nb-506 and Leukemia-P388

Specific phosphorylation of serine- and arginine-rich pre-mRNA splicing factors (SR proteins) is one of the key determinants regulating splicing events. Several kinases involved in SR protein phosphorylation have been identified and characterized, among which human DNA topoisomerase I is known to have DNA-relaxing activity. In this study, we have investigated the mechanism of splicing inhibition by a glycosylated indolocarbazole derivative (NB-506), a potent inhibitor of both kinase and relaxing activities of topoisomerase I. NB-506 completely inhibits the capacity of topoisomerase I to phosphorylate, in vitro, the human splicing factor 2/alternative splicing factor (SF2/ASF). This inhibition is specific, because NB-506 does not demonstrate activity against other kinases known to phosphorylate SF2/ASF such as SR protein kinase 1 and cdc2 kinase. Importantly, HeLa nuclear extracts competent in splicing but not splicing-deficient cytoplasmic S100 extracts treated with the drug fail to phosphorylate SF2/ASF and to support splicing of pre-mRNA substrates containing SF2/ASF-target sequences. Native gel analysis of splicing complexes revealed that the drug affects the formation of the spliceosome, a dynamic ribonucleoprotein structure where splicing takes place. In the presence of the drug, neither pre-spliceosome nor spliceosome is formed, demonstrating that splicing inhibition occurs at early steps of spliceosome assembly. Splicing inhibition can be relieved by adding phosphorylated SF2/ASF, showing that extracts treated with NB-506 lack a phosphorylating activity required for splicing. Moreover, NB-506 has a cytotoxic effect on murine P388 leukemia cells but not on P388CPT5 camptothecin-resistant cells that carry two point mutations in conserved regions of topoisomerase I gene (Gly361Val and Asp709Tyr). After drug treatment, P388 cells accumulated hypophosphorylated forms of SR proteins and polyadenylated RNA in the nucleus. In contrast, neither SR protein phosphorylation nor polyadenylated mRNA distribution was affected in P388 CPT5-treated cells. Consistently, NB506 treatment altered the mRNA levels and/or splicing pattern of several tested genes (Bcl-X, CD 44, SC35, and Sty) in P388 cells but not in P388 CPT5 cells. The study shows for the first time that indolocarbazole drugs targeting topoisomerase I can affect gene expression by modulating pre-mRNA splicing through inhibition of SR proteins phosphorylation.

Topics: Animals; Carbazoles; Glucosides; HeLa Cells; Humans; Leukemia P388; Mice; Nuclear Proteins; Phosphoproteins; Phosphorylation; RNA Precursors; RNA Splicing; RNA-Binding Proteins; RNA, Messenger; Serine-Arginine Splicing Factors; Spliceosomes; Topoisomerase I Inhibitors; Tumor Cells, Cultured

Topoisomerase (topo)-I targeting antitumor agents are very effective in vivo against various human cancers. The indolocarbazole compound 6-N-formylamino-12,13-dihydro-1,11-dihydroxy-13-(beta-D-glucopyranosyl)- 5H-indolo[2,3-alpha]pyrrolo-[3,4-c]carbazole-5,7(6H)-dione (NB-506) is a potent inhibitor of the religation step of topo I reaction, like camptothecin (CPT). We established a NB-506-resistant cell line from murine leukemia cell line P388. This resistant cell line, P388/F11, exhibited 73-fold higher resistance to NB-506 and 3.5-fold higher cross-resistance to CPT than the parental cell line. No induction of cleavable complex formations induced by NB-506 and CPT were detected by K-SDS precipitation assays in P388/F11 cells. Analysis of nuclear extracts from P388/F11 cells revealed that the relaxation activity of topo I was one-quarter of that of the parental cells, and that the activity was resistant to induction of DNA cleavage by these drugs. Furthermore, Western blot and Northern blot analyses showed the expression of an abnormal-sized 170-kDa topo I protein and its 6.0-kb transcript and the absence of the normal topo I protein and transcript in P388/F11 cells. Analyses of the structure of the abnormal topo I transcript by reverse transcription-PCR and direct sequencing methods revealed that a large portion of the gene from codon 21 to codon 609 was duplicated in its coding region. This internal duplication resulted in in-frame fusion and, thus, production of a partially duplicated protein of 1357 amino acids. Finally, we expressed and purified the recombinant P388/F11 topo I in a baculovirus system. P388/F11 topo I showed similar catalytic activity to wild-type topo I, but reduced sensitivities to NB-506 and CPT. These results show that the altered sensitivity of duplicated topo I is involved in the NB-506 resistance of P388/F11 cells and indicate a novel resistant mechanism which involves duplication of the topo I gene.

Topics: Animals; Antineoplastic Agents; Camptothecin; Carbazoles; DNA Damage; DNA Topoisomerases, Type I; DNA, Complementary; Drug Resistance, Neoplasm; Enzyme Inhibitors; Gene Rearrangement; Glucosides; Humans; Leukemia P388; Neoplasm Proteins; Reverse Transcriptase Polymerase Chain Reaction; Topoisomerase I Inhibitors