Polyamine Biosynthetic Pathway as a Drug Target for Osteosarcoma Therapy

Inhibitory Effect of Ursolic Acid on the Migration and Invasion of Doxorubicin-Resistant Breast Cancer

The cause of death in most breast cancer patients is disease metastasis and the occurrence of multidrug resistance (MDR). Ornithine decarboxylase (ODC), which is involved into multiple pathways, is closely related to carcinogenesis and development. Ursolic acid (UA), a natural triterpenoid compound, has been shown to reverse the MDR characteristics of tumor cells. However, the effect of UA on the invasion and metastasis of tumor cells with MDR is not known. Therefore, we investigated the effects of UA on invasion and metastasis, ODC-related polyamine metabolism, and MAPK-Erk-VEGF/MMP-9 signaling pathways in a doxorubicin-resistant breast cancer cell (MCF-7/ADR) model. The obtained results showed that UA significantly inhibited the adhesion and migration of MCF-7/ADR cells, and had higher affinities with key active cavity residues of ODC compared to the known inhibitor di-fluoro-methyl-ornithine (DFMO). UA could downregulate ODC, phosphorylated Erk (P-Erk), VEGF, and matrix metalloproteinase-9 (MMP-9) activity. Meanwhile, UA significantly reduced the content of metabolites of the polyamine metabolism. Furthermore, UA increased the intracellular accumulation of Dox in MCF-7/ADR cells. Taken together, UA can inhibit against tumor progression during the treatment of breast cancer with Dox, and possibly modulate the Erk-VEGF/MMP-9 signaling pathways and polyamine metabolism by targeting ODC to exert these effects.

Pharmacological targeting of polyamine and hypusine biosynthesis reduces tumor activity of endometrial cancer

Endometrial cancer (EC) is a common and deadly cancer in women and novel therapeutic approaches are urgently needed. Polyamines (putrescine, spermidine, spermine) are critical for mammalian cell proliferation and MYC coordinately regulates polyamine metabolism through ornithine decarboxylase (ODC). ODC is a MYC target gene and rate-limiting enzyme of polyamine biosynthesis and the FDA-approved anti-protozoan drug α-difluoromethylornithine (DFMO) inhibits ODC activity and induces polyamine depletion that leads to tumor growth arrest. Spermidine is required for the hypusine-dependent activation of eukaryotic translation initiation factors 5A1 (eIF5A1) and 5A2 (eIF5A2) and connects the MYC/ODC-induced deregulation of spermidine to eIF5A1/2 protein translation, which is increased during cancer cell proliferation. We show that the eIF5A1 is significantly upregulated in EC cells compared to control cells (p = 0.000038) and that combined pharmacological targeting of ODC and eIF5A hypusination with cytostatic drugs DFMO and N1-guanyl-1,7-diaminoheptane (GC7), respectively, reduces eIF5A1 activation and synergistically induces apoptosis in EC cells. In vivo, DFMO/GC7 suppressed xenografted EC tumor growth in mice more potently than each drug alone compared to control (p = 0.002) and decreased putrescine (p = 0.045) and spermidine levels in tumor tissues. Our data suggest DFMO and GC7 combination therapy may be useful in the treatment or prevention of EC.

Inhibition of the polyamine synthesis enzyme ornithine decarboxylase sensitizes triple-negative breast cancer cells to cytotoxic chemotherapy

Treatment of patients with triple-negative breast cancer (TNBC) is limited by a lack of effective molecular therapies targeting this disease. Recent studies have identified metabolic alterations in cancer cells that can be targeted to improve responses to standard-of-care chemotherapy regimens. Using MDA-MB-468 and SUM-159PT TNBC cells, along with LC-MS/MS and HPLC metabolomics profiling, we found here that exposure of TNBC cells to the cytotoxic chemotherapy drugs cisplatin and doxorubicin alter arginine and polyamine metabolites. This alteration was because of a reduction in the levels and activity of a rate-limiting polyamine biosynthetic enzyme, ornithine decarboxylase (ODC). Using gene silencing and inhibitor treatments, we determined that the reduction in ODC was mediated by its negative regulator antizyme, targeting ODC to the proteasome for degradation. Treatment with the ODC inhibitor difluoromethylornithine (DFMO) sensitized TNBC cells to chemotherapy, but this was not observed in receptor-positive breast cancer cells. Moreover, TNBC cell lines had greater sensitivity to single-agent DFMO, and ODC levels were elevated in TNBC patient samples. The alterations in polyamine metabolism in response to chemotherapy, as well as DFMO-induced preferential sensitization of TNBC cells to chemotherapy, reported here suggest that ODC may be a targetable metabolic vulnerability in TNBC.

Polyamine synthesis as a target of MYC oncogenes

This paper is in recognition of the 100th birthday of Dr. Herbert Tabor, a true pioneer in the polyamine field for over 70 years, who served as the editor-in-chief of the Journal of Biological Chemistry from 1971 to 2010. We review current knowledge of MYC proteins (c-MYC, MYCN, and MYCL) and focus on ornithine decarboxylase 1 (ODC1), an important bona fide gene target of MYC, which encodes the sentinel, rate-limiting enzyme in polyamine biosynthesis. Although notable advances have been made in designing inhibitors against the "undruggable" MYCs, their downstream targets and pathways are currently the main avenue for therapeutic anticancer interventions. To this end, the MYC-ODC axis presents an attractive target for managing cancers such as neuroblastoma, a pediatric malignancy in which MYCN gene amplification correlates with poor prognosis and high-risk disease. ODC and polyamine levels are often up-regulated and contribute to tumor hyperproliferation, especially of MYC-driven cancers. We therefore had proposed to repurpose α-difluoromethylornithine (DFMO), an FDA-approved, orally available ODC inhibitor, for management of neuroblastoma, and this intervention is now being pursued in several clinical trials. We discuss the regulation of ODC and polyamines, which besides their well-known interactions with DNA and tRNA/rRNA, are involved in regulating RNA transcription and translation, ribosome function, proteasomal degradation, the circadian clock, and immunity, events that are also controlled by MYC proteins.