A quarter century ago, Toohey (10) first recognized that certain murine malignant hematopoietic cell lines lacked methylthioadenosine phosphorylase (MTAP) 1 activity. MTAP is a key enzyme in the methionine salvage pathway (see Fig. 1). This pathway functions to salvage methylthioadenosine (MTA), which is formed as a by-product of polyamine metabolism. Phosphorolysis of MTA by MTAP results in the conversion of MTA into adenine and methylthioribose 1-phosphate. A series of reactions then salvages the methyl-thio group from methylthioribose 1-phosphate to form methionine. This pathway has been most extensively studied in Klebsiella pneumoniae (11-14) but has also been shown to exist in rat liver (15-17) and in Saccharomyces cerevisiae (18,19).Loss of MTAP activity through gene deletion is common in many kinds of human cancers including non-small cell lung cancer, glioma, T-cell acute leukemia, bladder cancer, osteosarcoma, and endometrial cancer (2-5, 20 -22). Recently, we found that expression of MTAP in an MTAP-deleted breast adenocarcinoma cell line (MCF-7) resulted in a dramatic inhibition of tumorigenicity in vitro and in vivo, showing that MTAP can function as a tumor suppressor gene (1). We also found that MTAP expression causes a significant decrease in intracellular polyamine levels and alters the ratio of putrescine to total polyamines. Consistent with this observation, the polyamine biosynthesis inhibitor ␣-difluoromethylornithine inhibits the ability of MTAP-deficient MCF-7 cells to form colonies in soft agar, whereas the addition of the polyamine putrescine stimulates colony formation in MTAP-expressing MCF-7 cells. These results indicate that the tumor suppressor activity of MTAP may be mediated by its effect on the intracellular polyamine pools.Polyamines are small, aliphatic amines involved in a variety of cellular processes including transcription and apoptosis (8). The rate-limiting enzyme in the production of polyamines is ornithine decarboxylase (ODC). Elevated ODC activity has been observed in a wide variety of human and animal tumors, and overexpression of ODC in NIH/3T3 cells is sufficient to cause transformation in vitro (7,23). Transgenic mice overexpressing ODC in skin develop skin tumors at a high frequency (9, 24). These observations show that overexpression of ODC is tumorigenic.Examination of the S. cerevisiae genome for MTAP homologues suggests that the MEU1 gene, with 35% identity and 53% similarity over 275 amino acids, may encode the yeast MTAP homologue. Furthermore, recent studies show that cells lacking MEU1, in combination with a mutation that allows yeast to take up methylthioadenosine, are unable to grow on medium containing MTA as the sole sulfur source (25). MEU1 was initially identified in a screen for genes that regulate expression of the yeast ADH2 gene (26). Overexpression of MEU1 increased ADH2 expression, whereas deletion of MEU1 resulted in reduced ADH2 expression. At the time these experiments were published, the MTAP gene had not yet been identified, so the relatio...

Purpose: Loss of the methylthioadenosine phosphorylase (MTAP) gene at 9p21 is observed frequently in a variety of human cancers. We have shown previously that MTAP can act as a tumor suppressor gene and that its tumor suppressor function is related to its effect on polyamine homeostasis. Ornithine decarboxylase is a key enzyme in the regulation of polyamine metabolism. The aim of this study is to analyze MTAP and ornithine decarboxylase (ODC) expression in primary pancreatic tumor specimens.Experimental Design: We measured MTAP and ODC activity in protein extracts derived from 30 surgically resected tumor samples and eight normal pancreas samples. In a subset of six samples, we also examined MTAP DNA using interphase fluorescence in situ hybridization. In addition, we examined the effect of the ODC inhibitor difluoromethylornithine on two pancreatic adenocarcinoma-derived cell lines.Result: MTAP activity was 2.8-fold reduced in adenocarcinomas and 6.3-fold reduced in neuroendocrine tumors compared with control pancreas. Conversely, ODC activity was 3.6-fold elevated in adenocarcinomas and 3.9-fold elevated in neuroendocrine tumors compared with control pancreas. Using interphase fluorescence in situ hybridization, we found in tumor samples that 43 to 75% of the nuclei had lost at least one copy of MTAP locus, indicating that loss of MTAP activity was at least partially because of deletion of the MTAP locus. We also show that inhibition of ODC by difluoromethylornithine caused decreased cell growth and increased apoptosis in two MTAP-deleted pancreatic adenocarcinoma-derived cell lines.Conclusions: MTAP activity is frequently lost, and ODC activity is frequently elevated in both pancreatic adenocarcinoma and neuroendocrine tumors. Inhibition of ODC activity caused decreased cell growth and increased apoptosis in pancreatic tumor-derived cell lines. These findings suggest that MTAP and polyamine metabolism could be potential therapeutic targets in the treatment of pancreatic cancer.