Reciprocal regulation of p21 and Chk1 controls the cyclin D1-RB pathway to mediate senescence onset after G2 arrest.

Senescence is an irreversible withdrawal from cell proliferation that can be initiated after DNA damage-induced cell cycle arrest in G2 phase to prevent genomic instability. Senescence onset in G2 requires p53 (also known as TP53) and retinoblastoma protein (RB, also known as RB1) family tumour suppressors, but how they are regulated to convert a temporary cell cycle arrest into a permanent one remains unknown. Here, we show that a previously unrecognised balance between the cyclin-dependent kinase (CDK) inhibitor p21 and the checkpoint kinase Chk1 controls cyclin D-CDK activity during G2 arrest. In non-transformed cells, p21 activates RB in G2 by inhibiting cyclin D1 complexed with CDK2 or CDK4. The resulting G2 exit, which precedes the appearance of senescence markers, is associated with a mitotic bypass, Chk1 downregulation and reduction in the number of DNA damage foci. In p53/RB-proficient cancer cells, a compromised G2 exit correlates with sustained Chk1 activity, delayed p21 induction, untimely cyclin E1 re-expression and genome reduplication. Conversely, Chk1 depletion promotes senescence by inducing p21 binding to cyclin D1- and cyclin E1-CDK complexes and downregulating CDK6, whereas knockdown of the checkpoint kinase Chk2 enables RB phosphorylation and delays G2 exit. In conclusion, p21 and Chk2 oppose Chk1 to maintain RB activity, thus promoting the onset of senescence induced by DNA damage in G2.

p53 Family in Resistance to Targeted Therapy of Melanoma.

Metastatic melanoma is one of the most aggressive tumors, with frequent mutations affecting components of the MAPK pathway, mainly protein kinase BRAF. Despite promising initial response to BRAF inhibitors, melanoma progresses due to development of resistance. In addition to frequent reactivation of MAPK or activation of PI3K/AKT signaling pathways, recently, the p53 pathway has been shown to contribute to acquired resistance to targeted MAPK inhibitor therapy. Canonical tumor suppressor p53 is inactivated in melanoma by diverse mechanisms. The TP53 gene and two other family members, TP63 and TP73, encode numerous protein isoforms that exhibit diverse functions during tumorigenesis. The p53 family isoforms can be produced by usage of alternative promoters and/or splicing on the C- and N-terminus. Various p53 family isoforms are expressed in melanoma cell lines and tumor samples, and several of them have already shown to have specific functions in melanoma, affecting proliferation, survival, metastatic potential, invasion, migration, and response to therapy. Of special interest are p53 family isoforms with increased expression and direct involvement in acquired resistance to MAPK inhibitors in melanoma cells, implying that modulating their expression or targeting their functional pathways could be a potential therapeutic strategy to overcome resistance to MAPK inhibitors in melanoma.

Characterization of Vemurafenib-Resistant Melanoma Cell Lines Reveals Novel Hallmarks of Targeted Therapy Resistance.

Regardless of the significant improvements in treatment of melanoma, the majority of patients develop resistance whose mechanisms are still not completely understood. Hence, we generated and characterized two melanoma-derived cell lines, primary WM793B and metastatic A375M, with acquired resistance to the RAF inhibitor vemurafenib. The morphology of the resistant primary WM793B melanoma cells showed EMT-like features and exhibited a hybrid phenotype with both epithelial and mesenchymal characteristics. Surprisingly, the vemurafenib-resistant melanoma cells showed a decreased migration ability but also displayed a tendency to collective migration. Signaling pathway analysis revealed the reactivation of MAPK and the activation of the PI3K/AKT pathway depending on the vemurafenib-resistant cell line. The acquired resistance to vemurafenib caused resistance to chemotherapy in primary WM793B melanoma cells. Furthermore, the cell-cycle analysis and altered levels of cell-cycle regulators revealed that resistant cells likely transiently enter into cell cycle arrest at the G0/G1 phase and gain slow-cycling cell features. A decreased level of NME1 and NME2 metastasis suppressor proteins were found in WM793B-resistant primary melanoma, which is possibly the result of vemurafenib-acquired resistance and is one of the causes of increased PI3K/AKT signaling. Further studies are needed to reveal the vemurafenib-dependent negative regulators of NME proteins, their role in PI3K/AKT signaling, and their influence on vemurafenib-resistant melanoma cell characteristics.

Prevention of DNA damage in Barrett's esophageal cells exposed to acidic bile salts.

