cx-5461 and Glioblastoma

PICT-1 was originally identified as a tumor suppressor. Here, we found that PICT-1 overexpression triggered pro-death autophagy without nucleolar disruption or p53 accumulation in U251 and MCF7 cells. Truncated PICT-1 fragments 181-346 and 1-346, which partly or totally lack nucleolar localization, showed weaker autophagy-inducing effects than full-length PICT-1 and a well-defined nucleolar mutant (181-479). Furthermore, PICT-1 partly localizes to the nucleolar fibrillar center (FC) and directly binds to ribosomal DNA (rDNA) gene loci, where it interacts with upstream binding factor (UBF). Overexpression of PICT-1 or the 181-479 mutant, but not the 1-346 or 181-346 mutants, markedly inhibited the phosphorylation of UBF and the recruitment of rRNA polymerase I (Pol I) to the rDNA promoter in response to serum stimulation, thereby suppressing rRNA transcription, suggesting that rRNA transcription inhibition might be an important contributor to PICT-1-induced autophagy. This is supported by the finding that CX-5461, a specific Pol I inhibitor, also induced autophagy. In addition, both CX-5461 and PICT-1, but not the 1-346 or 181-346 mutants, significantly suppressed the activation of the Akt/mTOR/p70S6K signaling pathway. Our data show that PICT-1 triggers pro-death autophagy through inhibition of rRNA transcription and the inactivation of AKT/mTOR/p70S6K pathway, independent of nucleolar disruption and p53 activation.

Topics: Antineoplastic Agents; Autophagy; Benzothiazoles; Binding Sites; Brain Neoplasms; Breast Neoplasms; Cell Nucleolus; Cell Proliferation; Dose-Response Relationship, Drug; Enzyme Inhibitors; Female; Gene Expression Regulation, Neoplastic; Glioblastoma; Humans; MCF-7 Cells; Mutation; Naphthyridines; Phosphorylation; Pol1 Transcription Initiation Complex Proteins; Promoter Regions, Genetic; Proto-Oncogene Proteins c-akt; Ribosomal Protein S6 Kinases, 70-kDa; RNA Polymerase I; RNA, Ribosomal; Signal Transduction; Time Factors; TOR Serine-Threonine Kinases; Transcription, Genetic; Transfection; Tumor Suppressor Proteins