Upregulation of the p53R2 ribonucleotide reductase subunit by nitric oxide

Challenges to DNA replication in hypoxic conditions

The term hypoxia refers to any condition where insufficient oxygen is available and therefore encompasses a range of actual oxygen concentrations. The regions of tumours adjacent to necrotic areas are at almost anoxic levels and are known to be extremely therapy resistant (radiobiological hypoxia). The biological response to radiobiological hypoxia includes the rapid accumulation of replication stress and subsequent DNA damage response, including both ATR- and ATM-mediated signalling, despite the absence of detectable DNA damage. The causes and consequences of hypoxia-induced replication stress will be discussed.

Overexpression of p53R2 is associated with poor prognosis in lung sarcomatoid carcinoma

Background

This study aimmed to evaluate the expression of p53-inducible RR small subunit 2 homologue (p53R2) in Lung sarcomatoid carcinoma (LSC) and its association with clinicopathological parameters and prognosis.

Methods

In this study, clinicopathological factors and prognostic significance of the expression of p53R2 was investigated by immunohistochemistry (IHC) in 100 cases of LSC.

Results

The results showed that the expression of p53R2 was significantly correlated with clinical stage (P<0.05). But there was no statistically correlation with gender, age, smoking, tumor size, pT stage, pN stage, pM stage, therapy and relapse. Kaplan-Meier analysis revealed that the expression of p53R2, clinical stage, pT stage, pN stage, pM stage and tumor size were closely related to patients’ survival, and the analysis also revealed that patients with low expression of p53R2 had a longer overall survival than that with high expression (Mean overall survival: 84.8 months vs. 34.7 months, P<0.05). Further multivariate analysis indicated that the expression of p53R2 was identified as an independent prognostic factor in the prediction of the overall survival for patients with LSC (HR = 3.217, P<0.05).

Conclusions

The expression of p53R2 was inversely associated with the proliferation and progression of LSC, and the results indicated that the high expression of p53R2 was an independent factor for unfavorable prognosis of patients with LSC.

Deep Sequencing Reveals Novel Genetic Variants in Children with Acute Liver Failure and Tissue Evidence of Impaired Energy Metabolism

Background & Aims

The etiology of acute liver failure (ALF) remains elusive in almost half of affected children. We hypothesized that inherited mitochondrial and fatty acid oxidation disorders were occult etiological factors in patients with idiopathic ALF and impaired energy metabolism.

Methods

Twelve patients with elevated blood molar lactate/pyruvate ratio and indeterminate etiology were selected from a retrospective cohort of 74 subjects with ALF because their fixed and frozen liver samples were available for histological, ultrastructural, molecular and biochemical analysis.

Results

A customized next-generation sequencing panel for 26 genes associated with mitochondrial and fatty acid oxidation defects revealed mutations and sequence variants in five subjects. Variants involved the genes ACAD9, POLG, POLG2, DGUOK, and RRM2B; the latter not previously reported in subjects with ALF. The explanted livers of the patients with heterozygous, truncating insertion mutations in RRM2B showed patchy micro- and macrovesicular steatosis, decreased mitochondrial DNA (mtDNA) content <30% of controls, and reduced respiratory chain complex activity; both patients had good post-transplant outcome. One infant with severe lactic acidosis was found to carry two heterozygous variants in ACAD9, which was associated with isolated complex I deficiency and diffuse hypergranular hepatocytes. The two subjects with heterozygous variants of unknown clinical significance in POLG and DGUOK developed ALF following drug exposure. Their hepatocytes displayed abnormal mitochondria by electron microscopy.

Conclusion

Targeted next generation sequencing and correlation with histological, ultrastructural and functional studies on liver tissue in children with elevated lactate/pyruvate ratio expand the spectrum of genes associated with pediatric ALF.

