Role of NF-kappaB in the apoptotic-resistant phenotype of keratinocytes

Proliferation and cornification during development of the mammalian epidermis

The skin is the largest organ of the body and consists of the underlying dermis and outer epidermis. Proper embryonic development and continual renewal of the adult epidermis are essential to provide an impenetrable barrier against fluid loss and serve as our most important defense against insults from the external environment. During mammalian embryogenesis the epidermis develops from the surface ectoderm, which initially consists of a multipotent single-layer epithelium. Once these epithelial cells receive the appropriate developmental cues, they become committed to stratify, initiate a massive expansion program, and finally embark on a journey of terminal differentiation to produce the morphologically distinct layers of the epidermis. The culmination of this journey is the formation of an impervious cornified envelope via a highly specialized form of programmed cell death, termed "cornification" (reviewed in Candi et al.), which is distinct in many ways from the classic apoptotic pathways. The epidermal developmental program that is first seen in the fetus is recapitulated for the entire life of the organism. The basal layer of adult skin harbors stem cells, which can divide to produce daughter stem cells and transit amplifying (TA) cells that go on to differentiate and cornify (reviewed in Fuchs and Raghavan). In this review we summarize current knowledge about the molecular regulation of proliferation and cornification in the developing mammalian epidermis. We focus on events in the interfollicular epidermis, with special emphasis on transcriptional regulation by p63, Notch, NF-kappaB/IKK, Hox, AP-1, AP-2, and C/EBP factors. We end with a discussion about perturbations in epidermal proliferation and cornification as they pertain to human skin pathologies.

Defining the caspase-containing apoptotic machinery contributing to cornification in human epidermal equivalents

Whether terminal differentiation/stratum corneum formation of keratinocytes (KCs) represents a form of programmed cell death, utilizing mediators of classical apoptosis, is unclear. Apoptosis, an evolutionarily conserved death process, is comprised of extrinsic and intrinsic pathways, which converge using caspase 3. To define upstream and downstream caspases involved in terminal differentiation, we utilized human epidermal equivalents (EEs). Using submerged cultures comprised of human KCs, EEs were sequentially analyzed before and after being raised to an air/liquid (A/L) interface at 3-24 h intervals. At each time point, EEs were analyzed morphologically and for specific enzyme activity to distinguish different initiator (caspases 1, 2, 8, 9) and effector caspases (3, 6, 7). Terminal differentiation began at 6-8 h, as defined by stratum corneum with loricirin expression and completed at 18-24 h producing an epidermis resembling normal skin. Enzyme activity for caspases 1, 2, 3, 6, 7, 8, and 9 (but not 4, 5) was enhanced (>two-fold nmol/mg/h) at 3-6 h compared with submerged cultures. Processing of caspase 14 occurred at 18 h, and cleaved caspase 14 was increased at 24 h. Activated caspase 3-positive and terminal deoxynucleotidyl transferase-mediated nick end labeling-positive KCs were identified in EEs at 3-6 h corresponding to initiation sites of terminal differentiation. Addition of caspase inhibitors reduced levels of involucrin and loricrin in EEs raised to an A/L interface. We conclude caspases function as important death effectors strategically positioned at intersection of intrinsic and extrinsic pathways in KCs undergoing stratum corneum formation.

The ubiquitin-proteasome system at the crossroads of stress-response and ageing pathways: a handle for skin care?

The regulation of gene expression at the transcriptional level has been considered for long as the main mechanism of cellular adaptive responses. Since the turn of the century, however, it is becoming clear that higher organisms developed a complex, sensitive and maybe equally important network of regulatory pathways, relying largely on protein interactions, post-translational modifications and proteolysis. Here we review the involvement of the ubiquitin-proteasome pathway of protein degradation at different levels of cellular life in relation with ageing, and with a special focus on skin. It comes out that the ubiquitin system plays a major role in signal transduction associated with stress and ageing, in skin in particular through the control of retinoid and NF-kappaB pathways. The understanding of specific proteolytic targeting by E3 ubiquitin-ligases paves the way for a new generation of active molecules that may control particular steps of normal and pathological ageing.

Knock down of p53 levels in human keratinocytes increases susceptibility to type I and type II interferon-induced apoptosis mediated by a TRAIL dependent pathway

BACKGROUND

Keratinocytes (KCs) in healthy skin only undergo death following differentiation to produce stratum corneum. By contrast, in inflammatory pathological conditions featuring type I (IFN-alpha) and type II (IFN-gamma) interferons KCs undergo premature apoptosis.

OBJECTIVE

To define apoptotic susceptibility of KCs, response to interferons was examined. Since molecular cross-talk occurs between interferons and p53, potential mechanistic roles for p53 in KC apoptosis were investigated.

METHODS

Knock down of p53 was performed, and apoptotic response to addition of interferons was assessed using FACS and by staining for activated caspase 3 and TUNEL. Elucidation of death pathway was accomplished by using a dominant negative death receptor construct and a neutralizing TRAIL antibody.

RESULTS

Reduction in p53 levels in KCs by siRNA treatment enhanced, rather than reduced, apoptotic responses to IFN-alpha plus IFN-gamma. In an immortalized human KC cell line (HaCaT cells with both p53 alleles mutated) enhanced apoptotic susceptibility to interferon exposure was also observed. The mechanism for this enhanced apoptosis involved induction of TRAIL and its interaction with death receptors, as blocking the death receptor pathway using dominant negative FADD, or by addition of neutralizing antibody against TRAIL, reduced the apoptotic response to IFN-alpha and IFN-gamma.

CONCLUSION

These results indicate IFN-alpha plus IFN-gamma triggers apoptosis independent of p53 in HaCaT cells, and also demonstrate an unexpected survival role for p53 in human KCs as regards apoptotic responsiveness to cytokines such as IFN-alpha and IFN-gamma involving activation of TRAIL-related death receptors. Strategies enhancing p53 regulated survival proteins in KCs may be of therapeutic benefit in skin disorders characterized by activated immunocytes triggering premature KC apoptosis.

Notch1 augments NF-kappaB activity by facilitating its nuclear retention

Notch1 specifically upregulates expression of the cytokine interferon-gamma in peripheral T cells through activation of NF-kappaB. However, how Notch mediates NF-kappaB activation remains unclear. Here, we examined the temporal relationship between Notch signaling and NF-kappaB induction during T-cell activation. NF-kappaB activation occurs within minutes of T-cell receptor (TCR) engagement and this activation is sustained for at least 48 h following TCR signaling. We used gamma-secretase inhibitor (GSI) to prevent the cleavage and subsequent activation of Notch family members. We demonstrate that GSI blocked the later, sustained NF-kappaB activation, but did not affect the initial activation of NF-kappaB. Using biochemical approaches, as well as confocal microscopy, we show that the intracellular domain of Notch1 (N1IC) directly interacts with NF-kappaB and competes with IkappaBalpha, leading to retention of NF-kappaB in the nucleus. Additionally, we show that N1IC can directly regulate IFN-gamma expression through complexes formed on the IFN-gamma promoter. Taken together, these data suggest that there are two 'waves' of NF-kappaB activation: an initial, Notch-independent phase, and a later, sustained activation of NF-kappaB, which is Notch dependent.

Inhibition of the nuclear factor-kappaB signaling pathway by leflunomide or triptolide also inhibits the anthralin-induced inflammatory response but does not affect keratinocyte growth inhibition

We performed this study to determine the relationship between activation of nuclear factor (NF)-kappaB and inhibition of keratinocyte growth by anthralin, which not only might be useful for a better understanding of the role of NF-kappaB in the pathogenesis of psoriasis, but also indicate whether the inflammatory reaction induced by anthralin is inseparable from its antipsoriatic activity. The involvement of NF-kappaB was assessed using the antipsoriatic drugs leflunomide and triptolide (T0) as effectors, since they can inhibit NF-kappaB activation induced by anthralin. The results showed that the inhibition of keratinocyte growth by anthralin was not related to the activation of NF-kappaB. Using sodium salicylate, a known NF-kappaB inhibitor, further confirmed this conclusion. Thus it might be possible to inhibit the inflammatory response induced by anthralin via repression of NF-kappaB activation. We found that leflunomide or T0 could significantly inhibit the mRNA overexpression of interleukin-8 and intercellular adhesion molecule-1 in keratinocytes induced by anthralin. Taken together, our data indicate that the growth inhibition of anthralin is related to the NF-kappaB-independent signaling pathway, and that leflunomide or T0 could control proinflammatory cytokine expression induced by anthralin via inhibiting the activation of NF-kappaB.

