MEK6 regulates human involucrin gene expression via a p38alpha - and p38delta -dependent mechanism

A novel MKK gene (EcMKK6) in Epinephelus coioides: Identification, characterization and its response to Vibrio alginolyticus and SGIV infection

Mitogen-activated protein kinase 6 (MKK6) is one of the major important central regulatory proteins response to environmental and physiological stimuli. In this study, a novel MKK6, EcMKK6, was isolated from Epinephelus coioides, an economically important cultured fish in China and Southeast Asian counties. The open reading frame (ORF) of EcMKK6 is 1077 bp encoding 358 amino acids. EcMKK6 contains a serine/threonine protein kinase (S_TKc) domain, a tyrosine kinase catalytic domain, a conserved dual phosphorylation site in the SVAKT motif and a conserved DVD domain. By in situ hybridization (ISH) with Digoxigenin-labeled probe, EcMKK6 mainly located at the cytoplasm of cells, and a little appears in the nucleus. EcMKK6 mRNA can be detected in all eleven tissues examined, but the expression level is different in these tissues. After challenge with Vibrio alginolyticus and Singapore grouper iridovirus (SGIV), the transcription level of EcMKK6 was apparently up-regulated in the tissues examined. The data demonstrated that the sequence and the characters of EcMKK6 were conserved, EcMKK6 showed tissue-specific expression profiles in healthy grouper, and the expression was significantly varied after pathogen infection, indicating that EcMKK6 may play important roles in E. coioides during pathogen-caused inflammation.

A genetic variation associated with plasma erythropoietin and a non-coding transcript of PRKAR1A in sickle cell disease

Blood erythropoietin (EPO) increases primarily to hypoxia. In sickle cell anaemia (homozygous HBBE6V; HbSS), plasma EPO is elevated due to hemolytic anaemia-related hypoxia. Hydroxyurea treatment reduces haemolysis and anaemia by increasing foetal haemoglobin, which leads to lower hypoxic transcriptional responses in blood mononuclear cells but paradoxically further increases EPO. To investigate this apparent hypoxia-independent EPO regulation, we assessed two sickle cell disease (SCD) cohorts for genetic associations with plasma EPO, by prioritizing 237,079 quantitative trait loci for expression level and/or transcript isoform variations of 12,727 genes derived from SCD blood mononuclear cells. We found an association between the T allele of SNP rs60684937 and increased plasma EPO (n = 567, combined P = 5.5 × 10 − 8 adjusted for haemoglobin and hydroxyurea) and validated it in independent SCD patients (n = 183, P = 0.018). The T allele of rs60684937 was associated with a relatively increased expression of a non-coding transcript of PRKAR1A (cAMP-dependent protein kinase type I-alpha regulatory subunit) in 58 SCD patients (P = 7.9 × 10 − 7) and 58 HapMap Yoruba samples (P = 0.0011). In conclusion, we demonstrate that plasma EPO elevation with hydroxyurea in SCD is independent of hypoxic responses and that genetic variation at SNP rs60684937 may contribute to EPO regulation through a cAMP-dependent protein kinase A pathway.

Promoting effect of lactoferrin on barrier function and epithelial differentiation of human keratinocytes

The purpose of this study was to elucidate the effects of bovine lactoferrin on keratinocyte differentiation and barrier function. Addition of bovine lactoferrin to differentiating HaCaT human keratinocytes led to increased transepithelial electrical resistance (TER), a marker of epithelial barrier function. This elevation was followed by upregulation of two differentiation markers, involucrin and filaggrin. The expression level of sterol regulatory element-binding protein-1 was also enhanced by bovine lactoferrin. The lactoferrin-induced upregulation of involucrin and filaggrin expression were confirmed in normal human epidermal keratinocytes (NHEK). Treatment with SB203580, a p38 mitogen-activated protein kinase (MAPK) α inhibitor, impaired the upregulation of involucrin and filaggrin expression in response to lactoferrin. The elevation of p38 MAPK phosphorylation was further enhanced by lactoferrin in the initial stage of differentiation of HaCaT keratinocytes. The findings suggest that bovine lactoferrin promotes epithelial differentiation by a p38-MAPK-dependent mechanism.

Combined deletion of p38γ and p38δ reduces skin inflammation and protects from carcinogenesis

The contribution of chronic skin inflammation to the development of squamous cell carcinoma (SCC) is poorly understood. While the mitogen-activated protein kinase p38α regulates inflammatory responses and tumour development, little is known about the role of p38γ and p38δ in these processes. Here we show that combined p38γ and p38δ (p38γ/δ) deletion blocked skin tumour development in a chemically induced carcinogenesis model. p38γ/δ deletion reduced TPA-induced epidermal hyperproliferation and inflammation; it inhibited expression of proinflammatory cytokines and chemokines in keratinocytes in vitro and in whole skin in vivo, resulting in decreased neutrophil recruitment to skin. Our data indicate that p38γ/δ in keratinocytes promote carcinogenesis by enabling formation of a proinflammatory microenvironment that fosters epidermal hyperproliferation and tumourigenesis. These findings provide genetic evidence that p38γ and p38δ have essential roles in skin tumour development, and suggest that targeting inflammation through p38γ/δ offers a therapeutic strategy for SCC treatment and prevention.

p38 mitogen-activated protein kinase inhibits USP22 transcription in HeLa cells

Elevated expression of ubiquitin-specific processing enzyme 22 (USP22) was identified in multiple types of human cancers, and was correlated with tumorigenesis and progression. Despite an increase in the numbers of studies in the physiological function of USP22, little is known regarding the regulation of its expression. The 5' flanking sequence of the USP22 gene was recently characterized. In the present study, USP22 transcription was regulated by p38 mitogen-activated protein kinase (MAPK). Treatment of human cervical carcinoma (HeLa) cells with SB203580, an inhibitor of p38 MAPK, enhanced basal USP22 promoter activity and mRNA abundance. Transfection of MAPK kinase 6 (MKK6), an upstream activator of p38 MAPK, resulted in a 40% decrease in USP22 mRNA, while the dominant negative MKK6 increased the transcription level of the USP22, similar to SB203580. Dual luciferase report assays showed that mutations of the Sp1 binding site ahead of the transcription start site abolished the promoting effect of the USP22 promoter by SB203580. Cisplatin, the activator of p38 MAPK, also suppressed USP22 expression. This suppression was blocked by SB203580. In conclusion, p38 MAPK acts as an upstream negative regulator of USP22 transcription in HeLa cells.

