Discovery of Potent 17β-Hydroxywithanolides for Castration-Resistant Prostate Cancer by High-Throughput Screening of a Natural Products Library for Androgen-Induced Gene Expression Inhibitors

Bortezomib in Combination with Physachenolide C Reduces the Tumorigenic Properties of KRASmut/P53mut Lung Cancer Cells by Inhibiting c-FLIP

BACKGROUND

Defects in apoptosis regulation are one of the classical features of cancer cells, often associated with more aggressiveness and failure to therapeutic options. We investigated the combinatorial antitumor effects of a natural product, physachenolide C (PCC) and bortezomib, in KRASmut/P53mut lung cancer cells and xenograft mice models.

METHODS

The in vitro anticancer effects of the bortezomib and PCC combination were investigated using cell viability, migration, and invasion assays in 344SQ, H23, and H358 cell lines. Furthermore, the effects of combination treatment on the critical parameters of cellular metabolism, including extracellular acidification rate (ECAR) and mitochondrial oxidative phosphorylation based on the oxygen consumption rate of cancer cells were assessed using Seahorse assay. Finally, the antitumor effect of the bortezomib (1 mg/kg) and PCC (10 mg/kg) combination was evaluated using xenograft mice models.

RESULTS

Our data showed that the bortezomib-PCC combination was more effective in reducing the viability of lung cancer cells in comparison with the individual treatments. Similarly, the combination treatment showed a significant inhibition of cell migration and invasion of cancer cells. Additionally, the key anti-apoptotic protein c-FLIP was significantly inhibited along with a substantial reduction in the key parameters of cellular metabolism in cancer cells. Notably, the bortezomib or PCC inhibited the tumor growth compared to the control group, the tumor growth inhibition was much more effective when bortezomib was combined with PCC in tumor xenograft mice models.

CONCLUSION

These findings demonstrate that PCC sensitizes cancer cells to bortezomib, potentially improving the antitumor effects against KRASmut/P53mut lung cancer cells, with an enhanced efficacy of combination treatments without causing significant side effects.

Withanolides: Promising candidates for cancer therapy

Natural products have played a significant role throughout history in the prevention and treatment of numerous diseases, particularly cancers. As a natural product primarily derived from various medicinal plants in the Withania genus, withanolides have been shown in several studies to exhibit potential activities in cancer treatment. Consequently, understanding the molecular mechanism of withanolides could herald the discovery of new anticancer agents. Withanolides have been studied widely, especially in the last 20 years, and attracted the attention of numerous researchers. Currently, over 1200 withanolides have been classified, with approximately a quarter of them having been reported in the literature to be able to modulate the survival and death of cancer cells through multiple avenues. To what extent, though, has the anticancer effects of these compounds been studied? How far are they from being developed into clinical drugs? What are their potential, characteristic features, and challenges? In this review, we elaborate on the current knowledge of natural compounds belonging to this class and provide an overview of their natural sources, anticancer activity, mechanism of action, molecular targets, and implications for anticancer drug research. In addition, direct targets and clinical research to guide the design and implementation of future preclinical and clinical studies to accelerate the application of withanolides have been highlighted.

Physachenolide C is a Potent, Selective BET Inhibitor

A pulldown using a biotinylated natural product of interest in the 17β-hydroxywithanolide (17-BHW) class, physachenolide C (PCC), identified the bromodomain and extra-terminal domain (BET) family of proteins (BRD2, BRD3, and BRD4), readers of acetyl-lysine modifications and regulators of gene transcription, as potential cellular targets. BROMOscan bromodomain profiling and biochemical assays support PCC as a BET inhibitor with increased selectivity for bromodomain (BD)-1 of BRD3 and BRD4, and X-ray crystallography and NMR studies uncovered specific contacts that underlie the potency and selectivity of PCC toward BRD3-BD1 over BRD3-BD2. PCC also displays characteristics of a molecular glue, facilitating proteasome-mediated degradation of BRD3 and BRD4. Finally, PCC is more potent than other withanolide analogues and gold-standard pan-BET inhibitor (+)-JQ1 in cytotoxicity assays across five prostate cancer (PC) cell lines regardless of androgen receptor (AR)-signaling status.

