Spermidine nuclear acetylation in rat hepatocytes and in logarithmically growing rat hepatoma cells: comparison with histone acetylation

Histone deacetylase-10 liberates spermidine to support polyamine homeostasis and tumor cell growth

Cytosolic histone deacetylase-10 (HDAC10) specifically deacetylates the modified polyamine N8-acetylspermidine (N8-AcSpd). Although intracellular concentrations of N8-AcSpd are low, extracellular sources can be abundant, particularly in the colonic lumen. Extracellular polyamines, including those from the diet and microbiota, can support tumor growth both locally and at distant sites. However, the contribution of N8-AcSpd in this context is unknown. We hypothesized that HDAC10, by converting N8- AcSpd to spermidine, may provide a source of this growth-supporting polyamine in circumstances of reduced polyamine biosynthesis, such as in polyamine-targeting anticancer therapies. Inhibitors of polyamine biosynthesis, including α-difluoromethylornithine (DFMO), inhibit tumor growth, but compensatory uptake of extracellular polyamines has limited their clinical success. Combining DFMO with inhibitors of polyamine uptake have improved the antitumor response. However, acetylated polyamines may use different transport machinery than the parent molecules. Here, we use CRISPR/Cas9-mediated HDAC10-knockout cell lines and HDAC10-specific inhibitors to investigate the contribution of HDAC10 in maintaining tumor cell proliferation. We demonstrate inhibition of cell growth by DFMO-associated polyamine depletion is successfully rescued by exogenous N8-AcSpd (at physiological concentrations), which is converted to spermidine and spermine, only in cell lines with HDAC10 activity. Furthermore, we show loss of HDAC10 prevents both restoration of polyamine levels and growth rescue, implicating HDAC10 in supporting polyamine-associated tumor growth. These data suggest the utility of HDAC10-specific inhibitors as an antitumor strategy that may have value in improving the response to polyamine-blocking therapies. Additionally, the cell-based assay developed in this study provides an inexpensive, high-throughput method of screening potentially selective HDAC10 inhibitors.

Polyamine acetylations in normal and neoplastic growth processes

The expression patterns of cytosolic and nuclear polyamine acetyltransferases were studied in normal and neoplastic growth processesin vivo andin vitro to evidentiate the roles played by these enzymes in cell proliferation. In regenerating liver, cytosolic spermidine/spermine N(1)-acetyltransferase showed similar augments of mRNA level and enzymatic activity during the prereplicative period (4-8 h), whereas spermidine N(8)-acetyltransferase activity increased later (24 h) when DNA synthesis was maximally enhanced. In fibroblasts continuously dividing, the messenger for spermidine/spermine N(1)-acetyltransferase rapidly accumulated after serum-stimulation. In cultured Morris hepatoma cells stimulated to logarithmic growth, spermidine N(8)-acetyltransferase activity remained at plateau for 1 day declining thereafter, while spermidine/spermine N(1)-acetyltransferase activity immediately decreased. In Yoshida AH-130 hepatoma cells transplanted in rat peritoneum, spermidine N(8)-acetyltransferase and spermidine/spermine N(1)-acetyltransferase activities rose, respectively, in concomitance with elevated proliferation-rate and quasi-stationary phase of growth. Since the expression of cytosolic and nuclear acetyltransferases underwent different temporal activation, an involvement of these enzymes in separate metabolic processes controlling normal and neoplastic growth may be suggested.

Polyamine metabolism and cancer

Polyamines are aliphatic cations present in all cells. In normal cells, polyamine levels are intricately controlled by biosynthetic and catabolic enzymes. The biosynthetic enzymes are ornithine decarboxylase, S-adenosylmethionine decarboxylase, spermidine synthase, and spermine synthase. The catabolic enzymes include spermidine/spermine acetyltransferase, flavin containing polyamine oxidase, copper containing diamine oxidase, and possibly other amine oxidases. Multiple abnormalities in the control of polyamine metabolism and uptake might be responsible for increased levels of polyamines in cancer cells as compared to that of normal cells. This review is designed to look at the current research in polyamine biosynthesis, catabolism, and transport pathways, enumerate the functions of polyamines, and assess the potential for using polyamine metabolism or function as targets for cancer therapy.

