Epidermal and Hepatocyte Growth Factors, but Not Keratinocyte Growth Factor, Modulate Protein Kinase Cα Translocation to the Plasma Membrane through 15(S)-Hydroxyeicosatetraenoic Acid Synthesis

Docosanoid signaling modulates corneal nerve regeneration: effect on tear secretion, wound healing, and neuropathic pain

The cornea is densely innervated, mainly by sensory nerves of the ophthalmic branch of the trigeminal ganglia (TG). These nerves are important to maintain corneal homeostasis, and nerve damage can lead to a decrease in wound healing, an increase in corneal ulceration and dry eye disease (DED), and neuropathic pain. Pathologies, such as diabetes, aging, viral and bacterial infection, as well as prolonged use of contact lenses and surgeries to correct vision can produce nerve damage. There are no effective therapies to alleviate DED (a multifunctional disease) and several clinical trials using ω-3 supplementation show unclear and sometimes negative results. Using animal models of corneal nerve damage, we show that treating corneas with pigment epithelium-derived factor plus DHA increases nerve regeneration, wound healing, and tear secretion. The mechanism involves the activation of a calcium-independent phospholipase A2 that releases the incorporated DHA from phospholipids and enhances the synthesis of the docosanoids, neuroprotectin D1 (NPD1) and a new resolvin stereoisomer, resolvin D6i (RvD6i). NPD1 stimulates the synthesis of brain-derived neurotrophic factor, nerve growth factor, and semaphorin 7A. RvD6i treatment of injured corneas modulates gene expression in the TG resulting in enhanced neurogenesis, decreased neuropathic pain, and increased sensitivity. Taken together, these results represent a promising therapeutic option to reestablish the homeostasis of the cornea.

ORAI1 calcium channel orchestrates skin homeostasis

To achieve and maintain skin architecture and homeostasis, keratinocytes must intricately balance growth, differentiation, and polarized motility known to be governed by calcium. Orai1 is a pore subunit of a store-operated Ca(2+) channel that is a major molecular counterpart for Ca(2+) influx in nonexcitable cells. To elucidate the physiological significance of Orai1 in skin, we studied its functions in epidermis of mice, with targeted disruption of the orai1 gene, human skin sections, and primary keratinocytes. We demonstrate that Orai1 protein is mainly confined to the basal layer of epidermis where it plays a critical role to control keratinocyte proliferation and polarized motility. Orai1 loss of function alters keratinocyte differentiation both in vitro and in vivo. Exploring underlying mechanisms, we show that the activation of Orai1-mediated calcium entry leads to enhancing focal adhesion turnover via a PKCβ-Calpain-focal adhesion kinase pathway. Our findings provide insight into the functions of the Orai1 channel in the maintenance of skin homeostasis.

The growing role of eicosanoids in tissue regeneration, repair, and wound healing

Tissue repair and regeneration are essential processes in maintaining tissue homeostasis, especially in response to injury or stress. Eicosanoids are ubiquitous mediators of cell proliferation, differentiation, and angiogenesis, all of which are important for tissue growth. Eicosanoids regulate the induction and resolution of inflammation that accompany the tissue response to injury. In this review, we describe how this diverse group of molecules is a key regulator of tissue repair and regeneration in multiple organ systems and biologic contexts.

Role of 15-hydroxyeicosatetraenoic acid in hemozoin-induced lysozyme release from human adherent monocytes

Natural hemozoin (nHZ), a lipid-bound ferriprotoporphyrin IX crystal produced by Plasmodium parasites after hemoglobin catabolism, seriously compromises the functions of human monocytes, and 15-hydroxyeicosatetraenoic acid (15-HETE) and 4-hydroxynonenal (4-HNE), two nHZ lipoperoxidation products, have been related to such a functional impairment. nHZ was recently shown to promote inflammation-mediated lysozyme release from human monocytes through p38 mitogen-activated protein kinase- (MAPK)- and nuclear factor (NF)-κB-dependent mechanisms. This study aimed at identifying the molecule of nHZ lipid moiety that was responsible for these effects. Results showed that 15-HETE mimicked nHZ effects on lysozyme release, whereas 4-HNE did not. 15-HETE-enhanced lysozyme release was abrogated by anti-TNF-α and anti-IL-1β-blocking antibodies and mimicked by recombinant cytokines; on the contrary, MIP-1α/CCL3 was not involved as a soluble mediator of 15-HETE effects. Moreover, 15-HETE early activated p38 MAPK and NF-κB pathways by inducing p38 MAPK phosphorylation; cytosolic I-κBα phosphorylation and degradation; NF-κB nuclear translocation and DNA-binding. Inhibition of both routes through chemical inhibitors (SB203580, quercetin, artemisinin, and parthenolide) prevented 15-HETE-dependent lysozyme release. Collectively, these data suggest that 15-HETE plays a major role in nHZ-enhanced monocyte degranulation.

