We previously identified PSMD2, a subunit of the 19S regulatory complex of proteasomes, as a constituent of a signature associated with the acquisition of metastatic phenotype and poor prognosis in lung cancers. In the present study, we found that knockdown of PSMD2 decreased proteasome activity, and induced growth inhibition and apoptosis in lung cancer cell lines. These effects of siRNA-mediated PSMD2 inhibition were associated with changes in the balance between phosphorylated AKT and p38, as well as with induction of p21. In addition, patients with higher PSMD2 expression had poorer prognosis and a small fraction of lung cancer specimens carried increased copies of PSMD2. Notably, our findings clearly illustrate that lung adenocarcinomas can be divided into two groups; those with and without general upregulation of proteasome pathway genes including PSMD2. This general upregulation was significantly more prevalent in the non-terminal respiratory unit (non-TRU)-type, a recently proposed genetically and clinicopathologically relevant expression profile-defined classification of adenocarcinomas (P<0.001 by Fisher's exact test). Patients with adenocarcinomas with general upregulation had significantly shorter survival after potentially curative resection (P=0.0001 by log-rank test) independent of disease stage, as shown by multivariate Cox regression analysis. Our results suggest that PSMD2 may be a good molecular target candidate and that other co-regulated proteasome pathway genes and/or their common regulator(s) might also be potential targets, warranting future study including elucidation of the underlying common regulatory mechanism.
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