Differential expression of phosphorylated NF-kappaB/RelA in normal and psoriatic epidermis and downregulation of NF-kappaB in response to treatment with etanercept

Etanercept, a recombinant human tumor necrosis factor (TNF) receptor fusion protein, is FDA approved for psoriasis and psoriatic arthritis. TNFalpha increases the synthesis of proinflammatory cytokines and leads to the activation of multiple signaling pathways, including nuclear factor kappa B (NF-kappaB). The Rel/NF-kappaB transcription factors play a central role in numerous cellular processes, including the stress response and keratinocyte proliferation and differentiation. Utilizing a phosphorylation-specific antibody, we examined the expression of active nuclear NF-kappaB/RelA via immunohistochemistry in normal skin, non-lesional psoriatic skin, lesional psoriatic skin, and lesional skin from patients treated with etanercept. There was no expression of active nuclear NF-kappaB in the normal epidermis, whereas a basal level of constitutive active phosphorylated NF-kappaB/RelA was present in uninvolved epidermis from psoriasis patients. There was also significant upregulation of active phosphorylated NF-kappaB/RelA in the epidermis from psoriatic plaques. Serial biopsies from psoriasis patients treated with etanercept at 1, 3, and 6 mo demonstrated a significant downregulation of phosphorylated NF-kappaB/RelA, which correlated with decreases in epidermal thickness, restoration of normal markers of keratinocyte differentiation, and clinical outcomes. These data suggest that activation of NF-kappaB plays a significant role in the pathogenesis of psoriasis and that a potential mechanism of action for TNF-targeting agents is downregulation of NF-kappaB transcriptional activity.

Toxic epidermal necrolysis: current evidence, practical management and future directions

Toxic epidermal necrolysis (TEN) is a rare disorder characterized by extensive epidermal death. Almost all cases appear to be caused by an idiosyncratic drug reaction. Proposed pathogenic mechanisms are conflicting, and the evidence for the benefits of individual treatments is inadequate, and in some cases contradictory. The mortality rate remains high. We review the literature pertaining to the pathogenesis of TEN and drug reactions in general. The rationale for therapeutic interventions, together with reported evidence of efficacy, are considered. We present a composite model of TEN, based on previous work and suggested pathogeneses of TEN, mechanisms of drug reactions and reported cytotoxic lymphocyte (CTL) cytolytic pathways. In this system, TEN, like some other cutaneous drug eruptions, is an HLA class I-restricted, specific drug sensitivity, resulting in clonal expansion of CD8+ CTLs. Cytotoxicity is mediated by CTL granzyme and possibly death receptor (DR) ligand (DR-L), probably Fas ligand (FasL). Particular to TEN, there is then an amplification sequence involving further DR-L expression. FasL is likely to be particularly important but tumour necrosis factor (TNF) may well contribute, via the TNF receptor 1 (TNF-R1) death pathway. Alternatively, we suggest the possibility of upregulation of an antiapoptotic TNF-R1-nuclear factor kappaB pathway, which would proscribe treatments which downregulate this pathway. None of the published data on individual treatment efficacies is sufficiently strong to suggest a definitive single treatment. Currently a multifaceted regimen appears indicated, targeting various likely intermediary mechanisms, including elimination of residual drug, immunosuppression, inhibition of DR pathways, general antiapoptotic strategies, and aggressive supportive care. Particular attention has been directed at avoiding potential conflicts between different treatments and avoiding agents that theoretically might have a net proapoptotic rather than antiapoptotic effect. Nursing on a specialized unit is of paramount importance.

UV induced responses of the human epidermal IGF system: impaired anti-apoptotic effects of IGF-I in HaCaT keratinocytes

The insulin-like growth factor receptor (IGF-IR) is critical in epidermal development and IGF binding protein-3 (IGFBP-3), a modulator of cellular activity with or without IGF-dependence, co-localises with epidermal IGF-IRs. We have investigated whether the greater UV susceptibility of a human keratinocyte cell line (HaCaT) in comparison to normal human keratinocytes (NHKs) may involve differences in the IGF system. At 24 h after UV (960 mJ/cm(2) UVB), in comparison to NHKs, HaCaT keratinocytes exhibited significantly higher levels of apoptosis, refractoriness to IGF-I treatment and reduced IGF-IR phosphorylation. Secreted, intact IGFBP-3 (38-42 kDa) and IGFBP-3 mRNA abundance were reduced in HaCaT keratinocytes, but not consistently altered in NHKs. Immunoreactive IGFBP-3 fragments (16-11 kDa) were detected in both UV-exposed cultures. These data suggest that an altered IGF system contributes to HaCaT keratinocyte UV susceptibility and that following UV insult the IGF system may enhance keratinocyte viability and contribute to a return to epidermal homeostasis.

High levels of p105 (NFKB1) and p100 (NFKB2) proteins in HPV16-transformed keratinocytes: role of E6 and E7 oncoproteins

We have previously shown that functional components of the NF-kappaB signaling pathway are up-regulated and sequestered in the cytoplasm of human papillomavirus 16 (HPV16)-transformed cell lines leading to a reduced activity of NF-kappaB. In this study, we examined the expression of the NF-kappaB precursors p100 and p105 in keratinocytes transformed or not by HPV16. Western immunoblotting experiments demonstrated high levels of p100 and p105 proteins not only in HPV16+ cervical carcinoma-derived keratinocytes but also in keratinocytes stably transfected by HPV16 E6 or E7 oncogenes. Moreover, p100 and p105 proteins were predominantly cytoplasmic and nuclear in keratinocytes expressing E7 and E6, respectively. A predominantly cytoplasmic localization of E7 protein was also detected in all keratinocytes expressing E7. Our results suggest that HPV16 E6 and E7 proteins modulate the expression and the subcellular localization of p100 and p105 NF-kappaB precursors.

Microarray analysis of UVB-regulated genes in keratinocytes: downregulation of angiogenesis inhibitor thrombospondin-1

BACKGROUND

Ultraviolet (UV) B light is an environmental mutagen that induces changes in cutaneous gene expression leading to immune suppression and carcinogenesis. Keratinocytes are a primary target for UVB.

OBJECTIVE

To further delineate UVB-induced gene expression changes in keratinocytes.

METHODS

cDNA microarray technology was utilized to examine gene expression in normal human KC (NHKC) following 20 mJcm(-2) UVB irradiation. Data was confirmed by semi-quantitative RT-PCR.

RESULTS

Microarray analysis revealed 57 genes were upregulated, and 27 genes were downregulated, by at least two-fold following UVB. One downregulated gene was the endogenous angiogenesis inhibitor thrombospondin-1 (TSP-1). Semi-quantitative RT-PCR confirmed persistent downregulation of TSP-1 up to 18h following UVB. Microarray analysis also revealed upregulation of platelet-derived endothelial cell growth factor (PD-ECGF)--an angiogenesis activator.

CONCLUSION

Our results suggest a gene expression mechanism by which UVB induces an angiogenic switch in keratinocytes. This may represent an important early event promoting neovascularization and growth of cutaneous neoplasms.

Plakin proteins are coordinately cleaved during apoptosis but preferentially through the action of different caspases

In epithelial cells, cell-cell and cell-matrix junctions, desmosomes and hemidesmosomes, provide anchorage sites for the keratin-intermediate filaments. The plakin proteins desmoplakin (DP), plectin, and periplakin represent intracellular constituents of these adhesion junctions. In staurosporine-treated apoptotic HaCaT cells, DP, plectin, and periplakin became cleaved coordinately with the elimination of keratins 10 and 14, while involucrin, actin, and keratin 18 displayed considerable stability. The caspase inhibitor zVAD-fmk prevented both the cell detachment and protein cleavage, indicating the function of caspases in these events. Closer examination in vitro revealed that while caspases 2 and 4 most efficiently cleaved DP, and plectin served as a target for caspases 3 and 7, periplakin as well as keratins were cleaved by caspase 6. The involvement of multiple caspases in the destruction of epithelial cell integrity ensures the efficient elimination of cytoskeleton, but also provides specificity for selectively targeting individual adhesion molecules.

A Role for Tumor Necrosis Factor-α-Mediated Endothelial Apoptosis in the Development of Experimental Idiopathic Pneumonia Syndrome

BACKGROUND

Idiopathic pneumonia syndrome (IPS) is a frequent and often fatal complication of allogeneic bone marrow transplantation (BMT). We have previously shown that experimental IPS is associated with alloreactive donor T cells and the inflammatory mediators TNF-alpha and lipopolysaccharide. Both TNF-alpha and lipopolysaccharide are known contributors to endothelial injury. Although damage to vascular endothelia has been associated with other complications after BMT, its relationship to lung injury has not been explored.

