Effect of a dCTP:dTTP pool imbalance on DNA replication fidelity in Friend murine erythroleukemia cells

Different responses of colorectal cancer cells to alternative sequences of cetuximab and oxaliplatin

Therapeutic protocols including EGFR antibodies in the context of oxaliplatin-based regimens have variable clinical effect in colorectal cancer. Here, we tested the effect of the EGFR antibody cetuximab in different sequential combinations with oxaliplatin on the growth of colorectal cancer cells in vitro and in vivo. Cetuximab reduced the efficacy of oxaliplatin when administered before oxaliplatin but provided additive effect when administered after oxaliplatin regardless of the KRAS or BRAF mutation status of the cells. Systemic gene expression and protein phosphorylation screens revealed alternatively activated pathways regulating apoptosis, cell cycle and DNA damage response. Functional assays indicated that cetuximab-induced arrest of the cells into the G1 phase of the cell cycle was associated with reduced responsiveness of the cells to subsequent treatment with oxaliplatin. In contrast, oxaliplatin-enhanced responsiveness to subsequent treatment with cetuximab was associated with increased apoptosis, inhibition of STAT3 activity and increased EGFR down-regulation. This preclinical study indicates that optimizing the sequence of administration may enhance the antitumor effect of combination therapy with EGFR antibodies and oxaliplatin.

Differences in the Detection of BrdU/EdU Incorporation Assays Alter the Calculation for G1, S, and G2 Phases of the Cell Cycle in Trypanosomatids

Trypanosomatids are the etiologic agents of various infectious diseases in humans. They diverged early during eukaryotic evolution and have attracted attention as peculiar models for evolutionary and comparative studies. Here, we show a meticulous study comparing the incorporation and detection of the thymidine analogs BrdU and EdU in Leishmania amazonensis, Trypanosoma brucei, and Trypanosoma cruzi to monitor their DNA replication. We used BrdU- and EdU-incorporated parasites with the respective standard detection approaches: indirect immunofluorescence to detect BrdU after standard denaturation (2 M HCl) and "click" chemistry to detect EdU. We found a discrepancy between these two thymidine analogs due to the poor detection of BrdU, which is reflected on the estimative of the duration of the cell cycle phases G1, S, and G2. To solve this discrepancy, we increase the exposure of incorporated BrdU using different concentrations of HCl. Using a new value for HCl concentration, we re-estimated the phases G1, S, G2 + M, and cytokinesis durations, confirming the values found by this approach using EdU. In conclusion, we suggest that the studies using BrdU with standard detection approach, not only in trypanosomatids but also in others cell types, should be reviewed to ensure an accurate estimation of DNA replication monitoring.

FISH comets show that the salvage enzyme TK1 contributes to gene-specific DNA repair

Thymidine kinase 1 (TK1) is a salvage enzyme that phosphorylates thymidine, imported from surrounding fluids, to create dTMP, which is further phosphorylated to the DNA precursor dTTP. TK1 deficiency has for a long time been known to cause increased cellular sensitivity to DNA damage. We have examined preferential strand break repair of DNA domains in TK1(+) and TK1(-) clones of the Raji cell line, by the Comet-FISH technique, in bulk DNA and in the actively transcribed tumor suppressor (TP53) and human telomerase reverse transcriptase (hTERT) gene regions, over 1 h after 5Gy γ-irradiation. Results showed that repair of the TP53 and hTERT gene regions was more efficient in TK1(+) compared to TK1(-) cells, a trend also reflected to a lesser degree in genomic DNA repair between the cell-lines. The targeted gene-specific repair in TK(+) cells occurred rapidly, mainly over the first 15 min repair-period. Therefore, TK1 is needed for preferential repair of actively transcribed regions, through a previously unsuspected mechanism. In principle, TK1 could exert its protective effects through supply of a supplementary dTTP pool for accurate repair of damaged genes; but Raji TK1(+) cells in thymidine free media still show preferential repair of transcribed regions. TK1 therefore does not exert its protective effects through dTTP pools, but through another unidentified mechanism, which affects sensitivity to and mutagenicity by DNA damaging agents.