Retinoic acid activates p53 in human embryonal carcinoma through retinoid receptor-dependent stimulation of p53 transactivation function

Acne Transcriptomics: Fundamentals of Acne Pathogenesis and Isotretinoin Treatment

This review on acne transcriptomics allows for deeper insights into the pathogenesis of acne and isotretinoin's mode of action. Puberty-induced insulin-like growth factor 1 (IGF-1), insulin and androgen signaling activate the kinase AKT and mechanistic target of rapamycin complex 1 (mTORC1). A Western diet (hyperglycemic carbohydrates and milk/dairy products) also co-stimulates AKT/mTORC1 signaling. The AKT-mediated phosphorylation of nuclear FoxO1 and FoxO3 results in their extrusion into the cytoplasm, a critical switch which enhances the transactivation of lipogenic and proinflammatory transcription factors, including androgen receptor (AR), sterol regulatory element-binding transcription factor 1 (SREBF1), peroxisome proliferator-activated receptor γ (PPARγ) and signal transducer and activator of transcription 3 (STAT3), but reduces the FoxO1-dependent expression of GATA binding protein 6 (GATA6), the key transcription factor for infundibular keratinocyte homeostasis. The AKT-mediated phosphorylation of the p53-binding protein MDM2 promotes the degradation of p53. In contrast, isotretinoin enhances the expression of p53, FoxO1 and FoxO3 in the sebaceous glands of acne patients. The overexpression of these proapoptotic transcription factors explains isotretinoin's desirable sebum-suppressive effect via the induction of sebocyte apoptosis and the depletion of BLIMP1(+) sebocyte progenitor cells; it also explains its adverse effects, including teratogenicity (neural crest cell apoptosis), a reduced ovarian reserve (granulosa cell apoptosis), the risk of depression (the apoptosis of hypothalamic neurons), VLDL hyperlipidemia, intracranial hypertension and dry skin.

TRAF inhibition drives cancer cell apoptosis and improves retinoic acid sensitivity in multiple cancers models

TNF receptor-associated factors (TRAFs) are signaling adaptor proteins that play a crucial role in regulating cellular receptors' signaling transduction to downstream pathways and exert multifaceted roles in regulating signaling pathways, cell survival, and carcinogenesis. The 13-cis-retinoic acid (RA), an active metabolite of vitamin A, exhibits anti-cancer properties, but the development of retinoic acid resistance poses a challenge in clinical application. This study aimed to investigate the relationship between TRAFs and retinoic acid sensitivity in various cancers. Here, we revealed that TRAFs' expression varied significantly across The Cancer Genome Atlas (TCGA) cancer cohorts and human cancer cell lines. Additionally, inhibiting TRAF4, TRAF5, or TRAF6 improved retinoic acid sensitivity and reduced colony formation in ovarian cancer and melanoma cells. Mechanistically, knocking down TRAF4, TRAF5, or TRAF6 in retinoic acid-treated cancer cell lines increased the levels of procaspase 9 and induced cell apoptosis. Further in vivo studies using the SK-OV-3 and MeWo xenograft models confirmed the anti-tumor effects of TRAF knockdown combined with retinoic acid treatment. These findings support that combination therapy with retinoic acid and TRAF silencing may offer significant therapeutic advantages in treating melanoma and ovarian cancers.

A master of all trades - linking retinoids to different signalling pathways through the multi-purpose receptor STRA6

Retinoids are a group of vitamin A-related chemicals that are essential to chordate mammals. They regulate a number of basic processes, including embryogenesis and vision. From ingestion to metabolism and the subsequent cellular effects, retinoid levels are tightly regulated in the organism to prevent toxicity. One component of this network, the membrane receptor STRA6, has been shown to be essential in facilitating the cellular entry and exit of retinol. However, recent data suggests that STRA6 may not function merely as a retinoid transporter but also act as a complex signalling hub in its own right, being able to affect cell fate through the integration of retinoid signalling with other key pathways, such as those involving p53, JAK/STAT, Wnt/β catenin and calcium. This may open new therapeutic strategies in diseases like cancer, where these pathways are often compromised. Here, we look at the growing evidence regarding the novel roles of STRA6 beyond its well characterized classic functions.

Radiation effects on early phase of NT2/D1 neural differentiation in vitro

Purpose: Widespread medical use of radiation in diagnosis, imaging and treatment of different central nervous system malignancies lead to various consequences. Aim of this study was to further elucidate mechanism of cell response to radiation and possible consequence on neural differentiation.Materials and methods: NT2/D1 cells that resemble neural progenitors were used as a model system. Undifferentiated NT2/D1 cells and NT2/D1 cells in the early phase of neural differentiation were irradiated with low (0.2 Gy) and moderate (2 Gy) doses of γ radiation. The effect was analyzed on apoptosis, cell cycle, senescence, spheroid formation and the expression of genes and miRNAs involved in the regulation of pluripotency or neural differentiation.Results: Two grays of irradiation induced apoptosis, senescence and cell cycle arrest of NT2/D1 cells, accompanied with altered expression of several genes (SOX2, OCT4, SOX3, PAX6) and miRNAs (miR-219, miR-21, miR124-a). Presented results show that 2 Gy of radiation significantly affected early phase of neural differentiation in vitro.Conclusions: These results suggest that 2 Gy of radiation significantly affected early phase of neural differentiation and affect the population of neural progenitors. These findings might help in better understanding of side effects of radiotherapy in treatments of central nervous system malignancies.

Disulfiram Overcomes Cisplatin Resistance in Human Embryonal Carcinoma Cells

Cisplatin resistance in testicular germ cell tumors (TGCTs) is a clinical challenge. We investigated the underlying mechanisms associated with cancer stem cell (CSC) markers and modalities circumventing the chemoresistance. Chemoresistant models (designated as CisR) of human embryonal carcinoma cell lines NTERA-2 and NCCIT were derived and characterized using flow cytometry, gene expression, functional and protein arrays. Tumorigenicity was determined on immunodeficient mouse model. Disulfiram was used to examine chemosensitization of resistant cells. ALDH1A3 isoform expression was evaluated by immunohistochemistry in 216 patients' tissue samples. Chemoresistant cells were significantly more resistant to cisplatin, carboplatin and oxaliplatin compared to parental cells. NTERA-2 CisR cells exhibited altered morphology and increased tumorigenicity. High ALDH1A3 expression and increased ALDH activity were detected in both refractory cell lines. Disulfiram in combination with cisplatin showed synergy for NTERA-2 CisR and NCCIT CisR cells and inhibited growth of NTERA-2 CisR xenografts. Significantly higher ALDH1A3 expression was detected in TGCTs patients' tissue samples compared to normal testicular tissue. We characterized novel clinically relevant model of chemoresistant TGCTs, for the first time identified the ALDH1A3 as a therapeutic target in TGCTs and more importantly, showed that disulfiram represents a viable treatment option for refractory TGCTs.

Germline genome protection: implications for gamete quality and germ cell tumorigenesis

BACKGROUND

Germ cells have a unique and critical role as the conduit for hereditary information and therefore employ multiple strategies to protect genomic integrity and avoid mutations. Unlike somatic cells, which often respond to DNA damage by arresting the cell cycle and conducting DNA repair, germ cells as well as long-lived pluripotent stem cells typically avoid the use of error-prone repair mechanisms and favor apoptosis, reducing the risk of genetic alterations. Testicular germ cell tumors, the most common cancers of young men, arise from pre-natal germ cells.

