Lysophosphatidic acid as a mediator for proinflammatory agonists in a human corneal epithelial cell line

The role of lysophosphatidic acid and its receptors in corneal nerve regeneration

The integrity of corneal nerves is critical for ocular surface health, and damages can lead to Neurotrophic Keratopathy (NK). Despite the regenerative abilities of the peripheral nerve system (PNS), corneal nerve regeneration is often incomplete, and the underlying mechanisms are poorly understood. This study aims to identify potential factors that can enhance corneal nerve regeneration for NK treatment, with a focus on Lysophosphatidic acid (LPA). Thus, the effect of LPA and its underlying pathways in nerve regeneration is investigated in detail using in vitro mouse sensory neurons. To elucidate the impact of LPA as well as to reveal the responsible receptor, several functional assays as well as siRNA-based knock-down experiments were conducted. Additionally, possible changes in underlying pathways were investigated on mRNA levels. LPA-treated neurons significantly reduced fiber growth. However, LPAR2 knockdown neurons (Lpar2-KD) following LPA treatment showed a significant increase in fiber length. Additionally, LPA-treated neurons demonstrated enhanced levels of Lpar2 mRNA. On the other hand, nerve regeneration indicators such as Ngf, Gap-43, and Cdc42, along with LPA downstream signaling components like Pi3k and Ras, were elevated in Lpar2-KD neurons. In conclusion, this study elucidates the inhibitory effects of LPA on fiber outgrowth of sensory neurons. Furthermore, LPAR2 was identified as the responsible receptor for the LPA effect. Thus, Lpar2 knockdown might be a promising therapeutic approach to enhance neuronal regeneration in patients with NK.

Lysophosphatidic acid receptor 3 (LPAR3) regulates ocular surface chloride transport via calcium signaling

Dry eye is a multifactorial disease associated with impaired tear film homeostasis, damaging the ocular surface epithelium. Lysophosphatidic acid receptors (LPARs) are G-protein coupled receptors involved in Ca2+ and cAMP signaling via PLC and adenylyl cyclase activation. LPAR activation is involved in cell proliferation and wound healing in human corneal epithelial cells (HCECs) and in neuropathic pain. This study investigates the expression and functions of LPARs in ocular surface epithelial cells. Functional measurements of ocular surface potential difference (OSPD) were done in mice with topically applied, selective LPAR modulators. LPAR3 immunostaining was performed in mouse and human cornea and conjunctiva, and mouse lacrimal gland. LPAR-induced Ca2+ signaling was studied in primary and immortalized HCECs. The general LPAR agonist, linoleoyl LPA, and the LPAR3 selective agonist, 2S-OMPT, stimulated ocular surface Cl- secretion via Ca2+-activated Cl- channels (CaCCs). LPAR3 was expressed in the corneal and conjunctival epithelia of mice and humans, as well as in mouse lacrimal gland. Activation of LPAR and LPAR3 in HCECs transiently elevated intracellular Ca2+ through the Gq/PLC signaling pathway. LPAR3 agonists may potentially have therapeutic efficacy in ocular surface diseases, including dry eye disease.

The role of the PI3K/AKT signalling pathway in the corneal epithelium: recent updates

Phosphatidylinositol 3 kinase (PI3K)/AKT (also called protein kinase B, PKB) signalling regulates various cellular processes, such as apoptosis, cell proliferation, the cell cycle, protein synthesis, glucose metabolism, and telomere activity. Corneal epithelial cells (CECs) are the outermost cells of the cornea; they maintain good optical performance and act as a physical and immune barrier. Various growth factors, including epidermal growth factor receptor (EGFR) ligands, insulin-like growth factor 1 (IGF1), neurokinin 1 (NK-1), and insulin activate the PI3K/AKT signalling pathway by binding their receptors and promote antiapoptotic, anti-inflammatory, proliferative, and migratory functions and wound healing in the corneal epithelium (CE). Reactive oxygen species (ROS) regulate apoptosis and inflammation in CECs in a concentration-dependent manner. Extreme environments induce excess ROS accumulation, inhibit PI3K/AKT, and cause apoptosis and inflammation in CECs. However, at low or moderate levels, ROS activate PI3K/AKT signalling, inhibiting apoptosis and stimulating proliferation of healthy CECs. Diabetes-associated hyperglycaemia directly inhibit PI3K/AKT signalling by increasing ROS and endoplasmic reticulum (ER) stress levels or suppressing the expression of growth factors receptors and cause diabetic keratopathy (DK) in CECs. Similarly, hyperosmolarity and ROS accumulation suppress PI3K/AKT signalling in dry eye disease (DED). However, significant overactivation of the PI3K/AKT signalling pathway, which mediates inflammation in CECs, is observed in both infectious and noninfectious keratitis. Overall, upon activation by growth factors and NK-1, PI3K/AKT signalling promotes the proliferation, migration, and anti-apoptosis of CECs, and these processes can be regulated by ROS in a concentration-dependent manner. Moreover, PI3K/AKT signalling pathway is inhibited in CECs from individuals with DK and DED, but is overactivated by keratitis.

