Abstract C1: Harnessing the pro-inflammatory tumor microenvironment of castrate-resistant prostate cancer to promote apoptosis
Authors: McCann Christopher , Crawford Nyree , Waugh David , Longley Daniel 
Published: May 2016

Introduction

Castrate-Resistant Prostate Cancer (CRPC) represents a highly aggressive, currently incurable disease state. Overexpression of anti-apoptotic proteins including FLIP (FLICE-inhibitory protein) and inhibitors of apoptosis proteins (cIAP-1, cIAP-2 and XIAP) in prostate cancer have been associated with progression to castrate-resistant disease. These proteins are known to contribute to resistance to standard-of-care therapies and aid cancer progression through their anti-apoptotic activity. SMAC mimetics are small molecule compounds which mimic the activity of the endogenous inhibitor of IAPs known as SMAC (Second Mitochondrial-derived Activator of Caspase). SMAC mimetic activity is dependent on TNFα signalling and has also been shown to induce apoptosis in response to TRAIL therapy. Both TRAIL and TNFα are derived from immune cells that are frequently present in the tumour microenvironment, making SMAC mimetic therapy an attractive strategy for treating cancers associated with pro-inflammatory immune infiltrates.

Methods

A panel of prostate cancer cell lines were treated with SMAC mimetic alone, or in combination with exogenous TNFα or TRAIL. Cell viability was determined by MTT assay, and apoptosis was assessed using Western blotting and AnnexinV/PI flow cytometry. siRNA and Histone Deacetylase Inhbitor (HDACi) therapy were employed to reduce FLIP expression. The addition of exogenous TNFα, TRAIL or macrophage-conditioned media was used to model the pro-inflammatory microenvironment.

Results

Prostate cancer cells were found to be resistant to SMAC mimetic therapy alone and in the presence of TNFα or TRAIL. FLIP expression was found to be a resistance mechanism to SMAC-mimetic treatment. Depletion of FLIP expression by siRNA or HDACi therapy resulted in an increased sensitivity to TNFα, TRAIL and macrophage conditioned media alone and in combination with SMAC mimetics. Targeting FLIP alone may therefore be a more effective means of harnessing the pro-inflammatory microenvironment. Moreover, the apoptosis induced in the presence of macrophage conditioned media by SMAC mimetics in FLIP-depleted cells was demonstrated to be TNFα-dependent using TNFα neutralising antibodies.

Conclusion

Prostate cancer cell lines are inherently resistant to SMAC mimetic therapy even in the presence of TNFα or TRAIL. However, dual targeting of FLIP and IAPs sensitises CRPC cell lines to microenvironment-derived TNFα. Combined targeting of IAPs and FLIP, or single targeting of FLIP, may be an effective means of treating pro-inflammatory prostate cancer.

Citation Format: Christopher McCann, Nyree Crawford, David Waugh, Daniel Longley. Harnessing the pro-inflammatory tumor microenvironment of castrate-resistant prostate cancer to promote apoptosis. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C1.