Okadaic Acid and Calyculin A Reverse Sulfur Mustard-Induced G2/M Cell-cycle Block in Human Keratinocytes

Abstract

Sulfur mustard is a chemical warfare agent that has been shown to produce G 2 /M cell-cycle block of human cells. Because protein phosphatase 2A (PP2A) has been implicated in the control of the normal G 2 /M transition, studies were undertaken to determine the role of PP2A in sulfur mustard-induced G 2 /M block of normal human epidermal keratinocytes. Sulfur mustard-induced G 2 /M cell-cycle block was characterized by markedly increased levels of cycling Bl and hyperphosphorylated cdc2 kinase. The G 2 /M block was completely reversed by a 4-h exposure to 500 nM okadaic acid or 10 nM calyculin A, potent inhibitors of PP2A. Reversal of the block was indicated by a loss of both hyperphosphorylated cdc2 kinase and cyclin B1. Lower concentrations of okadaic acid or calyculin A produced little or no reversal of sulfur mustard-induced G 2 /M block. Tautomycin, a potent inhibitor of protein phosphatase 1 (PP1), was ineffective at concentrations up to 500 nM. Total cellular PP2A activity was not affected by sulfur mustard treatment. These results suggest that PP2A is involved in the G 2 /M block produced by exposure of normal human epidermal keratinocytes to low concentration of sulfur mustard.