Abstract 4886: PTHrP sensitizes breast cancer cells to TRAIL-induced apoptosis
Authors: Cheung Vanessa , Bouralexis Steve , Gillespie Matthew T. 
Published: August 2012

Abstract
Parathyroid hormone-related protein (PTHrP) is a multifunctional protein that has been implicated in breast tumor biology. Breast cancers commonly express PTHrP, and when such tumors are grown in bone in murine models, PTHrP results in enhanced osteolysis. Whilst clinical studies have determined that PTHrP expression by primary breast cancers was an independent predictor of improved prognosis (Henderson et al., 2006). Thus, the action of PTHrP upon breast cancers directly remains to be elucidated. The aim of this study was to assess the interaction of PTHrP with molecules associated with cell survival and cell death pathways, such as Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). TRAIL is a member of the tumor necrosis factor (TNF) family of cytokines that induces apoptosis in a wide range of transformed cells but not in normal cells. To understand the role of PTHrP in relation to TRAIL in breast cancer cells, PTHrP was overexpressed in two breast cancer cell lines; MDA-MB-231 (TRAIL-sensitive) and MCF-7 (TRAIL-resistant), and functional assays were performed. Over-expression of PTHrP in both MCF-7 and MDA-MB-231 cells enhanced cell cycle progression and cell growth rate in comparison to parental and vector controls. However, treatment of PTHrP overexpressing breast cancer cells with TRAIL resulted in an enhancement of TRAIL-induced apoptosis in the TRAIL sensitive cell line, MDA-MB-231, and a sensitisation to TRAIL-induced apoptosis in the TRAIL-resistant cell line, MCF-7. Western blot showed decreased levels of Caspase-10, Caspase-9, Caspase-6, Capase-7 precursor proteins and increased PARP cleavage levels in PTHrP overexpressing cells treated with TRAIL when compared to controls. There was no change observed in levels of Caspase-8 percursor protein, suggesting that the apoptosis signal is acting via the intrinsic pathway in PTHrP overexpressing cells. To determine TRAIL sensitivity in MCF-7 cells overexpressing PTHrP, TRAIL receptor mRNA and protein expression was assessed by qRT-PCR and FACS analysis. TRAIL receptors; DR4, DR5, DcR1 and DcR2, mRNA expression was not altered in PTHrP overexpressing cells. However, flow cytometric analysis demonstrated an increase in cell surface expression of both death receptors (DR4 and DR5) in cells overexpressing PTHrP. To determine whether TRAIL-induced apoptosis in PTHrP overexpressing cells was signaling either through DR4, DR5 or a combination of both death receptors, antagonistic antibodies against DR4 or DR5 were used. Pre-incubation of PTHrP overexpressing cells with DR5 antagonistic antibody attenuated TRAIL-induced apoptosis. This demonstrated that TRAIL preferentially signals through DR5 in the PTHrP overexpressing cells to activate the intrinsic apoptotic pathway. In conclusion, these results demonstrate a new function for PTHrP in breast cancer and may provide utility when TRAIL is used as a treatment for breast cancer.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4886. doi:1538-7445.AM2012-4886