D-Serine reduces the expression of the cytopathic genotoxin colibactin

Some Escherichia coli strains harbour the pks island, a 54 kb ge-nomic island encoding the biosynthesis genes for a genotoxic compound named colibactin. In eukaryotic cells, colibactin can induce DNA damage, cell cycle arrest and chromosomal instability. Production of colibactin has been implicated in the development of colorectal cancer (CRC). In this study, we demonstrate the inhibitory effect of D-Serine on the expression of the pks island in both prototypic and clinically-associated colibactin-producing strains and determine the implications for cytopathic effects on host cells. We also tested a comprehensive panel of proteinogenic L-amino acids and corre-sponding D-enantiomers for their ability to modulate clbB transcription. Whilst several D-amino acids exhibited the ability to inhibit expression of clbB, D-Serine exerted the strongest repressing activity (>3.8-fold) and thus, we focussed additional experiments on D-Serine. To investigate the cellular effect, we investigated if repression of colibactin by D-Serine could reduce the cytopathic responses normally observed during infection of HeLa cells with pks+ strains. Levels of gamma-H2AX (a marker of DNA double strand breaks) were reduced 2.75-fold in cells infected with D-Serine treatment. Moreover, expo-sure of pks+E. coli to D-Serine during infection caused a reduction in cellular senescence that was observable at 72 h post infection. The recent finding of an association between pks-carrying commensal E. coli and CRC, highlights the necessity for the development of colibactin targeting therapeutics. Here we show that D-Serine can reduce expression of colibactin, and inhibit down-stream cellular cytopathy, illuminating its potential to prevent colibactin-associated disease.

Automated summary

In this study, the inhibitory effect of D-Serine on the expression of the pks island in colibactin-producing strains was demonstrated. It was also found that D-Serine could reduce the cytopathic effects on host cells during infection with pks+ strains.