Stress, Depression, Resilience and Ageing: A Role for the LPA-LPA1 Pathway

BACKGROUND

Chronic stress affects health and the quality of life, with its effects being particularly relevant in ageing due to the psychobiological characteristics of this population. However, while some people develop psychiatric disorders, especially depression, others seem very capable of dealing with adversity. There is no doubt that along with the identification of neurobiological mechanisms involved in developing depression, discovering which factors are involved in positive adaptation under circumstances of extreme difficulty will be crucial for promoting resilience.

METHODS

Here, we review recent work in our laboratory, using an animal model lacking the LPA1 receptor, together with pharmacological studies and clinical evidence for the possible participation of the LPA1 receptor in mood and resilience to stress.

RESULTS

Substantial evidence has shown that the LPA1 receptor is involved in emotional regulation and in coping responses to chronic stress, which, if dysfunctional, may induce vulnerability to stress and predisposition to the development of depression. Given that there is commonality of mechanisms between those involved in negative consequences of stress and in ageing, this is not surprising, considering that the LPA1 receptor may be involved in coping with adversity during ageing.

CONCLUSION

Alterations in this receptor may be a susceptibility factor for the presence of depression and cognitive deficits in the elderly population. However, because this is only a promising hypothesis based on previous data, future studies should focus on the involvement of the LPA-LPA1 pathway in coping with stress and resilience in ageing.

Epidermal growth factor increases lysophosphatidic acid production in human ovarian cancer cells: roles for phospholipase D2 and receptor transactivation

Lysophosphatidic acid (LPA), is a lipid mediator that binds to G-protein coupled receptors. Epidermal growth factor (EGF), a polypeptide growth factor, binds to the EGF receptor (EGFR), a receptor tyrosine kinase. Both LPA and EGF induce responses in tumor cells that include proliferation, migration, metastasis, and induction of angiogenesis. LPA has the potential to act as an autocrine/paracrine factor and can transactivate the EGFR. This study explores the role of phospholipase D2 (PLD2) activation in LPA production, as well as cross-talk between EGF and LPA receptors. We demonstrate that EGF and LPA both stimulate production of LPA by OVCAR3 and SKOV3 human ovarian cancer cell lines. PD158780, an EGFR-selective tyrosine kinase inhibitor, blocks LPA production in response to both EGF and LPA in OVCAR3 and SKOV3 cells. Pertussis toxin, an inhibitor of LPA receptor signaling, inhibits LPA production in response to both EGF and LPA. Similar results were observed for the LPA receptor antagonist, Ki16425. Overexpression of PLD2 increases LPA production, while knockdown of PLD2 blocks EGF-induced LPA production. A phospholipase A2 (PLA2) inhibitor also blocks LPA- and EGF-induced LPA production. These results indicate that EGF stimulates LPA production in a manner that requires PLD2, and suggest that cross-talk can occur bidirectionally between EGF and LPA receptors.

Lysophosphatidic acid up-regulates expression of growth-regulated oncogene-alpha, interleukin-8, and monocyte chemoattractant protein-1 in human first-trimester trophoblasts: possible roles in angiogenesis and immune regulation

The serum lysophospholipase D activity and production of lysophosphatidic acid (LPA) increase in women with pregnancy. The effects of LPA on human placenta tissue remained unclear. We investigate the expression of LPA receptors and function of LPA in human first-trimester trophoblasts. Normal villous trophoblasts were obtained from termination of first-trimester gestation. We examined the expression of LPA receptors in primary culture of trophoblasts and the tissue. The effects of LPA on the expressions of chemokines of trophoblasts were examined using RT-PCR and enzyme immunoassay. We delineate signal pathways of LPA-inducing relevant chemokines in trophoblasts. The secretory chemokines were tested for angiogenic function using human endometrial microvascular endothelial cells and for immunological chemotaxis using decidual natural killer cells and THP-1 monocytes. The results revealed the expression of LPA1 receptors in trophoblast cells. LPA enhanced growth-regulated oncogene (GRO)-alpha, IL-8 and monocyte chemoattractant protein (MCP)-1 expressions in a time- and dose-dependent manner. Mechanistic dissection disclosed that LPA functioned mainly via the LPA1 receptor, Gi protein, various signal mediators of ERK, protein kinase C, p38, Akt, and c-Jun N-terminal kinase, and nuclear factor-kappaB pathways to secrete these chemokines. LPA-induced IL-8 protein secretion of trophoblasts enhanced permeability, migration, proliferation, and capillary tube formation of human endometrial microvascular endothelial cells. LPA-induced GRO-alpha and MCP-1 incited chemotaxis of natural killer cells and monocytes. We demonstrate that LPA mediates trophoblast cells to produce GRO-alpha, IL-8, and MCP-1 via LPA1 receptors and nuclear factor-kappaB-dependent signal pathways. Through LPA-induced chemokine production, human first-trimester trophoblast cells may regulate angiogenesis and innate immune system in early pregnancy.

