Apratoxin A Reversibly Inhibits the Secretory Pathway by Preventing Cotranslational Translocation

Abstract

Apratoxin A is a potent cytotoxic marine natural product that rapidly inhibits signal transducer and activator of transcription (STAT) 3 phosphorylation by an undefined mechanism. We have used biochemical and proteomics approaches to illuminate upstream molecular events. Apratoxin A inhibits Janus kinase (JAK)/STAT signaling through rapid down-regulation of interleukin 6 signal transducer (gp130). Apratoxin A also depletes cancer cells of several cancer-associated receptor tyrosine kinases by preventing their N-glycosylation, leading to their rapid proteasomal degradation. A proteomics approach revealed that several proteins in the endoplasmic reticulum, the site of N-glycoprotein synthesis, are down-regulated upon apratoxin A exposure. Using in vitro cell free systems, we demonstrated that apratoxin A prevents cotranslational translocation of proteins destined for the secretory pathway. This process is reversible in living cells. Our study indicates that apratoxins are new tools to study the secretory pathway and raises the possibility that inhibition of cotranslational translocation may be exploited for anticancer drug development.

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

“…The Sec61 complex is also responsible for export of mis-folded proteins from the ER lumen to the cytosol for further degradation (27). It has been demonstrated previously that apratoxin A can block cotranslational translocation (9). Downregulation of several membrane-associated receptors, such as gp130, c-MET, HER-2, PDGFR-b, and IGF1R-b, was also reported following apratoxin A exposure.…”

“…It has also been suggested that apratoxin A can stabilize Hsc70/Hsp70 interaction with Hsp90 client proteins to prevent their binding to Hsp90; such stabilization results in degradation of EGFR and ErbB2 through chaperone-mediated autophagy (8). In a cell-free in vitro system, apratoxin A was also shown to inhibit the secretory pathway by preventing cotranslational translocation of newly synthesized proteins (9).…”

A Proteomic Survey Indicates Sortilin as a Secondary Substrate of the ER Translocation Inhibitor Cyclotriazadisulfonamide (CADA)

“…Most of these inhibitors interfere with a specific step in the translocation process and thus affect a wide spectrum of translocon substrates, often associated with cellular toxicity (35,36). The fungal cyclic heptadepsipeptide HUN-7293 was originally identified as an inhibitor of vascular cell adhesion molecule 1 (VCAM-1) expression in endothelial cells (37,38).…”

Eeyarestatin I inhibits Sec61-mediated protein translocation at the endoplasmic reticulum

“…Furthermore, the accumulation of cytosolic proteins that would result from an inhibition of ER translocation might also contribute to the cytotoxicity of ES I and its potential anticancer properties (Rane et al, 2008;Wang et al, 2009). The inhibition of ER translocation by the cyclodepsipeptide apratoxinA has also been linked to an anticancer effect, in this case by preventing the biosynthesis of cancer-associated receptor tyrosine kinases (Liu et al, 2009). Thus, modulation of protein homeostasis by targeting fundamental cellular components such as the Sec61 complex appears to be a valid target for therapeutic intervention.…”