Abstract 2149: Essential role of 5-lipoxygenase in prostate cancer cell survival: Signaling via PKC-epsilon

Background: Previous studies have demonstrated that human prostate cancer cells constitutively generate 5(S)-HETE series of metabolites from arachidonic acid via 5-lipoxygenase (5-Lox) activity, and inhibition of 5-Lox blocks production of 5-Lox metabolites and triggers apoptosis both in androgen-sensitive and androgen-independent prostate cancer cells. Interestingly, this apoptosis is prevented by 5(S)-HETE and 5-oxoETE, but not by leukotrienes or metabolites of 12-Lox or 15-Lox, which is consistent with the idea that the 5-HETE series of metabolites play an essential role in the survival of prostate cancer cells. Downstream signaling mechanisms of 5-Lox metabolites in the survival of prostate cancer cells are still unknown. Recently, we reported that inhibition of 5-Lox triggers apoptosis in prostate cancer cells without inhibition of PI3K-Akt, or MEK-ERK, two well-characterized pro-survival mechanisms. Protein kinase C-epsilon (PKC∈) is a transforming oncogene, up-regulated in a variety of cancer including prostate cancer, and is known to promote cell-survival via well-defined anti-apoptosis mechanisms. Thus, we wanted to address the question whether 5-Lox metabolites promote prostate cancer cell survival via signaling through PKC∈. Methods: Prostate cancer cells were treated with MK591 (a specific 5-Lox inhibitor) or by 5-Lox siRNA, and apoptosis was measured by annexin-V binding, PARP-cleavage, and DNA-degradation. Activation of PKC∈ was analyzed by membrane localization and by enzymatic assays. The role of PKC∈ in cell survival was tested by dominant-negative PKC∈, and by treating cells with activators of PKC∈ before 5-Lox inhibition. Results: We observed that inhibition of 5-Lox induced apoptosis in prostate cancer cells, but not in non-cancer cells such as NIH3T3 and astrocytes which do not express 5-Lox. Induction of apoptosis was preceded by a decrease in membrane localization and enzymatic activity of PKC∈, while no inhibition in phosphorylation of Akt or PDK1 was observed in the same experimental conditions. Both the MK591-mediated decrease in PKC∈ activity and induction of apoptosis were prevented by 5-oxoETE. Interestingly, pre-treatment of prostate cancer cells with activators of PKC∈ (diazoxide and KAE1-1) prevented 5-Lox inhibition-mediated apoptosis, suggesting that metabolites of 5-Lox may regulate prostate cancer cell survival via PKC∈. Moreover, over-expression of dominant-negative PKC∈ killed prostate cancer cells, indicating a critical role of PKC∈ in the survival of these cells. Conclusion: Our findings suggest that inhibition of 5-Lox triggers apoptosis in prostate cancer cells via inhibition of PKC∈, and indicate the existence of an Akt-independent, PKC∈-dependent survival mechanism in prostate cancer cells regulated by 5-Lox activity through metabolism of arachidonic acid, an omega-6 fatty acid plentiful in Western diets.Citation Format: \Authors\. \Abstract title\ [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2149. doi:10.1158/1538-7445.AM2011-2149