Programmed cell death in 2 ',3 '-dideoxycytidine-resistant human monoblastoid U937 cells

2',3'-Dideoxycytidine is a powerful in vitro inhibitor of human immunodefic iency virus and is currently used in the treatment of acquired immunodefici ency syndrome. A long-term exposure of U937 monoblastoid cells to dideoxycy tidine induces the selection of drug-resistant cells (U937-R). In previous studies, we investigated some important biochemical properties and function al activities, such as basal respiration, protein kinase C activity, supero xide anion release, and the level of reduced glutathione, which were found to be higher in the drug-resistant cell line, compared to the parental one. In the present study, we evaluated the response of the two cell lines to t he induction of apoptosis by treatment with staurosporine and okadaic acid, which interfere with the protein kinase and phosphatase pathways, respecti vely. Moreover, knowing that GSH plays a crucial role in the regulation of nitric oxide-dependent apoptosis, U937-R and parental lines have been treat ed with SIN-1, which is known to generate significant amounts of O-2 and ni tric oxide. Resistant and parental cells have been analysed by light and electron micro scopy and agarose gel electrophoresis of isolated DNA has been performed. T he obtained results demonstrate a different susceptibility of U937-R cell l ine to apoptosis induced with the three triggers. U937-R cells show more ad vanced apoptotic features if compared with parental cells, after staurospor ine treatment. Differently, the okadaic acid does not induce a different be haviour in the two models. On the contrary, the agent SIN-1 determines an i ncreased number of apoptotic cells in the U937 line. The results suggest th at a higher level of protein kinase C and glutathione could prevent program med cell death in U937-R.