Arsenite suppresses Notch1 signaling in human keratinocytes

[Involvement of Notch1 and ALK4/5 Signaling Pathways in Renal Tubular Cell Death: Their Application to Clarification of Cadmium Toxicity]

Renal tubular cell death is caused by various extracellular stresses including toxic amounts of cadmium, an occupational and environmental pollutant metal, and is responsible for renal dysfunction. While cadmium exposure disrupts many intracellular signaling pathways, the molecular mechanism underlying cadmium-induced renal tubular cell death has not yet been fully elucidated. We have recently identified two important intracellular signaling pathways that promote cadmium-induced renal tubular cell death: the Notch1 signaling and activin receptor-like kinase (ALK) 4/5 signaling (also known as the activin-transforming growth factor β receptor pathways). In this review paper, we introduce our previous experimental findings, focusing on Notch1 and ALK4/5 signaling pathways, which may uncover the molecular mechanisms involved in cadmium-induced renal tubular cell death.

Deducing signaling pathways from parallel actions of arsenite and antimonite in human epidermal keratinocytes

Inorganic arsenic oxides have been identified as carcinogens in several human tissues, including epidermis. Due to the chemical similarity between trivalent inorganic arsenic (arsenite) and antimony (antimonite), we hypothesized that common intracellular targets lead to similarities in cellular responses. Indeed, transcriptional and proteomic profiling revealed remarkable similarities in differentially expressed genes and proteins resulting from exposure of cultured human epidermal keratinocytes to arsenite and antimonite in contrast to comparisons of arsenite with other metal compounds. These data were analyzed to predict upstream regulators and affected signaling pathways following arsenite and antimonite treatments. A majority of the top findings in each category were identical after treatment with either compound. Inspection of the predicted upstream regulators led to previously unsuspected roles for oncostatin M, corticosteroids and ephrins in mediating cellular response. The influence of these predicted mediators was then experimentally verified. Together with predictions of transcription factor effects more generally, the analysis has led to model signaling networks largely accounting for arsenite and antimonite action. The striking parallels between responses to arsenite and antimonite indicate the skin carcinogenic risk of exposure to antimonite merits close scrutiny.

Arsenic Trioxide Suppresses Tumor Growth through Antiangiogenesis via Notch Signaling Blockade in Small-Cell Lung Cancer

Small-cell lung cancer (SCLC) is a highly malignant type of lung cancer with no effective second-line chemotherapy drugs. Arsenic trioxide (As2O3) was reported to exert antiangiogenesis activities against lung cancer and induce poor development of vessel structures, similar to the effect observed following the blockade of Notch signaling. However, there are no direct evidences on the inhibitory effects of As2O3 on tumor growth and angiogenesis via blockade of Notch signaling in SCLC. Here, we found that As2O3 significantly inhibited the tumor growth and angiogenesis in SCLC and reduced the microvessel density. As2O3 disturbed the morphological development of tumor vessels and downregulated the protein levels of delta-like canonical Notch ligand 4 (Dll4), Notch1, and Hes1 in vivo. DAPT, a Notch signaling inhibitor, exerted similar effects in SCLC. We found that both As2O3 treatment and Notch1 expression knockdown resulted in the interruption of tube formation by human umbilical vein endothelial cells (HUVECs) on Matrigel. As2O3 had no effects on Dll4 level in HUVECs but significantly inhibited the expression of Notch1 and its downstream gene Hes1 regardless of Dll4 overexpression or Notch1 knockdown. These findings suggest that the antitumor activity of As2O3 in SCLC was mediated via its antiangiogenic effect through the blockade of Notch signaling, probably owing to Notch1 targeting.

