RRP9 suppresses hepatocellular carcinoma progression by inhibiting the PI3K/AKT/mTOR pathway

Ribosomal RNA processing 9 (RRP9) encodes a WD‑repeat domain‑containing protein, which is a potential carcinogenic biomarker for various tumors. As a key structural component of small nucleolar ribonucleoproteins, RRP9 serves a key role in ribosome biogenesis by facilitating 18S rRNA processing. Despite its association with the pathogenesis of various malignancies, its function and molecular mechanisms in hepatocellular carcinoma (HCC) remain unknown. The present study aimed to examine the biological role of RRP9 in HCC progression and the underlying regulatory mechanisms. Immunohistochemical and western blot analyses revealed a significant downregulation of RRP9 expression in patients with HCC compared with matched adjacent non‑tumorous tissues. To investigate RRP9 biological functions in HCC, stable RRP9‑knockdown and ‑overexpressing isogenic HCC cell line models were established using lentiviral transduction and puromycin selection. Functional assays, including Cell Counting Kit‑8 viability, colony formation, wound healing migration and Transwell invasion experiments, consistently demonstrated that RRP9 significantly suppressed HCC cell viability, proliferation, invasion and migration. Transcriptome sequencing and western blot analyses indicated that RRP9 inhibited the PI3K/AKT/mTOR pathway. Furthermore, functional rescue assays using the PI3K activator 740 Y‑P and the inhibitor PI3K/AKT/mTOR‑IN‑2 verified that RRP9 exerts its tumor‑suppressive role via this pathway. Protein‑protein interaction analysis revealed an association between RRP9 and cyclin A2 (CCNA2). Western blotting confirmed that RRP9 downregulated CCNA2 expression. Additionally, subcutaneous tumorigenesis in mice showed that RRP9 inhibits liver cancer progression via the PI3K/AKT/mTOR signaling pathway.