The 19S proteasome subunit Rpn7 stabilizes DNA damage foci upon genotoxic insult

Hepatitis B virus infection disrupts homologous recombination in hepatocellular carcinoma by stabilizing resection inhibitor ADRM1

Many cancers harbor homologous recombination defects (HRDs). A HRD is a therapeutic target that is being successfully utilized in treatment of breast/ovarian cancer via synthetic lethality. However, canonical HRD caused by BRCAness mutations do not prevail in liver cancer. Here we report a subtype of HRD caused by the perturbation of a proteasome variant (CDW19S) in hepatitis B virus-bearing (HBV-bearing) cells. This amalgamate protein complex contained the 19S proteasome decorated with CRL4WDR70 ubiquitin ligase, and assembled at broken chromatin in a PSMD4Rpn10- and ATM-MDC1-RNF8-dependent manner. CDW19S promoted DNA end processing via segregated modules that promote nuclease activities of MRE11 and EXO1. Contrarily, a proteasomal component, ADRM1Rpn13, inhibited resection and was removed by CRL4WDR70-catalyzed ubiquitination upon commitment of extensive resection. HBx interfered with ADRM1Rpn13 degradation, leading to the imposition of ADRM1Rpn13-dependent resection barrier and consequent viral HRD subtype distinguishable from that caused by BRCA1 defect. Finally, we demonstrated that viral HRD in HBV-associated hepatocellular carcinoma can be exploited to restrict tumor progression. Our work clarifies the underlying mechanism of a virus-induced HRD subtype.

Dysregulation of Caveolin-1 Phosphorylation and Nuclear Translocation Is Associated with Senescence Onset

We recently reported that the inability of osteoarthritic (OA) chondrocytes to repair oxidative stress (OS) induced DNA damage is linked to Cav-1 overexpression/improper localization. We speculated that the senescent status of OA cells was responsible for this Cav-1 dysregulation. Here, to further investigate this hypothesis, we used Wharton Jelly derived mesenchymal stem cells (WJ-MSCs) and investigated Cav-1 function as cells reached replicative senescence or upon stress induced senescence (SIPS). We showed that Cav-1 is upregulated, phosphorylated and translocated to the nucleus in young WJ-MSCs upon acute exogenous OS, and that it returns back to basal/nonphosphorylated levels and exports the nucleus in the recovery phase. However, as cells reach senescence, this regulation is lost. OS did not induce any Cav-1-mediated response, which is concomitant with the inability of older cells to restore DNA damage. Furthermore, downregulation of Cav-1 resulted in persistent OS-induced DNA damage and subsequent onset of senescence. We also report that the establishment of senescence is mediated by autophagy stimulation, since downregulation of autophagy key molecule Atg5, simultaneously with Cav-1 downregulation, was found to inhibit SIPS. Basically, we propose that Cav-1 involvement in DNA damage response can lead to senescence, either because the damage is extensive or because Cav-1 is absent/unable to perform its homeostatic role.

The Intra- and Extra-Telomeric Role of TRF2 in the DNA Damage Response

Telomere repeat binding factor 2 (TRF2) has a well-known function at the telomeres, which acts to protect the telomere end from being recognized as a DNA break or from unwanted recombination. This protection mechanism prevents DNA instability from mutation and subsequent severe diseases caused by the changes in DNA, such as cancer. Since TRF2 actively inhibits the DNA damage response factors from recognizing the telomere end as a DNA break, many more studies have also shown its interactions outside of the telomeres. However, very little has been discovered on the mechanisms involved in these interactions. This review aims to discuss the known function of TRF2 and its interaction with the DNA damage response (DDR) factors at both telomeric and non-telomeric regions. In this review, we will summarize recent progress and findings on the interactions between TRF2 and DDR factors at telomeres and outside of telomeres.

Oxidative Stress Response Is Mediated by Overexpression and Spatiotemporal Regulation of Caveolin-1

Oxidative stress (OS) has been linked to the aetiology of many diseases including osteoarthritis (OA). Recent studies have shown that caveolin-1—a structural protein of plasma membrane’s caveolae—is upregulated in response to OS. Here, we explore the function of caveolin-1 in chondrocytes derived from healthy individuals (control) and OA patients that were subjected to exogenous OS. We showed that caveolin-1 was upregulated in response to acute OS in the control, but not in OA chondrocytes. Moreover, OS-induced DNA damage analysis revealed that control cells started repairing the DNA lesions 6 h post-oxidative treatment, while OA cells seemed unable to restore these damages. Importantly, in the control cells, we observed a translocation of caveolin-1 from the membrane/cytoplasm in and out of the nucleus, which coincided with the appearance and restoration of DNA lesions. When caveolin-1 was prevented from translocating to the nucleus, the control cells were unable to repair DNA damage. In OA cells, no such translocation of caveolin-1 was observed, which could account for their inability to repair DNA damage. Taken together, these results provide novel insights considering the role of caveolin-1 in response to OS-induced DNA damage while revealing its implication in the pathophysiology of OA.

