RESUMEN
The B2 family represents a group of short repetitive sequences that are found throughout the rodent genome and are analogous to the human Alu sequences. Certain B2 subfamilies are transcribed by RNA polymerase III (pol III), and this transcription is in part controlled by the retinoblastoma protein. In addition to their putative role in retrotranspositional events, these actively transcribed B2 RNAs show a predicted highly stable secondary structure. Although B2 transcripts are normally confined to the nucleus, they demonstrate altered compartmentation after carcinogen treatment, in cancers, and in immortalized and/or transformed cell lines, the significance of which is unclear. Because modulation of B2 transcripts did not seem feasible with an antisense approach, we designed a triple ribozyme (TRz) construct to down-regulate B2 transcripts. The B2-targeted TRz undergoes efficient self-cleavage, resulting in liberation of the internal hammerhead Rz, which we targeted to a single-stranded region of the consensus B2 sequence. The liberated internal targeted Rz was 20 times more active than the corresponding double-G mutant construct that could not undergo self-cleavage, and 5 times more active than the same Rz flanked by nonspecific vector sequences. The B2-targeted TRz was used to develop stable transfectant clones from an SV40-immortalized hepatocyte cell line. These transfectant clones all showed variably reduced growth rates, accompanied by significant reductions in both cytoplasmic and nuclear B2 RNA levels: linear regression analyses showed that their growth rates were directly related to residual cytoplasmic B2 levels. Reverse-transcription polymerase chain reaction (RT-PCR) analyses documented efficient self-liberation of the internal targeted Rz in vivo, and showed that the relative cytoplasmic expression levels generally paralleled the magnitude of the decrease in B2 transcripts. The RT-PCR analyses further demonstrated that up to 20% of the Rz was located in the nucleus, which presumably reflects competition between autocatalytic processing and nucleocytoplasmic transport of the initial TRz transcript.
Asunto(s)
Inhibidores de Crecimiento/genética , ARN Catalítico/genética , ARN Catalítico/metabolismo , Secuencias Repetitivas de Ácidos Nucleicos/genética , Animales , Antígenos Virales de Tumores/metabolismo , Northern Blotting , División Celular/genética , Línea Celular Transformada , Núcleo Celular/metabolismo , Citoplasma/metabolismo , Regulación de la Expresión Génica , Immunoblotting , Mutagénesis Sitio-Dirigida , Conformación de Ácido Nucleico , ARN/metabolismo , Ratas , Proteína de Retinoblastoma/metabolismo , Transcripción Genética , TransfecciónRESUMEN
Rats were treated with low doses of the hepatocarcinogens dimethylnitrosamine or thioacetamide, and livers were examined 48 h later. These treatments are known to produce altered RNA compartmentation, wherein a class of repetitive RNA sequences normally restricted to the nucleus appears in the cytoplasm. Reverse transcription-PCR amplifications demonstrated that the sequences showing altered compartmentation consisted largely of a subfamily of the rodent B2 sequence family, the counterpart of human Alu sequences involved in retrotransposition. Northern blot analyses showed that these B2 sequences were found in cytoplasmic RNA as 170- to 360-nucleotide "sense" transcripts, and competition hybridization experiments established that B2 sequences represented most (if not all) of the sequences showing altered compartmentation. The major increase in B2 transcriptions in cytoplasmic RNA was not associated with any change in B2 transcription by RNA polymerase III. In situ hybridizations showed that the altered compartmentation of B2 sequences occurred in well-delineated foci within the rat liver; these foci consisted of a central region containing a prominent infiltrate of macrophages admixed with small hepatocytes and a peripheral region of histologically normal hepatocytes that showed evidence of oxidative damage. Altered compartmentation of B2 sequences may represent an important focal initiatory change in a subset of hepatocytes, whereas subsequent retrotranspositional events (associated with Alu-like sequences) could predispose initiated cell foci to alterations in promotion/progression phases.