ROSEOTOXIN

Identifying effective anti-fibrotic therapies is a major clinical need that remains unmet. In the present study, roseotoxin B was shown to possess an improving effect on cholestatic liver fibrosis in bile duct-ligated mice, as proved by histochemical and immunohistochemical staining, hepatic biochemical parameters, and TUNEL apoptotic cell detection in tissue sections. Using cellular thermal shift assay, computational molecular docking, microscale thermophoresis technology, and surface plasmon resonance biosensor, we confirmed that PDGFR-β was a direct target of roseotoxin B in fibrotic livers. Of note, human tissue microarrays detected pathologically high expression of p-PDGFR-β in liver samples of ~80% of patients with liver fibrosis and cirrhosis. PDGF-B/PDGFR-β pathway promotes transdifferentiation and excessive proliferation of hepatic stellate cells (HSCs), which is a very crucial driver for liver fibrosis. Meaningfully, roseotoxin B blocked the formation of PDGF-BB/PDGFR-ββ complex by targeting the D2 domain of PDGFR-β, thereby inhibiting the PDGF-B/PDGFR-β pathway in HSCs. In summary, our study provided roseotoxin B as a unique candidate agent for the treatment of liver fibrosis.

An immunosuppressant agent with negligible or acceptable toxicity may provide a better therapeutic strategy for treatment of allergic contact dermatitis. We identified a natural cyclopeptide, roseotoxin B, that effectively suppressed cell proliferation and the production of proinflammatory cytokines in activated T cells but exhibited little naive T-cell toxicity at concentrations of 0.3-1 μmol/L. In addition, roseotoxin B inhibited the activation of AKT and signal transducer and activator of transcription-3, suppressed cell cycle-related signaling, caused G0/G1 phase arrest, reduced ribosomal protein-S3 (RPS3)-dependent NF-κB-mediated IL-2 production, and increased autophagy in activated T cells. Furthermore, picryl chloride-induced allergic contact dermatitis was significantly ameliorated by roseotoxin B in mice. The effects of roseotoxin B were inhibited in LC3-knockout mice, indicating that roseotoxin B acts in an autophagy-dependent manner in T-cell-mediated skin diseases. Overall, this study showed a mechanism for roseotoxin B-induced autophagic cell death and provided a unique perspective on autophagy-mediated down-regulation of NF-κB signaling in activated T cells. The unique anti-inflammatory mechanism of roseotoxin B against activated T lymphocytes in allergic contact dermatitis suggests that it could be a potential target for the treatment of immune-related skin diseases.

Natural peptide libraries often contain cyclodepsipeptides containing alpha- or beta-hydroxy residues. Extracts of fungal hyphae of Isaria yield a microheterogenous cyclodepsipeptide mixture in which two classes of molecules can be identified by mass spectral fragmentation of negative ions. In the case of isaridins, which contain an alpha-hydroxy residue and a beta-amino acid residue, a characteristic product ion corresponding to a neutral loss of 72 Da is obtained. In addition, neutral loss of water followed by a 72 Da loss is also observed. Two distinct modes of fragmentation rationalize the observed product ion distribution. The neutral loss of 72 Da has also been obtained for a roseotoxin component, which is also an alpha-hydroxy residue containing cyclodepsipeptide. In the case of isariins, which contain a beta-hydroxy acid residue, ring opening and subsequent loss of the terminal residue as an unsaturated ketene fragment, rationalizes the observed product ion formation. Fragmentation of negative ions provide characteristic neutral losses, which are diagnostic of the presence of alpha-hydroxy or beta-hydroxy residues.