Aglycone Polyether Nanchangmycin and Its Homologues Exhibit Apoptotic and Antiproliferative Activities against Cancer Stem Cells

Synthetic Biology in Natural Product Biosynthesis

Synthetic biology has played an important role in the renaissance of natural products research during the post-genomics era. The development and integration of new tools have transformed the workflow of natural product discovery and engineering, generating multidisciplinary interest in the field. In this review, we summarize recent developments in natural product biosynthesis from three different aspects. First, advances in bioinformatics, experimental, and analytical tools to identify natural products associated with predicted biosynthetic gene clusters (BGCs) will be covered. This will be followed by an extensive review on the heterologous expression of natural products in bacterial, fungal and plant organisms. The native host-independent paradigm to natural product identification, pathway characterization, and enzyme discovery is where synthetic biology has played the most prominent role. Lastly, strategies to engineer biosynthetic pathways for structural diversification and complexity generation will be discussed, including recent advances in assembly-line megasynthase engineering, precursor-directed structural modification, and combinatorial biosynthesis.

Combinatorial Biosynthesis Creates a Novel Aglycone Polyether with High Potency and Low Side Effects Against Bladder Cancer

Polyethers play a crucial role in the development of anticancer drugs. To enhance the anticancer efficacy and reduce the toxicity of these compounds, thereby advancing their application in cancer treatment, herein, guided by the structure-activity relationships of aglycone polyethers, novel aglycone polyethers are rationally redesigned with potentially improved efficacy and reduced toxicity against tumors. To realize the biosynthesis of the novel aglycone polyethers, the gene clusters and the post-polyketide synthase tailoring pathways for aglycone polyethers endusamycin and lenoremycin are identified and subjected to combinatorial biosynthesis studies, resulting in the creation of a novel aglycone polyether termed End-16, which demonstrates significant potential for treating bladder cancer (BLCA). End-16 demonstrates the ability to suppress the proliferation, migration, invasion, and cellular protrusions formation of BLCA cells, as well as induce cell cycle arrest in the G1 phase in vitro. Notably, End-16 exhibits superior inhibitory activity and fewer side effects against BLCA compared to the frontline anti-BLCA drug cisplatin in vivo, thereby warranting further preclinical studies. This study highlights the significant potential of integrating combinatorial biosynthesis strategies with rational design to create unnatural products with enhanced pharmacological properties.

Repurposing Polyether Ionophores as a New-Class of Anti-SARS-Cov-2 Agents as Adjunct Therapy

The emergence of SARS-CoV-2 and its variants have posed a significant threat to humankind in tackling the viral spread. Furthermore, currently repurposed drugs and frontline antiviral agents have failed to cure severe ongoing infections effectively. This insufficiency has fuelled research for potent and safe therapeutic agents to treat COVID-19. Nonetheless, various vaccine candidates have displayed a differential efficacy and need for repetitive dosing. The FDA-approved polyether ionophore veterinary antibiotic for treating coccidiosis has been repurposed for treating SARS-CoV-2 infection (as shown by both in vitro and in vivo studies) and other deadly human viruses. Based on selectivity index values, ionophores display therapeutic effects at sub-nanomolar concentrations and exhibit selective killing ability. They act on different viral targets (structural and non-structural proteins), host-cell components leading to SARS-CoV-2 inhibition, and their activity is further enhanced by Zn²⁺ supplementation. This review summarizes the anti-SARS-CoV-2 potential and molecular viral targets of selective ionophores like monensin, salinomycin, maduramicin, CP-80,219, nanchangmycin, narasin, X-206 and valinomycin. Ionophore combinations with Zn²⁺ are a new therapeutic strategy that warrants further investigation for possible human benefits.

Identification of the polyether ionophore lenoremycin through a new screening strategy for targeting cancer stem cells

Targeting and eradicating cancer stem cells (CSCs), also termed tumor-initiating cells, are promising strategies for preventing cancer progression and recurrence. To identify candidate compounds targeting CSCs, we established a new screening strategy with colorectal CSC spheres and non-CSC spheres in three-dimensional (3D) culture system. Through chemical screening using our system with in-house microbial metabolite library, we identified polyether cation ionophores that selectively inhibited CSC sphere formation, whereas CSC spheres were resistant to conventional anticancer agents. One of the hit compounds, the most selective and effective microbial metabolite lenoremycin, decreased CSC populations via inducing reactive oxygen species production. This study demonstrated that our newly established screening system is useful for discovering agents that selectively eliminate CSCs.

Undescribed polyether ionophores from Streptomyces cacaoi and their antibacterial and antiproliferative activities

Polyether ionophores represent a large group of naturally occurring compounds mainly produced by Streptomyces species. With previously proven varieties of bioactivity including antibacterial, antifungal, antiparasitic, antiviral and anti-tumor effects, the discovery of undescribed polyethers leading to development of efficient therapeutics has become important. As part of our research on polyether-rich Streptomyces cacaoi, we previously performed modification studies on fermentation conditions to induce synthesis of specialized metabolites. Here, we report four undescribed and nine known polyether compounds from S. cacaoi grown in optimized conditions. Antimicrobial activity assays revealed that four compounds, including the undescribed (6), showed strong inhibitory effects over both Bacillus subtilis and methicillin-resistant Staphylococcus aureus (MRSA) growth. Additionally, K41-A and its C15-demethoxy derivative exhibited significant cytotoxicity. These results signified that selectivity of C15-demethoxy K41-A towards cancer cells was higher than K41-A, which prompted us to conduct mechanistic experiments. These studies showed that this uninvestigated compound acts as a multitarget compound by inhibiting autophagic flux, inducing reactive oxygen species formation, abolishing proteasome activity, and stimulating ER stress. Consequently, the optimized fermentation conditions of S. cacaoi led to the isolation of undescribed and known polyethers displaying promising activities.