OBJECTIVES

To summarize the current understanding of DNA damage response mechanisms in pre-meiotic germ cells and to discuss how they impact both the origins of testicular germ cell tumors and their remarkable responsiveness to genotoxic chemotherapy.

MATERIALS AND METHODS

We conducted a review of literature gathered from PubMed regarding the DNA damage response properties of testicular germ cell tumors and the germ cells from which they arise, as well as the influence of these mechanisms on therapeutic responses by testicular germ cell tumors.

RESULTS AND DISCUSSION

This review provides a comprehensive evaluation of how the developmental origins of male germ cells and their inherent germ cell-like DNA damage response directly impact the development and therapeutic sensitivity of testicular germ cell tumors.

CONCLUSIONS

The DNA damage response of germ cells directly impacts the development and therapeutic sensitivity of testicular germ cell tumors. Recent advances in the study of primordial germ cells, post-natal mitotically dividing germ cells, and pluripotent stem cells will allow for new investigations into the initiation, progression, and treatment of testicular germ cell tumors.

Molecular Mechanisms of Cisplatin Chemoresistance and Its Circumventing in Testicular Germ Cell Tumors

PURPOSE OF REVIEW

Testicular germ cell tumors (TGCTs) represent the most common solid tumors affecting young men. Majority of TGCTs respond well to cisplatin-based chemotherapy. However, patients with refractory disease have limited treatment modalities associated with poor prognosis. Here, we discuss the main molecular mechanisms associated with acquired cisplatin resistance in TGCTs and how their understanding might help in the development of new approaches to tackle this clinically relevant problem. We also discuss recent data on the strategies of circumventing the cisplatin resistance from different tumor types potentially efficient also in TGCTs.

RECENT FINDINGS

Recent data regarding deregulation of various signaling pathways as well as genetic and epigenetic mechanisms in cisplatin-resistant TGCTs have contributed to understanding of the mechanisms related to the resistance to cisplatin-based chemotherapy in these tumors. Understanding of these mechanisms enabled explaining why majority but not all TGCTs patients are curable with cisplatin-based chemotherapy. Moreover, it could lead to the development of more effective treatment of refractory TGCTs and potentially other solid tumors resistant to platinum-based chemotherapy. This review provides additional insights into mechanisms associated with cisplatin resistance in TGCTs, which is a complex phenomenon, and there is a need for novel modalities to overcome it.

p53: key conductor of all anti-acne therapies

This review based on translational research predicts that the transcription factor p53 is the key effector of all anti-acne therapies. All-trans retinoic acid (ATRA) and isotretinoin (13-cis retinoic acid) enhance p53 expression. Tetracyclines and macrolides via inhibiting p450 enzymes attenuate ATRA degradation, thereby increase p53. Benzoyl peroxide and hydrogen peroxide elicit oxidative stress, which upregulates p53. Azelaic acid leads to mitochondrial damage associated with increased release of reactive oxygen species inducing p53. p53 inhibits the expression of androgen receptor and IGF-1 receptor, and induces the expression of IGF binding protein 3. p53 induces FoxO1, FoxO3, p21 and sestrin 1, sestrin 2, and tumour necrosis factor-related apoptosis-inducing ligand (TRAIL), the key inducer of isotretinoin-mediated sebocyte apoptosis explaining isotretinoin's sebum-suppressive effect. Anti-androgens attenuate the expression of miRNA-125b, a key negative regulator of p53. It can thus be concluded that all anti-acne therapies have a common mode of action, i.e., upregulation of the guardian of the genome p53. Immortalized p53-inactivated sebocyte cultures are unfortunate models for studying acne pathogenesis and treatment.

Refractory testicular germ cell tumors are highly sensitive to the second generation DNA methylation inhibitor guadecitabine

Testicular germ cell tumors (TGCTs) are the most common cancers of young males. A substantial portion of TGCT patients are refractory to cisplatin. There are no effective therapies for these patients, many of whom die from progressive disease. Embryonal carcinoma (EC) are the stem cells of TGCTs. In prior in vitro studies we found that EC cells were highly sensitive to the DNA methyltransferase inhibitor, 5-aza deoxycytidine (5-aza). Here, as an initial step in bringing demethylation therapy to the clinic for TGCT patients, we evaluated the effects of the clinically optimized, second generation demethylating agent guadecitabine (SGI-110) on EC cells in an animal model of cisplatin refractory testicular cancer. EC cells were exquisitely sensitive to guadecitabine and the hypersensitivity was dependent on high levels of DNA methyltransferase 3B. Guadecitabine mediated transcriptional reprogramming of EC cells included induction of p53 targets and repression of pluripotency genes. As a single agent, guadecitabine completely abolished progression and induced complete regression of cisplatin resistant EC xenografts even at doses well below those required to impact somatic solid tumors. Low dose guadecitabine also sensitized refractory EC cells to cisplatin in vivo. Genome-wide analysis indicated that in vivo antitumor activity was associated with activation of p53 and immune-related pathways and the antitumor effects of guadecitabine were dependent on p53, a gene rarely mutated in TGCTs. These preclinical findings suggest that guadecitabine alone or in combination with cisplatin is a promising strategy to treat refractory TGCT patients.

Lysine methylation represses p53 activity in teratocarcinoma cancer cells

TP53 (which encodes the p53 protein) is the most frequently mutated gene among all human cancers, whereas tumors that retain the wild-type TP53 gene often use alternative mechanisms to repress the p53 tumor-suppressive function. Testicular teratocarcinoma cells rarely contain mutations in TP53, yet the transcriptional activity of wild-type p53 is compromised, despite its high expression level. Here we report that in the teratocarcinoma cell line NTera2, p53 is subject to lysine methylation at its carboxyl terminus, which has been shown to repress p53's transcriptional activity. We show that reduction of the cognate methyltransferases reactivates p53 and promotes differentiation of the NTera2 cells. Furthermore, reconstitution of methylation-deficient p53 mutants into p53-depleted NTera2 cells results in elevated expression of p53 downstream targets and precocious loss of pluripotent gene expression compared with re-expression of wild-type p53. Our results provide evidence that lysine methylation of endogenous wild-type p53 represses its activity in cancer cells and suggest new therapeutic possibilities of targeting testicular teratocarcinoma.

Cisplatin induces resistance by triggering differentiation of testicular embryonal carcinoma cells

Although testicular germ cell tumors are generally quite responsive to treatment with cisplatin, a small fraction of them acquire resistance during therapy. Even when cisplatin treatment is successful the patient is often left with a residual teratoma at the site of the primary tumor suggesting that cisplatin may trigger differentiation in some tumors. Using the human embryonal carcinoma cell line NTera2/D1, we confirmed that exposure to the differentiating agent retinoic acid produced a reduction in pluripotency markers NANOG and POU5F1 (Oct3/4) and an acute concentration-dependent increase in resistance to both cisplatin and paclitaxel that reached as high as 18-fold for cisplatin and 61-fold for paclitaxel within four days. A two day exposure to cisplatin also produced a concentration-dependent decrease in the expression of the NANOG and POU5F1 and increased expression of three markers whose levels increase with differentiation including Nestin, SCG10 and Fibronectin. In parallel, exposure to cisplatin induced up to 6.2-fold resistance to itself and 104-fold resistance to paclitaxel. Paclitaxel did not induce differentiation or resistance to either itself or cisplatin. Neither retinoic acid nor cisplatin induced resistance in cervical or prostate cancer cell lines or other germ cell tumor lines in which they failed to alter the expression of NANOG and POU5F1. Forced expression of NANOG prevented the induction of resistance to cisplatin by retinoic acid. We conclude that cisplatin can acutely induce resistance to itself and paclitaxel by triggering a differentiation response in pluripotent germ cell tumor cells.

