Keratinocyte apoptosis induced by ultraviolet B radiation and CD95 ligation -- differential protection through epidermal growth factor receptor activation and Bcl-x(L) expression

Network pharmacology-based preventive effect of XZF on cutaneous toxicities induced by EGFR inhibitor

Skin toxicities induced by epidermal growth factor receptor inhibitors such as Erlotinib plagues clinical challenges. Chinese formulas have a unique advantage in reducing side effects. Here, we aim to investigate the skin protecting function of XiaoZhenFang (XZF), a clinical adjuvant prescription made up of Lonicerae Japonicae Flos, Lithospermum Erythrorhizon, Smilacis Glabrae Rhizoma, Forsythiae Fructus, Spirodelae Herba, Cortex Moutan and Prunellae Spica. Our data showed that XZF aqueous extract effectively reduced skin toxicities induced by Erlotinib in vivo using established mice model. Next, we used a systems pharmacology approach to investigate the pharmacological mechanism of XZF with the goal of understanding its effects at the system, organ, and molecular levels. 44 candidate compounds and 103 potential targets were identified by network pharmacology. Inflammation, cell stress and the EGFR-related signal pathways, which may participate in the skin protection afforded by XZF, were analyzed by gene enrichment. Importantly, our in vivo experimental results largely validated XZF's mechanism of action, as predicted by the system pharmacology analysis. Our study uncovered the effect and mechanism of XZF in attenuating skin toxicities induced by EGFRI, providing a basis for the development of in-hospital preparations and new drugs for the prevention of skin toxicities.

Skin problems and EGFR-tyrosine kinase inhibitor

Epidermal growth factor receptor inhibition is a good target for the treatment of lung, colon, pancreatic and head and neck cancers. Epidermal growth factor receptor-tyrosine kinase inhibitor was first approved for the treatment of advanced lung cancer in 2002. Epidermal growth factor receptor-tyrosine kinase inhibitor plays an essential role in the treatment of cancer, especially for patients harbouring epidermal growth factor receptor activating mutation. Hence, skin toxicity is the most concerning issue for the epidermal growth factor receptor-tyrosine kinase inhibitor treatment. Skin toxicity is bothersome and sometimes affects the quality of life and treatment compliance. Thus, it is important for physicians to understand the background and how to manage epidermal growth factor receptor-tyrosine kinase inhibitor-associated skin toxicity. Here, the author reviewed the mechanism and upfront preventive and reactive treatments for epidermal growth factor receptor inhibitor-associated skin toxicities.

Molecular signatures in the prevention of radiation damage by the synergistic effect of N-acetyl cysteine and qingre liyan decoction, a traditional chinese medicine, using a 3-dimensional cell culture model of oral mucositis

Qingre Liyan decoction (QYD), a Traditional Chinese medicine, and N-acetyl cysteine (NAC) have been used to prevent radiation induced mucositis. This work evaluates the protective mechanisms of QYD, NAC, and their combination (NAC-QYD) at the cellular and transcriptional level. A validated organotypic model of oral mucosal consisting of a three-dimensional (3D) cell tissue-culture of primary human keratinocytes exposed to X-ray irradiation was used. Six hours after the irradiation, the tissues were evaluated by hematoxylin and eosin (H and E) and a TUNEL assay to assess histopathology and apoptosis, respectively. Total RNA was extracted and used for microarray gene expression profiling. The tissue-cultures treated with NAC-QYD preserved their integrity and showed no apoptosis. Microarray results revealed that the NAC-QYD caused the upregulation of genes encoding metallothioneins, HMOX1, and other components of the Nrf2 pathway, which protects against oxidative stress. DNA repair genes (XCP, GADD45G, RAD9, and XRCC1), protective genes (EGFR and PPARD), and genes of the NFκB pathway were upregulated. Finally, tissue-cultures treated prophylactically with NAC-QYD showed significant downregulation of apoptosis, cytokines and chemokines genes, and constrained damage-associated molecular patterns (DAMPs). NAC-QYD treatment involves the protective effect of Nrf2, NFκB, and DNA repair factors.