Apoptosis induction by combination of drugs or a conjugated molecule associating non-steroidal anti-inflammatory and nitric oxide donor effects in medullary thyroid cancer models: implication of the tumor suppressor p73

BACKGROUND

Medullary thyroid cancer (MTC) is a C-cell neoplasm. Surgery remains its main treatment. Promising therapies based on tyrosine kinase inhibitors demand careful patient selection. We previously observed that two non-steroidal anti-inflammatory drugs (NSAID), indomethacin, celecoxib, and nitric oxide (NO) prevented tumor growth in a model of human MTC cell line (TT) in nude mice.

METHODS

In the present study, we tested the NO donor: glyceryl trinitrate (GTN), at pharmacological dose, alone and in combination with each of the two NSAIDs on TT cells. We also assessed the anti-proliferative potential of NO-indomethacin, an indomethacin molecule chemically conjugated with a NO moiety (NCX 530, Nicox SA) on TT cells and indomethacin/GTN association in rMTC 6-23 cells. The anti-tumoral action of the combined sc. injections of GTN with oral delivery of indomethacin was also studied on subcutaneous TT tumors in nude mice. Apoptosis mechanisms were assessed by expression of caspase-3, TAp73α, TAp73α inhibition by siRNA or Annexin V externalisation.

RESULTS

The two NSAIDs and GTN reduced mitotic activity in TT cells versus control (cell number and PCNA protein expression). The combined treatments amplified the anti-tumor effect of single agents in the two tested cell lines and promoted cell death. Moreover, indomethacin/GTN association stopped the growth of established TT tumors in nude mice. We observed a significant cleavage of full length PARP, a caspase-3 substrate. The cell death appearance was correlated with a two-fold increase in TAp73α expression, with inhibition of apoptosis after TAp73α siRNA addition, demonstrating its crucial role in apoptosis.

CONCLUSION

Association of NO with NSAID exhibited amplified anti-tumoral effects on in vitro and in vivo MTC models by inducing p73-dependent apoptotic cell death.

Caspase-dependent Proteolysis of Human Ribonucleotide Reductase Small Subunits R2 and p53R2 during Apoptosis

Ribonucleotide reductase (RnR) is a key enzyme synthesizing deoxyribonucleotides for DNA replication and repair. In mammals, the R1 catalytic subunit forms an active complex with either one of the two small subunits R2 and p53R2. Expression of R2 is S phase-specific and required for DNA replication. The p53R2 protein is expressed throughout the cell cycle and in quiescent cells where it provides dNTPs for mitochondrial DNA synthesis. Participation of R2 and p53R2 in DNA repair has also been suggested. In this study, we investigated the fate of the RnR subunits during apoptosis. The p53R2 protein was cleaved in a caspase-dependent manner in K-562 cells treated with inhibitors of the Bcr-Abl oncogenic kinase and in HeLa 229 cells incubated with TNF-α and cycloheximide. The cleavage site was mapped between Asp(342) and Asn(343). Caspase attack released a C-terminal p53R2 peptide of nine residues containing the conserved heptapeptide essential for R1 binding. As a consequence, the cleaved p53R2 protein was inactive. In vitro, purified caspase-3 and -8 could release the C-terminal tail of p53R2. Knocking down these caspases, but not caspase-2, -7, and -10, also inhibited p53R2 cleavage in cells committed to die via the extrinsic death receptor pathway. The R2 subunit was subjected to caspase- and proteasome-dependent proteolysis, which was prevented by siRNA targeting caspase-8. Knocking down caspase-3 was ineffective. Protein R1 was not subjected to degradation. Adding deoxyribonucleosides to restore dNTP pools transiently protected cells from apoptosis. These data identify RnR activity as a prosurvival function inactivated by proteolysis during apoptosis.

