Diastereomers of Coibamide A Show Altered Sec61 Client Selectivity and Ligand-Dependent Activity against Patient-Derived Glioma Stem-like Cells.

Coibamide A (CbA) is a cyanobacterial lariat depsipeptide that selectively inhibits multiple secreted and integral membrane proteins from entering the endoplasmic reticulum secretory pathway through binding the alpha subunit of the Sec61 translocon. As a complex peptide-based macrocycle with 13 stereogenic centers, CbA is presumed to adopt a conformationally restricted orientation in the ligand-bound state, resulting in potent antitumor and antiangiogenic bioactivity. A stereochemical structure-activity relationship for CbA was previously defined based on cytotoxicity against established cancer cell lines. However, the ability of synthetic isomers to inhibit the biosynthesis of specific Sec61 substrates was unknown. Here, we report that two less toxic diastereomers of CbA, [L-Hiv(2)]-CbA and [L-Hiv(2), L-MeAla(11)]-CbA, are pharmacologically active Sec61 inhibitors. Both compounds inhibited the expression of a secreted reporter (Gaussia luciferase), VEGF-A, and a Type 1 membrane protein (VCAM1), while [L-Hiv(2)]-CbA also decreased the expression of ICAM1 and BiP/GRP78. Analysis of 43 different chemokines in the secretome of SF-268 glioblastoma cells revealed different inhibitory profiles for the two diastereomers. When the cytotoxic potential of CbA compounds was compared against a panel of patient-derived glioblastoma stem-like cells (GSCs), Sec61 inhibitors were remarkably toxic to five of the six GSCs tested. Each ligand showed a distinct cytotoxic potency and selectivity pattern for CbA-sensitive GSCs, with IC(50) values ranging from subnanomolar to low micromolar concentrations. Together, these findings highlight the extreme sensitivity of GSCs to Sec61 modulation and the importance of ligand stereochemistry in determining the spectrum of inhibited Sec61 client proteins.