THE EFFECT OF PHOSPHONOFORMIC ACID OH WILD TYPE AND MUTANT S49 CELL LINES: 1

Abstract

Phosphonoformic acid (PFA) and its congener phosphonoacetic acid (PAA) are inhibitors of viral (and to a less extent mammalian) DNA polymerase. We selected mutants of S49 cells (a mouse T lymphoma line) resistant to 3mM phosphonoformic acid. These 11 lines had a range of growth rates, cell cycle distribution abnormalities, and concomitant resistance to the inhibitory effects of thymidine, acycloguanosine (acyclovir), aphidicolin, deoxyadenosine, and novobiocin. Most lines had normal to slightly elevated pools of ribonucleoside triphosphates and deoxyribonucleoside triphosphates. However, one line (PFA 3-9) had a greatly elevated dCTP pool and increased CDP reducatase activity in permeabilized cells. This activity in the PFA 3-9 cells diminished to wild type control levels in the presence of phosphonoformic acid, while PFA greatly diminished wild type CDP reductase activity (see table). In addition, 4mM PFA diminished the dCTP pool in wild type and in PFA 3-9 cells at 8 hours. The reduced dCTP pool could be increased by exogenous deoxycytidine but this only marginally reversed PFA toxicity. These observations suggest that PFA is an inhibitor of mammalian ribonucleotide reductase and partial resistance to PFA can be effected by mutation to increased CDP reductase activity resulting in a large dCTP pool.