Signal transduction responses to lysophosphatidic acid and sphingosine 1-phosphate in human prostate cancer cells

BACKGROUND

Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are lipid mediators that bind to G-protein-coupled receptors. In this study, signaling responses to 18:1 LPA and S1P were examined in parallel in three human prostate cancer cell lines: PC-3, Du145, and LNCaP.

METHODS

Receptor expression was assessed by RT-PCR, Northern blotting, and immunoblotting. Cellular responses to mediators were studied by proliferation assays, phosphoprotein immunoblotting, and phospholipid metabolism assays.

RESULTS

All cell lines express mRNA for both LPA and S1P receptors. PC-3 and Du145, but not LNCaP, proliferate in response to LPA and S1P. Epidermal growth factor (EGF), phorbol 12-myristate 13-acetate (PMA), LPA, and S1P induce activation of Erks in PC-3 and Du145; only EGF and PMA activate Erks in LNCaP. In Du145 and PC-3, Akt is activated by EGF, LPA, and S1P. Akt is constitutively active in LNCaP; EGF but not LPA or S1P stimulates further phosphorylation. FAK is phosphorylated in response to both LPA and S1P in PC-3 and Du145, but not in LNCaP. LPA and S1P stimulate phospholipase D (PLD) activity to varying extents in the different cell lines. Notably, both lipid mediators activate PLD in LNCaP. In Du145, LPA, but not S1P, activates PLD and enhances cellular production of LPA.

CONCLUSIONS

Although both LPA and S1P induce signal transduction in all prostate cancer cell lines studied, a proliferation response is observed only when the Erk, Akt, and FAK pathways are activated. Other responses to the lipid mediators, such as PLD activation, likely contribute to other cellular outcomes.

Lysophosphatidic acid mediates interleukin-8 expression in human endometrial stromal cells through its receptor and nuclear factor-kappaB-dependent pathway: a possible role in angiogenesis of endometrium and placenta

Lysophosphatidic acid (LPA) is a pleiotropic phospholipid molecule involved in inflammation, angiogenesis, would healing, and cancer invasion. Whereas serum lysophospholipase D activity increases in women with pregnancy, the role of LPA in pregnancy remains unclear. We investigated the expression of LPA receptors and function of LPA in endometrial stromal cells. Histologically normal endometrium was obtained from surgical specimens of women undergoing hysterectomy for leiomyoma. First-trimester decidua was obtained from women receiving elective termination of pregnancy. We examined the expressions of LPA1, LPA2, and LPA3 receptors in endometrial stromal cells. The effects of LPA on the expression of vascular endothelial growth factor, IL-6, and IL-8 were examined. Signal pathways of LPA were delineated. Functions of secretory angiogenic factors were tested using human endometrial microvascular endothelial cells. Immunoreactivity and mRNA of LPA1 receptors were identified in endometrial stromal cells. LPA enhanced IL-8 expression in a dose- and time-dependent manner, whereas vascular endothelial growth factor or IL-6 expression was not affected by LPA treatment. Mechanistic dissection disclosed that LPA functioned via the Gi protein, MAPK/p38 and nuclear factor-kappaB pathway. LPA-induced IL-8 enhanced migration, permeability, capillary tube formation, and proliferation of human endometrial microvascular endothelial cells. Endometrial stromal cells express LPA1 receptors. Through the LPA1 receptor, LPA induces IL-8 expression via a nuclear factor-kappaB-dependent signal pathway. These results could suggest that LPA may play a role in angiogenesis of endometrium and placenta through induction of IL-8 in endometrial stromal cells during pregnancy.

