometrium from rat at late gestation, the release of S1P is also strongly reduced when SphK and ABCC1 are inhibited. The data reveal that in rat leiomyoma cells and late pregnant rat myometrium, the release of S1P involves a similar signalling pathway and occurs through ABCC1.

Endothelin-1: physiological and pathological roles in myometrium

Endothelin-1 (ET-1), a member of endothelin peptide family is released by many different tissues including uterine smooth muscle. ET-1 acts through ETA and ETB receptors and is implicated in a wide range of biological and pathological functions that explain the great attention of the pharmacological industry for ET-1 receptors as potential therapeutic targets in vascular pathologies and cancers. It is now well established that ET-1 is also able to regulate myometrial functions. In the present review, we focused on ET axis and related signaling pathways involved in the regulation of myometrial contraction, as well as cell proliferation and survival. Such ET-1-mediated cellular functions play a critical role in normal pregnancy, preterm birth and uterine leiomyoma.

Differential endothelin receptor expression and function in rat myometrial cells and leiomyoma ELT3 cells

Uterine leiomyoma are the most common benign tumors of the myometrium. We previously identified endothelin (ET)-1 as a proliferative and antiapoptotic factor in Eker rat-derived leiomyoma (ELT3) cells. A major role of ETB receptor in the prosurvival effect was revealed. Here we investigated, in ELT3 and myometrial cells, the respective contribution of ETA and ETB in the proliferative effect of ET-1. In myometrial cells, binding experiments show that ETA is almost exclusively expressed and stimulates phospholipase C (PLC) activity and ERK1/2 phosphorylation and proliferation. In ELT3 cells, ETB is expressed at about the same level as ETA, and the two receptors are differently coupled to Gi protein. The ETB agonist, sarafotoxin S6c, stimulates PLC activity 60% less than ET-1 but is as potent as ET-1 to increase ERK1/2 phosphorylation and induce proliferation. However, the ability of ETA to activate ERK1/2 is observed after ETB desensitization. Although ETA and ETB antagonists partially reduce ET-1 stimulated PLC activity, they are without effect on ET-1-induced ERK1/2 phosphorylation and proliferation. Only the simultaneous use of ETA and ETB antagonists reduces ET-1-triggered ERK1/2 activation. These unconventional properties of ETRs may reveal the existence of functional ETA-ETB heterodimers. Finally, treatment of ELT3 cells with ETB but not ETA-directed small interfering RNA reduces the proliferative effect of ET-1. All the data obtained in ELT3 cells strengthen the relation between ETB overexpression, which decreases the ETA to ETB ratio, and the ability of leiomyoma cells to highly proliferate and resist apoptosis.

Role of lysophosphatidic acid in the regulation of uterine leiomyoma cell proliferation by phospholipase D and autotaxin

Phospholipase D (PLD) hydrolyzes phosphatidylcholine into phosphatidic acid (PA), a lipidic mediator that may act directly on cellular proteins or may be metabolized into lysophosphatidic acid (LPA). We previously showed that PLD contributed to the mitogenic effect of endothelin-1 (ET-1) in a leiomyoma cell line (ELT3 cells). In this work, we tested the ability of exogenous PA and PLD from Streptomyces chromofuscus (scPLD) to reproduce the effect of endogenous PLD in ELT3 cells and the possibility that these agents acted through LPA formation. We found that PA, scPLD, and LPA stimulated thymidine incorporation. LPA and scPLD induced extracellular signal-regulated kinase (ERK(1/2)) mitogen-activated protein kinase activation. Using Ki16425, an LPA(1)/LPA(3) receptor antagonist and small interfering RNA targeting LPA(1) receptor, we demonstrated that scPLD acted through LPA production and LPA(1) receptor activation. We found that scPLD induced LPA production by hydrolyzing lysophosphatidylcholine through its lysophospholipase D (lysoPLD) activity. Autotaxin (ATX), a naturally occurring lysoPLD, reproduced the effects of scPLD. By contrast, endogenous PLD stimulated by ET-1 failed to produce LPA. These results demonstrate that scPLD stimulated ELT3 cell proliferation by an LPA-dependent mechanism, different from that triggered by endogenous PLD. These data suggest that in vivo, an extracellular lysoPLD such as ATX may participate in leiomyoma growth through local LPA formation.