Involvement of p38 MAPK in haemozoin-dependent MMP-9 enhancement in human monocytes

The lipid moiety of natural haemozoin (nHZ, malarial pigment) was previously shown to enhance expression and release of human monocyte matrix metalloproteinase-9 (MMP-9), and a major role for 15-(S,R)-hydroxy-6,8,11,13-eicosatetraenoic acid (15-HETE), a nHZ lipoperoxidation product, was proposed. Here, the underlying mechanisms were investigated, focusing on the involvement of mitogen-activated protein kinases (MAPKs). Results showed that nHZ promoted either early or late p38 MAPK phosphorylation; however, nHZ did not modify basal phosphorylation/expression ratios of extracellular signal-regulated kinase-1/2 and c-jun N-terminal kinase-1/2. 15-HETE mimicked nHZ effects on p38 MAPK, whereas lipid-free synthetic (s)HZ and delipidized (d)HZ did not. Consistently, both nHZ and 15-HETE also promoted phosphorylation of MAPK-activated protein kinase-2, a known p38 MAPK substrate; such an effect was abolished by SB203580, a synthetic p38 MAPK inhibitor. SB203580 also abrogated nHZ-dependent and 15-HETE-dependent enhancement of MMP-9 mRNA and protein (latent and activated forms) levels in cell lysates and supernatants. Collectively, these data suggest that in human monocytes, nHZ and 15-HETE upregulate MMP-9 expression and secretion through activation of p38 MAPK pathway. The present work provides new evidence on mechanisms underlying MMP-9 deregulation in malaria, which might be helpful to design new specific drugs for adjuvant therapy in complicated malaria.

Specifying protein kinase C functions in melanoma

The protein kinase C (PKC) family of serine/threonine protein kinases is a heterogeneous group of enzymes receiving and integrating signals involved in both normal melanocyte biology and melanoma pathology. Alterations in PKC enzyme expression and activation contribute to the malignant phenotype of melanoma in both oncogenic and tumor suppressive roles. Delineating the diverse and often context-dependent functions of PKC enzymes in melanocyte/melanoma biology is key to capitalize on these kinases as drug targets. This review summarizes several of the diverse functions of PKC in melanocyte and melanoma biology with a focus on PKC enzyme regulation and function.

Direct evidence that PKCα positively regulates wound re-epithelialization: correlation with changes in desmosomal adhesiveness

Non-healing wounds cause considerable patient morbidity and represent a significant economic burden. Central to wound repair is re-epithelialization, a crucial process involving the modulation of cell adhesion to allow keratinocyte migration to cover the exposed underlying tissues. The cellular mechanisms regulating the earliest stages of re-epithelialization are unclear. We present the first direct evidence that protein kinase Cα (PKCα) plays an important role in regulating wound re-epithelialization. In PKCα(-/-) mice re-epithelialization is delayed, while in novel bitransgenic mice over-expressing constitutively active PKCα it is accelerated. These effects are not due to changes in keratinocyte proliferation, apoptosis or intrinsic cell motility. Instead, they correlate with changes in desmosomal adhesiveness, delay being preceded by retained desmosomal hyper-adhesiveness and acceleration with a rapid switch to desmosomal Ca(2+) -dependence. We demonstrate mechanistic conservation in acute human wounds where PKCα localizes to wound edge desmosomes, which become Ca(2+) -dependent. However, in chronic wounds PKCα remains cytoplasmic and desmosomes fail to switch from the hyper-adhesive state. These results throw new mechanistic light on the earliest stages of wound re-epithelialization and suggest activation of PKCα as a new therapeutic strategy for non-healing wounds.

Histamine stimulates the proliferation of small and large cholangiocytes by activation of both IP3/Ca2+ and cAMP-dependent signaling mechanisms

Although large cholangiocytes exert their functions by activation of cyclic adenosine 3',5'-monophosphate (cAMP), Ca(2+)-dependent signaling regulates the function of small cholangiocytes. Histamine interacts with four receptors, H1-H4HRs. H1HR acts by Gαq activating IP(3)/Ca(2+), whereas H2HR activates Gα(s) stimulating cAMP. We hypothesize that histamine increases biliary growth by activating H1HR on small and H2HR on large cholangiocytes. The expression of H1-H4HRs was evaluated in liver sections, isolated and cultured (normal rat intrahepatic cholangiocyte culture (NRIC)) cholangiocytes. In vivo, normal rats were treated with histamine or H1-H4HR agonists for 1 week. We evaluated: (1) intrahepatic bile duct mass (IBDM); (2) the effects of histamine, H1HR or H2HR agonists on NRIC proliferation, IP(3) and cAMP levels and PKCα and protein kinase A (PKA) phosphorylation; and (3) PKCα silencing on H1HR-stimulated NRIC proliferation. Small and large cholangiocytes express H1-H4HRs. Histamine and the H1HR agonist increased small IBDM, whereas histamine and the H2HR agonist increased large IBDM. H1HR agonists stimulated IP(3) levels, as well as PKCα phosphorylation and NRIC proliferation, whereas H2HR agonists increased cAMP levels, as well as PKA phosphorylation and NRIC proliferation. The H1HR agonist did not increase proliferation in PKCα siRNA-transfected NRICs. The activation of differential signaling mechanisms targeting small and large cholangiocytes is important for repopulation of the biliary epithelium during pathologies affecting different-sized bile ducts.