Production and Structural Diversification of Withanolides by Aeroponic Cultivation of Plants of Solanaceae: Cytotoxic and Other Withanolides from Aeroponically Grown Physalis coztomatl

Withanolides constitute one of the most interesting classes of natural products due to their diversity of structures and biological activities. Our recent studies on withanolides obtained from plants of Solanaceae including Withania somnifera and a number of Physalis species grown under environmentally controlled aeroponic conditions suggested that this technique is a convenient, reproducible, and superior method for their production and structural diversification. Investigation of aeroponically grown Physalis coztomatl afforded 29 withanolides compared to a total of 13 obtained previously from the wild-crafted plant and included 12 new withanolides, physacoztolides I−M (9–13), 15α-acetoxy-28-hydroxyphysachenolide C (14), 28-oxophysachenolide C (15), and 28-hydroxyphysachenolide C (16), 5α-chloro-6β-hydroxy-5,6-dihydrophysachenolide D (17), 15α-acetoxy-5α-chloro-6β-hydroxy-5,6-dihydrophysachenolide D (18), 28-hydroxy-5α-chloro-6β-hydroxy-5,6-dihydrophysachenolide D (19), physachenolide A-5-methyl ether (20), and 17 known withanolides 3–5, 8, and 21–33. The structures of 9–20 were elucidated by the analysis of their spectroscopic data and the known withanolides 3–5, 8, and 21–33 were identified by comparison of their spectroscopic data with those reported. Evaluation against a panel of prostate cancer (LNCaP, VCaP, DU-145, and PC-3) and renal carcinoma (ACHN) cell lines, and normal human foreskin fibroblast (WI-38) cells revealed that 8, 13, 15, and 17–19 had potent and selective activity for prostate cancer cell lines. Facile conversion of the 5,6-chlorohydrin 17 to its 5,6-epoxide 8 in cell culture medium used for the bioassay suggested that the cytotoxic activities observed for 17–19 may be due to in situ formation of their corresponding 5β,6β-epoxides, 8, 27, and 28.

Recent Advances in the Chemistry and Therapeutic Evaluation of Naturally Occurring and Synthetic Withanolides

Natural products are a major source of biologically active compounds that make promising lead molecules for developing efficacious drug-like molecules. Natural withanolides are found in many flora and fauna, including plants, algae, and corals, that traditionally have shown multiple health benefits and are known for their anti-cancer, anti-inflammatory, anti-bacterial, anti-leishmaniasis, and many other medicinal properties. Structures of these withanolides possess a few reactive sites that can be exploited to design and synthesize more potent and safe analogs. In this review, we discuss the literature evidence related to the medicinal implications, particularly anticancer properties of natural withanolides and their synthetic analogs, and provide perspectives on the translational potential of these promising compounds.

Ring A/B-Modified 17β-Hydroxywithanolide Analogues as Antiproliferative Agents for Prostate Cancer and Potentiators of Immunotherapy for Renal Carcinoma and Melanoma

Physachenolide C (1) is a 17β-hydroxywithanolide natural product with a unique anticancer potential, as it exhibits potent and selective in vitro antiproliferative activity against prostate cancer (PC) cells and promotes TRAIL-induced apoptosis of renal carcinoma (RC) and poly I:C-induced apoptosis of melanoma cells. To explore the effect of ring A/B modifications of physachenolide C (1) on these biological activities, 23 of its natural and semisynthetic analogues were evaluated. Analogues 4-23 were prepared by chemical transformations of a readily accessible compound, physachenolide D (2). Compound 1 and its analogues 2-23 were evaluated for their antiproliferative activity against PC (LNCaP and 22Rv1), RC (ACHN), and melanoma (M14 and SK-MEL-28) cell lines and normal human foreskin fibroblast (HFF) cells. Most of the active analogues had selective and potent activity in reducing cell number for PC cell lines, some showing selectivity for androgen-independent and enzalutamide-resistant 22Rv1 cells compared to androgen-dependent LNCaP cells. Analogues with IC₅₀s below 5.0 μM against ACHN cells, when tested in the presence of TRAIL, showed a significantly increased ability to reduce cell number, and those analogues active against the M14 and SK-MEL-28 cell lines exhibited enhanced activity when combined with poly I:C. These data provide additional structure-activity relationship information for 17β-hydroxywithanolides and suggest that selective activities of some analogues may be exploited to develop natural products-based tumor-specific agents for cancer chemotherapy.

