We have utilised cell cultures and growth of tumours in nude mice to assess further the potential of the Semliki Forest virus (SFV) vector as a cancer therapy agent. This vector is a transient RNA-based expression vector, and we have previously shown that SFV and its derived vector can induce p53-independent apoptosis by expression of the nonstructural region of the virus genome. Apoptosis induction is therefore an inherent property of the vector and is not dependent on heterologous gene expression. SFV recombinant suicide particles (rSFV) were shown to induce apoptosis in H358a cells, which are human non-small cell lung carcinoma cells deleted in p53. EGFP-expressing rSFV also inhibited the growth of developing H358a spheroids. Direct injection of rSFV into H358a tumours subcutaneously implanted as xenografts in nu/nu mice inhibited tumour growth, and in some cases caused complete regression. It is concluded that tumour growth suppression induced by rSFV was due to apoptosis induction and that the vector has an inherent cell death-promoting and antitumour activity. These results, as well as previous work by other authors on targeting and immune stimulation using alphavirus vectors, indicate that SFV recombinant particles in particular have considerable potential for further exploitation as a cancer therapy agent.
Several types of virus vector are currently under development as tumour therapy agents. Previous studies have indicated that p53 gene replacement strategies, using recombinant viral vectors, can induce apoptosis and suppress tumour growth in p53-defective tumour cells. Adenovirus and retrovirus vectors have been the leading candidates for cancer gene therapy,12 and adeno-associated virus has also proved successful in the transduction of this tumour suppresser gene.3 Inactivation of p53 has been implicated in many types of tumour,4 and non-small cell lung carcinoma (NSCLC) is one of the most common human cancers in which p53 mutations have been frequently identified. However, not all tumours are amenable to therapy with p53, and there is a need for the development of vectors which induce apoptosis independently of p53. In this report, we further evaluate the potential of a newly developed system, Semliki Forest virus recombinant particles (labelled rSFV), as a cancer therapy agent. This vector system may have several advantages over the retro- and adenovirus systems currently under development, particularly its inherent capacity to induce p53-independent apoptosis.
SFV, an enveloped positive-sense RNA virus, has been developed as a transient expression vector,567 and has been utilised in the development of prototype recombinant vaccines.789 In the vector, the SFV structural proteins, which are formed in large amounts in infected cells by a gene amplification mechanism, are replaced with a polylinker sequence, which facilitates insertion of a heterologous gene. This does not affect the replication capacity of the vector and structural proteins are supplied in trans by helper RNAs to form recombinant encapsidated particles. Such particles can be produced at high titres, and will infect cells and express a heterologous gene cloned into the multicloning site of the vector. They are capable of only one round of multiplication upon infection, and are thus suicide particles. In terms of cancer therapy, SFV offers an important advantage over other vectors, since previous studies show that SFV induces apoptosis in cultured cells,71011 and this can occur in the absence of viral structural protein genes and independently of the p53 status of infected cells.11 Thus, unlike alternative vectors, rSFV does not depend solely on the heterologous gene for apoptosis induction.
In this study, we used encapsidated rSFV particles to infect p53-deleted human NSCLC cells to determine the effects on tumour cell growth in vitro and in vivo. The multicloning site contained the reporter gene EGFP to facilitate detection of the vector in situ. The results indicate that SFV recombinant particles can inhibit the growth of human lung carcinoma cells and induce tumour regression by apoptosis induction, in the absence of heterologous gene expression.
Transduction of H358a cells
The conditions for optimal viral transduction of the H358a lung carcinoma cell line with SFV recombinant particles were determined by infecting cells with EGFP-expressing rSFV particles (rSFV-EGFP) using a range of different MOIs (Figure 1). A linear relationship existed between the number of infected cells and the MOI used, up to a MOI of 100. At MOI 400, almost 50% of H358a cells were positive for the vector, and this was improved using multiple infections (data not shown).
Effects of rSFV particles on cell growth in vitro
H358a cells were incubated with rSFV-EGFP, rSFV-1 or medium alone for 1 h and the medium replaced. rSFV-1 particles lack a reporter gene and contain the vector backbone only. Cell growth and viability were estimated by cell count (Figure 2a) and colorimetric assays (Figure 2b). In both assays, mock-infected cells continued to grow and proliferate, whereas the survival rate of rSFV-EGFP-infected cells was greatly reduced, with less than 50% of cells surviving after 1 day and only 10% after 1 week. Tumour cell growth was similarly inhibited on infection with rSFV-1. Values for both assays were reproduced in four independent experiments. Similar results were obtained with rSFV-LacZ particles (data not shown). Thus, infection with rSFV particles suppresses H358a cell growth and this occurs independently of the heterologous gene.