Estrogen negatively regulates epithelial wound healing and protective lipid mediator circuits in the cornea

Estrogen receptors (ERs) are expressed in leukocytes and in every ocular tissue. However, sex-specific differences and the role of estradiol in ocular inflammatory-reparative responses are not well understood. We found that female mice exhibited delayed corneal epithelial wound closure and attenuated polymorphonuclear (PMN) leukocyte responses, a phenotype recapitulated by estradiol treatment both in vivo (topically in male mice) and in vitro (corneal epithelial cell wound healing). The cornea expresses 15-lipoxygenase (15-LOX) and receptors for lipoxin A(4) (LXA(4)), which have been implicated in an intrinsic lipid circuit that regulates corneal inflammation and wound healing. Delayed epithelial wound healing correlated with lower expression of 15-LOX in the regenerated epithelium of female mice. Estradiol in vitro and in vivo down-regulated epithelial 15-LOX expression and LXA(4) formation, while estradiol abrogation of epithelial wound healing was completely reversed by treatment with LXA(4). More important, ERβ and ERα selectively regulated epithelial wound healing, PMN cell recruitment, and activity of the intrinsic 15-LOX/LXA(4) circuit. Our results demonstrate for the first time a sex-specific difference in the corneal reparative response, which is mediated by ERβ and ERα selective regulation of the epithelial and PMN 15-LOX/LXA(4) circuit. These findings may provide novel insights into the etiology of sex-specific ocular inflammatory diseases.

Knockdown of heme oxygenase-2 impairs corneal epithelial cell wound healing

Heme oxygenase (HO) represents an intrinsic cytoprotective system based on its anti-oxidative and anti-inflammatory properties mediated via its products biliverdin/bilirubin and carbon monoxide (CO). We showed that deletion of HO-2 results in impaired corneal wound healing with associated chronic inflammatory complications. This study was undertaken to examine the role of HO activity and the contribution of HO-1 and HO-2 to corneal wound healing in an in vitro epithelial scratch injury model. A scratch wound model was established using human corneal epithelial (HCE) cells. These cells expressed both HO-1 and HO-2 proteins. Injury elicited a rapid and transient increase in HO-1 and HO activity; HO-2 expression was unchanged. Treatment with biliverdin or CORM-A1, a CO donor, accelerated wound closure by 10% at 24 h. Inhibition of HO activity impaired wound closure by more than 50%. However, addition of biliverdin or CORM-A1 reversed the effect of HO inhibition on wound healing. Moreover, knockdown of HO-2 expression, but not HO-1, significantly impaired wound healing. These results indicate that HO activity is required for corneal epithelial cell migration. Inhibition of HO activity impairs wound healing while amplification of its activity restores and accelerates healing. Importantly, HO-2, which is highly expressed in the corneal epithelium, appears to be critical for the wound healing process in the cornea. The mechanisms by which it contributes to cell migration in response to injury may reside in the cytoprotective properties of CO and biliverdin.

Matrix Metalloproteinase-9 and Haemozoin: Wedding Rings for Human Host and Plasmodium falciparum Parasite in Complicated Malaria

It is generally accepted that the combination of both Plasmodium falciparum parasite and human host factors is involved in the pathogenesis of complicated severe malaria, including cerebral malaria (CM). Among parasite products, the malarial pigment haemozoin (HZ) has been shown to impair the functions of mononuclear and endothelial cells. Different CM models were associated with enhanced levels of matrix metalloproteinases (MMPs), a family of proteolytic enzymes able to disrupt subendothelial basement membrane and tight junctions and shed, activate, or inactivate cytokines, chemokines, and other MMPs through cleavage from their precursors. Among MMPs, a good candidate for targeted therapy might be MMP-9, whose mRNA and protein expression enhancement as well as direct proenzyme activation by HZ have been recently investigated in a series of studies by our group and others. In the present paper the role of HZ and MMP-9 in complicated malaria, as well as their interactions, will be discussed.

EGF stimulates lipoxin A4 synthesis and modulates repair in corneal epithelial cells through ERK and p38 activation

PURPOSE

To investigate the effect of epidermal growth factor (EGF) on lipoxin A4 (LXA4) synthesis and how it regulates corneal epithelial wound healing through mitogen-activated kinases, extracellular regulated kinase (ERK) 1/2, and p38.

METHODS

Rabbit corneal epithelial (RCE) cells were stimulated with EGF or LXA4 at different times. In some experiments, cells were pretreated with 12/15-lipoxygenase (12/15-LOX) inhibitor cinnamyl-3,4-dihydroxy-α-cyanocinnamate (CDC), ERK1/2 inhibitor PD98059, or p38 inhibitor SB203580. For wound-healing experiments, corneas from rabbits and 12/15-LOX (ALOX-15)-deficient mice were injured by epithelial removal and maintained in organ culture in the presence of EGF or LXA4 with or without inhibitors. Epithelial cell proliferation was assayed by immunofluorescence with Ki67 and cell counting. Scrape migration assays were performed in 6-well plates. LXA4 synthesis was analyzed by liquid chromatography-tandem mass spectrometry analysis.

RESULTS

EGF activated ERK1/2 and p38 in RCE cells in a sustained manner. EGF activation was partially inhibited by CDC. EGF and LXA4 increased corneal epithelial wound closure. ERK1/2 inhibition with PD98059 or p38 with SB203580 blocked the effect of LXA4 on wound healing. ALOX-15 corneas displayed inhibition of epithelial wound closure promoted by EGF, whereas LXA4 stimulation induced similar wound closure in wild-type and knockout mice. EGF-stimulated LXA4 synthesis in RCE cells was inhibited by CDC or the EGF receptor antagonist AG1478.

CONCLUSIONS

These results demonstrate that EGF-stimulated epithelial wound healing is partially mediated through a 12/15-LOX-LXA4 pathway, and activation of ERK1/2 and p38 is required for LXA4 action.

