XPC multifaceted roles beyond DNA damage repair: p53-dependent and p53-independent functions of XPC in cell fate decisions.

Xeroderma pigmentosum group C protein (XPC) acts as a DNA damage recognition factor for bulky adducts and as an initiator of global genome nucleotide excision repair (GG-NER). Novel insights have shown that the role of XPC is not limited to NER, but is also implicated in DNA damage response (DDR), as well as in cell fate decisions upon stress. Moreover, XPC has a proteolytic role through its interaction with p53 and casp-2S. XPC is also able to determine cellular outcomes through its interaction with downstream proteins, such as p21, ARF, and p16. XPC interactions with effector proteins may drive cells to various fates such as apoptosis, senescence, or tumorigenesis. In this review, we explore XPC's involvement in different molecular pathways in the cell and suggest that XPC can be considered not only as a genomic caretaker and gatekeeper but also as a tumor suppressor and cellular-fate decision maker. These findings envisage that resistance to cell death, induced by DNA-damaging therapeutics, in highly prevalent P53-deficent tumors might be overcome through new therapeutic approaches that aim to activate XPC in these tumors. Moreover, this review encourages care providers to consider XPC status in cancer patients before chemotherapy in order to improve the chances of successful treatment and enhance patients' survival.

Diphenyleneiodonium Triggers Cell Death of Acute Myeloid Leukemia Cells by Blocking the Mitochondrial Respiratory Chain, and Synergizes with Cytarabine.

Acute myeloid leukemia (AML) is characterized by the accumulation of undifferentiated blast cells in the bone marrow and blood. In most cases of AML, relapse frequently occurs due to resistance to chemotherapy. Compelling research results indicate that drug resistance in cancer cells is highly dependent on the intracellular levels of reactive oxygen species (ROS). Modulating ROS levels is therefore a valuable strategy to overcome the chemotherapy resistance of leukemic cells. In this study, we evaluated the efficiency of diphenyleneiodonium (DPI)-a well-known inhibitor of ROS production-in targeting AML cells. Results showed that although inhibiting cytoplasmic ROS production, DPI also triggered an increase in the mitochondrial ROS levels, caused by the disruption of the mitochondrial respiratory chain. We also demonstrated that DPI blocks mitochondrial oxidative phosphorylation (OxPhos) in a dose-dependent manner, and that AML cells with high OxPhos status are highly sensitive to treatment with DPI, which synergizes with the chemotherapeutic agent cytarabine (Ara-C). Thus, our results suggest that targeting mitochondrial function with DPI might be exploited to target AML cells with high OxPhos status.

Characterization of NADPH Oxidase Expression and Activity in Acute Myeloid Leukemia Cell Lines: A Correlation with the Differentiation Status.

In acute myeloid leukemia (AML), a low level of reactive oxygen species (ROS) is associated with leukemic stem cell (LSC) quiescence, whereas a high level promotes blast proliferation. ROS homeostasis relies on a tightly-regulated balance between the antioxidant and oxidant systems. Among the oxidants, NADPH oxidases (NOX) generate ROS as a physiological function. Although it has been reported in AML initiation and development, the contribution of NOX to the ROS production in AML remains to be clarified. The aim of this study was to investigate the NOX expression and function in AML, and to examine the role of NOX in blast proliferation and differentiation. First, we interrogated the NOX expression in primary cells from public datasets, and investigated their association with prognostic markers. Next, we explored the NOX expression and activity in AML cell lines, and studied the impact of NOX knockdown on cell proliferation and differentiation. We found that NOX2 is ubiquitously expressed in AML blasts, and particularly in cells from the myelomonocytic (M4) and monocytic (M5) stages; however, it is less expressed in LSCs and in relapsed AML. This is consistent with an increased expression throughout normal hematopoietic differentiation, and is reflected in AML cell lines. Nevertheless, no endogenous NOX activity could be detected in the absence of PMA stimulation. Furthermore, CYBB knockdown, although hampering induced NOX2 activity, did not affect the proliferation and differentiation of THP-1 and HL-60 cells. In summary, our data suggest that NOX2 is a marker of AML blast differentiation, while AML cell lines lack any NOX2 endogenous activity.

Repurposing of Acriflavine to Target Chronic Myeloid Leukemia Treatment.

