Polyamines and Polyamine Regulators in Prostate Cancer
Polyamines and Polyamine Regulators in Prostate Cancer
By David A. Corral, MD
The steps to progression for any given tumor are under the control of complex regulatory events, with the overall growth rate determined by the balance of the number of cells undergoing proliferation, cell death, and quiescence. Among the many factors that influence these events are polyamines, the most basic (positively charged) small organic molecules that are found in high concentrations in several tissues and affect diverse physiologic processes that influence cell proliferation and growth. Increases in the level of the aliphatic polyamines putrescine, spermidine (SPD), and spermine (SPM), which are necessary for normal and pathological cell growth, have been associated with proliferation and transformation induced by growth factors, carcinogens, viruses, and oncogenes.1,2 Polyamine metabolism is under complex regulation by multiple enzymes. In the prostate, polyamines are present in high concentration, and levels are positively controlled by androgens. Multiple attempts have been made at correlation of individual polyamine levels with tumor growth and progression, but because of the complex nature of the effects of these molecules, no single polyamine has proved to be a useful molecular marker of tumor progression.1
Expression of Polyamine Metabolism Regulatory Genes
Recently, Saverio et al published a report on the expression of various polyamine metabolism regulatory genes that encode enzymes that control polyamine synthesis and degradation in prostate cancer specimens and attempted to correlate these findings with clinical grade and stage.3 A total of 23 prostatectomy specimens were dissected to obtain 0.5 cc tumor specimens and control tissue from adjacent, normal areas of the gland.
It should be noted that each patient in this study had received three months of neoadjuvant hormone ablation therapy prior to surgery. The authors used Northern blot analysis on RNA obtained from the specimens to examine relative levels of expression of the following genes: 1) ornithine decarboxylase (ODC), the rate-limiting enzyme of polyamine synthesis and a putative proto-oncogene;4 2) ornithine decarboxylase antizyme (OAZ), which inhibits ODC activity by accelerating its degradation and is induced by high levels of intracellular polyamines; 3) adenosylmethionine decarboxylase (AdoMetDC), also a rate-limiting enzyme of polyamine synthesis; 4) spermidine/spermine N1-acetyltransferase (SSAT), an enzyme involved in polyamine degradation and excretion; and 5) clusterin, also known as sulfated glycoprotein 2, a heterodimeric glycoprotein upregulated during proliferation but downregulated during atrophy, such as in the prostate following androgen ablation. For comparison, the authors also determined levels of expression of histone H3, a marker of cell proliferation, and Gas1, which is involved in growth suppression and maintenance of the quiescent state.
Prostate cancer is graded according to the Gleason grading system, where a score is assigned to the two most predominant patterns seen in the tumor. These are added together to obtain the Gleason score for the tumor, which ranges from a low of 2 (lowest grade, least aggressive) to a high of 10 (most aggressive). Saverio et al compared expression of the polyamine metabolism regulator genes described above to the grade of each tumor. Not surprisingly, low grade Gleason score 2 tumors had increased levels of Gas1 but decreased levels of H3 mRNA relative to controls, whereas in Gleason score 5 or 8 tumors Gas1 was downregulated and H3 was overexpressed.
With regard to the expression of the regulatory proteins of polyamine metabolism, the authors demonstrated that in low-grade prostate cancer specimens (assumed low proliferative activity), ODC is upregulated with a concomitant increase in the expression of OAZ. Expression of AdoMetDC was unchanged and only a minor increase in SSAT mRNA was detected.
