Effect of methylmercury (II) on the synthesis of deoxyribonucleic acid, ribonucleic acid and protein in HeLa S3 cells

Abstract

Exposing HeLa S3 cells to increasing concentrations of methylmercury CH 3Hg(II) for varying periods of time results in an abrupt cessation of intracellular events, such as DNA, RNA and protein syntheses, once the organomercurial concentration in the growth medium has exceeded a certain threshold, the value of which lies between 1 and 10 μM. The synthetic activities, expressed as the ability of the cells to incorporate 3H-labeled thymidine, uridine and leucine into the acid-insoluble fraction of the cells, decrease exponentially with time at a given methylmercury concentration and, at a given time, they cease in an abrupt, highly cooperative manner when plotted against the logarithm of the CH 3Hg(II) concentration, yielding sigmoidally shaped dose-response curves. The inhibition of intracellular macromolecular synthesis is reversible up to a certain CH 3Hg(II) level. For instance, cells that had been kept for 6 hr at CH 3Hg(II) concentrations ranging from 2.5 to 10 μM did recover, albeit with lag periods that increased in length with increasing organomercurial concentration, as evidenced by their ability to continue RNA synthesis once the intoxicating medium had been removed. However, cells that had been exposed to 20 μM CH 3Hg(II) and above did not recover, at least not at incubation periods up to 91 hr (4.3 generation times), after termination of the 6 hr methylmercury treatment. In general, macromolecular synthesis decreases in the sequence DNA ~ RNA > protein, followed by cell death as soon as protein synthesis has become inhibited to a major degree. The acute effects of CH 3Hg(II) on intracellular DNA, RNA and protein syntheses were marginally different when HeLa S3 cells were exposed to methylmercury in three modes: as monolayers, as suspensions prepared from freshly trypsinized monolayer cells, and as suspension-cultures, demonstrating that, for instance, trypsin treatment of the cells increases their sensitivity toward methylmercury intoxication only slightly.