Lysophosphatidic acid induces rapid and sustained decreases in epidermal growth factor receptor binding via different signaling pathways in BEAS-2B airway epithelial cells

Lysophosphatidic acid (LPA) and epidermal growth factor (EGF) are important mediators of lung cell function and lung diseases. We showed previously that LPA decreases epidermal growth factor receptor (EGFR) binding rapidly in BEAS-2B airway epithelial cells, and this decrease is sustained to at least 18 h. The current studies investigate which LPA signaling pathways mediate the rapid versus sustained decreases in EGFR binding in BEAS-2B cells. The G(i/o) inhibitor pertussis toxin and the Rho kinase inhibitor Y-27632 [(R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide] had no effect on the rapid or sustained decreases. However, the mitogen-activated protein kinase kinase (MEK) inhibitor U0126 [1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)-butadiene ethanolate] decreased extracellular signal-regulated kinase (ERK) 1/2 phosphorylation, completely inhibited the rapid decrease in binding, and partially inhibited the sustained decrease. The direct Ca2+- and phospholipid-dependent protein kinase (PKC) activator phorbol-12-myristate-13-acetate stimulated ERK1/2 phosphorylation and decreased EGFR binding at both 15 min and 18 h. Furthermore, inhibitors of PKC partially inhibited ERK1/2 phosphorylation and the 15-min decrease but completely inhibited the 18-h decrease. Inhibitor time course studies showed that PKC induction of the 18-h decrease occurred during the first 3 h of treatment. We showed previously that LPA-stimulated EGFR transactivation contributes to the rapid decrease. Two transactivation inhibitors partially inhibited ERK1/2 phosphorylation, and U0126 partially inhibited EGFR transactivation, indicating that MEK may be involved both upstream and downstream of EGFR activation. Together, the data presented here indicate that LPA mediates the rapid decrease in EGFR binding via EGFR transactivation, MEK/ERK, and PKC, whereas the sustained decrease is regulated primarily by PKC.

Lysophosphatidic acid up-regulates expression of interleukin-8 and -6 in granulosa-lutein cells through its receptors and nuclear factor-kappaB dependent pathways: implications for angiogenesis of corpus luteum and ovarian hyperstimulation syndrome

CONTEXT

Lysophosphatidic acid (LPA) was found at significant amounts in follicular fluid of preovulatory follicle. The lysophospholipase D activity of serum from women receiving ovarian stimulation was higher than women with natural cycles. Angiogenic cytokines, including IL-6, IL-8, and vascular endothelial growth factor, increased in plasma and ascites of patients with ovarian hyperstimulation syndrome. The role of LPA in ovarian follicles is unclear.

OBJECTIVE

Our objective was to investigate the expression of LPA receptors and function of LPA in granulosa-lutein cells.

DESIGN

Granulosa-lutein cells were obtained from women undergoing in vitro fertilization. We examined the expression of LPA receptors using RT-PCR. The effects of LPA on the expression of IL-6, IL-8, and vascular endothelial growth factor were examined. Signal pathways of LPA were delineated. The functions of secretory angiogenic factors were tested using human umbilical vein endothelial cells.

RESULTS

The LPA1, LPA2, and LPA3 receptors' mRNA was identified in granulosa-lutein cells. LPA enhanced IL-8 and IL-6 expressions in a dose- and time-dependent manner. LPA functioned via LPA receptors, Gi protein, MAPK/ERK, p38, phosphatidylinositol 3-kinase/Akt, and nuclear factor-kappaB, and transactivation of epidermal growth factor receptor. LPA induced IL-8 and IL-6 through different pathways. LPA-induced IL-8 and IL-6 increased permeability of human umbilical vein endothelial cell monolayer.

CONCLUSIONS

LPA induces IL-8 and IL-6 expressions through LPA receptors and nuclear factor-kappaB dependent pathways in granulosa-lutein cells. The LPA in preovulatory follicles may play a role in the angiogenesis of corpus luteum. Large amounts of LPA-induced IL-8 and IL-6 from multiple corpora luteae of stimulated ovaries may be one of the pathophysiological causes of ovarian hyperstimulation syndrome.

Lysophosphatidic acid regulates inflammation-related genes in human endothelial cells through LPA1 and LPA3

Lysophosphatidic acid (LPA) is a low-molecular-weight lysophospholipid (LPL), which regulates endothelial cells participating in inflammation processes via interactions with endothelial differentiation gene (Edg) family G protein-coupled receptors. In this study, we attempted to determine which LPA receptors mediate the inflammatory response in human endothelial cells. Introduction of siRNA against LPA1 significantly suppressed LPA-induced ICAM-1 mRNA, total protein, and cell surface expressions, and subsequent U937 monocyte adhesion to LPA-treated human umbilical endothelial cells (HUVECs). By knock down of LPA1 and LPA3 in HUVECs, LPA-enhanced IL-1beta mRNA expression was significantly attenuated. Moreover, LPA1 and LPA3 siRNA also inhibited LPA-enhanced IL-1-dependent long-term IL-8 and MCP-1 mRNA expression, and subsequent THP-1 cell chemotaxis toward LPA-treated HUVEC-conditioned media. These results suggest that the expression of LPA-induced inflammatory response genes is mediated by LPA1 and LPA3. Our findings suggest the possible utilization of LPA1 or LPA3 as drug targets to treat severe inflammation.

