Phosphatidylinositol-3-kinase (PI3K), and its downstream effector Akt, or protein kinase B␣ (PKB␣), play a major regulatory role in control of apoptosis, proliferation, and angiogenesis. PI3K and Akt are amplified or overexpressed in a number of malignancies, including sarcomas, ovarian cancer, multiple myeloma, and melanoma. This pathway regulates production of the potent angiogenic factor vascular endothelial growth factor (VEGF), and protects tumor cells against both chemotherapy and reactive oxygen-induced apoptosis through phosphorylation of substrates such as apoptotic peptidaseactivating factor-1 (APAF-1), forkhead proteins, and caspase 9. Given its diverse actions, compounds that suppress the PI3K/Akt pathway have potential pharmacologic utility as angiogenesis inhibitors and antineoplastic agents. Using the SVR angiogenesis assay, a screen of natural products, we isolated the alkaloid solenopsin, and found that it is a potent angiogenesis inhibitor. We also found that solenopsin inhibits the PI3K signaling pathway in
IntroductionThe serine/threonine kinase c-Akt-1, or protein kinase B␣ (PKB), is the cellular homolog of a transforming oncogene initially isolated from a lymphoma. Akt is a downstream target of phosphatidylinositol-3-kinase (PI3K), a family of at least 4 different enzymes, with the prototypical PI3K heterodimer consisting of a p85 (regulatory) and a p110 (catalytic) subunit. The PI3K/Akt pathway is involved in the regulation of diverse cellular functions including proliferation, cytoskeletal organization, survival, and malignant transformation. [1][2][3][4] Upon binding of PI3K products to its pleckstrin homology domain, Akt is translocated to the plasma membrane where it is activated by upstream phosphorylated kinases, including PI3K-dependent kinases 1 and 2 (PDK1 and PDK2) and mammalian target of rapamycin complex 2 (mTORC2). The PI3K/Akt pathway is stimulated by numerous receptor tyrosine kinases and oncogenes, including receptors for insulin-like growth factor 1 (IGF-1), platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), ras, Her2/neu, and polyoma middle T oncogenes. [5][6][7][8][9][10] Because Akt plays a central role in regulating apoptosis, angiogenesis, and metabolism of cells, Akt is an attractive pharmacologic target for the treatment of cancer and inflammation. 11,12 Small-molecular-weight inhibitors of PI3K include LY 294002 and the fungal metabolite wortmannin, 13 as well as ether phospholipids, including perifosine, which has entered clinical trials. 14,15 Using the SVR angiogenesis assay, [16][17][18] we found that solenopsin A, 19,20 the primary alkaloid from the fire ant Solenopsis invicta, has antiangiogenic activity. We also discovered that solenopsin disrupted angiogenesis in vivo in embryonic zebrafish. In order to determine the mode of action, we examined the ability of solenopsin to inhibit a battery of cellular kinases and found that solenopsin A inhibited Akt relatively selectively in an ATP-competitive manner without affecting its u...
