Differential effects of nucleoside analogs on oxidative phosphorylation in human pancreatic cells

Although nucleoside analogs as a group inhibit mtDNA replication, individually they target specific organs for toxicity. For example, dideoxyinosine (ddI) is most closely associated with clinical pancreatitis and dideoxycytosine (ddC) with peripheral neuropathy. Comparison of the differential effects of these analogs on mitochondrial function in relevant human cell lines could provide general clues as to the mechanisms of their differential toxicity. We compared the effects of ddI [and its intracellular metabolite dideoxyadenosine (ddA)], with other nucleoside analogs ddC, Azidothymidine (AZT) and didehydrodeoxythymidine (d4T) on mtDNA elongation, cytotoxicity, oxidative phosphorylation, and cellular ATP concentration in a human pancreatic cell line, Capan-1 cells. AZT, like all the other analogs tested, altered mtDNA elongation, but had no other effect on these cells. Both ddC and d4T, but not ddI (20 microm and 50 microM), reduced total dish protein (a measure of cell numbers) in cells grown to confluence. The effect of ddA was intermediate. All (except AZT) increased lactate concentration in the cell culture medium. Dideoxycytosine (ddC) and d4T did not significantly affect cell oxygen consumption, expressed as a fraction of total dish protein. By contrast, ddI and ddA reduced basal and/or FCCP-stimulated oxygen consumption. Dideoxycytosine (ddC) but not ddI or ddA (50 microM) was cytotoxic to cells after six days of growth. Nevertheless, the ATP content (expressed as a fraction of surviving cells) for ddC-, ddI-, and ddA-treated cells was similar to control cells. Cytotoxicity was apparent for ddI, ddA, as well as ddC after seven days. Paradoxically, cell ATP content was now significantly higher than control cells. Electron microscopy of cells treated with ddI confirmed significant ultrastructural changes affecting the inner mitochondria membrane and cristae. In conclusion, these data suggest that nucleoside analogs uniformly induce damage to mtDNA. However, the mitochondrial phenotypic damage induced by ddI and ddA appear to result in less Capan-1 cytotoxicity than ddC and d4T. The link between these differential effects and ddI pancreatitis is unclear.