New glycosylated derivatives of versipelostatin, the GRP78/Bip molecular chaperone down-regulator, from Streptomyces versipellis 4083-SVS6

Digitoxose as powerful glycosyls for building multifarious glycoconjugates of natural products and un-natural products

Digitoxose, a significant 2,6-dideoxyhexose found in nature, exists in many small-molecule natural products. These digitoxose-containing natural products can be divided into steroids, macrolides, macrolactams, anthracyclines, quinones, enediynes, acyclic polyene, indoles and oligosaccharides, that exhibit antibacterial, anti-viral, antiarrhythmic, and antitumor activities respectively. As most of digitoxose-containing natural products for clinical application or preclinical tests, this review also summarizes the biosynthesis of digitoxose, and application of compound diversification by introducing sugar plasmids. It may provide a practical approach to expanding the diversity of digitoxose-containing products.

Natural [4 + 2]-Cyclases

[4 + 2]-Cycloadditions are increasingly being recognized in the biosynthetic pathways of many structurally complex natural products. A relatively small collection of enzymes from these pathways have been demonstrated to increase rates of cyclization and impose stereochemical constraints on the reactions. While mechanistic investigation of these enzymes is just beginning, recent studies have provided new insights with implications for understanding their biosynthetic roles, mechanisms of catalysis, and evolutionary origin.

A comprehensive review of glycosylated bacterial natural products

A systematic analysis of all naturally-occurring glycosylated bacterial secondary metabolites reported in the scientific literature up through early 2013 is presented. This comprehensive analysis of 15 940 bacterial natural products revealed 3426 glycosides containing 344 distinct appended carbohydrates and highlights a range of unique opportunities for future biosynthetic study and glycodiversification efforts.

Role of GRP78 in promoting therapeutic-resistant breast cancer

Over 230,000 new cases of invasive breast cancer are diagnosed annually within the USA. Recurrent breast cancer remains a mostly incurable disease with drug resistance, tumor latency and distant metastases driving breast tumor recurrence and morbidity. Understanding drug resistance is a critical component of combating breast cancer. Recently, the protein chaperone GRP78 and the unfolded protein response were implicated as drivers of drug resistance. Preclinical studies show inhibiting GRP78 can reverse drug resistance. Furthermore, drugs developed to target GRP78 show clinical promise in several ongoing clinical trials.

Novel Lobophorins Inhibit Oral Cancer Cell Growth and Induce Atf4- and Chop-Dependent Cell Death in Murine Fibroblasts

As part of the International Cooperative Biodiversity Groups (ICBG) Program, we were interested in identifying biologically active unfolded protein response (UPR) inducing compounds from marine microorganisms isolated from Costa Rican biota. With this aim in mind we have now generated more than 33,000 unique prefractionated natural product extracts from marine and terrestrial organisms that have been submitted to the Center of Chemical Genomics (CCG) at the University of Michigan for high throughput screening (HTS). An effective complementary cell-based assay to identify novel modulators of UPR signaling was used for screening extracts. Active fractions were iteratively subjected to reverse-phase HPLC chromatographic analysis, and together with lobophorin A, B, E, and F (1-4), three new lobophorin congeners, designated as CR1 (5), CR2 (6), and CR3 (7) were isolated. Herein, we report that secondary assays revealed that the new lobophorins induced UPR-associated gene expression, inhibited oral squamous cell carcinoma cell growth, and led to UPR-dependent cell death in murine embryonic fibroblast (MEF) cells.

Spirotetronate polyketides as leads in drug discovery

The discovery of chlorothricin (1) defined a new family of microbial metabolites with potent antitumor antibiotic properties collectively referred to as spirotetronate polyketides. These microbial metabolites are structurally distinguished by the presence of a spirotetronate motif embedded within a macrocyclic core. Glycosylation at the periphery of this core contributes to the structural complexity and bioactivity of this motif. The spirotetronate family displays impressive chemical structures, potent bioactivities, and significant pharmacological potential. This review groups the family members based on structural and biosynthetic considerations and summarizes synthetic and biological studies that aim to elucidate their mode of action and explore their pharmacological potential.