Esophageal adenocarcinoma (EA) is one of the fastest rising tumors in the USA. The major risk factor for EA is gastroesophageal reflux disease (GERD). During GERD, esophageal cells are exposed to refluxate which contains gastric acid frequently mixed with duodenal bile. This may lead to mucosal injury and Barrett's metaplasia (BE) that are important factors contributing to development of EA. In this study, we investigated DNA damage in BE cells exposed to acidic bile salts and explored for potential protective strategies. Exposure of BE cells to acidic bile salts led to significant DNA damage, which in turn, was due to generation of reactive oxygen species (ROS). We found that acidic bile salts induce a rapid increase in superoxide radicals and hydrogen peroxide, which were determined using electron paramagnetic resonance spectroscopy and Amplex Red assay. Analyzing a panel of natural antioxidants, we identified apocynin to be the most effective in protecting esophageal cells from DNA damage induced by acidic bile salts. Mechanistic analyses showed that apocynin inhibited ROS generation and increases the DNA repair capacity of BE cells. We identified BRCA1 and p73 proteins as apocynin targets. Downregulation of p73 inhibited the protective effect of apocynin. Taken together, our results suggest potential application of natural compounds such as apocynin for prevention of reflux-induced DNA damage and GERD-associated tumorigenesis.

Differential effects of diverse p53 isoforms on TAp73 transcriptional activity and apoptosis.

The p53 activities are due, at least in part, to its ability to form oligomers that bind to specific DNA sequences and activate transcription. Since some mutant p53 proteins and ΔNp73 isoforms form heterocomplexes with TAp73, we asked whether p53 isoforms can do the same and potentially act as dominant-negative inhibitors of TAp73. Moreover, it has already been found that some isoforms form complex with wtp53 and some of them inhibit p53 tumor-suppressor functions. Therefore, we studied the complex formation and co-immunoprecipitation assays show that all six p53 isoforms examined can form complexes with TAp73ß, whereas only Δ133p53α/ß/γ isoforms form complex with TAp73α. All p53 isoforms counteract TAp73ß transactivation function but with different efficiency and in a promoter-dependent manner. Furthermore, apoptotic activity of TAp73ß was augmented by coexpression of p53ß, whereas Δ133p53α and ß inhibit its apoptotic activity most efficiently. We have determined the half-life of different p53 isoforms: p53γ isoform has the shortest half-life, whereas Δ133p53γ has the longest half-life. Inhibitory interactions of two proteins in complex often lead to their stabilization. However, only three isoforms (Δ133p53α, Δ133p53ß and Δ40p53α) stabilize TAp73ß. We are convinced that defining the interactions between p53/p73 would give a new insight into how the p53 isoforms modulate the p73 functions in tumorigenesis.

Flavonols as a Potential Pharmacological Intervention for Alleviating Cognitive Decline in Diabetes: Evidence from Preclinical Studies.

Diabetes mellitus is a complex metabolic disease associated with reduced synaptic plasticity, atrophy of the hippocampus, and cognitive decline. Cognitive impairment results from several pathological mechanisms, including increased levels of advanced glycation end products (AGEs) and their receptors, prolonged oxidative stress and impaired activity of endogenous mechanisms of antioxidant defense, neuroinflammation driven by the nuclear factor kappa-light-chain enhancer of activated B cells (NF-κB), decreased expression of brain-derived neurotrophic factor (BDNF), and disturbance of signaling pathways involved in neuronal survival and cognitive functioning. There is increasing evidence that dietary interventions can reduce the risk of various diabetic complications. In this context, flavonols, a highly abundant class of flavonoids in the human diet, are appreciated as a potential pharmacological intervention against cognitive decline in diabetes. In preclinical studies, flavonols have shown neuroprotective, antioxidative, anti-inflammatory, and memory-enhancing properties based on their ability to regulate glucose levels, attenuate oxidative stress and inflammation, promote the expression of neurotrophic factors, and regulate signaling pathways. The present review gives an overview of the molecular mechanisms involved in diabetes-induced cognitive dysfunctions and the results of preclinical studies showing that flavonols have the ability to alleviate cognitive impairment. Although the results from animal studies are promising, clinical and epidemiological studies are still needed to advance our knowledge on the potential of flavonols to improve cognitive decline in diabetic patients.

p53/p73 Protein Network in Colorectal Cancer and Other Human Malignancies.

The p53 tumor suppressor protein is crucial for cell growth control and the maintenance of genomic stability. Later discovered, p63 and p73 share structural and functional similarity with p53. To understand the p53 pathways more profoundly, all family members should be considered. Each family member possesses two promoters and alternative translation initiation sites, and they undergo alternative splicing, generating multiple isoforms. The resulting isoforms have important roles in carcinogenesis, while their expression is dysregulated in several human tumors including colorectal carcinoma, which makes them potential targets in cancer treatment. Their activities arise, at least in part, from the ability to form tetramers that bind to specific DNA sequences and activate the transcription of target genes. In this review, we summarize the current understanding of the biological activities and regulation of the p53/p73 isoforms, highlighting their role in colorectal tumorigenesis. The analysis of the expression patterns of the p53/p73 isoforms in human cancers provides an important step in the improvement of cancer therapy. Furthermore, the interactions among the p53 family members which could modulate normal functions of the canonical p53 in tumor tissue are described. Lastly, we emphasize the importance of clinical studies to assess the significance of combining the deregulation of different members of the p53 family to define the outcome of the disease.

