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 andthe linear tumonolide aldehyde (2) are new interconvertingnatural products from a marine cyanobacterium with a peptide-polyketide skeleton, representing a hybrid of apratoxins andpalmyrolides or laingolides. The planar structures were estab-lished by NMR and mass spectrometry. The relative config-uration of the stereogenically-rich apratoxin-like polyketideportion was determined using J-based configuration analysis.The absolute configuration of tumonolide (1) was determinedby chiral analysis of the amino acid units and computationalmethods, followed by NMR chemical shift and ECD spectrumprediction, indicating all-R configuration for the polyketideportion, as in palmyrolide A and contrary to the all-Sconfiguration in apratoxins. Functional screening against apanel of 168 GPCR targets revealed tumonolide (1) as aselective antagonist of TACR2 with an IC50 of 7.0 μM, closelycorrelating with binding affinity. Molecular docking studiesestablished the binding mode and rationalized the selectivityfor TACR2 over TACR1 and TACR3. RNA sequencing upontreatment of HCT116 colorectal cancer cells demonstratedactivation of the pulmonary fibrosis idiopathic signaling path-way and the insulin secretion signaling pathway at 20 μM,indicating its potential to modulate these pathways.