Headway and hurdles in the clinical development of dietary phytochemicals for cancer therapy and prevention: lessons learned from vitamin A derivatives

Accumulating epidemiologic and preclinical evidence support the pharmacologic use of a variety of dietary chemicals for the prevention and treatment of cancer. However, it will be challenging to translate these findings into routine clinical practice since phytochemicals have pleiotropic biological activities that have to be balanced for optimal efficacy without unacceptable and potentially unanticipated toxicities. Correctly matching patient populations and settings with optimal, natural product-based phytochemical therapies will require a greater understanding of the specific mechanisms underlying the efficacy, toxicity, and resistance of each agent in a variety of normal, premalignant, and malignant settings. This, in turn, necessitates continued commitment from the basic research community to guide carefully designed and informed clinical trials. The most developed class of anticancer phytochemicals consists of the derivatives of vitamin A called retinoids. Unlike other natural product chemicals currently under study, the retinoids have been extensively tested in humans. Over 30 years of clinical investigation has resulted in several disappointments, but there were some spectacular successes where certain retinoid-based protocols are now FDA-approved standard of care therapies to treat specific malignancies. Furthermore, retinoids are one of the most evaluated pharmacologic agents in the ultra-challenging setting of interventional cancer prevention. This review will summarize the development of retinoids in cancer therapy and prevention with an emphasis on currently proposed mechanisms mediating their efficacy, toxicity, and resistance.

Unravelling mechanisms of cisplatin sensitivity and resistance in testicular cancer

Testicular cancer is the most frequent solid malignant tumour type in men 20-40 years of age. At the time of diagnosis up to 50% of the patients suffer from metastatic disease. In contrast to most other metastatic solid tumours, the majority of metastatic testicular cancer patients can be cured with highly effective cisplatin-based chemotherapy. This review aims to summarise the current knowledge on response to chemotherapy and the biological basis of cisplatin-induced apoptosis in testicular cancer. The frequent presence of wild-type TP53 and the low levels of p53 in complex with the p53 negative feed-back regulator MDM2 contribute to cisplatin sensitivity. Moreover, the high levels of the pluripotency regulator Oct4 and as a consequence of Oct4 expression high levels of miR-17/106b seed family and pro-apoptotic Noxa and the low levels of cytoplasmic p21 (WAF1/Cip1) appear to be causative for the exquisite sensitivity to cisplatin-based therapy of testicular cancer. However, resistance of testicular cancer to cisplatin-based therapy does occur and can be mediated through aberrant levels of the above mentioned key players. Drugs targeting these key players showed, at least pre-clinically, a sensitising effect to cisplatin treatment. Further clinical development of such treatment strategies will lead to new treatment options for platinum-resistant testicular cancers.

All-trans retinoic acid upregulates the expression of p53 via Axin and inhibits the proliferation of glioma cells

All-trans retinoic acid (ATRA) is a potent chemopreventive and therapeutic agent and exerts its effects by inducing growth arrest. In the present study, we demonstrated that ATRA activated the expression of p53 via Axin and induced cell cycle arrest at the G1/S phase and apoptosis of glioma cells. Briefly, C6 cells were treated with ATRA, and the levels of p53 mRNA and protein were determined by RT-PCR, western blotting and immunohistochemistry. The results showed that ATRA activated the expression of p53. In addition, ectopic expression of Axin by transient transfection of C6 cells with rAxin revealed that overexpression of Axin induced cell cycle arrest and apoptosis with an upregulation of p53. Furthermore, loss-of-function of Axin in glioma cells by RNAi blocked ATRA-induced cell cycle phase arrest and apoptosis via downregulation of p53. The present study revealed a novel function of Axin and identified it as an important regulator of ATRA-activated p53 expression.

Acute hypersensitivity of pluripotent testicular cancer-derived embryonal carcinoma to low-dose 5-aza deoxycytidine is associated with global DNA Damage-associated p53 activation, anti-pluripotency and DNA demethylation

Human embryonal carcinoma (EC) cells are the stem cells of nonseminoma testicular germ cells tumors (TGCTs) and share remarkable similarities to human embryonic stem (ES) cells. In prior work we found that EC cells are hypersensitive to low nanomolar doses of 5-aza deoxycytidine (5-aza) and that this hypersensitivity partially depended on unusually high levels of the DNA methyltransferase, DNMT3B. We show here that low-dose 5-aza treatment results in DNA damage and induction of p53 in NT2/D1 cells. In addition, low-dose 5-aza results in global and gene specific promoter DNA hypomethylation. Low-dose 5-aza induces a p53 transcriptional signature distinct from that induced with cisplatin in NT2/D1 cells and also uniquely downregulates genes associated with pluripotency including NANOG, SOX2, GDF3 and Myc target genes. Changes in the p53 and pluripotency signatures with 5-aza were to a large extent dependent on high levels of DNMT3B. In contrast to the majority of p53 target genes upregulated by 5-aza that did not show DNA hypomethylation, several other genes induced with 5-aza had corresponding decreases in promoter methylation. These genes include RIN1, SOX15, GPER, and TLR4 and are novel candidate tumors suppressors in TGCTs. Our studies suggest that the hypersensitivity of NT2/D1 cells to low-dose 5-aza is multifactorial and involves the combined activation of p53 targets, repression of pluripotency genes, and activation of genes repressed by DNA methylation. Low-dose 5-aza therapy may be a general strategy to treat those tumors that are sustained by cells with embryonic stem-like properties.GEO NUMBER FOR THE MICROARRAY DATA: GSE42647.

Potential for Cell-Transplant Therapy with Human Neuronal Precursors to Treat Neuropathic Pain in Models of PNS and CNS Injury: Comparison of hNT2.17 and hNT2.19 Cell Lines

Effective treatment of sensory neuropathies in peripheral neuropathies and spinal cord injury (SCI) is one of the most difficult problems in modern clinical practice. Cell therapy to release antinociceptive agents near the injured spinal cord is a logical next step in the development of treatment modalities. But few clinical trials, especially for chronic pain, have tested the potential of transplant of cells to treat chronic pain. Cell lines derived from the human neuronal NT2 cell line parentage, the hNT2.17 and hNT2.19 lines, which synthesize and release the neurotransmitters gamma-aminobutyric acid (GABA) and serotonin (5HT), respectively, have been used to evaluate the potential of cell-based release of antinociceptive agents near the lumbar dorsal (horn) spinal sensory cell centers to relieve neuropathic pain after PNS (partial nerve and diabetes-related injury) and CNS (spinal cord injury) damage in rat models. Both cell lines transplants potently and permanently reverse behavioral hypersensitivity without inducing tumors or other complications after grafting. Functioning as cellular minipumps for antinociception, human neuronal precursors, like these NT2-derived cell lines, would likely provide a useful adjuvant or replacement for current pharmacological treatments for neuropathic pain.