In vitro investigations on the effect of dermal fibroblasts on keratinocyte responses to ultraviolet B radiation

Exposure to ultraviolet radiation is closely linked to the development of skin cancers in humans. The ultraviolet B (UVB) radiation wavelength (280-320 nm), in particular, causes DNA damage in epidermal keratinocytes, which are linked to the generation of signature premalignant mutations. Interactions between dermal fibroblasts and keratinocytes play a role in epidermal repair and regeneration after UVB-induced damage. To investigate these processes, established two and three-dimensional culture models were utilized to study the impact of fibroblast-keratinocyte crosstalk during the acute UVB response. Using a coculture system it was observed that fibroblasts enhanced keratinocyte survival and the repair of cyclobutane pyrimidine dimers (CPDs) after UVB radiation exposure. These findings were also mirrored in irradiated human skin coculture models employed in this study. Fibroblast coculture was shown to play a role in the expression and activation of members of the apoptotic cascade, including caspase-3 and Bad. Interestingly, the expression and phosphorylation of p53, a key player in the regulation of keratinocyte cell fate postirradiation, was also shown to be influenced by fibroblast-produced factors. This study highlights the importance of synergistic interactions between fibroblasts and keratinocytes in maintaining a functional epidermis while promoting repair and regeneration following UVB radiation-induced damage.

The Polyphenol Chlorogenic Acid Attenuates UVB-mediated Oxidative Stress in Human HaCaT Keratinocytes

We investigated the protective effects of chlorogenic acid (CGA), a polyphenol compound, on oxidative damage induced by UVB exposure on human HaCaT cells. In a cell-free system, CGA scavenged 1,1-diphenyl-2-picrylhydrazyl radicals, superoxide anions, hydroxyl radicals, and intracellular reactive oxygen species (ROS) generated by hydrogen peroxide and ultraviolet B (UVB). Furthermore, CGA absorbed electromagnetic radiation in the UVB range (280–320 nm). UVB exposure resulted in damage to cellular DNA, as demonstrated in a comet assay; pre-treatment of cells with CGA prior to UVB irradiation prevented DNA damage and increased cell viability. Furthermore, CGA pre-treatment prevented or ameliorated apoptosis-related changes in UVB-exposed cells, including the formation of apoptotic bodies, disruption of mitochondrial membrane potential, and alterations in the levels of the apoptosis-related proteins Bcl-2, Bax, and caspase-3. Our findings suggest that CGA protects cells from oxidative stress induced by UVB radiation.

Environmental factors and unhealthy lifestyle influence oxidative stress in humans--an overview

Oxygen is the most essential molecule for life; since it is a strong oxidizing agent, it can aggravate the damage within the cell by a series of oxidative events including the generation of free radicals. Antioxidative agents are the only defense mechanism to neutralize these free radicals. Free radicals are not only generated internally in our body system but also trough external sources like environmental pollution, toxic metals, cigarette smoke, pesticides, etc., which add damage to our body system. Inhaling these toxic chemicals in the environment has become unavoidable in modern civilization. Antioxidants of plant origin with free radical scavenging properties could have great importance as therapeutic agents in several diseases caused by environmental pollution. This review summarizes the generation of reactive oxygen species and damage to cells by exposure to external factors, unhealthy lifestyle, and role of herbal plants in scavenging these reactive oxygen species.

Algorithm for dermocosmetic use in the management of cutaneous side-effects associated with targeted therapy in oncology

Currently, numerous patients who receive targeted chemotherapy for cancer suffer from disabling skin reactions due to cutaneous toxicity, which is a significant problem for an increasing number of patients and their treating physicians. In addition, using inappropriate personal hygiene products often worsens these otherwise manageable side-effects. Cosmetic products for personal hygiene and lesion camouflage are part of a patients’ well-being and an increasing number of physicians feel that they do not have adequate information to provide effective advice on concomitant cosmetic therapy. Although ample information is available in the literature on pharmaceutical treatment for cutaneous side-effects of chemotherapy, little is available for the concomitant use of dermatological skin-care products with medical treatments. The objective of this consensus study is to provide an algorithm for the appropriate use of dermatological cosmetics in the management of cutaneous toxicities associated with targeted chemotherapy such as epidermal growth factor receptor inhibitors and other monoclonal antibodies. These guidelines were developed by a French and German expert group of dermatologists and an oncologist for oncologists and primary care physicians who manage oncology patients. The information in this report is based on published data and the expert group’s opinion. Due to the current lack of clinical evidence, only a review of published recommendations including suggestions for concomitant cosmetic use was conducted.