DUOX expression in human keratinocytes and bronchial epithelial cells: Influence of vanadate

Dual oxygenases (DUOX) 1 and 2, expressed in many animal tissues, participate in host defense at mucosal surfaces and may have important signaling roles through generation of reactive oxygen. Present work addresses their expression in cultured human epidermal keratinocytes and effects of cytokines and metal/metalloid compounds. Both DUOX1 and 2 were expressed at much higher levels after confluence than in the preconfluent state. Maximal DUOX1 mRNA levels were 50 fold those of DUOX2. DUOX1 and 2 were induced ≈3 fold by interleukin 4, but only DUOX1 was induced by interferon gamma (IFNγ). In human bronchial HBE1 cells, by contrast, interleukin 4 induced only DUOX 1, and IFNγ induced only DUOX2. A survey in the keratinocytes of metal/metalloid compounds showed that arsenite, antimonite, chromate, cadmium, copper, lead and vanadate suppressed DUOX1 levels but did not prevent interleukin 4 stimulation. Effects on DUOX2 were less dramatic, except that vanadate potentiated the stimulation by IFNγ up to 7 fold. The results indicate that epithelial cell types of different tissue origins can differ in their cytokine regulation and that epidermal cells can exhibit striking alterations in response due to certain metal/metalloid exposures.

Parallel responses of human epidermal keratinocytes to inorganic SbIII and AsIII

SbIII and AsIII are known to exhibit similar chemical properties, but the degree of similarity in their effects on biological systems merits further exploration. Present work compares the responses of human epidermal keratinocytes, a known target cell type for arsenite-induced carcinogenicity, to these metalloids after treatment for a week at environmentally relevant concentrations. Previous work with these cells has shown that arsenite and antimonite have parallel effects in suppressing differentiation, altering levels of several critical enzymes and maintaining colony forming ability. More globally, protein profiling now reveals parallels in SbIII and AsIII effects. The more sensitive technique of transcriptional profiling also shows considerable parallels. Thus, gene expression changes were almost entirely in the same directions for the two treatments, although the degree of change was sometimes significantly different. Inspection of the changes revealed that RYR1 and LRIG1 were among the genes strongly suppressed, consistent with reduced calcium-dependent differentiation and maintenance of EGF-dependent proliferative potential. Moreover, levels of miRNAs in the cells were altered in parallel, with nearly 90% of the 198 most highly expressed ones being suppressed. Among these was miR-203, which is known to decrease proliferative potential. Finally, both SbIII and AsIII were seen to attenuate bone morphogenetic protein 6 induction of dual specificity phosphatases 2 and 14, consistent with maintaining epidermal growth factor receptor signaling. These findings raise the question whether SbIII, like AsIII, could act as a human skin carcinogen.

Efficient fabrication of epidermal cell sheets using γ-secretase inhibitor

BACKGROUND

Epidermal cell sheets have been utilized for regeneration of skin when skin defects occur and prevention of esophageal stricture after endoscopic submucosal dissection. To reduce the cost of cultivation, a novel culture method to shorten a culture process needs to be developed.

OBJECTIVES

To shorten a culture process of epidermal cell sheets, we developed a novel culture method to accelerate the fabrication of epidermal cell sheets using γ-secretase inhibitor.

METHODS

Normal human epidermal keratinocytes (NHEKs) were cultured using γ-secretase inhibitor, DAPT, during expansion of the cells to confluence and culture without DAPT during stratification. The cell growth, quantitative gene expression of stem/progenitor or differentiation markers, and protein expression of these markers were analyzed to verify the effectiveness of the novel method.

RESULTS

The proliferation of NHEKs on cell-culture inserts was promoted using DAPT. However, NHEKs were not stratified completely in the presence of DAPT. In contrast, NHEKs cultured using DAPT were stratified and differentiated by eliminating the inhibitor after the cells reached confluence. Real-time PCR analyses showed that the gene expressions of putative epithelial stem/progenitor cell markers and epidermis differentiation markers in the cell sheets fabricated using this novel method were significantly higher than those in the cell sheets fabricated without DAPT. Histological and immunofluorescence analyses revealed that it was possible to fabricate well-differentiated epidermal cell sheets efficiently by the novel culture method. The culture period was shortened to 67% of the time required for the control group. In feeder-free conditions, stratified epidermal cell sheets were also fabricated using DAPT.

CONCLUSIONS

The novel culture method using γ-secretase inhibitor, DAPT, was found to be effective for fabricating epidermal cell sheets.

