A hammerhead ribozyme that cleaves perforin and fas-ligand RNAs in vitro

Use of ribozymes and antisense oligodeoxynucleotides to investigate mechanisms of drug resistance

Chemotherapy can cure a number of human cancers but resistance (either intrinsic or acquired) remains a significant problem in many patients and in many types of solid tumour. Combination chemotherapy (using drugs with different cellular targets/mechanisms) was introduced in order to kill cells which had developed resistance to a specific drug, and to allow delivery of a greater total dose of anti-cancer chemicals by combining drugs with different side-effects (Pratt et al., 1994). Nearly all anti-cancer drugs kill tumour cells by activating an endogenous bio-chemical pathway for cell suicide, known as programmed cell death or apoptosis.

Down-regulation of Fas-L in glioma cells by ribozyme reduces cell apoptosis, tumour-infiltrating cells, and liver damage but accelerates tumour formation in nude mice

Fas-L (CD95L, APO-1L) expresses in a variety of tumours and has been proposed to play a role in tumour formation and metastasis. The contribution of Fas-L to tumour growth, however, is not conclusive especially in systems using cells with over-expressed Fas-L. In this study we down-regulated the expression o Fas-L in human glioma cells by a hammerhead ribozyme (Fas-L(ribozyme)) targeting against Fas-L mRNA. Fas-L(ribozyme)-carrying cells exhibited slightly enhanced growth rate and less degree of spontaneous apoptosis in vitro as compared with vector controls. In nude mice, Fas-L(ribozyme)-carrying cells grew faster with lesser apoptosis, formed bigger tumour with significantly fewer infiltrating cells in the tumour area, and triggered relatively milder tumour-associated liver damage than vector controls did. Thus, down-regulation of Fas-L not only improved viability of glioma cells but also reduces local immune responses that may consequently affect tumour formation. Taken together, our findings imply that endogenous expression of Fas-L in malignant cells is not always growth promoting.

Inhibition of Fas-mediated apoptosis in mouse insulinoma betaTC-3 cells via an anti-Fas ribozyme

In this study we have designed and constructed an anti-Fas ribozyme and show that it can specifically cleave the Fas mRNA in vitro. Moreover, to test its efficacy ex vivo, we transfected the anti-Fas ribozyme into betaTC-3 insulinoma cells, using a RNA polymerase III promoter to drive its expression. Like pancreatic beta cells, betaTC-3 cells do not constitutively express Fas, but Fas expression can be induced with IL-1 and IFN-gamma. Transfected cells expressed an average of 5000 copies of anti-Fas ribozyme transcript per cell as assessed by reverse transcriptase-real-time PCR. After IL-1/IFN-gamma treatment, betaTC-3 cells transfected with the anti-Fas ribozyme expressed 80% less Fas compared with mock-transfected cells. In addition, the anti-Fas ribozyme also inhibited Fas expression in NIT-1 insulinoma cells and in primary cultures of dispersed pancreatic islet cells. Inhibition of de novo Fas expression in betaTC-3 cells expressing the anti-Fas ribozyme correlated with resistance to Fas-mediated apoptosis as determined by the number of cells exhibiting caspase 3 proteolytic activity. Hence, we have engineered a ribozyme capable of preventing Fas expression in the betaTC-3 pancreatic insulinoma cell line and conferring resistance to Fas-mediated apoptosis. We suggest that ribozymes may be potentially useful to engineer resistance to apoptosis in transplantable beta cells, a feature that may significantly improve the survival of islet cell grafts.

Generation and characterization of hammerhead ribozymes targeting rodent metallothionein-I and -II ribonucleic acid

Two trans-acting hammerhead ribozymes (Rzs), Rz1-2 and Rz4-9, were designed and shown to be effective in cleaving rat or mouse metallothenein (MT)-I and MT-II mRNA, respectively, at position +147/148, in a sequence-specific manner. The catalytic efficiency for Rz1-2 on rat MT-I was found to be 678 M(-1)s(-1) and that for Rz4-9 on rat MT-II cRNA was found to be 372 M(-1)s(-1). An expression vector, pRz(4-9/1-2), containing both Rzs linked in tandem, was constructed and used to transfect NbE-1 cells, an immortalized ventral prostate epithelial cell line. Two stable-transfected lines, NbE-1(Rz2) and NbE-1(Rz3), expressing substantial levels of (Rz1-2/Rz4-9) and minimal levels of MT-I and MT-II transcripts were found to exhibit increased sensitivity to cadmium (Cd)-induced cytotoxicity compared to the parent line. This article is the first to report successful degradation of rodent MT mRNA in vitro and in cellulo by hammerhead Rzs.

Efficient ex vivo inhibition of perforin and Fas ligand expression by chimeric tRNA-hammerhead ribozymes

Graft-versus-host disease (GVHD) is a feared complication of allogeneic bone marrow transplantation. Research in rodent models has linked perforin and Fas ligand (FasL), two components of independent lytic pathways, with the induction of GVHD. In this study we characterized two hammerhead ribozymes that cleave their target perforin and Fas ligand RNAs with high efficiency in CTLL-2 cells. The perforin and Fas ligand ribozymes were expressed from a tRNA-directed RNA polymerase III promoter that was inserted in an episomal multicopy plasmid derived from papilloma virus. Chimeric anti-perforin and anti-FasL tRNA-ribozymes had sequences engineered in order to have specific secondary structure effects. These sequence modifications allow the formation of a 5' --> 3' stem structure and also place the ribozyme in a flexible bulge region that keeps the ribozyme separated from the tRNA domain. Northern and RT in situ PCR analyses showed high levels of transcription and efficient transportation to the cytoplasm. The expression of perforin and FasL in CTLL-2 cells was significantly reduced as assessed by RNA and protein analyses.

Perforin/granzyme-dependent and independent mechanisms are both important for the development of graft-versus-host disease after murine bone marrow transplantation

Graft-versus-host disease (GvHD) is the major limiting toxicity of allogeneic bone marrow transplantation. T cells are important mediators of GvHD, but the molecular mechanisms that they use to induce GvHD are controversial. Three effector pathways have been described for cytotoxic T lymphocytes: one requires perforin and granzymes, the second Fas (APO-1; CD95) and its ligand. Thirdly, secreted molecules (e.g., TNF-alpha, gamma-IFN) can also mediate cytotoxicity. Together, these mechanisms appear to account for virtually all cytotoxicity induced by activated CTL in standard in vitro lytic assays. Using transplants across histocompatibility barriers, we were able to analyze the contributions of these effector molecules to cell-mediated cytotoxicity in vivo in a GvHD model. We found that Fas ligand is an important independent mediator of class II-restricted acute murine GvHD, while perforin/granzyme-dependent mechanisms have only a minor role in that compartment. In contrast, perforin/ granzyme-dependent mechanisms are required for class I-restricted acute murine GvHD, while Fas ligand is not. The perforin/granzyme pathway may therefore represent a novel target for anti-GvHD drug design. In support of this approach, we provide additional data suggesting that specific perforin/granzyme inhibitors should not adversely affect hematopoietic recovery after transplantation.