Cross-talk between epidermal growth factor receptor and protein kinase C during calcium-induced differentiation of keratinocytes

Epidermal growth factor receptor-dependent stimulation of differentiation by human papillomavirus type 16 E5

Human papillomaviruses (HPVs) infect keratinocytes of stratified squamous epithelia, and persistent infection with high-risk HPV types, such as HPV16, may lead to the development of malignancies. HPV evades host immunity in part by linking its gene expression to the host differentiation program, and therefore relies on differentiation to complete its life cycle. Based on previous reports indicating that the HPV16 protein E5 is important in the late stages of the differentiation-dependent life cycle, we found that organotypic cultures harboring HPV16 genomes lacking E5 showed reduced markers of terminal differentiation compared to wild type HPV16-containing cultures. We found that epidermal growth factor receptor (EGFR) levels and activation were increased in an E5-depdendent manner in these tissues, and that EGFR promoted terminal differentiation and expression of the HPV16 L1 gene. These findings suggest a function for E5 in preserving the ability of HPV16 containing keratinocytes to differentiate, thus facilitating the production of new virus progeny.

EGFR Signaling Is Overactive in Pachyonychia Congenita: Effective Treatment with Oral Erlotinib

Pachyonychia congenita (PC) is a rare keratinizing disorder characterized by painful palmoplantar keratoderma for which there is no standard current treatment. PC is caused by dominant mutations in keratin (K) K6A, K6B, K6C, K16, or K17 genes involved in stress, wound healing, and epidermal barrier formation. Mechanisms leading to pain and painful palmoplantar keratoderma in PC remain elusive. In this study, we show overexpression of EGFR ligands epiregulin and TGF-α as well as HER1‒EGFR and HER2 in the upper spinous layers of PC lesions. EGFR activation was confirmed by upregulated MAPK/ERK and mTOR signaling. Abnormal late terminal keratinization was associated with elevated TGM1 activity. In addition, the calcium ion permeable channel TRPV3 was significantly increased in PC-lesional skin, suggesting a predominant role of the TRPV3/EGFR signaling complex in PC. We hypothesized that this complex contributes to promoting TGM1 activity and induces the expression and shedding of EGFR ligands. To counteract this biological cascade, we treated three patients with PC with oral erlotinib for 6‒8 months. The treatment was well-tolerated and led to an early, drastic, and sustained reduction of neuropathic pain with a major improvement of QOL. Our study provides evidence that targeted pharmacological inhibition of EGFR is an effective strategy in PC.

Immune Checkpoint and Other Receptor-Ligand Pairs Modulating Macrophages in Cancer: Present and Prospects

Immunotherapy, especially immune checkpoint blocking, has become the primary anti-tumor treatment in recent years. However, the current immune checkpoint inhibitor (ICI) therapy is far from satisfactory. Macrophages are a key component of anti-tumor immunity as they are a common immune cell subset in tumor tissues and act as a link between innate and adaptive immunity. Hence, understanding the regulation of macrophage activation in tumor tissues by receptor-ligand interaction will provide promising macrophage-targeting strategies to complement current adaptive immunity-based immunotherapy and traditional anti-tumor treatment. This review aims to offer a systematic summary of the current advances in number, structure, expression, biological function, and interplay of immune checkpoint and other receptor-ligand between macrophages and tumor cells.

Bovine colostrum induces the differentiation of human primary keratinocytes

Bovine colostrum, the first milk secreted by the mammary glands of cows shortly after they have given birth, provides a natural source of bioactive substances helpful to promote tissue development and repair, and to maintain a healthy immune system. Owing to its properties, the use of colostrum in the treatment of human diseases is under investigation. We evaluated the biological activity of colostrum on human primary keratinocytes, focusing on its effects with regard to a proliferation/differentiation balance. Using cellular and molecular approaches, we showed that colostrum favors a cell cycle withdrawal by increasing the expression of p21/WAF1 and p27/KIP1. It also promotes the transition of keratinocytes from a proliferating to a differentiating state, as assessed by a decrease in keratin 5 and an increase in keratin 16. We demonstrated the ability of colostrum to induce the expression of early and late differentiation markers (keratin 1, involucrin, and filaggrin) and the synthesis of caspase 14 and bleomycin hydrolase, the two main enzymes involved in filaggrin maturation. Moreover, we showed that bovine colostrum is able to promote keratinocyte stratification and terminal differentiation not only in two-dimensional (2D), but also in a more physiological system of three-dimensional (3D) skin equivalents. Finally, we demonstrated that colostrum stimulates cell differentiation through the PI3K/PLC-γ1/PKCα pathways mainly associated to tyrosine kinase receptors. These results suggest the possibility to benefit from colostrum properties for the treatment of skin diseases characterized by altered differentiation and perturbed barrier function.

Heat injured stromal cells-derived exosomal EGFR enhances prostatic wound healing after thulium laser resection through EMT and NF-κB signaling

BACKGROUND

This study investigated shallow heat injury to prostate stromal fibroblasts and epithelial cells and their interaction to regulate the wound healing and the underlying molecular events.

METHODS

Prostate stromal fibroblasts and epithelial cells were cultured individually or cocultured and subjected to shallow heat injury for assessments of cell proliferation, migration, apoptosis, cell cycle distribution, and gene expression. The supernatant of heat-injured WPMY-1 cells was collected for exosome extraction and assessments. Furthermore, beagle dogs received thulium laser resection of the prostate (TmLRP) and randomly divided into Gefitinib, GW4869, and control treatment for the histological analysis, tissue re-epithelialization, and epidermal growth factor receptor (EGFR) expression on the prostatic wound surface. Immunofluorescence was to evaluate p63-positive basal progenitor cell trans-differentiation and macrophage polarization and ELISA was to detect cytokine levels in beagles' urine.