Due to the marked decrease in H358a cell viability on infection with rSFV, we examined if this were due to the induction of apoptosis. Chromosomal DNA was extracted from cells infected with rSFV-EGFP (Figure 2c) or medium alone, and was analysed by agarose gel electrophoresis. The appearance of 180–200-bp DNA fragments and their multiples, formed by internucleosomal cleavage, is characteristic of apoptotic cell death. This was demonstrated at 24 and 48 h after infection with rSFV-EGFP particles (Figure 2d). Fragmented DNA was not observed in mock-infected controls. These results were confirmed using TUNEL staining (not shown), and were similar to those obtained previously using transfected vector RNA.11
Effect of rSFV infection on spheroids
Multicellular spheroids are composed of tumour cells growing in culture as three-dimensional structures, simulating the growth and microenvironmental conditions of tumours in vivo.12 The cytotoxic efficiency of rSFV particles was further evaluated in H358a cells growing as tumour spheroids. Following 36 h in culture, individual H358a spheroids were placed in medium alone, or medium containing rSFV-EGFP. We then compared the relative abilities of these spheroids to proliferate. At 24 h after treatment, mock-infected spheroids incorporated at least eight times the amount of 3H-thymidine compared with those treated with the vector (Figure 3a). The results of the MTS viability assay were similar to those of the cell proliferation assay (Figure 3b). A four-fold difference in spheroid viability was demonstrated after 6 days in culture. Similar results were obtained in three separate experiments.
Effect of rSFV administration on tumour growth in nude mice
To address the potential therapeutic efficiency of the rSFV particles in vivo, H358a cells were injected subcutaneously into nu/nu mice to form xenografts, and tumours of 4 mm in diameter were allowed to develop. Mice received a direct intratumoral injection of 50 μl of either TNE buffer alone, or TNE buffer containing rSFV-EGFP particles (1 × 1010 IU/ml) daily for 3 days (Figure 4a). In mice treated with TNE buffer, tumours continued to grow rapidly throughout the treatment. Mice treated with rSFV-EGFP showed a marked inhibition of tumour growth as early as day 4 after treatment. At 30 days after injection, tumour growth was reduced five-fold in comparison to controls.
The growth inhibitory effect was more pronounced after two treatment cycles (Figure 4b). Mice received intratumoral injections of TNE buffer or rSFV-EGFP particles each on days 1, 2, 3 and days 7, 8 and 9. Six injections with rSFV-EGFP reduced the size of tumours by 95% in comparison to controls. Three of the seven treated tumours showed complete regression on macroscopic examination, and no signs of regrowth were observed, even at 135 days after treatment. Similar results were observed in two separate experiments.
Histopathological examination of tumours injected with TNE buffer only revealed well-demarcated, densely cellular nodular masses located in the subcutis. The tumour cells had distinct cell borders and oval hyperchromatic nuclei with prominent nucleoli. Numerous mitotic figures and apoptotic forms were present together with occasional small foci of oncotic necrosis. Tumours treated once with rSFV-EGFP and sampled at 4 days after treatment were characterised by large coalescent areas of oncotic necrosis (Figure 4d); apoptosis was also common. Small aggregates of lymphoid cells and neutrophils were present among fibroblastic cells that partially encircled the tumour nodules. The vector was detected by EGFP fluorescence in treated tumours no earlier than 16 h after injection and persisted for 3 days (Figure 4c). Positive TUNEL reactivity was detected as early as 8 h after treatment with the vector, reaching maximum reactivity at 24 h and approached control levels after 1 week (Figure 4c, e). Control tumours exhibited neither EGFP fluorescence nor large areas of TUNEL reactivity.
