Efficient long-term coexpression of a hammerhead ribozyme targeted to the U5 region of HIV-1 LTR by linkage to the multidrug-resistance gene

High-efficiency protein expression mediated by enterovirus 71 internal ribosome entry site

An internal ribosome entry site (IRES) has been used to facilitate the expression of more than one protein in a single transcript. In this study, we examined the translational activities of several IRES elements derived from different RNA viruses. The protein expression of encephalomyocarditis virus (EMCV) IRES is similar to that of hepatitis C virus (HCV) IRES in mammalian cells. Notably, the protein expression of enterovirus 71 (EV71) IRES was 23-fold higher than the efficiency of EMCV IRES following normalization of mRNA transcriptional level. Thus, expression of the secreted alkaline phosphatase (SEAP) reporter protein in mammalian cells may be controlled at desirable levels by using appropriate IRES in the expression vector.

MAPPING OF MULTIDRUG RESISTANCE GENE 1 AND MULTIDRUG RESISTANCE-ASSOCIATED PROTEIN ISOFORM 1 TO 5 mRNA EXPRESSION ALONG THE HUMAN INTESTINAL TRACT

Efflux transporters such as P-glycoprotein and multidrug resistance-associated proteins (MRPs) in the intestinal wall restrict intestinal drug transport. To overcome this limitation for enteral drug absorption, galenical targeting approaches have been proposed for site-specific luminal drug release in segments of the gut, where expression of the respective absorption-limiting transporter is minimal. Therefore, expression of multidrug resistance gene 1 (MDR1) and MRP1-5 was systematically investigated in 10 healthy subjects. Biopsies were taken from different segments of the gastrointestinal tract (from duodenum and terminal ileum, as well as ascending, transverse, descending, and sigmoid colon). Gene expression was investigated by quantitative real-time PCR (TaqMan). MRP3 appeared to be the most abundantly expressed transporter in investigated parts of the human intestine, except for the terminal ileum, where MDR1 showed the highest expression. The ranking of transporter gene expression in the duodenum was MRP3 >> MDR1 > MRP2 > MRP5 > MRP4 > MRP1. In the terminal ileum, the ranking order was as follows: MDR1 > MRP3 >> MRP1 approximately MRP5 approximately MRP4 > MRP2. In all segments of the colon (ascending, transverse, descending, and sigmoid colon), the transporter gene expression showed the following order: MRP3 >> MDR1 > MRP4 approximately MRP5 > MRP1 >> MRP2. We have shown, for the first time, systematic site-specific expression of MDR1 and MRP mRNA along the gastrointestinal tract in humans. All transporters showed alterations in their expression levels from the duodenum to sigmoid colon. The most pronounced changes were observed for MRP2, with high levels in the small intestine and hardly any expression in colonic segments. This knowledge may be useful to develop new targeting strategies for enteral drug delivery.

Improved co-expression of multiple genes in vectors containing internal ribosome entry sites (IRESes) from human genes

Incorporation of an internal ribosome entry site (IRES) into the gene therapy vector represents a promising strategy to efficiently co-express several gene products from the same promoter. However, vector systems that utilize the encephalomyocarditis virus IRES express the downstream gene much less efficiently than the upstream gene. In this study, we compared four IRESes isolated from human genes against the EMCV IRES, using beta-galactosidase and chloramphenicol acetyl transferase genes as reporters, to evaluate their potential for providing better expression of the downstream gene. We found that an IRES from the eukaryotic initiation factor 4G gene mediates greater than 100-fold higher expression of the downstream gene compared with the EMCV IRES in four different cell lines tested. Other mammalian IRESes displayed more variable results and smaller enhancement of downstream gene expression in three different cell lines tested. Furthermore, while the efficiency of the IRES from the vascular endothelium growth factor gene was not significantly higher than the EMCV IRES under normoxic conditions, expression was significantly increased under hypoglycemic conditions, suggesting that the VEGF IRES could be exploited in cancer gene therapy to preferentially target expression of therapeutic genes at the relatively hypoglycemic cores of tumors.

An episomally maintained MDR1 gene for gene therapy

Potential applications of the MDR1 multidrug transporter in gene therapy include protecting sensitive bone marrow cells against cytotoxic drugs during cancer chemotherapy and serving as a dominant selectable marker when coexpressed with a corrective passenger gene. To address safety concerns associated with integrating viral systems, such as retroviruses, we tested the feasibility of maintaining a nonvirally delivered MDR1 gene (pEpiHaMA) episomally. An MDR1 vector containing the Epstein-Barr virus (EBV) origin of replication (OriP) and its nuclear retention protein (EBNA-1) was transfected into human (KB-3-1) cells. MDR1 was expressed at a higher level in cells carrying the episomal vector, pEpiHaMA, compared with the vector lacking sequences needed for episomal maintenance (pHaMA). Furthermore, more drug-resistant KB-3-1 colonies were obtained on selection after transfection with pEpiHaMA. These observations correlated with longer maintenance of episomes in cells transfected with pEpiHaMA. In addition, episomes could still be recovered for more than 1 month from tumor explants in nude mice that were injected with pEpiHaMA-liposome complexes after drug selection, suggesting that these constructs can be maintained extrachromosomally in vivo.