Six1 overexpression at early stages of HPV16-mediated transformation of human keratinocytes promotes differentiation resistance and EMT

Previous studies in our laboratory discovered that SIX1 mRNA expression increased during in vitro progression of HPV16-immortalized human keratinocytes (HKc/HPV16) toward a differentiation-resistant (HKc/DR) phenotype. In this study, we explored the role of Six1 at early stages of HPV16-mediated transformation by overexpressing Six1 in HKc/HPV16. We found that Six1 overexpression in HKc/HPV16 increased cell proliferation and promoted cell migration and invasion by inducing epithelial-mesenchymal transition (EMT). Moreover, the overexpression of Six1 in HKc/HPV16 resulted in resistance to serum and calcium-induced differentiation, which is the hallmark of the HKc/DR phenotype. Activation of MAPK in HKc/HPV16 overexpressing Six1 is linked to resistance to calcium-induced differentiation. In conclusion, this study determined that Six1 overexpression resulted in differentiation resistance and promoted EMT at early stages of HPV16-mediated transformation of human keratinocytes.

p38δ MAPK: Emerging Roles of a Neglected Isoform

p38δ mitogen activated protein kinase (MAPK) is a unique stress responsive protein kinase. While the p38 MAPK family as a whole has been implicated in a wide variety of biological processes, a specific role for p38δ MAPK in cellular signalling and its contribution to both physiological and pathological conditions are presently lacking. Recent emerging evidence, however, provides some insights into specific p38δ MAPK signalling. Importantly, these studies have helped to highlight functional similarities as well as differences between p38δ MAPK and the other members of the p38 MAPK family of kinases. In this review we discuss the current understanding of the molecular mechanisms underlying p38δ MAPK activity. We outline a role for p38δ MAPK in important cellular processes such as differentiation and apoptosis as well as pathological conditions such as neurodegenerative disorders, diabetes, and inflammatory disease. Interestingly, disparate roles for p38δ MAPK in tumour development have also recently been reported. Thus, we consider evidence which characterises p38δ MAPK as both a tumour promoter and a tumour suppressor. In summary, while our knowledge of p38δ MAPK has progressed somewhat since its identification in 1997, our understanding of this particular isoform in many cellular processes still strikingly lags behind that of its counterparts.

p38δ regulates p53 to control p21Cip1 expression in human epidermal keratinocytes

PKCδ suppresses keratinocyte proliferation via a mechanism that involves increased expression of p21(Cip1). However, the signaling mechanism that mediates this regulation is not well understood. Our present studies suggest that PKCδ activates p38δ leading to increased p21(Cip1) promoter activity and p21(Cip1) mRNA/protein expression. We further show that exogenously expressed p38δ increases p21(Cip1) mRNA and protein and that p38δ knockdown or expression of dominant-negative p38 attenuates this increase. Moreover, p53 is an intermediary in this regulation, as p38δ expression increases p53 mRNA, protein, and promoter activity, and p53 knockdown attenuates the activation. We demonstrate a direct interaction of p38δ with PKCδ and MEK3 and show that exogenous agents that suppress keratinocyte proliferation activate this pathway. We confirm the importance of this regulation using a stratified epidermal equivalent model, which mimics in vivo-like keratinocyte differentiation. In this model, PKCδ or p38δ knockdown results in reduced p53 and p21(Cip1) levels and enhanced cell proliferation. We propose that PKCδ activates a MEKK1/MEK3/p38δ MAPK cascade to increase p53 levels and p53 drives p21(Cip1) gene expression.

Delayed skin wound repair in proline-rich protein tyrosine kinase 2 knockout mice

Proline-rich protein tyrosine kinase 2 (Pyk2) is a member of the focal adhesion kinase family. We used Pyk2 knockout (Pyk2-KO) mice to study the role of Pyk2 in cutaneous wound repair. We report that the rate of wound closure was delayed in Pyk2-KO compared with control mice. To examine whether impaired wound healing of Pyk2-KO mice was caused by a keratinocyte cell-autonomous defect, the capacities of primary keratinocytes from Pyk2-KO and wild-type (WT) littermates to heal scratch wounds in vitro were compared. The rate of scratch wound repair was decreased in Pyk2-KO keratinocytes compared with WT cells. Moreover, cultured human epidermal keratinocytes overexpressing the dominant-negative mutant of Pyk2 failed to heal scratch wounds. Conversely, stimulation of Pyk2-dependent signaling via WT Pyk2 overexpression induced accelerated scratch wound closure and was associated with increased expression of matrix metalloproteinase (MMP)-1, MMP-9, and MMP-10. The Pyk2-stimulated increase in the rate of scratch wound repair was abolished by coexpression of the dominant-negative mutant of PKCδ and by GM-6001, a broad-spectrum inhibitor of MMP activity. These results suggest that Pyk2 is essential for skin wound reepithelialization in vivo and in vitro and that it regulates epidermal keratinocyte migration via a pathway that requires PKCδ and MMP functions.

Disruption of prostate epithelial differentiation pathways and prostate cancer development

One of the foremost problems in the prostate cancer (PCa) field is the inability to distinguish aggressive from indolent disease, which leads to difficult prognoses and thousands of unnecessary surgeries. This limitation stems from the fact that the mechanisms of tumorigenesis in the prostate are poorly understood. Some genetic alterations are commonly reported in prostate tumors, including upregulation of Myc, fusion of Ets genes to androgen-regulated promoters, and loss of Pten. However, the specific roles of these aberrations in tumor initiation and progression are poorly understood. Likewise, the cell of origin for PCa remains controversial and may be linked to the aggressive potential of the tumor. One important clue is that prostate tumors co-express basal and luminal protein markers that are restricted to their distinct cell types in normal tissue. Prostate epithelium contains layer-specific stem cells as well as rare bipotent cells, which can differentiate into basal or luminal cells. We hypothesize that the primary oncogenic cell of origin is a transient-differentiating bipotent cell. Such a cell must maintain tight temporal and spatial control of differentiation pathways, thus increasing its susceptibility for oncogenic disruption. In support of this hypothesis, many of the pathways known to be involved in prostate differentiation can be linked to genes commonly altered in PCa. In this article, we review what is known about important differentiation pathways (Myc, p38MAPK, Notch, PI3K/Pten) in the prostate and how their misregulation could lead to oncogenesis. Better understanding of normal differentiation will offer new insights into tumor initiation and may help explain the functional significance of common genetic alterations seen in PCa. Additionally, this understanding could lead to new methods for classifying prostate tumors based on their differentiation status and may aid in identifying more aggressive tumors.