RESULTS

Shallow heat injury caused these cells to enter a stressed state and enhanced their crosstalk. The prostate stromal fibroblasts produced and secreted more exosomal-EGFR and other cytokines and chemokines after shallow heat injury, resulting in increased proliferation and migration of prostate epithelial cells during wound healing. The wound healing of the canine prostatic urethra following the TmLRP procedure was slower in the Gefitinib and GW4869 treatment group than in the control group of animals. Immunofluorescence and ELISA showed that reduced EGFR expression interrupted macrophage polarization but increased the inflammatory response.

CONCLUSIONS

Shallow heat injury was able to promote the interaction of prostate stromal cells with prostate epithelial cells to enhance wound healing. Stromal-derived exosomal-EGFR plays a crucial role in the balance of the macrophage polarization and prostatic wound healing.

ADAM17/EGFR axis promotes transglutaminase-dependent skin barrier formation through phospholipase C γ1 and protein kinase C pathways

The vitally important skin barrier is formed by extensive cross-linking activity of transglutaminases (TGs) during terminal epidermal differentiation. We have previously shown that epidermal deficiency of a disintegrin and metalloproteinase 17 (ADAM17), the principal EGFR ligand sheddase, results in postnatal skin barrier defects in mice due to impeded TG activity. However, the mechanism by which ADAM17/EGFR signalling maintains TG activity during epidermal differentiation remains elusive. Here we demonstrate that ADAM17-dependent EGFR signalling promotes TG activity in keratinocytes committed to terminal differentiation by direct induction of TG1 expression. Restored TG1 expression of EGF-stimulated differentiated Adam17-/- keratinocytes was strongly repressed by inhibitors for PLCγ1 or protein kinase C (PKC) pathways, while treatment with the PKC stimulator 12-O-tetradecanoylphorbol-13-acetate restored TG activity in the epidermis of keratinocyte-specific Adam17-/- (AD17ΔKC) mice. Further investigations emphasized the expression of PKCη, a mediator of TGM1 transcription, to be sensitive to EGFR activation. In agreement, topical skin application of cholesterol sulfate, an activator of PKCη, significantly improved TG activity in epidermis of AD17ΔKC mice. Our results suggest ADAM17/EGFR-driven PLCγ1 and PKC pathways as important promoters of TG1 expression during terminal keratinocyte differentiation. These findings may help to identify new therapeutic targets for inflammatory skin diseases related to epidermal barrier defects.

MEP50/PRMT5 Reduces Gene Expression by Histone Arginine Methylation and this Is Reversed by PKCδ/p38δ Signaling

PKCδ and p38δ are key proteins in a cascade that stimulates keratinocyte differentiation. This cascade activates transcription of involucrin (hINV) and other genes associated with differentiation. Protein arginine methyltransferase 5 (PRMT5) is an arginine methyltransferase that symmetrically dimethylates arginine residues. This protein interacts with a cofactor, MEP50, and symmetrically dimethylates arginine eight of histone 3 (H3R8me2s) and arginine three of histone 4 (H4R3me2s) to silence gene expression. We use the involucrin gene as a tool to understand the relationship between PKCδ/p38δ and PRMT5/MEP50 signaling. MEP50 suppresses hINV mRNA level and promoter activity. This is associated with increased arginine dimethylation of hINV gene-associated H3/H4. We further show that the PKCδ/p38δ keratinocyte differentiation cascade reduces PRMT5 and MEP50 expression, association with the hINV gene promoter, and H3R8me2s and H4R2me2s formation. We propose that PRMT5/MEP50-dependent methylation is an epigenetic mechanism that assists in silencing of hINV expression, and that PKCδ signaling activates gene expression by directly activating transcription and by suppressing PRMT5/MEP50 dependent arginine dimethylation of promoter associated histones. This is an example of crosstalk between PKCδ/p38δ signaling and PRMT5/MEP50 epigenetic silencing.

RNAi-mediated knockdown of the c-jun gene sensitizes radioresistant human nasopharyngeal carcinoma cell line CNE-2R to radiation

This study aimed to investigate the effect of RNA interference (RNAi)-mediated downregulation of the expression of the c-jun gene (a proto-oncogene) on the radiosensitivity of a radioresistant human nasopharyngeal carcinoma cell line (CNE-2R) and to validate its potential as an anticancer target. A lentiviral vector with c-jun small hairpin RNA (shRNA) was constructed and transfected into CNE-2R cells. The gene silencing efficiency of these recombinants was confirmed by RT-PCR and western blotting. Radiosensitivity, cell proliferation, cell cycle profile and apoptosis were assessed using colony formation assay, CCK-8 assay and flow cytometry, respectively. The lentiviral shRNA efficiently knocked down the expression of c-jun at both the mRNA and protein levels (P<0.05). c-jun-downregulated CNE-2R cells exhibited significantly decreased cell proliferation and enhanced radiosensitivity compared to the control group (P<0.05), and the effects were likely due to G2/M phase arrest and enhanced cell apoptosis. These data provide evidence that c-jun may be involved in the radioresistance of nasopharyngeal carcinoma (NPC) and knockdown of the c-jun gene may be a potential strategy to enhance the radiation sensitivity of NPC.

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.

