Agonist-induced hydrolysis of phosphoinositides and formation of 1,2-diacylglycerol in adult human keratinocytes

Significance of thrombomodulin release from gingival epithelial cells in periodontitis patients

BACKGROUND AND OBJECTIVE

Thrombomodulin, a cell transmembrane glycoprotein, binds to thrombin and converts it from a procoagulant protease to an anticoagulant enzyme that activates protein C. Thrombomodulin is very important in regulating the function of thrombin. Elevated soluble thrombomodulin is present in the gingival crevicular fluid of subjects with periodontitis. The objective of the present study was to investigate the mechanisms about the elevated soluble thrombomodulin in gingival crevicular fluid.

MATERIAL AND METHODS

Gingival sections from six patients with chronic periodontitis and from three periodontally healthy subjects were immunostained for thrombomodulin detection. Thrombomodulin levels were investigated in the gingival crevicular fluid of 11 subjects with chronic periodontitis. The effects of neutrophil enzymes on thrombomodulin release and on thrombomodulin in the gingival crevicular fluid were examined by an enzyme-linked immunosorbent assay or by Western blotting.

RESULTS

The expression of gingival epithelial thrombomodulin was lost or decrease near infiltrating neutrophils. Thrombomodulin was rapidly released from gingival epithelial cells by neutrophil enzymes, and gingival crevicular fluid with periodontitis included the proteolytic cleavage thrombomodulin using immunoblotting analysis. The thrombomodulin release was not caused by rapid cell damage, on lactate dehydrogenase assay. There were significant differences in thrombomodulin content between gingival crevicular fluid samples from healthy and diseased sites, regardless of the degree of probing depth.

CONCLUSION

Neutrophil enzymes induced rapid thrombomodulin release from the membrane surface of gingival epithelial cells. This might explain the thrombomodulin increase in gingival crevicular fluid with local diseased gingiva. Elevation of thrombomodulin in gingival crevicular fluid may be a potential marker of epithelial cell membrane injury.

Regulation of phospholipase C activation by the number of H(2) receptors during Ca(2+)-induced differentiation of mouse keratinocytes

We have reported previously that the histamine H(2) receptor (H(2)R) can stimulate the phospholipase C (PLC) signaling pathway in mouse keratinocytes. In the present work, we examined the physiological mechanisms involved in this activation by studying histamine metabolism and H(2)R expression and coupling during mouse keratinocyte differentiation. Ca(2+)-induced differentiation decreased histidine decarboxylase (HDC) mRNA, the enzyme responsible for histamine synthesis, by 68.9+/-5.0%. Concomitantly, intracellular histamine content and its release into the extracellular medium were reduced significantly by 68.2+/-2.0 and 74.1+/-1.7%, respectively. Binding of [3H]tiotidine to H(2)Rs present on the surface of whole cells was also decreased by cellular differentiation [(18.17+/-2.1)x10(4) vs. (6.27+/-0.87)x10(4) sites/cell, undifferentiated and differentiated cells, respectively], without affecting H(2)R affinity. Northern blot and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of the H(2)R mRNA showed that the expression was also down-regulated at the transcriptional level. Moreover, the inhibition of H(2)R expression strongly affected the ability of the receptor to induce PLC activation. Our findings suggest that H(2)R signaling through the PLC second messenger system is inhibited during keratinocyte differentiation by an autocrine loop involving down-regulation of H(2)R expression and inhibition of histamine metabolism.

Expression and biological significance of Ca2+-activated ion channels in human keratinocytes

In whole-cell recordings from HaCaT keratinocytes, ATP, bradykinin, and histamine caused a biphasic change of the membrane potential consisting of an initial transient depolarization, followed by a pronounced and long-lasting hyperpolarization. Flash photolysis of caged IP3 mimicked the agonist-induced voltage response, suggesting that intracellular Ca2+ release and subsequent opening of Ca2+-activated ion channels serve as the common transduction mechanism. In contrast, cAMP- and PKC-dependent pathways were not involved in the electrophysiological effects of the extracellular signaling molecules. The depolarization was predominantly mediated by a DIDS- and niflumic acid-sensitive Cl- current, whereas a charybdotoxin- and clotrimazole-sensitive K+ current underlay the prominent hyperpolarization. Consistent with the electrophysiological data, RT-PCR showed that HaCaT keratinocytes express two types of Ca2+-activated Cl- channels, CaCC2 and CaCC3 (CLCA2), as well as the Ca2+-activated K+ channel hSK4. That the pronounced hSK4-mediated hyperpolarization bears significance on the growth and differentiation properties of keratinocytes is suggested by RNase protection assays showing that hSK4 mRNA expression is strongly down-regulated under conditions that allow keratinocyte differentiation. hSK4 might thus play a role in linking changes in membrane potential to the biological fate of keratinocytes.

