Swainsonine, an indolizidine alkaloid, inhibits tumor metastasis, augments macrophage-mediated tumor cell killing and stimulates hematopoietic functions in a murine model system. The objective of our studies is to determine the mechanism(s) by which swainsonine induces these effects and to optimize its possible use as an adjuvant in treatments of cancer metastasis. We have demonstrated that swainsonine is taken up by macrophages by using a newly developed assay to measure the inhibition of Golgi alpha-mannosidase activity in vitro. This assay makes possible a careful examination of swainsonine uptake in multiple cell types. Furthermore, we have begun to look for receptor-mediated binding of swainsonine and have obtained preliminary data that suggests that swainsonine is bound by a specific receptor on the cell surface. This binding is saturable and is competitive with mannose, suggesting that the receptor may naturally bind this sugar. We are beginning to characterize this receptor and will begin to determine the signals generated by this binding. We have already found that calcium activity increases over a matter of seconds to minutes when swainsonine is added to murine macrophages.We are investigating this process to see if internal stores are released or if external stores are utilized to increase the internal calcium concentration. Finally, we have shown that nitric oxide is released by macrophages treated with swainsonine and are investigating whether this is due to enhanced gene expression of the inducible nitric oxide synthase or to other processes within the cell.