Role of the NF-κB transcription pathway in the haemozoin- and 15-HETE-mediated activation of matrix metalloproteinase-9 in human adherent monocytes

Haemozoin (HZ, malarial pigment) is a crystalline ferriprotoporphyrin IX polymer derived from undigested host haemoglobin haem, present in late stages of Plasmodium falciparum-parasitized RBCs and in residual bodies shed after schizogony. It was shown previously that phagocytosed HZ or HZ-containing trophozoites increased monocyte matrix metalloproteinase-9 (MMP-9) activity and enhanced production of MMP-9-related cytokines TNF and IL-1beta. Here we show that in human monocytes the HZ/trophozoite phagocytosis effects and their recapitulation by 15(S,R)-hydroxy-6,8,11,13-eicosatetraenoic acid (15-HETE), a potent lipoperoxidation derivative generated by HZ from arachidonic acid via haem catalysis, were mediated via activation of NF-κB transcription pathway. After phagocytosis of HZ/trophozoites or treatment with 15-HETE, the NF-κB complex migrated to the nuclear fraction while the inhibitory cytosolic IκBalpha protein was phosphorylated and degraded. All HZ/trophozoite/15-HETE effects on MMP-9 activity and TNF/IL-1beta production were abrogated by quercetin, artemisinin and parthenolide, inhibitors of IκBalpha phosphorylation and subsequent degradation, NF-κB nuclear translocation, and NF-κB-p65 binding to DNA respectively. In conclusion, enhanced activation of MMP-9, and release of pro-inflammatory cytokines TNF and IL-1beta, a triad of effects involved in malaria pathogenesis, elicited in human monocytes by trophozoite and HZ phagocytosis and recapitulated by 15-HETE, appear to be causally connected to persisting activation of the NF-κB system.

Altered morphology and function of the lacrimal functional unit in protein kinase C{alpha} knockout mice

PURPOSE

Protein kinase C (PKC) α plays a major role in the parasympathetic neural stimulation of lacrimal gland (LG) secretion. It also has been reported to have antiapoptotic properties and to promote cell survival. Therefore, the hypothesis for the present study was that PKCα knockout ((-/-)) mice have impaired ocular surface-lacrimal gland signaling, rendering them susceptible to desiccating stress and impaired corneal epithelial wound healing. In this study, the lacrimal function unit (LFU) and the stressed wound-healing response were examined in PKCα(-/-) mice.

METHODS

In PKCα(+/+) control mice and PKCα(-/-) mice, tear production, osmolarity, and clearance rate were evaluated before and after experimental desiccating stress. Histology and immunofluorescent staining of PKC and epidermal growth factor were performed in tissues of the LFU. Cornified envelope (CE) precursor protein expression and cell proliferation were evaluated. The time course of healing and degree of neutrophil infiltration was evaluated after corneal epithelial wounding.

RESULTS

Compared with the PKCα(+/+) mice, the PKCα(-/-) mice were noted to have significantly increased lacrimal gland weight, with enlarged, carbohydrate-rich, PAS-positive acinar cells; increased corneal epithelia permeability, with reduced CE expression; and larger conjunctival epithelial goblet cells. The PKCα(-/-) mice showed more rapid corneal epithelial healing, with less neutrophil infiltration and fewer proliferating cells than did the PKCα(+/+) mice.

CONCLUSIONS

The PKCα(-/-) mice showed lower tear production, which appeared to be caused by impaired secretion by the LG and conjunctival goblet cells. Despite their altered tear dynamics, the PKCα(-/-) mice demonstrated more rapid corneal epithelial wound healing, perhaps due to decreased neutrophil infiltration.

Significance of lipid mediators in corneal injury and repair

Corneal injury induces an inflammatory reaction and damages the sensory nerves that exert trophic influences in the corneal epithelium. Alterations in normal healing disrupt the integrity and function of the tissue with undesirable consequences, ranging from dry eye and loss of transparency to ulceration and perforation. Lipids play important roles in this complex process. Whereas lipid mediators such as platelet activating factor (PAF) and cyclooxygenease-2 metabolites contribute to tissue damage and neovascularization, other mediators, such as the lipoxygenase (LOX) derivatives from arachidonic acid, 12- and 15-hydroxy/hydroperoxyeicosatetraenoic acids, and lipoxin A4, act as second messengers for epidermal growth factor to promote proliferation and repair. Stimulation of the cornea with pigment epithelial derived factor in the presence of docosahexaenoic acid gives rise to the synthesis of neuroprotectin D1, a derivative of LOX activity, and increases regeneration of corneal nerves. More knowledge about the role that lipids play in corneal wound healing can provide insight into the development of new therapeutic approaches for treating corneal injuries. PAF antagonists, lipoxins, and neuroprotectins can be effective therapeutic tools for maintaining the integrity of the cornea.

Neuroprotectin D1 inhibits retinal ganglion cell death following axotomy

Neuroprotectin D1 (NPD1), a docosahexaenoic acid-derived autacoid, is an endogenous neuroprotective and anti-inflammatory mediator that is generated in the retina and brain. The effects of exogenous NPD1 on retinal ganglion cell (RGC) apoptosis and the role of 12/15-lipoxygenase (Alox15) in retina were evaluated after optic nerve transection (ONT). Treatment with NPD1 was associated with significant protection against RGC death. The percentage of RGC survival in NPD1-treated group was 30% at 2 weeks after ONT as compared with 12% of RGC survival in the ONT group without treatment. Endogenous NPD1 was a predominant lipid autocoid in uninjured and axotomized retinas. Alox15 mRNA expression was upregulated in retinas following ONT suggesting that amplification of 12/15-lipoxygenase (12/15-LOX) may represent a neuroprotective response in the rat retina. The density of RGCs was higher in the normal retina of 12/15-LOX-deficient mice as compared with congenic controls. Hence, the resident NPD1 has a potential role in the physiological and pathophysiological responses of the retina.