Protein kinase C δ increases Kruppel-like factor 4 protein, which drives involucrin gene transcription in differentiating keratinocytes

KLF4 is a member of the Kruppel-like factor family of transcriptional regulators. KLF4 has been shown to be required for normal terminal differentiation of keratinocytes, but the molecular mechanism whereby KLF4 regulates genes associated with the differentiation process has not been studied. In the present study, we explore the impact of KLF4 on expression of involucrin, a gene that is specifically expressed in differentiated keratinocytes. KLF4 overexpression and knockdown studies show that involucrin mRNA and protein level correlates directly with KLF4 level. Moreover, studies of mutant KLF4 proteins indicate that transcriptionally inactive forms do not increase involucrin expression. PKCδ is a regulator of keratinocyte differentiation that increases expression of differentiation-associated target genes, including involucrin. Overexpression of KLF4 augments the PKCδ-dependent increase in involucrin expression, whereas KLF4 knockdown attenuates this response. The KLF4 induction of human involucrin (hINV) promoter activity is mediated via KLF4 binding to a GC-rich element located in the hINV promoter distal regulatory region, a region of the promoter required for in vivo involucrin expression. Mutation of the GC-rich element, an adjacent AP1 factor binding site, or both sites severely attenuates the response. Moreover, loss of KLF4 in an epidermal equivalent model of differentiation results in loss of hINV expression. These studies suggest that KLF4 is part of a multiprotein complex that interacts that the hINV promoter distal regulatory region to drive differentiation-dependent hINV gene expression in epidermis.

Protein kinase C (PKC) delta suppresses keratinocyte proliferation by increasing p21(Cip1) level by a KLF4 transcription factor-dependent mechanism

PKCδ increases keratinocyte differentiation and suppresses keratinocyte proliferation and survival. However, the mechanism of proliferation suppression is not well understood. The present studies show that PKCδ overexpression increases p21(Cip1) mRNA and protein level and promoter activity and that treatment with dominant-negative PKCδ, PKCδ-siRNA, or rottlerin inhibits promoter activation. Analysis of the p21(Cip1) promoter upstream regulatory region reveals three DNA segments that mediate PKCδ-dependent promoter activation. The PKCδ response element most proximal to the transcription start site encodes six GC-rich DNA elements. Mutation of these sites results in a loss of PKCδ-dependent promoter activation. Gel mobility supershift and chromatin immunoprecipitation reveal that these DNA elements bind the Kruppel-like transcription factor KLF4. PKCδ increases KLF4 mRNA and protein level and KLF4 binding to the GC-rich elements in the p21(Cip1) proximal promoter. In addition, KLF4-siRNA inhibits PKCδ-dependent p21(Cip1) promoter activity. PKCδ increases KLF4 expression leading to enhanced KLF4 interaction with the GC-rich elements in the p21(Cip1) promoter to activate transcription.

Opposing actions of insulin and arsenite converge on PKCdelta to alter keratinocyte proliferative potential and differentiation

When cultured human keratinocytes reach confluence, they undergo a program of changes replicating features of differentiation in vivo, including exit from the proliferative pool, increased cell size, and expression of specialized differentiation marker proteins. Previously, we showed that insulin is required for some of these steps and that arsenite, a human carcinogen in skin and other epithelia, opposes the differentiation process. In present work, we show that insulin signaling, probably through the IGF-I receptor, is required for the increase in cell size accompanying differentiation and that this is opposed by arsenite. We further examine the impact of insulin and arsenite on PKCdelta, a known key regulator of keratinocyte differentiation, and show that insulin increases the amount, tyrosine phosphorylation, and membrane localization of PKCdelta. All these effects are prevented by exposure of cells to arsenite or to inhibitors of downstream effectors of insulin (phosphotidylinositol 3-kinase and mammalian target of rapamycin). Retrovirally mediated expression of activated PKCdelta resulted in increased loss of proliferative potential after confluence and greatly increased formation of cross-linked envelopes, a marker of keratinocyte terminal differentiation. These effects were prevented by removal of insulin, but not by arsenite addition. We further demonstrate a role for src family kinases in regulation of PKCdelta. Finally, inhibiting epidermal growth factor receptor kinase activity diminished the ability of arsenite to prevent cell enlargement and to suppress insulin-dependent PKCdelta amount and tyrosine 311 phosphorylation. Thus suppression of PKCdelta signaling is a critical feature of arsenite action in preventing keratinocyte differentiation and maintaining proliferative capability.

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.

TRP channel regulates EGFR signaling in hair morphogenesis and skin barrier formation

A plethora of growth factors regulate keratinocyte proliferation and differentiation that control hair morphogenesis and skin barrier formation. Wavy hair phenotypes in mice result from naturally occurring loss-of-function mutations in the genes for TGF-alpha and EGFR. Conversely, excessive activities of TGF-alpha/EGFR result in hairless phenotypes and skin cancers. Unexpectedly, we found that mice lacking the Trpv3 gene also exhibit wavy hair coat and curly whiskers. Here we show that keratinocyte TRPV3, a member of the transient receptor potential (TRP) family of Ca(2+)-permeant channels, forms a signaling complex with TGF-alpha/EGFR. Activation of EGFR leads to increased TRPV3 channel activity, which in turn stimulates TGF-alpha release. TRPV3 is also required for the formation of the skin barrier by regulating the activities of transglutaminases, a family of Ca(2+)-dependent crosslinking enzymes essential for keratinocyte cornification. Our results show that a TRP channel plays a role in regulating growth factor signaling by direct complex formation.