Serine/threonine kinase 17A is a novel p53 target gene and modulator of cisplatin toxicity and reactive oxygen species in testicular cancer cells

Testicular cancer is highly curable with cisplatin-based therapy, and testicular cancer-derived human embryonal carcinoma (EC) cells undergo a p53-dominant transcriptional response to cisplatin. In this study, we have discovered that a poorly characterized member of the death-associated protein family of serine/threonine kinases, STK17A (also called DRAK1), is a novel p53 target gene. Cisplatin-mediated induction of STK17A in the EC cell line NT2/D1 was prevented with p53 siRNA. Furthermore, STK17A was induced with cisplatin in HCT116 and MCF10A cells but to a much lesser extent in isogenic p53-suppressed cells. A functional p53 response element that binds endogenous p53 in a cisplatin-dependent manner was identified 5 kb upstream of the first coding exon of STK17A. STK17A is not present in the mouse genome, but the closely related gene STK17B is induced with cisplatin in mouse NIH3T3 cells, although this induction is p53-independent. Interestingly, in human cells containing both STK17A and STK17B, only STK17A is induced with cisplatin. Knockdown of STK17A conferred resistance to cisplatin-induced growth suppression and apoptotic cell death in EC cells. This was associated with the up-regulation of detoxifying and antioxidant genes, including metallothioneins MT1H, MT1M, and MT1X that have previously been implicated in cisplatin resistance. In addition, knockdown of STK17A resulted in decreased cellular reactive oxygen species, whereas STK17A overexpression increased reactive oxygen species. In summary, we have identified STK17A as a novel direct target of p53 and a modulator of cisplatin toxicity and reactive oxygen species in testicular cancer cells.

Hsa-miR-34b is a plasma-stable microRNA that is elevated in pre-manifest Huntington's disease

Huntington's disease (HD) is a devastating, neurodegenerative condition, which lacks effective treatment. Normal Huntingtin (HTT) and mutant Huntingtin (mHTT) are expressed in multiple tissues and can alter transcription of microRNAs (miRs). Importantly, miRs are present in a bio-stable form in human peripheral blood plasma and have recently been shown to be useful biomarkers in other diseases. We therefore sought to identify potential miR biomarkers of HD that are present in, and have functional consequences for, neuronal and non-neuronal tissues. In a cell line over-expressing mHTT-Exon-1, miR microarray analysis was used to identify candidate miRs. We then examined their presence and bio-stability in control and HD plasma. We found that miR-34b is significantly elevated in response to mHTT-Exon-1, and its blockade alters the toxicity of mHTT-Exon-1 in vitro. We also show that miR-34b is detectable in plasma from small input volumes and is insensitive to freeze-thaw-induced RNA degradation. Interestingly, miR-34b is significantly elevated in plasma from HD gene carriers prior to symptom onset. This is the first study suggesting that plasma miRs might be used as biomarkers for HD.

Effects of ginsenoside Rg2 on the ultraviolet B-induced DNA damage responses in HaCaT cells

Our previous study demonstrated the increase in the repair of UVB damage by mRg2, a mixture of ginsenosides containing 60% Rg2 in NIH3T3 cells. In the present study, the effects of purified Rg2 on the repair and apoptosis in ultraviolet B (UVB)-exposed HaCaT cells were investigated on gene expression levels. When cells were exposed to UVB and post-incubated in normal medium for 24 h, the cell viability decreased to about 50% of that in nontreated control. When Rg2 was post-incubated, however, the UVB-induced cytotoxicity was significantly prevented in an Rg2 concentration- and time-dependent manner. The apoptotic nuclear fragmentation resulting from UVB exposure was also significantly protected by the Rg2 post-incubation. Microarray analysis showed that the genes stimulated by the Rg2-alone treatment include those involved in p53 signaling pathway such as GADD45alpha, GADD45beta, and cell communication genes. RT-PCR analysis showed that the Rg2-alone treatment slightly upregulated the p53 and GADD45 transcript and protein levels by about 1.5-fold as compared with the nontreated control. The mRNA levels of p53 and GADD45 in cells exposed to UVB and post-incubated with Rg2 for 24 h decreased in an Rg2 concentration-dependent manner as compared with that post-incubated in normal medium. However, the mRNA level of the UVB-exposed cells post-incubated with 5 microM retinol was essentially the same as that post-incubated in normal medium. Time course experiment showed that the mRNA levels of p53 and GADD45 in UVB-exposed cells were upregulated by post-incubation with 50 microM Rg2 until 6 and 9 h, respectively, and then gradually decreased until 24 h. By Western blot analysis, it was also revealed that the Rg2 post-incubation decreases the expression of p53, phospho-p53, GADD45, and ATM in UVB-exposed cells. Time course analysis also indicated that these decreased expressions were due to the earlier upregulation of p53 and GADD45 proteins. When UVB-exposed cells were post-incubated with Rg2 for 24 h after UVB exposure, the level of remaining cyclobutane pyrimidine dimers decreased in both Rg2 concentration- and time-dependent manner. All these results suggest that Rg2 protects cells against UVB-induced genotoxicity by increasing DNA repair, in possible association with modulation of protein levels involved in p53 signaling pathway.

Nucleoside drugs induce cellular differentiation by caspase-dependent degradation of stem cell factors

BACKGROUND

Stem cell characteristics are an important feature of human cancer cells and play a major role in the therapy resistance of tumours. Strategies to target cancer stem cells are thus of major importance for cancer therapy. Differentiation therapy by nucleoside drugs represents an attractive approach for the elimination of cancer stem cells. However, even if it is generally assumed that the activity of these drugs is mediated by their ability to modulate epigenetic pathways, their precise mode of action remains to be established. We therefore analysed the potential of three nucleoside analogues to induce differentiation of the embryonic cancer stem cell line NTERA 2 D1 and compared their effect to the natural ligand retinoic acid.

METHODOLOGY/PRINCIPAL FINDINGS

All nucleoside analogues analyzed, but not retinoic acid, triggered proteolytic degradation of the Polycomb group protein EZH2. Two of them, 3-Deazaneplanocin A (DZNep) and 2'-deoxy-5-azacytidine (decitabine), also induced a decrease in global DNA methylation. Nevertheless, only decitabine and 1beta-arabinofuranosylcytosine (cytarabine) effectively triggered neuronal differentiation of NT2 cells. We show that drug-induced differentiation, in contrast to retinoic acid induction, is caused by caspase activation, which mediates depletion of the stem cell factors NANOG and OCT4. Consistent with this observation, protein degradation and differentiation could be counteracted by co-treatment with caspase inhibitors or by depletion of CASPASE-3 and CASPASE-7 through dsRNA interference. In agreement with this, OCT4 was found to be a direct in-vitro-target of CASPASE-7.

CONCLUSIONS/SIGNIFICANCE

We show that drug-induced differentiation is not a consequence of pharmacologic epigenetic modulation, but is induced by the degradation of stem-cell-specific proteins by caspases. Our results thus uncover a novel pathway that induces differentiation of embryonic cancer stem cells and is triggered by the established anticancer drugs cytarabine and decitabine. These findings suggest new approaches for directly targeting the stem cell fraction of human tumours.

p53 inactivation by MDM2 and MDMX negative feedback loops in testicular germ cell tumors

Testicular germ cell tumors (TGCT) are unique in their excellent response to DNA-damaging chemotherapy. Mutation of p53 is rare in both untreated and relapsed TGCTs, suggesting that p53 fails to respond effectively against malignant transformation in germ cells. Previous studies implicated the presence of a poorly defined TGCT-specific mechanism of p53 inactivation. Here we show that disruption of p53-MDM2 binding using the MDM2-specific inhibitor Nutlin activates p53 in TGCT cells and is sufficient to induce strong apoptosis. Knockdown of MDMX cooperates with Nutlin to activate p53. Surprisingly, we found that p53 activation induced a two-fold increase in MDMX mRNA and protein expression in TGCT cells. A p53-responsive promoter is identified in MDMX intron 1 that contains a functional p53-binding site, suggesting that MDMX also functions as a negative feedback regulator of p53 in a cell line-dependent fashion. These findings suggest that MDM2 and MDMX are responsible for the functional inactivation of p53 in TGCT. Furthermore, TGCT cells are unique in having a strong apoptosis response to p53. Direct activation of p53 by targeting MDM2 and MDMX may provide a backup approach for the treatment of TGCTs resistant to DNA-damaging drugs.

