The polyamine oxidase inhibitor MDL-72,527 selectively induces apoptosis of transformed hematopoietic cells through lysosomotropic effects

Polyamine oxidase functions in the polyamine catabolic pathway, converting N-1-acetyl-spermidine and -spermine into putrescine (Put) and spermidine (S pd), respectively, thereby facilitating homeostasis of intracellular polyam ine pools. Inhibition of polyamine oxidase in hematopoietic cells by a spec ific inhibitor, N, N'-bis(2,3-butadienyl)-1,4-butanediamine (MDL-72,527), r educes the levels of Put and Spd and induces the accumulation of N-1-acetyl ated Spd. Although previously thought to be relatively nontoxic, we now rep ort that this inhibitor overrides survival factors to induce cell, death of several immortal and malignant murine and human hematopoietic cells, but n ot of primary myeloid progenitors. Cells treated with MDL-72,527 displayed biochemical changes typical of apoptosis, and cell death was associated wit h the down-regulation of the antiapoptotic protein Bcl-X-L. However, enforc ed overexpression of Bcl-X-L, or treatment with the universal caspase inhib itor zVAD-fmk, failed to block MDL-72,527-induced apoptosis in these hemato poietic cells. Despite decreases in Put and Spd pools, MDL-72,527-induced a poptosis was not blocked by cotreatment with exogenous Put or Spd, nor was it influenced by overexpression or inhibition of the polyamine biosynthetic enzyme ornithine decarboxylase. Significantly, MDL72,527-induced apoptosis was associated with the rapid formation of numerous lysosomally derived va cuoles. Malignant leukemia cells were variably sensitive to the lysosomotro pic effects of MDL-72,527, yet pretreatment with the ornithine decarboxylas e inhibitor L-alpha-difluoromethylornithine sensitized all of these leukemi a cells to the deleterious effects of the inhibitor by stimulating its intr acellular accumulation. The lysosomotropic nature of select polyamine analo gues may, thus, provide a novel chemotherapeutic strategy to selectively in duce apoptosis of malignant hematopoietic cells.