Expression of the homeobox gene, HOPX, is modulated by cell differentiation in human keratinocytes and is involved in the expression of differentiation markers

Homeodomain only protein X (HOPX), an unusual homeodomain protein, was originally identified as a key regulator of cardiac development. We first demonstrated that the expression of HOPX was dependent on the differentiation of human keratinocytes and has an effect on the expression of differentiation markers. HOPX was suppressed in proliferating human keratinocytes and was gradually induced by calcium-triggered differentiation of human keratinocytes. In the epidermis, HOPX is highly expressed in the terminally differentiated suprabasal layers. Among the transcript variants of HOPX, the variant 3 driven by promoter A was the main transcript and it was regulated by cell differentiation in human keratinocytes. The expression of HOPX was induced through the phorbol-12-myristate-13-acetate (PMA)-dependent protein kinase C (PKC) signaling pathway, and not by the demethylating agent, 5-aza-dC (5-aza-2'-deoxycitidine) suggesting the suppression of HOPX is not associated with DNA methylation in human keratinocytes. The RNA interference (RNAi) silencing experiment showed that the knockdown of HOPX expression resulted in the increase of such differentiation markers as involucrin and loricrin. Exogenous expression of HOPX down-regulated the expression of differentiation marker genes in immortalized human keratinocytes (HaCaT). Collectively, HOPX is modulated by cell differentiation in human keratinocytes and this might contribute to homeostasis of keratinocytes by controlling differentiation-dependent genes.

Coordinated integrin and growth factor regulation of primary keratinocyte migration mediated through extracellular signal regulated kinase and phosphoinositide 3-kinase

We have examined coordinated integrin and growth factor regulation of primary keratinocyte migration mediated by phosphoinositide 3-kinase (PI3K) and mitogen-activated extracellular-regulated kinase (MEK)/extracellular signal-regulated kinase (ERK). On collagen I and fibronectin substrates, both epidermal growth factor (EGF) and hepatocyte growth factor (HGF) stimulated chemokinetic (random) and chemotactic (directional) migration. On provisional matrix, a combination of fibronectin and fibrin found in the early phase of wound healing, EGF and HGF-stimulated significant chemotactic but little or no chemokinetic cell movement. Blocking mAbs to integrin alpha2beta1 and alpha5beta1 effectively inhibited EGF- and HGF-stimulated chemokinetic and chemotactic cell movement on collagen I and fibronectin, respectively; however, HGF-stimulated chemotactic migration on collagen I was only partially inhibited by alpha2beta1 blocking mAb. Differentiated keratinocytes underwent reduced chemokinetic and chemotactic migration compared with undifferentiated keratinocytes; however, EGF-stimulated migration was reduced more than HGF-stimulated migration. When the migratory response on collagen I and fibronectin was assessed in the presence of the MEK-specific inhibitor PD98059, EGF- and HGF-stimulated chemotaxis was significantly reduced, whereas PD98059 had little effect on the stimulated chemokinesis. PI3K-specific inhibitor LY294002 reduced EGF- and HGF-stimulated chemokinesis and chemotaxis on collagen I and fibronectin. Thus beta1 integrins acted in concert with EGF and HGF to regulate migration of primary keratinocytes on extracellular matrix components via PI3K and MEK/ERK.

Identification of ERK and JNK as signaling mediators on protein kinase C activation in cultured granulosa cells

PKC signaling is critical for follicular development and the induction of ovulatory genes including Pgr, Prkg2, and Cyp11a1 (SCC). We investigated PKC signaling mechanisms in the JC-410 porcine granulosa cell line stably expressing an SCC-luciferase reporter gene containing 2kb of the porcine SCC promoter. Addition of phorbol 12-myristate 13-acetate (PMA), which activates protein kinase C, induced the promoter approximately 6-fold over the basal levels in 4h. This effect was predominantly mediated by the PKC beta and delta isoforms. PMA-mediated induction of the SCC promoter was sensitive to inhibition of ERK1/2 or JNK. Inhibition of p38 MAP kinase or Src tyrosine kinase did not alter the PMA-mediated inducibility of the promoter. SCC promoter induction in response to PMA treatment required basal EGF-receptor activity, but did not involve ectodomain shedding. Western blot analyses using phospho-specific antibodies showed that PMA treatment of JC-410 cells induced phosphorylation of MEK1/2, ERK1/2, and its downstream target p90 RSK at 15min. We also documented the rapid phosphorylation of JNK1/2 in response to PMA treatment. Phosphorylation of ERK and JNK was robust and sustained in contrast to activation of PKA and EGF-receptor signaling in these cells. Pretreatment of JC-410 granulosa cells with IGF-1 had a synergistic effect on PMA-mediated induction of the SCC promoter. We demonstrated the importance of PMA activation of ERK signaling and the synergism with IGF-1 by showing similar responses for Prkg2 expression in primary granulosa cells. In conclusion, our studies demonstrated PMA activation of ERK and JNK signaling which is relevant in the regulation of gene expression during follicular development, ovulation, and luteinization.

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.

Darier's disease: a calcium-signaling perspective

Ca(2+) influx evoked across the plasma membrane upon internal store depletion is essential for a myriad of cellular functions including gene expression, cell proliferation, differentiation and even apoptosis. Darier's disease (DD), an autosomal dominant inherited disorder of the skin, arising due to mutations in the isoform 2 of the sarco (endo) plasmic reticulum Ca(2+) ATPase (SERCA2), exemplifies an anomaly of Ca(2+) signaling disturbances. Owing to loss of function mutations in SERCA2, keratinocytes in DD patients have a reduced pool of endoplasmic reticulum (ER) Ca(2+). Importantly, the status of ER Ca(2+) is critical for the activation of a class of plasma membrane Ca(2+) channels referred to as store operated Ca(2+) channels (SOCs). The widely expressed transient receptor potential (TRP) family of channels is proposed to be SOCs. In this review we discuss DD from the viewpoint of Ca(2+) signaling and present a potential role for TRPC1 in the disease pathogenesis.