Physachenolide C induces complete regression of established murine melanoma tumors via apoptosis and cell cycle arrest

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PCC and LG-134 had direct cytotoxicity in murine melanoma cell lines (IC₅₀ values ranged from 0.19–1.8 µM).

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PCC treatment induced apoptosis of tumor cells both in vitro and in vivo.

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PCC treatment induced G0-G1 cell cycle arrest of melanoma cells.

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PCC treatment caused complete regression of established melanoma tumors in all mice.

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17β-hydroxywithanolides have the potential to improve melanoma therapeutic outcome.

Melanoma is an aggressive skin cancer that metastasizes to other organs. While immune checkpoint blockade with anti-PD-1 has transformed the treatment of advanced melanoma, many melanoma patients fail to respond to anti-PD-1 therapy or develop acquired resistance. Thus, effective treatment of melanoma still represents an unmet clinical need. Our prior studies support the anti-cancer activity of the 17β-hydroxywithanolide class of natural products, including physachenolide C (PCC). As single agents, PCC and its semi-synthetic analog demonstrated direct cytotoxicity in a panel of murine melanoma cell lines, which share common driver mutations with human melanoma; the IC₅₀ values ranged from 0.19–1.8 µM. PCC treatment induced apoptosis of tumor cells both in vitro and in vivo. In vivo treatment with PCC alone caused the complete regression of established melanoma tumors in all mice, with a durable response in 33% of mice after discontinuation of treatment. T cell-mediated immunity did not contribute to the therapeutic efficacy of PCC or prevent tumor recurrence in YUMM2.1 melanoma model. In addition to apoptosis, PCC treatment induced G0-G1 cell cycle arrest of melanoma cells, which upon removal of PCC, re-entered the cell cycle. PCC-induced cycle cell arrest likely contributed to the in vivo tumor recurrence in a portion of mice after discontinuation of treatment. Thus, 17β-hydroxywithanolides have the potential to improve the therapeutic outcome for patients with advanced melanoma.

Strobiloscyphones A-F, 6-Isopentylsphaeropsidones and Other Metabolites from Strobiloscypha sp. AZ0266, a Leaf-Associated Fungus of Douglas Fir

Six new 6-isopentylsphaeropsidones, strobiloscyphones A-F (1-6), and a new hexadecanoic acid, (2Z,4E,6E)-8,9-dihydroxy-10-oxohexadeca-2,4,6-trienoic acid (7), together with sphaeropsidone (8) and its known synthetic analogue 5-dehydrosphaeropsidone (9) were isolated from Strobiloscypha sp. AZ0266, a fungus inhabiting the leaf litter of Douglas fir (Pseudotsuga menziesii). The structures of 1-7 were established on the basis of their high-resolution mass and 1D and 2D NMR spectroscopic data, and their relative and/or absolute configurations were determined by NOE, comparison of experimental and calculated ECD spectra, and application of the modified Mosher's ester method. Of these, strobiloscyphone F (6) contains a novel highly oxygenated tetracyclic oxireno-octahydrodibenzofuran ring system. Natural products 1, 6, and 9 and the semisynthetic analogue 12 derived from 8 exhibited cytotoxic activity, whereas 9 and 12 showed antimicrobial activity. Possible biosynthetic pathways to 1-6, 8, and 9 are proposed.

Molecular Basis of Prostate Cancer and Natural Products as Potential Chemotherapeutic and Chemopreventive Agents

Prostate cancer is the second most common malignant cancer in males. It involves a complex process driven by diverse molecular pathways that closely related to the survival, apoptosis, metabolic and metastatic characteristics of aggressive cancer. Prostate cancer can be categorized into androgen dependent prostate cancer and castration-resistant prostate cancer and cure remains elusive due to the developed resistance of the disease. Natural compounds represent an extraordinary resource of structural scaffolds with high diversity that can offer promising chemical agents for making prostate cancer less devastating and curable. Herein, those natural compounds of different origins and structures with potential cytotoxicity and/or in vivo anti-tumor activities against prostate cancer are critically reviewed and summarized according to the cellular signaling pathways they interfere. Moreover, the anti-prostate cancer efficacy of many nutrients, medicinal plant extracts and Chinese medical formulations were presented, and the future prospects for the application of these compounds and extracts were discussed. Although the failure of conventional chemotherapy as well as involved serious side effects makes natural products ideal candidates for the treatment of prostate cancer, more investigations of preclinical and even clinical studies are necessary to make use of these medical substances reasonably. Therefore, the elucidation of structure-activity relationship and precise mechanism of action, identification of novel potential molecular targets, and optimization of drug combination are essential in natural medicine research and development.