Expression of thrombomodulin and consequences of thrombomodulin deficiency during healing of cutaneous wounds

Thrombomodulin is a cell surface anticoagulant that is expressed by endothelial cells and epidermal keratinocytes. Using immunohistochemistry, we examined thrombomodulin expression during healing of partial-thickness wounds in human skin and full-thickness wounds in mouse skin. We also examined thrombomodulin expression and wound healing in heterozygous thrombomodulin-deficient mice, compound heterozygous mice that have <1% of normal thrombomodulin anticoagulant activity, and chimeric mice derived from homozygous thrombomodulin-deficient embryonic stem cells. In both human and murine wounds, thrombomodulin was absent in keratinocytes at the leading edge of the neoepidermis, but it was expressed strongly by stratifying keratinocytes within the neoepidermis. No differences in rate or extent of reepithelialization were observed between wild-type and thrombomodulin-deficient mice. In chimeric mice, both thrombomodulin-positive and thrombomodulin-negative keratinocytes were detected within the neoepidermis. Compared with wild-type mice, heterozygous and compound heterozygous thrombomodulin-deficient mice exhibited foci of increased collagen deposition in the wound matrix. These findings demonstrate that expression of thrombomodulin in keratinocytes is regulated during cutaneous wound healing. Severe deficiency of thrombomodulin anticoagulant activity does not appear to alter reepithelialization but may influence collagen production by fibroblasts in the wound matrix.

H2 histamine receptor-mediated increase in intracellular Ca2+ in cultured human keratinocytes

Histamine is present in the epidermis in intracellular and extracellular area and is released from mast cells and keratinocytes in the early stage of inflammation of the skin. Such release may contribute to common itching or intensify the inflammatory responses. Histamine binds to its receptors and participate in regulation of the inflammatory responses by acting on endothelial cells, nerve endings, lymphocytes, monocytes, and leukocytes. Histamine has direct effects on keratinocytes as well. Histamine modulates the proliferation of keratinocytes. The binding of histamine to the receptor on keratinocyte membrane induces activation of adenylate cyclase and phospholipase C through GTP binding protein. We previously reported that histamine induces transient increase in intracellular Ca2+ in cultured normal human epidermal keratinocytes (NHEK) and normal epidermis. H1 and H2 histamine receptors are widely distributed in many tissues and cells. In this study, we investigated which types of histamine receptors are related to the increase in intracellular Ca2+ by histamine stimulation in cultured human epidermal keratinocytes. NHEK were cultured in serum-free KGM medium. With H1 antihistamines, mepyramine and diphenhydramine, histamine responses were moderately but not statistically significantly inhibited. With H2 antihistamine, cimetidine, histamine response was significantly inhibited. Epinephrine response was not affected by these antihistamines. Thus, it is considered that H2 antihistamines specifically block histamine-mediated increase in intracellular Ca2+ of cultured normal human keratinocytes.

Phospholipase C-gamma1 is required for calcium-induced keratinocyte differentiation

Phospholipase C-gamma1 is the most abundant member of the phospholipase C family in keratinocytes and is induced by calcium. Phospholipase C-gamma1, therefore, may be involved in the signal transduction system leading to calcium regulation of keratinocyte differentiation. To test this hypothesis, expression of phospholipase C-gamma1 in human keratinocytes was blocked by transfecting cells with the antisense human phospholipase C-gamma1 cDNA construct. These cells demonstrated a dramatic reduction in phospholipase C-gamma1 protein level compared with the empty vector-transfected cells and a marked reduction in the mRNA and protein levels of the differentiation markers involucrin and transglutaminase following administration of calcium. Similarly, cotransfection of antisense phospholipase C-gamma1 constructs with a luciferase reporter vector containing involucrin or transglutaminase promoters led to a substantial reduction in calcium-stimulated involucrin and transglutaminase promoter activities. Similar results were seen following treatment with a specific phospholipase C inhibitor U73122. To determine whether phospholipase C-gamma1 regulated differentiation by controlling intracellular calcium, we examined the ability of antisense phospholipase C-gamma1 to block the calcium-induced rise in intracellular calcium and found that it could. These findings indicate that phospholipase C-gamma1 is a critical component of the signaling pathway mediating calcium regulation of keratinocyte differentiation via its mobilization of intracellular calcium.

1,25 dihydroxyvitamin D3 enhances the calcium response of keratinocytes

The steroid hormone 1,25 dihydroxyvitamin D3 (1,25(OH)2D3) regulates cell proliferation and differentiation. Intracellular calcium (Cai) concentrations play a crucial role in these events. From our previous studies, we have demonstrated a calcium receptor (CaR) in keratinocytes which appears to regulate the initial release of Cai from intracellular stores in response to extracellular calcium (Cao) and so is likely to participate in the differentiation process. In this study, we determined whether the ability of 1,25(OH)2D3 to enhance Ca++ -induced differentiation was mediated at least in part through changes in the CaR. Keratinocytes were grown in keratinocyte growth medium (KGM) with 0.03 mM, 0.1 mM, or 1.2 mM Ca and treated with 10(-8) M 1,25(OH)2D3 till harvest after 5, 7, 14, and 21 days. CaR mRNA levels were quantitated by polymerase chain reaction. The results were compared to the ability of 1,25(OH)2D3 to enhance calcium-stimulated increases in Cai. In cells grown in 0.03 mM Ca, the CaR mRNA levels decreased with time. 1,25(OH)2D3 stimulated the levels at 5 days and prevented the falloff over the subsequent 16 days. On the other hand, in cells grown in 0.1 or 1.2 mM Ca, the message levels remained high, and 1,25(OH)2D3 had no further effect. To study the functional relationship, we harvested cells after 5 and 7 days in culture following a 24 h treatment with 1,25(OH)2D3 or vehicle to measure the Cai response to 2 mM Cao. The preconfluent cells grown in 0.03 mM Ca showed a nearly twofold increase in the Cai response to Cao when pretreated with 1,25(OH)2D3, whereas the confluent cells and those grown in 1.2 mM Ca showed no enhancement by 1,25(OH)2D3. Studies with 45Ca influx into keratinocytes revealed that 1,25(OH)2D3 enhanced the influx in preconfluent and confluent cells when grown in KGM containing 0.03 mM Ca but not in cells grown in 1.2 mM calcium. We conclude that 1,25(OH)2D3 maintains the CaR mRNA levels in cells grown in 0.03 mM Ca, thus maintaining their responsiveness to Cao and so ensuring their ability to differentiate in response to the calcium signal.

