Background:We have shown that a sodium ionophore monensin inhibits prostate cancer cell growth. A structurally related compound to monensin, salinomycin, was recently identified as a putative cancer stem cell inhibitor.Methods:The growth inhibitory potential of salinomycin was studied in a panel of prostate cells. To get insights into the mechanism of action, a variety of assays such as gene expression and steroid profiling were performed in salinomycin-exposed prostate cancer cells.Results:Salinomycin inhibited the growth of prostate cancer cells, but did not affect non-malignant prostate epithelial cells. Salinomycin impacted on prostate cancer stem cell functions as evidenced by reduced aldehyde dehydrogenase activity and the fraction of CD44+ cells. Moreover, salinomycin reduced the expression of MYC, AR and ERG, induced oxidative stress as well as inhibited nuclear factor-κB activity and cell migration. Furthermore, profiling steroid metabolites revealed increased levels of oxidative stress-inducing steroids 7-ketocholesterol and aldosterone and decreased levels of antioxidative steroids progesterone and pregnenolone in salinomycin-exposed prostate cancer cells.Conclusion:Our results indicate that salinomycin inhibits prostate cancer cell growth and migration by reducing the expression of key prostate cancer oncogenes, inducing oxidative stress, decreasing the antioxidative capacity and cancer stem cell fraction.

Fungal secondary metabolites are a rich source of medically useful compounds due to their pharmaceutical and toxic properties. Sequencing of fungal genomes has revealed numerous secondary metabolite gene clusters, yet products of many of these biosynthetic pathways are unknown since the expression of the clustered genes usually remains silent in normal laboratory conditions. Therefore, to discover new metabolites, it is important to find ways to induce the expression of genes in these otherwise silent biosynthetic clusters. We discovered a novel secondary metabolite in Aspergillus nidulans by predicting a biosynthetic gene cluster with genomic mining. A Zn(II)2Cys6–type transcription factor, PbcR, was identified, and its role as a pathway-specific activator for the predicted gene cluster was demonstrated. Overexpression of pbcR upregulated the transcription of seven genes in the identified cluster and led to the production of a diterpene compound, which was characterized with GC/MS as ent-pimara-8(14),15-diene. A change in morphology was also observed in the strains overexpressing pbcR. The activation of a cryptic gene cluster by overexpression of its putative Zn(II)2Cys6–type transcription factor led to discovery of a novel secondary metabolite in Aspergillus nidulans. Quantitative real-time PCR and DNA array analysis allowed us to predict the borders of the biosynthetic gene cluster. Furthermore, we identified a novel fungal pimaradiene cyclase gene as well as genes encoding 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG-CoA) reductase and a geranylgeranyl pyrophosphate (GGPP) synthase. None of these genes have been previously implicated in the biosynthesis of terpenes in Aspergillus nidulans. These results identify the first Aspergillus nidulans diterpene gene cluster and suggest a biosynthetic pathway for ent-pimara-8(14),15-diene.

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Therapeutic options for prostate cancer are limited and treatment responses to currently existing therapies are often unsatisfactory. Thus, there is an urgent need for novel agents to target advanced and metastatic prostate cancer cells. We have recently carried out a chemical-biological high-throughput screening of 4,910 known drugs and drug-like molecules in four prostate cancer cell models and two non-tumorigenic prostate epithelial cell lines to identify prostate cancer cell growth selective inhibitors. Only four compounds, antibiotic ionophore monensin, aldehyde dehydrogenase (ALDH) inhibitor disulfiram, histone deacetylase inhibitor trichostatin A and fungicide thiram inhibited selectively cancer cell growth at nanomolar concentrations. The mechanistic studies indicated that monensin inhibited prostate cancer cell growth by inducing oxidative stress and apoptosis. In addition, monensin reduced androgen receptor signaling, showed a synergistic anti-proliferative effect with anti-androgens as well as reduced the levels of MYC and ERG oncogenes and reduced the activity of ALDH in prostate cancer cells. Moreover, antioxidant vitamin C rescued the monensin induced growth inhibition, indicating that oxidative stress plays a key role in the antineoplastic effect of monensin in cultured prostate cancer cells. Our previous Connectivity Map results indicated that monensin has agonistic effects to NF-κB inactivator and oxidative stress inducer niclosamide. Here, we show that monensin indeed reduced the activity of NF-κB pathway. NF-κB maintains cellular antioxidant defence capacity and its inhibition induces oxidative stress as well as reduces tumorigenesis, metastasis and cancer stem cell potential. Cancer stem cells have a controlled redox balance system including high ALDH and CD44 expression which protect cancer stem cells from oxidative stress. Our results confirmed that monensin reduced the cancer stem cell markers in prostate cancer cells. Moreover, monensin induced epithelial cell differentiation shown as well as reduced motility in cultured prostate cancer cells, suggesting that monensin inhibits prostate tumorigenesis by multiple ways. Furthermore, the steroid profiling indicated that monensin increases the levels of oxidative stress inducing steroids and reduces androgen precursors in cultured prostate cancer cells. In conclusion, our results suggest that impairing the redox control, which has a crucial role in cancer cells enabling survival under high intracellular ROS, is a potent way to target prostate cancer cells and potentially also prostate cancer stem cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A62.