Mechanism of Selectivity of Arsenic Trioxide for Acute Promyelocytic Leukemia Cells from Screening a Genome Wide Set of Saccharomyces cerevisiae Deletion Strains.

All-trans retinoic acid (ATRA) targets the underlying molecular lesion in acute promyelocytic leukaemia (APL) and leads to differentiation of leukemic blasts into mature granulocytes. Treatment with ATRA, however, is associated with the retinoic acid syndrome and relapse is also a problem. In relapsed patients, arsenic trioxide is considered the treatment of choice. Arsenic trioxide is a trivalent arsenical that selectively kills APL cells. To better understand how arsenic trioxide perturbs cellular functions we chose to screen a genome-wide set of Saccharomyces cerevisiae deletion strains for sensitivity or resistance to the drug. The idea was that genes whose loss conferred sensitivity to the drug would reflect mechanisms of cell death, while genes whose loss conferred resistance would suggest mechanisms of selectivity. 7.6% of the 4,564 mutants were more sensitive to arsenic trioxide than the wild-type strain, while 1.5% was more resistant. In accordance with published studies in mammalian cells, yeast mutants lacking genes required for cytoskeleton stability, response to oxidative stress and DNA repair were prominent in the sensitive list. The most prominent genes in the resistant list were that encoding the plasma membrane transporter, Fps1, Hog1 and repressors of Hog1. Fps1 is an aquaglyceroporin that mediates plasma membrane glycerol flux and uptake of inorganic arsenic in yeast. Its activity is controlled by the high osmolarity MAPK signalling or HOG pathway. There was a clear correlation between genes involved in modulating HOG pathway activity and altered arsenic trioxide tolerance in yeast. In mammalian cells, aquaglyceroporin 9 is the human homolog of yeast Fps1 and the p38 MAPK signalling cascade constitutes the analogous pathway to HOG. APL is characterized by chromosomal rearrangements of 17q21 leading to the formation of fusion proteins involving retinoic acid receptor α. Notably, the fusion protein interacts with p38 kinase and modulates its activity. Our findings imply that modulation of p38 by the APL fusion protein leads to increased arsenic trioxide uptake via aquaglyceroporin 9 in APL cells. This mechanism may account for the selectivity of arsenic trioxide for APL cells.