Lysophosphatidic acid promoting corneal epithelial wound healing by transactivation of epidermal growth factor receptor

PURPOSE

To identify the underlying mechanisms by which lipid mediator lysophosphatidic acid (LPA) acts as a growth factor in stimulating extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3'-kinase (PI3K) during corneal epithelial wound healing.

METHODS

Epithelial debridement wounds in cultured porcine corneas and scratch wounds in an epithelial monolayer of SV40-immortalized human corneal epithelial (THCE) cells were allowed to heal in the presence or absence of an epidermal growth factor receptor (EGFR) inhibitor (tyrphostin AG1478), a matrix metalloproteinase inhibitor (GM6001), or a heparin-binding EGF-like growth factor (HB-EGF) antagonist (CRM197) with or without LPA. EGFR activation was analyzed by immunoprecipitation using EGFR antibodies and Western blotting with phosphotyrosine antibodies. Phosphorylation of ERK and AKT (a major substrate of PI3K) was analyzed by Western blotting with antibodies specific to the phosphorylated proteins. Wound- and LPA-induced shedding of HB-EGF was assessed by measuring the release of alkaline phosphatase (AP) in a stable THCE cell line that expressed HB-EGF with AP inserted in the heparin-binding site.

RESULTS

In organ and cell culture models, LPA enhanced corneal epithelial wound healing. LPA-stimulated and spontaneous wound closure was attenuated by AG1478, GM6001, or CRM197. Consistent with the effects on epithelial migration, these inhibitors, as well as the Src kinase inhibitor (PP2), retarded LPA-induced activation of EGFR and its downstream effectors ERK and AKT in THCE cells. Unlike exogenously added HB-EGF, LPA stimulated moderate EGFR phosphorylation; the level of phosphorylated EGFR was similar to that induced by wounding. However, LPA appeared to prolong wound-induced EGFR signaling. The release of HB-EGF assessed by AP activity increased significantly in response to wounding, LPA, or both, and the release of HB-EGF-AP induced by LPA was inhibited by PP2 and GM6001.

CONCLUSIONS

LPA accelerates corneal epithelial wound healing through its ability to induce autocrine HB-EGF signaling. Transactivation of EGFR by LPA represents a convergent signaling pathway accessible to stimuli such as growth factors and ligands of G-protein-coupled receptors in response to pathophysiological challenge in human corneal epithelial cells.

Thymosin beta 4 suppression of corneal NFkappaB: a potential anti-inflammatory pathway

The purpose of this study was to determine the effect of thymosin beta 4 (Tbeta4) on NFkappaB protein levels, activation, phosphorylation, and nuclear translocation in a model of tumor necrosis factor (TNF)-alpha-mediated corneal inflammation. Transformed and primary (HCET and HCEC) human corneal epithelial cells were stimulated with the pro-inflammatory cytokine TNF-alpha and treated or not with Tbeta4. Nuclear NFkappaB p65 subunit protein levels were assayed using ELISA, and activity was measured by determining NFkappaB binding to consensus oligonucleotides. NFkappaB p65 protein phosphorylation was also measured by ELISA. Nuclear translocation of NFkappaB p65 subunit was assayed by immunofluorescence microscopy. Compared to non-treated controls, Tbeta4 treatment significantly decreased nuclear NFkappaB protein levels, NFkappaB activity and p65 subunit phosphorylation in corneal epithelial cells after TNF-alpha stimulation. In TNF-alpha-stimulated corneal epithelial cells, NFkappaB p65 subunit translocation to the nucleus was observed using immunofluorescence microscopy. In contrast, Tbeta4 blocked nuclear translocation of the NFkappaB p65 subunit in TNF-alpha-stimulated corneal epithelial cells. TNF-alpha initiates cell signaling pathways that converge on the activation of NFkappaB, thus both are known mediators of the inflammatory process. Tbeta4, a protein with diverse cellular functions including wound healing and suppression of inflammation, inhibits the activation of NFkappaB in TNF-alpha-stimulated cells. These results have important clinical implications for the potential role of Tbeta4 as a corneal anti-inflammatory agent.