Identification and functional characterisation of G protein-coupled receptor three prime untranslated region transcript variants in cancer cell models

Abstract

G protein-coupled receptors (GPCRs) are integral membrane proteins, comprising one of the largest gene families. GPCR signalling affects almost all human physiological systems and, as such, the expression of the receptors and their cognate ligands are subjected to strict regulation. Dysregulated GPCR expression has been associated with an array of pathological conditions, including cancer. Despite this, the majority of research focussing on the control of GPCR expression has centred on transcriptional regulation, despite a wealth of research delineating the importance of translational regulation and the frequent lack of correlation between messenger RNA (mRNA) and protein levels. Regulation of mRNA translation is largely governed by the non-coding regions (five prime- and three prime- untranslated regions; 5' and 3’ UTRs). Messenger RNA UTRs have been demonstrated to play important roles in the temporospatial expression of mRNAs and, importantly, regulate recruitment of translational machinery to the transcripts, ultimately determining protein levels. Aberrant expression of GPCRs, including kisspeptin receptor (KISS1R), leucine-rich repeat-containing G-protein coupled receptor 5 (LGR5) and growth hormone-releasing hormone receptor (GHRHR), has been implicated in cancer proliferation and metastasis. This study set out to establish whether these receptors were expressed in various commonly used in vitro breast and prostate cancer cell lines. Rapid-amplification of cDNA ends (RACE) was then employed to characterise the 3’ UTR sequences transcribed for these receptors. While the annotated prototypical 3’ UTRs were identified, no 3’ UTR variants were observed. The role of the annotated 3’ UTRs in regulating protein expression was assessed using a dual luciferase reporter assay. Here we establish that the KISS1R 3’ UTR, LGR5 3’ UTR and GHRHR 3’ UTR result in significant differences in luciferase protein expression when cloned downstream of the firefly luciferase ORF in breast and prostate cancer cell backgrounds compared to a non-cancer cell background. This suggests that differential expression of these receptors in different cell backgrounds may be due to 3' UTR-mediated translational regulation.