As an expression system, rSFV has already proved successful in treating murine tumours via the anti-angiogenic effects of interleukin-12.13 In this study, we have shown that direct infection with rSFV particles can inhibit tumour growth and induce regression in the p53-deleted human NSCLC cell line H358a. Growth suppression and tumour regression have been previously demonstrated in this cell line by restoring wild-type p53 activity and inducing apoptosis. This was achieved using adenoviral,14 retroviral15 and adeno-associated3 viral vectors. SFV-induced apoptosis can occur independently of p53 expression, and in our study rSFV particles contained the reporter gene EGFP. Expression of this gene did not contribute to the induction of apoptosis but was necessary to facilitate detection of the vector in situ. Pathological analysis of rSFV-treated tumours indicated that inhibition of tumour growth was not due to the induction of an immune response, but was consistent with an initial induction of apoptosis followed by a large depletion of tumour cells by oncotic necrosis.16
SFV also offers several other advantages over retrovirus and adenovirus vector systems. It is a transient RNA-based expression system, and this obviates nuclear complications such as random RNA splicing or host chromosomal integration. rSFV particles are very stable and can be concentrated to titres exceeding 1010 infectious units per millilitre. The vector also has the ability to infect both dividing and non-dividing cells. The treated mice in this study remained healthy, and no gross lesions were detected in the tissue surrounding the tumour. This may be because rSFV particles adsorb rapidly to cells at the injection site and are capable of one round of multiplication only. Our unpublished results indicate that dispersal of rSFV particles from the injection site to other organs is transient, and that no detectable tissue damage occurs. Unlike DNA vectors, which disperse in the host and persist for many weeks following administration,17 rSFV vector RNA persists for 7 days only as detected by RT-PCR (MM Morris-Downes and GJ Atkins, unpublished results).
This study and previous work by other authors has promising implications for tumour therapy. We have shown here that the SFV vector has inherent cell death-promoting and antitumour activity which acts independently of p53. However, the cell killing ability of the vector in its present form is relatively low, necessitating multiple treatments to induce efficient cell killing of cultured cells. In tumour tissue, cell killing may be enhanced by a possible bystander effect, or an effect on angiogenesis leading to oncotic necrosis. Replacing the reporter gene EGFP with a known pro-apoptotic gene such as bax could serve to augment further the cell death-promoting capability of this vector in NSCLC and other tumours, and may serve to counteract the action of anti-apoptotic genes such as bcl-2. As has been recently shown with both naked SFV RNA and rSFV particles, it may also be possible to augment antitumour immune responses by expression of immunogenic proteins.1819 Attempts have been made with other vectors, including the Sindbis vector (an alphavirus-based vector like SFV), to target vectors to tumour cells by modification of viral structural proteins.20 This strategy could also be applied to the SFV system, possibly by modification of the viral E2 protein to recognise tumour-specific receptors. We feel that the unique properties of SFV could be further exploited to develop a safe and effective vector for cancer gene therapy.
Materials and methods
The human lung carcinoma cell line H358a was kindly provided by Dr JA Roth (Department of Thoracic Surgery, MD Anderson Cancer Center, University of Texas, Houston, TX, USA). These cells contain a homozygous deletion in the p53 gene4 and were propagated in RPMI 1640 supplemented with 10% heat-inactivated foetal bovine serum (Life Technologies, Paisley, UK).
Preparation of recombinant SFV vector
The SFV vector system, and a vector expressing EGFP, were obtained from Professor P Liljeström, Microbiology and Tumorbiology Center, Karolinska Institute, Stockholm, Sweden. The production of recombinant SFV RNA and generation of fully infectious vector particles have been described previously.6 Briefly, BHK-21 cells were cotransfected with rSFV RNA and two helper RNAs, one of which codes for the SFV capsid protein, the other for the envelope proteins. These RNAs were transcribed in vitro from the appropriate plasmids; particles expressing EGFP (rSFV-EGFP), or with no heterologous cloned gene (ie vector alone, rSFV-1), were produced. After 24 h incubation, fully infectious virus particles containing the recombinant gene were harvested. Titration was performed by direct (rSFV-EGFP) or indirect epifluorescence using antiserum to the SFV non-structural proteins (rSFV-1). For in vivo experiments, virus particles were concentrated by ultracentrifugation through a 20% (w/v) sucrose cushion (Beckman SW40 rotor; 25000 r.p.m. for 1.5 h at 4°C; Beckman Coulter, Fullerton, CA, USA). Virus pellets were resuspended in TNE buffer (50 mM Tris-HCL, pH 7.4, 100 mM NaCl, 0.1 mM EDTA) and titres were determined as before.
Cell death analysis
To measure growth and cytotoxicity in cultured cells, H358a cells were plated at a density of 1 × 104 cells per well in six-well plates and infected with rSFV-EGFP or rSFV-1 particles (MOI = 100). Medium alone was used as a mock-infection control. Cells were harvested and counted daily for 7 days. Cell viability was determined by trypan blue exclusion and each sample was counted in triplicate. The cytotoxicity of both vector constructs was also assessed using a colorimetric assay.21 H358a cells (1 × 104) in six-well dishes were infected as before. At 24 h intervals, cells were fixed in 5% trichloroacetic acid, washed, air dried, and stained for 20 min with 0.4% (w/v) sulphorhodamine B (Sigma Chemical, St Louis, MO, USA) in 1% acetic acid. Absorbance values were measured at 492 nm using an ELISA reader, and were directly related to the total number of surviving cells.