METHODS

We used a well-established murine BMT system, in which lung injury and graft-versus-host disease are induced by minor histocompatibility antigenic differences between donor and host, and the DNA terminal transferase nick-end labeling (TUNEL) procedure to evaluate whether significant pulmonary vascular endothelial cell (EC) apoptosis is present during the development of IPS.

RESULTS

Our data demonstrate that pulmonary histopathology after allogeneic BMT is accompanied by significant EC apoptosis and the appearance of activated caspase 3. Further evaluation reveals that EC injury coincides with the onset of pulmonary pathology, is associated with elevations in bronchoalveolar lavage fluid tumor necrosis factor (TNF)-alpha levels, and is accompanied by evidence for EC activation. Administration of a soluble TNF-alpha binding protein (recombinant human TNF-alpha receptor:Fc) from week 4 to week 6 after allogeneic BMT significantly reduces EC apoptosis and lung histopathology observed in this setting.

CONCLUSIONS

EC damage mediated by TNF-alpha is directly linked to the development of experimental IPS. Methods that protect or maintain the integrity of the pulmonary vascular endothelium may therefore prove effective in reducing the severity of lung injury after BMT.

Low-dose UV-radiation sensitizes keratinocytes to TRAIL-induced apoptosis

The impact of low-dose ultraviolet light (UV-light) on apoptotic susceptibility of keratinocytes (KCs) induced by TRAIL is unclear. Skin expresses a functional form of TRAIL, and while sun exposure influences TRAIL death receptors, a role for decoy receptors has not been evaluated. Unraveling mechanisms involving apoptotic sensitivity of KCs is important because skin is the first target of UV-light, and a site for commonly occurring cancers. Since apoptosis is a homeostatic process eliminating UV-light induced DNA damaged cells, elucidating molecular events regulating apoptosis enhances understanding of cutaneous photocarcinogenesis. Here we demonstrate low-dose UV-light enhances susceptibility of KCs to TRAIL-induced apoptosis. Low-dose UV-light selectively reduces decoy receptors, without influencing death receptor levels. UV-induced enhanced apoptotic susceptibility was reduced by over-expression of decoy receptor TRAIL-R4, but not TRAIL-R3; or treatment with thiol compound pyrrolidine dithiocarbamate (PDTC), which also enhanced TRAIL-R4 levels. Besides influencing decoy receptors, low-dose UV-light plus TRAIL also synergistically promoted cytochrome c and Smac release from mitochondria. Inhibitors directed against caspases 2, 3, 8, and 9 reduced the synergistic apoptotic response following low-dose UV-light plus TRAIL exposure; as did forced over-expression of Bcl-x and dominant negative (DN) constructs of FADD and caspase 9. Thus, relative levels of decoy receptors significantly influence susceptibility of KCs to TRAIL-induced apoptosis with concomitant low-dose UV-light exposure; in addition to the apoptotic pathway mediated by mitochondrial permeabilization.

Immunopathogenic mechanisms in psoriasis

Psoriasis is a common autoimmune skin disease characterized by T cell-mediated hyperproliferation of keratinocytes. The disease has a strong but complex genetic background with a concordance of approximately 60% in monozygotic twins, and recent linkage and high resolution association studies indicate that HLA-Cw*0602 is itself a major susceptibility allele for psoriasis. Patients carrying this allele have been shown to have different clinical features and earlier age of disease onset, and patients homozygous for this allele have about 2.5 times higher disease risk than heterozygotes. Published data indicate that CD8+ T cells may play a major effector role in psoriasis. Epidermal infiltration of predominantly oligoclonal CD8+ T cells, and probably also of CD4+ T cells in the dermis, is a striking feature of chronic psoriasis lesions, indicating that these cells are responding to specific antigens. We argue that CD4+ T cells are essential for initiating and maintaining the pathogenic process of psoriasis but that cross-primed CD8+ T cells are the main effector cells responding to antigens in the HLA-Cw*0602 binding pocket of keratinocytes. It is further proposed that CD8+ T cells are involved in the control of the Th1 polarization, which is observed in psoriasis lesions, through a complex interplay between CD4+, CD8+ T cells and cross-presenting dendritic cells. It is also suggested that spontaneous remissions or fluctuations in disease activity may be determined by a balance within the lesions between effector and suppressor CD4+ and CD8+ T cells.

Tumor necrosis factor related apoptosis inducing ligand triggers apoptosis in dividing but not in differentiating human epidermal keratinocytes

Using serial analysis of gene expression we have previously identified the expression of several pro-apoptotic and anti-apoptotic genes in cultured human primary epidermal keratinocytes, including tumor necrosis factor related apoptosis inducing ligand (TRAIL). TRAIL is a potent inducer of apoptosis in transformed and tumor cell lines, but usually not in other cells. Here we present a study on the effect of TRAIL on cultured keratinocytes. It is shown that differentiated and undifferentiated keratinocytes undergo apoptosis after addition of TRAIL to the medium as determined by morphologic and biochemical criteria, such as cellular shrinkage and activation of caspases. The sensitivity for TRAIL differs greatly between undifferentiated and differentiating keratinocytes, however, with undifferentiated cells being much more susceptible to apoptosis. Commitment to terminal differentiation in the absence of TRAIL does not in itself induce apoptosis. In contrast to the promyelocytic cell line HL60, internucleosomal DNA fragmentation is not observed in keratinocytes, as assessed by flow cytometric analysis and agarose gel electrophoresis. Interestingly, the prime effector of DNA fragmentation, DNA fragmentation factor of 40 kDa (DFF40), is expressed in keratinocytes, yet internucleosomal cleavage fails to occur. Our data indicate that programmed cell death during keratinocyte differentiation is distinct from receptor-mediated apoptosis in response to a death ligand.

Corticotropin-releasing hormone inhibits nuclear factor-kappaB pathway in human HaCaT keratinocytes

Treatment of human HaCaT keratinocytes with corticotropin-releasing hormone modulates cell proliferation and expression of inflammation markers. In this study we report that corticotropin-releasing hormone also inhibits nuclear factor-kappaB binding and transcriptional activity. Incubating cells in the absence of growth factors increased nuclear factor-kappaB activity; this effect was significantly attenuated by corticotropin-releasing hormone. Specifically, corticotropin-releasing hormone downregulated p50/p50 and p50/p65 dimers of nuclear factor-kappaB, diminished kappaB-driven CAT reporter gene activity and inhibited IkappaB-beta degradation. Moreover, corticotropin-releasing hormone inhibited the trans-cription of the nuclear factor-kappaB responsive genes, interleukin-2 and heat shock protein 90.

Resistance to UV-induced apoptosis in human keratinocytes during accelerated senescence is associated with functional inactivation of p53

Compared to proliferating keratinocytes (KCs), growth-arrested KCs are relatively resistant to UV-light induced apoptosis. When KCs undergo confluency, or following exposure to anti-proliferative agents such as IFN-gamma plus a phorbol ester-12-O-tetradecanoylyphorbol-13-acetate (TPA), they convert from a proliferative to a nonproliferative state resembling senescence. Since p53 regulates UV-induced apoptosis of KCs, this report further characterizes p53 half-life, post-translational modifications, and transcriptional activity using cultured human KCs and living epidermal equivalents. The half-life of p53 in KCs was longer than fibroblasts (greater than approximately 3 h vs. 30 min). Exposure of proliferating KCs to UV-light induces post-translational modifications of p53 including acetylation of lysine-382 residues. By contrast, KCs undergoing irreversible growth arrest following confluency, or exposure to IFN-gamma plus TPA, were resistant to UV-induced apoptosis, and failed to undergo the acetylation modification of p53. Exposure of KCs to IFN-gamma plus TPA reduced total cellular p53 levels and reduced the transcriptional activity of p53. Addition of Trichostatin A (TSA), an inhibitor of de-acetylation, increased acetylation of lysine-382 in confluent KCs, thereby enhancing susceptibility of confluent cultures to UV-induced apoptosis. Pre-treatment of epidermal equivalents with IFN-gamma plus TPA also blocked UV-light induced increase in p53 levels, and reduced apoptosis. In conclusion, these studies demonstrate that growth arrested KCs may resist UV-light induced apoptosis by inactivating the pro-apoptotic function of p53.