alpha(1)-Adrenergic receptor stimulation of cell motility requires phospholipase D-mediated extracellular signal-regulated kinase activation

Phospholipase D is suspected to play a role in tumorigenesis, and the inhibition of phospholipase D has been associated with changes in several cellular events including invasion and migration. We report here that the specific alpha(1)-adrenergic receptor agonist, phenylepherine, signals to a growth factor pathway in a manner that requires phospholipase D activity in CCL39 fibroblasts. Phenylepherine increased extracellular signal-regulated kinase phosphorylation eightfold and promoted stress fiber formation threefold. Stress fiber formation was blocked when extracellular signal-regulated kinase activation was inhibited. Stimulation of CCL39 fibroblasts by phenylepherine increased the rate of wound healing fourfold in a wounding assay, while treatment with the MEK inhibitor, PD98059 reduced the closure of phenylepherine-induced wound healing to control levels. Addition of 1-butanol but not 2-butanol inhibited extracellular signal-regulated kinase activation by phenylepherine, presumably by blocking the formation of phosphatidic acid. Exogenously added cell permeable phosphatidic acid increased extracellular signal-regulated kinase activation in a time- and dose-dependent manner as well as stimulated the formation of stress fibers. 1-butanol also significantly inhibited the ability of phenylepherine to stimulate stress fiber formation and wound healing. Taken together, these results indicate a novel role for phospholipase D in the activation of the extracellular signal-regulated kinase growth factor pathway to stimulate early cellular events induced by phenylepherine.

Endothelin-1 inhibits apoptosis through a sphingosine kinase 1-dependent mechanism in uterine leiomyoma ELT3 cells

Uterine leiomyomas, or fibroids, are the most common tumors of the myometrium. The ELT3 cell line, derived from Eker rat leiomyoma, has been successfully used as a model for the study of leiomyomas. We have demonstrated previously the potent mitogenic properties of the peptidic hormone endothelin (ET)-1 in this cell line. Here we investigated the antiapoptotic effect of ET-1 in ELT3 cells. We found that 1) serum starvation of ELT3 cells induced an apoptotic process characterized by cytochrome c release from mitochondria, caspase-3/7 activation, nuclei condensation and DNA fragmentation; 2) ET-1 prevented the apoptotic process; and 3) this effect of ET-1 was fully reproduced by ETB agonists. In contrast, no antiapoptotic effect of ET-1 was observed in normal myometrial cells. A pharmacological approach showed that the effect of ET-1 on caspase-3/7 activation in ELT3 cells was not dependent on phosphatidylinositol 3-kinase, ERK1/2, or phospholipase D activities. However, inhibitors of sphingosine kinase-1 (SphK1), dimethylsphingosine and threo-dihydrosphingosine, reduced the effect of ET-1 by about 50%. Identical results were obtained when SphK1 expression was down-regulated in ELT3 cells transfected with SphK1 small interfering RNA. Furthermore, serum starvation induced a decrease in SphK1 activity that was prevented by ET-1 without affecting the level of SphK1 protein expression. Finally, sphingosine 1-phosphate, the product of SphK activity, was as efficient as ET-1 in inhibiting serum starvation-induced caspase-3/7 activation. Together, these results demonstrate that ET-1 possesses a potent antiapoptotic effect in ELT3 cells that involves sphingolipid metabolism through the activation of SphK1.

Cellular biology of myomas: interaction of sex steroids with cytokines and growth factors

Many investigators who have been trying to delineate the pathophysiology of leiomyomata believe in the autocrine-paracrine model of tumor growth,where ovarian hormones act as regulators of gene expression in cells. These affected cells overproduce the stimulatory and fibrogenic cytokines and growth factors to which they respond, resulting in sustained, self-stimulated proliferation and fibrogenesis. A number of cytokines and growth factors have been investigated in leiomyomata to determine which cytokines or factors may be responsible for mediating the growth-promoting effects of ovarian hormones. A review of the literature reveals that TGF-3 is the only growth factor shown to be overexpressed in leiomyomata versus myometrium, hormonally regulated both in vivo and in vitro, and both mitogenic and fibrogenic in these tissues. The authors believe that, given the extent and depth of the current research on the cellular biology of leiomyoma, the cellular mechanisms responsible in the pathogenesis of leiomyoma will be identified clearly within the foreseeable future. This will enable researchers to develop therapy directed against the molecules and mechanisms at the cellular level, which undoubtedly will have a major impact on the number of hysterectomies being performed for a"fibroid uterus."