Heterogeneity in apoptotic responses of microvascular endothelial cells to oxidative stress

Oxidative stress contributes to disease and can alter endothelial cell (EC) function. EC from different vascular beds are heterogeneous in structure and function, thus we assessed the apoptotic responses of EC from lung and heart to oxidative stress. Since protein kinase Cδ (PKCδ) is activated by oxidative stress and is an important modulator of apoptosis, experiments assessed the level of apoptosis in fixed lung and heart sections of PKCδ wild-type (PKCδ(+/+)) and null (PKCδ(-/-)) mice housed under normoxia (21% O(2)) or hyperoxia (~95% O(2)). We noted a significantly greater number of TUNEL-positive cells in lungs of hyperoxic PKCδ(+/+) mice, compared to matched hearts or normoxic organs. We found that 33% of apoptotic cells identified in hyperoxic lungs of PKCδ(+/+) mice were EC, compared to 7% EC in hyperoxic hearts. We further noted that EC apoptosis was significantly reduced in lungs of PKCδ(-/-) hyperoxic mice, compared to lungs of PKCδ(+/+) hyperoxic mice. In vitro, both hyperoxia and H(2)O(2) promoted apoptosis in EC isolated from microvasculature of lung (LMVEC), but not from the heart (HMVEC). H(2)O(2) treatment significantly increased p38 activity in LMVEC, but not in HMVEC. Inhibition of p38 attenuated H(2)O(2)-induced LMVEC apoptosis. Baseline expression of total PKCδ protein, as well as the caspase-mediated, catalytically active PKCδ cleavage fragment, was higher in LMVEC, compared to HMVEC. PKCδ inhibition significantly attenuated H(2)O(2)-induced LMVEC p38 activation. Conversely, overexpression of wild-type PKCδ or the catalytically active PKCδ cleavage product greatly increased H(2)O(2)-induced HMVEC caspase and p38 activation. We propose that enhanced susceptibility of lung EC to oxidant-induced apoptosis is due to increased PKCδ→p38 signaling, and we describe a PKCδ-centric pathway which dictates the differential response of EC from distinct vascular beds to oxidative stress.

Protein arginine methyltransferase 5 (PRMT5) signaling suppresses protein kinase Cδ- and p38δ-dependent signaling and keratinocyte differentiation

PKCδ is a key regulator of keratinocyte differentiation that activates p38δ phosphorylation leading to increased differentiation as measured by an increased expression of the structural protein involucrin. Our previous studies suggest that p38δ exists in association with protein partners. A major goal is to identify these partners and understand their role in regulating keratinocyte differentiation. In this study we use affinity purification and mass spectrometry to identify protein arginine methyltransferase 5 (PRMT5) as part of the p38δ signaling complex. PRMT5 is an arginine methyltransferase that symmetrically dimethylates arginine residues on target proteins to alter target protein function. We show that PRMT5 knockdown is associated with increased p38δ phosphorylation, suggesting that PRMT5 impacts the p38δ signaling complex. At a functional level we show that PRMT5 inhibits the PKCδ- or 12-O-tetradecanoylphorbol-13-acetate-dependent increase in human involucrin expression, and PRMT5 dimethylates proteins in the p38δ complex. Moreover, PKCδ expression reduces the PRMT5 level, suggesting that PKCδ activates differentiation in part by reducing PRMT5 level. These studies indicate antagonism between the PKCδ and PRMT5 signaling in control of keratinocyte differentiation.

New Insights into the p38γ and p38δ MAPK Pathways

The mammalian p38 mitogen-activated protein kinases (MAPKs) family is composed of four members (p38α, p38β, p38γ, and p38δ), which are very similar in amino acid sequence but differ in their expression patterns. This suggests that they may have specific functions in different organs. In the last years most of the effort has been centred on the study of the function of the p38α isoform, which is widely referred to as p38 in the literature. However, the role that other p38 isoforms play in cellular functions and their implication in some of the pathological conditions have not been precisely defined so far. In this paper we highlight recent advances made in defining the functions of the two less studied alternative p38MAPKs, p38γ and p38δ. We describe that these p38MAPKs show similarities to the classical p38α isoform, although they may play central and distinct role in certain physiological and pathological processes.

PKC-delta and -eta, MEKK-1, MEK-6, MEK-3, and p38-delta are essential mediators of the response of normal human epidermal keratinocytes to differentiating agents

Previous studies suggest that the novel protein kinase C (PKC) isoforms initiate a mitogen-activated protein kinase (MAPK) signaling cascade that regulates keratinocyte differentiation. However, assigning these functions has relied on treatment with pharmacologic inhibitors and/or manipulating kinase function using overexpression of wild-type or dominant-negative kinases. As these methods are not highly specific, an obligatory regulatory role for individual kinases has not been assigned. In this study, we use small interfering RNA knockdown to study the role of individual PKC isoforms as regulators of keratinocyte differentiation induced by the potent differentiating stimulus, 12-O-tetradecanoylphorbol-13-acetate (TPA). PKC-delta knockdown reduces TPA-activated involucrin promoter activity, nuclear activator protein-1 factor accumulation and binding to DNA, and cell morphology change. Knockdown of PKC downstream targets, including MEKK-1, MEK-6, MEK-3, or p38-delta, indicates that these kinases are required for these responses. Additional studies indicate that knockdown of PKC-eta inhibits TPA-dependent involucrin promoter activation. In contrast, knockdown of PKC-alpha (a classical PKC isoform) or PKC-epsilon (a novel isoform) does not inhibit these TPA-dependent responses. Further studies indicate that PKC-delta is required for calcium and green tea polyphenol-dependent regulation of end responses. These findings are informative as they suggest an essential role for selected PKC and MAPK cascade enzymes in mediating a range of end responses to a range of differentiation stimuli in keratinocytes.

Type I transglutaminase accumulation in the endoplasmic reticulum may be an underlying cause of autosomal recessive congenital ichthyosis

Type I transglutaminase (TG1) is an enzyme that is responsible for assembly of the keratinocyte cornified envelope. Although TG1 mutation is an underlying cause of autosomal recessive congenital ichthyosis, a debilitating skin disease, the pathogenic mechanism is not completely understood. In the present study we show that TG1 is an endoplasmic reticulum (ER) membrane-associated protein that is trafficked through the ER for ultimate delivery to the plasma membrane. Mutation severely attenuates this processing and a catalytically inactive point mutant, TG1-FLAG(C377A), accumulates in the endoplasmic reticulum and in aggresome-like structures where it is ubiquitinylated. This accumulation results from protein misfolding, as treatment with a chemical chaperone permits it to exit the endoplasmic reticulum and travel to the plasma membrane. ER accumulation is also observed for ichthyosis-associated TG1 mutants. Our findings suggest that misfolding of TG1 mutants leads to ubiquitinylation and accumulation in the ER and aggresomes, and that abnormal intracellular processing of TG1 mutants may be an underlying cause of ichthyosis.

