Chk2 protects against radiation-induced genomic instability

Synthetic selenomelanin nanoparticles radio-sensitize non-melanocytic lung cancer (A549) cells by promoting G2/M arrest

Recent studies have postulated the natural existence of selenomelanin and its role in the radio-protection of healthy cells. The present study aimed to understand its radio-modulatory activity in non-melanocytic cancerous (A549) cells of lung origin. Briefly, selenomelanin was synthesized under laboratory conditions following the previously reported methodology. The various spectroscopic (electron paramagnetic resonance, X-ray photoelectron spectroscopy, atomic absorption spectroscopy, transmission electron microscopy and dynamic light scattering) analyses confirmed the formation of selenomelanin nanoparticles. The short-term (72 h) and long-term (14 days) toxicity profiling of selenomelanin by 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide (MTT) and clonogenic assays respectively revealed its half maximal inhibitory concentrations (IC50) of 72.03 ± 7.13 μg/ml and 0.85 ± 0.16 μg/ml respectively in A549 cells and of 81.56 ± 1.63 μg/ml and > 5 μg/ml respectively in healthy lung fibroblast (WI26) cells. Further, pre-treatment of selenomelanin (at concentrations non-toxic for WI26 cells) selectively augmented the radiosensitivity of A549 cells. Finally, mechanistic investigations in A549 cells revealed that selenomelanin increased the levels of reactive oxygen species, DNA damage and modulated the phospho-levels of CHK1 and CHK2 (effectors of cell cycle arrest) in the irradiated cells to favour G2/M arrest followed by cleavage of caspase 3 (effector of apoptosis). Together, the present study proposes the novel application of selenomelanin as a radiosensitizer to enhance the efficacy of radiotherapy in cancerous cells of lung origin.

Functional Activation of Mutant p53 by Platinum Analogues in Cisplatin-Resistant Cells Is Dependent on Phosphorylation

Dysfunctionality of the p53 tumor suppressor is a major cause of therapeutic drug resistance in cancer. Recently, we reported that mutant, but otherwise functional, p53^v172F was inactivated in cisplatin-resistant 2780CP/Cl-16 and 2780CP/Cl-24 human ovarian tumor cells by increased recruitment of the inhibitor MDM4. The current study demonstrates that, unlike cisplatin, platinum analogues oxaliplatin and DACH-diacetato-dichloro-Pt(IV) (DAP) strongly stabilize and activate p53^v172F in resistant cells, as indicated by prolonged p53 half-life and transactivation of targets p21 (CDKN1A) and MDM2. This increase in MDM2 reduced MDM4 levels in cell lysates as well as the p53 immunocomplex and prevented reversion of p53 to the inactive p53-MDM2-MDM4-bound state. Phosphorylation of p53 at Ser15 was demonstrated by all three drugs in sensitive A2780 and corresponding resistant 2780CP/Cl-16 and 2780CP/Cl-24 cell lines. However, cisplatin induced Ser20 phosphorylation in A2780 cells only, but not in resistant cells; in contrast, both DAP and oxaliplatin induced this phosphorylation in all three cell lines. The inference that Ser20 phosphorylation is more important for p53 activation was confirmed by ectopic expression of a phosphomimetic (S20D) mutant p53 that displayed reduced binding, relative to wild-type p53, to both MDM2 and MDM4 in p53-knockout A2780 cells. In consonance, temporal studies demonstrated drug-induced Ser15 phosphorylation coincided with p53 stabilization, whereas Ser20 phosphorylation coincided with p53 transactivation.Implications: Cisplatin fails to activate the pathway involved in phosphorylating mutant p53^v172F at Ser20 in resistant cells, but this phosphorylation is restored by oxaliplatin and DAP that reactivates p53 function and circumvents cisplatin resistance. Mol Cancer Res; 15(3); 328-39. ©2016 AACR.

Resistance and gain-of-resistance phenotypes in cancers harboring wild-type p53

Chemotherapy is the bedrock for the clinical management of cancer, and the tumor suppressor p53 has a central role in this therapeutic modality. This protein facilitates favorable antitumor drug response through a variety of key cellular functions, including cell cycle arrest, senescence, and apoptosis. These functions essentially cease once p53 becomes mutated, as occurs in ∼50% of cancers, and some p53 mutants even exhibit gain-of-function effects, which lead to greater drug resistance. However, it is becoming increasingly evident that resistance is also seen in cancers harboring wild-type p53. In this review, we discuss how wild-type p53 is inactivated to render cells resistant to antitumor drugs. This may occur through various mechanisms, including an increase in proteasomal degradation, defects in post-translational modification, and downstream defects in p53 target genes. We also consider evidence that the resistance seen in wild-type p53 cancers can be substantially greater than that seen in mutant p53 cancers, and this poses a far greater challenge for efforts to design strategies that increase drug response in resistant cancers already primed with wild-type p53. Because the mechanisms contributing to this wild-type p53 "gain-of-resistance" phenotype are largely unknown, a concerted research effort is needed to identify the underlying basis for the occurrence of this phenotype and, in parallel, to explore the possibility that the phenotype may be a product of wild-type p53 gain-of-function effects. Such studies are essential to lay the foundation for a rational therapeutic approach in the treatment of resistant wild-type p53 cancers.