Targeting RNA-Polymerase I in Both Chemosensitive and Chemoresistant Populations in Epithelial Ovarian Cancer

Abstract

Purpose A hallmark of neoplasia is increased ribosome biogenesis, and targeting this process with RNA polymerase I (Pol I) inhibitors has shown some efficacy. We examined the contribution and potential targeting of ribosomal machinery in chemotherapy resistant and sensitive models of ovarian cancer. Experimental Design Pol I machinery expression was examined, and subsequently targeted with the Pol I inhibitor CX-5461, in ovarian cancer cell lines, an immortalized surface epithelial line, and patient derived xenograft (PDX) models with and without chemotherapy. Effects on viability, Pol I occupancy of rDNA, ribosomal content, and chemosensitivity were examined. Results In PDX models, ribosomal machinery components were increased in chemotherapy-treated tumors compared to controls. 13 cell lines were sensitive to CX-5461, with IC50s 25nM – 2μM. Interestingly two chemoresistant lines were 10.5- and 5.5-fold more sensitive than parental lines. CX-5461 induced DNA damage checkpoint activation and G2/M arrest with increased γH2AX staining. Chemoresistant cells had 2-4-fold increased rDNA Pol I occupancy and increased rRNA synthesis, despite having slower proliferation rates, while ribosome abundance and translational efficiency were not impaired. In five PDX models treated with CX-5461, one showed a complete response, one a 55% reduction in tumor volume, and one maintained stable disease for 45 days. Conclusions Pol I inhibition with CX-5461 shows high activity in ovarian cancer cell lines and PDX models, with an enhanced effect on chemoresistant cells. Effects occur independent of proliferation rates or dormancy. This represents a novel therapeutic approach that may have preferential activity in chemoresistant populations.

CX-5461 Inhibits Pancreatic Ductal Adenocarcinoma Cell Growth, Migration and Induces DNA Damage

“…CX-5461 is capable of inducing DNA double-strand breaks as indicated by an increased formation of phospho-H2A.X in PC cells and in the primary cell culture PDAC-3, as reported before in U2OS cells and ovarian cancer cells [10,16].…”

The antimalarial drug amodiaquine stabilizes p53 through ribosome biogenesis stress, independently of its autophagy-inhibitory activity

“…Notably, this is the first example of a FDA-approved drug capable of triggering Pol I degradation. Targeting ribosome biogenesis has been proposed as a therapeutic strategy for hematological malignancies [57], ovarian [58], prostate [59], and MYC-driven cancers [60]. Relevance to CRC has been highlighted by the discovery that oxaliplatin, used as standard-of-care for CRC patients, kills cancer cells mainly due to its capacity to impair ribosome synthesis [13].…”

Ribosome biogenesis: An emerging druggable pathway for cancer therapeutics

“…Once characterized, these molecular profiles might lead to identification of tumor subtypes that would benefit from treatment with these compounds. To date, CX-5461 has been the most extensively studied, and its anticancer activity has been reported in multiple cancer backgrounds, including: leukemia, lymphomas and myelomas [36,[42][43][44], prostate [45], osteosarcoma [46], ovary [47] and neuroblastoma [48], illustrating the potentially broad applicability of such compounds. In addition to interfering with the function of Pol I, CX derivatives also cause severe replication and DNA repair issues since they stabilize G4 DNA motifs which form naturally within guanine-rich DNA sequences and consist of a four-stranded helical structure [41].…”