Significance: Tumors promote angiogenesis to facilitate their growth and metastasis; thus, inhibition of angiogenesis is a promising strategy for treating cancer. During angiogenesis, endothelial cells (EC) are stimulated by proangiogenic factors to proliferate and migrate, leading to the formation of new blood vessels. Understanding the mechanisms regulating EC function therefore is essential for the development of new antiangiogenic interventions. Here, we identify a novel mechanism of EC regulation by the recently discovered angiogenesis inhibitor itraconazole, mediated by direct binding to the mitochondrial protein voltage-dependent anion channel 1 (VDAC1). VDAC1 inhibition perturbs mitochondrial ATP production, leading to activation of the AMP-activated protein kinase pathway and subsequent inhibition of mechanistic target of rapamycin, a regulator of EC proliferation. This study suggests VDAC1 may serve as a new therapeutic target for angiogenesis inhibition.

Ipomoeassin F is a potent natural cytotoxin that inhibits growth of many tumor cell lines with single-digit nanomolar potency. However, its biological and pharmacological properties have remained largely unexplored. Building upon our earlier achievements in total synthesis and medicinal chemistry, we used chemical proteomics to identify Sec61α (protein transport protein Sec61 subunit alpha isoform 1), the pore-forming subunit of the Sec61 protein translocon, as a direct binding partner of ipomoeassin F in living cells. The interaction is specific and strong enough to survive lysis conditions, enabling a biotin-analogue of ipomoeassin F to pull down Sec61α from live cells, yet it is also reversible as judged by several experiments including fluorescent streptavidin staining, delayed competition in affinity pulldown, and inhibition of TNF-biogenesis after washout. Sec61α forms the central subunit of the ER protein translocation complex, and the binding of ipomoeassin F results in a substantial, yet selective, inhibition of protein translocation in vitro and a broad ranging inhibition of protein secretion in live cells. Lastly, the unique resistance profile demonstrated by specific single nucleotide point mutations in Sec61α provides compelling evidence that Sec61α is the primary molecular target of ipomoeassin F and strongly suggests that the binding of this natural product to Sec61α is distinctive. Therefore, ipomoeassin F represents the first plant-derived, carbohydrate-based member of a novel structural class that offers new opportunities to explore Sec61α function and to further investigate its potential as a therapeutic target for drug discovery.

The heterotrimeric Sec61 complex is a major site for the biogenesis of transmembrane proteins (TMPs), accepting nascent TMP precursors that are targeted to the endoplasmic reticulum (ER) by the signal recognition particle (SRP). Unlike most single-spanning membrane proteins, the integration of type III TMPs is completely resistant to small molecule inhibitors of the Sec61 translocon. Using siRNA-mediated depletion of specific ER components, in combination with the potent Sec61 inhibitor ipomoeassin F (Ipom-F), we show that type III TMPs utilise a distinct pathway for membrane integration at the ER. Hence, following SRP-mediated delivery to the ER, type III TMPs can uniquely access the membrane insertase activity of the ER membrane complex (EMC) via a mechanism that is facilitated by the Sec61 translocon. This alternative EMC-mediated insertion pathway allows type III TMPs to bypass the Ipom-F-mediated blockade of membrane integration that is seen with obligate Sec61 clients.