Bcl-2 family members: essential players in skin cancer

Skin cancer has reached epidemic proportions and is considered to be a direct consequence of ultraviolet (UV) radiation exposure. Excessive exposure of epidermal cells to UV results in apoptosis of irreparably damaged cells to avoid malignant transformation. The Bcl-2 family of proteins is emerging as a crucial regulator of epidermal homeostasis and cell's fate in the stressed skin. Not surprisingly, deregulation of Bcl-2 family members is also chiefly involved in skin carcinogenesis and response to cancer therapy. Here we discuss the physiopathological role of epidermal Bcl-2 family members, their implications in skin carcinogenesis and as potential targets in cancer therapy.

Loss of EGFR induced autophagy sensitizes hormone refractory prostate cancer cells to adriamycin

BACKGROUND

The epidermal growth factor receptor (EGFR), a receptor tyrosine kinase, is over-expressed in advanced prostate cancer but tyrosine kinase inhibitors are not clinically effective in the treatment of prostate cancer. Recently it was found that EGFR in cancer cells has a kinase-independent pro-survival function, preventing cells from undergoing autophagy. In the present study we investigated whether the anti-autophagic function of EGFR may contribute to resistance of hormone-refractory prostate cancer cells to chemotherapeutic-induced apoptosis.

METHODS

We first characterized the autophagic phenotype induced by knocking down EGFR in hormone refractory prostate cancer cells (PC-3MM2 and DU-145), then we tested whether loss of EGFR-induced autophagy could sensitize cancer cells to adriamycin.

RESULTS

Using continuous live cell imaging techniques, we observed that knocking down EGFR lead to typical autophagic morphological/molecular changes, cell shrinkage without detachment, aggregation of microtubule-associated protein 1 light chain 3 (LC3) protein and absence of activation of apoptotic caspases 3/7. Loss of EGFR also increased the activity of calpain, which is pro-apoptotic. Knocking down EGFR, but not inhibiting its tyrosine kinase activity, significantly sensitized cells to adriamycin-induced apoptosis. Adriamycin-induced apoptosis could be inhibited by increased extracellular glucose level, suggesting intracellular glucose deficiency is a key mediator of the sensitization. The loss of EGFR induced autophagy and sensitization to adriamycin were also reproduced by using another hormone refractory prostate cancer cell line, Du145.

CONCLUSION

Taken together, these data suggest that decreasing the expression level of EGFR protein, rather than inhibiting its tyrosine kinase activity, may enhance the efficiency of EGFR targeted therapy for prostate cancer.

Phase II study of erlotinib plus gemcitabine in Japanese patients with unresectable pancreatic cancer

Erlotinib combined with gemcitabine has not been evaluated in Japanese patients with unresectable pancreatic cancer. This two-step phase II study assessed the safety and pharmacokinetics of erlotinib 100 mg/day (oral) plus gemcitabine 1000 mg/m(2) (i.v. days 1, 8, 15) in a 28-day cycle in the first step, and efficacy and safety in the second step. The primary end-point was safety. One hundred and seven patients were enrolled (first step, n = 6; second step, n = 101). The most common adverse event was RASH (compiled using the preferred terms rash, acne, exfoliative rash, dermatitis acneiform, erythema, eczema, dermatitis and pustular rash) in 93.4% of patients. One treatment-related death occurred. While interstitial lung disease-like events were reported in nine patients (8.5%; grade 1/2/3, 3.8/2.8/1.9%), all patients recovered or improved. The median overall survival, the 1-year survival rate and median progression-free survival were 9.23 months, 33.0% and 3.48 months, respectively. The overall response and disease control rates were 20.3% and 50.0%, respectively. In Japanese patients with unresectable pancreatic cancer, erlotinib plus gemcitabine had acceptable toxicity and efficacy that was not inferior to that seen in Western patients.