Up-regulation of p21WAF1 by phorbol ester and calcium in human keratinocytes through a protein kinase C-dependent pathway

Terminal differentiation in a variety of cell types has been associated with p53-independent up-regulation of p21WAF1 p21WAF1 mRNA and protein are expressed at low levels in normal human skin, but overexpression of p21WAF1 has been observed in differentiating keratinocytes in involved psoriatic epidermis and in human squamous cell carcinoma. In this study we investigated by immunohistochemistry and Western blotting whether calcium and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, well characterized differentiation signals, induce p21WAF1 in cultured normal human keratinocytes and whether induction of p21WAF1 in this system depends on protein kinase C activation or functional p53. Phorbol ester induced p21WAF1 expression, which was maximal at 4 to 8 h with reduction back to baseline by 24 to 48 h. In contrast, increasing the extracellular Ca2+ concentration from 70 micromol/L to 1.5 mmol/L resulted in upregulation of p21WAF1 expression with a slower time course, with peak induction at 18 to 24 h. No parallel increase in p53 expression was observed in normal human keratinocytes. Up-regulation of p21WAF1 was also observed in response to phorbol ester in HaCaT cells, which carry homozygous and inactivating mutations for p53. Induction of p21WAF1 by phorbol ester and Ca2+ was inhibited by the specific protein kinase C inhibitor Ro 31-8220. The results demonstrate a differential time course of p21WAF1 protein up-regulation in response to phorbol ester and Ca2+, signals that result in keratinocyte differentiation, and suggest that induction of p21WAF1 in differentiating human keratinocytes occurs through protein kinase C-dependent and p53-independent mechanisms.

Phospholipase C-mediated signaling is altered during HaCaT cell proliferation and differentiation

To elucidate the signaling mechanisms associated with keratinocyte differentiation, we studied in vitro phospholipase C-mediated signal transduction, which results in the generation of inositol phosphates, comparing proliferating versus differentiated HaCaT cells, a human keratinocyte line. Bradykinin- or A23187-induced formation of inositol 1,4,5-trisphosphate, inositol 1,4-bisphosphate, and inositol monophosphates, as determined by anion exchange high performance liquid chromatography, were found to be highest in the early logarithmic growth phase of the cells. In more highly differentiated HaCaT cells, which expressed maximal amounts of the differentiation marker involucrin, inositol phosphate formation was reduced to about one third of that in proliferating cells. Thin layer chromatography of membrane phosphatidylinositol phosphates revealed that this reduction was associated with a steady decrease in phospholipase C substrates. Immunoblot analysis of phospholipase C isozymes, however, and of expression of Gq alpha, the G protein subunit that activates phospholipase C beta, revealed no decrease during the differentiation phase. The results suggest that the inositol-phospholipid signal transduction pathway is involved in keratinocyte proliferation and in the induction of differentiation, with attenuated signal transduction activity via phospholipase C-coupled receptors in more differentiated keratinocytes.

Glucosamine for psoriasis?

Amphiregulin and transforming growth factor-alpha, agonists for the epidermal growth factor receptor, are the major autocrine growth factors for cultured keratinocytes, and their substantial overexpression in psoriatic lesions suggests that they are crucial to the basal hyperplasia that characterizes psoriasis. Amphiregulin binds to heparin and related highly sulfated polysaccharides, and exogenous heparin blocks its growth factor activity, rationalizing previous reports that psoriasis responds to heparin therapy. Differentiating keratinocytes produce increased amounts of protein-bound as well as free-chain heparan sulfates, which may function physiologically as amphiregulin antagonists. By promoting keratinocyte synthesis of these heparan sulfates, glucosamine administration may inhibit amphiregulin function and thus provide therapeutic benefit in psoriasis. Concurrent ingestion of fish oil, by impeding the excessive activation of protein kinase C, may decrease keratinocyte production of amphiregulin and other autocrine growth factors, thus complementing the postulated benefits of glucosamine.

Thrombin and melittin activate phospholipase C in human HaCaT keratinocytes

Following the activation of specific receptors, phospholipase C has been shown to cleave the membrane phospholipid phosphatidylinositol bisphosphate into the 2nd messengers inositol 1,4,5-trisphosphate and diacylglycerol. Both 2nd messengers contribute to the regulation of cellular proliferation. The receptor for bradykinin is coupled to this pathway in keratinocytes, but knowledge about other activators of phospholipase C is limited. Additional mediators and agents were therefore examined regarding their ability to activate phospholipase C in HaCaT keratinocytes. Analysis for 3H-inositol phosphates was performed by anion-exchange HPLC. Thrombin and melittin induced a time- and dose-dependent release of inositol 1,4,5-trisphosphate. Several other mediators examined such as angiotension II, neurotensin, C3a, pituitary adenylate cyclase activating peptide, phenylephrin, and prostaglandin E2, did not induce the formation of inositol phosphates. In view of the mitogenic activity and the increased formation of thrombin after tissue injury, the coupling of the thrombin receptor to phospholipase C in HaCaT keratinocytes suggests a role of this protease in epidermal wound healing.