Small-Molecule Natural Product Physachenolide C Potentiates Immunotherapy Efficacy by Targeting BET Proteins

Screening for sensitizers of cancer cells to TRAIL-mediated apoptosis identified a natural product of the 17β-hydroxywithanolide (17-BHW) class, physachenolide C (PCC), as a promising hit. In this study, we show that PCC was also able to sensitize melanoma and renal carcinoma cells to apoptosis in response not only to TRAIL, but also to the synthetic polynucleotide poly I:C, a viral mimetic and immune activator, by reducing levels of antiapoptotic proteins cFLIP and Livin. Both death receptor and TLR3 signaling elicited subsequent increased assembly of a proapoptotic ripoptosome signaling complex. Administration of a combination of PCC and poly I:C in human M14 melanoma xenograft and a syngeneic B16 melanoma model provided significant therapeutic benefit as compared with individual agents. In addition, PCC enhanced melanoma cell death in response to activated human T cells in vitro and in vivo in a death ligand-dependent manner. Biochemical mechanism-of-action studies established bromo and extraterminal domain (BET) proteins as major cellular targets of PCC. Thus, by targeting of BET proteins to reduce antiapoptotic proteins and enhance caspase-8-dependent apoptosis of cancer cells, PCC represents a unique agent that can potentially be used in combination with various immunotherapeutic approaches to promote tumor regression and improve outcome. SIGNIFICANCE: These findings demonstrate that PCC selectively sensitizes cancer cells to immune-mediated cell death, potentially improving the efficacy of cancer immunotherapies. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/12/3374/F1.large.jpg.

Chemical Screening of Nuclear Receptor Modulators

Nuclear receptors are ligand-inducible transcriptional factors that control multiple biological phenomena, including proliferation, differentiation, reproduction, metabolism, and the maintenance of homeostasis. Members of the nuclear receptor superfamily have marked structural and functional similarities, and their domain functionalities and regulatory mechanisms have been well studied. Various modulators of nuclear receptors, including agonists and antagonists, have been developed as tools for elucidating nuclear receptor functions and also as drug candidates or lead compounds. Many assay systems are currently available to evaluate the modulation of nuclear receptor functions, and are useful as screening tools in the discovery and development of new modulators. In this review, we cover the chemical screening methods for nuclear receptor modulators, focusing on assay methods and chemical libraries for screening. We include some recent examples of the discovery of nuclear receptor modulators.

Control of the stereochemistry of C14 hydroxyl during the total synthesis of withanolide E and physachenolide C

The stereochemical outcome of the epoxidation of Δ_14–15 cholestanes with mCPBA is controlled by the steric bulk of a C17 substituent. When the C17 is in the β configuration, the epoxide is formed in the α face, whereas if the C17 is trigonal (flat) or the substituent is in the α configuration, the epoxide is formed in the β face. The presence of a hydroxyl substituent at C20 does not influence the stereochemical outcome of the epoxidation.

The epoxide configuration in oxidation of C14–C15 alkenes is determined by the configuration of the C17 substituent.

Cytotoxic and other withanolides from aeroponically grown Physalis philadelphica

Eleven withanolides including six previously undescribed compounds, 16β-hydroxyixocarpanolide, 24,25-dihydroexodeconolide C, 16,17-dehydro-24-epi-dioscorolide A, 17-epi-philadelphicalactone A, 16-deoxyphiladelphicalactone C, and 4-deoxyixocarpalactone A were isolated from aeroponically grown Physalis philadelphica. Structures of these withanolides were elucidated by the analysis of their spectroscopic (HRMS, 1D and 2D NMR, ECD) data and comparison with published data for related withanolides. Cytotoxic activity of all isolated compounds was evaluated against a panel of five human tumor cell lines (LNCaP, ACHN, UO-31, M14 and SK-MEL-28), and normal (HFF) cells. Of these, 17-epi-philadelphicalactone A, withaphysacarpin, philadelphicalactone C, and ixocarpalactone A exhibited cytotoxicity against ACHN, UO-31, M14 and SK-MEL-28, but showed no toxicity to HFF cells.