Skin toxicity evaluation protocol with panitumumab (STEPP), a phase II, open-label, randomized trial evaluating the impact of a pre-Emptive Skin treatment regimen on skin toxicities and quality of life in patients with metastatic colorectal cancer

PURPOSE

Panitumumab, a fully human monoclonal antibody targeting the epidermal growth factor receptor (EGFR), is approved in the United States and Europe for the treatment of refractory metastatic colorectal cancer (mCRC). Skin toxicities are the most common adverse events with EGFR inhibitors. This is the first study designed to examine differences between pre-emptive and reactive skin treatment for specific skin toxicities in patients with mCRC for any EGFR inhibitor.

PATIENTS AND METHODS

Patients receiving panitumumab-containing therapy were randomly assigned 1:1 to pre-emptive or reactive treatment (after skin toxicity developed). Pre-emptive treatment included use of skin moisturizers, sunscreen, topical steroid, and doxycycline. The primary end point of the study was the incidence of protocol-specified >or= grade 2 skin toxicities during the 6-week skin treatment period. Quality of life (QOL) was assessed with the Dermatology Life Quality Index (DLQI).

RESULTS

Of 95 enrolled patients, 48 received pre-emptive treatment, and 47 received reactive treatment. The incidence of protocol-specified >or= grade 2 skin toxicities during the 6-week skin treatment period was 29% and 62% for the pre-emptive and reactive groups, respectively. Mean DLQI score change from baseline to week 3 was 1.3 points and 4.2 points in the pre-emptive and reactive groups, respectively.

CONCLUSION

The pre-emptive skin treatment regimen was well tolerated. The incidence of specific >or= grade 2 skin toxicities during the 6-week skin treatment period was reduced by more than 50% in the pre-emptive group compared with the reactive group. Patients in the pre-emptive group reported less QOL impairment than patients in the reactive group.

Variable Bax antigenicity is linked to keratinocyte position within epidermal strata and UV-induced apoptosis

Pro- and anti-apoptotic members of the Bcl-2 family are fundamental in the control of apoptosis. Among them, Bax plays a key role in apoptosis induction by mediating the release of apoptogenic factors from mitochondria to the cytosol. In this report, we investigated, by immunohistofluorescence, the in vivo distribution of Bax in normal human epidermis before and 24 h after exposure to solar-simulated radiation. Bax expression was evaluated with three different, Western blot pretested, anti-Bax antibodies (Ab) and correlated with markers of keratinocyte differentiation and apoptosis using anti-beta(1) integrin and anti-active caspase-3 Abs respectively. Using anti-Bax N20 and A-3533 polyclonal Ab, we found that, whereas undifferentiated keratinocytes of the basal proliferative compartment contained Bax in the cytosol, the differentiated suprabasal cells had Bax mainly in the nucleus. This immunoreactivity pattern was not modified by skin irradiation. Interestingly, the well known apoptosis-related Bax redistribution to mitochondria in response to a cell death signal, could be detected only with yet another, the 2D2 monoclonal Ab. This relocalization occurred specifically in apoptotic, active caspase-3 positive cells of irradiated epidermis. Our data highlight the differentiation- and apoptosis-associated changes in the pattern of Bax subcellular and cellular distribution as uncovered by different anti-Bax Abs and suggest that Bax undergoes successive activation that progresses in parallel with keratinocyte differentiation and apoptosis.

Suprabasal Dsg2 expression in transgenic mouse skin confers a hyperproliferative and apoptosis-resistant phenotype to keratinocytes

Desmoglein 2 (Dsg2), a component of the desmosomal cell-cell adhesion structure, has been linked to invasion and metastasis in squamous cell carcinomas. However, it is unknown whether – and if so how – Dsg2 contributes to the malignant phenotype of keratinocytes. In this study, we addressed the consequences of Dsg2 overexpression under control of the involucrin promoter (Inv-Dsg2) in the epidermis of transgenic mice. These mice exhibited epidermal hyperkeratosis with slightly disrupted early and late differentiation markers, but intact epidermal barrier function. However, Inv-Dsg2 transgene expression was associated with extensive epidermal hyperplasia and increased keratinocyte proliferation in basal and suprabasal epidermal strata. Cultured Inv-Dsg2 keratinocytes showed enhanced cell survival in the anchorage-independent state that was critically dependent on EGF receptor activation and NF-κB activity. Consistent with the hyperproliferative and apoptosis-resistant phenotype of Inv-Dsg2 transgenic keratinocytes, we observed enhanced activation of multiple growth and survival pathways, including PI 3-kinase/AKT, MEK-MAPK, STAT3 and NF-κB, in the transgenic skin in situ. Finally, Inv-Dsg2 transgenic mice developed intraepidermal skin lesions resembling precancerous papillomas and were more susceptible to chemically induced carcinogenesis. In summary, overexpression of Dsg2 in epidermal keratinocytes deregulates multiple signaling pathways associated with increased growth rate, anchorage-independent cell survival, and the development of skin tumors in vivo.

