Effect of protein kinase C on transmembrane calcium fluxes in HaCaT keratinocytes

Generation and Application of a Reporter Cell Line for the Quantitative Screen of Extracellular Vesicle Release

Extracellular vesicles (EVs) are identified as mediators of intercellular communication and cellular regulation. In the immune system, EVs play a role in antigen presentation as a part of cellular communication. To enable drug discovery and characterization of compounds that affect EV biogenesis, function, and release in immune cells, we developed and characterized a reporter cell line that allows the quantitation of EVs shed into culture media in phenotypic high-throughput screen (HTS) format. Tetraspanins CD63 and CD9 were previously reported to be enriched in EVs; hence, a construct with dual reporters consisting of CD63-Turbo-luciferase (Tluc) and CD9-Emerald green fluorescent protein (EmGFP) was engineered. This construct was transduced into the human monocytic leukemia cell line, THP-1. Cells expressing the highest EmGFP were sorted by flow cytometry as single cell, and clonal pools were expanded under antibiotic selection pressure. After four passages, the green fluorescence dimmed, and EV biogenesis was then tracked by luciferase activity in culture supernatants. The Tluc activities of EVs shed from CD63Tluc-CD9EmGFP reporter cells in the culture supernatant positively correlated with the concentrations of released EVs measured by nanoparticle tracking analysis. To examine the potential for use in HTS, we first miniaturized the assay into a robotic 384-well plate format. A 2210 commercial compound library (Maybridge) was then screened twice on separate days, for the induction of extracellular luciferase activity. The screening data showed high reproducibility on days 1 and 2 (78.6%), a wide signal window, and an excellent Z′ factor (average of 2-day screen, 0.54). One hundred eighty-seven compounds showed a response ratio that was 3SD above the negative controls in both day 1 and 2 screens and were considered as hit candidates (approximately 10%). Twenty-two out of 40 re-tested compounds were validated. These results indicate that the performance of CD63Tluc-CD9EmGFP reporter cells is reliable, reproducible, robust, and feasible for HTS of compounds that regulate EV release by the immune cells.

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.

Identifying disease-specific genes based on their topological significance in protein networks

BACKGROUND

The identification of key target nodes within complex molecular networks remains a common objective in scientific research. The results of pathway analyses are usually sets of fairly complex networks or functional processes that are deemed relevant to the condition represented by the molecular profile. To be useful in a research or clinical laboratory, the results need to be translated to the level of testable hypotheses about individual genes and proteins within the condition of interest.

RESULTS

In this paper we describe novel computational methodology capable of predicting key regulatory genes and proteins in disease- and condition-specific biological networks. The algorithm builds shortest path network connecting condition-specific genes (e.g. differentially expressed genes) using global database of protein interactions from MetaCore. We evaluate the number of all paths traversing each node in the shortest path network in relation to the total number of paths going via the same node in the global network. Using these numbers and the relative size of the initial data set, we determine the statistical significance of the network connectivity provided through each node. We applied this method to gene expression data from psoriasis patients and identified many confirmed biological targets of psoriasis and suggested several new targets. Using predicted regulatory nodes we were able to reconstruct disease pathways that are in excellent agreement with the current knowledge on the pathogenesis of psoriasis.

CONCLUSION

The systematic and automated approach described in this paper is readily applicable to uncovering high-quality therapeutic targets, and holds great promise for developing network-based combinational treatment strategies for a wide range of diseases.

Protein kinase C isoenzymes differentially regulate the differentiation-dependent expression of adhesion molecules in human epidermal keratinocytes

