Application of a New Multiplexed Array for Rapid, Sensitive, Simultaneous and Quantitative Assessment of Spliced and Unspliced XBP1

IRE1α-mediated unconventional splicing of XBP1 is emerging as a biomarker in several disease states and is indicative of activation of the unfolded protein response sensor IRE1. Splicing of mRNA results in the translation of two distinct XBP1 protein isoforms (XBP1s and XBP1u) which, due to post-translational regulation, do not correlate with mRNA levels. As both XBP1 isoforms are implicated in pathogenic or disease progression mechanisms there is a need for a reliable, clinically applicable method to detect them. A multiplexed isoform-specific XBP1 array utilising Biochip array technology (BAT™) was assessed for specificity and suitability when using cell protein lysates. The array was applied to RIPA protein lysates from several relevant pre-clinical models with an aim to quantify XBP1 isoforms in comparison with RT-PCR or immunoblot reference methods. A novel reliable, specific and sensitive XBP1 biochip was successfully utilised in pre-clinical research. Application of this biochip to detect XBP1 splicing at the protein level in relevant breast cancer models, under basal conditions as well as pharmacological inhibition and paclitaxel induction, confirmed the findings of previous studies. The biochip was also applied to non-adherent cells and used to quantify changes in the XBP1 isoforms upon activation of the NLRP3 inflammasome. The XBP1 biochip enables isoform specific quantification of protein level changes upon activation and inhibition of IRE1α RNase activity, using a routine clinical methodology. As such it provides a research tool and potential clinical tool with a quantified, simultaneous, rapid output that is not available from any other published method.