Multiple PKCdelta tyrosine residues are required for PKCdelta-dependent activation of involucrin expression--a key role of PKCdelta-Y311

Protein kinase C-delta (PKCdelta) is a key regulator of human involucrin (hINV) gene expression and is regulated by tyrosine phosphorylation. However, a comprehensive analysis of the requirement for individual PKCdelta tyrosine residues is lacking. We show that multiple tyrosine residues influence the ability of PKCdelta to increase hINV gene expression. Mutation of individual PKCdelta tyrosine residues 52, 64, 155, 187, or 565 does not reduce the ability of PKCdelta to increase hINV promoter activity; however, simultaneous mutation of these five tyrosines markedly reduces activity. Moreover, restoration of any one of these residues results in nearly full activity restoration. It is significant that phosphorylation of PKCdelta-Y(311) is reduced in the five-tyrosine mutant and that mutation of Y(311) results in reduced PKCdelta activity comparable to that observed in the five-tyrosine mutant. Restoration of any one of the tyrosine residues in the five-tyrosine mutant restores Y(311) phosphorylation and biological activity. In addition, reduced phosphorylation of endogenous PKCdelta-Y(311) is associated with reduced biological activity. These findings indicate that PKCdelta activity requires Y(311) and a second tyrosine residue; however, any one of the several tyrosine residues can serve in the secondary role.

The Rb family connects with the Tp53 family in skin carcinogenesis

In contrast with the low frequency of alterations found in the Rb gene, the pRb pathway is inactivated in the vast majority of human tumors. A similar situation takes place in mouse models of cancer, including two-stage skin tumorigenesis. This might be explained if the Rb functions are carried out, in its absence, by other proteins that are also controlled by the same upstream regulators and display similar effectors. The other Rb family members, p107 and or p130, are plausible candidates. The embryonic lethality of pRb-deficient animals, which precludes the analysis of the roles of Rb gene in mouse models, has been avoided using tissue-specific deletion of pRb. In epidermis, pRb deletion leads to altered proliferation and differentiation. However, these deficient mice do not develop spontaneous tumors, and chemical carcinogenesis experiments revealed that the absence of pRb renders fewer and smaller tumors than control animals, but showing increased malignant conversion to squamous cell carcinomas (SCC). Detailed biochemical analyses have indicated that, in the absence of pRb, multiple pathways, including the aberrant p53 activation mediated by E2F/p19(ARF), are activated leading to increased tumor apoptosis. As Rb loss in epidermis is functionally compensated by Rbl1 (p107), this might also suggest that p107 could behave as a tumor suppressor. We summarize here our findings in support of this hypothesis. The pRb-;p107-/- epidermis form spontaneous tumors, and the reduction of p107 levels restores the susceptibility of pRb-mice to chemical skin carcinogenesis experiments. Moreover, Rb-deficient keratinocytes are highly susceptible to Ha-ras-induced transformation, and this susceptibility is enhanced by p107 loss. Further functional studies have indicated that the loss of p107 in the absence of pRb produces the reduction of p53-dependent proapoptotic signals through the modulation of p63 and p73 isoforms. In addition, expression profiling analysis has revealed multiple oncogenic alterations that can contribute to tumor susceptibility in epidermis in the absence of pRb and p107.

Protein kinase C family: on the crossroads of cell signaling in skin and tumor epithelium

The protein kinase C (PKC) family represents a large group of phospholipid dependent enzymes catalyzing the covalent transfer of phosphate from ATP to serine and threonine residues of proteins. Phosphorylation of the substrate proteins induces a conformational change resulting in modification of their functional properties. The PKC family consists of at least ten members, divided into three subgroups: classical PKCs (alpha, betaI, betaII, gamma), novel PKCs (delta, epsilon, eta, theta), and atypical PKCs (zeta, iota/lambda). The specific cofactor requirements, tissue distribution, and cellular compartmentalization suggest differential functions and fine tuning of specific signaling cascades for each isoform. Thus, specific stimuli can lead to differential responses via isoform specific PKC signaling regulated by their expression, localization, and phosphorylation status in particular biological settings. PKC isoforms are activated by a variety of extracellular signals and, in turn, modify the activities of cellular proteins including receptors, enzymes, cytoskeletal proteins, and transcription factors. Accordingly, the PKC family plays a central role in cellular signal processing. Accumulating data suggest that various PKC isoforms participate in the regulation of cell proliferation, differentiation, survival and death. These findings have enabled identification of abnormalities in PKC isoform function, as they occur in several cancers. Specifically, the initiation of squamous cell carcinoma formation and progression to the malignant phenotype was found to be associated with distinct changes in PKC expression, activation, distribution, and phosphorylation. These studies were recently further extended to transgenic and knockout animals, which allowed a more direct analysis of individual PKC functions. Accordingly, this review is focused on the involvement of PKC in physiology and pathology of the skin.

EGF Receptor Activation Decreases Retroviral Gene Transfer through Protein Kinase C-δ

Although much progress has been made in the design of retrovirus vectors, the interactions of recombinant retrovirus with host cells remain largely elusive. The inability of recombinant retrovirus to transduce non-dividing cells prompted several studies to determine optimal cocktails of growth factors and/or extracellular matrix molecules to promote gene transfer to slowly diving cells and stem cells. In contrast to previous reports that growth factors increased gene transfer, we found that treatment of human epidermal keratinocytes and several cell lines with epidermal growth factor receptor (EGFR) ligands EGF, transforming growth factor-alpha, or heparin-binding-EGF decreased gene transfer. Conversely, treatment with an EGFR function-blocking antibody or inhibition of EGFR tyrosine phosphorylation enhanced gene transfer in a dose-dependent manner. In addition, blocking protein kinase C (PKC)-delta but not PKC-zeta, with chemical inhibitors or small interfering RNA reversed the effects of EGF and restored gene transfer, indicating that the effect of EGFR activation is mediated through PKC-delta. Lastly, cell cycle analysis showed that the effect of EGFR activation on retroviral gene transfer was independent of the cell cycle status of target cells. Our results implicate EGFR and PKC-delta in retroviral infection and may have implications for retrovirus gene transfer or design of antiretroviral therapies.

