Abstract
Gastric cancer is the fifth most common cancer worldwide, posing a significant health burden. The diffuse subtype, more likely to spread quicker and have poorer patient prognosis, is often treated using the same chemotherapies as the intestinal subtype despite differences in molecular basis and progression. In Aotearoa New Zealand, Māori and Pacific communities are disproportionately affected, with over twice the chance of developing diffuse gastric cancer. Understanding factors contributing to the poorer prognostic outcomes for diffuse gastric cancer patients could identify better therapeutic targets. One such target is the proteasomal degradation pathway. The immunoproteasome is an intracellular complex that breaks down proteins into small fragments which can be presented on MHC-class I molecules to trigger an immune response. Two proteins, PA28a and PA28b, encoded by PSME1 and PSME2, respectively, form a cap on the immunoproteasome catalytic core, allowing activation. Our lab previously identified that high expression of PSME1 and PSME2 in diffuse gastric cancer is associated with poorer prognosis, while the opposite was observed in intestinal gastric cancer. Prior experiments in our lab found an alternative splice variant of PSME2, which was named PSME2d33. PSME2d33 had been reported, but not characterised. A possible explanation for the survival difference between subtypes is that the cells express different isoforms with varying functions, possibly driving progression of diffuse gastric cancer and leading to worse patient prognosis. Understanding the isoform effects might be therapeutically important as it could provide a specific target. This project aimed to determine whether overexpression of the PSME2d33 isoform affects cell growth and survival. After determining optimal doxycycline doses to induce overexpression (OE) of PSME2 and PSME2d33 through RT-qPCR and Western Blot experiments, proliferation and migration assays were carried out to investigate the effect of PSME2d33 isoform expression. While these characterisation experiments did not show statistically significant differences between empty vector control, PSME2OE and PSME2d33OE cells, a trend was seen towards decreased migration of PSME2d33OE cells compared with PSME2OE. Additionally, flow cytometry to determine cell viability suggested PSME2OE and PSME2d33OE cells displayed increased early apoptotic and necrotic cells. Overall, further research is required to fully explore the role of the PSME2d33 isoform in gastric cancer cells and to provide a greater understanding into the mechanisms underlying the survival difference between intestinal and diffuse gastric cancer.