Pyoluteorin

Objectives:To screen the cytotoxic activity of six secondary metabolites isolated from soil fungus Aspergillus niger. Importantly, to investigate the mechanism that pyoluteorin induced human triple-negative breast cancer MDA-MB-231 cells apoptosis in vitro.Methods:The cell viability assay was tested with CTG assay. Cell cycle, apoptosis and intracellular reactive oxygen species (ROS) production assay were tested with flow cytometry. Additionally, intracellular ROS production assay and mitochondrial membrane potential assay were determined with laser scanning confocal microscopy. The expression of apoptosis-related proteins was determined with Western blot.Key findings:Pyoluteorin displayed significantly selective cytotoxicity against human triple-negative breast cancer MDA-MB-231 cells (IC50= 0.97 µm) with low toxicity against human breast epithelial cell MCF-10A. It was found that pyoluteorin could arrest MDA-MB-231 cells cycle at G2/M phase and induce cell apoptosis. Further experiments demonstrated that the apoptosis-inducing effect of pyoluteorin was related to reduction of mitochondrial membrane potential, accumulation of ROS and change of apoptosis-related protein expressions.Conclusion:Our studies revealed that pyoluteorin had potent proliferation inhibition against MDA-MB-231 cells through arresting cell cycle at G2/M phase and inducing caspase-3-dependent apoptosis by mitochondrial pathway, implying that pyoluteorin may be a potential lead compound for drug discovery of human triple-negative breast cancer.

Three new pyoluteorin analogues, mindapyrroles A-C (1-3), were purified from Pseudomonas aeruginosa strain 1682U.R.0a.27, a gill-associated bacterium isolated from the tissue homogenate of the giant shipworm Kuphus polythalamius. Mindapyrroles B and C inhibit the growth of multiple pathogenic bacteria, with mindapyrrole B (2) showing the most potent antimicrobial activity and widest selectivity index over mammalian cells. Preliminary structure-activity relationship analysis showed that dimerization of the pyoluteorin moiety through a C-C linkage is detrimental to the antimicrobial activity, but addition of an aerugine unit in the methylene bridge is favorable for both the antimicrobial activity and selectivity index.

Pseudomonas protegensPf-5 is an effective biocontrol agent that protects many crops against pathogens, including the fungal pathogenBotrytis cinereacausing gray mold disease inCannabis sativacrops. Previous studies have demonstrated the important role of antibiotics pyoluteorin (PLT) and 2,4-diacetylphloroglucinol (DAPG) in Pf-5-mediated biocontrol. To assess the potential involvement of PLT and DAPG in the biocontrol exerted by Pf-5 againstB. cinereain the phyllosphere ofC. sativa, two knockout Pf-5 mutants were generated by in-frame deletion of genespltDorphlA, required for the synthesis of PLT or DAPG respectively, using a two-step allelic exchange method. Additionally, two complemented mutants were constructed by introducing a multicopy plasmid carrying the deleted gene into each deletion mutant.In vitroconfrontation assays revealed that deletion mutant ∆pltDinhibitedB. cinereagrowth significantly less than wild-type Pf-5, supporting antifungal activity of PLT. However, deletion mutant ∆phlAinhibited mycelial growth significantly more than the wild-type, hypothetically due to a co-regulation of PLT and DAPG biosynthesis pathways. Both complemented mutants recoveredin vitroinhibition levels similar to that of the wild-type. In subsequent growth chamber inoculation trials, characterization of gray mold disease symptoms on infected cannabis plants revealed that both ∆pltDand ∆phlAsignificantly lost a part of their biocontrol capabilities, achieving only 10 and 19% disease reduction respectively, compared to 40% achieved by inoculation with the wild-type. Finally, both complemented mutants recovered biocontrol capabilitiesin plantasimilar to that of the wild-type. These results indicate that intact biosynthesis pathways for production of PLT and DAPG are required for the optimal antagonistic activity ofP. protegensPf-5 againstB. cinereain the cannabis phyllosphere.