Epidermal expression of adhesion molecules such as desmogleins (Dsg) and cadherins is strongly affected by the differentiation status of keratinocytes. We have previously shown that certain protein kinase C (PKC) isoforms differentially alter the growth and differentiation of human epidermal HaCaT keratinocytes. In this paper, using recombinant overexpression and RNA interference, we define the specific roles of the different PKC isoenzymes in modulation of expression of adhesion molecules in HaCaT keratinocytes. The level of Dsg1, a marker of differentiating keratinocytes, was antagonistically regulated by two Ca-independent 'novel' nPKC isoforms; i.e. it increased by the differentiation-promoting nPKCdelta and decreased by the growth-promoting nPKCepsilon. The expression of Dsg3 (highly expressed in proliferating epidermal layers) was conversely regulated by these isoenzymes, and was also inhibited by the differentiation inducer Ca-dependent 'conventional' cPKCalpha. Finally, the expression of P-cadherin (a marker of proliferating keratinocytes) was regulated by all of the examined PKCs, also in an antagonistic manner (inhibited by cPKCalpha/nPKCdelta and stimulated by cPKCbeta/nPKCepsilon). Collectively, the presented results strongly argue for the marked, differential, and in some instances antagonistic roles of individual Ca-dependent and Ca-independent PKC isoforms in the regulation of expression of adhesion molecules of desmosomes and adherent junctions in human epidermal keratinocytes.

Altered calcium handling following the recombinant overexpression of protein kinase C isoforms in HaCaT cells

Both changes in intracellular calcium concentration ([Ca(2+)](i)) and activation of certain protein kinase C (PKC) isoforms play a crucial role in keratinocyte functions. To better understand the interaction between these two signalling pathways we investigated the resting [Ca(2+)](i) and the extracellular ATP-induced changes in [Ca(2+)](i) on HaCaT cell clones overexpressing either the classical alpha or the beta PKC isoform. These PKC isoenzymes were previously shown to decrease (alpha) or increase (beta) cell proliferation and augment (alpha) or suppress (beta) cell differentiation. Keratinocyte clones with decreased proliferation rate were found to have unaltered resting [Ca(2+)](i), but responded with greater calcium transients to the application of 180 mum of ATP. In contrast, clones with increased proliferation rate had elevated resting [Ca(2+)](i) and suppressed calcium responses to ATP. Calcium transients on PKCbeta clones displayed a faster falling phase. Each clone had a distinct purinergic receptor expression pattern, some of which paralleled the altered proliferation rate and calcium handling. Keratinocytes overexpressing PKCbeta revealed decreased P2X1 and increased P2Y1 receptor expression as compared with the control or PKCalpha clones. The expression level of P2X7 was significantly increased in keratinocytes overexpressing PKCalpha. On the other hand neither the P2X2 nor the P2Y2 expression was altered significantly in the cell types investigated. These data indicate that a modified proliferation and differentiation pattern is associated with altered calcium handling in keratinocytes. The observations also suggest that different PKC isoenzymes have different effects on the phosphatidyl-inositol signalling pathway.

Functional genomics of calcium channels in human melanoma cells

Ca(2+)-signaling of human melanoma is in the focus of intensive research since the identification of the role of WNT-signaling in melanomagenesis. Genomic and functional studies pointed to the important role of various Ca(2+) channels in melanoma, but these data were contradictory. In the present study we clearly demonstrate, in a number of different ways including microarray analysis, DNA sequencing and immunocytochemistry, that various human melanoma cell lines and melanoma tissues overexpress ryanodine receptor type 2 (RyR2) and express P2X(7) channel proteins as compared to melanocytes. These channels, although retain some of their usual characteristics and pharmacological properties, display unique features in melanoma cells, including a functional interaction between the two molecules. Unlike P2X(7), RyR2 does not function as a calcium channel. On the other hand, the P2X(7) receptor has an antiapoptotic function in melanoma cells, since ATP-activation suppresses induced apoptosis, while knock down of the gene expression significantly enhances that.

Hypotonic stress influence the membrane potential and alter the proliferation of keratinocytes in vitro