Withanolides from Aeroponically Grown Physalis peruviana and Their Selective Cytotoxicity to Prostate Cancer and Renal Carcinoma Cells

Investigation of aeroponically grown Physalis peruviana resulted in the isolation of 11 new withanolides, including perulactones I-L (1-4), 17-deoxy-23β-hydroxywithanolide E (5), 23β-hydroxywithanolide E (6), 4-deoxyphyperunolide A (7), 7β-hydroxywithanolide F (8), 7β-hydroxy-17-epi-withanolide K (9), 24,25-dihydro-23β,28-dihydroxywithanolide G (10), and 24,25-dihydrowithanolide E (11), together with 14 known withanolides (12-25). The structures of 1-11 were elucidated by the analysis of their spectroscopic data, and 12-25 were identified by comparison of their spectroscopic data with those reported. All withanolides were evaluated for their cytotoxic activity against a panel of tumor cell lines including LNCaP (androgen-sensitive human prostate adenocarcinoma), 22Rv1 (androgen-resistant human prostate adenocarcinoma), ACHN (human renal adenocarcinoma), M14 (human melanoma), SK-MEL-28 (human melanoma), and normal human foreskin fibroblast cells. Of these, the 17β-hydroxywithanolides (17-BHWs) 6, 8, 9, 11-13, 15, and 19-22 showed selective cytotoxic activity against the two prostate cancer cell lines LNCaP and 22Rv1, whereas 13 and 20 exhibited selective toxicity for the ACHN renal carcinoma cell line. These cytotoxicity data provide additional structure-activity relationship information for the 17-BHWs.

17β-Hydroxywithanolides as Sensitizers of Renal Carcinoma Cells to Tumor Necrosis Factor-α Related Apoptosis Inducing Ligand (TRAIL) Mediated Apoptosis: Structure-Activity Relationships

Renal cell carcinoma (RCC) is a cancer with poor prognosis, and the 5-year survival rate of patients with metastatic RCC is 5-10%. Consequently, treatment of metastatic RCC represents an unmet clinical need. Screening of a 50 000-member library of natural and synthetic compounds for sensitizers of RCC cells to TRAIL-mediated apoptosis led to identification of the 17β-hydroxywithanolide (17-BHW), withanolide E (1), as a promising lead. To explore structure-activity relationships, we obtained natural and semisynthetic withanolides 1, 2a, 2c, and 3-36 and compared their ability to sensitize TRAIL-mediated apoptosis in a panel of renal carcinoma cells. Our findings revealed that 17-BHWs with a α-oriented side chain are superior to known TRAIL-sensitizing withanolides belonging to withaferin A class with a β-oriented side chain and demonstrated that the 17-BHW scaffold can be modified to enhance sensitization of RCCs to TRAIL-mediated apoptosis, thereby assisting development of natural-product-inspired drugs to treat metastatic RCC.

17β-Hydroxy-18-acetoxywithanolides from Aeroponically Grown Physalis crassifolia and Their Potent and Selective Cytotoxicity for Prostate Cancer Cells

When cultivated under aeroponic growth conditions, Physalis crassifolia produced 11 new withanolides (1-11) and seven known withanolides (12-18) including those obtained from the wild-crafted plant. The structures of the new withanolides were elucidated by the application of spectroscopic techniques, and the known withanolides were identified by comparison of their spectroscopic data with those reported. Withanolides 1-11 and 16 were evaluated for their potential anticancer activity using five tumor cell lines. Of these, the 17β-hydroxy-18-acetoxywithanolides 1, 2, 6, 7, and 16 showed potent antiproliferative activity, with some having selectivity for prostate adenocarcinoma (LNCaP and PC-3M) compared to the breast adenocarcinoma (MCF-7), non-small-cell lung cancer (NCI-H460), and CNS glioma (SF-268) cell lines used. The cytotoxicity data obtained for 12-15, 17, and 19 have provided additional structure-activity relationship information for the 17β-hydroxy-18-acetoxywithanolides.