Nanchangmycin regulates FYN, PTK2, and MAPK1/3 to control the fibrotic activity of human hepatic stellate cells

Chronic liver injury causes fibrosis, characterized by the formation of scar tissue resulting from excessive accumulation of extracellular matrix (ECM) proteins. Hepatic stellate cell (HSC) myofibroblasts are the primary cell type responsible for liver fibrosis, yet there are currently no therapies directed at inhibiting the activity of HSC myofibroblasts. To search for potential anti-fibrotic compounds, we performed a high-throughput compound screen in primary human HSC myofibroblasts and identified 19 small molecules that induce HSC inactivation, including the polyether ionophore nanchangmycin (NCMC). NCMC induces lipid re-accumulation while reducing collagen expression, deposition of collagen in the extracellular matrix, cell proliferation, and migration. We find that NCMC increases cytosolic Ca²⁺ and reduces the phosphorylated protein levels of FYN, PTK2 (FAK), MAPK1/3 (ERK2/1), HSPB1 (HSP27), and STAT5B. Further, depletion of each of these kinases suppress COL1A1 expression. These studies reveal a signaling network triggered by NCMC to inactivate HSC myofibroblasts and reduce expression of proteins that compose the fibrotic scar. Identification of the antifibrotic effects of NCMC and the elucidation of pathways by which NCMC inhibits fibrosis provide new tools and therapeutic targets that could potentially be utilized to combat the development and progression of liver fibrosis.

Genomics-driven discovery of the biosynthetic gene cluster of maduramicin and its overproduction in Actinomadura sp. J1-007

Maduramicin is the most efficient and possesses the largest market share of all anti-coccidiosis polyether antibiotics (ionophore); however, its biosynthetic gene cluster (BGC) has yet to been identified, and the associated strains have not been genetically engineered. Herein, we performed whole-genome sequencing of a maduramicin-producing industrial strain of Actinomadura sp. J1-007 and identified its BGC. Additionally, we analyzed the identified BGCs in silico to predict the biosynthetic pathway of maduramicin. We then developed a conjugation method for the non-spore-forming Actinomadura sp. J1-007, consisting of a site-specific integration method for gene overexpression. The maduramicin titer increased by 30% to 7.16 g/L in shake-flask fermentation following overexpression of type II thioesterase MadTE that is the highest titer at present. Our findings provide insights into the biosynthetic mechanism of polyethers and provide a platform for the metabolic engineering of maduramicin-producing microorganisms for overproduction and development of maduramicin analogs in the future.

Histone deacetylase (HDAC) inhibitors and doxorubicin combinations target both breast cancer stem cells and non-stem breast cancer cells simultaneously

PURPOSE

Breast cancer stem cells (CSCs) are a small subpopulation of cancer cells that have high capability for self-renewal, differentiation, and tumor initiation. CSCs are resistant to chemotherapy and radiotherapy, and are responsible for cancer recurrence and metastasis.

METHODS

By utilizing a panel of breast cancer cells and mammospheres culture as cell-based screening platforms, we performed high-throughput chemical library screens to identify agents that are effective against breast CSCs and non-CSCs. The hit molecules were paired with conventional chemotherapy to evaluate the combinatorial treatment effects on breast CSCs and non-CSCs.

RESULTS

We identified a total of 193 inhibitors that effectively targeting both breast CSCs and non-CSCs. We observed that histone deacetylase inhibitors (HDACi) synergized conventional chemotherapeutic agents (i.e., doxorubicin and cisplatin) in targeting breast CSCs and non-CSCs simultaneously. Further analyses revealed that quisinostat, a potent inhibitor for class I and II HDACs, potentiated doxorubicin-induced cytotoxicity in both breast CSCs and non-CSCs derived from the basal-like (MDA-MB-468 and HCC38), mesenchymal-like (MDA-MB-231), and luminal-like breast cancer (MCF-7). It was also observed that the basal-like breast CSCs and non-CSCs were more sensitive to the co-treatment of quisinostat with doxorubicin compared to that of the luminal-like breast cancer subtype.

CONCLUSION

In conclusion, this study demonstrates the potential of HDACi as therapeutic options, either as monotherapy or in combination with chemotherapeutics against refractory breast cancer.

Synthesis and Anticancer Activity of Tertiary Amides of Salinomycin and Their C20-oxo Analogues

The polyether ionophore salinomycin (SAL) has captured much interest because of its potent activity against cancer cells and cancer stem cells. Our previous studies have indicated that C1/C20 double-modification of SAL is a useful strategy to generate diverse agents with promising biological activity profiles. Thus, herein we describe the synthesis of a new class of SAL analogues that combine key modifications at the C1 and C20 positions. The activity of the obtained SAL derivatives was evaluated using primary acute lymphoblastic leukemia, human breast adenocarcinoma and normal mammary epithelial cells. One single- [N,N-dipropyl amide of salinomycin (5 a)] and two novel double-modified analogues [N,N-dipropyl amide of C20-oxosalinomycin (5 b) and piperazine amide of C20-oxosalinomycin (13 b)] were found to be more potent toward the MDA-MB-231 cell line than SAL or its C20-oxo analogue 2. When select analogues were tested against the NCI-60 human tumor cell line panel, 4 a [N,N-diethyl amide of salinomycin] showed particular activity toward the ovarian cancer cell line SK-OV-3. Additionally, both SAL and 2 were found to be potent ex vivo against human ER/PR⁺ , Her2^- invasive mammary carcinoma, with 2 showing minimal toxicity toward normal epithelial cells. The present findings highlight the therapeutic potential of SAL derivatives for select targeting of different cancer types.