Insulin receptor substrate 1 (IRS-1) plays a unique role in normal epidermal physiology

Insulin receptor substrate (IRS) proteins play a central role in insulin signaling. Previously we have demonstrated that insulin is essential for normal skin development and function. In the present study we investigated the involvement of the IRS-1 and IRS-2 proteins in skin physiology and in mediating insulin action in skin. For this purpose we have investigated the effects of inactivation of each of the IRSs on skin, studying skin sections and primary skin cells derived from IRS-1 or IRS-2 null mice. We have demonstrated that while the skin of the IRS-2 null mice appeared normal, the skin of the IRS-1 null mice was thinner and translucent. Histological analysis revealed that the thinning of the IRS-1 null skin was a consequence of the thinning of the spinous compartment, consisting of fewer layers. Proliferation of the IRS-1 and IRS-2 null skin epidermal cells was normal. However, the differentiation process of the IRS-1 skin and skin cells was impaired. There was a marked decrease in the induction of the expression of K1, the marker of advanced stages of skin differentiation. In contrary, IRS-2 inactivation had no effects on skin differentiation. In conclusion, we have shown for the first time that IRS-1 but not IRS-2 has an effect on skin formation and development, being one of the main activators of the differentiation process in skin keratinocytes. Furthermore, we suggest that IRS-1 and IRS-2 have distinct roles in skin physiology.

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.

Insulin-like growth factor 1 receptor signaling regulates skin development and inhibits skin keratinocyte differentiation

The insulin-like growth factor 1 receptor (IGF-1R) is a multifunctional receptor that mediates signals for cell proliferation, differentiation, and survival. Genetic experiments showed that IGF-1R inactivation in skin results in a disrupted epidermis. However, because IGF-1R-null mice die at birth, it is difficult to study the effects of IGF-1R on skin. By using a combined approach of conditional gene ablation and a three-dimensional organotypic model, we demonstrate that IGF-1R-deficient skin cocultures show abnormal maturation and differentiation patterns. Furthermore, IGF-1R-null keratinocytes exhibit accelerated differentiation and decreased proliferation. Investigating the signaling pathway downstream of IGF-1R reveals that insulin receptor substrate 2 (IRS-2) overexpression compensates for the lack of IGF-1R, whereas IRS-1 overexpression does not. We also demonstrate that phosphatidylinositol 3-kinase and extracellular signal-regulated kinase 1 and 2 are involved in the regulation of skin keratinocyte differentiation and take some part in mediating the inhibitory signal of IGF-1R on differentiation. In addition, we show that mammalian target of rapamycin plays a specific role in mediating IGF-1R impedance of action on keratinocyte differentiation. In conclusion, these results reveal that IGF-1R plays an inhibitory role in the regulation of skin development and differentiation.

Unexpected roles for pRb in mouse skin carcinogenesis

The mouse skin carcinogenesis represents one of the best models for the understanding of malignant transformation, including the multistage nature of tumor development. The retinoblastoma gene product (pRb) plays a critical role in cell cycle regulation, differentiation, and inhibition of oncogenic transformation. In epidermis, Rb-/- deletion leads to proliferation and differentiation defects. Numerous evidences showed the involvement of the retinoblastoma pathway in this model. However, the actual role of pRb is still unknown. To study the possible involvement of pRb in keratinocyte malignant transformation, we have carried out two-stage chemical skin carcinogenesis on Rb(F19/F19) (thereafter Rb+/+) and Rb(F19/F19);K14Cre (thereafter Rb-/-) animals. Unexpectedly, we found that Rb-/- mice developed fewer and smaller papillomas than the Rb+/+ counterparts. Moreover, the small size of the pRb-deficient tumors is associated with an increase in the apoptotic index. Despite this, pRb-deficient tumors display an increased conversion rate to squamous cell carcinomas. Biochemical analyses revealed that these characteristics correlate with the differential expression and activity of different pathways, including E2F/p19arf/p53, PTEN/Akt, c-jun NH2-terminal kinase/p38, and nuclear factor-kappaB. Collectively, our findings show unexpected and hitherto nondescribed roles of pRb during the process of epidermal carcinogenesis.

Phosphatidylinositol 3-kinase regulates early differentiation in human laryngeal keratinocytes

Epidermal growth factor receptor (EGFR) signaling regulates a variety of cellular functions, including proliferation, gene expression, and differentiation. Infection of laryngeal epithelial cells by human papillomaviruses causes recurrent respiratory papillomas, benign tumors characterized by an altered pattern of differentiation. Papilloma cells overexpress the EGFR and have constitutively active extracellular signal-regulated kinase (ERK) and enhanced phosphatidylinositol 3-kinase (PI3K) activity, but overexpression of the lipid phosphatase PTEN (Phosphatase and Tensin Homolog) reduces activation of Akt by PI3K. We hypothesized that the altered differentiation of papillomas reflects these changes in signaling from the EGFR-ERK and PI3K-Akt pathways and that one or both of these pathways is required for the normal differentiation process in mucosal epithelium. Inhibiting either the enzymatic activity or the synthesis of PI3K in uninfected laryngeal cells blocked expression of keratin-13 (K13), a protein induced during normal differentiation. In contrast, inhibiting activation of ERK had minimal effect. Using ribonucleic acid interference to reduce protein levels of integrin-linked kinase 1 or phosphoinositide-dependent protein kinase 1, intermediates in the activation of Akt by PI3K, or reducing levels of Akt-1 itself did not inhibit K13 expression by normal laryngeal keratinocytes. We conclude that PI3K activation is an important regulator of expression of K13, a marker for the normal differentiation process in human mucosal keratinocytes, that this function does not require activation of Akt-1, and that the failure to express K13 in papilloma cells is not because of reduction in activated Akt.

