CX-5461 induces autophagy and inhibits tumor growth via mammalian target of rapamycin-related signaling pathways in osteosarcoma

Osteosarcoma (OS) is the most common primary bone tumor, but molecular mechanisms of the disease have not been well understood, and treatment of metastatic OS remains a challenge. Rapid ribosomal RNA synthesis in cancer is transcribed by RNA polymerase I, which results in unbridled cell growth. The recent discovery of CX-5461, a selective RNA polymerase I inhibitor, exerted its inhibitory effect of ribosomal RNA synthesis and antiproliferative potency. Here, we demonstrate that CX-5461 induces G(2) arrest in the cell cycle and expression of microtubule-associated protein 1 light chain 3 II isoform in OS cell lines. Autophagic vacuoles could be observed in electron microscopy and 3-methyladenine prevented cell death mediated by CX-5461. Moreover, it significantly augmented phosphorylated AMP-Activated Protein Kinases α (p-AMPK α). (Thr(172)) expression in U2-OS cells and decreased p-Akt (Ser(473)) expression in MNNG cells, respectively, which repressed their downstream effector, mammalian target of rapamycin. On the other hand, CX-5461 increased p53 accumulation and messenger RNA level of its target genes, p21, MDM2, and Sestrin1/2 in U2-OS cells. Knockdown of p53 expression markedly impaired cell death as well as the expression of light chain 3-II and p21 induced by CX-5461. It also significantly enhanced doxorubicin-mediated cytotoxic effect in vitro and in vivo together with additive expression of p53, p21, and light chain 3-II in U2-OS cells. Our data indicate that CX-5461 might induce autophagy via mammalian target of rapamycin-associated signaling pathways dependent on p53 status and exert p53-dependent synergistic antitumor effect combined with doxorubicin in OS. These results suggest that CX-5461 might be promising in clinical therapy for OS, especially cases harboring wild-type p53.