Abstract
Activation of the serine/threonine kinase Akt is associated with aggressive clinical behavior of prostate cancer. We found that the human prostate cancer cell lines LNCaP and PC-3 express predominantly Akt1 and Akt2. Selective down-regulation of Akt1, but not Akt2, by short-hairpin RNA reduced the viability of prostate cancer cells. In addition, structurally different Akt inhibitors were cytotoxic for the prostate cancer cells, confirming that the Akt pathway is indispensable for their viability. We have purified the tetracyclic triterpenoids 3-oxo-tirucallic acid, 3-α-acetoxy-tirucallic acid, and 3-β-acetoxy-tirucallic acid from the oleogum resin of Boswellia carterii to chemical homogeneity. The acetoxy-derivatives in particular potently inhibited the activities of human recombinant Akt1 and Akt2 and of constitutively active Akt immunoprecipitated from PC-3 cells, whereas inhibitor of nuclear factor-κB kinases remained unaffected. Docking data indicated that these tetracyclic triterpenoids form hydrogen bonds within the phosphatidylinositol binding pocket of the Akt pleckstrin homology domain. Accordingly, 3-β-acetoxy-tirucallic acid did not inhibit the activity of Akt1 lacking the pleckstrin homology domain. In the prostate cancer cell lines investigated, these compounds inhibited the phosphorylation of cellular Akt and the Akt signaling pathways, including glycogen synthase kinase-3β and BAD phosphorylation, nuclear accumulation of p65, the androgen receptor, β-catenin, and c-Myc. These events culminated in the induction of apoptosis in prostate cancer, but not in nontumorigenic cells. The tirucallic acid derivatives inhibited proliferation and induced apoptosis in tumors xenografted onto chick chorioallantoic membranes and decreased the growth of pre-established prostate tumors in nude mice without overt systemic toxicity. Thus, tirucallic acid derivatives represent a new class of Akt inhibitors with antitumor properties.
Footnotes
↵ The online version of this article (available at http://molpharm.aspetjournals.org) contains supplemental material.
This work was supported by the Deutsche Krebshilfe [Grant 102383] (to T.S. and T.S.).
Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org.
doi:10.1124/mol.109.060475
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ABBREVIATIONS:
- PTEN
- phosphatase and tensin homolog deleted on chromosome ten
- PDK1
- phosphoinositide-dependent kinase 1
- GSK-3
- glycogen synthase kinase-3
- IKK
- IκB kinase
- IκB
- inhibitor of nuclear factor-κB
- CAM
- chorioallantoic membrane
- ERK
- extracellular signal-regulated kinase
- OTA
- 3-oxo-tirucallic acid
- αATA
- 3-α-acetoxy-tirucallic acid
- βATA
- 3-β-acetoxy-tirucallic acid
- PVP
- polyvinylpyrrolidone K10
- PBMC
- peripheral blood mononuclear cell
- TUNEL
- terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling
- PCR
- polymerase chain reaction
- siRNA
- small interfering RNA
- shRNA
- short-hairpin RNA
- PH
- pleckstrin homology
- SPR
- surface plasmon resonance.
- Received August 20, 2009.
- Accepted December 16, 2009.
- Copyright © 2010 The American Society for Pharmacology and Experimental Therapeutics
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