JWA sensitizes P-glycoprotein-mediated drug-resistant choriocarcinoma cells to etoposide via JNK and mitochondrial-associated signal pathway

A major obstacle in cancer chemotherapy is the phenomenon of multidrug resistance (MDR), increased P-glycoprotein expression, and abnormal apoptotic processes that may contribute to MDR. Our previous studies demonstrated that JWA is a pro-apoptotic molecule and required for arsenic trioxide and all-trans-retinoic acid-induced cancer cell apoptosis. In this study, the role of JWA in mediating MDR during treatment of choriocarcinoma cells was examined. Data showed that JWA expression was reduced significantly by etoposide (VP16) in JAR MDR cells (JAR/VP16) compared to parent JAR cells. VP16-induced apoptosis in JAR cells was dependent upon the presence of JWA. Knockdown of JWA attenuated VP16-induced apoptosis, and was accompanied by significantly reduced caspase-9 activity and inhibition of JNK phosphorylation. Loss of mitochondrial transmembrane potential induced by VP16 was accompanied by higher JWA expression. JWA was also involved in downregulation of P-glycoprotein through JNK signal pathway. These results suggest that JWA may play an important role in the therapeutic responses to chemotherapeutic agents used to treat choriocarcinoma.

p38 MAP-kinases pathway regulation, function and role in human diseases

Mammalian p38 mitogen-activated protein kinases (MAPKs) are activated by a wide range of cellular stresses as well as in response to inflammatory cytokines. There are four members of the p38MAPK family (p38alpha, p38beta, p38gamma and p38delta) which are about 60% identical in their amino acid sequence but differ in their expression patterns, substrate specificities and sensitivities to chemical inhibitors such as SB203580. A large body of evidences indicates that p38MAPK activity is critical for normal immune and inflammatory response. The p38MAPK pathway is a key regulator of pro-inflammatory cytokines biosynthesis at the transcriptional and translational levels, which makes different components of this pathway potential targets for the treatment of autoimmune and inflammatory diseases. However, recent studies have shed light on the broad effect of p38MAPK activation in the control of many other aspects of the physiology of the cell, such as control of cell cycle or cytoskeleton remodelling. Here we focus on these emergent roles of p38MAPKs and their implication in different pathologies.

Activation of PKCδ and p38δ MAPK during okadaic acid dependent keratinocyte apoptosis

There is substantial interest in identifying agents that differentially activate keratinocyte differentiation versus apoptosis. Okadaic acid (OA) is a tumor promoter in mouse skin that also stimulates apoptosis of murine keratinocytes. OA also enhances human keratinocyte differentiation; however, the impact of OA treatment on apoptosis in these cells has not been examined. We show that OA promotes normal human keratinocyte apoptosis as evidenced by increased accumulation of cells having sub-G1/S DNA content, decreased mitochondrial integrity, increased annexin V binding, increased cytoplasmic cytochrome c level, and increased procaspase 3 and PARP cleavage. Cyclin A, cyclin D1, cdk2, cdk4, p53 and p21 levels are reduced. These changes are associated with release of the PKCdelta catalytic domain and increased phosphorylation of PKCdelta-T(505)-responses consistent with PKCdelta activation. In contrast, phosphorylation of PKCdelta-Y(311) is not increased. The apoptotic response is enhanced in OA treated cells in the presence of p38delta, a PKCdelta target. OA treatment selectively activated p38delta, and OA-dependent apoptosis is not inhibited by treatment with the p38alpha/beta inhibitor, SB203580. These findings are consistent with the idea that the response is mediated by p38delta. Our data indicate that OA is an agent that regulates both keratinocyte differentiation and apoptosis, and that this regulation is mediated via activation of a PKCdelta/p38delta signaling cascade.

Apigenin Inhibition of Involucrin Gene Expression Is Associated with a Specific Reduction in Phosphorylation of Protein Kinase Cδ Tyr311

Apigenin is a plant-derived flavanoid that has significant promise as a skin cancer chemopreventive agent. In the present study, we examine the mechanism whereby apigenin regulates normal human keratinocyte differentiation. Expression of involucrin (hINV), a marker of keratinocyte differentiation, is increased by differentiating agents via a protein kinase Cdelta (PKCdelta), Ras, MEKK1, MEK3 cascade that increases AP1 factor level and AP1 factor binding to DNA elements in the hINV promoter. We show that apigenin inhibits this response. Apigenin suppresses the 12-O-tetradeconylphorbol-13-acetate-dependent increase in AP1 factor expression and binding to the hINV promoter and the increase in hINV promoter activity. Apigenin also inhibits the increase in promoter activity observed following overexpression of PKCdelta, constitutively active Ras, or MEKK1. The suppression of PKCdelta activity is associated with reduced phosphorylation of PKCdelta-Y311. The physiological importance of this phosphorylation event was confirmed by showing that the PKCdelta phosphorylation-defective mutant, PKCdelta-Y311F, is less able to increase hINV promoter activity. Activation of hINV promoter activity by the green tea polyphenol, (-)-epigellocathecin-3-gallate, is also inhibited by apigenin, suggesting that the two chemopreventive agents can produce opposing actions in keratinocytes. Additional studies show that the apigenin-dependent suppression of differentiation is associated with reduced cell proliferation but that there is no evidence of apoptosis.

Mechanism of action of sulforaphane: inhibition of p38 mitogen-activated protein kinase isoforms contributing to the induction of antioxidant response element-mediated heme oxygenase-1 in human hepatoma HepG2 cells

Exposure of sulforaphane to HepG2 cells increased heme oxygenase-1 (HO-1) expression by activating antioxidant response element (ARE) through induction of Nrf2 and suppression of Kelch-like ECH-associated protein 1 (Keap1). Using human HO-1 promoter reporter plasmids and ChIP assay, we have identified that sulforaphane transcriptionally activated the upstream ARE-rich enhancer region, located at -9.0 kb upstream human HO-1 promoter. Induction of HO-1 by sulforaphane was attenuated by overexpression of mutant Nrf2 plasmid in HepG2 cells and totally abolished in Nrf2 knockout mouse embryonic keratinocytes and fibroblasts. Overexpression of individual p38 mitogen-activated protein (MAP) kinase (MAPK) isoforms also suppressed constitutive as well as sulforaphane- or Nrf2-induced ARE-dependent gene expression. Among the upstream kinases, although MKK3 was not involved in suppression of ARE by any of p38 MAPK isoforms, MKK6 selectively suppressed ARE by p38 gamma or p38 delta, but not by p38 alpha or p38 beta. Importantly, sulforaphane not only activated MAP/extracellular signal-regulated kinase (ERK) kinases 1/2 and ERK1/2, but also strongly suppressed anisomycin-induced activation of p38 MAPK isoforms by blocking phosphorylation of upstream kinases, MKK3/6. Finally, we found that stimulation of p38 MAPK isoforms phosphorylated purified Nrf2 protein and caused an increase in the interaction between Nrf2 and Keap1 in vitro and the suppression of Nrf2 translocation into the nucleus. Collectively, our results indicate that transcriptional activation of Nrf2/ARE is critical in sulforaphane-mediated induction of HO-1, which can be modulated in part by the blockade of p38 MAPK signaling pathway. In addition, our study shows that p38 MAPK can phosphorylate Nrf2 and promotes the association between Nrf2 and Keap1 proteins, thereby potentially inhibiting nuclear translocation of Nrf2.