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.

Overexpression of the calcium sensing receptor accelerates epidermal differentiation and permeability barrier formation in vivo

The calcium sensing receptor (CaSR) has emerged as an important mediator of a wide range of Ca(2+)-dependent physiological responses (Ca(2+) signaling) in various tissues. To explore the role of CaSR in the epidermis, we utilised the keratin 14 promoter to express CaSR cDNA constitutively in the basal cells of the stratified squamous epithelium of transgenic mice. Analysis of the transgenic mice revealed that a sensitized response to CaSR signaling accelerates the epidermal differentiation program with the precocious formation of the epidermal permeability barrier (EPB) during development and an accelerated hair growth at birth. Our observations indicate that overexpression of CaSR in the undifferentiated basal cells leads to changes in the differentiation program of the transgenic epidermis, including the stimulation of keratins 1 and 6 as well as the overexpression of several markers of terminal differentiation such as filaggrin, loricrin and involucrin. Our data suggest that the observed modifications in the differentiation pathway are a consequence of a CaSR-induced enhancement of Ca(2+) signaling involving cross-talk with other signaling pathways (e.g. EGF and Wnt/Ca(2+)). These studies provide new insights into the role of CaSR in epidermal differentiation including EPB development and hair follicle morphogenesis.

Calcium: a potential central regulator in wound healing in the skin

Calcium has an established role in the normal homeostasis of mammalian skin and serves as a modulator in keratinocyte proliferation and differentiation. Gradients of calcium concentration increasing from 0.5 mM in the basal layer to > 1.4 mM in the stratum granulosum are consistent with migration patterns in response to minor abrasion (normal wear). Dermal fibroblasts require calcium but are approximately 100 times less sensitive than keratinocytes. Normal calcium metabolism in the skin is dependent on cell membrane and cytosolic calcium binding proteins (calmodulin, cadherins, etc.), but their modulation through parathyroid hormone, vitamin D or growth factors in normal or damaged tissue is not well documented. In wound repair, calcium is predominantly involved as Factor IV in the hemostatic phase, but it is expected to be required in epidermal cell migration and regeneration patterns in later stages of healing. Calcium alginate dressings are designed to liberate calcium early in the acute phase to promote hemostasis, but it is presently unclear whether the supplementary calcium influences the intracellular environment at later stages of wound repair, notably during the remodeling phase. Although experimental studies suggest that control of calcium is obligatory in wound management, we know very little as to how calcium in the wound bed is modulated through hormones, vitamin D, or various growth factors. Also, there is limited information as to how calcium released either from dressings, platelets, or from the circulation through the action of parathyroid hormone, growth factors or other modulators influences cell migration and remodeling in skin wounds, although experimental models suggest that management of calcium is essential in wound management.

Novel protein kinase C isoforms and mitogen-activated kinase kinase mediate phorbol ester-induced osteopontin expression

BACKGROUND AND AIMS

The expression of osteopontin (OPN), a protein postulated to play a role in tumorigenesis, is induced by the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA) in vivo and in the in vitro initiation-promotion skin carcinogenesis model (JB6 cells). Although TPA-induced OPN expression in JB6 cells has been suggested to involve protein kinase C (PKC), the PKC isoforms and the downstream pathway mediating OPN expression have not been extensively studied.

METHODS

Using the JB6 cell model, we determined the involvement of PKC isoforms, mitogen-activated protein kinase kinase (MAPK kinase/MEK) and MAPK in TPA-induced OPN expression using inhibitors specific to PKC isoforms and MEK and performing Northern blot analyses. Western blot analyses of cells treated with specific inhibitors were also performed to determine whether PKC isoforms or MEK were involved in activation of MAPK.

KEY RESULTS

TPA increased the steady-state level of OPN mRNA as early as 2-4h and this expression persisted for at least 4 days. TPA induction of OPN expression in JB6 cells is mediated through PKC epsilon and PKC delta, which also mediated the phosphorylation of MAPK. Additionally, inhibition of MEK activity, which activates MAPK, attenuated TPA-induced OPN expression. These findings suggest that activation of MAPK is important in mediating OPN expression.

CONCLUSION

TPA-induced steady-state OPN mRNA expression in mouse JB6 cells involves the activation of MAPK mediated through PKC epsilon and/or PKC delta.

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.

Src family kinases phosphorylate protein kinase C delta on tyrosine residues and modify the neoplastic phenotype of skin keratinocytes

Protein kinase C delta (PKC delta) is tyrosine-phosphorylated and catalytically inactive in mouse keratinocytes transformed by a ras oncogene. In several other model systems, Src kinases are upstream regulators of PKC delta. To examine this relationship in epidermal carcinogenesis, v-ras transformed mouse keratinocytes were treated with a selective Src kinase inhibitor (PD 173958). PD 173958 decreased autophosphorylation of Src, Fyn, and Lyn kinases and prevented tyrosine phosphorylation of the Src kinase substrate p120. PD 173958 also prevented PKC delta tyrosine phosphorylation and activated PKC delta as detected by membrane translocation. Expression of keratinocyte differentiation markers increased in PD 173958-treated v-ras-keratinocytes, and fluid-filled domes emerged, indicative of tight junction formation. Antisense PKC delta or bryostatin 1 inhibited dome formation, while overexpression of PKC delta in the presence of PD 173958 enhanced the formation of domes. Plasmids encoding phenylalanine mutants of PKC delta tyrosine residues 64 and 565 induced domes in the absence of PD 173958, while phenylalanine mutants of tyrosine residues 52, 155, and 187 were inactive. Thus, Src kinase mediated post-translational modification of PKC delta on specific tyrosine residues in ras-transformed mouse keratinocytes inactivates PKC delta and contributes to alterations in the differentiated phenotype and tight junction formation associated with neoplasia.