Changes in calcium responsiveness and handling during keratinocyte differentiation. Potential role of the calcium receptor

Extracellular calcium concentrations (Cao) > 0.1 mM are required for the differentiation of normal human keratinocytes in culture. Increments in Cao result in acute and sustained increases in the intracellular calcium level (Cai), postulated to involve both a release of calcium from intracellular stores and a subsequent increase in calcium influx through nonspecific cation channels. The sustained rise in Cai appears to be necessary for keratinocyte differentiation. To understand the mechanism by which keratinocytes respond to Cao, we measured the acute effects of Cao on Cai and calcium influx in keratinocytes at various stages of differentiation. We then demonstrated the existence of the calcium receptor (CaR) in keratinocytes and determined the effect of calcium-induced differentiation on its mRNA levels. Finally, we examined the role of Cai in regulating both the initial rise in Cai after the switch to higher Cao and the activity of the nonspecific cation channel through which calcium influx occurs. Our data indicate that the acute Cai response to Cao is lost as the cells differentiate and increase their basal Cai. These data correlated with the decrease in CaR mRNA levels in cells grown in low calcium. However, calcium influx as measured by 45Ca uptake increased with differentiation in 1.2mM calcium, consistent with the increase in CaR mRNA in these cells as well as the calcium-induced opening of the nonspecific cation channels. We conclude that the keratinocyte contains a CaR that regulates both the initial release of Cai from intracellular stores and the subsequent increase in calcium flux through nonspecific calcium channels. A rising level of Cai may turn off the release of calcium from intracellular stores while potentiating the influx through the nonspecific cation channels. Differentiation of keratinocytes appears to increase the CaR, which may facilitate the maintenance of the high Cai required for differentiation.

Evidence for the presence of a protease-activated receptor distinct from the thrombin receptor in human keratinocytes

Thrombin receptor activation was explored in human epidermal keratinocytes and human dermal fibroblasts, cells that are actively involved in skin tissue repair. The effects of thrombin, trypsin, and the receptor agonist peptides SFLLRN and TFRIFD were assessed in inositolphospholipid hydrolysis and calcium mobilization studies. Thrombin and SFLLRN stimulated fibroblasts in both assays to a similar extent, whereas TFRIFD was less potent. Trypsin demonstrated weak efficacy in these assays in comparison with thrombin. Results in fibroblasts were consistent with human platelet thrombin receptor activation. Keratinocytes, however, exhibited a distinct profile, with trypsin being a far better activator of inositolphospholipid hydrolysis and calcium mobilization than thrombin. Furthermore, SFLLRN was more efficacious than thrombin, whereas no response was observed with TFRIFD. Since our data indicated that keratinocytes possess a trypsin-sensitive receptor, we addressed the possibility that these cells express the human homologue of the newly described murine protease-activated receptor, PAR-2 [Nystedt, S., Emilsson, K., Wahlestedt, C. & Sundelin, J. (1994) Proc. Natl. Acad. Sci. USA 91, 9208-9212]. PAR-2 is activated by nanomolar concentrations of trypsin and possesses the tethered ligand sequence SLIGRL. SLIGRL was found to be equipotent with SFLLRN in activating keratinocyte inositolphospholipid hydrolysis and calcium mobilization. Desensitization studies indicated that SFLLRN, SLIGRL, and trypsin activate a common receptor, PAR-2. Northern blot analyses detected a transcript of PAR-2 in total RNA from keratinocytes but not fibroblasts. Levels of thrombin receptor message were equivalent in the two cell types. Our results indicate that human keratinocytes possess PAR-2, suggesting a potential role for this receptor in tissue repair and/or skin-related disorders.

1,25-Dihydroxyvitamin D3 upregulates the phosphatidylinositol signaling pathway in human keratinocytes by increasing phospholipase C levels

1,25-Dihydroxyvitamin D3 (1,25(OH)2D3) induces the differentiation of normal human keratinocytes, in part by increasing their basal intracellular calcium levels (Cai) over a period of hours. Agonists such as ATP acting through membrane receptors cause an immediate but transient increase in Cai accompanied by an increase in inositol trisphosphate (IP3). Treatment of keratinocytes for 24 h with 1 nM 1,25(OH)2D3 resulted in a two- to four-fold potentiation of the Cai response of these cells to ATP. This potentiation was inhibitable with cycloheximide, unaccompanied by a change in total intracellular calcium pools, but associated with an increase in basal IP3 levels and ATP-stimulated IP3 production. Treatment with 1,25(OH)2D3 raised the protein and mRNA levels of phospholipase C isoenzymes, particularly phospholipase C-beta 1 in a dose-dependent manner. These studies indicate that 1,25(OH)2D3 modulates the keratinocyte signal transduction pathway by induction of phospholipase isoenzymes, a previously undescribed action for this hormone.

