Abstract 3164: Posttranscriptional regulation of opiate tolerance by OPRM1 splice variants following morphine/fig (Ficus carica) leaf extract treatment of human neuroblastoma (SH-SY5Y) cells

Abstract

Abstract Aberrant splicing events have been implicated in a growing number of diseases, including cancer. The pharmacological significance of newly identified mu opioid receptor (MOR-1) alternative splice variants has not been characterized relative to drug response mechanisms and may inform the issue of morphine tolerance. Among the 70-90% of cancer patients requiring individualized opioid therapy for intense chronic pain, the response to prototypical opiates like morphine is highly variable, necessitating dose escalation with an increased risk of developing tolerance. It has been suggested that chronic morphine exposure has little, if any, effect on MOR-1 regulation. If this is true, then chronic exposure should also have no effect on variant forms of receptor. We therefore tested the hypothesis that, following chronic exposure to morphine and/or fig leaf extract, MOR-1 variants would be differentially regulated as evidenced by altered relative mRNA expression levels. Retinoic-acid differentiated human neuroblastoma (SH-SY5Y) cells were treated with morphine (10 µM), fig leaf extract (3 µL/30 mL media), or both for 48 hours before analysis by quantitative real-time polymerase chain reaction (qRT-PCR) using the Bio-Rad iCycler/MyiQ™. MOR-1 variant forms identified by analysis included: MOR1A, MOR1-B1, MOR1-B2, MOR1-B3, MOR1-B5, MOR1-K1. MOR1B4 expression was negligible. Variant mRNA expression was differentially regulated in 92% of the samples treated with fig leaf extracts from 7 cultivars (Green Ishia, Brown Turkey, Mission, Alma, Giant Celeste, Nero, Hollier). Fig administered alone up-regulated MOR-1 splice variant expression (1.6 to 16.9-fold), but these levels were suppressed in combination with morphine (0.2 to 0.8-fold). Also in combined treatment groups, MOR1-K1 and MOR1-B2 mRNA levels consistently up-regulated or down-regulated together. Morphine given alone down-regulated variant expression (0.2 to 0.7-fold). Closer inspection of MOR-1 alternative splice variants under experimental conditions appear to disqualify the assumption that MOR-1 receptor expression remains unchanged under chronic exposure conditions. The results of this study confirm our hypothesis that MOR-1 splice variants are differentially regulated following chronic exposure to morphine and other agents. Further examination of the functional role of MOR-1B2 and MOR1-K1 variants may contribute to our understanding of the mechanistic basis of morphine tolerance and may give clues as to the therapeutic benefit of targeting transcriptional isoforms the receptor for cancer pain relief. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3164.