Camptothecin releases P-TEFb from the inactive 7SK snRNP complex

• Published in: Cell cycle (Georgetown, Tex.) Vol. 8; no. 8; pp. 1249 - 1255
• Main Authors: Amente, Stefano, Gargano, Barbara, Napolitano, Giuliana, Lania, Luigi, Majello, Barbara
• Format: Journal Article
• Language: English
• Published: United States Taylor & Francis 15.04.2009
• Subjects: Animals; Binding; Biology; Bioscience; Calcium; Camptothecin - pharmacology; Cancer; Cell; Cycle; Cyclin-Dependent Kinase 9 - antagonists & inhibitors; Flavonoids - pharmacology; Gene Expression Regulation, Neoplastic - drug effects; HeLa Cells; Humans; Landes; Organogenesis; Phosphorylation - drug effects; Piperidines - pharmacology; Positive Transcriptional Elongation Factor B - metabolism; Protein Processing, Post-Translational - drug effects; Proteins; Rats; Ribonucleoproteins, Small Nuclear - metabolism; RNA Polymerase II - metabolism; Transcription, Genetic - drug effects
• DOI: 10.4161/cc.8.8.8286
• ISSN: 1538-4101

An immediate effect of DNA Topoisomerase I inhibitors camptothecin (CPT) and its derivates is the inhibition of transcription. These fast-acting drugs are believed to inhibit transcription by blocking topoisomerase-mediated relief of DNA supercoiling that occurs during transcription elongation. The CPT effects are commonly considered to be due to a collision between the drug-trapped enzyme on the DNA template and the elongating RNAPII. Here we present evidences that CPT treatment induces an early affect on the positive elongation factor b (P-TEFb). The P-TEFb activity is tightly and dynamically regulated, and a reservoir of P-TEFb is kept in an inactive state in the multisubunit 7SK snRNP. We found that, shortly after treatment, CPT disrupts the large inactive P-TEFB complex, and such effect is reversible and independent from DNA replication. Thus, CPT modulates P-TEFb equilibrium in a manner similar to Flavopiridol (FP), a pan-Cdk inhibitor proposed as chemotherapeutic agents against cancers. We determined that while FP inhibits Cdk9 leading to hypo-phosphorylation of RNA polymerase II, CPT-mediated release of free P-TEFb correlates with a concomitant hyper-phosphorylation of RNAPII, which in turn alters the levels and distribution of the RNAPII along transcribed genes. The findings that CPT affects P-TEFb activity provide a direct evidence of the mechanism of this drug to inhibit transcription.