Translocation and downregulation of protein kinase C isoenzymes-alpha and -epsilon by phorbol ester and bryostatin-1 in human keratinocytes and fibroblasts

Protein kinase C isoenzymes can be subdivided into two classes, based on their requirement for calcium. Protein kinase C-alpha, beta I, -beta II, and -gamma are calcium dependent, whereas protein kinase C-gamma, -epsilon, -zeta, -eta, and -theta are calcium independent. We have examined the expression, translocation, downregulation, and activation of calcium-dependent and -independent protein kinase C isoenzymes in human skin keratinocytes and fibroblasts. Human keratinocytes and fibroblasts expressed protein kinase C-alpha, -delta, -epsilon, and -zeta mRNA and protein, whereas protein kinase C-eta (L) was detected only in keratinocytes. Protein kinase C-beta I, -beta II, -gamma, and -theta were not detected in either cell type. The protein kinase C activators 12-0-tetradecanoylphorbol 13-acetate and bryostatin-1 (50 nM, for 5 min) induced translocation of protein kinase C-alpha and -epsilon cytosol to membrane in both keratinocytes and fibroblasts. 12-0-tetradecanoylphorbol 13-acetate and bryostatin-1, for 18 h, induced complete downregulation (i.e., loss) of protein kinase C-alpha and -epsilon in keratinocytes, but only partial downregulation was observed in fibroblasts. The subcellular distribution of protein kinase C-delta, -zeta or protein kinase C-eta, in keratinocytes or fibroblasts, did not change in response to 12-0-tetradecanoylphorbol 13-acetate or bryostatin-1. These data indicate differential expression, subcellular distribution, and regulation of protein kinase C isoenzymes in human skin cells.

Thrombomodulin expression by human keratinocytes. Induction of cofactor activity during epidermal differentiation

Thrombomodulin is an endothelial cell surface glycoprotein that inhibits the procoagulant activities of thrombin and accelerates activation of the anticoagulant protein C. Because protein C deficiency is associated with cutaneous thrombosis, we investigated the expression of thrombomodulin in human skin. Thrombomodulin was detected by immunohistochemical staining both in dermal endothelial cells and in epidermal keratinocytes. Within the epidermis, thrombomodulin staining was limited to keratinocytes of the spinous layer, suggesting that thrombomodulin is induced when basal keratinocytes begin to terminally differentiate. Thrombomodulin expression also correlated with squamous differentiation in epidermal malignancies; little or no thrombomodulin staining was seen in five basal cell carcinomas, whereas strong thrombomodulin staining was observed in each of five squamous cell carcinomas. Human foreskin keratinocytes cultured in medium containing 0.07 mM calcium chloride synthesized functional thrombomodulin with cofactor activity comparable to thrombomodulin in human umbilical vein endothelial cells. Stimulation of keratinocyte differentiation with 1.4 mM calcium chloride for 48 h produced 3.5-, 3.2-, and 5.6-fold increases in thrombomodulin cofactor activity, antigen, and mRNA, respectively. These observations suggest that thrombin is regulated by keratinocyte thrombomodulin at sites of cutaneous injury, and indicate a potential role for thrombomodulin in epidermal differentiation.

Differentiation-associated localization of nPKC eta, a Ca(++)-independent protein kinase C, in normal human skin and skin diseases

The expression of nPKC eta, a Ca(++)-independent isoform of protein kinase C in normal human skin, and skin from patients with psoriasis, squamous cell carcinoma, basal cell epithelioma, nevus pigmentosus, and seborrheic keratosis, were examined by immunohistochemical staining using a polyclonal antibody raised against a synthetic peptide at a diverse region of the nPKC eta molecule. In normal epidermis, the strongest staining was observed in the uppermost granular layer with no staining of the spinous or basal layers. The inner layer of the intra-epidermal eccrine duct was also strongly stained. Weak staining was observed in several layers of the outer root sheath of the follicular infundibulum. No staining was detected in the inner root sheath of the hair follicles, hair matrix, sebaceous gland, eccrine gland, intradermal eccrine duct, arrectores pilorum, melanocytes, Langerhans cells, fibroblasts, or blood vessels. In psoriatic skin, stained keratinocytes were distributed in the suprabasal layers with the most being observed in the uppermost layer and the least in layers closed to the basal layer. In squamous cell carcinoma, weak staining was observed in the keratotic cells around horny pearls. In the basal cell epithelioma and nevus pigmentosus, the cells were not stained, whereas in seborrheic keratosis, cells that stained were located in the granular layer. We conclude from the evidence presented above that nPKC eta is expressed in close association with epidermal differentiation in normal skin and skin diseases.

Phosphatidic acid and phospholipase D both stimulate phosphoinositide turnover in cultured human keratinocytes

Phosphatidic acid (PA) induced a rapid dose-dependent increase in production of inositol phosphates in cultured adult human keratinocytes, peaking at 30 s. Natural and dioleoyl PA were equally effective, while other phospholipid classes had no effect. Lipid A was also active. Lyso-PA also induced inositol phosphate production, but contamination of the PA preparation by lyso-PA could not account for the effect of PA. The effect of PA could not be reproduced by treatment of cells with calcium ionophore. PA-induced inositol phosphate production could be inhibited (> 50%) by pre-treatment of cells with either pertussis toxin or 12-O-tetradecanoylphorbol 13-acetate, suggesting the involvement of a GTP-binding protein and a protein kinase C-mediated negative feedback mechanism. PA also stimulated release of arachidonic acid from keratinocytes. Treatment of cells with exogenous phospholipase D similarly induced inositol phosphate production in the keratinocytes. Since PA may be formed by receptor-mediated activation of phospholipase D, or by phosphorylation of diacylglycerol, the results suggest that PA may play a significant role in signalling mechanisms of human keratinocytes.