Mechanisms of cutaneous toxicities to EGFR inhibitors

The increased target specificity of epidermal growth factor receptor (EGFR) inhibitors (EGFRIs) is associated with the reduction or abolition of nonspecific and haematopoietic side effects. However, coincident inhibition of receptor activity in tissues that depend on EGFR signalling for normal function has undesirable consequences. Because of the key role of EGFR signalling in skin, dermatological toxicities have frequently been described with EGFRIs. The resultant significant physical and psycho-social discomfort might lead to interruption or dose modification of anticancer agents. There is an urgent need for an improved understanding of these toxicities to develop adequate staging systems and mechanistically driven therapies, and to ensure quality of life and consistent antineoplastic therapy.

Ultraviolet radiation-induced apoptosis in keratinocytes: on the role of cytosolic factors

Epidemiological and experimental evidences have established solar ultraviolet (UV) radiation as the leading cause of skin cancers. Specifically, the frequency of non-melanoma skin cancer, one of the malignancies with the most rapidly increasing incidence, is directly related to the total exposure to solar UV light. As part of a general effort to elucidate the components of cellular signal transduction pathways, the mechanisms of cellular responses to UV radiation have received considerable attention over the last few years. These efforts were driven mainly by the conviction that understanding how normal cells respond to extracellular stimuli such as exposure to UV radiation will undoubtedly help in deciphering what goes wrong in a variety of clinical disorders including skin cancers and will assist in the development of novel therapeutic strategies. Studies over the last decade have established that UV radiation induces a bewildering array of signal transduction pathways, some of which could lead to apoptotic cell death. UV-induced cell death by apoptosis is considered to be a natural protective mechanism that removes damaged keratinocytes and circumvents the risk of malignant transformation. In this review, we summarize some of the most important findings regarding the response and role of mitogen-activated protein kinases in UVA and UVB radiation-induced signaling to apoptosis in keratinocytes. We will also briefly discuss what is known about the role of the BCL-2 family of proteins, the emerging role of lysosomal proteases and other important cytosolic signaling proteins in UV-induced apoptosis.

Targeting HER1/EGFR in cancer therapy: experience with erlotinib

Research into the role of the human epidermal receptor growth factor receptor 1/epidermal growth factor receptor (HER1/EGFR) in tumorigenesis has generated a new class of anticancer drugs. One such agent, erlotinib (TarcevaTM), is a potent, selective and reversible inhibitor of HER1/EGFR tyrosine kinase activity. Erlotinib has demonstrated the clinical activity in a variety of solid tumors, and has recently demonstrated improvements in survival in large Phase III trials, in non-small-cell lung cancer and pancreatic cancer.

Activation of p38 MAPK is required for Bax translocation to mitochondria, cytochrome c release and apoptosis induced by UVB irradiation in human keratinocytes

This study establishes that activation of p38 MAPK by UVB represents a crucial signal required for the conformational change and translocation of Bax to the mitochondria in human keratinocytes. UVB-induced Bax translocation and mitochondrial cytochrome c release, which precede caspase activation and other endpoints of the apoptotic program such as chromatin fragmentation and loss of mitochondrial transmembrane potential, are blocked by genetic or pharmacological inhibition of the p38alpha MAPK. Inhibition of p38 MAPK strongly reduces the UVB-induced formation of sunburn cells and blocks Bax conformational change both in cultured human keratinocytes and in human skin, providing clear evidence for the physiological role of the p38 MAPK-Bax pathway in the removal of precancerous, UVB-damaged keratinocytes. Furthermore, we show that Bcl-2 overexpression, but not the pan-caspase inhibitor zVAD-fmk, blocks Bax conformational change and its subsequent translocation downstream of p38 MAPK. These data indicate that the activation of p38 MAPK by UVB engages a caspase-independent death signal leading to mitochondrial membrane permeabilization and apoptosis in human keratinocytes and suggest that p38 MAPK might have a preventive role in the process of photocarcinogenesis.