A novel role for activating transcription factor-2 in 15(S)-hydroxyeicosatetraenoic acid-induced angiogenesis

To investigate the mechanisms underlying 15(S)-HETE-induced angiogenesis, we have studied the role of the small GTPase, Rac1. We find that 15(S)-HETE activated Rac1 in human retinal microvascular endothelial cells (HRMVEC) in a time-dependent manner. Blockade of Rac1 by adenovirus-mediated expression of its dominant negative mutant suppressed HRMVEC migration as well as tube formation and Matrigel plug angiogenesis. 15(S)-HETE stimulated Src in HRMVEC in a time-dependent manner and blockade of its activation inhibited 15(S)-HETE-induced Rac1 stimulation in HRMVEC and the migration and tube formation of these cells as well as Matrigel plug angiogenesis. 15(S)-HETE stimulated JNK1 in Src-Rac1-dependent manner in HRMVEC and adenovirus-mediated expression of its dominant negative mutant suppressed the migration and tube formation of these cells and Matrigel plug angiogenesis. 15(S)-HETE activated ATF-2 in HRMVEC in Src-Rac1-JNK1-dependent manner and interference with its activation via adenovirus-mediated expression of its dominant negative mutant abrogated migration and tube formation of HRMVEC and Matrigel plug angiogenesis. In addition, 15(S)-HETE-induced MEK1 stimulation was found to be dependent on Src-Rac1 activation. Blockade of MEK1 activation inhibited 15(S)-HETE-induced JNK1 activity and ATF-2 phosphorylation. Together, these findings show that 15(S)-HETE activates ATF-2 via the Src-Rac1-MEK1-JNK1 signaling axis in HRMVEC leading to their angiogenic differentiation.

Role of cPKCalpha and nPKCepsilon in EGF-stimulated goblet cell proliferation

PURPOSE

The authors determined the role of the protein kinase C (PKC) isoforms cPKCalpha and nPKCepsilon in EGF-stimulated proliferation of cultured rat and human conjunctival goblet cells.

METHODS

Rat and human conjunctivas were minced, and goblet cells were allowed to grow. Passage 1 cells were serum starved for 24 to 48 hours and were incubated with the PKC inhibitors calphostin C and Gö 6983 (10(-10)-10(-7) M) for 20 minutes before stimulation with EGF (10(-7) M) for 24 hours. The presence and localization of PKC isoforms in cultured rat goblet cells were determined by Western blot analysis and immunofluorescence microscopy, respectively. Cultured rat goblet cells were serum starved and incubated with adenoviruses containing genes for dominant-negative cPKCalpha (Ad DNPKCalpha, 10(4) pfu), dominant-negative nPKCepsilon (Ad DNPKCepsilon, 10(4) pfu), and wild-type cPKCalpha (Ad WTPKCalpha, 10(7) pfu), and proliferation was measured.

RESULTS

In rat goblet cells, EGF-stimulated proliferation was completely inhibited by calphostin C, whereas Gö 6983 inhibited proliferation by 53%+/-15%. In human goblet cells, EGF-stimulated proliferation was completely inhibited by calphostin C. PKCalpha, -betaI, -betaII, -delta, -epsilon, -iota/lambda, -theta, -gamma, and -zeta were found in cultured rat goblet cells. Ad DNPKCalpha and Ad DNPKCepsilon inhibited EGF-stimulated proliferation in rat goblet cells by 78%+/-6% and 92%+/-8%, respectively. Incubation with Ad WTPKCalpha alone significantly increased proliferation.

CONCLUSIONS

cPKCalpha and nPKCepsilon play key roles in conjunctival goblet cell proliferation.

Novel translocation responses of cytosolic phospholipase A2alpha fluorescent proteins

Cytosolic phospholipase A2 (cPLA2)alpha responds to the rise in cytosolic Ca2+ ([Ca2+]i) attending cell stimulation by moving to intracellular membranes, releasing arachidonic acid (AA) from these membranes, and thereby initiating the synthesis of various lipid mediators. Under some conditions, however, cPLA2alpha translocation occurs without any corresponding changes in [Ca2+]i. The signal for such responses has not been identified. Using confocal microscopy to track fluorescent proteins fused to cPLA2alpha or cPLA2alpha's C2 domain, we find that AA mimics Ca2+ ionophores in stimulating cPLA(2)alpha translocations to the perinuclear ER and to a novel site, the lipid body. Unlike the ionophores, AA acted independently of [Ca2+](i) rises and did not translocate the proteins to the Golgi. AA's action did not involve its metabolism to eicosanoids or acylation into cellular lipids. Receptor agonists also stimulated translocations targeting lipid bodies. We propose that AA is a signal for Ca2+-independent cPLA2alpha translocation and that lipid bodies are common targets of cPLA2alpha and contributors to stimulus-induced lipid mediator synthesis.