Inositol phosphate formation and release of intracellular free calcium by bradykinin in HaCaT keratinocytes

Phospholipase C-mediated release of inositol trisphosphate, followed by an increase in free intracellular calcium, is an important signal transduction pathway for several membrane receptors. In the present investigation, the coupling of various receptors to phospholipase C was studied in the human keratinocyte line HaCaT. Inositol trisphosphate formation was determined by anion-exchange chromatography, and the release of intracellular calcium was analysed with the fluorescence probe Fura-2 AM. Activation of HaCaT keratinocytes with bradykinin resulted in a time- and dose-dependent release of inositol trisphosphate and intracellular calcium, with an EC50 value of 50 nM for bradykinin-induced inositol trisphosphate formation. The mediators and cytokines IL-1, IL-4, IL-6, IL-8, EGF and TGF alpha, as well as bombesin, prolactin, carbachol, substance P and retinoic acid, did not activate this pathway. The inability of the mediators examined to activate phospholipase C may be due to lack of the respective cognate receptors or to the use of other signal transduction pathways.

Differential induction of phosphatidylcholine hydrolysis, diacylglycerol formation and protein kinase C activation by epidermal growth factor and transforming growth factor-alpha in normal human skin fibroblasts and keratinocytes

We have investigated coupling between the epidermal growth factor (EGF) receptor and the phospholipase C (PLC)/protein kinase C (PKC) signal-transduction system in normal skin fibroblasts and keratinocytes, for which EGF and transforming growth factor alpha (TGF-alpha) are mitogenic. EGF and TGF-alpha induced a rapid increase in tyrosine phosphorylation of the EGF receptor, in both fibroblasts and keratinocytes, but failed to induce tyrosine phosphorylation of PLC-gamma 1 or detectable phosphoinositide hydrolysis, as measured by two sensitive assays. In fibroblasts, EGF induced phosphatidylcholine (PC) hydrolysis, resulting in increased diacylglycerol (DAG). In contrast, in keratinocytes, there was no detectable PC hydrolysis or elevation of DAG in response to EGF or TGF-alpha. EGF and TGF-alpha activated PKC in fibroblasts, as evidenced by increased phosphorylation of a specific cellular PKC substrate (myristoylated alanine-rich C-kinase substrate, 'MARCKS'). In keratinocytes, TGF-alpha and EGF induced only a modest increase in MARCKS protein phosphorylation. This apparent modest activation of PKC, in the absence of detectable DAG formation, may have been mediated by arachidonic acid, which was released from keratinocytes in response to TGF-alpha, and has been shown to stimulate PKC activity in vitro. These data demonstrate that (1) in dermal fibroblasts and keratinocytes, which express normal levels of EGF receptors, EGF receptor activation is not coupled to tyrosine phosphorylation of PLC-gamma 1 or PtdIns hydrolysis, suggesting that these events are not required for the mitogenic activity of EGF or TGF-alpha in these cells, (2) coupling of EGF receptor to PC hydrolysis is cell-type specific, and (3) in skin fibroblasts, DAG, formed through EGF-induced PC hydrolysis, is capable of activating PKC.

Substance P induces intracellular calcium increase and translocation of protein kinase C in epidermis

Substance P is a neuropeptide present in, and released from, peripheral C nerve endings. The presence of substance P-positive nerve fibres in the epidermis has been reported. We investigated the effect of substance P on the transmembrane signalling system of pig epidermal keratinocytes. Treatment of pig epidermis with substance P resulted in an increase in inositol 1,4,5-trisphosphate (IP3), and in intracellular free calcium. The treatment also resulted in translocation of protein kinase C from a cytosol to a membrane fraction. Substance P, however, did not affect the beta-adrenergic- or histamine (H2)- adenylate cyclase responses of the epidermis. Neither forskolin-induced, nor cholera toxin-induced cyclic AMP accumulation were affected by substance P treatment. These results consistent with the view that substance P stimulates phosphatidylinositol-4,5-bisphosphate (PIP2) hydrolysis of keratinocytes, resulting in IP3-Ca2+ and diacylglycerol-protein kinase C signal activation. Although protein kinase C is known to affect the epidermal adenylate cyclase system, no evidence for such 'cross-talk regulation' was detected in keratinocytes by substance P treatment.

Signal transduction pathways in keratinocytes

Mammalian cells do not live as isolated organisms, but are instead organized into complex, highly specialized tissue organs composed of a homogeneous or a mixed cell population. In order to maintain tissue homeostasis in physiological and pathophysiological conditions, intercellular communication is an absolute requirement. This review will summarize our current knowledge as to how an extracellular signal is transduced via a specific receptor to the interior of the cell and how this signal will induce special cell functions. Attention will be paid to the major signal transduction pathways known to be active in keratinocytes, namely the adenylate cyclase, guanylate cyclase, tyrosine kinase, and phospholipase C systems. Finally, examples will be given of how interactions between these signal transduction pathways can take place and how 'signal cross-talk' might regulate keratinocyte function.

Aluminum fluoride stimulates inositol phosphate metabolism and inhibits expression of differentiation markers in mouse keratinocytes

