Evidence for APOBEC3B mRNA and protein expression in oral squamous cell carcinomas

It has been demonstrated that APOBEC3B possesses cytidine deaminase activity, which is likely to result in C-to-T signature mutations. Increased expression of the APOBEC3B gene has been shown to correlate with higher incidence of such mutations in various cancer types, such as breast, bladder, lung, and head and neck carcinomas. In the current study, we used in silico methods, immunohistochemistry and qRT-PCR to detect the presence of APOBEC3B signature mutations and examine the levels and patterns of APOBEC3B expression in oral squamous cell carcinomas (OSCCs). Using the Cancer Genome Atlas (TCGA) database, we have found a high incidence of C-to-T transitions in head and neck squamous cell carcinomas (HNSCCs), of which OSCCs constitute the largest subgroup. Additionally, we compared APOBEC3B expression, at both mRNA and protein level, between OSCCs and non-cancerous samples. APOBEC3B was detected in both groups, but nuclear localization was consistent only in normal oral cells. APOBEC3B mRNA levels were clearly higher in OSCCs than in controls. These results suggest that while in normal oral cells APOBEC3B has an important nuclear function to fulfill, this activity may be hindered in a subgroup of tumor cells, due to the more prominent localization of the enzyme in the cytoplasm.

Nicotinamide treatment reduces the levels of histone H3K4 trimethylation in the promoter of the mper1 circadian clock gene and blocks the ability of dexamethasone to induce the acute response

Circadian rhythms, which measure time on a scale of 24h, are generated by one of the most ubiquitous endogenous mechanisms, the circadian clock. SIRT1, a class III histone deacetylase, and PARP-1, a poly(ADP-ribose) polymerase, are two NAD(+)-dependent enzymes that have been shown to be involved in the regulation of the clock. Here we present evidence that the metabolite nicotinamide, an inhibitor of SIRT1, PARP-1 and mono(ADP-ribosyl) transferases, blocks the ability of dexamethasone to induce the acute response of the circadian clock gene, mper1, while it concomitantly reduces the levels of histone H3 trimethylation of lysine 4 (H3K4me3) in the mper1 promoter. Moreover, application of alternative inhibitors of SIRT1 and ADP-ribosylation did not lead to similar results. Therefore, inhibition of these enzymes does not seem to be the mode by which NAM exerts these effects. These results suggest the presence of a novel mechanism, not previously documented, by which NAM can alter gene expression levels via changes in the histone H3K4 trimethylation state.