Abstract 1015: Targeting actin cytoskeleton pathway overcomes resistance to histone deacetylase inhibitors in multiple myeloma cells.

Abstract Histone deacetylase inhibitors (HDACi) are novel therapeutics compounds being utilised in multiple myeloma (MM) clinical trials in combination with approved and investigational agents. While proteasome inhibitors are known to synergise effectively with HDACi, there is a lack of understanding concerning other potential partner drugs. In this report, utilising human myeloma cell lines (HMCL) as a paradigm, gene expression profile was performed to determine if a genetic signature associated with HDACi-resistance could be identified. Nine HMCLs were treated with HDACi (LBH589, SAHA or FK228) and cell death proportion determined after 48 hours through flow cytometric enumeration of propidium iodide staining. Of the nine cell lines, five were sensitive (70-90% cell death), two showed an intermediate response (70-40%) and two were resistant (40-10%) to HDACi. Following the determination of HDACi-response of HMCLs, RNA was extracted from untreated cell lines and gene expression profiling was performed utilising the Illumina HT-12 platform. Analysis revealed that 97 genes were differentially regulated between the sensitive and resistant cell lines. Gene ontology enrichment analysis identified six pathways that were significantly different and these included notch signalling pathway (p=0.01), protein processing in endoplasmic reticulum (p=0.02), glutathione metabolism (p=0.01), melanoma (p=0.02), regulation of actin cytoskeleton (p=0.03) and apoptosis (p=0.03). Furthermore, 4889 probes were tested for their correlation to grade of sensitivity using Spearmann rank algorithm and 35 genes were found to have >80% correlation. Enrichment analyses indicated two pathways as being significantly different based on their correlation to HDACi-sensitivity and these included regulation of actin cytoskeleton (p=0.03) and protein processing in endoplasmic reticulum (p=0.02). The actin cytoskeleton represents a major network of proteins that impinge on motility, invasion, polarity, survival and growth of normal cells, and is often altered in tumour cells. Actin cytoskeleton modifications are known to be involved in MM cell metastasis and a number of agents that destabilize the action cytoskeleton have been utilised in MM pre-clinical studies. When HDACi-resistance cell lines were treated with a combination of HDACi and actin cytoskeleton pathway inhibitors (MEK inhibitor - GSK1120212, focal adhesion inhibitors - TAE-226 and PF-573228 and actin cytoskeleton destabilising agents - NVP-HSP990 and AUY922), synergistic cell death was observed, indicating that these agents can be used in combination with HDACi for MM therapy. This report provides a rationalistic approach for identification of HDACi partner drugs for anti-MM therapy and has provided the framework to identify promising new therapeutic combinations for the treatment of MM utilising HDACi. Citation Format: Sridurga Mithraprabhu, Tiffany Khong, Andrew Spencer. Targeting actin cytoskeleton pathway overcomes resistance to histone deacetylase inhibitors in multiple myeloma cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1015. doi:10.1158/1538-7445.AM2013-1015