Mouse keratinocytes are induced to differentiate in vitro by elevating the level of extracellular calcium from 0.05 mM, where keratinocytes express a basal cell phenotype, to greater than 0.10 mM, where they express the differentiated phenotype. This process has been associated with a rapid, sustained increase in inositol phosphate (InsP) turnover, which precedes the expression of differentiation-specific proteins. In 0.05 mM Ca2+ medium, aluminum and fluoride salts (AIF4-), which combine to activate nonspecifically heterotrimeric guanine nucleotide-binding (G) proteins, cause a concentration-dependent increase in InsP metabolism in keratinocytes, and generate elevated intracellular diacylglycerol levels. This is associated with an inhibition of cell growth. Treatment with both AIF4- and Ca2+ greater than 0.10 mM resulted in an additive increase in InsP turnover, implying the presence of at least two responsive InsP pools. AIF4- inhibited the expression of differentiation markers induced by Ca2+ greater than 0.10 mM and altered the morphology of keratinocytes from squamous to dendritic, which was reversible upon withdrawal of AIF4-. Neoplastic keratinocytes, in which basal levels of InsP metabolism are higher than in normal cells, do not differentiate in response to Ca2+. Neoplastic keratinocytes responded to AIF-4 treatment with an even greater rise in InsP metabolism. AIF-4 also inhibited cell growth and reversibly altered morphology in neoplastic keratinocytes. These data suggest that InsP metabolism in keratinocytes is at least partially regulated by a G protein mechanism. Furthermore, an increase in InsP metabolism is not sufficient to stimulate differentiation and may be inhibitory to differentiation if exceeding limited increases. However, these observations cannot exclude the possibility that other AIF-4 stimulated pathways involving G or non-G proteins can also influence keratinocyte biology.

Desensitization of the epidermal adenylate cyclase system: agonists and phorbol esters desensitize by independent mechanisms

Exposure of pig epidermis to adenylate cyclase stimulators results in receptor-specific desensitization. We investigated the nature of the agonist-induced desensitization, which was compared with the phorbol ester-induced, receptor-nonspecific desensitization. Both phorbol ester-induced desensitization and the agonist-induced desensitization were accompanied by an increase in forskolin- and cholera toxin-induced cyclic AMP accumulations. The magnitude of the increase in the agonist-induced desensitization was parallel to the degree of the initial cyclic AMP accumulation; histamine and adenosine, which increase more cyclic AMP than epinephrine, resulted in a more marked increase in forskolin- and cholera toxin-induced cyclic AMP accumulations. Similarly, epidermis desensitized to multiple receptors revealed more marked forskolin- and cholera toxin-induced cyclic AMP accumulations than epidermis desensitized to a single receptor. In contrast to the phorbol ester-induced desensitization, agonist-induced desensitization was not affected by the protein kinase C inhibitors H-7 and staurosporin. Further, agonist-induced desensitization was still inducible in phorbol ester-desensitized epidermis and vice versa. In contrast to the agonist-induced desensitization, which is accompanied by the preceding adenylate cyclase stimulation, no evidence for the stimulation of the adenylate cyclase during phorbol ester treatment was obtained. Neither agonist-induced desensitization nor phorbol ester-induced desensitization affected the content of inhibitory guanine nucleotide binding protein of the epidermis, which was monitored by the pertussis toxin (IAP)-catalyzed ADP ribosylation reaction. Our results indicate that agonist-induced desensitization and the phorbol ester-induced desensitization are independent of each other. Although both processes are characterized by increased forskolin- and toxin-induced cyclic AMP accumulations, the former is accompanied by initial cyclic AMP accumulation; the latter is not.

Cellular, immunologic and biochemical characterization of topical retinoic acid-treated human skin

Histologic and clinical improvement of sun-exposed skin following topical treatment with retinoic acid has been reported. Daily application of retinoic acid typically results within 2-5 d in an erythematous scaling reaction, which lessens with continued usage. The cellular, immunologic, and biochemical basis of this retinoid reaction and its role in the repair of photodamaged skin are not known. To investigate the retinoid reaction in man, we have treated non-sun-exposed skin with 0.1% retinoic acid cream (Retin-A, Ortho Pharmaceutical Corporation, Raritan, NJ) under occlusion for 4 d to induce erythema and then examined changes in 1) histology, 2) expression of cell-surface molecules, 3) the enzymes and second messengers of the phospholipase C/protein kinase C signal-transduction system, 4) levels of eicosanoids, and 5) levels of interleukin-1 protein and mRNA. These parameters were chosen for measurement both because they are indicators of epidermal function and previous studies suggest they may be responsive to retinoic acid treatment. Epidermal cell growth as judged by increased epidermal thickness and mitotic figures was significantly increased in retinoic acid-treated skin compared to vehicle-treated controls. Increased numbers of CD4+ T cells accompanied by prominence of dermal dendrocytes in the papillary dermis and focal keratinocyte expression of intercellular adhesion molecule-1 were observed in retinoic acid-treated biopsies. Phosphoinositide-specific phospholipase C activity and 1,2-diacylglycerol content were also elevated in retinoic acid-treated epidermis. Protein kinase C activity was reduced by one third in both the soluble and membrane fraction, suggesting down-regulation. Surprisingly, in view of the inflammatory nature of the retinoid reaction, no increases were observed in arachidonic acid, its metabolites, interleukin-1 alpha, or interleukin-1 beta. To examine the specificity of the retinoid reaction, subjects were treated with the irritant sodium lauryl sulfate, under conditions that resulted in a reaction clinically similar to that observed with retinoic acid. The histologic alterations induced by sodium lauryl sulfate were found to be indistinguishable from those induced by retinoic acid. These data indicate that, although a wide range of cellular and molecular alterations occur in retinoic acid-treated skin, these changes may not be necessarily specific or unique for retinoic acid.

