Abstract 768: Sweet gum extract inhibits proliferation of prostate cancer PC3 cells through suppression of mTOR pathway, but through MAPK pathways in DU145 prostate cancer cells

Abstract

Abstract A methanolic fruit extract (LIS-100) of sweet gum (Liquidambar styraciflua L.) containing lupine and oleanane-type triterpenoids has shown potential in inhibiting the proliferation of multiple human cancer cell lines particularly against human prostate cancer cells (Proc. Am. Assoc. Cancer Res, 1431, 2008). Here we further investigated the inhibition of LIS-100 on prostate cancer cells with wildtype PTEN (DU145) and PTEN -mutant (PC3) prostate cancer cells and associated molecular mechanisms. LIS-100 inhibited the proliferation of PC3 cells at IC50 of 3.0 ± 0.2 µg/ml and DU145 cells at IC50 of 39.1 ± 2.5 μg/ml, a 13-fold potency difference. LIS-100 altered the cell cycle at G1 phase and induced apoptosis at 10 µg/ml in the PC3 and LNCaP cells, whereas it induced autophagic cell death at 25 to 40 µg/ml in the DU145 cells evidenced by increased expression of LC3 proteins (hallmark for autophagic cell death) and acrine orange staining. These effects were time- (24 to 72 hrs) and concentration-dependent (2.5 to 10 µg/ml for PC3 and 10 to 40 µg/ml for DU145). In PC3 cells, the expression of CDK4 and Cyclin D1 was inhibited. In contrast, it was CDK2 and CDK7 that were reduced in DU145 cells, suggesting different molecular targets of LIS-100 in PC3 and DU145 cells. When PC3 cells were treated with LIS-100 (0.5 to 5 µg/ml), the activity of both PI3K and mTOR were inhibited at almost equal ED50 (5.5 and 4.25 µg/ml, respectively). However, the expression of mTOR was inhibited to a greater degree, at least 3-fold, over PI3K/Akt proteins in PC3 cells. Functional Proteomic Array analyses confirmed that LIS-100 (2.5 to 5 µg/ml) not only inhibited proteins associated with PI3K/Akt/mTOR pathways (e.g. Akt, p70S6K, S6, elF4EP, GSK3, TSC2), also suppressed upstream of PI3-kinase cell signaling pathways (IGFr, and IRS-1 proteins) in PC3 cells. Furthermore, the anti-proliferative effect of LIS-100 was reduced by 46% in mTOR siRNA-transfected PC3 cells from the control siRNA-transfected cells, confirming the important role of mTOR cell signaling pathway in LIS-100 mediated cell growth suppression in this particular cell. In contrast, when DU145 cells were treated with LIS-100 at 25 µg/ml, the phosphorylation of Akt, p70S6K, and pS6 was not affected, whereas pERK and pSTAT3 levels were reduced. Interestingly, LIS-100 reduced the formation of both anti-apoptotic lipid mediators 5-HETE and 12-HETE and increased the production of tumor suppressor 15-HETE in both PC3 and LNCaP cells, no such effects were observed in DU145 cells. These results suggest antiproliferative effect of sweet gum in prostate cancer cells is mediated through different molecular mechanisms. The important role of PTEN status on sweet gum elicited anti-proliferative activity on prostate cancer is currently under investigation. This study was supported by NIH grant 1R21 AT004514-01A1 to P. Yang. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 768.