Abstract P2-24-08: Altersolanol B, a fungal metabolite, induces proteasome-dependent degradation of estrogen receptor α (ERα) and inhibits downstream signaling targets in ER+ breast adenocarcinoma cells
Authors: Siraj Md Afjalus , Rahman Md Sajjadur , Jacobs Aaron T. , Tan Ghee T. 
Published: March 2023

Abstract
We recently reported the cellular signal-modulating properties of altersolanol B (AB), a minor fungal tetrahydroanthraquinone (THAQ) metabolite, in the estrogen receptor positive (ER+) human breast adenocarcinoma cell lines, MCF-7 and T47D. MCF-7 (IC50 5.5 µM) and T47D (IC50 8.8 µM) were observed to be 4- and 2.4-fold more sensitive to the antiproliferative effects of AB, respectively, compared to MDA-MB-231 (triple-negative, IC50 21.3 µM). AB disrupted both AKT and ERK1/2 signaling leading to intrinsic apoptosis in MCF-7. The clinical limitations of multi-agent combination therapy that targets multiple pathways in cancer may potentially be circumvented by using a single molecule, such as AB, that inhibits both AKT and ERK1/2 signaling. The THAQ pharmacophore, with its disrupted conjugated ring system and relative redox inactivity, may possess greater mechanistic advantage against ER+ breast cancer when compared to the fully conjugated ring systems of the anthracyclines (doxorubicin) and anthraquinone (mitoxantrone) that are associated with nonselective mechanisms. The present phase of our study provides evidence that AB downregulated ERα expression at the post-translational level through the induction of proteasome-dependent degradation similar to a classical selective estrogen receptor degrader (SERD). AB dose-dependently downregulated both wild-type (WT)- and 17β-estradiol (E2)-stimulated ERα protein expression without altering mRNA expression in ER+ cells. This was corroborated by the marked reduction in the expression of downstream ERα target genes (cathepsin D and pS2). AB also dose-dependently downregulated cyclin D1, the estrogen-independent activator of ER. Antiproliferative effects were enhanced when ER+ cells were exposed to a combination of AB and the ERα-selective antagonist, MPP dihydrochloride. The AKT activator, SC79, showed that the AB-induced ERα protein downregulation was independent of the inhibition of AKT-FOXO1 signaling. The expression of Hsp90 was not affected. Co-treatment with the protein synthesis inhibitor, cycloheximide, revealed that AB downregulated ERα expression at the post-translational level. The proteosome inhibitor, MG-132, effectively suppressed AB-induced ERα degradation and restored cellular proliferation. In silico approaches were adopted to probe the direct binding of AB to ERα. AB demonstrated significant binding affinity with human ERα (PDB ID - 3ert) compared to the binding of the standard SERD, fulvestrant. Non-covalent bonding interaction analysis further revealed that, much like fulvestrant, AB successfully interacted with 5 major amino acid residues (i.e., Ala350, Asp351, Leu525, Leu536 and Trp383) at the ligand-binding domain of ERα which is stabilized by 7 non-bonding interactions. Principal component analysis indicated that binding with AB improved the compactness of 3ert folding. Molecular dynamics simulation indicated that the 3ert-AB interaction was stable even in the 100 ns simulated physiological environment.
Citation Format: Md Afjalus Siraj, Md Sajjadur Rahman, Aaron T. Jacobs, Ghee T. Tan. Altersolanol B, a fungal metabolite, induces proteasome-dependent degradation of estrogen receptor α (ERα) and inhibits downstream signaling targets in ER+ breast adenocarcinoma cells [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-24-08.