Ribonucleotide reductase and cancer: biological mechanisms and targeted therapies

Accurate DNA replication and repair is essential for proper development, growth and tumor-free survival in all multicellular organisms. A key requirement for the maintenance of genomic integrity is the availability of adequate and balanced pools of deoxyribonucleoside triphosphates (dNTPs), the building blocks of DNA. Notably, dNTP pool alterations lead to genomic instability and have been linked to multiple human diseases, including mitochondrial disorders, susceptibility to viral infection and cancer. In this review, we discuss how a key regulator of dNTP biosynthesis in mammals, the enzyme ribonucleotide reductase (RNR), impacts cancer susceptibility and serves as a target for anti-cancer therapies. Because RNR-regulated dNTP production can influence DNA replication fidelity while also supporting genome-protecting DNA repair, RNR has complex and stage-specific roles in carcinogenesis. Nevertheless, cancer cells are dependent on RNR for de novo dNTP biosynthesis. Therefore, elevated RNR expression is a characteristic of many cancers, and an array of mechanistically distinct RNR inhibitors serve as effective agents for cancer treatment. The dNTP metabolism machinery, including RNR, has been exploited for therapeutic benefit for decades and remains an important target for cancer drug development.

ΔNp63α regulates keratinocyte proliferation by controlling PTEN expression and localization

ΔNp63α, implicated as an oncogene, is upregulated by activated Akt, part of a well-known cell survival pathway. Inhibition of Akt activation by phosphatase and tensin homolog deleted on chromosome 10 (PTEN) and the presence of putative p63-binding sites in the pten promoter led us to investigate whether ΔNp63α regulates PTEN expression. Knockdown of ΔNp63α led to increases in PTEN levels and loss of activated Akt, while overexpression of ΔNp63α decreased PTEN levels and elevated active Akt. The repression of PTEN by ΔNp63α occurs independently of p53 status, as loss of ΔNp63α increases PTEN expression in cell lines with and without functional p53. In addition, decreased levels of ΔNp63α resulted in an increase in nuclear PTEN. Conversely, in vivo nuclear PTEN was absent in the proliferative basal layer of the epidermis where ΔNp63α expression is highest. Additionally, we show that in keratinocytes a balance between ΔNp63α and PTEN regulates Akt activation and maintains normal proliferation rates. This balance is disrupted in non-melanoma skin cancers through increased ΔNp63α levels, and could enhance proliferation and subsequent neoplastic development. Our studies show that ΔNp63α negatively regulates PTEN, thereby providing a feedback loop between PTEN, Akt and ΔNp63α, which has an integral role in skin cancer development.

TAp73 induction by nitric oxide: regulation by checkpoint kinase 1 (CHK1) and protection against apoptosis

Nitric oxide (NO) is a potent activator of the p53 tumor suppressor protein, thereby inducing cell cycle arrest and apoptosis. However, little is known about the regulation of the two other p53-family members, p63 and p73, by nitrogen oxides. We report here an up-regulation of p73 by NO in p53-null K-562 leukemia cells. Chemical NO prodrugs or macrophage iNOS activity induced an accumulation of the TAp73α isoform in these cells, whereas macrophages from iNOS(-/-) mice did not. NO also up-regulated TAp73 mRNA expression, suggesting a transcriptional regulation. The checkpoint kinases Chk1 and Chk2 can regulate TAp73 induction after DNA damage. We show that these kinases were rapidly phosphorylated upon NO treatment. Genetic silencing or pharmacological inhibition of Chk1 impaired NO-mediated accumulation of TAp73α. Because NO is known to block DNA synthesis through ribonucleotide reductase inhibition, the up-regulation of TAp73α might be caused by DNA damage induced by an arrest of DNA replication forks. In support of this hypothesis, DNA replication inhibitors such as hydroxyurea and aphidicolin similarly enhanced TAp73α expression and Chk1 phosphorylation. Moreover, inhibition of Chk1 also prevented TAp73α accumulation in response to replication inhibitors. The knockdown of TAp73 with siRNA sensitized K-562 cells to apoptosis induced by a nitrosative (NO) or oxidative (H(2)O(2)) injury. Therefore, TAp73α has an unusual cytoprotective role in K-562 cells, contrasting with its pro-apoptotic functions in many other cell models. In conclusion, NO up-regulates several p53 family members displaying pro- and anti-apoptotic effects, suggesting a complex network of interactions and cross-regulations between NO production and p53-related proteins.