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The BRCA1/BARD1 complex plays a key role in DNA double-stand break (DSB) repair in both somatic cells and germ cells. However, the underlying molecular mechanism by which this complex mediates DSB repair remains elusive. Here, we show that the recruitment of the BRCA1/BARD1 complex to the unsynapsed axis of the XY body in male germ cells is mediated by pre-rRNA. Similarly, the BRCA1/BARD1 complex associates with pre-rRNA in somatic cells, which targets the BRCA1/BARD1 complex to DSBs. The interactions between the BRCT domains of the BRCA1/BARD1 complex and pre-rRNA induces liquid-liquid phase separations, which may be the molecular basis of DSB-induced nuclear foci formation of the BRCA1/BARD1 complex. Pre-rRNA also mediates BRCA1-dependent homologous recombination (HR). Cancer-associated mutations in the BRCT domains of BRCA1 and BARD1 abolish the interaction with pre-rRNA. We developed novel RNA pol I inhibitors to suppress pre-rRNA biogenesis, which not only abolished HR repair but also sensitized tumor cells to PARP inhibitor treatment. Collectively, this study reveals that pre-rRNA is a functional partner of the BRCA1/BARD1 complex in the DSB repair. It is also a potential therapeutic target for cancer therapy. Citation Format: Duo Wu, Huang Huang, Tenglong Chen, Xiaochen Gai, Song Liu, Hu He, Song Shi, Xiaochun Yu. Pre-ribosomal RNA mediates HR repair via the BRCA1/BARD1 complex. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6216.

Golgi membrane protein 1 (GOLM1/GP73) is a serum marker of hepatocellular carcinoma (HCC). We have previously shown that mTOR promoted tumorigenesis of HCC through stimulating GOLM1 expression. In this study, we demonstrated that the mammalian target of rapamycin (mTOR) was a negative regulator of microRNA-145 (miR-145) expression. miR-145 inhibited GOLM1 expression by targeting a coding sequence of GOLM1 gene. GOLM1 and miR-145 were inversely correlated in human HCC tissues. GOLM1-enriched exosomes activated the glycogen synthase kinase-3β/matrix metalloproteinases (GSK-3β/MMPs) signaling axis of recipient cells and accelerated cell proliferation and migration. In contrast, miR-145 suppressed tumorigenesis and metastasis. We suggest that mTOR/miR-145/GOLM1 signaling pathway should be targeted for HCC treatment.

Background & Aims Levels of the Golgi protein 73 (GP73) increase during development of hepatocellular carcinoma (HCC); GP73 is a serum marker for HCC. However, little is known about the mechanisms or effects of GP73 during hepatic carcinogenesis. Methods GP73 was overexpressed from a retroviral vector in HepG2 cells, which were analyzed in proliferation and migration assays. Xenograft tumors were grown from these cells in nude mice. The effects of monoclonal antibodies against GP73 were studied in mice and cell lines. GP73 −/– , GP73 +/− , and GP73 +/+ mice were given injections of diethylnitrosamine to induce liver injury. Levels of GP73 were reduced in MHCC97H, HCCLM3, and HepG2.215 cell lines using small hairpin RNAs; xenograft tumors were grown in mice from MHCC97H–small hairpin GP73 or MHCC97H-vector cells. We used microarray analysis to compare expression patterns between GP73-knockdown and control MHCC97H cells. We studied the effects of the mechanistic target of rapamycin (mTOR) inhibitor rapamycin on GP73 expression in different cancer cell lines and on growth of tumors in mice. Levels of GP73 and activated mTOR were quantified in human HCC tissues. Results Xenograft tumors grown from HepG2 cells that expressed GP73 formed more rapidly and more metastases than control HepG2 cells in mice. A monoclonal antibody against GP73 reduced proliferation of HepG2 cells and growth of xenograft tumors in mice. GP73 −/– mice had less liver damage after administration of diethylnitrosamine than GP73 +/− or GP73 +/+ mice. In phosphatase and tensin homolog–null mouse embryonic fibroblasts with constitutively activated mTOR, GP73 was up-regulated compared with control mouse embryonic fibroblasts; this increase was reversed after incubation with rapamycin. Expression of GP73 also was reduced in HCC and other cancer cell lines incubated with rapamycin. mTORC1 appeared to regulate expression of GP73 in cell lines. Activated mTOR correlated with the level of GP73 in human HCC tissues. Injection of rapamycin slowed the growth of xenograft tumors from MHCC97H-vector cells, compared with MHCC97H–short hairpin GP73 cells. Conclusions Increased expression of GP73 promotes proliferation and migration of HCC cell lines and growth of xenograft tumors in mice. mTORC1 regulates the expression of GP73, so GP73 up-regulation can be blocked with rapamycin. mTOR inhibitors or other reagents that reduce the level or activity of GP73 might be developed for the treatment of HCC.