Interplay between PI3K/Akt and MAPK signaling pathways in DNA-damaging drug-induced apoptosis

In order to elucidate the role of the mitogen-activated protein kinases, including JNK, p38 MAPK and ERK, as well as the survival-associated PI3K/Akt signaling pathway, in the response to chemotherapy, we have conducted a comparative study regarding the effects of doxorubicin on these pathways. Doxorubicin was determined to elicit the apoptosis of NIH3T3 cells in a dose-dependent manner. Prior to cell death, both Akt and p38 MAPK were transiently activated, and subsequently inactivated almost wholly, whereas ERK and JNK evidenced sustained activations in response to the drug treatment. The inhibition of PI3K/Akt and p38 MAPK both accelerated and enhanced doxorubicin-induced apoptosis and ERK inhibition apparently exerted negative effect on apoptosis. The modulation of PI3K/Akt activation by treatment of LY294002 or expression of Akt mutants such as Akt-DN or Myr-Akt exerted a significant effect on the activation of ERK1/2. We also observed that PI3K/Akt and sustained ERK activation were associated intimately with the etoposide-induced apoptosis. Taken together, our results clearly suggest that the differential regulation of the PI3K/Akt, ERK1/2, and p38 MAPK signaling pathways are crucial in the context of DNA-damaging drug-induced apoptosis, and this has compelled us to propose that the sustained activation of ERK1/2 pathway may be generally involved in the apoptosis induced by anticancer DNA-damaging drugs, including doxorubicin and etoposide.

Opposing action of curcumin and green tea polyphenol in human keratinocytes

Persistent environmental insult can convert a normal cell into a cancer cell. However, various natural chemopreventive agents called antioxidants can retard this progression. We have recently explored the effects of several chemopreventive agents, including green tea polyphenol and curcumin, on normal human keratinocyte function. Our findings suggest that a bioactive polyphenol from green tea, (-)-epigallocatechin-3-gallate (EGCG), acts to increase involucrin gene expression, suggesting that EGCG treatment enhances normal human keratinocyte differentiation. Mechanistic studies indicate that EGCG alters mitogen-activated protein kinase cascade function to activate involucrin gene transcription via a Ras, MEKK1, MEK3, ERK1/2-p38delta cascade that targets AP1 and CAATT enhancer binding protein transcription factors. These findings suggest that EGCG may inhibit disease progression by promoting keratinocyte differentiation. Parallel studies indicate that not all antioxidants produce a similar response. Curcumin, an antioxidant derived from the turmeric, antagonizes the EGCG-dependent response by interfering in this signaling pathway. These studies suggest that different antioxidant may produce antagonistic effects in tissues.

A novel transglutaminase activator forms a complex with type 1 transglutaminase

Type I transglutaminase is a plasma membrane-anchored intracellular protein-protein crosslinking enzyme that is responsible for assembly of the keratinocyte cornified envelope during terminal keratinocyte differentiation. We recently described a novel protein, TIG3, that when expressed in keratinocytes causes increased transglutaminase activity and keratinocyte cell death. However, the mechanism of activation of transglutaminase by TIG3 is not known. We now extend our previous study and show that full-length TIG3 forms a complex with type I transglutaminase that is demonstrated by TIG3-transglutaminase co-precipitation. We also demonstrate that treating TIG3-expressing cells with monodansyl cadaverine, a competitive transglutaminase substrate, attenuates the TIG3-dependent response, suggesting that transglutaminase is an important mediator of TIG3 action. These findings suggest that TIG3 forms a complex with transglutaminase resulting in transglutaminase activation and that transglutaminase activity is required for the TIG3-dependent biological response.

Novel signaling stimulated by arsenite increases cholesterol metabolism through increases in unphosphorylated steroidogenic acute regulatory (StAR) protein

Cholesterol metabolism to pregnenolone is dependent on the steroidogenic acute regulatory protein (StAR), which activates mitochondrial transfer of cholesterol to cytochrome CYP450scc. In mouse Y-1 adrenal cells and testis MA10 cells stimulation by 8-Bromo-cAMP (Br-cAMP) is augmented by a novel signaling initiated by low concentrations of arsenite (3-20 microM) and anisomycin (0.2 microM), a more selective stress agent. Each elevated StAR mRNA (three-fold after 6 h treatment) even with simultaneous stimulation by Br-cAMP. Arsenite produced parallel increases in StAR protein expression and cholesterol metabolism, but not for P450scc-mediated metabolism of 20alpha-hydroxycholesterol. Although arsenite and anisomycin each stimulated the phosphorylation of p38, the p38 inhibitor SB203580 (SB) produced additive increases in StAR expression. Cholesterol metabolism increased in parallel but without the increased StAR protein phosphorylation produced by Br-cAMP. Arsenite and anisomycin each elevated StAR mRNA but preferentially increased the 3.5 kb form relative to the 1.6 kb form. Arsenite and anisomycin each enhanced the stability of the more labile 3.5 kb mRNA which contains AU-rich elements that control mRNA stability. Although there were increases in both forms of StAR mRNA, arsenite did not stimulate a StAR promoter-reporter that exhibited a typical three-fold response to Br-cAMP. Arsenite and anisomycin may therefore activate a novel SB-independent MAP kinase which in part increases StAR expression through stabilizing the 3.5 kb mRNA but which may also activate a mechanism that by-passes transcription factors detected by the reporter. SB stimulation, which was completely blocked by a MEK inhibitor, was also selective towards the 3.5 kb StAR mRNA suggesting a second pathway for mRNA stabilization. These activations contrast with inhibition of StAR expression by arsenite at higher concentrations or longer incubation times.

The mitogen-activated protein kinase kinase kinase dual leucine zipper-bearing kinase (DLK) acts as a key regulator of keratinocyte terminal differentiation

In the skin, epithelial cells undergo a terminal differentiation program leading to the formation of the stratum corneum. Although it is expected that the last phases of this process must be tightly regulated since it results in cell death, the signaling pathways involved in this induction remain ill defined. We now report that a single kinase, the mitogen-activated protein kinase kinase kinase dual leucine zipper-bearing kinase (DLK), acts in the epidermis to promote the terminal differentiation of human keratinocytes. In support of this notion, we showed that DLK expression was restricted to the granular layer in situ. In addition, cultured keratinocytes infected with a recombinant adenovirus expressing DLK exhibited morphological and biochemical changes, including a suprabasal localization, altered cell shape, compacted cytoplasm, DNA fragmentation, and the up-regulation of filaggrin, that are reminiscent of a terminally differentiated phenotype. Moreover the expression of wild-type DLK in keratinocytes stimulated transglutaminase activity and the consequent formation of the cornified cell envelope, while a kinase-inactive variant of DLK did not. Together these results identify DLK as a signaling molecule implicated in the regulation of keratinocyte terminal differentiation and cornification.