High DNA methyltransferase 3B expression mediates 5-aza-deoxycytidine hypersensitivity in testicular germ cell tumors

Testicular germ cell tumors (TGCT) are the most common solid tumors of 15- to 35-year-old men. TGCT patients are frequently cured with cytotoxic cisplatin-based therapy. However, TGCT patients refractory to cisplatin-based chemotherapy have a poor prognosis, as do those having a late relapse. Pluripotent embryonal carcinomas (EC) are the malignant counterparts to embryonic stem cells and are considered the stem cells of TGCTs. Here, we show that human EC cells are highly sensitive to 5-aza-deoxycytidine (5-aza-CdR) compared with somatic solid tumor cells. Decreased proliferation and survival with low nanomolar concentrations of 5-aza-CdR is associated with ATM activation, H2AX phosphorylation, increased expression of p21, and the induction of genes known to be methylated in TGCTs (MGMT, RASSF1A, and HOXA9). Notably, 5-aza-CdR hypersensitivity is associated with markedly abundant expression of the pluripotency-associated DNA methyltransferase 3B (DNMT3B) compared with somatic tumor cells. Knockdown of DNMT3B in EC cells results in substantial resistance to 5-aza-CdR, strongly indicating that 5-aza-CdR sensitivity is mechanistically linked to high levels of DNMT3B. Intriguingly, cisplatin-resistant EC cells retain an exquisite sensitivity to low-dose 5-aza-CdR treatment, and pretreatment of 5-aza-CdR resensitizes these cells to cisplatin-mediated toxicity. This resensitization is also partially dependent on high DNMT3B levels. These novel findings indicate that high expression of DNMT3B, a likely byproduct of their pluripotency and germ cell origin, sensitizes TGCT-derived EC cells to low-dose 5-aza-CdR treatment.

Molecular portrait of cisplatin induced response in human testis cancer cell lines based on gene expression profiles

BACKGROUND

Testicular germ cell tumors (TGCTs) respond well to cisplatin-based chemotherapy and show a low incidence of acquired resistance compared to most somatic tumors. The reasons for these specific characteristics are not known in detail but seem to be multifactorial. We have studied gene expression profiles of testicular and colon cancer derived cell lines treated with cisplatin. The main goal of this study was to identify novel gene expression profiles with their functional categories and the biochemical pathways that are associated with TGCT cells' response to cisplatin.

RESULTS

Genes that were differentially expressed between the TGCT cell lines vs the (somatic) HCT116 cell line, after cisplatin treatment, were identified using the significance analysis of microarrays (SAM) method. The response of TGCT cells was strikingly different from that of HCT116, and we identified 1794 genes that were differentially expressed. Functional classification of these genes showed that they participate in a variety of different and widely distributed functional categories and biochemical pathways. Database mining showed significant association of genes (n = 41) induced by cisplatin in our study, and genes previously reported to by expressed in differentiated TGCT cells. We identified 37 p53-responsive genes that were altered after cisplatin exposure. We also identified 40 target genes for two microRNAs, hsa-mir-372 and 373 that may interfere with p53 signaling in TGCTs. The tumor suppressor genes NEO1 and LATS2, and the estrogen receptor gene ESR1, all have binding sites for p53 and hsa-mir-372/373. NEO1 and LATS2 were down-regulated in TGCT cells following cisplatin exposure, while ESR1 was up-regulated in TGCT cells. Cisplatin-induced genes associated with terminal growth arrest through senescence were identified, indicating associations which were not previously described for TGCT cells.

CONCLUSION

By linking our gene expression data to publicly available databases and literature, we provide a global pattern of cisplatin induced cellular response that is specific for testicular cancer cell lines. We have identified cisplatin-responsive functional classes and pathways, such as the angiogenesis, Wnt, integrin, and cadherin signaling pathways. The identification of differentially expressed genes in this study may contribute to a better understanding of the unusual sensitivity of TGCT to some DNA-damaging agents.

The direct p53 target gene, FLJ11259/DRAM, is a member of a novel family of transmembrane proteins

The tumor suppressor p53 regulates diverse biological processes primarily via activation of downstream target genes. Even though many p53 target genes have been described, the precise mechanisms of p53 biological actions are uncertain. In previous work we identified by microarray analysis a candidate p53 target gene, FLJ11259/DRAM. In this report we have identified three uncharacterized human proteins with sequence homology to FLJ11259, suggesting that FLJ11259 is a member of a novel family of proteins with six transmembrane domains. Several lines of investigation confirm FLJ11259 is a direct p53 target gene. p53 siRNA prevented cisplatin-mediated up-regulation of FLJ11259 in NT2/D1 cells. Likewise in HCT116 p53+/+ cells and MCF10A cells, FLJ11259 is induced by cisplatin treatment but to a much lesser extent in isogenic p53-suppressed cells. A functional p53 response element was identified 22.3 kb upstream of the first coding exon of FLJ11259 and is shown to be active in reporter assays. In addition, chromatin immunoprecipitation assays indicate that p53 binds directly to this element in vivo and that binding is enhanced following cisplatin treatment. Confocal microscopy showed that an FLJ-GFP fusion protein localizes mainly in a punctate pattern in the cytoplasm. Overexpression studies in Cos-7, Saos2, and NT2/D1 cells suggest that FLJ11259 is associated with increased clonal survival. In summary, we have identified FLJ11259/DRAM as a p53-inducible member of a novel family of transmembrane proteins. FLJ11259/DRAM may be an important modulator of p53 responses in diverse tumor types.

The tumor suppressor protein p53 is required for neurite outgrowth and axon regeneration

Axon regeneration is substantially regulated by gene expression and cytoskeleton remodeling. Here we show that the tumor suppressor protein p53 is required for neurite outgrowth in cultured cells including primary neurons as well as for axonal regeneration in mice. These effects are mediated by two newly identified p53 transcriptional targets, the actin-binding protein Coronin 1b and the GTPase Rab13, both of which associate with the cytoskeleton and regulate neurite outgrowth. We also demonstrate that acetylation of lysine 320 (K320) of p53 is specifically involved in the promotion of neurite outgrowth and in the regulation of the expression of Coronin 1b and Rab13. Thus, in addition to its recognized role in neuronal apoptosis, surprisingly, p53 is required for neurite outgrowth and axonal regeneration, likely through a different post-translational pathway. These observations may suggest a novel therapeutic target for promoting regenerative responses following peripheral or central nervous system injuries.

Testicular Germ Cell Tumors: A Paradigm for the Successful Treatment of Solid Tumor Stem Cells

Treatment of testicular germ cell tumors (TGCTs) has been a success primarily due to the exquisite responsiveness of this solid tumor to cisplatin-based therapy. Despite the promise of cure for the majority of TGCT patients, the effectiveness of therapy for some patients is limited by toxicity and the problem of resistance. There is compelling rationale to further understand the biology of TGCTs in order to better treat other solid tumors and to address the shortcomings of present TGCT therapies. TGCTs contain undifferentiated pluripotent stem cells, known as embryonal carcinoma, that share many properties with human embryonic stem cells. The importance of cancer stem cells in the initiation, progression and treatment of solid tumors is beginning to emerge. We discuss TGCTs in the context of solid tumor curability and targeted cancer stem cell therapy.