Ultraviolet A-induced modulation of Bcl-XL by p38 MAPK in human keratinocytes: post-transcriptional regulation through the 3'-untranslated region

We examined the effect of inhibiting p38 MAPK on UVA-irradiated HaCaT cells, a spontaneously immortalized human keratinocyte cell line. Recent work from our laboratory has shown that UVA (250 kJ/m2) induces a rapid phosphorylation of p38 MAPK in the HaCaT cell line. Inhibition of p38 MAPK activity through the use of a specific inhibitor, SB202190, in combination with UVA treatment induced a rapid cleavage of caspase-9, caspase-8, and caspase-3, whereas UVA irradiation alone had no effect. Similarly, cleavage of the caspase substrate poly(ADP-ribose) polymerase was observed in UVA-irradiated HaCaT cells treated with SB202190 or in cells expressing a dominant-negative p38 MAPK. No effect of p38 MAPK inhibition upon caspase cleavage was observed in mock-irradiated HaCaT cells. In addition, increases in apoptosis were observed in UVA-irradiated cells treated with SB202190 by morphological analysis with no significant apoptosis occurring from UVA irradiation alone. Similar results were obtained by using normal human epidermal keratinocytes. UVA induced expression of the anti-apoptotic Bcl-2 family member, Bcl-XL, with abrogation of expression by using the p38 MAPK inhibitor SB202190. Overexpression of Bcl-XL prevented poly(ADP-ribose) polymerase cleavage induced by the combination of UVA and p38 MAPK inhibition. UVA enhanced the stability of Bcl-XL mRNA through increases in p38 MAPK activity. We determined that increases in UVA-induced expression of Bcl-XL occur through a posttranscriptional mechanism mediated by the 3'-untranslated region (UTR). We used Bcl-XL 3'-UTR luciferase constructs to determine the mechanism by which UVA increased Bcl-XL mRNA stability. Additionally, RNA binding studies indicate that UVA increases the binding of RNA-binding proteins to Bcl-XL 3'-UTR mRNA, which can be decreased by using SB202190. In conclusion, p38 MAPK and Bcl-XL expression play critical roles in the survival of UVA-irradiated HaCaT cells.

Complex Regulation of Signal Transducers and Activators of Transcription 3 Activation in Normal and Malignant Keratinocytes

Previous work implicated activation of the signal transducer and activator of transcription (STAT)3 downstream of the epidermal growth factor receptor (EGFR) in the malignant phenotype of squamous carcinoma cells (SCC). Here, we show that EGFR-dependent STAT3 activation is restricted to malignant keratinocytes. Specifically, constitutive and epidermal growth factor-induced phosphorylation of STAT3 on Y705 was observed only in SCC but not in either immortalized (HaCaT) or normal keratinocyte strains. Furthermore, STAT3 activation as determined by DNA binding assays was restricted to SCC and dependent on EGFR activation. Forced expression of EGFR in immortalized keratinocytes (HaCaT cells) was associated with enhanced EGFR activation but not STAT3-Y705 phosphorylation. EGFR-dependent activation of mitogen-activated protein kinase (MAPK) kinase 1 negatively regulated STAT3-Y705 phosphorylation in normal and malignant keratinocytes. Together, these results underscore that EGFR activation is required but not sufficient for STAT3 activation to occur in malignant keratinocytes. They also highlight complex regulation of STAT3 phosphorylation through EGFR activation including negative regulation via the MAPK kinase/MAPK signaling pathway.

