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. 2011 Dec;2(12):1176-90.
doi: 10.18632/oncotarget.397.

Phospholipase PLA2G7, associated with aggressive prostate cancer, promotes prostate cancer cell migration and invasion and is inhibited by statins

Paula Vainio  1 , Laura Lehtinen, Tuomas Mirtti, Mika Hilvo, Tuulikki Seppänen-Laakso, Johannes Virtanen, Anna Sankila, Stig Nordling, Johan Lundin, Antti Rannikko, Matej Orešič, Olli Kallioniemi, Kristiina Iljin
Affiliations

Phospholipase PLA2G7, associated with aggressive prostate cancer, promotes prostate cancer cell migration and invasion and is inhibited by statins

Paula Vainio et al. Oncotarget. 2011 Dec.

Abstract

Prostate cancer is the second leading cause of cancer mortality in men in developed countries. Due to the heterogeneous nature of the disease, design of novel personalized treatments is required to achieve efficient therapeutic responses. We have recently identified phospholipase 2 group VII (PLA2G7) as a potential drug target especially in ERG oncogene positive prostate cancers. Here, the expression profile of PLA2G7 was studied in 1137 prostate cancer and 409 adjacent non-malignant prostate tissues using immunohistochemistry to validate its biomarker potential and putative association with disease progression. In order to reveal the molecular alterations induced by PLA2G7 impairment, lipidomic and gene expression profiling was performed in response to PLA2G7 silencing in cultured prostate cancer cells. Moreover, the antineoplastic effect of statins combined with PLA2G7 impairment was studied in prostate cancer cells to evaluate the potential of repositioning of in vivo compatible drugs developed for other indications towards anti-cancer purposes. The results indicated that PLA2G7 is a cancer-selective biomarker in 50 % of prostate cancers and associates with aggressive disease. The alterations induced by PLA2G7 silencing highlighted the potential of PLA2G7 inhibition as an anti-proliferative, pro-apoptotic and anti-migratorial therapeutic approach in prostate cancer. Moreover, the anti-proliferative effect of PLA2G7 silencing was potentiated by lipid-lowering statins in prostate cancer cells. Taken together, our results support the potential of PLA2G7 as a biomarker and a drug target in prostate cancer and present a rationale for combining PLA2G7 inhibition with the use of statins in prostate cancer management.

