Antizyme and antizyme inhibitor activities influence cellular responses to polyamine analogs.

Close structural analogs of spermidine and spermine, polyamine mimetics, are potential chemotheraputic agents as they depress cellular polyamines required for tumor growth. Specific mimetic analogs stimulate synthesis of the regulatory protein antizyme (AZ), which not only inactivates the initial enzyme in polyamine biosynthesis but also inhibits cellular uptake of polyamines. The role of AZ induction in influencing cellular uptake of representative analogs was investigated using three analogs produced by Cellgate Inc., CGC-11047, CGC-11102, and CGC-11144, which exhibit markedly distinct AZ-inducing potential. An inverse correlation was noted between the AZ-inducing activity of a compound and the steady-state levels accumulated in cells. As some tumor cells over express AZI as a means of enhancing the polyamines required for aggressive growth, analog sensitivity was examined in transgenic CHO cells expressing exogenous antizyme inhibitor protein (AZI). Although AZI over expression increased cell sensitivity to analogs, the degree of this affect varied with the analog used.

Unusual aspects of the polyamine transport system affect the design of strategies for use of polyamine analogues in chemotherapy.

One strategy for inhibiting tumour cell growth is the use of polyamine mimetics to depress endogenous polyamine levels and, ideally, obstruct critical polyamine-requiring reactions. Such polyamine analogues make very unusual drugs, in that extremely high intracellular concentrations are required for growth inhibition or cytotoxicity. Cells exposed to even sub-micromolar concentrations of such analogues can achieve effective intracellular levels because these compounds are incorporated by the very aggressive polyamine uptake system. Once incorporated to these levels, many of these analogues induce the synthesis of a regulatory protein, antizyme, which inhibits both polyamine synthesis and the transporter they used to enter the cell. Thus this feedback system allows steady-state maintenance of effective cellular doses of such analogues. Accordingly, effective cellular levels of polyamine analogues are generally inversely related to their capacity to induce antizyme. Antizyme activity is down-regulated by interaction with several binding partners, most notably antizyme inhibitor, and at least a few tumour tissues exhibit deficiencies in antizyme expression. Our studies explore the role of antizyme induction by several polyamine analogues in their physiological response and the possibility that cell-to-cell differences in antizyme expression may contribute to variable sensitivities to these agents.