Expression profiling of p53-target genes in copper-mediated neuronal apoptosis

Copper toxicity associated with Wilson's disease is known to cause neuronal damage and death in the basal ganglia and frontal cortex leading to Parkinson-like symptoms and cognitive deficits. Our previous work in cultured human NTERA-2-N neurons showed that copper-induced neuronal apoptosis is dependent on the induction and nuclear translocation of the tumor suppressor protein, p53. Because p53 acts as a DNA-binding transcription factor, this work used an oligonucleotide array to identify p53 target genes that are differentially regulated in copper-loaded neurons. Arrays representing 145 human genes expressed downstream of p53 were hybridized with labeled mRNA from control and copper-treated neurons. Differentially regulated mRNAs included those involved in the regulation of the cell cycle, cytoprotective mechanisms, and apoptotic mechanisms. Transfection of cells with a dominant-negative p53 construct enabled us to determine which molecular events were dependent on p53 expression. Copper treatment resulted in the upregulation of p21, reprimo, stathmin, and Tp53INP1, all known to participate in cell cycle arrest. Protective mechanisms included the upregulation of stat-3, and the heat-shock proteins, heat-shock protein (Hsp) 70 and Hsp 27. Both p53-dependent and -independent mechanisms leading to apoptosis were identified including insulin-like growth factor binding protein-6, glutathione peroxidase, bcl-2, RB-1, PUMA, and several members of the redox active PIG family of proteins. Thus it appears that following copper-mediated neuronal DNA damage, the regulation of a variety of pro- and antiapoptotic genes are responsible for determining neuronal fate.

Sensitivity to DNA damage is a common component of hormone-based strategies for protection of the mammary gland

An early full-term pregnancy significantly reduces the risk of getting breast cancer in women. In animals, this protection can be mimicked by a short-term exposure to physiologic doses of estrogen plus progesterone. Sensitization of p53 and up-regulation of transforming growth factor beta are believed to be important aspects of the mechanism by which protection is imparted. Little is known, however, about the use of this pathway in response to other chemopreventive agents. In this article, we investigated the ability of retinoids, such as 9-cis retinoic acid, all-trans retinoic acid, and N-4-hydroxyphenylretinamide (4-HPR), to sensitize the ductal epithelial cells of virgin mammary glands to DNA damage responses. Using a whole-organ culture system, we observed enhanced cell death in response to gamma-irradiation in the virgin tissues treated with retinoids for 72 hours. These retinoids were partially dependent on p53 and transforming growth factor beta to exert their radiosensitizing effects. However, 4-HPR seemed to sensitize other cells or activate these pathways in a different manner as costimulation with ovarian hormones and 4-HPR was additive, whereas coculture of ovarian hormones and the natural retinoids did not increase amount of death. Taken together, these data suggest that sensitization of the mammary epithelium to p53-dependent apoptosis is a common pathway, which is engaged by retinoids as well as ovarian hormones.

A p53-dominant transcriptional response to cisplatin in testicular germ cell tumor-derived human embryonal carcinoma

Testicular germ cell cancers remain one of the few solid tumors routinely cured in advanced stages with conventional cisplatin-based chemotherapy. The mechanisms remain largely unknown. Through use of gene-expression array profiling we define immediate transcriptional targets in response to cisplatin in testicular germ cell-derived human embryonal carcinoma cells. We report 46 genes upregulated and five genes repressed by cisplatin. Several of these gene products, including FAS, TRAILR3, PHLDA3, LRDD, and IER3 are previously implicated in the apoptotic death receptor pathway, while others including SESN1, FDXR, PLK3, and DDIT4 are known mediators of reactive oxygen species generation. Approximately 54% of the upregulated genes are established or suspected downstream targets of p53. Specific siRNA to p53 prevents cisplatin-mediated activation of p53 and p53 pathway genes and renders embryonal carcinoma cells relatively resistant to cisplatin cytotoxicity. Interestingly, in p53 knockdown cells nearly the entire set of identified cisplatin targets fail to respond or have a diminished response to cisplatin, suggesting that many are new direct or indirect targets of p53 including GPR87, STK17A, INPP5D, FLJ11259, and EPS8L2. The data indicate that robust transcriptional activation of p53 is linked to the known hypersensitivity of testicular germ cell tumors to chemotherapy. Many of the gene products may participate in the unique curability of this disease.

Clinical significance of altered expression of retinoid receptors in oral precancerous and cancerous lesions: Relationship with cell cycle regulators

Alterations in expression of retinoid receptors are implicated in human cancers. We hypothesized that altered expression of retinoic acid receptors (RARalpha,beta,gamma) and retinoid X receptor RXRalpha and their relationship with cell cycle regulators (p53, p16, p21) is associated with development, progression and prognosis of oral cancer. Immunohistochemical analysis of RAR alpha, beta, gamma and RXRalpha proteins was carried out on serial sections from 244 oral squamous cell carcinomas (OSCCs), 102 potentially malignant lesions (65 hyperplasias, 37 dysplasias), 83 matched histologically normal oral tissues and 29 normal mucosa from non-exposed individuals without oral lesions and correlated with expression of cell cycle regulators p53, p16 and p21 as well as with clinicopathological parameters. Expression of retinoid receptors RARbeta, RARgamma, RXRalpha and cell cycle regulators p16 and p21 was decreased in majority of oral SCCs as well as in potentially malignant lesions. Multivariate stepwise logistic regression analysis carried out for comparison of non-exposed normal oral mucosa with histologically normal oral tissues from patients with oral lesions showed significant loss of RARbeta or p53 accumulation (RARbeta(-)/p53(+) Odd's ratio, OR = 266.6, p = 0.000); non-exposed normal mucosa from individuals without oral lesions with potentially malignant lesion was RARbeta(-)/p21(-)/p53(+) (OR = 215.7, p = 0.000); matched normal to potentially malignant stage was RARalpha(+)/p21(-) (OR = 4.414, p = 0.005); hyperplasia to dysplasia was RARalpha(+)/p53(+) (OR = 4.72, p = 0.005) and potentially malignant to malignant phenotype was RARalpha(+) (OR = 2.061, p = 0.004). The prognostic relevance of these factors was assessed in 115 of these SCC patients who were followed-up for a maximum period of 94 months (median 21 months). Multivariate analysis using Cox's proportional Hazard's model showed that RARalpha(+)/p21(-) phenotype was associated with shorter disease-free survival (Hazard's ratio, HR = 1.863, p = 0.0471). To our knowledge, this is the first large study showing alterations in expression of retinoid receptors at the protein level at different stages in development and progression of oral SCC. It also underscored the prognostic significance of retinoid receptors and their interactions with cell cycle regulators in multistep oral tumorigenesis.

