Metabolic and Antiproliferative Consequences of Activated Polyamine Catabolism in LNCaP Prostate Carcinoma Cells

Abstract

Depletion of intracellular polyamine pools invariably inhibits cell growth. Although this is usually accomplished by inhibiting polyamine biosynthesis, we reasoned that this might be more effectively achieved by activation of polyamine catabolism at the level of spermidine/spermine N 1 -acetyltransferase (SSAT); a strategy first validated in MCF-7 breast carcinoma cells. We now examine the possibility that, due to unique aspects of polyamine homeostasis in the prostate gland, tumor cells derived from it may be particularly sensitive to activated polyamine catabolism. Thus, SSAT was conditionally overexpressed in LNCaP prostate carcinoma cells via a tetracycline-regulatable (Tet-off) system. Tetracycline removal resulted in a rapid ϳ10-fold increase in SSAT mRNA and an increase of ϳ20-fold in enzyme activity. SSAT products N 1 -acetylspermidine, N 1 -acetylspermine, and N 1 ,N 12 -diacetylspermine accumulated intracellularly and extracellularly. SSAT induction also led to a growth inhibition that was not accompanied by polyamine pool depletion as it was in MCF-7 cells. Rather, intracellular spermidine and spermine pools were maintained at or above control levels by a robust compensatory increase in ornithine decarboxylase and S-adenosylmethionine decarboxylase activities. This, in turn, gave rise to a high rate of metabolic flux through both the biosynthetic and catabolic arms of polyamine metabolism. Treatment with the biosynthesis inhibitor ␣-difluoromethylornithine during tetracycline removal interrupted flux and prevented growth inhibition. Thus, flux-induced growth inhibition appears to derive from overaccumulation of metabolic products and/or from depletion of metabolic precursors. Metabolic effects that were not excluded as possible contributing factors include high levels of putrescine and acetylated polyamines, a 50% reduction in S-adenosylmethionine, and a 45% decline in the SSAT cofactor acetyl-CoA. Overall, the study demonstrates that activation of polyamine catabolism in LNCaP cells elicits a compensatory increase in polyamine biosynthesis and downstream metabolic events that culminate in growth inhibition.

Activated Polyamine Catabolism Depletes Acetyl-CoA Pools and Suppresses Prostate Tumor Growth in TRAMP Mice

“…The effort was catalyzed by our recent reports showing that conditional overexpression of SSAT inhibits in vitro growth of both MCF-7 breast carcinoma cells (20) and LNCaP prostate carcinoma cells (21). Consistent with these findings, we now demonstrate that overexpression of SSAT markedly suppresses tumor outgrowth of early and advanced prostatic cancer in TRAMP mice.…”

“…As a prelude to the present study, we showed that conditional enzyme overexpression in LNCaP prostate carcinoma cells causes growth inhibition that differed from that seen in MCF-7 cells in that it was not accompanied by polyamine pool depletion (21). Instead cells averted the latter by increasing polyamine biosynthesis at the levels of ODC and AdoMet decarboxylase activities causing heightened metabolic flux through the biosynthetic and catabolic pathways.…”

“…(25) with minor modifications as described previously (21). Data were collected and processed by Beckman P/ACE 32 Karat software version 4.0.…”

“…Instead cells averted the latter by increasing polyamine biosynthesis at the levels of ODC and AdoMet decarboxylase activities causing heightened metabolic flux through the biosynthetic and catabolic pathways. In a critical experiment, it was shown that interruption of flux by blocking ODC activity prevented growth inhibition (21). Additional studies concluded that growth inhibition deriving from overexpression of SSAT was probably attributable to overproduction of pathway products such as acetylated polyamines or to depletion of polyamine precursor metabolites such as AdoMet and/or the SSAT cofactor acetylCoA (21).…”

“…In a critical experiment, it was shown that interruption of flux by blocking ODC activity prevented growth inhibition (21). Additional studies concluded that growth inhibition deriving from overexpression of SSAT was probably attributable to overproduction of pathway products such as acetylated polyamines or to depletion of polyamine precursor metabolites such as AdoMet and/or the SSAT cofactor acetylCoA (21). Whatever the downstream mechanism, these in vitro data suggest that activation of polyamine catabolism by selective induction of SSAT may constitute an effective antitumor strategy against prostate cancer.…”