Apratoxin A Shows Novel Pancreas-Targeting Activity through the Binding of Sec 61

Abstract

Apratoxin A is a natural product with potent antiproliferative activity against many human cancer cell lines. However, we and other investigators observed that it has a narrow therapeutic window in vivo. Previous mechanistic studies have suggested its involvement in the secretory pathway as well as the process of chaperone-mediated autophagy. Still the link between the biologic activities of apratoxin A and its in vivo toxicity has remained largely unknown. A better understanding of this relationship is critically important for any further development of apratoxin A as an anticancer drug. Here, we describe a detailed pathologic analysis that revealed a specific pancreas-targeting activity of apratoxin A, such that severe pancreatic atrophy was observed in apratoxin A-treated animals. Follow-up tissue distribution studies further uncovered a unique drug distribution profile for apratoxin A, showing high drug exposure in pancreas and salivary gland. It has been shown previously that apratoxin A inhibits the protein secretory pathway by preventing cotranslational translocation. However, the molecule targeted by apratoxin A in this pathway has not been well defined. By using a 3 H-labeled apratoxin A probe and specific Sec 61a/b antibodies, we identified that the Sec 61 complex is the molecular target of apratoxin A. We conclude that apratoxin A in vivo toxicity is likely caused by pancreas atrophy due to high apratoxin A exposure.

A Mononuclear Iron-Dependent Methyltransferase Catalyzes Initial Steps in Assembly of the Apratoxin A Polyketide Starter Unit

“…This path to incorporation of branched-chain starter units was identified recently in the annotation of gene clusters for biosynthesis of gephyronic acid (Gph) 18 and apratoxin A (Apr) 20, 21 , a Sec61 inhibitor 22, 23 . Based on the natural product structures, the AprA loading module installs a pivaloyl group, whereas GphF introduces an isobutyryl group into gephyronic acid (Figure 1a, 1b).…”

ATG5 Promotes Death Signaling in Response to the Cyclic Depsipeptides Coibamide A and Apratoxin A

“…Apratoxin A, which has been studied more extensively, is known to act by inhibiting cotranslational translocation of a subset of proteins that are sorted to the secretory compartment of mammalian cells, including receptor tyrosine kinases, growth factors and cytokines [ 34 , 35 ]. This action occurs as a result of direct binding of apratoxin A to the alpha, or channel-forming, subunit of the Sec61 protein translocation channel located at the entrance to the ER secretory pathway [ 36 , 37 ]. Inhibition of Sec61 client proteins by apratoxin A leads to a pattern of cellular consequences, some of which appear to be shared with coibamide A, including induction of mTOR-independent autophagy and a cell cycle phase-specific block in G 1 that precedes cell death [ 23 , 30 , 33 , 34 ].…”

Development of apratoxin S10 (Apra S10) as an anti-pancreatic cancer agent and its preliminary evaluation in an orthotopic patient-derived xenograft (PDX) model

“…injection routes were performed, considering both routes have been used in previous apratoxins in vivo studies. [19,31,32] Tissues and plasma were collected at six time points following a single i.v. or i.p.…”