Distinct roles for p53 transactivation and repression in preventing UCN-01-mediated abrogation of DNA damage-induced arrest at S and G2 cell cycle checkpoints

The topoisomerase I inhibitor SN38 arrests cell cycle progression primarily in S or G(2) phases of the cell cycle in a p53-independent manner. The Chk1 inhibitor, 7-hydroxystaurosporine (UCN-01), overcomes both S and G(2) arrest preferentially in cells mutated for p53, driving cells through a lethal mitosis and thereby enhancing cytotoxicity. The mechanism by which p53 maintains S and G(2) arrest was investigated here. The p53 wild-type MCF10A cells were arrested in S phase by incubation with SN38 for 24 h. Subsequent incubation with UCN-01 failed to abrogate arrest. To examine the impact of p53, MCF10A cells were developed, which express the tetramerization domain of p53 to inhibit endogenous p53 function. These cells were attenuated in SN38-mediated induction of p21(WAF1), and UCN-01 induced S, but not G(2) progression. In contrast, MCF10A cells expressing short hairpin RNA to ablate p53 expression underwent both S and G(2) phase progression with UCN-01. The difference in G(2) progression was attributed to p53-mediated gene repression; the MCF10A cells expressing the tetramerization domain retained p53 protein and repressed both cyclin B and Chk1, while cells ablated for p53 did not repress these proteins. Hence, inhibition of p53 activator function permits S phase abrogation, while additional inhibition of p53 repressor function is required for abrogation of G(2) arrest. These studies provide a mechanistic explanation for how this therapeutic strategy can selectively target tumor cells.

p53 in human embryonal carcinoma: identification of a transferable, transcriptional repression domain in the N-terminal region of p53

Testicular germ cell tumors (TGCTs) arise despite possessing high levels of wild-type p53, suggesting p53 latency. We have previously shown that p53 repression in TGCT-derived human embryonal carcinoma (EC) is relieved upon treatment with all-trans retinoic acid (RA), resulting in enhanced p53 transactivation activity. To further investigate p53 repression in EC, a series of gal4-p53 truncation constructs were generated. Deletion of the core DNA-binding region, residues 117-274, had no effect on basal or RA-induced p53 activity. Progressively, larger truncations were made in the C- or N-terminal direction. Deletion of residues toward the C-terminus of p53 as far as residue 354 did not affect either the basal or RA-inducible activity of gal4-p53. When a small region in the N-terminus was deleted (residues 105-116), relief of the basal repression of p53 activity characteristic of EC was observed. Fusion of this region to the VP16 activation domain (VPAD) resulted in a 10-20-fold repression of VPAD activity in NT2/D1 human EC cells, indicating that this region acts as a heterologous repressor. Owing to its location in the N-terminal half of p53, we have named this region the p53 N-terminal Repression Domain (p53-NRD). The p53-NRD mediated repression in a variety of cell lines, with the most prominent repression observed in human EC cells. While RA alone had no effect on p53-NRD activity, cotreatment with RA and the histone deacetylase inhibitor trichostatin-A (TSA) completely relieved p53-NRD-mediated repression. In contrast, NRD-mediated repression was not sensitive to RA and TSA in a derived RA-resistant cell line with a retinoic acid receptor gamma (RARgamma) defect, but sensitivity could be restored with transfection of RARgamma. These data indicate that a unique repressor domain resides in p53 at residues 90-116 whose activity can be modulated in the presence of 'differentiation therapy' and 'transcription therapy' agents.

Retinoids in cancer therapy and chemoprevention: promise meets resistance

Retinoids (natural and synthetic derivatives of vitamin A) signal potent differentiation and growth-suppressive effects in diverse normal, premalignant, and malignant cells. A strong rationale exists for the use of retinoids in cancer treatment and chemoprevention based on preclinical, epidemiological, and early clinical findings. Despite the success of all-trans-retinoic acid (RA)-based differentiation therapy in acute promyelocytic leukemia (APL), the broad promise of retinoids in the clinic has not yet been realized. In addition to the expected limited activity of any single therapeutic agent, translation of retinoid activities from the laboratory to the clinic has met with intrinsic or acquired retinoid resistance. Evidence suggests that solid tumors develop intrinsic resistance to retinoids during carcinogenesis. In contrast, relapse of APL is often associated with acquired resistance to retinoid maturation induction. This review discusses what is known about retinoid resistance mechanisms in cancer therapy and chemoprevention. Strategies to overcome this resistance will be discussed, including combination therapy with other differentiation-inducing, cytotoxic or chromatin-remodeling agents, as well as the use of receptor-selective and nonclassical retinoids. Opportunities exist in the post-genomic era to bypass resistance to classical retinoids by identifying target genes and associated pathways that directly mediate the antineoplastic effects of retinoids. In this regard, the retinoids are useful pharmacological tools to reveal important pathways targeted in cancer therapy and chemoprevention.

Negative Feedback at the Level of Nuclear Receptor Coregulation

Nuclear receptor-mediated gene expression is proposed to be regulated by the ordered recruitment of large protein complexes in which activity depends on mutual interactions and posttranslational modifications. In contrast, relatively little attention has been given to mechanisms regulating the expression of the coregulator proteins themselves. Previously we have shown that the ligand-dependent corepressor, RIP140, is a direct transcriptional target of all-trans retinoic acid (RA). Here we demonstrate that RA induction of RIP140 constitutes a rate-limiting step in the regulation of retinoic acid receptor signaling. Silencing of the RA induction of RIP140 dramatically enhances and accelerates retinoid receptor transactivation, endogenous expression of other RA target genes, and RA-induced neuronal differentiation and cell cycle arrest in human embryonal carcinoma cells. The data suggest that RA induction of RIP140 constitutes a functional negative feedback loop that limits activation of retinoid receptors in the continued presence of RA and that acutely regulated expression of coregulators may be a general regulatory mechanism in hormonal signaling.

Cancer prevention and therapy: strategies and problems

During the next years, molecular diagnostic science and the pharmaceutical industry will face increasing demand for personalized medicine. Therapeutic treatments should be tailored to the needs of individual patient. Patients will inquire for information about potential tumor detection at an early stage when disease can be more likely to be arrested or cured with specific regimens of drug therapy. To respond to this demand, science and industry need to modulate therapeutic approaches to the continuous development of cancer. Now more than ever, it is necessary to fill the knowledge hiatus between the "beginning" and the "end" of cancer development, i.e we need to critically analyze the extensive multi-step process of cancer development that still remains poorly understood.

TP53 family members and human cancers

Based on gene sequence homologies, a p53 (TP53) gene family become apparent with the addition of the most recently identified p63 (TP73L; formerly TP63) and p73 (TP73) genes to the already known p53. The p53 gene encodes for a unique protein eliciting well-known tumor suppressor gene (TSG) properties that mediate cellular response to DNA damage, e.g., cell cycle arrest or apoptosis. In contrast, both homologues specify an array of isoforms different in their N- and C-terminal domains. Transactivating isoforms, such as TAp63/p73, show TSG properties similar to p53, while isoforms lacking N-terminal transactivating domain such as DeltaNp63/p73, induce a functional block against p53 as well as TAp63/p73 activities. Both p63/p73 types of isoforms are involved in development: p63 is critical for epithelial stem cell renewal and epithelial homeostasis, and p73 is involved in neurogenesis and natural immune response. These facts support interdependent functions for the p53 family members, which appear linked together in a complex and tight regulation network to fulfill cellular functions related to DNA damage and tissue homeostasis maintenance. The lack of p63/p73 mutations in human cancers rule out a typical TSG role for either of the p53 homologues. Nonetheless, p63 and p73 genes seem strongly involved in malignancy acquisition and maintenance process because of: 1) their tissue identities, and 2) their close interplay activities within the p53 family members, and primarily through the negative regulatory role played by DeltaNp63/p73 isoforms for cell death control and differentiation.