Effects of cyclosporine on immunologic mechanisms in psoriasis

A major impetus for further investigation of cellular immunologic mechanisms in psoriasis has been the discovery that cyclosporine, a potent immunosuppressive, is highly effective in the treatment of psoriasis. Cyclosporine has significant inhibitory effects on the ability of T cells to become activated. However, a direct activity of this drug on human keratinocyte signal transduction or growth has been difficult to demonstrate at relevant concentrations. Nevertheless, treatment of psoriasis or of 12-O-tetradecanoyl phorbol-13-acetate-treated murine skin with cyclosporine does reverse many epidermal abnormalities that are common to these two systems. This suggests that the compound exerts an indirect effect on epidermal keratinocytes in vivo, perhaps through immunocyte inhibition. During treatment of psoriasis patients, cyclosporine therapy resulted in selective changes in the numbers and functions of certain antigen-presenting cell subsets (which were distinct from Langerhans cells) and T-cell subsets. These changes were accompanied by indirect evidence of decreased T-cell lymphokine release. Lesional activity of cyclosporine-treated psoriasis patients was closely correlated with the degree of T-cell activation caused by antigen-presenting cells. Cyclosporine inhibition of lymphokine or cytokine release may result in decreased recruitment of non-Langerhans antigen-presenting cells into the epidermis and thus decreased immunoreactivity in the lesion.

Bradykinin induces phosphoinositide turnover, 1,2-diglyceride formation, and growth in cultured adult human keratinocytes

The effects of bradykinin on activation of phosphoinositide turnover, 1,2-diglyceride formation, and growth of cultured adult human keratinocytes were investigated. Keratinocytes specifically bound [3H]bradykinin with high affinity (kd = 3.4 nM) and displayed 1.5 X 10(5) binding sites/cell. Bradykinin caused a rapid dose-dependent increase in inositol trisphosphate (IP3) inositol bisphosphate, and inositol monophosphate. IP3 was maximally increased (fivefold) at 30 s and remained elevated for at least 10 min. Half maximal stimulation of IP3 formation was observed at 27 nM bradykinin. IP3 accumulation was equally elevated by bradykinin and lys-bradykinin but was not stimulated by des-Arg9-bradykinin, indicating that phospholipase C in cultured keratinocytes is coupled to B2 bradykinin receptors. Treatment of keratinocytes with active phorbol ester (TPA) caused a significant inhibition (50%) of bradykinin-induced IP3 accumulation, suggesting negative regulation of phospholipase C by protein kinase C. Bradykinin also caused a significant elevation in 1,2-diacylglycerol (DAG) content. DAG content was maximally elevated (twofold) at 1 min and remained elevated for at least 10 min. Bradykinin also caused a significant (twofold, p less than 0.02) increase in keratinocyte growth. These data demonstrate that bradykinin is a potent agonist of the phospholipase C/protein kinase C signal transduction system in cultured adult human keratinocytes and that activation of this pathway by bradykinin is associated with increased keratinocyte growth.

Increased phospholipase C-catalyzed hydrolysis of phosphatidylinositol-4,5-bisphosphate and 1,2-sn-diacylglycerol content in psoriatic involved compared to uninvolved and normal epidermis

Evidence suggests that the phospholipase C/protein kinase C signal transduction system participates in the regulation of epidermal cell growth and differentiation. Psoriatic epidermis is characterized by hyperproliferation, defective differentiation, and inflammation. In this report, we have determined the activity of phospholipase C-catalyzed hydrolysis of phosphatidylinositol-4,5-bisphosphate (PIP2) and 1,2-diacylglycerol content in normal and psoriatic involved and uninvolved epidermis. 1,2-diacylglycerol is formed from phospholipase C-catalyzed hydrolysis of PIP2 and is the physiologic activator of protein kinase C. PIP2 hydrolysis was assayed in soluble and particulate fractions prepared from keratome biopsies of normal and psoriatic skin. Total lipids were extracted from normal and psoriatic epidermis and 1,2-diradylglycerol (a mixture of 1,2-diacylglycerol and 1-ether, 2-acyl-glycerol) quantitated by enzyme assay. Because 1,2-diacylglycerol is a more potent activator of protein kinase C, the relative proportions of 1,2-diacyl and 1-ether, 2-acylglycerol in uninvolved and involved psoriatic epidermis were determined. This was accomplished by separation of acetate derivatives of 1,2-diacylglycerol and 1-ether, 2-acyl-glycerol by thin layer chromatography. Soluble and membrane-associated phospholipase C-catalyzed PIP2 hydrolysis were increased 3.7 times (p less than 0.001) and 3 times (p less than 0.004), respectively, in psoriatic involved compared to uninvolved and normal epidermis. 1,2-diradylglycerol content was also significantly elevated (3 times, p less than 0.01) in psoriatic involved versus uninvolved and normal epidermis. Analysis of the acetate derivatives of 1,2-diradylglycerol in psoriatic uninvolved and involved epidermis revealed that 1,2-diacylglycerol was the major species (86% and 95%, respectively). There were no significant differences in either phospholipase C-catalyzed PIP2 hydrolysis or 1,2-diacylglycerol content between uninvolved and normal epidermis. 1,2-diacylglycerol purified from normal and involved psoriatic epidermis was capable of activating protein kinase C from normal epidermis in vitro. In epidermal slices, activation of protein kinase C by addition of 12-0-tetradecanoylphorbol-13-acetate and 1,2-diacylglycerol (1,2-dioctanoylglycerol) resulted in subsequently decreased protein kinase C activity, a process termed down-regulation. These data are consistent with the possibility that the elevation in lesional 1,2-diacylglycerol content may account, in part, for the previously reported reduction of protein kinase C activity in psoriasis (Horn, Marks, Fisher, et al: J Invest Dermatol 88:220-222, 1987).