Folate and Pegylated Aliphatic Polyester Nanoparticles for Targeted Anticancer Drug Delivery

Purpose

The use of chemotherapeutic agents to combat cancer is accompanied by high toxicity due to their inability to discriminate between cancer and normal cells. Therefore, cancer therapy research has focused on the targeted delivery of drugs to cancer cells. Here, we report an in vitro study of folate-poly(ethylene glycol)-poly(propylene succinate) nanoparticles (FA-PPSu-PEG-NPs) as a vehicle for targeted delivery of the anticancer drug paclitaxel in breast and cervical cancer cell lines.

Methods

Paclitaxel-loaded-FA-PPSu-PEG-NPs characterization was performed by in vitro drug release studies and cytotoxicity assays. The NPs cellular uptake and internalization mechanism were monitored by live-cell imaging in different cancer cell lines. Expression of folate receptor-α (FOLR1) was examined in these cell lines, and specific FOLR1-mediated entry of the FA-PPSu-PEG-NPs was investigated by free folic acid competition. Using inhibitors for other endocytic pathways, alternative, non-FOLR1 dependent routes for NPs uptake were also examined.

Results

Drug release experiments of Paclitaxel-loaded PPSu-PEG-NPs indicated a prolonged release of Paclitaxel over several days. Cytotoxicity of Paclitaxel-loaded PPSu-PEG-NPs was similar to free drug, as monitored in cancer cell lines. Live imaging of cells treated with either free Paclitaxel or Paclitaxel-loaded PPSu-PEG-NPs demonstrated tubulin-specific cell cycle arrest, with similar kinetics. Folate-conjugated NPs (FA-PPSu-PEG-NPs) targeted the FOLR1 receptor, as shown by free folic acid competition of the FA-PPSu-PEG-NPs cellular uptake in some of the cell lines tested. However, due to the differential expression of FOLR1 in the cancer cell lines, as well as the intrinsic differences between the different endocytic pathways utilized by different cell types, other mechanisms of nanoparticle cellular entry were also used, revealing that dynamin-dependent endocytosis and macropinocytosis pathways mediate, at least partially, cellular entry of the FA-PPSu-PEG NPs.

Conclusion

Our data provide evidence that Paclitaxel-loaded-FA-PPSu-PEG-NPs can be used for targeted delivery of the drug, FA-PPSu-PEG-NPs can be used as vehicles for other anticancer drugs and their cellular uptake is mediated through a combination of FOLR1 receptor-specific endocytosis, and macropinocytosis. The exploration of the different cellular uptake mechanisms could improve treatment efficacy or allow a decrease in dosage of anticancer drugs.

'Stemness' and 'senescence' related escape pathways are dose dependent in lung cancer cells surviving post irradiation

AIMS

Lung cancer is one of the main causes of cancer-related deaths worldwide and radiotherapy is a major treatment of choice. However, radioresistance is a main reason for radiotherapy failure or tumor relapse. Here, we investigated possible mechanisms associated with cancer cell radioresistance.

MATERIALS AND METHODS

We compared two newly derived cell lines, namely A549-IR3 and A549-IR6, which survived repeated (3 or 6 times) 4 Gy exposure of parental A549 lung cancer cell line. DNA repair ability, stemness and senescence were comparatively studied.

KEY FINDINGS

A549-IR3 exhibited higher proliferation ability and radioresistance compared to parental and A549-IR6 cells. Enhanced radioresistance was not accompanied by chemoresistance to cisplatin or docetaxel. DNA repair kinetics (γΗ2ΑΧ expression) were similar in all cell lines. A549-IR3 cells exhibited a significant rise in stem cell markers (CD44, CD133, OCT4, SOX2 and NANOG) whereas A549-IR6 displayed an increased senescent population.

SIGNIFICANCE

Cancer cells surviving after radiotherapy may follow two different escape pathways: selection for radioresistance resulting in regrowth, and in clinical terms relapse, or above an irradiation threshold, stem-cells die and cancer cells become senescent, leading the tumor to a state of dormancy.