Reciprocal altered expression of T-cadherin and P-cadherin in psoriasis vulgaris

BACKGROUND

The most characteristic change in psoriasis vulgaris is markedly increased, persistent keratinocyte proliferation. The underlying mechanism of excessive epidermal growth is controversial. We previously found and reported that T-cadherin was expressed in keratinocytes and confined to the basal layer of mouse and human skin. Invasive cutaneous squamous cell carcinoma showed a loss of T-cadherin expression. Another study showed that T-cadherin was a negative growth regulator of epidermal growth factor in T-cadherin transfectant neuroblastoma cells.

OBJECTIVES

To obtain insight into the role of T-cadherin in keratinocyte proliferation and to investigate further the pathogenesis of psoriasis vulgaris, we examined the expression of T-cadherin, as well as E- and P-cadherin, in psoriasis vulgaris.

METHODS

Four untreated active psoriatic skin samples from psoriasis vulgaris patients and four normal human skin samples from plastic surgery were collected, cryosectioned and immunohistochemically stained by antihuman T-, P- and E-cadherin antibodies. Further, the immunofluorescence intensities of T- and P-cadherin on the basal layer of the epidermis were quantitatively measured by the histogram function of LSM 510 software installed in a Zeiss laser scanning confocal microscope. The data were statistically analysed by Student's t-test.

RESULTS

It was observed that T-cadherin was weakly and discontinuously expressed on the basal layer of psoriatic skin, while it was intensively expressed on all basal keratinocytes in normal human skin. In contrast, P-cadherin was strongly expressed throughout the entire epidermal layer in psoriatic skin samples, although its expression is restricted to the basal cell layer in normal human skin. There were no obvious differences in E-cadherin expression between normal human skin and psoriatic skin. Statistical analyses showed that the immunofluorescence intensity of T-cadherin in the basal cell layer of psoriatic skin (35 +/- 9.08) was significantly decreased compared with that in normal human skin (131.75 +/- 3.49, P = 2.46 x 10(-6)). There was a significant increase (P = 0.00139) in the immunofluorescence intensity of P-cadherin in the basal layer of psoriatic skin (68.25 +/- 12.13) compared with normal human skin (26 +/- 4.90).

CONCLUSIONS

The present study demonstrates that there is downregulation of T-cadherin expression and upregulation of P-cadherin expression in psoriatic skin, which are considered to be involved in the hyperproliferation of keratinocytes in psoriasis vulgaris.

Aberrant signalling and transcription factor activation as an explanation for the defective growth control and differentiation of keratinocytes in psoriasis: a hypothesis

Psoriasis is a chronic inflammatory skin disease characterized by the accumulation of red, scaly plaques on the skin. The plaques result from hyperproliferation and incomplete differentiation of keratinocytes (KC) in a process that seems to be driven, in part by skin-infiltrating leucocytes. We believe that the KC have inherent defects in intracellular signalling which could be usefully targeted to allow the development of more effective therapies. We suggest that there are defects in the regulation of the transcription factors: signal transducer and activator of transcription (STAT-1alpha), interferon regulated factor-1 (IRF-1) and NFkappaB which lead to loss of growth and differentiation control when the cells are subjected to physico-chemical and immunological stress. We also highlight recent studies that suggest that peroxisome proliferator-activated receptors, the notch receptor and defects in calcium and other ion transporting proteins may contribute to impairment in the ability of psoriatic KC to differentiate. The role of these systems in the development of the psoriatic phenotype and tests of these hypotheses are proposed.

Divergent gene regulation and growth effects by NF-kappa B in epithelial and mesenchymal cells of human skin

NF-kappa B regulates normal and pathological processes, including neoplasia, in a tissue-context-dependent manner. In skin, NF-kappa B is implicated in epidermal homeostasis as well as in the pathogenesis of squamous cell carcinoma; however, its function in the underlying mesenchymal dermis has been unclear. To gain insight into NF-kappa B roles in these two adjacent cutaneous tissue compartments, NF-kappa B effects on expression of 12 435 genes were determined in epidermal keratinocytes and dermal fibroblasts. Although NF-kappa B induced proinflammatory and antiapoptotic genes in both settings, it exhibited divergent effects on growth regulatory genes. In keratinocytes, but not in fibroblasts, NF-kappa B induced p21(CIP1), which was sufficient to inhibit growth of both cell types. Levels of growth inhibitory factor (GIF), in contrast, were increased by NF-kappa B in both settings but inhibited growth only in keratinocytes. These findings indicate that transcription factors such as NF-kappa B can program tissue-selective effects via both differential target gene induction as well as by inducing common targets that exert differing effects depending on cellular lineage.

Expression of NF-kappaB in epidermis and the relationship between NF-kappaB activation and inhibition of keratinocyte growth

BACKGROUND

Nuclear factor-kappaB (NF-kappaB) is a transcription factor involved in a number of signalling pathways in many cell types. NF-kappaB in mice has been implicated as an important regulator of keratinocyte proliferation and differentiation.

OBJECTIVES

To evaluate the role of NF-kappaB in keratinocyte growth in human beings, we examined its expression in keratinocytes both in culture and in situ, and studied the relationship between NF-kappaB activation and the inhibition of keratinocyte proliferation induced by known modulators of keratinocyte growth.

METHODS

The expression of subunits of the NF-kappaB family was examined in human skin, primary cultured keratinocytes and an immortalized keratinocyte line by immunohistochemistry and reverse transcriptase-polymerase chain reaction analysis. NF-kB activation was examined in keratinocytes treated with various modulating agents by electrophoretic mobility shift assay (for DNA-binding activity) and by immunocytochemistry (nuclear translocation). The proliferative capacity of treated keratinocytes was also examined by 3H-thymidine incorporation, cell cycle analysis, and expression of Ki-67, a nuclear marker for cell proliferation. The involvement of NF-kappaB was assessed using sodium salicylate, which inhibits NF-kappaB activation.

RESULTS

The NF-kappaB subunits, p50, p65, RelB, and c-Rel (but not p52), were detected in keratinocytes and in normal epidermis at mRNA and protein levels. The four subunits were expressed in a cytoplasmic (rather than a nuclear) pattern in both basal and suprabasal keratinocytes. Phorbol myristate acetate (PMA), tumour necrosis factor alpha, and interferon gamma each activated NF-kappaB and inhibited keratinocyte proliferation. Lipopolysaccharide and dexamethasone did not activate NF-kappaB and had the least effect on proliferation. Finally, a high concentration of calcium (Ca2+) and retinoic acid each failed to activate NF-kappaB, but were potent inhibitors of keratinocyte proliferation, respectively. PMA-induced cell cycle arrest of keratinocytes was blocked by pretreatment with sodium salicylate.

CONCLUSIONS

NF-kappaB is constitutively expressed in a resting state in both human cultured keratinocytes and the epidermis. Activation of NF-kappaB is required for PMA-induced keratinocyte growth arrest.

Innate immune-related receptors in normal and psoriatic skin

CONTEXT

A precise role for the innate immune system in psoriasis remains to be determined. Surface receptors, including Toll-like receptors (TLRs) that recognize bacterial ligands and CD91, which recognizes heat shock proteins (HSPs), are implicated in both innate and adaptive immunity.

OBJECTIVE

Since skin is exposed to various exogenous stimuli, which can provoke or exacerbate psoriasis, we characterized expression and function of TLRs, CD91, and HSPs in normal and psoriatic skin.

DESIGN

A variety of skin-derived cells and blood-derived cells were analyzed both in vivo and in vitro; samples were obtained from 24 different individuals for innate immune-related receptor expression and function. By comparing and contrasting individuals with healthy skin and psoriatic patients, several specific differences were identified.

RESULTS

Immunohistochemistry-based expression profiling revealed TLR1 expression in epidermal dendritic cells (DCs) and dermal dendritic cells (DDCs) in normal skin, as well as in pre-psoriatic skin and psoriatic plaques, with enhanced basal layer keratinocyte (KC) expression in pre-psoriatic and psoriatic plaques compared with normal skin; TLR2 expression primarily by DDCs; and TLR4 expression by epidermal DCs and DDCs, with mid-epidermal-layer KCs displaying cell surface staining. No TLR9 or CD14 was detected on DCs or KCs, although psoriatic plaques contained CD14-positive macrophages. Analysis of psoriatic epidermis revealed HSPs 27, 60, and 70. Keratinocytes were CD91 negative, but CD91 was expressed by fibroblasts and DDCs in normal and pre-psoriatic skin, with prominent accumulation of CD91-positive DDCs in psoriatic plaques. Cultured KCs revealed no surface expression of TLR2, TLR4, TLR9, or CD91. Exposure of fibroblasts, but not KCs, to lipopolysaccharide or HSPs triggered nuclear factor (NF)-kappaB activation. Heat shock proteins did induce maturation of blood-derived DCs accompanied by increased interleukin-12 production and enhanced antigen-presenting function.

