Borrelidin

Chemoselective purification technologies have seen great success in biomolecule isolation, with a classic example being the genetically-encoded His tag utilized to enrich desired proteins from a crude lysate. We sought to translate this purification tactic into a powerful tool for the isolation of natural products and demonstrate that chemoselective enrichment can reduce the number of purification steps required and increase the yield obtained for important natural products, as compared to the use of traditional chromatography methods alone. To date, we have reported reversible enrichment tags for three functional groups, carboxylic acids and aliphatic or aryl hydroxyls. To illustrate the power of chemoselectivity-mediated purification of natural products, we present here an improved isolation of borrelidin. Application of our carboxylic acid tag yielded pure borrelidin in only two steps and with double the yield acquired with traditional chromatography methods.

Oral squamous cell carcinoma (OSCC) is the most common cancer affecting the oral cavity, and US clinics will register about 30,000 new patients in 2015. Current treatment modalities include chemotherapy, surgery, and radiotherapy, which often result in astonishing disfigurement. Cancers of the head and neck display enhanced levels of glucose-regulated proteins and translation initiation factors associated with endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). Previous work demonstrated that chemically enforced UPR could overwhelm these adaptive features and selectively kill malignant cells. The threonyl-tRNA synthetase (ThRS) inhibitor borrelidin and two congeners were discovered in a cell-based chemical genomic screen. Borrelidin increased XBP1 splicing and led to accumulation of phosphorylated eIF2α and UPR-associated genes, prior to death in panel of OSCC cells. Murine embryonic fibroblasts (MEFs) null for GCN2 and PERK were less able to accumulate UPR markers and were resistant to borrelidin.

Borrelidin (1) is a nitrile-containing bacterially derived polyketide that is a potent inhibitor of bacterial and eukaryotic threonyl-tRNA synthetases. We now report the discovery of borrelidin B (2), a tetrahydro-borrelidin derivative containing an aminomethyl group in place of the nitrile functionality in borrelidin. The discovery of this new metabolite has implications for both the biosynthesis of the nitrile group and the bioactivity of the borrelidin compound class. Screening in the SToPS assay for tRNA synthetase inhibition revealed that the nitrile moiety is essential for activity, while profiling using our in-house image-based cytological profiling assay demonstrated that 2 retains biological activity by causing a mitotic stall, even in the absence of the nitrile motif.

An inhibitor of 3T3-L1 adipocyte differentiation was isolated from Streptomyces sp. TK08330 and identified by spectroscopy as the 18-membered macrolide borrelidin. Treatment with 1.0 μM borrelidin suppressed intracellular lipid accumulation by 80% and inhibited the expression of adipocyte-specific genes. Borrelidin suppressed the mRNA expression of two master regulators of adipocyte differentiation, peroxisome proliferator-activated receptor gamma (PPARγ) and CCAAT/enhancer binding protein (C/EBPα). Studies on well-known upstream regulators of PPARγ revealed that borrelidin down-regulated C/EBPδ mRNA expression but did not affect expression of C/EBPβ. Borrelidin increased mRNA expression of negative regulators of differentiation such as GATA-binding protein (GATA) 3, Krüppel-like factor (KLF) 3 and KLF7, as well as positive regulators, KLF4, KLF6 and KLF15, at early stages of differentiation. To elucidate a primary mediator of borrelidin differentiation inhibitory activity, small interfering RNA (siRNA) transfection experiments were performed.