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Background and purpose:Exogenous peroxynitrite from nanomolar to micromolar concentrations exerts cardioprotection. Here, we have assessed its effects on ischaemia- and reperfusion-induced ventricular arrhythmias in vivo and a possible role for mitochondrial KATP channels in these effects, using the channel inhibitor 5-hydroxydecanoate (5-HD).Experimental approach:Chloralose-urethane-anaesthetized dogs were treated twice for 5 min with peroxynitrite (100 nM, by intracoronary infusions) in both the absence and presence of 5-HD (150 μg kg−1 min−1), and then subjected to 25 min occlusion of the left anterior descending coronary artery. The severity of ischaemia and of arrhythmias, as well as the levels of nitrotyrosine were assessed and compared with a group of control dogs, subjected only to a 25 min occlusion and reperfusion insult.Key results:Compared with controls, infusion of peroxynitrite markedly suppressed the number of ventricular premature beats (388±88 vs 133±44), the incidence of ventricular fibrillation both during occlusion (50% vs 10%) and reperfusion (100% vs 44%), and increased survival (0% vs 50%; all P<0.05). The severity of ischaemia (epicardial ST-segment changes, inhomogeneity of electrical activation) during occlusion and nitrotyrosine levels on reperfusion were significantly less in the peroxynitrite-treated dogs than in the controls. 5-HD did not modify the cardioprotective effects of peroxynitrite.Conclusion and implications:Exogenous peroxynitrite provided antiarrhythmic protection in vivo, which might have been on account of a reduction in endogenous peroxynitrite formation. This protection seemed not to be mediated through mitoKATP channels.British Journal of Pharmacology (2008) 155, 1015–1024; doi:10.1038/bjp.2008.344; published online 1 September 2008 [ABSTRACT FROM AUTHOR]

Multidrug resistance (MDR) is a serious hurdle to successful cancer therapy. Here, we examined the efficiency of novel semi-synthetic dihydrotestosterone derivatives, more specifically androstano-arylpyrimidines in inhibiting the efflux activity of ATP-binding cassette (ABC) transporters and sensitizing inherently MDR colon cancer cells to various chemotherapy drugs. Using the Rhodamine123 accumulation assay, we evaluated the efflux activity of cancer cells following treatments with androstano-arylpyrimidines. We found that acetylated compounds were capable of attenuating the membrane efflux of inherently MDR cells; however, deacetylated counterparts were ineffective. To delineate the possible molecular mechanisms underlying these unique activities of androstano-arylpyrimidines, the degree of apoptosis induction was assessed by AnnexinV-based assays, both upon the individual as well as by steroid and chemotherapy agent combination treatments. Five dihydrotestosterone derivatives applied in combination with Doxorubicin or Epirubicin triggered massive apoptosis in MDR cells, and these combinations were more efficient than chemotherapy drugs together with Verapamil. Furthermore, our results revealed that androstano-arylpyrimidines induced significant endoplasmic reticulum stress (ER stress) but did not notably modulate ABC transporter expression. Therefore, ER stress triggered by acetylated androstano-arylpyrimidines is probably involved in the mechanism of efflux pump inhibition and drug sensitization which can be targeted in future drug developments to defeat inherently multidrug-resistant cancer.

The p53 tumour suppressor regulates the transcription initiation of selected genes by binding to specific DNA sequences at their promoters. Here we report a novel role of p53 in transcription elongation in human cells. Our data demonstrate that upon transcription elongation blockage, p53 is associated with genes that have not been reported as its direct targets. p53 could be co-immunoprecipitated with active forms of DNA-directed RNA polymerase II subunit 1 (RPB1), highlighting its association with the elongating RNA polymerase II. During a normal transcription cycle, p53 and RPB1 are localised at distinct regions of selected non-canonical p53 target genes and this pattern of localisation was changed upon blockage of transcription elongation. Additionally, transcription elongation blockage induced the proteasomal degradation of RPB1. Our results reveal a novel role of p53 in human cells during transcription elongation blockage that may facilitate the removal of RNA polymerase II from DNA.