CONCLUSIONS

These data demonstrate distinctive patterns of innate immune-related receptors by specific subsets of cells in normal and psoriatic skin, suggesting functional roles for HSPs and DCs in psoriasis.

Role of INK4a/Arf locus-encoded senescent checkpoints activated in normal and psoriatic keratinocytes

During malignant transformation in skin, epidermal keratinocytes (KCs) frequently acquire the capacity to by-pass cellular senescence, a response that normally limits their unrestricted proliferation. Despite growing interest in the role for senescence during aging of skin and cutaneous carcinogenesis, little is known regarding regulation of three proteins encoded by the INK4a/ARF locus (p12, p14(ARF), p16) in KCs. In this study, several molecular pathways are explored using cultured KCs and KCs freshly isolated from psoriatic plaques. p16 and p14(ARF) are predominantly expressed spontaneously when foreskin-derived early-passage KCs undergo confluency-induced premature senescence. Induction of p14(ARF) on confluency occurred with low E2F-1 levels. Suspension of KCs in methylcellulose induced p12 expression. Addition of various cytokines (interferon-gamma, tumor necrosis factor-alpha) or a phorbol ester [12-O-tetradecanoylphorbol-13-acetate (TPA)] only induced p16, but not p14(ARF). Confluent KCs up-regulated Ras activity and the downstream signaling involving ERK. Addition of MAPK inhibitor blocked cytokine and TPA-induced p16 expression. Confluency and interferon-gamma induced premature senescence and p16 expression was linked to induction of the transcription factor Egr-1. KCs derived from chronic psoriatic plaques were characterized by enhanced p16, p14(ARF), and p12 expression accompanied by elevated Egr-1 levels. These results demonstrate that multiple and highly divergent stimuli can trigger the senescent checkpoint in human KCs with differential regulation of p16, p14(ARF), and p12. Although abnormal mitogenic signaling by oncogenic Ras is generally cited as being responsible for induction of premature senescence, our findings indicate that a broader perspective is warranted, to include confluency and cytokine-/TPA-induced pathways for KCs.

Arginase 1 overexpression in psoriasis: limitation of inducible nitric oxide synthase activity as a molecular mechanism for keratinocyte hyperproliferation

Abnormal proliferation of keratinocytes in the skin appears crucial to the pathogenesis of psoriasis, but the underlying mechanisms remain unknown. Nitric oxide (NO), released from keratinocytes at high concentrations, is considered a key inhibitor of cellular proliferation and inducer of differentiation in vitro. Although high-output NO synthesis is suggested by the expression of inducible NO synthase (iNOS) mRNA and protein in psoriasis lesions, the pronounced hyperproliferation of psoriatic keratinocytes may indicate that iNOS activity is too low to effectively deliver anti-proliferative NO concentrations. Here we show that arginase 1 (ARG1), which substantially participates in the regulation of iNOS activity by competing for the common substrate L-arginine, is highly overexpressed in the hyperproliferative psoriatic epidermis and is co-expressed with iNOS. Expression of L-arginine transporter molecules is found to be normal. Treatment of primary cultured keratinocytes with Th1-cytokines, as present in a psoriatic environment, leads to de novo expression of iNOS but concomitantly a significant down-regulation of ARG1. Persistent ARG1 overexpression in psoriasis lesions, therefore, may represent a disease-associated deviation from normal expression patterns. Furthermore, the culturing of activated keratinocytes in the presence of an ARG inhibitor results in a twofold increase in nitrite accumulation providing evidence for an L-arginine substrate competition in human keratinocytes. High-output NO synthesis is indeed associated with a significant decrease in cellular proliferation as shown by down-regulation of Ki67 expression in cultured keratinocytes but also in short-term organ cultures of normal human skin. In summary, our data demonstrate for the first time a link between a human inflammatory skin disease, limited iNOS activity, and ARG1 overexpression. This link may have substantial implications for the pathophysiology of psoriasis and the development of new treatment strategies.

Pathways involved in proliferating, senescent and immortalized keratinocyte cell death mediated by two different TRAIL preparations

Properly regulated keratinocyte cell death is fundamentally important to maintain structural integrity and homeostatic function of epidermis. Moreover, from an oncological perspective, therapeutic approaches selectively targeting apoptosis of malignant cell types while sparing normal keratinocytes in surrounding skin is desirable. Apo2Ligand/tumor necrosis factor-related apoptosis-inducing ligand (Apo2L/TRAIL) has been observed to preferentially induce cytopathic effects on transformed/malignant cell types compared with their non-neoplastic counterparts. In this report, two different biologically active preparations of Apo2L/TRAIL, a non-tagged version, NT-Apo2L/TRAIL, and a leucine zipper fusion protein, LZ-Apo2L/TRAIL, were examined for their ability to trigger apoptosis in normal human keratinocytes, and in an immortalized cell line (HaCaT cells). Differences between these preparations were observed, including: NT-Apo2L/TRAIL induced less keratinocyte apoptosis compared with LZ-Apo2L/TRAIL; NT-Apo2L/TRAIL also induced less apoptosis of HaCaT cells compared with LZ-Apo2L/TRAIL; LZ-Apo2L/TRAIL but not NT-Apo2L/TRAIL induced cytotoxic effects when keratinocytes became growth arrested due to undergoing spontaneous replicative senescence--a biological state previously observed to be resistant to UV-light-induced apoptosis. Similarities between preparations included: an enhanced ability for both Apo2L/TRAIL preparations to kill a greater relative percentage of HaCaT cells compared with keratinocytes; enhanced cytotoxicity towards keratinocytes that had their NF-B activity inhibited; a dependence of both Apo2L/TRAIL preparations on FADD and caspase activation; triggering of the same caspase cascades including caspase 8 and 3; and an ability to induce apoptosis even when HaCaT cells and keratinocytes were transduced to overexpress either Bcl-2 or Bcl-x(L) (survival factors that reduce susceptibility to UV-light-induced apoptosis). These results indicate that while both preparations of Apo2L/TRAIL possess biological activity, there are important differences as regards their ability to induce apoptosis in normal and immortalized keratinocytes. Moreover, the death receptor pathway triggered by LZ-Apo2L/TRAIL can overcome the apoptotic resistance normally observed in response to UV-light mediated by Bcl-2/Bcl-x(L), as well as by the state of cellular senescence. Unraveling the molecular basis for these differential biological effects may reveal a new strategic role for these death receptor/ligands linked to apoptosis in maintaining the dynamic balance of keratinocyte proliferation, differentiation, and cell death necessary to achieve a homeostatic thickness and function of normal skin. In addition, it may be possible to utilize these Apo2L/TRAIL preparations for the treatment of various sun-induced skin cancers as they can differentially trigger apoptosis of transformed keratinocytes, or keratinocytes with abnormal NF-kappaB signaling, while sparing adjacent normal keratinocytes.

Inhibition of tumor necrosis factor-alpha stimulated NFkappaB/p65 in human keratinocytes by alpha-melanocyte stimulating hormone and adrenocorticotropic hormone peptides

Alpha-melanocyte stimulating hormone (alpha-MSH) has pigmentary, anti-inflammatory, antipyretic, and general immunomodulatory roles. It can oppose several cytokines including tumor necrosis factor-alpha in a number of tissues, including skin. We have previously shown that alpha-MSH can inhibit tumor necrosis factor-alpha stimulated intercellular adhesion molecule 1 upregulation and nuclear factor kappaB (NFkappaB) transcription factor activation in melanocyte and melanoma cells. It is thought, however, that this MSH biology may also extend to other cells of the skin and in this study we extend our work to keratinocytes. We have investigated in detail the ability of three alpha-MSH peptides to inhibit tumor necrosis factor alpha stimulated NFkappaB activation in nonpigmentary HaCaT keratinocytes (alpha-MSH, L-Lys-L-Pro-L-Val, and L-Lys-L-Pro-D-Val) and two adrenocorticotropic hormone (ACTH) peptides (1-17 and 1-39), reported to be present in skin tissue. NFkappaB/p65 activation was analyzed by electrophoretic mobility shift assay and immunofluorescent microscopy. alpha-MSH, L-Lys-L-Pro-L-Val, and L-Lys-L-Pro-D-Val all significantly inhibited tumor necrosis factor alpha stimulated NFkappaB activation, whereas ACTH 1-17 and 1-39 did not, in the HaCaT keratinocytes. MSH peptides and ACTH 1-39 were effective, however, at inhibiting NFkappaB activation in normal human keratinocytes. Immunolabeling of inhibitor kappaBalpha of NFkappaB (IkappaBalpha) revealed an abnormal localization to the nucleus of HaCaT cells, which was unaffected by MSH/ACTH peptides. In contrast, normal human keratinocytes showed a normal IkappaBalpha distribution that responded to MSH/ACTH with nuclear translocation. Our data support previous work on the role of MSH/ACTH peptides as immunomodulatory/anti-inflammatory regulators, and extend this work to keratinocytes identifying a novel IkappaBalpha mechanism and extends findings to ACTH peptides, identifying an abnormal IkappaBalpha mechanism in the immortal HaCaT versus normal keratinocyte.