Zinc inhibits the nuclear translocation of the tumor suppressor protein p53 and protects cultured human neurons from copper-induced neurotoxicity

High concentrations of the trace metal zinc (Zn) have previously been shown to provide transient protection of cells from apoptotic death. The molecular mechanisms responsible for this protection are not known. Thus, this work explored the ability of Zn to protect human neurons in culture (NT2-N) from Cu-mediated death and tested the hypotheses that the tumor-suppressor protein p53 plays a role in Cu-induced neuronal death and is part of the mechanism of Zn protection. Copper toxicity (100 microM) resulted in significant apoptotic neuronal death by 12 h. Addition of 100 microM Zn to Cu-treated cells increased neuronal death. However, the addition of 700 microM Zn to Cu-treated cells resulted in neuronal viability that was not different from untreated controls through 24 h. p53 mRNA abundance, while increased by the addition of Cu and 100 microM Zn, was decreased to 50% of control with the addition of 500 microM Zn in Cu-treated cells, and to 10% of control with 700 microM Zn. Consistent with its role as a transcription factor, both Western analysis and immunocytochemistry showed significant increases in nuclear p53 protein levels in Cu toxicity. The role of p53 in Cu-mediated apoptosis was further confirmed by elimination of apoptosis in Cu-treated cells that had been transfected with a dominant-negative p53 construct to prevent p53 expression. Furthermore, the addition of 500-700 microM Zn prevented the movement of p53 into the nucleus suggesting that Zn not only protects neurons from Cu toxicity by regulating p53 mRNA abundance but also by preventing the translocation of p53 to the nucleus.

A mechanism of resistance to TRAIL/Apo2L-induced apoptosis of newly established glioma cell line and sensitisation to TRAIL by genotoxic agents

Most tumour cells are sensitive to TRAIL-induced apoptosis, but not normal cells; thus, cancer therapy using TRAIL is expected clinically. Several tumour cells are resistant to TRAIL-induced apoptosis, and various mechanisms of such resistance were reported in individual cases. In this study, we established a TRAIL-resistant glioma cell line, which completely lacked TRAIL receptors. In addition, this tumour cell line had wild-type p53 tumour-suppressive gene, suggesting new mechanisms for tumour cells to expand and escape from immune surveillance. The present study further explored the mechanisms that determine the sensitivity to TRAIL. We show that genotoxic agents such as cisplatin, doxorubicin and camptothecin, in addition to UV radiation, can induce TRAIL-R2 on the cell surface of TRAIL receptor-negative tumour cells. Newly synthesised TRAIL-R2 is functional, so apoptosis is effectively induced by TRAIL, but it is significantly inhibited by constitutive expression of dominant-negative p53. In addition, apoptosis induced by pretreatment of genotoxic agents and additional stimulation of TRAIL is efficiently inhibited by either antagonistic anti-TRAIL-R2 antibody or pan-caspase inhibitor z-VAD-FMK. Taken together, these findings suggest that resistance to TRAIL by lack of TRAIL receptors on glioma is restored by genotoxic agents, which support the new strategies for tumour killing by TRAIL-bearing cytotoxic cells in combination with genotoxic treatment.

New directions in testicular cancer; molecular determinants of oncogenesis and treatment success

Metastatic testicular cancer is highly curable with conventional cytotoxic drugs. This is in contrast to most other metastatic solid tumours which can only be palliated with chemotherapy achieving only a modest impact on overall survival. If we could understand at the molecular level why chemotherapy is so effective in the treatment of testicular cancer, we may be better able to move other forms of metastatic cancer into the curable bracket. Most cytotoxic drugs appear to induce cell death by activating intracellular apoptotic mechanisms. Thus, the ability of a cancer to activate and execute such mechanisms in response to treatment is paramount in determining the effectiveness of chemotherapy. The basic study of cancer molecular biology is providing some insight into the proteins involved in this process and the ability to apply this information to actual human tumours is essential to rationalise clinical treatment failures at a molecular level. Testicular cancer provides an excellent model system in this analysis. Whereas there are large numbers of patients that are cured by chemotherapy, there are some whose cancers become resistant to treatment. An understanding of testicular cancer molecular biology may allow the identification of the genes regulating such a crucial behavioural switch. It may then be possible to manipulate specific signalling pathways to overcome drug resistance. This review focuses on recent developments in our understanding of the molecular biology of testicular cancer. A number of key players have been implicated including p53, pRb, cyclin D2, p INK proteins, c-kit and the bcl-2 family of proteins. The exact manner by which cellular transformation occurs has still not been established, but it is clear that many of the above proteins also have important roles in normal spermatogenesis. This is a developmental phase when the generation of genetic diversity is at a premium, but when selective apoptotic mechanisms are paramount. We discuss why this may be relevant to the behaviour of germ cell tumours and address possible reasons why they can become resistant to conventional therapy.

Retinoid target gene activation during induced tumor cell differentiation: human embryonal carcinoma as a model

Many agents that exhibit chemopreventive activity are able to mediate a differentiation response in premalignant and malignant tissues. One of the most widely studied classes of tumor differentiation agents is the retinoids. There is rapidly evolving evidence for beneficial retinoid actions in the prevention or treatment of clinical tumors. However, the use of retinoids in the clinic is limited by acquired resistance and toxicity, especially when administered chronically in preventive strategies. Although retinoids are known to regulate gene transcription by activating retinoid receptors, the identity of the target genes that mediate the beneficial effects of retinoids are largely unknown. Here we review a useful model of retinoid-induced tumor cell differentiation: human embryonal carcinoma. The pluripotent nature and ease of use make human embryonal carcinoma cells a valuable and practical complement to human embryonic stem cells as an in vitro model of early human development. In addition, retinoid treatment of human embryonal carcinoma is an important model of induced tumor cell differentiation because retinoids cause the reversal of the malignant phenotype coincident with terminal neuronal differentiation. We have used both de novo and candidate approaches with this system in an effort to uncover critical downstream targets of retinoid receptors during differentiation induction.

Developmentally-related candidate retinoic acid target genes regulated early during neuronal differentiation of human embryonal carcinoma

Embryonal carcinoma is a model of embryonic development as well as tumor cell differentiation. In response to all-trans retinoic acid (RA), the human embryonal carcinoma (EC) cell line, NT2/D1, differentiates toward a neuronal lineage with associated loss of cell growth and tumorigenicity. Through the use of cDNA-based microarrays we sought to identify the early downstream targets of RA during differentiation commitment of NT2/D1 cells. A total of 57 genes were induced and 37 genes repressed by RA. RA regulated genes were restricted at 8 h with 27 genes induced and five repressed. The total number of RA-responsive transcripts increased at 24 and 48 h and their pattern of expression was more symmetrical. For a given time point less than 1% of the 9128 cDNAs on the expression array were regulated by RA. Many of these gene products are associated with developmental pathways including those of TGF-beta (Lefty A, NMA, follistatin), homeo domain (HoxD1, Meis2, Meis1, Gbx2), IGF (IGFBP3, IGFBP6, CTGF), Notch (manic fringe, ADAM11), Hedgehog (patched) and Wnt (Frat2, secreted frizzled-related protein 1) signaling. In addition a large cassette of genes induced by RA at 24-48 h are associated with cell adhesion, cytoskeletal and matrix remodeling, growth suppression and intracellular signaling cascades. The majority of repressed genes are associated with protein/RNA processing, turnover or metabolism. The early induced genes identified may play a regulatory role in RA-mediated growth suppression and terminal differentiation and may have physiologic or pharmacologic importance during normal human development and retinoid-based cancer therapy or prevention.