Targeting Bcl-X(L) for prevention and therapy of skin cancer

Bcl-xL: A Focus on Melanoma Pathobiology

Apoptosis is the main mechanism by which multicellular organisms eliminate damaged or unwanted cells. To regulate this process, a balance between pro-survival and pro-apoptotic proteins is necessary in order to avoid impaired apoptosis, which is the cause of several pathologies, including cancer. Among the anti-apoptotic proteins, Bcl-xL exhibits a high conformational flexibility, whose regulation is strictly controlled by alternative splicing and post-transcriptional regulation mediated by transcription factors or microRNAs. It shows relevant functions in different forms of cancer, including melanoma. In melanoma, Bcl-xL contributes to both canonical roles, such as pro-survival, protection from apoptosis and induction of drug resistance, and non-canonical functions, including promotion of cell migration and invasion, and angiogenesis. Growing evidence indicates that Bcl-xL inhibition can be helpful for cancer patients, but at present, effective and safe therapies targeting Bcl-xL are lacking due to toxicity to platelets. In this review, we summarized findings describing the mechanisms of Bcl-xL regulation, and the role that Bcl-xL plays in melanoma pathobiology and response to therapy. From these findings, it emerged that even if Bcl-xL plays a crucial role in melanoma pathobiology, we need further studies aimed at evaluating the involvement of Bcl-xL and other members of the Bcl-2 family in the progression of melanoma and at identifying new non-toxic Bcl-xL inhibitors.

DSIR: assessing the design of highly potent siRNA by testing a set of cancer-relevant target genes

Chemically synthesized small interfering RNA (siRNA) is a widespread molecular tool used to knock down genes in mammalian cells. However, designing potent siRNA remains challenging. Among tools predicting siRNA efficacy, very few have been validated on endogenous targets in realistic experimental conditions. We previously described a tool to assist efficient siRNA design (DSIR, Designer of siRNA), which focuses on intrinsic features of the siRNA sequence. Here, we evaluated DSIR’s performance by systematically investigating the potency of the siRNA it designs to target ten cancer-related genes. mRNA knockdown was measured by quantitative RT-PCR in cell-based assays, revealing that over 60% of siRNA sequences designed by DSIR silenced their target genes by at least 70%. Silencing efficacy was sustained even when low siRNA concentrations were used. This systematic analysis revealed in particular that, for a subset of genes, the efficiency of siRNA constructs significantly increases when the sequence is located closer to the 5′-end of the target gene coding sequence, suggesting the distance to the 5′-end as a new feature for siRNA potency prediction. A new version of DSIR incorporating these new findings, as well as the list of validated siRNA against the tested cancer genes, has been made available on the web (http://biodev.extra.cea.fr/DSIR).

Single-cell analysis of dihydroartemisinin-induced apoptosis through reactive oxygen species-mediated caspase-8 activation and mitochondrial pathway in ASTC-a-1 cells using fluorescence imaging techniques

Dihydroartemisinin (DHA), a front-line antimalarial herbal compound, has been shown to possess promising anticancer activity with low toxicity. We have previously reported that DHA induced caspase-3-dependent apoptosis in human lung adenocarcinoma cells. However, the cellular target and molecular mechanism of DHA-induced apoptosis is still poorly defined. We use confocal fluorescence microscopy imaging, fluorescence resonance energy transfer, and fluorescence recovery after photobleaching techniques to explore the roles of DHA-elicited reactive oxygen species (ROS) in the DHA-induced Bcl-2 family proteins activation, mitochondrial dysfunction, caspase cascade, and cell death. Cell Counting Kit-8 assay and flow cytometry analysis showed that DHA induced ROS-mediated apoptosis. Confocal imaging analysis in a single living cell and Western blot assay showed that DHA triggered ROS-dependent Bax translocation, mitochondrial membrane depolarization, alteration of mitochondrial morphology, cytochrome c release, caspase-9, caspase-8, and caspase-3 activation, indicating the coexistence of ROS-mediated mitochondrial and death receptor pathway. Collectively, our findings demonstrate for the first time that DHA induces cell apoptosis by triggering ROS-mediated caspase-8/Bid activation and the mitochondrial pathway, which provides some novel insights into the application of DHA as a potential anticancer drug and a new therapeutic strategy by targeting ROS signaling in lung adenocarcinoma therapy in the future.

UV-induced squamous cell carcinoma--a role for antiapoptotic signalling pathways

The incidence of nonmelanoma skin cancer including squamous cell carcinoma (SCC) and basal cell carcinoma (BCC) has dramatically increased in the last decades, and chronic sun exposure was identified as a main etiologic agent. UV radiation may produce DNA damage either directly or through reactive oxygen species (ROS). As mutations caused by UV may lead to skin cancer due to oncogene activation and tumor suppressor gene inactivation, efficient safeguard mechanisms have been developed during evolution. These enclose induction of apoptosis and formation sunburn cells aiming at the removal of premalignant cells. The keratinocyte apoptotic machinery in response to UV consists of both intrinsic/mitochondrial and extrinsic/death receptor-mediated cell-death pathways, which are particularly regulated by mitogen-activated protein kinases (MAPKs, JNK and p38) and the tumor-suppressor protein p53. For development of skin cancer, it appears that critical steps in apoptosis control are dysregulated leading to resistance both to death ligand-mediated and intrinsic proapoptotic pathways. These particularly include inactivation of p53, as well as activation of EGFR, COX-2 and MAPKs, which result in specific regulation of Bcl-2 proteins, death ligands and death receptors. The final unravelling of apoptosis regulation in epithelial skin cancer may allow the development of new targeted therapeutic strategies.