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.

Life and death signaling pathways contributing to skin cancer

Apoptosis is generally regarded as a critical regulatory event in the development of malignancies in several different organ systems (Thompson, 1995). Initially, oncologists focused on alterations in rates of proliferation and cell cycle kinetics, but more recently an emphasis on apoptosis has dominated the fight against cancer (Evan and Vousden, 2001). As approximately 1,000,000 individuals in the U.S.A. develop skin cancer each year, it is important to elucidate the molecular mechanisms that govern cell survival and cell death in the epidermis (Miller and Weinstock, 1994). Moreover, given that most skin cancers occur on sun-exposed skin, the pro-apoptotic and antiapoptotic response of keratinocytes (KC) to UV light is of particular relevance to the development of skin cancer (Brash et al, 1996). Whereas both squamous cell carcinoma (SCC) and basal cell carcinoma (BCC) arise from epidermal KC, it is becoming increasingly apparent that the natural history of their development, their underlying molecular pathogenesis, and potential involvement of antiapoptotic pathways are significantly different. Nonetheless, as pointed out later in the text, significant progress is being made in our understanding of the pathophysiology of these relatively common epithelial-cell-derived neoplasms. In this review we will explore four topics: first, a review of the life and death signaling pathways operative in normal human skin that prevents premature apoptosis of KC with an emphasis on nuclear factor kappaB (NFkappaB) survival signals; second, the molecular pathways that are engaged and regulate apoptosis after normal KC are exposed to ultraviolet (UV) light; third, the apoptotic resistant mechanisms that premalignant and malignant KC utilize to avoid cell death; fourth, therapeutic strategies that can render malignant cells more susceptible to apoptosis with an emphasis on a death pathway mediated by the death ligand TRAIL.

Jagged-1 mediated activation of notch signaling induces complete maturation of human keratinocytes through NF-kappaB and PPARgamma

Establishing an effective epidermal barrier requires a series of coordinated molecular events involving keratinocytes (KCs) within a stratified epithelium. Epidermal maturation depends on convergence of pathways involving components of NF-kappaB and peroxisome proliferator activated receptor (PPAR) signaling systems that promote terminal differentiation and production of a stratum corneum. The Notch-1 receptor and its ligand Delta-1 have been proposed by others to participate in early events in KC differentiation. Here, we establish differential expression patterns for several Notch receptors and ligands in normal human skin. These immunolocalization findings, together with functional studies demonstrating increased levels of Notch ligand/receptors occurring during the onset of differentiation, prompted use of a soluble Notch ligand, a peptide derived from the most conspicuously expressed ligand in skin, Jagged-1. Exposing submerged KC monolayers to this peptide (JAG-1) in co-presence of elevated calcium ion concentration, produced stratification with loricrin expression. Using a living human epidermal equivalent (EE) model system, when submerged cultures were raised to an air/liquid interface to generate a fully mature epidermis, activation of Notch signaling was detected. Addition of JAG-1 peptide to submerged EEs was sufficient to induce epidermal maturation. Moreover, a soluble decoy Notch inhibitor prevented such differentiation and corneogenesis in human EEs exposed to either an air/liquid interface or to the JAG-1 peptide. In KC monolayers, addition of JAG-1 peptide induced IKKalpha mediated NF-kappaB activation, as well as increased PPARgamma expression. Immunoprecipitation/Western blot analysis revealed a physical association between the p65 subunit of NF-kappaB and PPARgamma. These results indicate that activation of Notch signaling is necessary for maturation of human epidermis, and activation by a soluble Notch ligand is sufficient to trigger complete KC differentiation including cornification.

The helix-loop-helix protein id-1 delays onset of replicative senescence in human endothelial cells

Id proteins are negative regulators of basic helix-loop-helix transcription factors, which are critical for expression of genes associated with cellular differentiation. Previous studies have shown that overexpression of Id-1 delays cellular senescence in several cell types, including fibroblasts, mammary epithelial cells, and keratinocytes. Although previous studies have demonstrated the expression of Id-1 in endothelium, the regulation of Id-1 has not been studied in these cells. In this report, a retroviral vector was used to overexpress Id-1 in human endothelial cells. Sustained expression of Id-1 resulted in a 2- to 3-fold increase in the total number of population doublings (replicative capacity) of the cells compared with vector-treated controls, which correlated with low levels of p16, p21, and p27 expression. The cells, however, were not immortalized and did eventually undergo senescence despite elevated Id-1 levels. Senescence was characterized by a dramatic increase in p16, but not p21 and p27. Under these experimental conditions, telomerase activity was not detected and the telomeres became progressively shorter with time. These results demonstrate the importance of Id-1 in endothelial cell proliferation and indicate that Id-1 represses p16 expression, resulting in delayed senescence. These findings may have implications in the development of endothelial cell-derived tumors.

Differential production of cytokines and activation of NF-kappaB in HPV-transformed keratinocytes

We have proposed that chronic infection of keratinocytes by HPV modifies the expression of potentially important cytokines by interfering with the NF-kappaB signal pathway. We evaluated the constitutive and IL-1beta-induced expression of GM-CSF and TNF-alpha and the expression/activity of NF-kappaB in HPV+ and HPV- cell lines. Despite the enhanced expression of the functional components of the NF-kappaB signaling pathway in HPV+ cell lines by a mechanism implicating the HPV oncoprotein E6, the constitutive activity of NF-kappaB and the expression of GM-CSF/TNF-alpha were significantly reduced relative to the HPV- cell line and normal keratinocytes. In contrast, we observed a superactivation of NF-kappaB activity after IL-1beta stimulation, a strong and transient induction of GM-CSF/TNF-alpha mRNA, but undetectable levels of secreted proteins in HPV+ cell lines. Our data demonstrate that E6 modulates the NF-kappaB signaling pathway and suggest that other HPV proteins also interfere with GM-CSF/TNF-alpha expression by transcriptional and/or posttranscriptional mechanisms.

Nuclear transcription factor-kappaB as a target for cancer drug development

Nuclear factor kappa B (NF-kappaB) is a family of inducible transcription factors found virtually ubiquitously in all cells. Since its discovery by Sen and Baltimore in 1986, much has been discovered about its mechanisms of activation, its target genes, and its function in a variety of human diseases including those related to inflammation, asthma, atherosclerosis, AIDS, septic shock, arthritis, and cancer. Due to its role in a wide variety of diseases, NF-kappaB has become one of the major targets for drug development. Here, we review our current knowledge of NF-kappaB, the possible mechanisms of its activation, its potential role in cancer, and various strategies being employed to target the NF-kappaB signaling pathway for cancer drug development.

Activation of caspase-9 is required for UV-induced apoptosis of human keratinocytes

UV radiation from the sun activates both the membrane death receptor and the intrinsic or mitochondrial apoptotic signaling pathways in epidermal keratinocytes, triggering apoptosis and affording protection against skin cancer formation. We have investigated the involvement of caspase-9 in the UV death effector pathway in human keratinocytes, since this is the initiating caspase in the mitochondrial pathway required for UV-induced apoptosis in some, but not all, cell types. UV radiation triggered activation of caspase-3, caspase-9, and caspase-8 with similar kinetics, although the rank order of activation was caspase-3 > caspase-9 > caspase-8. Inhibition of caspase-9 with either the peptide inhibitor benzyloxycarbonyl-Leu-Glu(OCH(3))-His-Asp(OCH(3))-fluoromethyl ketone, or expression of a catalytically inactive caspase-9 by retroviral transduction, protected normal keratinocytes from UV-induced apoptosis. HaCaT keratinocytes harboring mutant p53 alleles were also protected from UV-induced apoptosis by the dominant negative caspase-9. The dominant negative caspase-9 blocked UV-induced activation of caspase-3, caspase-9, and caspase-8, and also protected cells from the loss of mitochondrial membrane potential. In contrast, the dominant negative caspase-9 did not protect from anti-Fas-induced apoptosis or caspase activation. These results identify caspase-9 as the critical upstream caspase initiating apoptosis by UV radiation in human keratinocytes, the relevant cell type for this important environmental carcinogen.