HIV-1 LTR as a target for synthetic ribozyme-mediated inhibition of gene expression: site selection and inhibition in cell culture

A library of three synthetic ribozymes with randomized arms, targeting NUX, GUX and NXG triplets, respectively, were used to identify ribozyme-accessible sites on the HIV-1 LTR transcript comprising positions -533 to 386. Three cleavable sites were identified at positions 109, 115 and 161. Ribozymes were designed against these sites, either unmodified or with 2'-modifications and phosphorothioate groups, and their cleavage activities of the transcript were determined. Their biological activities were assessed in cell culture, using a HIV-1 model assay system where the LTR is a promoter for the expression of the reporter gene luciferase in a transient expression system. Intracellular efficiency of the ribozymes were determined by cotransfection of ribozyme and plasmid DNA, expressing the target RNA. Modified ribozymes, directed against positions 115 and 161, lowered the level of LTR mRNA in the cell resulting in inhibition of expression of the LTR-driven reporter gene luciferase of 87 and 61%, respectively. In the presence of Tat the inhibitions were 43 and 25%. The inactive variants of these ribozymes exhibited a similar inhibitory effect. RNase protection revealed a reduction of RNA which was somewhat stronger for the active than the inactive ribozymes, particularly for ribozyme 115. Unmodified ribozymes showed no inhibition in the cell. The third ribozyme, targeting a GUG-triplet at position 109, possessed only low cleavage activity in vitro and no inhibitory effect in cell culture.

Effect of ABC transporters on HIV-1 infection: inhibition of virus production by the MDR1 transporter

The MDR1 multidrug transporter P-gp (P-glycoprotein) is an efflux pump that extrudes diverse hydrophobic drugs and peptides from cells. Since the entry of HIV-1 into cells involves an initial interaction of the viral gp41 hydrophobic peptide with the plasma membrane, a potential effect of P-gp on HIV-1 infectivity was explored. Virus production was greatly decreased when P-gp was overexpressed at the surface of a continuous CD4(+) human T-leukemic cell line (12D7) infected with HIV-1(NL4-3), a T-tropic molecular clone of HIV-1. P-gp overexpression did not significantly alter the surface expression or distribution of either the HIV-1 receptor CD4 or the coreceptor CXCR4. Reduction of HIV-1 infectivity in P-gp-expressing cells occurred both during the fusion of viral and plasma membranes and at subsequent step(s) in the HIV-1 life cycle.

Internal ribosome entry sites (IRESs): reality and use

IRESs are known to recruit ribosomes directly, without a previous scanning of untranslated region of mRNA by the ribosomes. IRESs have been found in a number of viral and cellular mRNAs. Experimentally, IRESs are commonly used to direct the expression of the second cistrons of bicistronic mRNAs. The mechanism of action of IRESs is not fully understood and a certain number of laboratories were not successful in using them in a reliable manner. Three observations done in our laboratory suggested that IRESs might not work as functionally as it was generally believed. Stem loops added before IRESs inhibited mRNA translation. When added into bicistronic mRNAs, IRESs initiated translation of the second cistrons efficiently only when the intercistronic region contained about 80 nucleotides, and they did not work any more effectively with intercistronic regions containing at least 300-400 nucleotides. Conversely, IRESs inserted at any position into the coding region of a cistron interrupted its translation and initiated translation of the following cistron. The first two data are hardly compatible with the idea that IRESs are able to recruit ribosomes without using the classical scanning mechanism. IRESs are highly structured and cannot be scanned by the 40S ribosomal subunit. We suggest that IRESs are short-circuited and are essentially potent stimulators favoring translation in particular physiological situations.

Expression of ribozymes in gene transfer systems to modulate target RNA levels

The possibility of designing ribozymes to cleave any specific target RNA has rendered them valuable tools in both basic research and therapeutic applications. In the therapeutics area, they have been exploited to target viral RNAs in infectious diseases, dominant oncogenes in cancers and specific somatic mutations in genetic disorders. Most notably, several ribozyme gene therapy protocols for HIV patients are already in Phase 1 trials. More recently, ribozymes have been used for transgenic animal research, gene target validation and pathway elucidation.