Aulosirazoles B and C from the Cyanobacterium Nostoc sp. UIC 10771: Analogues of an Isothiazolonaphthoquinone Scaffold that Activate Nuclear Transcription Factor FOXO3a in Ovarian Cancer Cells

Abstract

The known solid-tumor-selective cytotoxin aulosirazole (1) was identified from bioactive extracts from the culture medium of the cyanobacterium Nostoc sp. UIC 10771. Here, we demonstrate that 1 induces the nuclear accumulation of FOXO3a in OVCAR3 using both Western blot analysis and immunofluorescence confocal microscopy. We also report the discovery of two additional analogues, aulosirazoles B (2) and C (3). Structures for compounds 2 and 3 were determined using HR-ESI-LC-MS/MS and 1D and 2D NMR experiments. Aulosirazoles B (2) and C (3) represent the first natural analogues of the FOXO-activating compound aulosirazole (1) and are the second and third isothiazole-containing metabolites reported from this phylum.

Chart 1.

Structures of Compounds Isolated from Nostoc sp. UIC 10771

Figure 1.

Key COSY and HMBC correlations observed in 2.

Figure 2.

Key HMBC and LR-HSQMBC correlations observed in 3.

Figure 3.

Aulosirazole (1) induces transactivation and nuclear accumulation of FOXO3a in OVCAR3. Following treatment with 1 (1 μM), strong nuclear accumulation and reduced cytosolic concentration of FOXO3a were observed compared to the vehicle (DMSO) control. Treatment with the positive control also led to the nuclear accumulation of FOXO3a. HDAC2 was selected as a positive control marker for proteins from the nuclear cellular fraction. GAPDH was selected as a positive control marker for the proteins from the cytoplasmic cellular fraction. One-way ANOVA with Dunnett’s multiple comparisons to vehicle control was used to generate P values (*P < 0.05).

Figure 4.

Aulosirazole (1) induces nuclear accumulation of FOXO3a in OVCAR3. Treatment with 1 (1 μM) led to an accumulation of FOXO3a in the nucleus. Treatment with the positive control, LY294002 (2 μM), led to a slight increase in the nuclear concentration of FOXO3a.