Paraquat-induced cytoskeletal injury in cultured cells

Although the redox cycling of paraquat (PQ) and the resultant "activated oxygen" generation are important in toxicity development, the intracellular events leading to cell injury remain unclear. To understand the mechanism of PQ-induced cell injury, we have studied the effects of PQ on DNA synthesis, cell proliferation, the cytoskeletal organization, particularly microtubules (MT) and microfilaments (MF), and the synthesis and composition of cytoskeletal proteins in mouse 3T3 cells. PQ treatment produced a dose-dependent inhibition of DNA synthesis and cell growth. Exposure of cells to PQ (313 microM, 20 hr) resulted in MT aggregation and bundling as well as MF redistribution in the perinuclear area as revealed by fluorescence microscopy. Although this PQ dose inhibited DNA synthesis by 95%, it caused only a 22% decrease in protein synthesis of the cytoskeletal fraction. Higher doses of PQ (1250 microM, 20 hr) caused (a) dramatic thinning out and loss of MT and (b) marked loss of MF cables and the appearance of numerous pine needle-like structures much finer and shorter than normal MF. Under these conditions, the synthesis of cytoskeletal proteins was decreased by about 83%. Further analysis of the cytoskeletal fraction from PQ-treated cells by sodium dodecyl sulfate gel electrophoresis showed (a) that tubulin was greatly diminished, in agreement with microscopic observations; (b) that two new protein bands appeared; and (c) another protein band which was also reduced considerably. These results indicate that the PQ-induced dose-dependent cytoskeletal injury may be important to the mechanism of cytotoxicity of this herbicide.