Fumiquinazoline D

Fumiquinazoline alkaloids have attracted much attention from medicinal and natural product chemists due to their interesting structures and biological potential. In this study, three new and 12 known fumiquinazoline alkaloids were isolated and characterized from the marine fungusScedosporium apiospermumF41-1. The structures of the new compounds and their absolute configurations were determined using NMR spectroscopy, ECD, and OR calculations. The compounds were evaluated for their antidiabetic potential by determining their triglyceride-promoting activity using 3T3-L1 adipocytes. One of the new compounds, scequinadoline J (14), as well as scequinadolines D (9) and E (10), was found to promote triglyceride accumulation in 3T3-L1 cells. Scequinadoline D (9) demonstrated the most potent activity, with an EC50value of 0.27 ± 0.03 μM. Quantitative polymerase chain reaction experiments suggested that scequinadoline D (9) acts through activation of the PPARγ pathway. It stimulated the mRNA expression of PPARγ, AMPKα, C/EBPα, LXRα, SCD-1, and FABP4. In addition, its triglyceride-promoting efficacy could be blocked by a double dose of the PPARγ antagonist GW9662. These results indicated that scequinadoline D (9) is a potent insulin sensitizer that targets adipocytes and may be useful for the treatment of type 2 diabetes mellitus after further investigation.

Further chemical examination of a coral-associated fungusAspergillus versicolorLZD-14-1 by the PHLC-DAD detection resulted in the isolation of six new polycyclic alkaloids, namely versiquinazolines L-Q (1-6). Their structures were determined by extensive analyses of spectroscopic data, including quantum ECD calculation and X-ray single crystal diffraction for the assignment of absolute configurations. Versiquinazoline L bearing a d-Ala residue and versiquinazoline M containing an l-serine residue are rarely found in the fumiquinazoline-type alkaloids, while versiquinazoline P displayed an unusual scaffold with a spiro-γ-lactone. Versiquinazolines P and Q exhibited significant inhibition against thioredoxin reductase (TrxR) with IC50values of 13.6 ± 0.6 and 12.2 ± 0.7 μM, which showed higher activity than the positive control curcumin (IC50= 25 μM). The weak cytotoxicity and potent inhibition toward TrxR suggested that versiquinazolines P and Q are potential for microenvironmental regulation of tumor progression and metastasis.

Cyclization of linear peptidyl precursors produced by nonribosomal peptide synthetases (NRPSs) is an important step in the biosynthesis of bioactive cyclic peptides. Whereas bacterial NRPSs use thioesterase domains to perform the cyclization, fungal NRPSs have apparently evolved to use a different enzymatic route. In verified fungal NRPSs that produce macrocyclic peptides, each megasynthetase terminates with a condensation-like (C(T)) domain that may perform the macrocyclization reaction. To probe the role of such a C(T) domain, we reconstituted the activities of the Penicillium aethiopicum trimodular NPRS TqaA in Saccharomyces cerevisiae and in vitro. Together with the reconstituted bimodular NRPS AnaPS, we dissected the cyclization steps of TqaA in transforming the linear anthranilate-D-tryptophan-L-alanyl tripeptide into fumiquinazoline F. Extensive biochemical and mutational studies confirmed the essential role of the C(T) domain in catalyzing cyclization in a thiolation domain-dependent fashion. Our work provides evidence of a likely universal macrocyclization strategy used by fungal NRPSs.