Abstract
Polyamines have been shown to play a role in the growth and survival of several solid tumors, including colorectal cancer. We identified the polyamine catabolic enzyme spermidine/spermine N(1)-acetyltransferase (SSAT) as being one of the most highly inducible genes in two DNA microarray screens to identify novel determinants of response to chemotherapeutic agents in colorectal cancer. SSAT was shown to be inducible in response to 5-fluorouracil (5-FU) or oxaliplatin in parental and drug-resistant HCT116 cell lines. It was also shown that SSAT mRNA was up-regulated in response to 5-FU or oxaliplatin in a panel of six colorectal cancer cell lines. The polyamine analogue N(1),N(11)-diethylnorspermine (DENSpm) depletes polyamine pools and potently induces SSAT. We evaluated the effect of combining DENSpm with chemotherapeutic agents in HCT116 p53(+/+) cells and in HCT116 drug-resistant daughter cell lines. Western blot analyses showed that SSAT protein expression was dramatically enhanced when DENSpm was combined with oxaliplatin or 5-FU in HCT116 p53(+/+) cells. Using cell viability assays and flow cytometry, synergistic induction of cell death was observed following cotreatment of HCT116 p53(+/+) cells with DENSpm and each chemotherapeutic agent. Of note, this combined therapy increased the chemosensitivity of cells rendered resistant to each of these chemotherapeutic agents. Small interfering RNA-mediated down-regulation of SSAT resulted in loss of synergy between DENSpm and these agents. These results show that SSAT plays an important role in regulating cell death following combined cytotoxic drug and DENSpm treatment. Furthermore, DENSpm sensitizes both sensitive and resistant cells to chemotherapeutic agents. Taken together, these results suggest that SSAT may be an important target for therapeutic intervention in colorectal cancer.
Highlights
Colorectal cancer is the second leading cause of cancerrelated deaths in the western world
The aim of the current study was to evaluate the role of spermidine/spermine N1-acetyltransferase (SSAT), which was recently identified in a DNA microarray screen carried out in our laboratory [27] in regulating the response to the combination of DENSpm and chemotherapeutic agents in colorectal cancer cell line models
SSAT mRNA was inducible in all six colorectal cancer cell lines in response to IC50(72 h) doses of oxaliplatin at 24 h, with the greatest induction occurring in the LoVo cells followed by the HCT175 and HCT248 cells
Summary
Colorectal cancer is the second leading cause of cancerrelated deaths in the western world. 25% of colorectal cancer patients present with metastatic disease, and between 40% and 50% of newly diagnosed patients eventually develop metastatic disease. In the metastatic setting, only 10% to 15% of patients derive benefit from 5-FU monotherapy [1]. Combination of 5-FU with the topoisomerase I inhibitor irinotecan (CPT-11) or the DNA-damaging agent oxaliplatin has significantly improved response rates to 40% to 50% in the advanced setting [2, 3]. Novel agents, such as the monoclonal antibodies cetuximab, panitumumab, and bevacizumab, which target the epidermal growth factor receptor and vascular endothelial growth factor receptor, respectively, have become available and have shown promising activity in metastatic colorectal cancer (4 – 6). The prognosis for many patients still remains poor, and this is frequently due to chemoresistance, which is either intrinsic or acquired during treatment
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