In-depth Analysis of the Lid Subunits Assembly Mechanism in Mammals

The 26S proteasome is a key player in the degradation of ubiquitinated proteins, comprising a 20S core particle (CP) and a 19S regulatory particle (RP). The RP is further divided into base and lid subcomplexes, which are assembled independently from each other. We have previously demonstrated the assembly pathway of the CP and the base by observing assembly intermediates resulting from knockdowns of each proteasome subunit and the assembly chaperones. In this study, we examine the assembly pathway of the mammalian lid, which remains to be elucidated. We show that the lid assembly pathway is conserved between humans and yeast. The final step is the incorporation of Rpn12 into the assembly intermediate consisting of two modular complexes, Rpn3-7-15 and Rpn5-6-8-9-11, in both humans and yeast. Furthermore, we dissect the assembly pathways of the two modular complexes by the knockdown of each lid subunit.

The autophagic response to oxidative stress in osteoarthritic chondrocytes is deregulated

It has been reported that oxidative stress (OS) is involved in the pathogenesis of osteoarthritis (OA) and that defective autophagy is accompanying this age-related disease. Moreover, it has been proposed that induction of autophagy could serve as therapeutic mean, as it was shown to alleviate several symptoms in OA animal models. On the contrary, it is also known that autophagic death, which results from over-activation of autophagy, is also a contributor in the development of this disease. Given this discrepancy, in this study we aimed at analysing the autophagic response against acute exogenous oxidative insult of chondrocytes from healthy individuals (control) and OA patients (OA). Cells were treated with sublethal concentrations of hydrogen peroxide (H₂O₂) and then allowed to recover for different periods of time. Firstly, mRNA levels of autophagy-related genes (ATG5, Beclin-1 and LC3) were found significantly reduced in OA chondrocytes compared to control chondrocytes under physiological conditions. After the exposure to OS, in control cells mRNA and protein levels of these genes initially increased and decreased back to their basal levels 6-24 h after treatment. On the contrary, in OA chondrocytes the levels of autophagy-related genes remained high even 24 h post-treatment, indicating their inability to attenuate autophagy. Under the same conditions, the staining pattern of LC3, known marker of autophagosome formation, was analysed, and possible morphological differences between mitochondria of control and OA cells were microscopically assessed. These analyses revealed higher number of impaired mitochondria as well as increased autophagosome formation in OA cells as compared to control cells at all time points. Taken together, our results demonstrate a deregulation of the autophagic response against the oxidative insult in OA chondrocytes and offers insights on autophagy's role in the progression of OA.

Human mesenchymal stem cells with enhanced telomerase activity acquire resistance against oxidative stress-induced genomic damage

BACKGROUND

Human mesenchymal stem cells (MSC) are important tools for several cell-based therapies. However, their use in such therapies requires in vitro expansion during which MSCs quickly reach replicative senescence. Replicative senescence has been linked to macromolecular damage, and especially oxidative stress-induced DNA damage. Recent studies on the other hand, have implicated telomerase in the cellular response to oxidative damage, suggesting that telomerase has a telomere-length independent function that promotes survival.

METHODS

Here, we studied the DNA damage accumulation and repair during in vitro expansion as well as after acute external oxidative exposure of control MSCs and MSCs that overexpress the catalytic subunit of telomerase (hTERT MSCs).

RESULTS

We showed that hTERT MSCs at high passages have a significant lower percentage of DNA lesions as compared to control cells of the same passages. Additionally, less damage was accumulated due to external oxidative insult in the nuclei of hTERT overexpressing cells as compared to the control cells. Moreover, we demonstrated that oxidative stress leads to diverse nucleus malformations, such as multillobular nuclei or donut-shaped nuclei, in the control cells whereas hTERT MSCs showed significant resistance to the formation of such defects. Finally, hTERT MSCs were found to possess higher activities of the basic antioxidant enzymes, superoxide dismutase and catalase, than control MSCs.

DISCUSSION

On the basis of these results, we propose that hTERT enhancement confers resistance to genomic damage due to the amelioration of the cell's basic antioxidant machinery.

Effect of ionizing radiation exposure on Trypanosoma cruzi ubiquitin-proteasome system