Epidermal tissue regeneration and stromal interaction in HaCaT cells is initiated by TGF-alpha

The human keratinocyte cell line HaCaT expresses essentially all epidermal differentiation markers but exhibits deficiencies in tissue organization as surface transplants in nude mice and even more so in organotypic co-cultures with fibroblasts. Whereas tissue differentiation by normal keratinocytes (NEKs) is regulated by stromal interactions, this mechanism is impaired in HaCaT cells. This regulatory process is initiated by interleukin-1 (IL-1) release in keratinocytes, which induces expression of keratinocyte growth factor (KGF/FGF-7) and granulocyte macrophage-colony stimulating factor (GM-CSF) in fibroblasts. Production and release of IL-1 is very low and, consequently, expression of the fibroblast-derived growth factors KGF/FGF-7 and GM-CSF is absent in HaCaT-fibroblast co-cultures. However, addition of KGF and GMCSF, respectively, is inefficient to improve stratification and differentiation by HaCaT cells due to the low expression of their cognate receptors. More importantly, expression and release of the autocrine keratinocyte growth factor TGF-alpha is dramatically decreased in HaCaT cells. Addition of TGF- alpha or EGF stimulated HaCaT cell proliferation but, even more effectively, suppressed apoptosis, thus facilitating the formation of a regularly stratified epithelium. Furthermore, TGF-alpha enhanced the expression of the receptors for KGF and GM-CSF so that addition of these growth factors, or of their inducer IL-1, further improved epidermal tissue differentiation leading to in vitro skin equivalents comparable with cultures of NEKs. Thus, supplementing TGF-alpha normalized epidermal tissue regeneration by immortal HaCaT keratinocytes and their interaction with stromal cells so that regular skin equivalents are produced as standardized in vitro models.

Inhibition of erbB receptor family members protects HaCaT keratinocytes from ultraviolet-B-induced apoptosis

In the human epidermis, the cells most at risk for the development of cancer due to sunlight exposure are the keratinocytes. In animal models, ultraviolet-B is a complete carcinogen, capable of inducing and promoting the development of malignant cells. A key element of ultraviolet-B-induced carcinogenesis is the ability of ultraviolet-B to induce the expression of a number of cellular proteins and activate growth factor receptor tyrosine kinases, including the erbB receptor family. Keratinocytes express the erbB1 (also called EGF-R, HER1), the erbB2 (also known as neu or HER2), and the erbB3 (HER3) subtypes. In general, activation of the erbB receptor family leads to a cellular proliferative response. In certain instances, however, activation of an erbB receptor can induce differentiation, cell cycle arrest, and even apoptosis. The inhibition of tyrosine kinase activity in rodent models and human skin has been shown to inhibit some ultraviolet-B response pathways. We have shown that the inhibition of erbB receptors, by both pharmaceutical and immunologic means, will inhibit ultraviolet-B-induced apoptosis in the HaCaT human keratinocyte cell line. This inhibition was specific for the erbB receptor family and specific for ultraviolet-B-induced apoptosis. These results suggest that, in certain instances, ultraviolet-B-induced apoptotic signaling requires erbB family receptor activity.

Mitogen-activated protein kinase activation in UV-induced signal transduction

Experimental evidence supported by epidemiological findings suggests that solar ultraviolet (UV) irradiation is the most important environmental carcinogen leading to the development of skin cancers. Because the ozone layer blocks UVC (wavelength, 180 to 280 nm) exposure, UVA (UVA I, 340 to 400 nm; UVA II, 320 to 340 nm) and UVB (280 to 320 nm) are probably the chief carcinogenic components of sunlight with relevance for human skin cancer. Substantial contributions to the elucidation of the specific signal transduction pathways involved in UV-induced skin carcinogenesis have been made over the past few years, and most evidence suggests that the cellular signaling response is UV wavelength-dependent. The mitogen-activated protein kinase (MAPK) signaling cascades are targets for UV and are important in the regulation of the multitude of UV-induced cellular responses. Experimental studies have used a range of UVA, UVB, UVC, and various combinations in multiple doses, and the observed effects on activation and phosphorylation of MAPKs are varied. This review focuses on the mechanistic data supporting a role for MAPKs in UV-induced skin carcinogenesis. Progress in understanding the mechanisms of UV-induced signal transduction could lead to the use of these protein kinases as specific targets for the prevention and control of skin cancer.