Expression of desmosomal proteins in rat keratinocytes during in vitro differentiation

The keratinocyte, the major component of the epidermis, expresses several proteins that characterize the keratinization during the differentiation. Proliferation and differentiation of cultured human keratinocytes are known to be regulated by the Ca2+ concentration in the culture medium. However, informations about the rat keratinocyte are relatively limited and their physiology is still an open question. To elucidate the characteristics of the rat keratinocyte, we established rat keratinocyte culture system and examined effects of extracellular calcium concentration on the expression of differentiation-related proteins. Keratinocytes were isolated from the newborn rat skin with 0.25% trypsin, followed by separation with a Percoll density gradient. The separated cells were grown in MCDB 153 medium containing several growth factors and Ca(2+)-free fetal bovine serum, then stimulated with Ca2+. Immunoblotting demonstrated strong expression of beta1 integrin in unstimulated cells, suggesting that the primary culture of rat keratinocytes was successfully established. Expression of desmoglein and transglutaminase was increased by Ca2+ stimulation, whereas beta1 integrin expression was decreased in response to increasing concentrations of Ca2+. These observations indicate that cultured rat keratinocytes maintain the ability to differentiate in vitro, which is similar to that of the basal keratinocytes in the epidermis.

Antitumor activity of protein kinase C inhibitors and cisplatin in human head and neck squamous cell carcinoma lines

Protein kinase C (PKC) plays a pivotal role in signal transduction involved in the control of cell proliferation, differentiation and apoptosis. Interference with such signaling pathways may result in altered tumor cell response to antineoplastic drugs. We investigated the effects of two selective PKC inhibitors as single agents and in combination with cisplatin in cell lines derived from squamous cell carcinomas of the head and neck (SCCHN). Safingol (Saf) is directed against the regulatory domain, whereas chelerythrine (Che) interacts with the catalytic domain of PKC. In six SCCHN cell lines (UM-SCC 11B, 14A, 14C and 22B, 8029NA, and a 5-fold cisplatin-resistant subline 8029DDP). PKC activities ranged between 1 and 158 IU/1 x 10(7) cells, and they were inversely proportional to the amount of cellular epidermal growth factor receptor. Using the colorimetric MTT assay, PKC inhibitors Saf and Che showed comparable dose-dependent growth inhibition. The 50% inhibitory concentrations (IC50) were between 3.8-8.6 microM for Saf and 8.5-13.6 microM for Che with no relationship to PKC activity or cisplatin sensitivity of the respective cell lines. Combinations of cisplatin (IC50 = 0.4-5.8 microg/ml) and either PKC inhibitor (5 microM Saf, 10 microM Che) led to a significant decrease of cisplatin IC50 values in most cell lines. However, comparison with theoretical additive dose-response curves showed additive rather than synergistic effects for both PKC inhibitors.

Protein Kinase C Is Involved in the Regulation of hairless mRNA Expression during Mouse Keratinocyte Differentiation

The hairless (hr) gene is a putative transcriptional factor whose mutations lead to hair loss in animals and humans. As a step toward understanding the role of the hr gene, we investigated the expression of hr mRNA in mouse keratinocyte differentiation. Treatment of mouse primary keratinocyte cultures with phorbol-12-myristate-13-acetate (PMA) reduced DNA synthesis and sequentially induced an up-regulation of p21Cip1/WAF1 (p21), hr and involucrin (inv) mRNAs in a time-dependent fashion, suggesting that an increase in hr gene expression is associated with keratinocyte differentiation. This up-regulation was blocked by the RNA synthesis inhibitor actinomycin D. However, an increase in hr mRNA, but not in inv mRNA, was seen in cells treated with the protein synthesis inhibitor cycloheximide, suggesting that new protein synthesis is involved in the suppression of hr transcription or in the degradation of hr mRNA in the steady state. The up-regulation of hr mRNA expression by PMA was blocked by the protein kinase C (PKC) inhibitor, GF109203X. These data indicate that PKC activation is involved in the up-regulation of hr mRNA expression during mouse keratinocyte differentiation.

Differentiation therapy of human cancer: basic science and clinical applications

Current cancer therapies are highly toxic and often nonspecific. A potentially less toxic approach to treating this prevalent disease employs agents that modify cancer cell differentiation, termed 'differentiation therapy.' This approach is based on the tacit assumption that many neoplastic cell types exhibit reversible defects in differentiation, which upon appropriate treatment, results in tumor reprogramming and a concomitant loss in proliferative capacity and induction of terminal differentiation or apoptosis (programmed cell death). Laboratory studies that focus on elucidating mechanisms of action are demonstrating the effectiveness of 'differentiation therapy,' which is now beginning to show translational promise in the clinical setting.

A PKC-eta/Fyn-dependent pathway leading to keratinocyte growth arrest and differentiation

Growth control of epithelial cells differs substantially from other cell types. Activation of Fyn, a Src kinase family member, is required for normal keratinocyte differentiation. We report that increased Fyn activity by itself suppresses growth of keratinocytes, but not dermal fibroblasts, through downmodulation of EGF receptor (EGFR) signaling. Protein kinase C-eta has also been implicated in keratinocyte growth/differentiation control. We show that growth suppression of keratinocytes by PKC-eta depends mostly on Fyn. PKC-eta activity is both necessary and sufficient for Fyn activation, PKC-eta and Fyn are found in association, and recombinant PKC-eta directly activates Fyn. Thus, our findings reveal a direct cross talk between PKC-eta and Fyn, which presides over the decision between keratinocyte (epithelial) cell growth and differentiation.