Evaluation of TBT toxicity on murine macrophages

Several classes of environmental contaminants are considered to be prototypical immunotoxic agents. These include the polycylic aromatic hydrocarbons, halogenated aromated hydrocarbons and various organometallic compounds such as the organotins. Immunotoxicity following exposure to planar aromatic hydrocarbons is associated with a distinct genetic locus for a cytosolic receptor known as the aryl-hydrocarbon receptor. Of the organotin, tributyltin (TBT), a biocidal organotin historically used as an antifungal agent and in marine antifouling paints, has received a great deal of attention in toxicological studies due to the likelihood of exposure to human through seafood consumption and dry-dock application procedures. No known genetic loci is associated with TBT toxicity and the exact mecanisms of toxicity are unknown. Nevertheless, an important target for tributyltin (TBT) toxicity, at least in rodents, is the immune system, with thymic atrophy serving as a hallmark feature of exposure. Moreover, it is known that TBT is able to induce both cytoskeletal alterations and apoptosis in mammalian thymocytes. In order to explain the molecular mechanism which is responsible for the effects observed in whole organisms, in this study we have evaluated the effects of TBT in cultures of murine peritoneal macrophages. After 24 h exposure at increasing doses of TBT, the MTT test showed a dose-dependent cytotoxicity with an IC50 value of about 3 µM. Since the inhibition of mitochondrial activity by TBT is well known, our investigations were focused on lysosomes prepared from the same murine peritoneal macrophages in which the toxicity has been evalued. By means of a probe for the measurement of internal pH by acridine orange, it was observed that TBT inhibits the lysosomal acidification. A possible explanation of these results is that TBT behaves as uncoupler in both mitochondria and lysosomes.