Regulation of the human involucrin gene promoter by co-activator proteins

Stratified squamous epithelial cells undergo an orderly process of terminal differentiation that is characterized by specific molecular and morphological changes, including expression of the cornified envelope protein involucrin. Significant progress has been made in characterizing the upstream regulatory region of the involucrin gene. Binding sites for AP-1 (activator protein 1) and Sp1 transcription factors were shown to be important for involucrin promoter activity and tissue-specific expression. Defective terminal differentiation is often characterized by decreased or lack of involucrin expression. Recently, a dominant-negative construct of the transcriptional co-activator P/CAF [p300/CBP-associated factor, where CBP stands for CREB (cAMP-response-element-binding protein)-binding protein] was shown to inhibit involucrin expression in immortalized keratinocytes [Kawabata, Kawahara, Kanekura, Araya, Daitoku, Hata, Miura, Fukamizu, Kanzaki, Maruyama and Nakajima (2002) J. Biol. Chem. 277, 8099-8105]. Loss of expression or inactivation of other co-activators has also been demonstrated [Suganuma, Kawabata, Ohshima, and Ikeda (2002) Proc. Natl. Acad. Sci. U.S.A. 99, 13073-13078]. In the present study, we re-expressed CBP and P/CAF in immortalized keratinocyte lines that had lost expression of these co-activator proteins. Re-expression of these proteins restored calcium- and RA (retinoic acid)-responsive involucrin expression in these cells. RA and calcium signalling induced exchange of CBP and P/CAF occupancy at the AP-1 sites of the involucrin promoter. CBP and P/CAF inductions of the involucrin expression were not dependent on MEK (mitogen-activated protein kinase/extracellular-signal-regulated kinase kinase), p38, protein kinase C or CaM kinase (calcium/calmodulin-dependent kinase) signalling. Kinase-induced changes in involucrin promoter activity directly resulted from changes in AP-1 protein expression. We concluded that CBP and P/CAF are important regulators of involucrin expression in stratified squamous epithelial cells.

Protein kinase Cdelta regulates keratinocyte death and survival by regulating activity and subcellular localization of a p38delta-extracellular signal-regulated kinase 1/2 complex

Protein kinase Cdelta (PKCdelta) is an important regulator of apoptosis in epidermal keratinocytes. However, little information is available regarding the downstream kinases that mediate PKCdelta-dependent keratinocyte death. This study implicates p38delta mitogen-activated protein kinase (MAPK) as a downstream carrier of the PKCdelta-dependent death signal. We show that coexpression of PKCdelta with p38delta produces profound apoptosis-like morphological changes. These morphological changes are associated with increased sub-G(1) cell population, cytochrome c release, loss of mitochondrial membrane potential, caspase activation, and PARP cleavage. This death response is specific for the combination of PKCdelta and p38delta and is not produced by replacing PKCdelta with PKCalpha or p38delta with p38alpha. A constitutively active form of MEK6, an upstream activator of p38delta, can also produce cell death when coupled with p38delta. In addition, concurrent p38delta activation and extracellular signal-regulated kinase 1/2 (ERK1/2) inactivation are required for apoptosis. Regarding this inverse regulation, we describe a p38delta-ERK1/2 complex that may coordinate these changes in activity. We further show that this p38delta-ERK1/2 complex relocates into the nucleus in response to PKCdelta expression. This regulation appears to be physiological, since H(2)O(2), a known inducer of keratinocyte apoptosis, promotes identical PKCdelta and p38delta-ERK1/2 activity changes, leading to similar morphological changes.

Antioxidants regulate normal human keratinocyte differentiation

Cancer begins with a normal cell that, due to persistent environmental insult, is transformed, via a series of progressively more insidious steps, into a cancer cell. A major goal of chemopreventive therapy is to alter the normal cell response to the environmental agent with the goal of inhibiting disease progression. (-)-Epigallocatechin-3-gallate (EGCG) is an important bioactive green tea antioxidant that possesses remarkable cancer chemopreventive properties. We have recently explored the hypothesis that EGCG prevents cancer by promoting keratinocyte differentiation. Based on our findings, we argue that EGCG acts to enhance the differentiation of normal keratinocytes. This is a potentially important finding, as it represents a novel mechanism of disease inhibition by EGCG--cancer preventive "differentiation therapy". However, not all antioxidant chemopreventive agents work by this mechanism. Curcumin, for example, inhibits the differentiation-promoting activity of EGCG. This report discusses the mechanism of EGCG and curcumin action in regulating expression of involucrin, a marker of keratinocyte differentiation.

Cholesterol Depletion Upregulates Involucrin Expression in Epidermal Keratinocytes Through Activation of p38

Cholesterol has been recently suggested to regulate the early steps of keratinocyte differentiation through lipid rafts. In many cell types, depletion of cholesterol activates signaling proteins like epidermal growth factor receptor (EGFR), human epidermal growth factor receptor 2 (HER2), or extracellular signal-regulated kinase (ERK) known to affect cell differentiation. In this study, we explored the effects of cholesterol depletion on the phenotype of cultured keratinocytes, using a treatment with methyl-beta-cyclodextrin (MbetaCD) to extract cholesterol and a treatment with lovastatin to inhibit cholesterol neosynthesis. Analysis of the expression of differentiation marker genes in early differentiating confluent cultures reveals that cholesterol depletion induces downregulation of keratin 14 (K14) and keratin 10 (K10) and upregulation of involucrin. MbetaCD treatment induces phosphorylation of EGFR, HER2, and ERK, but not HER3. Inhibition of EGFR with PD153035 impairs the MbetaCD-induced phosphorylation of EGFR, HER2, and ERK, but does not impair the alteration of K14, K10, or involucrin gene expression, indicating that other signaling proteins regulate this phenomenon. p38 has been suggested to regulate the expression of involucrin during keratinocyte differentiation. We found that MbetaCD treatment induces a prolonged phosphorylation of p38 in general and p38alpha in particular. An inhibition of p38 with PD169316 impairs the upregulation of involucrin mRNAs by a treatment with MbetaCD, but not by a p38delta-activating TPA treatment, which might suggest that cholesterol depletion alters involucrin gene expression through activation of p38alpha/beta.