Biosynthesis of versipelostatin: identification of an enzyme-catalyzed [4+2]-cycloaddition required for macrocyclization of spirotetronate-containing polyketides

Versipelostatin (VST) is an unusual 17-membered macrocyclic polyketide product that contains a spirotetronate skeleton. In this study, the entire VST biosynthetic gene cluster (vst) spanning 108 kb from Streptomyces versipellis 4083-SVS6 was identified by heterologous expression using a bacterial artificial chromosome vector. Here, we demonstrate that an enzyme, VstJ, catalyzes the stereoselective [4+2]-cycloaddition between the conjugated diene and the exocyclic olefin of a newly identified tetronate-containing intermediate to form the spirotetronate skeleton during VST biosynthesis.

Our recent progress on the intramolecular Diels-Alder reaction approach in natural products synthesis: synthetic studies of the octahydronaphthalene substructure of versipelostatins and the A/B/C-tricyclic substructure of GKK1032s

During this decade, the enantio- and stereoselective synthesis of natural products has been actively explored in the author's laboratory. In this account, the author outlines practical syntheses of the polycyclic substructures of two novel structurally formidable antibiotics, namely, the trans-fused octahydronaphthalene moiety of versipelostatins and the A/B/C-tricyclic decahydrofluorene moiety of GKK1032s. Both syntheses have been achieved with remarkable regio- and stereoselectivity via intramolecular Diels-Alder reactions using well-designed enantiomeric substrates.

Targeting GRP78 and antiestrogen resistance in breast cancer

Breast cancer is the most prevalent cancer in women, with over 200,000 new cases diagnosed each year. Over 70% of breast cancers express the estrogen receptor-α, and drugs targeting these receptors such as tamoxifen or Faslodex(®) often fail to cure these patients. Many estrogen receptor-positive tumors lose drug sensitivity, making endocrine resistance a major clinical problem. Recently, investigation into the molecular mechanisms of endocrine resistance has highlighted a causative role of the unfolded protein response in antiestrogen resistance. In particular, the master regulator of the unfolded protein response, GRP78, was observed to be elevated in endocrine-resistant breast cancer and directly affected antiestrogen therapy responsiveness. GRP78 was found to impact many different cellular processes that may affect breast cancer survival. Recently, various compounds have been reported to affect GRP78 activity and it may be advantageous to combine these drugs with antiestrogens to overcome endocrine therapy resistance.

Synthetic studies of the spirocyclic cyclohexene part of versipelostatin, a novel GRP78/Bip molecular chaperone downregulator

The spirocyclic part consisting of an α-acylated tetronic acid and a multisubstituted cyclohexene embedded in versipelostatin, a novel GRP78/Bip molecular chaperone downregulator, has been synthesized in enantiomerically pure form. The asymmetric synthesis of the targeted spiro[4.5]-1-oxa-7-decen-2,4-dione derivative was characterized by (1) stereoselective allylation at the α-carbon of methylmalonate diester, in which one carboxylic acid was esterified with a D-glucose-derived chiral template, (2) construction of the tetrasubstituted cyclohexenone substructure by high-yielding ring-closing metathesis and (3) stereoselective construction of the spirocyclic tetronic acid part starting from the cyclohexenone obtained as the ring-closing metathesis product.

Cell-centric view of apoptosis and apoptotic cell death-inducing antitumoral strategies

Programmed cell death and especially apoptotic cell death, occurs under physiological conditions and is also desirable under pathological circumstances. However, the more we learn about cellular signaling cascades, the less plausible it becomes to find restricted and well-limited signaling pathways. In this context, an extensive description of pathway-connections is necessary in order to point out the main regulatory molecules as well as to select the most appropriate therapeutic targets. On the other hand, irregularities in programmed cell death pathways often lead to tumor development and cancer-related mortality is projected to continue increasing despite the effort to develop more active and selective antitumoral compounds. In fact, tumor cell plasticity represents a major challenge in chemotherapy and improvement on anticancer therapies seems to rely on appropriate drug combinations. An overview of the current status regarding apoptotic pathways as well as available chemotherapeutic compounds provides a new perspective of possible future anticancer strategies.

In vitro reconstruction of tetronate RK-682 biosynthesis

The protein phosphatase inhibitor RK-682 is one of a number of potentially valuable tetronate polyketide natural products. Understanding how the tetronate ring is formed has been frustrated by the inaccessibility of the putative substrates. We report the heterologous expression of rk genes in Saccharopolyspora erythraea and reconstitution of the RK-682 pathway using recombinant enzymes, and we show that RkD is the enzyme required for RK-682 formation from acyl carrier protein-bound substrates.