Regulation of apoptosis by p53 in UV-irradiated human epidermis, psoriatic plaques and senescent keratinocytes

The carcinogenic effects of sunlight in human epidermis may be thwarted by either: transient growth arrest and repair of DNA photodamage in keratinocytes (KCs); elimination of KCs with damaged DNA via apoptosis; or by stimulating a senescence switch whereby KCs become irreversibly growth arrested. Using normal human skin organ cultures and living epidermal equivalents, we demonstrate that in the proliferative basal layer, removal of KCs via apoptosis had a rapid onset (beginning within 2 h) following UV-light exposure generating progressively greater numbers of KCs with thymine dimers as the dose of UV-light was increased; involved induction of Apaf-1, activation of caspase-3, and was dependent on p53 activation as addition of a p53 chemical inhibitor blocked the apoptotic response. Suprabasal layer KCs underwent apoptosis at much later time points (>8 h). KCs in the basal layer repaired DNA damage more rapidly than KCs in suprabasal layers. Steady state levels of p53 increased in irradiated cells, and the increase was accompanied by phosphorylation of serine 9 and serine 15, but not serine 6 residues. By contrast, cultured KCs undergoing spontaneous replicative senescence were resistant to UV-induced apoptosis. Senescent KCs constitutively contained low levels of p53, which were neither increased nor phosphorylated or acetylated after UV-exposure and possessed minimal DNA binding activity, indicative of functional inactivation. Furthermore, treatment of senescent KCs with DNA damaging agent adriamycin did not result in activation of latent p53 or apoptosis. When KCs within psoriatic plaques were examined, they resembled senescent KCs in that they expressed p53, which was not phosphorylated or acetylated. Thus, UV-light induces DNA damage in human epidermal KCs triggering p53 activation, and subsequent apoptosis involving distinct cell layers and kinetics. However, the lack of p53 activation as seen in senescent KCs and psoriatic plaques, is associated with a relative resistance of KCs to UV-induced apoptosis. In conclusion, the sensitivity and resistance of KCs to apoptosis depends not only on the location within various layers of epidermis and levels of p53, but may also involve p53 activation via post-translational modifications.

Nuclear Factor-κB Activation: A Question of Life or Death

Apoptosis is a mode of cell death that plays an important role in both pathological and physiological processes. Research during the last decade has delineated the entire machinery needed for cell death, and its constituents were found to pre-exist in cells. The apoptotic cascade is triggered when cells are exposed to an apoptotic stimulus. It has been known for several years that inhibitors of protein synthesis can potentiate apoptosis that is induced by cytokines and other inducers. Until 1996, it was not understood why protein synthesis inhibitors potentiate apoptosis. Then three reports appeared that suggested the role of the transcription factor NF-kappaB activation in protecting the cells from TNF-induced apoptosis. Since then several proteins have been identified that are regulated by NF-kappaB and are involved in cell survival, proliferation, and protection from apoptosis. It now seems that when a cell is attacked by an apoptotic stimulus, the cell responds first by activating anti-apoptotic mechanisms, which may or may not be followed by apoptosis. Whether or not a cell undergoes proliferation, the survival, or apoptosis, appears to involve a balance between the two mechanisms. Inhibitors of protein synthesis seem to suppress the appearance of protein that are involved in anti-apoptosis. The present review discusses how NF-kappaB controls apoptosis.

Caspase activation and disruption of mitochondrial membrane potential during UV radiation-induced apoptosis of human keratinocytes requires activation of protein kinase C

The induction of apoptosis in human keratinocytes by UV radiation involves caspase-mediated cleavage and activation of protein kinase C delta (PKCdelta). Here we examined the role of PKC activation in caspase activation and disruption of mitochondria function by UV radiation. Inhibition of PKC partially blocked UV radiation-induced cleavage of PKCdelta, pro-caspase-3, and pro-caspase-8, and the activation of these caspases. PKC inhibition also blocked the UV-induced loss of mitochondria membrane potential, but did not block the release of cytochrome c from mitochondria. Expression of the active catalytic domain of PKCdelta was sufficient to induce apoptosis and disrupt mitochondrial membrane potential, however a kinase inactive PKCdelta catalytic domain did not. Furthermore, the PKCdelta catalytic fragment generated following UV radiation localized to the mitochondria fraction, as did ectopically expressed PKCdelta catalytic domain. These results identify a functional role for PKC activation in potentiating caspase activation and disrupting mitochondrial function during UV-induced apoptosis.

The platelet-activating factor receptor protects epidermal cells from tumor necrosis factor (TNF) alpha and TNF-related apoptosis-inducing ligand-induced apoptosis through an NF-kappa B-dependent process

A number of chemical mediators can induce human keratinocytes and epidermal-derived carcinomas to undergo apoptosis, or programmed cell death. Recent evidence suggests pro-inflammatory cytokines, such as interleukin-1 beta or transforming growth factor alpha, protects carcinomas from numerous pro-apoptotic stimuli. Platelet-activating factor (1-alkyl-2-acetyl-3-glycerophosphocholine; PAF) is a lipid mediator with pro-inflammatory effects on numerous cell types. Although PAF can be metabolized to other bioactive lipids, the majority of PAF effects occur through activation of a G protein-coupled receptor. Using a model system created by retroviral transduction of the PAF receptor (PAF-R) into the PAF-R-negative human epidermal cell line KB and the PAF-R-expressing keratinocyte cell line HaCaT, we now demonstrate that activation of the epidermal PAF-R results in protection from apoptosis induced by tumor necrosis factor (TNF) alpha or TNF-related apoptosis-inducing ligand. The PAF-mediated protection was inhibited by PAF-R antagonists, and protection did not occur in PAF-R-negative KB cells. Additionally, we show protection from TNFalpha- or TRAIL-induced apoptosis by PAF-R activation is dependent on the transcription factor nuclear factor (NF)-kappa B, because PAF-R activation-induced NF-kappa B and epidermal cells transduced with a super-repressor form of inhibitor kappa B were not protected by the PAF-R. These studies provide a mechanism whereby the epidermal PAF-R, and possibly other G protein-coupled receptors, can exert anti-apoptotic effects through an NF-kappa B-dependent process.

Role of NF-kappaB activity in apoptotic response of keratinocytes mediated by interferon-gamma, tumor necrosis factor-alpha, and tumor-necrosis-factor-related apoptosis-inducing ligand

An important step in tumorigenesis involves loss of sensitivity to various apoptotic signals by malignant cells, imbuing them with an enhanced survival phenotype. NF-kappaB also regulates epidermal thickness, susceptibility to apoptosis, and tumor formation in skin. Keratinocytes were examined for their susceptibility to apoptosis using cytokines produced during an immunologic response to tumor antigens, i.e., interferon-gamma and/or tumor necrosis factor-alpha (TNF-alpha). The role for NF-kappaB in this response was examined using a retroviral vector containing a degradation-resistant form of IkappaBalpha. Whereas interferon-gamma and TNF-alpha either alone or in combination did not induce apoptosis in keratinocytes, after infection with the retrovirus to block NF-kappaB activation they became susceptible to TNF-alpha but not Fas-induced apoptosis. Moreover, when keratinocytes with repressed NF-kappaB activity were simultaneously treated with interferon-gamma, there was a synergistic induction of apoptosis by TNF-alpha that was dependent on FADD, tumor-necrosis-factor-related apoptosis-inducing ligand (TRAIL), and caspase activation. Molecular abnormalities accompanying repressed NF-kappaB activity included failure to induce TNF-RII receptor together with enhanced levels of TRAIL death receptor 4. The ability of interferon-gamma when combined with TNF-alpha to mediate keratinocyte apoptosis included induction of TRAIL coupled with diminished capacity of keratinocytes with repressed NF-kappaB activity to increase the TRAIL decoy receptor-1, as well as lower levels of several NF-kappaB-dependent antiapoptotic proteins accompanied by enhanced caspase 8 levels. These results indicate that interferon-gamma and TNF-alpha synergistically induce keratinocyte apoptosis when concomitant induction of NF-kappaB is blocked. Participants in the apoptotic response mediated by NF-kappaB, besides cell-survival proteins, include modulation of TRAIL and both death and decoy receptors. Thus, not only does NF-kappaB signaling influence the intrinsic survival pathway for keratinocytes in normal skin, but it may also play a role in determining the apoptotic response to cytokines generated during an immune response via TRAIL produced by the keratinocytes themselves.