Regulation of Involucrin Gene Expression

The epidermis is a dynamic renewing structure that provides life-sustaining protection from the environment. The major cell type of the epidermis, the epidermal keratinocyte, undergoes a carefully choreographed program of differentiation. Alteration of these events results in a variety of debilitating and life-threatening diseases. Understanding how this process is regulated is an important current goal in biology. In this review, we summarize the literature regarding regulation of involucrin, an important marker gene that serves as a model for understanding the mechanisms that regulate the differentiation process. Current knowledge describing the role of transcription factors and signaling cascades in regulating involucrin gene expression are presented. These studies describe a signaling cascade that includes the novel protein kinase C isoforms, Ras, MEKK1, MEK3, and a p38delta-extracellular signal regulated kinase 1/2 complex. This cascade regulates activator protein one, Sp1, and CCATT/enhancer-binding protein transcription factor DNA binding to two discrete involucrin promoter regions, the distal- and proximal-regulatory regions, to regulate involucrin gene expression.

Distinctive regulation and function of PI 3K/Akt and MAPKs in doxorubicin-induced apoptosis of human lung adenocarcinoma cells

Regulation and function of PI 3K/Akt and mitogen-activated protein kinases (MAPKs) in doxorubicin-induced cell death were investigated in human lung adenocarcinoma cells. Doxorubicin induced dose-dependent apoptosis of human lung adenocarcinoma NCI-H522 cells. Prior to cell death, both Akt and the MAPK family members (MAPKs: ERK1/2, JNK, and p38) were activated in response to the drug treatment. The kinetics of the inductions for Akt and MAPKs are, however, distinct. The activation of Akt was rapid and transient, activated within 30 min of drug addition, then declined after 3 h, whereas the activations of three MAPKs occurred later, 4 h after addition of the drug and sustained until cell death occurred. Inhibition of PI 3K/Akt activation had no effect on MAPKs' activation, suggesting that the two pathways are independently activated in response to the drug treatment. Inhibition of PI 3K/Akt and p38 accelerated and enhanced doxorubicin-induced cell death. On the contrary, inhibition of ERK1/2 or JNK had no apparent effect on the cell death. Taken together, these results suggest that PI 3K/Akt and MAPKs signaling pathways are all activated, but with distinct mechanisms, in response to doxorubicin treatment. Activation of PI 3K/Akt and p38 modulates apoptotic signal pathways and inhibits doxorubicin-induced cell death. These responses of tumor cells to cancer drug treatment may contribute to their drug resistance. Understanding of the mechanism and function of the responses will be beneficial for the development of novel therapeutic approaches for improvement of drug efficacy and circumvention of drug resistance.

Effects of p38MAPK isoforms on renal mesangial cell inducible nitric oxide synthase expression

Several related isoforms of p38MAPK have been identified and cloned in many species. Although they all contain the dual phosphorylation motif TGY, the expression of these isoforms is not ubiquitous. p38alpha and -beta2 are ubiquitously expressed, whereas p38gamma and -delta appear to have more restricted expression. Because there is evidence for selective activation by upstream kinases and selective preference for downstream substrates, the functions of these conserved proteins is still incompletely understood. We have demonstrated that the renal mesangial cell expresses the mRNA for all the isoforms of p38MAPK, with p38alpha mRNA expressed at the highest level, followed by p38gamma and the lowest levels of expression by p38beta2 and -delta. To determine the functional effects of these proteins on interleukin (IL)-1beta-induced inducible nitric oxide synthase (iNOS) expression, we transduced TAT-p38 chimeric proteins into renal mesangial cells and assessed the effects of wild-type and mutant p38 isoforms on ligand induced iNOS expression. We show that whereas p38gamma and -delta had minimal effects on iNOS expression, p38alpha and -beta2 significantly altered its expression. p38alpha mutant and p38beta2 wild-type dose dependently inhibited IL-1beta-induced iNOS expression. These data suggest that p38alpha and beta2 have reciprocal effects on iNOS expression in the mesangial cell, and these observations may have important consequences for the development of selective inhibitors targeting the p38MAPK family of proteins.

A regulatory role for p38 delta MAPK in keratinocyte differentiation. Evidence for p38 delta-ERK1/2 complex formation

p38 MAPK isoforms are important in the regulation of a variety of cellular processes. Among the four described p38 isoforms, p38 alpha, beta, and delta are expressed in keratinocytes (Dashti, S. R., Efimova, T., and Eckert, R. L. (2001) J. Biol. Chem. 276, 8059-8063). However, very little is known about how individual p38 isoforms regulate keratinocyte function. In the present study, we use okadaic acid (OA) as a tool to study the role of p38 MAPKs as regulators of keratinocyte differentiation. We demonstrate that OA activates p38 delta but not other p38 isoforms. p38 delta activation is increased as early as 0.5 h after OA addition, and activity is maximal at 8 and 24 h. ERK1 and ERK2 activity are reduced on an identical time course. We show that p38 delta forms a complex with ERK1/2, and overexpression of p38 delta inhibits ERK1/2 activity without reducing ERK1/2 level. Thus, p38 delta may directly suppress ERK1/2 activity. Additional studies show that p38 delta is expressed in the epidermis, suggesting a role for p38 delta in regulating differentiation. To evaluate its function, we show that increased p38 delta activity is associated with increased levels of AP1 and CAATT enhancer binding protein factors, increased binding of these factors to the involucrin (hINV) promoter, and increased expression. Moreover, these responses are maintained in the presence of SB203580, an agent that inhibits p38 alpha and beta, further suggesting a central role for the p38 delta isoform. Dominant-negative p38 also inhibits these responses. These unique observations suggest that p38 delta is the major p38 isoform driving suprabasal hINV gene expression and that p38 delta directly regulates ERK1/2 activity via formation of a p38 delta-ERK1/2 complex.

p38 Mitogen-activated protein kinases on the body surface--a function for p38 delta

The p38 family of mitogen-activated protein kinases includes p38 alpha (SAPK2a, CSBP), p38 beta (SAPK2b), p38 delta (SAPK4), and p38 gamma (SAPK3/ERK6). p38 alpha and p38 beta are widely expressed p38 isoforms that are involved in regulation of cell proliferation, differentiation, development, and response to stress. Relatively less is known regarding the function of the p38 delta isoform. In this review, we discuss the role of the p38 alpha, p38 beta, and p38 gamma isoforms and then present recent findings that define a role for p38 delta as a regulator of differentiation-dependent gene expression in keratinocytes.

Keratinocyte survival, differentiation, and death: many roads lead to mitogen-activated protein kinase

The epidermis is a dynamic and continually renewing surface that provides and maintains a life-sustaining interface with the environment. The epidermal keratinocyte, the major cell type of the epidermis, undergoes a complex and carefully choreographed program of differentiation. This process requires a balance between keratinocyte proliferation, differentiation, and apoptosis. This overview will concentrate on cascades that regulate the balance between keratinocyte cell proliferation and survival, and apoptosis and cell differentiation, with a particular emphasis on the role of the mitogen-activated protein kinase cascades. A summary of the literature suggests that extracellular regulated kinases function to promote keratinocyte proliferation and survival, whereas p38 mitogen-activated protein kinase functions to promote differentiation and apoptosis.