Effect of sodium arsenite on reproductive organs of female Wistar rats

The present study was performed to investigate the subchronic effect of sodium arsenite on female Wistar rats. Mature female rats were divided into 4 groups of 12 animals each. Group I received distilled water, whereas the other 3 groups received sodium arsenite at 10, 30, and 50 µg/L doses for 60 days through drinking water. Half of the animals from each group were dissected after 30 days and the remaining after 60 days. A disruption in estrous cycle was observed with prolonged diestrous and metestrous phases. A significant increase in ovarian surface epithelium and follicular atresia was observed in treated rats (p ≤ .05). A significant decrease (p ≤ .05) in the uterine myometrium was observed. A significant increase (p ≤ .05) in the levels of lipid peroxidation along with decrease in the activities of antioxidant enzymes was observed. The results revealed that subchronic exposure of sodium arsenite causes degenerative changes in reproductive organs and induces oxidative stress in female rats.

Impaired Notch-MKP-1 signalling in hidradenitis suppurativa: an approach to pathogenesis by evidence from translational biology

Recent findings in familial hidradenitis suppurativa (HS) demonstrated loss-of-function mutations of components of the γ-secretase (GS) complex leading to decreased protease cleaving activity, which may compromise canonical Notch signalling. Appropriate Notch signalling is of pivotal importance for maintaining the inner and outer root sheath of the hair follicle and skin appendages. This viewpoint on the pathogenesis of HS is primarily supported by circumstantial evidence derived from translational biology. Impaired Notch signalling is proposed to be the major pathogenic mechanism of HS. Deficient Notch signalling switches the fate of outer root sheath cells, resulting in conversion of hair follicles to keratin-enriched epidermal cysts. Impaired Notch signalling may compromise apocrine gland homoeostasis as well. Damage-associated molecular pattern molecules released by either ruptured epidermal cysts exposing keratin fibres or altered structural components of less maintained apocrine glands may both stimulate TLR-mediated innate immunity. All aggravating factors of HS, that is, smoking, obesity, skin occlusion, androgens and progesterone, may further promote inflammation by release of proinflammatory cytokines derived from activated monocyte/macrophages. Inappropriate Notch signalling may not only initiate inflammation in HS but may lead to insufficient feedback inhibition of overstimulated innate immunity. Regular Notch signalling via induction of MAPK phosphatase-1 (MKP-1) terminates TLR-MAPK-signalling in macrophages and IL-23 secreting DCs, the key players for Th17 cell polarization. Thus, impaired Notch signalling links HS to other Th17-driven comorbidities. All major therapeutic interventions in HS appear to attenuate increased MAPK activation of innate immune cells due to impaired Notch-mediated feedback regulation of innate immunity.

Arsenite suppression of BMP signaling in human keratinocytes

Arsenic, a human skin carcinogen, suppresses differentiation of cultured keratinocytes. Exploring the mechanism of this suppression revealed that BMP-6 greatly increased levels of mRNA for keratins 1 and 10, two of the earliest differentiation markers expressed, a process prevented by co-treatment with arsenite. BMP also stimulated, and arsenite suppressed, mRNA for FOXN1, an important transcription factor driving early keratinocyte differentiation. Keratin mRNAs increased slowly after BMP-6 addition, suggesting they are indirect transcriptional targets. Inhibition of Notch1 activation blocked BMP induction of keratins 1 and 10, while FOXN1 induction was largely unaffected. Supporting a requirement for Notch1 signaling in keratin induction, BMP increased levels of activated Notch1, which was blocked by arsenite. BMP also greatly decreased active ERK, while co-treatment with arsenite maintained active ERK. Inhibition of ERK signaling mimicked BMP by inducing keratin and FOXN1 mRNAs and by increasing active Notch1, effects blocked by arsenite. Of 6 dual-specificity phosphatases (DUSPs) targeting ERK, two were induced by BMP unless prevented by simultaneous exposure to arsenite and EGF. Knockdown of DUSP2 or DUSP14 using shRNAs greatly reduced FOXN1 and keratins 1 and 10 mRNA levels and their induction by BMP. Knockdown also decreased activated Notch1, keratin 1 and keratin 10 protein levels, both in the presence and absence of BMP. Thus, one of the earliest effects of BMP is induction of DUSPs, which increases FOXN1 transcription factor and activates Notch1, both required for keratin gene expression. Arsenite prevents this cascade by maintaining ERK signaling, at least in part by suppressing DUSP expression.