Arachidonic Acid metabolites in the cardiovascular system: the role of lipoxygenase isoforms in atherogenesis with particular emphasis on vascular remodeling

Vascular remodeling refers to lasting structural alterations in the vessel wall that are initiated in response to external and internal stimuli. These changes are distinct from acute functional responses of blood vessels when challenged by increased blood pressure, altered hemodynamics, or vasoactive mediators. In early atherogenesis, when lesion formation is starting to impact local hemodynamics, the vessel wall responds with outward vascular remodeling to maintain normal blood flow. However, inward remodeling may also occur during the time course of plaque formation, contributing to vascular stenosis. Lipoxygenases form a heterogeneous family of lipid-peroxidizing enzymes, which have been implicated in atherogenesis. Several lines of in vitro and in vivo evidence indicated their involvement in disease development, but the precise function of different lipoxygenase isoforms is still a matter of discussion. Vascular remodeling is an early response during plaque development; therefore, lipoxygenases may be involved in this process. Unfortunately, little is known about the potential role of lipoxygenase isoforms in vascular remodeling. This review will briefly summarize our knowledge of the role of lipoxygenases in vascular biology and will critically review the activities of the 3 most athero-relevant lipoxygenase isoforms in atherogenesis, with particular emphasis on vascular remodeling.

Lipoxygenase metabolism: roles in tumor progression and survival

The metabolism of arachidonic acid through lipoxygenase pathways leads to the generation of various biologically active eicosanoids. The expression of these enzymes vary throughout the progression of various cancers, and thereby they have been shown to regulate aspects of tumor development. Substantial evidence supports a functional role for lipoxygenase-catalyzed arachidonic and linoleic acid metabolism in cancer development. Pharmacologic and natural inhibitors of lipoxygenases have been shown to suppress carcinogenesis and tumor growth in a number of experimental models. Signaling of hydro[peroxy]fatty acids following arachidonic or linoleic acid metabolism potentially effect diverse biological phenomenon regulating processes such as cell growth, cell survival, angiogenesis, cell invasion, metastatic potential and immunomodulation. However, the effects of distinct LOX isoforms differ considerably with respect to their effects on both the individual mechanisms described and the tumor being examined. 5-LOX and platelet type 12-LOX are generally considered pro-carcinogenic, with the role of 15-LOX-1 remaining controversial, while 15-LOX-2 suppresses carcinogenesis. In this review, we focus on the molecular mechanisms regulated by LOX metabolism in some of the major cancers. We discuss the effects of LOXs on tumor cell proliferation, their roles in cell cycle control and cell death induction, effects on angiogenesis, migration and the immune response, as well as the signal transduction pathways involved in these processes. Understanding the molecular mechanisms underlying the anti-tumor effect of specific, or general, LOX inhibitors may lead to the design of biologically and pharmacologically targeted therapeutic strategies inhibiting LOX isoforms and/or their biologically active metabolites, that may ultimately prove useful in the treatment of cancer, either alone or in combination with conventional therapies.

Protein kinase C alpha and epsilon differentially modulate hepatocyte growth factor-induced epithelial proliferation and migration

Protein kinase C (PKC) isoenzymes require membrane translocation for physiological activation. We have recently shown that the growth factors such as epidermal growth factor and hepatocyte growth factor (HGF), but not keratinocyte growth factor (KGF), regulate PKCalpha activation to promote epithelial wound healing [Sharma, G.D., Ottino, P., Bazan, H.E.P., 2005. Epidermal and hepatocyte growth factors, but not keratinocyte growth factor, modulate protein kinase C alpha translocation to the plasma membrane through 15(S)-hydroxyeicosatetraenoic acid synthesis. J. Biol. Chem. 280, 7917--924]. Protein kinase C alpha (PKCalpha) and protein kinase C epsilon (PKCvarepsilon) are two differentially regulated isoenzymes. While PKCalpha requires Ca(2+) for its activation, PKEvarepsilon is Ca(2+) independent. However, growth factor-induced activation of these enzymes and their specific regulation of epithelial migration and proliferation have not been explored. In the present study, we overexpressed PKCvarepsilon fused to green fluorescent protein to examine its translocation in real-time to the plasma membrane in living human corneal epithelial cells. Stimulation with HGF and KGF demonstrated translocation of PKCvarepsilon to the plasma membrane. Because HGF activates both PKCs, this growth factor was used to stimulate wound healing. PKCalpha or PKCvarepsilon-genes were knocked down individually without affecting the basal expression of the other PKC isoforms. Gene knockdown of PKCalpha significantly inhibited HGF-stimulated proliferation of human corneal epithelial cells. In contrast, PKCvarepsilon-gene-silencing severely impaired the HGF-stimulated migratory ability of human corneal epithelial cells. When migrating epithelial cells in the cornea wound bed after injury were transfected with specific PKCalpha- or PKCvarepsilon-siRNA, there was a significant delay in wound healing. Corneal wound healing stimulated with HGF in similar conditions was also inhibited. On the other hand, overexpression of PKCalpha or PKCvarepsilon-genes fused with green fluorescent protein in migrating corneal epithelium accelerated repair of the epithelial defect. Our findings demonstrate that PKCalpha and PKCvarepsilon modulate different stages of wound healing stimulated by HGF and contribute to epithelial repair by playing selective regulatory roles in epithelial proliferation and migration, both crucial to corneal wound healing.