The promise of natural products for blocking early events in skin carcinogenesis

This perspective on Stratton et al. (beginning on p. 160), Kowalczyk et al. (beginning on p. 170), and Katiyar et al. (beginning on p. 179) highlights the common theme of translational investigation of natural substances and their molecular effects and mechanisms in preventing skin squamous cell carcinoma, which has potentially severe clinical consequences. These studies comprise results of naturally occurring phytochemicals and green tea polyphenols in mouse models of UV-induced and chemically induced skin carcinogenesis and results of perillyl alcohol in a phase IIa clinical trial-all pointing to the great promise of this exciting approach for better understanding of and preventing skin cancer.

Targeted disruption of Bcl-xL in mouse keratinocytes inhibits both UVB- and chemically induced skin carcinogenesis

Bcl-x(L) is one of several antiapoptotic proteins regulated by signal transducer and activator of transcription 3 (Stat3). We have recently shown that Stat3 is required for chemically induced and ultraviolet B (UVB)-induced skin carcinogenesis. In this study, the functional role of Bcl-x(L) in skin carcinogenesis was investigated using skin-specific Bcl-x(L)-deficient mice. In this model, Bcl-x(L) expression is disrupted in the basal compartment of mouse epidermis using the bovine keratin 5 (K5) promoter to drive expression of Cre recombinase (K5.Cre x Bcl-x(L) (fl/fl) mice). A significant increase in apoptosis induced by either UVB irradiation or 7,12-dimethylbenz[a]anthracene (DMBA) treatment was observed in the epidermis of Bcl-x(L)-deficient mice. Furthermore, an increase in apoptotic cells was noted in hair follicle keratinocytes, including those located in the bulge region. Cell proliferation was not affected by Bcl-x(L) deficiency following exposure to either UVB or 12-O-tetradecanoylphorbol-13-acetate (TPA). Bcl-x(L)-deficient mice were more resistant than wild-type controls to skin tumor development with delayed onset and reduced number of tumors using either UVB or the DMBA/TPA two-stage regimen. Moreover, Bcl-2, Mcl-1, and survivin protein levels were increased in the epidermis of Bcl-x(L)-deficient mice in the absence of stimuli. Furthermore, levels of these antiapoptotic proteins were also high in skin tumors from Bcl-x(L)-deficient mice that developed in response to either UVB or two-stage carcinogenesis protocols. Collectively, these studies demonstrate that Bcl-x(L) plays a role early in skin carcinogenesis through its anti-apoptotic functions to enhance survival of keratinocytes, including bulge region keratinocyte stem cells, following DNA damage.

Multi-stage chemical carcinogenesis in mouse skin: fundamentals and applications

For more than 60 years, the chemical induction of tumors in mouse skin has been used to study mechanisms of epithelial carcinogenesis and evaluate modifying factors. In the traditional two-stage skin carcinogenesis model, the initiation phase is accomplished by the application of a sub-carcinogenic dose of a carcinogen. Subsequently, tumor development is elicited by repeated treatment with a tumor-promoting agent. The initiation protocol can be completed within 1-3 h depending on the number of mice used; whereas the promotion phase requires twice weekly treatments (1-2 h) and once weekly tumor palpation (1-2 h) for the duration of the study. Using the protocol described here, a highly reproducible papilloma burden is expected within 10-20 weeks with progression of a portion of the tumors to squamous cell carcinomas within 20-50 weeks. In contrast to complete skin carcinogenesis, the two-stage model allows for greater yield of premalignant lesions, as well as separation of the initiation and promotion phases.

Cellular and sub-cellular responses to UVA in relation to carcinogenesis

PURPOSE

UVA radiation (315-400 nm) contributes to skin aging and carcinogenesis. The aim of this review is to consider the mechanisms that underlie UVA-induced cellular damage, how this damage may be prevented or repaired and the signal transduction processes that are elicited in response to it.

RESULTS

Exposure to ultraviolet (UV) light is well-established as the causative factor in skin cancer. Until recently, most work on the mechanisms that underlie skin carcinogenesis focused on shorter wavelength UVB radiation (280-315 nm), however in recent years there has been increased interest in the contribution made by UVA. UVA is able to cause a range of damage to cellular biomolecules including lipid peroxidation, oxidized protein and DNA damage, such as 8-oxoguanine and cyclobutane pyrimidine dimers. Such damage is strongly implicated in both cell death and malignant transformation and cells have a number of mechanisms in place to mitigate the effects of UVA exposure, including antioxidants, DNA repair, and stress signalling pathways.

CONCLUSIONS

The past decade has seen a surge of interest in the biological effects of UVA exposure as its significance to the process of photo-carcinogenesis has become increasingly evident. However, unpicking the unique complexity of the cellular response to UVA, which is only now becoming apparent, will be a major challenge for the field of photobiology in the 21st century.