Drug repurposing has lately received increasing interest in several diseases especially in cancers, due to its advantages in facilitating the development of new therapeutic strategies, by adopting a cost-friendly approach and avoiding the strict Food and Drug Administration (FDA) regulations. Acriflavine (ACF) is an FDA approved molecule that has been extensively studied since 1912 with antiseptic, trypanocidal, anti-viral, anti-bacterial and anti-cancer effects. ACF has been shown to block the growth of solid and hematopoietic tumor cells. Indeed, ACF acts as an inhibitor of various proteins, including DNA-dependent protein kinases C (DNA-PKcs), topoisomerase I and II, hypoxia-inducible factor 1α (HIF-1α), in addition to its recent discovery as an inhibitor of the signal transducer and activator of transcription (STAT). Chronic myeloid leukemia (CML) is a clonal myeloproliferative disorder characterized by the expression of the constitutively active tyrosine kinase BCR-ABL. This protein allows the activation of several signaling pathways known for their role in cell proliferation and survival, such as the JAK/STAT pathway. CML therapy, based on tyrosine kinase inhibitors (TKIs), such as imatinib (IM), is highly effective. However, 15% of patients are refractory to IM, where in some cases, 20-30% of patients become resistant. Thus, we suggest the repurposing of ACF in CML after IM failure or in combination with IM to improve the anti-tumor effects of IM. In this review, we present the different pharmacological properties of ACF along with its anti-leukemic effects in the hope of its repurposing in CML therapy.

VAS3947 Induces UPR-Mediated Apoptosis through Cysteine Thiol Alkylation in AML Cell Lines.

Nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOX) involvement has been established in the oncogenic cell signaling of acute myeloid leukemia (AML) cells and in the crosstalk with their niche. We have shown an expression of NOX subunits in AML cell lines while NOX activity is lacking in the absence of exogenous stimulation. Here, we used AML cell lines as models to investigate the specificity of VAS3947, a current NOX inhibitor. Results demonstrated that VAS3947 induces apoptosis in AML cells independently of its anti-NOX activity. High-performance liquid chromatography (HPLC) and mass spectrometry analyses revealed that VAS3947 thiol alkylates cysteine residues of glutathione (GSH), while also interacting with proteins. Remarkably, VAS3947 decreased detectable GSH in the MV-4-11 cell line, thereby suggesting possible oxidative stress induction. However, a decrease in both cytoplasmic and mitochondrial reactive oxygen species (ROS) levels was observed by flow cytometry without disturbance of mitochondrial mass and membrane potential. Thus, assuming the consequences of VAS3947 treatment on protein structure, we examined its impact on endoplasmic reticulum (ER) stress. An acute unfolded protein response (UPR) was triggered shortly after VAS3947 exposure, through the activation of inositol-requiring enzyme 1α (IRE1α) and PKR-like endoplasmic reticulum kinase (PERK) pathways. Overall, VAS3947 induces apoptosis independently of anti-NOX activity, via UPR activation, mainly due to aggregation and misfolding of proteins.

Novel Missense Mutations in BEST1 Are Associated with Bestrophinopathies in Lebanese Patients.

To identify Bestrophin 1 (BEST1) causative mutations in six Lebanese patients from three families, of whom four had a presumed clinical diagnosis of autosomal recessive bestrophinopathy (ARB) and two showed a phenotype with a single vitelliform lesion, patients were subjected to standard ophthalmic examinations. In addition, BEST1 exons and their flanking regions were amplified and sequenced by Sanger sequencing. Co-segregation and detailed bio-informatic analyses were performed. Clinical examination results were consistent with ARB diagnosis for all index patients showing multifocal vitelliform lesions and a markedly reduced light peak in the electrooculogram, including the two patients with a single vitelliform lesion. In all cases, most likely disease-causing BEST1 mutations co-segregated with the phenotype. The ARB cases showed homozygous missense variants (M1, c.209A>G, p.(Asp70Gly) in exon 3, M2, c.1403C>T; p.(Pro468Leu) in exon 10 and M3, c.830C>T, p.(Thr277Met) in exon 7), while the two patients with a single vitelliform lesion were compound heterozygous for M1 and M2. To our knowledge, this is the first study describing mutations in Lebanese patients with bestrophinopathy, where novel biallelic BEST1 mutations associated with two phenotypes were identified. Homozygous mutations were associated with multifocal lesions, subretinal fluid, and intraretinal cysts, whereas compound heterozygous ones were responsible for a single macular vitelliform lesion.