Keratinocyte proliferation and differentiation is strongly influenced by mechanical forces. We investigated the effect of osmotic changes in the development of HaCaT cells in culture using intracellular calcium measurements, electrophysiological recordings and molecular biology techniques. The application of hypotonic stress (174 mOsmol/l) caused a sustained hyperpolarization of HaCaT cells from a resting potential of -27 +/- 4 to -51 +/- 9 mV. This change was partially reversible. The surface membrane channels involved in the hyperpolarization were identified as chloride channels due to the lack of response in the absence of the anion. Cells responded with an elevation of intracellular calcium concentration to hypotonic stress, which critically depended on external calcium. The presence of phorbol-12-myristate-13-acetate in the culture medium for 12 h augmented the subsequent response to hypotonic stress. A sudden switch from iso- to hypotonic solution increased cell proliferation and suppressed the production of involucrin, filaggrin and transglutaminase, markers of keratinocyte differentiation. It is concluded that sudden mechanical forces increase the proliferation of keratinocytes through alterations in their membrane potential and intracellular calcium concentration. These changes together with additional modifications in channel expression and intracellular signalling mechanisms could underlie the increased proliferation of keratinocytes in hyperproliferative skin diseases.

TRPC7 is a receptor-operated DAG-activated channel in human keratinocytes

Muscarinic and purinergic receptors expressed in keratinocytes are an important part of a functional system for cell growth. While several aspects of this process are clearly dependent on Ca(2+) homeostasis, less is known about the mechanisms controlling Ca(2+) entry during epidermal receptor stimulation. We used patch-clamp technique to study responses to carbachol (CCh) and adenosine triphosphate (ATP) in HaCaT human keratinocytes. Both agonists induced large currents mediated by cation-selective channels about three times more permeable to Ca(2+) than Na(+), suggesting that they play an important role in receptor-operated Ca(2+) entry. CCh- and ATP-induced currents were inhibited by 1-[6-([(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino)hexyl]-1H-pyrrole-2,5-dione, a phospholipase C (PLC) blocker. Investigation of the pathways downstream of PLC activation revealed that InsP(3) did not affect the agonist responses. In contrast, 1-oleoyl-2-acetyl-sn-glycerol (OAG), a membrane-permeable analog of 1,2-diacylglycerol (DAG), evoked a similar cation current. This action appears to be direct, since the effects of activators or inhibitors of protein kinase C were comparatively small. Finally, transient receptor potential canonical 7 (TRPC7) specific knockdown by antisense oligonucleotides led to a decrease in ATP- and CCh-induced calcium entry, as well as OAG-evoked current. We concluded that activation of both muscarinic and purinergic receptors via a common DAG-dependent link opens Ca(2+)-permeable TRPC7 channels.

EGF stimulates proliferation of mouse embryonic stem cells: involvement of Ca2+influx and p44/42 MAPKs

We examined the effect of EGF on the proliferation of mouse embryonic stem (ES) cells and their related signal pathways. EGF increased [3H]thymidine and 5-bromo-2′-deoxyuridine incorporation in a time- and dose-dependent manner. EGF stimulated the phosphorylation of EGF receptor (EGFR). Inhibition of EGFR tyrosine kinase with AG-1478 or herbimycin A, inhibition of PLC with neomycin or U-73122, inhibition of PKC with bisindolylmaleimide I or staurosporine, and inhibition of L-type Ca2+channels with nifedipine or methoxyverapamil prevented EGF-induced [3H]thymidine incorporation. PKC-α, -βI, -γ, -δ, and -ζ were translocated to the membrane and intracellular Ca2+concentration ([Ca2+]i) was increased in response to EGF. Moreover, inhibition of EGFR tyrosine kinase, PLC, and PKC completely prevented EGF-induced increases in [Ca2+]i. EGF also increased inositol phosphate levels, which were blocked by EGFR tyrosine kinase inhibitors. Furthermore, EGF rapidly increased formation of H2O2, and pretreatment with antioxidant ( N-acetyl-l-cysteine) inhibited EGF-induced increase of [Ca2+]i. In addition, we observed that p44/42 MAPK phosphorylation by EGF and inhibition of EGFR tyrosine kinase, PLC, PKC, or Ca2+channels blocked EGF-induced phosphorylation of p44/42 MAPKs. Inhibition of p44/42 MAPKs with PD-98059 (MEK inhibitor) attenuated EGF-induced increase of [3H]thymidine incorporation. Finally, inhibition of EGFR tyrosine kinase, PKC, Ca2+channels, or p44/42 MAPKs attenuated EGF-stimulated cyclin D1, cyclin E, cyclin-dependent kinase (CDK)2, and CDK4, respectively. In conclusion, EGF partially stimulates proliferation of mouse ES cells via PLC/PKC, Ca2+influx, and p44/42 MAPK signal pathways through EGFR tyrosine kinase phosphorylation.