Aberrant redox regulation in human metastatic melanoma cells compared to normal melanocytes

Melanocytes and melanoma cells contain melanin, a complex polymer that modulates redox changes in these cells. Relative intracellular hydrogen peroxide levels measured by dichlorodihydrofluorescein are similar in the two cell types, but the levels of superoxide anion measured by dihydroethidium were markedly increased in melanoma cells. Chelator-induced oxidative stress is efficiently suppressed by melanocytes without substantial recruitment of the transcription factors NF-kappaB and AP-1 as measured by electrophoretic mobility shift assay and quantitated by densitometry or by a change in frequency of apoptosis as determined by annexin V binding. In contrast, NF-kappaB in melanoma cells is strongly recruited by changes in redox status and exhibits a correlative relationship to intracellular hydrogen peroxide (but not superoxide anion). However, the response of the NF-kappaB pathway to intracellular hydrogen peroxide is anomalous, including downregulation of p65 and IkappaBalpha RNA expression (Northern blot). Additionally, recruitment of AP-1 binding in melanoma cells was directly correlated with intracellular levels of superoxide anion (but not hydrogen peroxide). Neither the degree of NF-kappaB nor AP-1 binding in melanoma cells was related to the frequency of apoptosis. The responsiveness of NF-kappaB and AP-1 recruitment to intracellular levels of hydrogen peroxide and superoxide anion without concomitant control of apoptosis provides a general mechanism by which these cells can escape noxious injury (e.g., chemotherapy). The marked enhancement of apoptosis in melanoma cells by chelators indicates, however, that this alteration can be circumvented and offers a unique therapeutic window to explore.

Abnormal NF-kappaB signaling pathway with enhanced susceptibility to apoptosis in immortalized keratinocytes

The transcriptional activation and proper regulation of NF-kappaB is known to be important to the apoptotic resistant phenotype of epidermal-derived keratinocytes. By comparing and contrasting the responses of normal foreskin-derived keratinocytes versus an immortalized skin-derived keratinocyte cell line (i.e. HaCaT cells), several molecular defects involving NF-kappaB signaling pathway were delineated in the immortalized keratinocytes. While exposure to IFN-gamma plus TPA produces growth arrest in both normal and immortalized keratinocytes, with rapid phosphorylation of MEKKI and recruitment of distinctive protein kinase C isoforms into the signalosome complex, subsequent molecular events necessary for NF-kappaB activation were abnormal in HaCaT cells. This disrupted NF-kappaB activation in HaCaT cells was accompanied by enhanced susceptibility to UV-light induced apoptosis, which was associated with elevated levels of E2F-1 and decreased TRAF1/TRAF2 levels. Additional defects in HaCaT cells included markedly diminished levels of IKKbeta (and lack of induction of kinase activity) in response to inflammatory stimuli, a failure of p21(WAF1/CIP1) to associate with CDK2, and a decreased association between p65 and p300. These studies suggest caution in using HaCaT cells as a substitute for normal keratinocytes to study apoptosis in the skin. Thus, it appears that while the immortalized cells can escape cell cycle checkpoints by elevated levels of E2F-1, an adverse biological consequence of such dysregulated cell cycle control is the inability to activate the anti-apoptotic NF-kappaB signaling pathway. Therefore, exploiting this apoptosis vulnerability in pre-malignant, or immortalized cells, prior to acquiring a death-defying phenotype characteristic of more advanced malignant cell types, provides the basis for an early interventional therapeutic strategy for cutaneous oncologists.

Expression of the NF-kappa B target gene IEX-1 (p22/PRG1) does not prevent cell death but instead triggers apoptosis in Hela cells

P22PRG1/IEX-1 is a putative NF-kappaB target gene implicated in the regulation of cellular viability. Here, we show that in HeLa cells TNFalpha induces expression of p22PRG1/IEX-1 in an NF-kappaB dependent fashion. Blockade of NF-kappaB activation by various NF-kappaB inhibitors abolished TNFalpha-induced p22PRG1/IEX-1 expression and increased the sensitivity to apoptosis induced by TNFalpha, an activating Fas-antibody or the anti-cancer drug etoposide. Surprisingly, ectopic expression of p22PRG1/IEX-1 in HeLa cells transfected with an inducible p22PRG1/IEX-1-expression vector augments the susceptibility to apoptosis initiated by death-receptor ligands or by etoposide. In addition, p22PRG1/IEX-1 expressing HeLa cells exhibit an accelerated progression through the cell cycle. Transfection of an antisense hammerhead ribozyme targeted to p22PRG1/IEX-1 reduced the speed in cell cycle progression and decreased the apoptotic response to death ligands. Our data demonstrate that p22PRG1/IEX-1 is specifically induced during NF-kappaB activation, but this seems not to be related to the anti-apoptotic actions of NF-kappaB. Instead, NF-kappaB dependent recruitment of p22PRG1/IEX-1 might be related to a modulation in the cell cycle, and hereby, p22PRG1/IEX-1 may accelerate cell growth on the one hand, but may trigger apoptosis on the other. Oncogene (2001) 20, 69 - 76.

NF-kappa B defects in humans: the NEMO/incontinentia pigmenti connection

The components of the nuclear factor-kappaB (NF-kappaB) family of transcription factors are critical for regulating the response to immune challenges. Recently, a role for NF-kappaB in skin biology has been revealed. Within the cascade of proteins whose activities impinge upon the activation of NF-kappaB, the NEMO (NF-kappaB essential modulator)/IKKgamma protein is required for the activation of the IkappaB kinases, which in turn, promote the degradation of IkappaB proteins, leading to the derepression of NF-kappaB activity. Courtois and Israël discuss the role of NEMO/IKKgamma in normal physiological activation of NF-kappaB and the consequences of defective NF-kappaB activation, as an effect of NEMO/IKKgamma mutations, which can lead to incontinentia pigmenti, a disease marked by alopecia, tooth eruption, skin lesions, and changes in skin pigmentation.

Apoptosis and nuclear factor-kappa B: a tale of association and dissociation

It is not clear why on treatment with certain killer cytokines or chemotherapeutic agents, some cells undergo apoptosis while others do not. The delineation of sensitivity/resistance pathways should provide a more specific therapy for cancer and other hyperproliferative diseases. Most cells die either by apoptosis or by necrosis. The biochemical pathway that mediates these two modes of cell death has recently been described. The nuclear factor (NF)-kappa B and the genes regulated by this transcription factor have been shown to play a critical role in induction of resistance to killer agents. Thus, inhibitors of NF-kappa B activation have a potential in overcoming resistance to apoptosis induced by various agents. The evidence for and against such a notion is discussed.

Inflammatory versus proliferative processes in epidermis. Tumor necrosis factor alpha induces K6b keratin synthesis through a transcriptional complex containing NFkappa B and C/EBPbeta

Epidermal keratinocytes respond to injury by becoming activated, i.e. hyperproliferative, migratory, and proinflammatory. These processes are regulated by growth factors and cytokines. One of the markers of activated keratinocytes is keratin K6. We used a novel organ culture system to show that tumor necrosis factor alpha (TNFalpha) induces the expression of K6 protein and mRNA in human skin. Multiple isoforms of K6 are encoded by distinct genes and have distinct patterns of expression. By having shown previously that proliferative signals, such as epidermal growth factor (EGF), induce expression of the cytoskeletal protein keratin K6b, we here demonstrate that the same isoform, K6b, is also induced by TNFalpha, a proinflammatory cytokine. Specifically, TNFalpha induces the transcription of the K6b gene promoter. By using co-transfection, specific inhibitors, and antisense oligonucleotides, we have identified NFkappaB and C/EBPbeta as the transcription factors that convey the TNFalpha signal. Both transcription factors are necessary for the induction of K6b by TNFalpha and act as a complex, although only C/EBPbeta binds the K6b promoter DNA. By using transfection, site-directed mutagenesis, and footprinting, we have mapped the site that responds to TNFalpha, NFkappaB, and C/EBPbeta. This site is separate from the one responsive to EGF and AP1. Our results show that the proinflammatory (TNFalpha) and the proliferative (EGF) signals in epidermis separately and independently regulate the expression of the same K6b keratin isoform. Thus, the cytoskeletal responses in epidermal cells can be precisely tuned by separate proliferative and inflammatory signals to fit the nature of the injuries that caused them.