Patterns of immunoproteasome subunit expression and assembly into proteolytic particles in gastric cancer cells with different histotypes: implications for cancer metastasis

Gastric adenocarcinoma which represents 90% of gastric cancers remains an aggressive and poorly understood malignancy with a heterogeneous presentation and tumor biology. Understanding the molecular basis of this variability is important for effective treatment. Due to its role in controlling protein homeostasis, the proteasome is considered an attractive drug target in cancer. More recently, its variant called the immunoproteasome has been implicated in the biology of different types of tumors and its expression correlated to disease outcome. Nevertheless, the relevance of the immunoproteasome in neoplasia is poorly understood. It has been recently appreciated that besides expression, proteasome subunit assembly into the catalytic 20S core and association of this particle with different regulators have important implications in tumorigenesis and response to therapy. Based on this evidence, we have characterized proteasome patterns in gastric cancer cells representative of different histotypes, based on the Lauren’s classification: epithelial (23132/87), epithelial/diffuse (MKN45) and diffuse (KATO III). Native PAGE analysis followed by western immunoblotting and in gel activity assays with fluorogenic substrates demonstrated that cells with diffuse-type components express higher levels of immunoproteasome subunits which are preferentially incorporated into active 19S-capped regulatory particles. Inhibition of immunoproteasome activity with ONX 0914 significantly affects cell migration in diffuse-type gastric cell lines. Analysis of differentially expressed proteins in KATO III cells by LC-HR-MS-based proteomics highlighted that ONX 0914 affects several signaling pathways involved in tumor invasion and metastasis which are under investigation. These results suggest that pharmacological inhibition of the immunoproteasome may be useful in treating metastatic gastric cancers.