Novel protein kinase C isoforms regulate human keratinocyte differentiation by activating a p38 delta mitogen-activated protein kinase cascade that targets CCAAT/enhancer-binding protein alpha

The novel protein kinase C (nPKC) isoforms are important regulators of human involucrin (hINV) gene expression during keratinocyte differentiation (Efimova, T., and Eckert, R. L. (2000) J. Biol. Chem. 275, 1601-1607). Although the regulatory mechanism involves mitogen-activated protein kinase (MAPK) activation, the role of individual MAPK isoforms has not been elucidated. We therefore examined the effects of individual nPKCs on MAPK activation. We observe unique changes whereby nPKC expression simultaneously increases p38 activity and decreases ERK1 and ERK2 activity. Although p38 alpha, p38 beta, and p38 delta are expressed in keratinocytes, only a single isoform, p38 delta, accounts for the increased p38 activity. Parallel studies indicate that this isoform is also activated by treatment with the keratinocyte regulatory agents, 12-O-tetradecanoylphorbol-13-acetate, calcium, and okadaic acid. These changes in MAPK activity are associated with increased C/EBP alpha transcription factor expression and DNA binding to the hINV promoter and increased hINV gene expression. Expression of PKC delta, PKC epsilon, or PKC eta causes a 10-fold increase in hINV promoter activity, whereas C/EBP alpha expression produces a 25-fold increase. However, simultaneous expression of both proteins causes a synergistic 100-fold increase in promoter activity. These responses are eliminated by the dominant-negative C/EBP isoform, GADD153, and are also inhibited by dominant-negative forms of Ras, MEKK1, MEK3, and p38. These results suggest that the nPKC isoforms produce a unique shift in MAPK activity via a Ras, MEKK1, MEK3 pathway, to increase p38 delta and inhibit ERK1/2 and ultimately increase C/EBP alpha binding to the hINV promoter and hINV gene expression.

Calcium-dependent involucrin expression is inversely regulated by protein kinase C (PKC)alpha and PKCdelta

Calcium is an important physiologic regulator of keratinocyte function that may regulate keratinocyte differentiation via modulation of protein kinase C (PKC) activity. PKCalpha and PKCdelta are two PKC isoforms that are expressed at high levels in keratinocytes. In the present study, we examine the effect of PKCdelta and PKCalpha on calcium-dependent keratinocyte differentiation as measured by effects on involucrin (hINV) gene expression. Our studies indicate that calcium increases hINV promoter activity and endogenous hINV gene expression. This response requires PKCdelta, as evidenced by the observation that treatment with dominant-negative PKCdelta inhibits calcium-dependent hINV promoter activity, whereas wild type PKCdelta increases activity. PKCalpha, in contrast, inhibits calcium-dependent hINV promoter activation, a finding that is consistent with the ability of dominant-negative PKCalpha and the PKCalpha inhibitor, Go6976, to increase hINV gene expression. The calcium-dependent regulatory response is mediated by an AP1 transcription factor-binding site located within the hINV promoter distal regulatory region that is also required for PKCdelta-dependent regulation; moreover, both calcium and PKCdelta produce similar, but not identical, changes in AP1 factor expression. A key question is whether calcium directly influences PKC isoform function. Our studies show that calcium does not regulate PKCalpha or delta levels or cause a marked redistribution to membranes. However, tyrosine phosphorylation of PKCdelta is markedly increased following calcium treatment. These findings suggest that PKCalpha and PKCdelta are required for, and modulate, calcium-dependent keratinocyte differentiation in opposing directions.

Green tea polyphenol stimulates a Ras, MEKK1, MEK3, and p38 cascade to increase activator protein 1 factor-dependent involucrin gene expression in normal human keratinocytes

(-)-Epigallocatechin-3-gallate (EGCG) is an important bioactive constituent of green tea that efficiently reduces epidermal cancer cell proliferation. This inhibition is associated with a reduction in activator protein 1 (AP1) transcription factor level and activity. However, its effects on AP1 function in normal epidermal cells have not been extensively explored. Our present studies show that EGCG regulates normal keratinocyte function. To understand the mechanism of action, we examined the effects of EGCG on AP1 factor activity, MAPK signal transduction, and expression of the AP1 factor-regulated human involucrin (hINV) gene. EGCG increases hINV promoter activity in a concentration-dependent manner that requires the presence of an intact hINV promoter AP1 factor binding site. This response appears to be physiologic, as endogenous hINV gene expression is also increased. Fra-1, Fra-2, FosB, JunB, JunD, c-Jun, and c-Fos levels are increased by EGCG treatment, as is AP1 factor binding to hINV promoter AP1 site. Gel mobility shift studies show that this complex contains Fra-1 and JunD. Signal transduction analysis indicates that the EGCG response requires Ras, MEKK1, MEK3, and p38 kinases. Kinase assays and inhibitor studies suggest that p38delta is the p38 isoform responsible for the regulation. These changes are also associated with a cessation of cell proliferation and enhanced cornified envelope formation. These studies show that in normal human keratinocytes EGCG markedly increases, via a MAPK signaling mechanism, AP1 factor-associated responses.

The p38-MAPK/SAPK pathway is required for human keratinocyte migration on dermal collagen

Human keratinocyte motility plays an important role in the re-epithelialization of human skin wounds. The wound bed over which human keratinocytes migrate is rich in extracellular matrices, such as fibrin, fibronectin, and collagen, and serum factors, such as platelet-derived growth factor and transforming growth factor beta 1. Extracellular matrices and the serum factors bind to cell surface receptors and initiate a cascade of intracellular signaling events that regulate cell migration. In this study, we identified an intracellular signaling pathway that mediates collagen- driven motility of human keratinocytes. Pharmaco logic inhibition of the activation of p38-alpha and p38-beta mitogen-activated protein kinase activation potently blocked collagen-driven human keratinocyte migration. Transfection of the same keratinocytes with the kinase-negative mutants of p38-alpha or p38-beta mitogen-activated protein kinase markedly inhibited keratinocyte migration on collagen. Attachment of keratinocytes to collagen activated p38 mitogen- activated protein kinase, as well as p44/p42 ERKs. Interestingly, activation of the p38 mitogen-activated protein kinase cascade by overexpressing the constitutively active MKK3 and MKK6, MKK3b(E) and MKK6b(E), could neither initiate migration in the absence of collagen nor enhance collagen-driven migration. This study provides evidence that the p38-MAPK/SAPK pathway is necessary, but insufficient, for mediating human keratinocyte migration on collagen.