Mitotic aberrations induced by carbaryl reflect tyrosine kinase inhibitionwith coincident up-regulation of serine threonine protein phosphatase activity: implications for coordination of karyokinesis and cytokinesis

The insecticide carbaryl and its metabolite 1-naphthol cause partial uncoup ling of karyokinesis and cytokinesis in V79 Chinese hamster fibroblasts; ka ryokinesis is blocked in metaphase, the microtubules of the spindle depolym erize and the chromosomes and spindle remnants become displaced to the peri phery of the cell. A high frequency of these disturbed cells elongate and a smaller fraction initiate a cleavage furrow. Here, we attempt to determine the potential targets for carbaryl and 1-naphthol in cytokinesis-specific signalling, led by the fact that the potential protein phosphatase inhibito r 1-naphthyl phosphate was previously identified in treated cells. We found that the typical cytological pattern induced by carbaryl and 1-naphthol co uld be obtained with tyrphostins, specific tyrosine kinase inhibitors, indi cating that the carbaryl-induced effects could be due to tyrosine kinase in hibition. This was confirmed by tyrosine kinase assays showing that carbary l, 1-naphthol and 2-naphthol were equally efficient at inhibiting tyrosine kinase activity as tyrphostin B44(-). As tyrosine kinases can act as regula tory factors in determining dephosphorylation rates, the activities of type -1 (PP1) and type-2A (PP2A) serine/threonine protein phosphatases were also determined. There was a clear up-regulation of the overall PP1/PP2A activi ties in cells treated with carbaryl, 1-naphthol or tyrphostin B44(-), This stimulation was shown to be indirect because these compounds had no effect on the activity of purified human PP1 in the test tube. 2-Naphthol, which h as been found to be less efficient with regard to displacement of chromatin , did not cause up-regulation, but a significant decrease in PP1/PP2A activ ity. We suggest that a net decrease in tyrosine kinase activity in combinat ion with a net increase in PP1/PP2A activity is a precondition for cell elo ngation and cytokinesis in mammalian cells and that the corresponding enzym es are targets in the network of activities serving to coordinate karyokine sis and cytokinesis.