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Figures

Figure 1
Figure 1. PLA2G7 is expressed in a cancer specific manner and associates with aggressive disease
(A) Staining intensity of PLA2G7 in primary prostate cancer (n = 1,137) and adjacent non-malignant prostate (n = 409) samples was scored as follows: Strong (+++), moderate (++), weak (+) or no staining (negative). Representative section of each staining intensity is presented. (B) Immunohistochemical staining results of PLA2G7 expression in adjacent non-malignant and cancer tissue samples for two patients are shown. The areas presented at higher magnification have been indicated in the core images. (C) The proportion of tissue samples with no staining and positive PLA2G7 staining in non-malignant and cancer tissue samples included in the TMA. (D) The proportion of TMA tissue samples with no PLA2G7 staining or positive PLA2G7 staining in non-malignant and cancer tissue samples according to Gleason score. The amount of samples in each group is indicated in parentheses. Significant p-values between different histological stages are presented. (E) Kaplan-Meier curve presentation of prostate cancer specific survival in the patient groups with no PLA2G7 staining (n = 135) or positive PLA2G7 staining (n = 230) in the cancer samples.
Figure 2
Figure 2. PLA2G7 silencing decreases the level of lysophosphatidylcholine, induces apoptosis and reduces prostate tumorigenesis
(A) PLA2G7 mRNA expression in cancerous and non-malignant prostate cell lines. (B) Validation of PLA2G7 gene silencing in VCaP prostate cancer cells at mRNA and protein level. (C) Heatmap presentation of the cellular lipidomic changes in response to PLA2G7 silencing in VCaP cells. The profiles were obtained at 48 h after transfection with two PLA2G7 siRNAs and compared to the respective scrambled siRNA control sample. The results are presented as the mean percentual change of two replicates in relation to the scrambled siRNA samples. Red color indicates upregulation and blue downregulation in response to PLA2G7 silencing. PC, phosphatidylcholine; PE, phosphatidylethanolamine; PI, phosphatidylinositol; SM, sphingomyelin, Cer, ceramide. (D) The effect of PLA2G7 silencing on the mRNA expression of pro-apoptotic CASP8 at 24 h and anti-apoptotic BCL2L1 at 24 h (left) and 48 h (right). (E) The effect of PLA2G7 silencing on the mRNA expression of ALDH1A1 at 24 h (left) and 48 h (right), and the relative ALDH activity at 48 h. ALDH inhibitor DEAB was used as a positive control. Significant p-values in comparison to scrambled control are indicated.
Figure 3
Figure 3. PLA2G7 affects the expression of multiple genes associated with cell adhesion and migration
(A) The change in the relative mRNA expression of ACTR3, CDC42, DSCAM, LIMK1, NCAM1, and STAT3 in response to 24 h siRNA transfection. (B) The effect of 48 h PLA2G7 silencing on ITGB mRNA expression. (C) The change in pSTAT3, pPAK and PAK protein expression in response to 72 h PLA2G7 silencing. β-actin is presented as an endogenous control. (D) Schematic illustration of the Rac1 and CDC42 signaling related gene products as a pathway. Blue color indicates downregulation and red upregulation in response to PLA2G7 silencing. (E) Microscopic images (63 x) of PLA2G7 siRNA and scrambled siRNA transfected and LPC stimulated VCaP cells stained with phalloidin (F-actin, green; DAPI, blue) are shown. Arrows indicate the presence of cell protrusions.
Figure 4
Figure 4. PLA2G7 silencing decreases prostate cancer cell adhesion, migration and invasion
(A) Fibronectin cell adhesion analysis. The relative amount of attached PI labeled cells is presented. (B) The effect of 72 h PLA2G7 silencing on VCaP and PC-3 cell viability. (C) Wound healing assay with PC-3 cells following 72 h siRNA transfection. The results from 6 h time point after wound scratching are presented. (D) 3D cell invasion analysis. The spheroid roundness (%), and the relative index of invasive protrusions (AppIndex) in the 3D structures were measured from the microscopic (5 x) images. Significant p-values in comparison to scrambled control are indicated.
Figure 5
Figure 5. Statins decrease the enzymatic activity of PLA2G7 and act synergistically with PLA2G7 silencing on prostate cancer cell viability
(A) The effect of 48 h statin (10 μM) treatments on PLA2G7 protein expression in VCaP cells. The relative PLA2G7 (PLA2G7/β-actin) protein expression has been indicated with bars. (B) The effect of 10 μM statins alone and in combination with PLA2G7 silencing on PLA2G7 activity in VCaP cells. Significant p-values for individual treatments are given, compared with the scrambled control and diluent treated cells. (C) The relative effect of fluvastatin, lovastatin, pravastatin and simvastatin in combination with PLA2G7 silencing on VCaP cell viability. Cell viability in diluent treated cells (scrambled and PLA2G7 siRNA transfected samples) was set as 100% to distinguish the synergism detected. Significant p-values for individual treatments are given, compared with the scrambled control.
Figure 6
Figure 6. Schematic figure showing the putative potential of PLA2G7 impairment in prostate cancer management
PLA2G7 expression and enzymatic function can be activated by e.g. ERG oncogene, inflammation or by substrates produced by oxidative stress [9, 43]. Our results suggest that PLA2G7 is a potential novel prognostic and therapeutic biomarker associating with aggressive disease. Therapeutic intervention impairing the expression or function of PLA2G7 induces multiple antineoplastic effects in cultured prostate cancer cells. PLA2G7 inhibition sensitizes prostate cancer cells to oxidative stress induced damage, decreases tumorigenetic potential and proliferation, as well as induces apoptosis and inhibits cancer cell migration. Moreover, clinically widely used statins inhibit the enzymatic activity of PLA2G7 and synergistically inhibit prostate cancer cell viability with RNAi induced PLA2G7 inhibition.
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