Syntheses of gold nanoparticles and their impact on the cell cycle in breast cancer cells subjected to megavoltage X-ray irradiation.

Gold nanoparticles (AuNPs) were synthesized in the presence of citrate (Au-CIT), glutathione (Au-GSH) and aminodextran (Au-DEX) in order to modify AuNPs surfaces and to increase their cellular uptake in the breast cancer cells MDA-MB-231. AuNPs were characterized with respect to their particle size, shape and colloidal stability in an aqueous solution and cell media. The mass accumulation of each AuNP type inside cancer cells was determined quantitatively, using Inductive Coupled Plasma - mass spectroscopy. The sub-cellular accumulation was studied using Transmission Electron Microscopy (TEM). It was found that gold nanoparticles applied to cancer cells were localized in cytoplasmic vesicles and that the highest uptake was shown in the presence of Au-GSH nanoparticles. The effect of AuNPs on the cell cycle was investigated using flow cytometry and Western blot analysis. The gold nanoparticles alone did not affect the cell cycle, as shown by flow cytometry. Furthermore, the cancer cells were irradiated using conventional clinically relevant high-energy X-ray radiation of 6 MV in the dose of 4 Gy. The results on cells only irradiated showed an S phase arrest six and 8 h after irradiation, and a G2/M arrest 24 and 48 h after irradiation. The irradiation of breast cancer cells treated with AuNPs has shown no significant variation in cell cycle distribution as opposed to X-ray radiation alone.

Helicobacter pylori pathogen regulates p14ARF tumor suppressor and autophagy in gastric epithelial cells.

Infection with Helicobacter pylori is one of the strongest risk factors for development of gastric cancer. Although these bacteria infect approximately half of the world's population, only a small fraction of infected individuals develops gastric malignancies. Interactions between host and bacterial virulence factors are complex and interrelated, making it difficult to elucidate specific processes associated with H. pylori-induced tumorigenesis. In this study, we found that H. pylori inhibits p14ARF tumor suppressor by inducing its degradation. This effect was found to be strain-specific. Downregulation of p14ARF induced by H. pylori leads to inhibition of autophagy in a p53-independent manner in infected cells. We identified TRIP12 protein as E3 ubiquitin ligase that is upregulated by H. pylori, inducing ubiquitination and subsequent degradation of p14ARF protein. Using isogenic H. pylori mutants, we found that induction of TRIP12 is mediated by bacterial virulence factor CagA. Increased expression of TRIP12 protein was found in infected gastric epithelial cells in vitro and human gastric mucosa of H. pylori-infected individuals. In conclusion, our data demonstrate a new mechanism of ARF inhibition that may affect host-bacteria interactions and facilitate tumorigenic transformation in the stomach.

Activation of NADPH oxidases leads to DNA damage in esophageal cells.

Gastroesophageal reflux disease (GERD) is the strongest known risk factor for esophageal adenocarcinoma. In the center of tumorigenic events caused by GERD is repeated damage of esophageal tissues by the refluxate. In this study, we focused on a genotoxic aspect of exposure of esophageal cells to acidic bile reflux (BA/A). Analyzing cells generated from patients with Barrett's esophagus and human esophageal specimens, we found that BA/A cause significant DNA damage that is mediated by reactive-oxygen species. ROS originate from mitochondria and NADPH oxidases. We specifically identified NOX1 and NOX2 enzymes to be responsible for ROS generation. Inhibition of NOX2 and NOX1 with siRNA or chemical inhibitors significantly suppresses ROS production and DNA damage induced by BA/A. Mechanistically, our data showed that exposure of esophageal cells to acidic bile salts induces phosphorylation of the p47phox subunit of NOX2 and its translocation to the cellular membrane. This process is mediated by protein kinase C, which is activated by BA/A. Taken together, our studies suggest that inhibition of ROS induced by reflux can be a useful strategy for preventing DNA damage and decreasing the risk of tumorigenic transformation caused by GERD.

Targeting p73--a potential approach in cancer treatment.

The p53 family consists of three members: p53, p63 and p73. All three of them have a role in cell cycle arrest and induction of apoptosis. However, despite structural and partly functional similarity, there are striking differences in their functions and each of them has its own and unique identity. All three genes encode multiple variants with opposing functions in cancer development - full length transactivation forms with proapoptotic and antiproliferative functions, and dominant-negative transactivation-deficient forms with anti-apoptotic (oncogenic) functions. The functional interactions between family members are crucial to gain insight and understand their role in cancer biology. The discovery of p53/p73 network could have a major clinical impact in prognostic use and targeted drug design. In the current review we present the recent achievements in p73 research including very complex and sophisticated p73 regulation and response to DNA damage, and functional interactions among family members. We discuss how p73 has affected drug discovery. According to the p73 tumor suppressor function, we outline current aspects of anticancer therapy.