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Oncogenic Action of Secreted Phospholipase A₂ in Prostate Cancer

¹ Department of Medicine and Sydney Cancer Centre, ² Institute of Biomedical Research, School of Biomedical Sciences and Pathology, The University of Sydney, Australia; ³ St Vincent’s Hospital Clinical School, School of Medical Sciences and Department of Clinical Medicine, The University of New South Wales, Australia; ⁴ Department of Anatomical Pathology, Royal Prince Alfred Hospital, Sydney, Australia; ⁵ Departments of Chemistry and Biochemistry, University of Washington, Seattle, Washington; ⁶ Oncology Research Centre, Prince of Wales Hospital, Sydney, Australia; and ⁷ Therapeutics Centre, St. Vincent’s Hospital, Sydney, Australia

Mortality from prostate cancer is associated with progression of tumors to androgen-independent growth and metastasis. Eicosanoid products of both the cyclooxygenase (COX) and lipoxygenase (LOX) pathways are important mediators of the proliferation of prostate cancer cells in culture and regulate tumor vascularization and metastasis in animal models. Pharmacologic agents that block either COX or LOX products effectively reduce the size of prostate cancer xenografts. Phospholipase A₂ (PLA₂) enzymes regulate the provision of arachidonic acid to both COX- and LOX-derived eicosanoids, and a secreted form of the enzyme (sPLA₂-IIA) is elevated in prostate cancer tissues. Here, we show by immunohistochemistry, in patients receiving androgen ablation therapy, that sPLA₂-IIA remains elevated in remaining cancer cells relative to benign glands after treatment. Furthermore, sPLA₂-IIA expression seen in benign glands is substantially decreased after androgen depletion, whereas cytosolic PLA₂- (cPLA₂-) levels are unchanged. sPLA₂-IIA mRNA expression is detectable and inducible by androgen (0.01–10 nmol/L) in the androgen-sensitive cell line LNCaP, and exogenous addition of sPLA₂-IIA (1–100 nmol/L), but not an inactive sPLA₂-IIA mutant (H₄₈Q), results in a dose-dependent increase in cell numbers or the fraction of cells in G₂-M phase, which is inhibited by sPLA₂-IIA-selective inhibitors. The effect of exogenous sPLA₂-IIA can also be blocked by inhibition of cPLA₂-, suggesting a role for cPLA₂- in mediating sPLA₂-IIA action. sPLA₂-IIA inhibitors suppressed basal proliferation in LNCaP cells and in the androgen-independent, sPLA₂-positive cell line PC3 but not in the sPLA₂-IIA-negative androgen-independent cell line DU145. Established PC3 xenograft tumors grew more slowly in mice treated with sPLA₂-IIA inhibitors than those treated with saline only. The PLA₂ enzymes, and sPLA₂-IIA in particular, thus represent important targets for the treatment of sPLA₂-IIA-positive androgen-independent prostate cancer.

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