Differentiation of PC12 cells induced by N8-acetylspermidine and by N8-acetylspermidine deacetylase inhibition

Spermidine is one of the simple polyamines found in cells of virtually all living organisms. It undergoes a metabolic conversion to N8-acetylspermidine catalyzed by an enzyme in cell nuclei and is converted back to spermidine by a deacetylase in the cytoplasm. In this study, two different mechanisms were used to produce an elevation in the level of N8-acetylspermidine in PC12 cells: inhibition of N8-acetylspermidine deacetylase and direct addition of N8-acetylspermidine to the cell culture. The increasing intracellular concentration of N8-acetylspermidine was accompanied by signs of PC12 cell differentiation including increased content of dopamine and morphological changes (neurite outgrowths), suggesting a strong and perhaps causal relationship among these effects. This effect on differentiation appears to be specific for N8-acetylspermidine as the addition of other polyamines including spermidine and N1-acetylspermidine did not elicit these changes. Nerve growth factor (NGF) and dexamethasone, commonly used inducers of differentiation in PC12 cells, produced differentiation without measurable changes in N8-acetylspermidine levels, suggesting that different (or multiple) mechanisms may be involved in these differentiation processes.

Effect of N8-acetylspermidine deacetylase inhibition on the growth of L1210 cells

A selective inhibitor of N8-acetylspermidine deacetylase has been employed to study the role of N8-acetylspermidine deacetylation in the regulation of L1210 cell growth. This inhibitor, 7-[N-(3-aminopropyl) amino] heptan-2-one (APAH), was found to stimulate the growth of L1210 cells at concentrations between 10 microM and 0.5 mM. Maximum stimulation was seen at 100 microM, resulting in significantly increased rates of cell division and maximum cell density. N8-Acetylspermidine levels in L1210 cells were shown to increase significantly after the APAH treatment as would be expected for deacetylase inhibition. The effects of deacetylase inhibition were mimicked by addition of N8-acetylspermidine to the culture medium at concentrations greater than 1 mM as indicated by a subsequent increase in rate of cell growth and maximum cell density. The magnitudes of the increases in growth observed were not large, but this might be expected in cells that are already in a rapid growth phase. Other exogenously added polyamines including N1-acetylspermidine, spermidine, putrescine, and spermine did not stimulate cell growth. These data suggest that stimulation of cell growth occurs as a consequence of N8-acetylspermidine accumulation and N8-acetylspermidine deacetylase inhibition.

Characterization of a diamine exporter in Chinese hamster ovary cells and identification of specific polyamine substrates

Export of the diamine putrescine was studied using inside-out plasma membrane vesicles prepared from Chinese hamster cells. Putrescine uptake into vesicles was a saturable and an ATP- and antizyme-independent process. Excess amounts of a series of diamines or monoacetyl spermidine, but not monoacetyl putrescine, spermidine, or spermine, inhibited putrescine transport. Putrescine uptake into vesicles prepared at pH 7.4 was suppressed at pH 5, compared with pH 7.4; was stimulated approximately 2.5-fold at pH 7.4 in vesicles prepared at pH 6.25, compared with vesicles prepared at pH 7.4; and was not inhibited by valinomycin in the presence of potassium ions. Reserpine and verapamil blocked [3H]putrescine uptake into inverted vesicles. Verapamil treatment caused an increase in intracellular contents of putrescine, cadaverine, and N8-acetylspermidine, in unstressed proliferating cells, or of N1-acetylspermidine, in cells subjected to heat shock to induce acetylation of spermidine at N1. These data indicate that putrescine export in Chinese hamster cells is mediated by a non-electrogenic antiporter capable of using protons as the counter ion. Physiological substrates for this exporter include putrescine, cadaverine, and monoacetyl spermidine and have the general structure NH3+-(CH2)n-NH2 + R at acidic or neutral pH.