Apoptosis regulators and responses in human melanocytic and keratinocytic cells

Apoptosis in keratinocytes is required for epidermal turnover, stratum corneum formation, and removal of ultraviolet-damaged premalignant cells. Its role in melanocyte homeostasis and transformation, on the other hand, has not been defined, although apoptosis resistance is a commonly recognized feature of melanoma. We examined the expression of apoptosis regulators in melanocytes, keratinocytes, melanoma, and HaCat cells. Melanocytic cells expressed relatively high levels of Bcl-2, Bcl-X(L), Mcl-1, C-IAP-1, C-IAP-2, XIAP, Livin, and Apaf-1. The only apoptotic regulator that was differentially expressed in melanoma cells and not melanocytes was Survivin, whereas Bax was expressed in melanocytes but not in most melanoma lines. Keratinocytic cells, on the other hand, expressed high levels of FLIP and were relatively deficient in Bcl-2 family proteins. Levels of p53 were highest in HaCat cells and some of the melanoma lines, and barely detectable in melanocytes and keratinocytes. Next, susceptibility of these cells types to apoptosis induced by ultraviolet B, the tyrosine analog 4-tert-butylphenol, and cytotoxic drugs was examined. Melanocytes were relatively resistant to ultraviolet B, whereas keratinocytes were unresponsive to 4-tert-butylphenol. Melanocytes and keratinocytes were generally less susceptible than melanoma lines and HaCat cells to etoposide, cisplatin, and staurosporine. Induction of apoptosis in these cell types was generally associated with decreased levels of Mcl-1, XIAP, and Livin, and increased levels of p53, whereas levels of other apoptotic regulators were unaltered. These results provide insights into the potential roles of apoptosis in the function and transformation of epidermal melanocytes and keratinocytes.

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.

Expression and transcriptional regulation of caspase-14 in simple and complex epithelia

Caspase-14 is a recent addition to the caspase family of aspartate proteases involved in apoptotic processes. Human caspase-14 appears to be only weakly processed during apoptosis, and it does not cleave classical caspase substrates. Post partum, caspase-14 is prominently expressed by human keratinocytes and reportedly participates in terminal differentiation of complex epithelia. Here we provide evidence challenging the view that caspase-14 expression or processing is linked exclusively to terminal keratinocyte differentiation. We demonstrate that caspase-14 expression extended to multiple cell lines derived from simple epithelia of the breast, prostate, and stomach. In keratinocytes and breast epithelial cells, caspase-14 expression was upregulated in high-density cultures and during forced suspension culture. These effects were primarily due to transcriptional activation as indicated by reporter gene assays using a 2 kb caspase-14 promoter fragment. Importantly, caspase-14 was not cleaved during forced suspension culture of either cell type although this treatment induced caspase-dependent apoptosis (anoikis). Forced expression of caspase-14 in immortalized human keratinocytes had no effect on cell death in forced suspension nor was the transfected caspase-14 processed in this setting. In contrast to postconfluent and forced suspension culture, terminal differentiation of keratinocytes induced in vitro by Ca2+ treatment was not associated with increased caspase-14 expression or promoter activity. Our results indicate that (1) caspase-14 is expressed not only in complex but also simple epithelia; (2) cells derived from complex and simple epithelia upregulate caspase-14 expression in conditions of high cell density or lack of matrix interaction and; (3) in both cell types this phenomenon is due to transcriptional regulation.

The human papillomavirus type 16 (HPV-16) E5 protein sensitizes human keratinocytes to apoptosis induced by osmotic stress

We have studied the role of the HPV-16 E5 protein in apoptosis, using HaCaT cell lines stably transfected with either E5 (HaCaT/E5) or the empty vector (HaCaT/pMSG) as control. When subjected to a hyperosmolar concentration of sorbitol, HaCaT/E5 cells respond with cytochrome c release, activation of caspase-3, -8, and -9, and PARP-cleavage, showing that the mitochondria and death-receptor mediated apoptotic pathways are involved in subsequent cell death. Similar effects are observed for the control cells only after extended sorbitol treatment. Thus, E5-expressing cells are more sensitive to osmotic stress, perhaps because of modifications of the cellular membranes caused by this strongly hydrophobic molecule.