Interdependence of lipoxin A4 and heme-oxygenase in counter-regulating inflammation during corneal wound healing

In the immune-privileged cornea, epithelial wounds heal rapidly with almost no scarring and, unlike in most other tissues, acute inflammation in the absence of infection is beneficial to healing. Molecular mechanisms, which account for this striking property, remain to be clearly defined, but they likely include autacoids that control leukocyte activation. Two prominent enzymes, 12/15-lipoxygenase (LOX), which generates antiinflammatory lipid autacoids, and heme-oxygenase (HO), which generates antioxidants and carbon monoxide, are highly expressed in human and mouse corneas. LXA4, an endogenous 12/15-LOX product, proved to be a potent inhibitor of exacerbated inflammation and significantly increased re-epithelialization in corneal wounds. In vivo deletion of 12/15-LOX correlated with exacerbated inflammation and impaired wound healing in 12/15-LOX(-/-) mice, a phenotype that was rescued by treatment with LXA4. More importantly, 12/15-LOX(-/-) mice demonstrated impaired induction of HO-1 in both acute and exacerbated inflammation. Topical LXA4 restored HO-1 expression in 12/15-LOX(-/-) mice and amplified HO-1 gene expression in human corneal epithelial cells. HO-2(-/-) mice, which fail to induce HO-1, also demonstrated exacerbated inflammation in response to injury, a phenotype that, notably, correlated with a 50% reduction in endogenous LXA4 formation. Collectively, results demonstrate a critical role for LXA4 in inflammatory/reparative responses and provide the first evidence that 12/15-LOX and HO systems function in concert to control inflammation.

Epidermal Growth Factor Signaling for Matrix-Dependent Cell Proliferation and Differentiation in Primary Cultured Hepatocytes

Understanding hepatocellular signaling occurring in biomaterial systems is important for successful hepatic tissue engineering. Toward this end, we employed synthetic glycopolymers, as artificial matrices, to examine integrin-mediated epidermal growth factor (EGF) signaling in primary hepatocyte cultures. We dispersed hepatocytes on a collagen matrix or on a synthetic glycopolymer matrix and subsequently stimulated them with EGF. Only hepatocytes cultured on collagen proliferated, and we observed significant expression of cyclin B1 in these cells. Pharmacological agents, LY294004 (a phosphatidylinositol [PI] 3-kinase inhibitor) and AG1478 (an EGF kinase receptor inhibitor), blocked hepatocyte proliferation and cyclin B1 expression. In addition, EGF-stimulated hepatocytes formed spheroids, exhibited membrane ruffling, and increased tryptophan 2,3-oxygenase (TO) expression when cultured on glycopolymer matrices. Interestingly, PI 3-kinase inhibition suppressed membrane ruffling, spheroid formation, and TO expression. Taken together, this data suggests PI 3-kinase plays an important role in mediating cross talk between integrin and the EGF signaling pathways in primary hepatocyte cultures.

Epidermal growth factor receptor-dependent PI3K-activation promotes restitution of wounded human gastric epithelial monolayers

Restitution is a crucial event during the healing of superficial injury of the gastric mucosa involving epithelial cell sheet movement into the damaged area. We demonstrated that growth factors promote the restitution of human gastric epithelial cells. However, the intracellular signaling pathways that transmit extracellular cues as well as regulate basal and growth factor-stimulated gastric epithelial cell migration are still unclear. Herein, confluent human gastric epithelial cell monolayers (HGE-17) or primary cultures of gastric epithelial cells were wounded with a razor blade and the migration response was analyzed in presence or absence of TGFalpha or of pharmacological inhibitors of signaling proteins. Kinase activation profile analysis and phase-contrast microscopy were also performed in parallel. We report that ERK1/2 and Akt activities are rapidly stimulated following wounding of HGE-17 cells. Treatment of confluent HGE-17 cells or primary cultures of gastric epithelial cells with the phosphatidylinositol 3-kinase inhibitor LY294002, but not the MEK1 inhibitor, PD98059, significantly inhibits basal and TGFalpha-induced migration following wounding. Conversely, treatment of wounded HGE-17 cells with phosphatidylinositol(3,4,5)-triphosphate is sufficient to stimulate basal cell migration by 235%. In addition, pp60c-src kinase activity and tyrosine phosphorylation of epidermal growth factor receptors (EGFR) are also rapidly enhanced after wounding and pharmacological inhibition of both these activities strongly attenuates basal and TGFalpha-induced migration as well as Akt phosphorylation levels. In conclusion, the present results indicate that EGFR-dependent PI3K activation promotes restitution of wounded human gastric epithelial monolayers.

5-Hydroxyeicosatetraenoic acid is a key intermediate of the arachidonate-dependent protective signaling in monocytes/macrophages exposed to peroxynitrite

Endogenous generation of arachidonic acid via selective activation of cytosolic phospholipase A(2) has been implicated in the mechanism of monocytes/macrophage survival in the presence of peroxynitrite. In particular, the lipid messenger was shown to prevent the otherwise rapid onset of a mitochondrial permeability-transition (MPT)-dependent necrosis by causing the mitochondrial translocation of protein kinase Calpha (PKCalpha) and the ensuing cytosolic accumulation of the Bcl-2-antagonist of cell death (Bad), an event promoting the anti-MPT function of Bcl-2 (or Bcl-X(L)). Here, we show that the effects on PKCalpha are not mediated directly by arachidonate but rather, by downstream products of the enzyme 5-lipoxygenase (5-LO). Peroxynitrite elicited the nuclear membrane translocation of 5-LO and enhanced its enzymatic activity via a mechanism sensitive to low concentrations of inhibitors of 5-LO or the 5-LO-activating protein, as well as to genetic depletion of the latter enzyme. Inhibition of 5-LO activity was invariably associated with the cytosolic localization of PKCalpha, the mitochondrial accumulation of Bad, and a rapid MPT-dependent necrosis. All these events were prevented by nanomolar concentrations of the 5-LO product 5-hydroxyeicosatetraenoic acid.

