Several inhibitors of ornithine and adenosylmethionine decarboxylases may also have antiproliferative effects unrelated to polyamine depletion

Abstract

The efficacy of various inhibitors of polyamine synthesis to inhibit the in vitro proliferation of phytohaemagglutinin-activated lymphocytes and serum-stimulated fibroblasts was compared in respect to the concomitant polyamine depletion. The substrate-analogue inhibitors of ornithine decarboxylase, DL-α-hydrazino-δ-aminovaleric acid and DL-α-difluoromethylornithine effectively inhibited the accumulations of putrescine, spermidine and spermine that were induced in the stimulated lymphocytes, and subsequently reduced the rate of cell proliferation. The antiproliferative effect of difluoromethylornithine, but not that of hydrozino-δ-aminovaleric acid, could be prevented by adding low concentrations of polyamines to the cultures. α,ω-Diamines with carbon chains of one to twelve atoms and 1,3-diamino-2-propanol, which indirectly inhibit ornithine decarboxylase, all reduced the formation of polyamines and macromolecules in the activated lymphocytes if applied at millimolar concentrations. Diamines with long hydrophobic carbon chains were more potent inhibitors of polyamine accumulation than the shorter amines, but at the same time they were more toxic as well. The use of the diamines with two to six hydrocarbon chains and diaminopropanol for the study of polyamine deficiency was complicated by their ability at millimolar concentrations to partially replace polyamines in cell growth. In the presence of the diamines (diaminododecane, diaminoheptane and diaminopropanol) tested, the inhibition of cell proliferation was not abolished by exogenous polyamines. The inhibitors of S-adenosylmethionine decarboxylase, methylglyoxalbis-(guanylhydrazone) and 1,1′-[(methylethanediylidene)dinitrilo]bis(3-aminoguanidine) had mostly similar effects on the stimulated lymphocytes and fibroblasts: the increase in putrescine content was accentuated while that of both spermidine and spermine, as well as the rate of cell proliferation, were effectively inhibited. Supplementation of spermidine and spermine, but not of putrescine, resulted in a complete restoration of cell proliferation. Yet the antiproliferative effects of these drugs may not be solely based on the inhibition of polyamine synthesis, since they inhibited the macromolecular synthesis more than difluoromethylornithine, although the latter inhibitor was a more potent depressor of polyamine levels. Among the inhibitors studied, difluoromethylornithine appeared to be superior to the other agents as regards the selectivity of growth inhibition by polyamine depletion.