Expression of the human spermidine/spermine N1-acetyltransferase in spermidine acetylation-deficient Escherichia coli

A cDNA encoding the human spermidine/spermine N1-acetyltransferase (N1SSAT) was conditionally expressed in a strain of Escherichia coli deficient in spermidine-acetylating activity. Conditional expression of this cDNA was performed under the control of the lac promoter, by addition of the non-hydrolysable lactose analogue isopropyl beta-D-thiogalactoside. Expression of the N1SSAT cDNA oriented in the sense direction resulted in the acetylation of spermidine at the N1 but not the N8 position and a decrease in endogenous spermidine contents and growth rates in these bacteria. When this cDNA was expressed in the antisense orientation, spermidine acetylation was not detected and endogenous spermidine contents and growth rates were unaffected. Increasing the endogenous N1-acetylspermidine concentration by addition of this amine to the culture medium did not suppress growth, and increasing endogenous spermidine pools by exogenous addition was not sufficient to restore optimal growth in cells expressing the human N1SSAT. Exogenous spermidine, but neither N1- nor N8-acetylspermidine, stimulated cell growth in strains unable to synthesize spermidine. These results suggest that one physiological consequence of spermidine acetylation in E. coli is growth inhibition. The mechanism of this inhibition seems to involve the formation of acetylspermidine, and is not simply due to a decrease in the intracellular concentration of non-acetylated spermidine.

Flux control exerted by overt carnitine palmitoyltransferase over palmitoyl-CoA oxidation and ketogenesis is lower in suckling than in adult rats

We examined the potential of overt carnitine palmitoyltransferase (CPT I) to control the hepatic catabolism of palmitoyl-CoA in suckling and adult rats, using a conceptually simplified model of fatty acid oxidation and ketogenesis. By applying top-down control analysis, we quantified the control exerted by CPT I over total carbon flux from palmitoyl-CoA to ketone bodies and carbon dioxide. Our results show that in both suckling and adult rat, CPT I exerts very significant control over the pathways under investigation. However, under the sets of conditions we studied, less control is exerted by CPT I over total carbon flux in mitochondria isolated from suckling rats than in those isolated from adult rats. Furthermore the flux control coefficient of CPT I changes with malonyl-CoA concentration and ATP turnover rate.

Cytosolic and nuclear spermidine acetyltransferases in growing NIH 3T3 fibroblasts stimulated with serum or polyamines: relationship to polyamine-biosynthetic decarboxylases and histone acetyltransferase

The expression (mRNA level of enzymic activity) of cytosolic and nuclear spermidine acetyltransferases was studied in NIH 3T3 fibroblasts, either (1) serum-starved and stimulated to grow by serum refeeding, or (2) treated with inhibitors of ornithine decarboxylase (ODC) (MDL 72.175) and S-adenosylmethionine decarboxylase (AdoMetDC) (MDL 73.811) and stimulated to grow by spermidine. Expression of the known growth-regulated genes for ODC, AdoMetDC and histone acetyltransferase was also examined. The mRNA for spermidine/spermine N1-acetyltransferase (SAT) accumulated after serum refeeding (between 6 and 16 h) and even more after spermidine addition (16 h). Histone acetyltransferase activity increased after both growth stimuli, whereas spermidine N8-acetyltransferase activity remained unchanged. After serum stimulation, the ODC mRNA level and activity rose between 6 and 16 h, whereas AdoMetDC mRNA accumulation occurred later (16 h) than the increase in enzyme activity (6 h). Stimulation of ODC and AdoMetDC activities was suppressed by the inhibitors added alone or in combination with spermidine, whereas mRNA accumulation was down-regulated by spermidine. These results indicate that the expression of SAT was growth-controlled and that SAT mRNA level was regulated by polyamines.