Saverio et al conclude that these findings are consistent with the hypothesis that ODC induction, which should be balanced by activation of the regulatory steps of polyamine metabolism to prevent polyamine overaccumulation, is an early event during cell transformation. In higher grade Gleason 8 cancers (presumed more actively proliferating), induction of ODC was not counterbalanced by OAZ overexpression. AdoMetDC mRNA was dramatically elevated in the high grade specimen, and the increased levels of the polyamines SPD and SPM, which would follow the induction of the two biosynthetic enzymes, may have been partially balanced by SSAT overexpression. The authors propose that this constellation of findings would result in intracellular polyamine concentrations capable of supporting a high rate of cell proliferation without jeopardizing cell survival. Clusterin was downregulated in both Gleason grade 2 and grade 8 tumors, which the authors interpreted as evidence of clusterin downregulation being an early event of prostate tumor development. The trends detected at the mRNA level were confirmed at the protein level by Western blot analysis for clusterin and by enzymatic assays for SSAT and AdoMetDC. The authors also detected a trend toward increased histone H3 mRNA levels and decreased Gas1 mRNA relative to normal controls, but this was not statistically significant.
Saverio et al went on to correlate their findings with clinical prognostic indicators. Tumors from patients with negative prognostic indicators, such as elevated prostate specific antigen (PSA) following surgery, lymph node involvement, or the presence of distant metastases, demonstrated significant upregulation of ODC, AdoMetDC, and SSAT, whereas Gas 1 and SGP-2 were downregulated. SSAT was also overexpressed in patients with a favorable prognosis.
An Alternative Tumor Classification Scheme?
Similar findings were noted when patients were stratified by PSA level. Using logistic regression to evaluate the predictive ability of changes in expression of the entire panel of the genes studied, the authors sought to establish an alternative tumor classification system. Changes in H3 and Gas 1gene expression were predictive of tumor localization (i.e., organ confined vs capsular penetration) in 72% of cases, and the accuracy was raised to 83% when all genes were included. Perhaps more importantly, two patients who were placed in the poorly differentiaited group by virtue of the results of their polyamine regulatory gene profiles but who had a moderate Gleason histologic score have had a rising PSA within one year of radical surgery, indicating cancer recurrence.
Summary
The article by Saverio et al points out the complexities of polyamine biology and its involvement in the regulation of the cell cycle. The authors elegantly examined the relationship of the various regulatory molecules in varying grades of prostate cancer. In their report, the authors do, however, make the assumption that there is a true progression from Gleason grade 2 prostate cancer through to grade 10 cancer as the disease progresses. While it is unquestionably true that the higher grade cancers behave more aggressively, it is not so clear that prostate cancer begins as a low-grade tumor and progressively de-differentiates in a step-wise fashion to a poorly differentiated state.
The fact that latent low-grade prostate cancer is a frequent incidental finding at autopsy and would likely never have become apparent, whereas clinically significant cancer presents at higher grade and stage, would argue against such a regimented, step-wise progression. Nevertheless, Saverio’s finding of differences among differing grades is certainly important. While the biology surrounding polyamine action, synthesis, and degradation is certainly interesting, the question arises of how therapeutically useful this information is and how it could be specifically applied in the clinical setting.
The data from Saverio’s study are interesting in that they demonstrate that a molecular profile of the expression of the genes involved in polyamine metabolism may provide information that the current Gleason grading system and clinical preoperative staging cannot. If such a classification scheme could predict, at the time of diagnosis, who is likely to have a cancer recurrence following surgery, then therapy could be directed more appropriately toward other modalities, thus sparing the patient the potential associated morbidities. Of course, the utility of such a profile would need to be tested in a large scale prospective fashion.
References
1. Pegg AE. Polyamine metabolism and its importance in neoplastic growth and as a target for chemotherapy. Cancer Res 1998;48:759-774.
2. Holtta E, Auvinen M, Anderson LC. Polyamines are essential for cell transformation by pp60v-src: Delineation of molecular events relevant for the transformed phenotype. J Cell Biol 1993;122:903-914.
3. Saverio B, Pierpaola D, Serenella A, et al. Tumor progression is accompanied by significant changes in the levels of expression of polyamine metabolism regulatory genes and clusterin in human prostate cancer specimens. Cancer Res 2000;60:28-34.
4. Hayashi S, Murakami Y. Rapid and regulated degradation of ornithine decarboxylase. Biochem J 1995;306:1-10.
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