Abstract 1900: Understanding the biological role of latexin in the normal and malignant prostate
Authors: Seed Robert I. , Taurozzi Alberto J. , Nappo Giovanna , Erb Holger H. , Collins Anne , Maitland Norman J. , Frame Fiona 
Published: July 2017

Abstract
The retinoid-inducible protein Latexin (LXN) acts as a stem cell suppressor in normal murine hematopoiesis, and in human tumors where it can influence both tumor cell migration and invasion (Liang et al, Nature genetics 2007). We have shown that LXN is silenced in the stem cell fraction of primary prostate epithelial cells, and that expression increases as cells differentiate through the prostate basal epithelial hierarchy towards mature luminal cells. Furthermore, transient knockdown of LXN significantly increased clonogenic potential and promoted a more invasive phenotype in primary prostate epithelia (Oldridge et al, Oncogenesis 2013). LXN may therefore be involved in the differentiation of prostate epithelial cells, but could act as a tumor suppressor in Prostate Cancer (CaP), in addition to other tumours. In genome databases LXN is described as an inhibitor for Carboxypeptidase A4, but despite considerable molecular characterisation, the mechanism of action has remained an enigma since perhaps paradoxically LXN appears to be able to function independently of its classical CPA4 inhibitory role. To resolve this, we adopted an unbiased approach to determine if targeted re-expression of LXN or its binding partners could provide a viable novel drug target for the treatment of CaP. Our preliminary studies had revealed that LXN was an exclusively cytosolic protein in prostate epithelial cells. However, subsequent live cell imaging of primary prostate epithelial cells overexpressing YFP-LXN demonstrated for the first time that, whilst predominantly cytosolic, LXN was also present within the cell nucleus. To characterise the effects of LXN overexpression on gene transcription after LXN-expressing lentiviral transduction of primary prostate epithelial cells, we utilised Affymetrix HTA2 gene arrays. Perhaps surprisingly, short-term overexpression of LXN did not significantly effect global gene transcription compared to controls (n=4 P=NS). Additionally, we used IP/MS to investigate the potential interacting partners of LXN, using multiple approaches, including both overexpression of HA-tagged and WT LXN, and endogenous LXN as bait. These approaches failed to reveal any strong (and reproducible) intracellular protein interactors of LXN. From these data, we hypothesised that LXN may function as a secreted protein. We found that LXN was present in not only the conditioned media of LXN+ prostate epithelial cells, but also preliminary evidence that LXN is enriched in extracellular exosomes. Therefore, LXN may perform its biological roles extracellularly, or at the cell surface. Our current work is focusing on determining the functional consequences of secreted LXN on prostate epithelia and to establish potential protein: protein interactions of LXN in the extracellular space. Full characterisation of LXN function considering its potent biological effects on cancer cells will determine its suitability as a therapeutic in CaP and other cancers.
Citation Format: Robert I. Seed, Alberto J. Taurozzi, Giovanna Nappo, Holger H. Erb, Anne Collins, Norman J. Maitland, Fiona Frame. Understanding the biological role of latexin in the normal and malignant prostate [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1900. doi:10.1158/1538-7445.AM2017-1900