In recent years, proteasome involvement in the damage response induced by ionizing radiation (IR) became evident. However, whether proteasome plays a direct or indirect role in IR-induced damage response still unclear. Trypanosoma cruzi is a human parasite capable of remarkable high tolerance to IR, suggesting a highly efficient damage response system. Here, we investigate the role of T. cruzi proteasome in the damage response induced by IR. We exposed epimastigotes to high doses of gamma ray and we analyzed the expression and subcellular localization of several components of the ubiquitin-proteasome system. We show that proteasome inhibition increases IR-induced cell growth arrest and proteasome-mediated proteolysis is altered after parasite exposure. We observed nuclear accumulation of 19S and 20S proteasome subunits in response to IR treatments. Intriguingly, the dynamic of 19S particle nuclear accumulation was more similar to the dynamic observed for Rad51 nuclear translocation than the observed for 20S. In the other hand, 20S increase and nuclear translocation could be related with an increase of its regulator PA26 and high levels of proteasome-mediated proteolysis in vitro. The intersection between the opposed peaks of 19S and 20S protein levels was marked by nuclear accumulation of both 20S and 19S together with Ubiquitin, suggesting a role of ubiquitin-proteasome system in the nuclear protein turnover at the time. Our results revealed the importance of proteasome-mediated proteolysis in T. cruzi IR-induced damage response suggesting that proteasome is also involved in T. cruzi IR tolerance. Moreover, our data support the possible direct/signaling role of 19S in DNA damage repair. Based on these results, we speculate that spatial and temporal differences between the 19S particle and 20S proteasome controls proteasome multiple roles in IR damage response.

Loss of urokinase receptor sensitizes cells to DNA damage and delays DNA repair

DNA damage induced by numerous exogenous or endogenous factors may have irreversible consequences on the cell leading to cell cycle arrest, senescence and cell death. The DNA damage response (DDR) is powerful signaling machinery triggered in response to DNA damage, to provide DNA damage recognition, signaling and repair. Most anticancer drugs induce DNA damage, and DNA repair in turn attenuates therapeutic efficiency of those drugs. Approaches delaying DNA repair are often used to increase efficiency of treatment. Recent data show that ubiquitin-proteasome system is essential for signaling and repair of DNA damage. However, mechanisms providing regulation of proteasome intracellular localization, activity, and recruitment to DNA damage sites are elusive. Even less investigated are the roles of extranuclear signaling proteins in these processes. In this study, we report the involvement of the serine protease urokinase-type plasminogen activator receptor (uPAR) in DDR-associated regulation of proteasome. We show that in vascular smooth muscle cells (VSMC) uPAR activates DNA single strand break repair signaling pathway. We provide evidence that uPAR is essential for functional assembly of the 26S proteasome. We further demonstrate that uPAR mediates DNA damage-induced phosphorylation, nuclear import, and recruitment of the regulatory subunit PSMD6 to proteasome. We found that deficiency of uPAR and PSMD6 delays DNA repair and leads to decreased cell survival. These data may offer new therapeutic approaches for diseases such as cancer, cardiovascular and neurodegenerative disorders.

STUbLs in chromatin and genome stability

Chromatin structure and function is based on the dynamic interactions between nucleosomes and chromatin-associated proteins. In addition to the other post-translational modifications considered in this review issue of Biopolymers, ubiquitin and SUMO proteins also have prominent roles in chromatin function. A specialized form of modification that involves both, referred to as SUMO-targeted ubiquitin ligation, or STUbL [Perry, Tainer, and Boddy, Trends Biochem Sci, 2008, 33, 201-208], has significant effects on nuclear functions, ranging from gene regulation to genomic stability. Intersections between SUMO and ubiquitin in protein modification have been the subject of a recent comprehensive review [Praefcke, Hofmann, and Dohmen, Trends Biochem Sci, 2012, 37, 23-31]. Our goal here is to focus on features of enzymes with STUbL activity that have been best studied, particularly in relation to their nuclear functions in humans, flies, and yeasts. Because there are clear associations of disease and development upon loss of STUbL activities in metazoans, learning more about their function, regulation, and substrates will remain an important goal for the future.

Protein damage, repair and proteolysis

Proteins are continuously affected by various intrinsic and extrinsic factors. Damaged proteins influence several intracellular pathways and result in different disorders and diseases. Aggregation of damaged proteins depends on the balance between their generation and their reversal or elimination by protein repair systems and degradation, respectively. With regard to protein repair, only few repair mechanisms have been evidenced including the reduction of methionine sulfoxide residues by the methionine sulfoxide reductases, the conversion of isoaspartyl residues to L-aspartate by L-isoaspartate methyl transferase and deglycation by phosphorylation of protein-bound fructosamine by fructosamine-3-kinase. Protein degradation is orchestrated by two major proteolytic systems, namely the lysosome and the proteasome. Alteration of the function for both systems has been involved in all aspects of cellular metabolic networks linked to either normal or pathological processes. Given the importance of protein repair and degradation, great effort has recently been made regarding the modulation of these systems in various physiological conditions such as aging, as well as in diseases. Genetic modulation has produced promising results in the area of protein repair enzymes but there are not yet any identified potent inhibitors, and, to our knowledge, only one activating compound has been reported so far. In contrast, different drugs as well as natural compounds that interfere with proteolysis have been identified and/or developed resulting in homeostatic maintenance and/or the delay of disease progression.