Induction of apoptotic death and retardation of neuronal differentiation of human neural stem cells by sodium arsenite treatment

Chronic arsenic toxicity is a global health problem that affects more than 100 million people worldwide. Long-term health effects of inorganic sodium arsenite in drinking water may result in skin, lung and liver cancers and in severe neurological abnormalities. We investigated in the present study whether sodium arsenite affects signaling pathways that control cell survival, proliferation and neuronal differentiation of human neural stem cells (NSC). We demonstrated that the critical signaling pathway, which was suppressed by sodium arsenite in NSC, was the protective PI3K-AKT pathway. Sodium arsenite (2-4μM) also caused down-regulation of Nanog, one of the key transcription factors that control pluripotency and self-renewal of stem cells. Mitochondrial damage and cytochrome-c release induced by sodium arsenite exposure was followed by initiation of the mitochondrial apoptotic pathway in NSC. Beside caspase-9 and caspase-3 inhibitors, suppression of JNK activity decreased levels of arsenite-induced apoptosis in NSC. Neuronal differentiation of NSC was substantially inhibited by sodium arsenite exposure. Overactivation of JNK1 and ERK1/2 and down-regulation of PI3K-AKT activity induced by sodium arsenite were critical factors that strongly affected neuronal differentiation. In conclusion, sodium arsenite exposure of human NSC induces the mitochondrial apoptotic pathway, which is substantially accelerated due to the simultaneous suppression of PI3K-AKT. Sodium arsenite also negatively affects neuronal differentiation of NSC through overactivation of MEK-ERK and suppression of PI3K-AKT.

Arsenic affects expression and processing of amyloid precursor protein (APP) in primary neuronal cells overexpressing the Swedish mutation of human APP

Arsenic poisoning due to contaminated water and soil, mining waste, glass manufacture, select agrochemicals, as well as sea food, affects millions of people world wide. Recently, an involvement of arsenic in Alzheimer's disease (AD) has been hypothesized (Gong and O'Bryant, 2010). The present study stresses the hypothesis whether sodium arsenite, and its main metabolite, dimethylarsinic acid (DMA), may affect expression and processing of the amyloid precursor protein (APP), using the cholinergic cell line SN56.B5.G4 and primary neuronal cells overexpressing the Swedish mutation of APP, as experimental approaches. Exposure of cholinergic SN56.B5.G4 cells with either sodium arsenite or DMA decreased cell viability in a concentration- and exposure-time dependent manner, and affected the activities of the cholinergic enzymes acetylcholinesterase and choline acetyltransferase. Both sodium arsenite and DMA exposure of SN56.B5.G4 cells resulted in enhanced level of APP, and sAPP in the membrane and cytosolic fractions, respectively. To reveal any effect of arsenic on APP processing, the amounts of APP cleavage products, sAPPβ, and β-amyloid (Aβ) peptides, released into the culture medium of primary neuronal cells derived from transgenic Tg2576 mice, were assessed by ELISA. Following exposure of neuronal cells by sodium arsenite for 12h, the membrane-bound APP level was enhanced, the amount of sAPPβ released into the culture medium was slightly higher, while the levels of Aβ peptides in the culture medium were considerably lower as compared to that assayed in the absence of any drug. The sodium arsenite-induced reduction of Aβ formation suggests an inhibition of the APP γ-cleavage step by arsenite. In contrast, DMA exposure of neuronal cells considerably increased formation of Aβ and sAPPβ, accompanied by enhanced membrane APP level. The DMA-induced changes in APP processing may be the result of the enhanced APP expression. Alternatively, increased Aβ production may also be due to stimulation of caspase activity by arsenic compounds, or failure in Aβ degradation. In summary, the present report clearly demonstrates that sodium arsenite and DMA affect processing of APP in vitro.