15-Lipoxygenase-2 gene regulation by its product 15-(S)-hydroxyeicosatetraenoic acid through a negative feedback mechanism that involves peroxisome proliferator-activated receptor gamma

An inverse relationship exists between the expression of 15-lipoxygenase-2 (15-LOX-2) and peroxisome proliferator-activated receptor gamma (PPARgamma) in normal prostate epithelial cells (PrECs) compared with their expression in prostate carcinoma cells (PC-3). The reason for this difference, however, is unknown. We hypothesized that this inverse expression partly involves the 15-LOX-2 promoter and 15-S-hydroxyeicosatetraenoic acid (15-(S)-HETE), a product of 15-LOX-2 that binds to PPARgamma. We identified an active steroid nuclear receptor half-site present in the 15-LOX-2 promoter fragment F-5 (-618/+177) that can interact with PPARgamma. After forced expression of wild-type PPARgamma, 15-(S)-HETE (1 microM) decreased F-5 reporter activity in PrECs whereas forced expression of 15-LOX-2 resulted in 15-(S)-HETE production which enhanced F-5 activity in PC-3. In contrast, the expression of dominant-negative PPARgamma reversed the transcriptional activation of F-5 by enhancing it 202-fold in PrEC or suppressing it in PC-3; the effect in PC-3 was positively increased 150-fold in the presence of 15-(S)-HETE (1 microM). Peroxisome proliferator-activated receptor gamma interacted with 15-LOX-2 promoter sequences in pulldown experiments using biotinylated 15-LOX-2 (-560/-596 bp) oligonucleotides. In gelshift analyses PPARgamma and orphan receptor RORalpha were shown to interact with the F-5 fragment in PC-3 cells. These data suggest that crosstalk mechanisms exist between the 15-LOX-2 gene and PPARgamma to counterbalance expression and help explain the inverse relationship of these genes in normal versus cancer cells.

Involvement of arachidonic acid metabolism and EGF receptor in neurotensin-induced prostate cancer PC3 cell growth

Dietary fats, which increase the risk of prostate cancer, stimulate release of intestinal neurotensin (NT), a growth-promoting peptide that enhances the formation of arachidonic acid metabolites in animal blood. This led us to use PC3 cells to examine the involvement of lipoxygenase (LOX) and cyclooxygenase (COX) in the growth effects of NT, including activation of EGF receptor (EGFR) and downstream kinases (ERK, AKT), and stimulation of DNA synthesis. NT and EGF enhanced [3H]-AA release, which was diminished by inhibitors of PLA2 (quinacrine), EGFR (AG1478) and MEK (U0126). NT and EGF phosphorylated EGFR, ERK and AKT, and stimulated DNA synthesis. These effects were diminished by PLA2 inhibitor (quinacrine), general LOX inhibitors (NDGA, ETYA), 5-LOX inhibitors (Rev 5901, AA861), 12-LOX inhibitor (baicalein) and FLAP inhibitor (MK886), while COX inhibitor (indomethacin) was without effect. Cells treated with NT and EGF showed an increase in 5-HETE levels by HPLC. PKC inhibitor (bisindolylmaleimide) blocked the stimulatory effects of NT, EGF and 5-HETE on DNA synthesis. We propose that 5-LOX activity is required for NT to stimulate growth via EGFR and its downstream kinases. The mechanism may involve an effect of 5-HETE on PKC, which is known to facilitate MEK-ERK activation. NT may enhance 5-HETE formation by Ca2+-mediated and ERK-mediated activation of DAG lipase and cPLA2. NT also upregulates cPLA2 and 5-LOX protein expression. Thus, the growth effects of NT and EGF involve a feed-forward system that requires cooperative interactions of the 5-LOX, ERK and AKT pathways.

Lipoxins in the eye and their role in wound healing

The eye must contain highly evolved programs to limit inflammation and promote wound healing as an errant response can lead to blindness. However, pathways that protect the delicate visual axis and account for its atypical inflammatory responses remain to be clearly defined. Hence, research efforts have been initiated to elucidate the role of the anti-inflammatory LXA4 circuits in the eye. LXA4 is formed in healthy and injured corneas and both its receptor and 12/15-lipoxygenase are predominantly expressed in epithelial cells. An essential role for LXA4 in preserving ocular function is supported by 12/15-LOX deficient mice that exhibit a phenotype of impaired wound healing and LXA4 formation. A novel epithelial bioaction role for LXA4 has been uncovered in the cornea as topical LXA4 promotes wound healing and limits the sequelae of injury. These emerging studies indicate that the LXA4 circuit may hold a fundamental role in maintaining an ocular environment that actively restricts inflammation while promoting wound healing.

Cellular and molecular events in corneal wound healing: significance of lipid signalling

Alterations in the normal healing process after corneal injury can produce undesirable outcomes that range from corneal haze to ulceration and perforation. Lipids play important roles in the complex inflammatory processes that occur after corneal wounding. While some lipid mediators, such as the lipoxygenase derivatives of arachidonic acid, 12-hydroxyeicosatetraenoic acid (12[S]-HETE and 15[S]-HETE), act as second messengers to promote cell proliferation and are possibly involved in the synthesis of other molecules that suppress inflammation, others, such as platelet-activating factor (PAF), exert their actions through specific receptors, play key roles during sustained corneal inflammation (as might occur with chemical burns), and contribute to tissue destruction and neovascularization. PAF is also a strong inducer of selective metalloproteinases (MMPs) that degrade the extracellular matrix. The use of a new PAF antagonist has shown great promise for the treatment of diffuse lamellar keratitis (DLK) and alkali-burned corneas.