Activation of phospholipase D by 8-Br-cAMP occurs through novel pathway involving Src, Ras, and ERK in human endometrial stromal cells

We investigated the mechanism of 8-Br-cAMP-mediated phospholipase D (PLD) activation using a primary cell culture system of human endometrial stromal cells (ES cells). PLD activity was increased by the treatment of ES cells with 8-Br-cAMP, maximally at 5 min. To determine whether the effects of 8-Br-cAMP on PLD occurred as a consequence of PKC activation, ES cells were preincubated for 15 min with RO320432 (1 microM) and GF109203X (1 microM), the PKC inhibitors, or they were pretreated for 24h with phorbol myristate acetate (100 nM) to downregulate PKC. However, these treatments had no effects on PLD activation induced by 8-Br-cAMP. Furthermore, 8-Br-cAMP had no effects on the subcellular distribution of PKC alpha and PKC betaI, confirming no involvement of PKC. 8-Br-cAMP activated ERK1/2, maximally at 5 min, and PD98059 (MEK inhibitor: 50 microM) and transfection of ES cells with dominant negative (DN)-MEK completely inhibited 8-Br-cAMP-induced PLD activation, suggesting that ERK1/2 mediates the PLD activation. To investigate the involvement of protein kinase A (PKA), Src, and Ras in 8-Br-cAMP-induced PLD activation, we used PKA inhibitor, H89 and Rp-cAMPs, and transfections of DN-Src and DN-Ras. H-89 and Rp-cAMPs completely blocked 8-Br-cAMP-mediated PLD and ERK activation, implying the involvement of PKA in this PLD activation. In addition, transfection of ES cells with DN-Src, or DN-Ras partially inhibited 8-Br-cAMP-induced ERK1/2 and consequently PLD activation, whereas cotransfection of DN-Src and DN-Ras completely inhibited ERK1/2 and PLD activation, suggesting that Src and Ras independently regulate ERK/PLD activation. Taken together, these results demonstrate a novel pathway in ES cells that 8-Br-cAMP activate PLD through PKA and ERK1/2 and this ERK/PLD activation by 8-Br-cAMP is mediated by Src and Ras, separately.

Endothelin-1 inhibits apoptosis in prostate cancer

Endothelin-1 (ET-1), produced by the prostate epithelia, likely plays an important role in the progression of prostate cancer. ET-1 can bind two receptor subtypes; generally, binding of the endothelin receptor A (ET(A)) induces a survival pathway, whereas binding of the endothelin receptor B (ET(B)) mediates clearance of circulating ET-1 as well as promotes apoptosis. In prostate carcinoma, hypermethylation of the ET(B) promoter results in repression of ET(B) expression, thereby eliminating the negative growth response that ET-1 binding elicits through this receptor. Therefore, activation of ET(A) exclusively provides a pathway for survival advantage. Our current studies examine the mechanisms by which activation of the ET(A) may allow growth/survival. ET-1 treatment of prostate tumor cells significantly decreased paclitaxel-induced apoptosis through activation of the ET(A) subtype. The anti-apoptotic effects of ET-1 are mediated, at least in part, through the Bcl-2 family. Although no significant changes in Bcl-2 expression occurred with ET-1 treatment, the pro-apoptotic family members Bad, Bax, and Bak all decreased significantly. Further analysis of the survival pathway demonstrated that phosphorylation of Akt occurs with ET-1 treatment in a time- and dose-dependent manner through phosphatidyinositol 3-kinase activation. These data support the combination of ET(A) antagonists and apoptosis-inducing therapies for prostate cancer treatment.