Isolation, Structure Elucidation, and Biological Activity of the Selective TACR2 Antagonist Tumonolide and its Aldehyde from a Marine Cyanobacterium.

The macrocyclic tumonolide (1) with enamide functionality and the linear tumonolide aldehyde (2) are new interconverting natural products from a marine cyanobacterium with a peptide-polyketide skeleton, representing a hybrid of apratoxins and palmyrolides or laingolides. The planar structures were established by NMR and mass spectrometry. The relative configuration of the stereogenically-rich apratoxin-like polyketide portion was determined using J-based configuration analysis. The absolute configuration of tumonolide (1) was determined by chiral analysis of the amino acid units and computational methods, followed by NMR chemical shift and ECD spectrum prediction, indicating all-R configuration for the polyketide portion, as in palmyrolide A and contrary to the all-S configuration in apratoxins. Functional screening against a panel of 168 GPCR targets revealed tumonolide (1) as a selective antagonist of TACR2 with an IC of 7.0 μM, closely correlating with binding affinity. Molecular docking studies established the binding mode and rationalized the selectivity for TACR2 over TACR1 and TACR3. RNA sequencing upon treatment of HCT116 colorectal cancer cells demonstrated activation of the pulmonary fibrosis idiopathic signaling pathway and the insulin secretion signaling pathway at 20 μM, indicating its potential to modulate these pathways.