Effect of protein kinase C activation on intracellular Ca2+ signaling and integrity of intestinal epithelial cells

Protein kinase C (PKC) activation and increases in cytosolic Ca(2+) cause intestinal injury. Since PKC activation can alter Ca(2+) homeostasis and increase Ca(2+) levels, we examined the effects of PKC activation on intestinal cellular integrity and the role of Ca(2+) signaling in this response. The epithelial cell line, IEC-18 was incubated with the PKC activator phorbol myristate acetate (PMA; 0.1-1.0 microM). In some experiments, cells were incubated in Ca(2+)-free medium. PMA treatment produced a concentration-dependent increase in cell injury and PKC activity. This response was attenuated by addition of the pan-specific PKC inhibitor, GF 109203X. Furthermore, cell viability was maintained in cells preincubated with PKC isoform-specific inhibitors to PKCalpha, PKCdelta and PKCepsilon. Cell injury was also reduced if cells were incubated in Ca(2+)-free medium or in the presence of the Ca(2+) channel antagonist, verapamil or the intracellular chelator BAPTA-AM. PMA, but not the inactive phorbol ester, 4alphaPMA, induced a dose-dependent increase in cellular Ca(2+) that was characterized by a rapid, transient spike followed by a tonic plateau phase which approximated control levels. These responses were eliminated by the addition of BAPTA-AM. Furthermore the increase in the Ca(2+) spike was reduced or eliminated by co-incubation with the PKCdelta antagonist, rottlerin. Inhibition of PKCalpha or PKCepsilon was less effective or ineffective in this regard. These data suggest that PKC activation via PMA challenge affects the integrity of rat intestinal epithelial cells. PKCdelta, but not PKCepsilon or PKCalpha activation appears to mediate this effect via an increase in cellular Ca(2+).

Ca(2+)- and volume-sensitive chloride currents are differentially regulated by agonists and store-operated Ca2+ entry

Using patch-clamp and calcium imaging techniques, we characterized the effects of ATP and histamine on human keratinocytes. In the HaCaT cell line, both receptor agonists induced a transient elevation of [Ca²⁺]_i in a Ca²⁺-free medium followed by a secondary [Ca²⁺]_i rise upon Ca²⁺ readmission due to store-operated calcium entry (SOCE). In voltage-clamped cells, agonists activated two kinetically distinct currents, which showed differing voltage dependences and were identified as Ca²⁺-activated (I_Cl(Ca)) and volume-regulated (I_{Cl, swell}) chloride currents. NPPB and DIDS more efficiently inhibited I_Cl(Ca) and I_{Cl, swell}, respectively. Cell swelling caused by hypotonic solution invariably activated I_{Cl, swell} while regulatory volume decrease occurred in intact cells, as was found in flow cytometry experiments. The PLC inhibitor U-73122 blocked both agonist- and cell swelling–induced I_{Cl, swell}, while its inactive analogue U-73343 had no effect. I_Cl(Ca) could be activated by cytoplasmic calcium increase due to thapsigargin (TG)-induced SOCE as well as by buffering [Ca²⁺]_i in the pipette solution at 500 nM. In contrast, I_{Cl, swell} could be directly activated by 1-oleoyl-2-acetyl-sn-glycerol (OAG), a cell-permeable DAG analogue, but neither by InsP₃ infusion nor by the cytoplasmic calcium increase. PKC also had no role in its regulation. Agonists, OAG, and cell swelling induced I_{Cl, swell} in a nonadditive manner, suggesting their convergence on a common pathway. I_{Cl, swell} and I_Cl(Ca) showed only a limited overlap (i.e., simultaneous activation), although various maneuvers were able to induce these currents sequentially in the same cell. TG-induced SOCE strongly potentiated I_Cl(Ca), but abolished I_{Cl, swell}, thereby providing a clue for this paradox. Thus, we have established for the first time using a keratinocyte model that I_{Cl, swell} can be physiologically activated under isotonic conditions by receptors coupled to the phosphoinositide pathway. These results also suggest a novel function for SOCE, which can operate as a “selection” switch between closely localized channels.

