Characterization and targeting of ribosome biogenesis in cellular and tissue subtypes of pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) remains a fatal disease with a five-year overall survival rate of only 12%. This is mainly because the efficacy of existing therapies is strongly restricted by tumor heterogeneity, emphasizing the importance of identifying robust therapeutic targets and applicable combinational treatment approaches. Oncogene-driven cancers often display hyperactivation of ribosome biogenesis (RiBi), allowing the cells to meet their increased demand for protein synthesis and thus maintain their abnormal proliferation rate. The present study investigated the level of RiBi in different PDAC model systems, including conventional PDAC cell lines, primary patient-derived cells (PDC), and a genetically engineered mouse model (CKP), and also analyzed gene expression data from PDAC patients. It was evaluated whether the inhibition of RNA polymerase I (Pol I)-mediated transcription of ribosomal DNA, as the first and rate-limiting step of RiBi, presents a potential treatment strategy for PDAC. High expression levels of multiple RiBi regulators were related to poor survival of PDAC patients, indicating that aggressive tumors may present a stronger dependence on RiBi. Conventional PDAC cell lines showed remarkable differences in the expression of ribosomal proteins (RP) and 45S ribosomal (r)RNA as well as in the nucleolar morphology depending on their molecular characteristics. In turn, pronounced nucleolar heterogeneity was observed within individual PDC populations as well as within endogenous tumors of CKP mice, pointing to RiBi as a highly dynamic process in PDAC. However, despite the observed variations in RP expression, 45S rRNA levels, and nucleolar organization, the RNA Pol I inhibitor BMH-21 effectively reduced cell viability in all investigated conventional PDAC cell lines and PDCs. As expected, BMH-21 treatment decreased the expression of 45S rRNA in PDAC cell lines and consequently caused nucleolar stress, as indicated by the translocation of multiple nucleolar proteins into the nucleoplasm or nuclear spots, respectively. This was followed by the activation of different stress response pathways, including apoptosis, cell cycle arrest, and pro-death autophagy. Additionally, RNA Pol I inhibition induced transcriptional and/or functional alterations in various metabolic pathways, including glycolysis, oxidative phosphorylation, and the tricarboxylic acid cycle. This suggests a potentially targetable connection between RiBi and metabolic reprogramming. Together, the findings of the study support RNA Pol I inhibition as a potential treatment approach in PDAC and open a perspective for future co-targeting strategies.