Reduced reactive oxygen species-generating capacity contributes to the enhanced cell growth of arsenic-transformed epithelial cells

Reactive oxygen species (ROS) have been implicated in the activation of protein kinases, DNA damage responses, and cell apoptosis. The details of how ROS regulate these intracellular biochemical and genetic processes remain to be fully understood. By establishing transformed bronchial epithelial cells through chronic low-dose arsenic treatment, we showed that the capacity of ROS generation induced by arsenic is substantially reduced in the transformed cells relative to the nontransformed cells. Such a reduction in ROS generation endows cells with premalignant features, including rapid growth, resistance to arsenic toxicity, and increased colony formation of the transformed cells. To validate these observations, the capability of ROS generation was restored in the transformed cells by treatment with inhibitors or siRNAs to silence the function of superoxide dismutase (SOD) or catalase and cell growth was determined following these treatments. Enhancement in ROS generation suppressed cell growth and colony formation of the transformed cells significantly. Despite the fact that the transformed cells showed a decreased expression of NF-kappaB signaling proteins IKKbeta and IKKgamma, the proteolytic processing p105 and p100 and NF-kappaB DNA binding activity were elevated in these cells. Increasing ROS generation by silencing SOD and catalase reduced the DNA binding activity of NF-kappaB in the transformed cells. Taken together, the transformed cells induced by arsenic exhibited a decrease in ROS generation, which is responsible for the enhanced cell growth and colony formation of the transformed cells, most likely through a sustained alternative activation of the NF-kappaB transcription factor.

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

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

Genomics and proteomics approaches to the study of cancer-stroma interactions

Background

The development and progression of cancer depend on its genetic characteristics as well as on the interactions with its microenvironment. Understanding these interactions may contribute to diagnostic and prognostic evaluations and to the development of new cancer therapies. Aiming to investigate potential mechanisms by which the tumor microenvironment might contribute to a cancer phenotype, we evaluated soluble paracrine factors produced by stromal and neoplastic cells which may influence proliferation and gene and protein expression.

Methods

The study was carried out on the epithelial cancer cell line (Hep-2) and fibroblasts isolated from a primary oral cancer. We combined a conditioned-medium technique with subtraction hybridization approach, quantitative PCR and proteomics, in order to evaluate gene and protein expression influenced by soluble paracrine factors produced by stromal and neoplastic cells.

Results

We observed that conditioned medium from fibroblast cultures (FCM) inhibited proliferation and induced apoptosis in Hep-2 cells. In neoplastic cells, 41 genes and 5 proteins exhibited changes in expression levels in response to FCM and, in fibroblasts, 17 genes and 2 proteins showed down-regulation in response to conditioned medium from Hep-2 cells (HCM). Nine genes were selected and the expression results of 6 down-regulated genes (ARID4A, CALR, GNB2L1, RNF10, SQSTM1, USP9X) were validated by real time PCR.

Conclusions

A significant and common denominator in the results was the potential induction of signaling changes associated with immune or inflammatory response in the absence of a specific protein.

Arsenite suppression of involucrin transcription through AP1 promoter sites in cultured human keratinocytes

While preserving keratinocyte proliferative ability, arsenite suppresses cellular differentiation markers by preventing utilization of AP1 transcriptional response elements. In present experiments, arsenite had a dramatic effect in electrophoretic mobility supershift analysis of proteins binding to an involucrin promoter AP1 response element. Without arsenite treatment, binding of JunB and Fra1 was readily detected in nuclear extracts from preconfluent cultures and was not detected a week after confluence, while c-Fos was detected only after confluence. By contrast, band shift of nuclear extracts from arsenite treated cultures showed only JunB and Fra1 binding in postconfluent as well as preconfluent cultures. Immunoblotting of cell extracts showed that arsenite treatment prevented the loss of Fra1 and the increase in c-Fos proteins that occurred after confluence in untreated cultures. Chromatin immunoprecipitation assays demonstrated substantial reduction of c-Fos and acetylated histone H3 at the proximal and distal AP1 response elements in the involucrin promoter and of coactivator p300 at the proximal element. Alteration of AP1 transcription factors was also examined in response to treatment with four metal containing compounds (chromate, vanadate, hemin, divalent cadmium) that also suppress involucrin transcription. These agents all influenced transcription at AP1 elements in a transcriptional reporter assay, but exhibited less effect than arsenite on binding activity assessed by mobility shift and chromatin immunoprecipitation and displayed variable effects on AP1 protein levels. These findings help trace a mechanism by which transcriptional effects of arsenite become manifest and help rationalize the unique action of arsenite, compared to the other agents, to preserve proliferative ability.