S100C/A11 is a key mediator of Ca(2+)-induced growth inhibition of human epidermal keratinocytes

An increase in extracellular Ca2+ induces growth arrest and differentiation of human keratinocytes in culture. We examined possible involvement of S100C/A11 in this growth regulation. On exposure of the cells to high Ca2+, S100C/A11 was specifically phosphorylated at 10Thr and 94Ser. Phosphorylation facilitated the binding of S100C/A11 to nucleolin, resulting in nuclear translocation of S100C/A11. In nuclei, S100C/A11 liberated Sp1/3 from nucleolin. The resulting free Sp1/3 transcriptionally activated p21CIP1/WAF1, a representative negative regulator of cell growth. Introduction of anti-S100C/A11 antibody into the cells largely abolished the growth inhibition induced by Ca2+ and the induction of p21CIP1/WAF1. In the human epidermis, S100C/A11 was detected in nuclei of differentiating cells in the suprabasal layers, but not in nuclei of proliferating cells in the basal layer. These results indicate that S100C/A11 is a key mediator of the Ca(2+)-induced growth inhibition of human keratinocytes in culture, and that it may be possibly involved in the growth regulation in vivo as well.

Store-operated Ca2+ channels in prostate cancer epithelial cells: function, regulation, and role in carcinogenesis

Ca2+ homeostasis mechanisms, in which the Ca2+ entry pathways play a key role, are critically involved in both normal function and cancerous transformation of prostate epithelial cells. Here, using the lymph node carcinoma of the prostate (LNCaP) cell line as a major experimental model, we characterize prostate-specific store-operated Ca2+ channels (SOCs)--a primary Ca2+ entry pathway for non-excitable cells--for the first time. We show that prostate-specific SOCs share major store-dependent, kinetic, permeation, inwardly rectifying, and pharmacological (including dual, potentiation/inhibition concentration-dependent sensitivity to 2-APB) properties with "classical" Ca2+ release-activated Ca2+ channels (CRAC), but have a higher single channel conductance (3.2 and 12pS in Ca2+- and Na+-permeable modes, respectively). They are subject to feedback inhibition via Ca2+-dependent PKC, CaMK-II and CaM regulatory pathways and are functionally dependent on caveolae integrity. Caveolae also provide a scaffold for spatial co-localization of SOCs with volume-regulated anion channels (VRAC) and their Ca2+-mediated interaction. The TRPC1 and TRPV6 members of the transient receptor potential (TRP) channel family are the most likely molecular candidates for the formation of prostate-specific endogenous SOCs. Differentiation of LNCaP cells to an androgen-insensitive, apoptotic-resistant neuroendocrine phenotype downregulates SOC current. We conclude that prostate-specific SOCs are important determinants in the transition to androgen-independent prostate cancer.

Calcium signalling is altered in myeloid cells with a deficiency in NADPH oxidase activity

The relation of O(2.-)-production and Ca2+ homeostasis was investigated in PLB-985 cell lines and neutrophilic granulocytes from peripheral blood. In differentiated wild-type PLB-985 cells, a high level of O(2.-)-production was associated with a significant decrease in the membrane potential and the inhibition of capacitative Ca2+ entry. These correlations were not observed in gp91phox -/- cells or in cells transfected with a non-functional mutant of gp91phox (Thr341Lys). Membrane depolarization and inhibition of Ca2+ entry reappeared in cells transfected with wild-type gp91phox. These experiments demonstrate that inhibition of Ca2+ entry depends on the presence of a functional NADPH oxidase. The Ca2+ signal induced by stimulation of chemotactic receptors also showed remarkable differences: [Ca2+]ic in the sustained phase was higher in gp91phox-/- than in wild-type cells. Alteration of the Ca2+ signal was reproduced by treating peripheral blood neutrophils with the NADPH oxidase inhibitor diphenylene-iodonium. It is concluded that the deficiency in O(2.-)-production is accompanied by significant alterations of Ca2+ homeostasis in myeloid cells.