To detect the internucleosomal DNA fragments characteristic of apoptosis, DNA was isolated from rSFV-EGFP-infected and mock-infected H358a cells at 24 h and 48 h after infection. Cells were treated with lysis buffer (20 mM EDTA, 100 mM Tris, pH 8, 0.8% w/v sodium lauryl sarcosinate), followed by ribonuclease A (10 mg/ml; Sigma) at 37°C for 2 h and proteinase K (20 mg/ml; Sigma) digestion for 18 h. Samples were subjected to electrophoresis on a 1.5% agarose gel and visualised by ethidium bromide staining. TUNEL staining, characteristic of apoptosis in situ, was performed as described previously.11 Cells were fixed in 4% paraformaldehyde, permeabilized in 0.1% Triton X-100 solution and labelled using terminal transferase and horseradish peroxidase (Boehringer Mannheim, Indianapolis, IN, USA). Colorimetric detection was performed using diaminobenzidine.
Multicellular spheroids were prepared by the liquid overlay technique.22 A total of 104 H358a cells in 0.1 ml of complete medium were plated in 96-well plates, whereas 105 cells in 1 ml were used in 24-well plates. To prevent cell attachment, plates were coated with 1% medium agar. After 1–2 days incubation, aggregates formed in each well, with all suspended cells contributing to the formation of a single spheroid. To determine the proliferation of H358a cells as three-dimensional spheroids, cells were plated in 24-well plates for 36 h and then infected with rSFV-EGFP (MOI = 100) or with medium alone. The spheroids were then incubated with 3H-thymidine (2 μCi/well in 50 μl; Amersham Pharmacia Biotech, Uppsala, Sweden) for 24 h and samples were taken daily for 6 days. Spheroids were washed, resuspended in 1% sodium dodecyl sulfate, precipitated with 5% trichloroacetic acid, harvested on to filter mats and counted using a liquid scintillation counter. To determine viability, cells were incubated in 96-well plates with additions as above. After incubation, 20 μl of phenazine methosulphate (MTS reagent; Promega, Madison, WI, USA) was added to each well and incubated at 37°C for 4 h. MTS is bioreduced into formazan which is soluble in culture medium and was detected at 570 nm using an ELISA reader; absorbance is directly related to cell viability. Results are presented as the means of triplicate samples.
Inhibition of tumour growth in vivo
Female Balb/c nu/nu mice aged 50–60 days were obtained from Harlan (Bicester, UK), and were maintained under barrier conditions with sterilised food and water supplied ab libitum, in accordance with the principles set down in SI 17/94 European Communities regulations, 1994 for care and use of laboratory animals. To determine inhibition of tumour growth in vivo, H358a cells (1 × 106 in 0.4 ml complete medium) were injected into the right flank of each mouse to form xenografts. On reaching a diameter of 4 mm, 50 μl of TNE buffer alone or TNE containing rSFV-EGFP (1 × 1010 IU/ml) was injected intratumorally. Three separate analyses were performed; one with a single injection (on day 1), three injections (days 1, 2, 3) or six injections (days 1, 2, 3, and 7, 8, 9). Five to seven mice were used per treatment group. Tumours were measured every 2 days with calipers in two perpendicular diameters without knowledge of the treatment groups. Tumour volume was calculated by assuming a spherical shape, with the average tumour diameter calculated as the square root of the product of the cross-sectional diameters. Tumours which received one injection only were harvested from 8 h to 8 days after infection for pathological examination. Direct epifluorescence and TUNEL were performed on paraformaldehyde-fixed tumour cryosections. Haematoxylin and eosin (H&E) staining was performed on formalin-fixed, paraffin-embedded sections.
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We thank Dr Marina Fleeton and Professor Peter Liljeström for providing the vectors used in this study and for help with the vector system, and Ms Marie Moore for her assistance with pathological studies. We are also grateful to Ms Deborah Mansfield and Dr Jack Roth for providing the H358a cells. This work was supported by the Cancer Research Advancement Board of the Irish Cancer Society, the Irish Health Research Board, BioResearch Ireland, the Wellcome Trust and the European Union Biotechnology Programme.
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Cite this article
Murphy, A., Morris-Downes, M., Sheahan, B. et al. Inhibition of human lung carcinoma cell growth by apoptosis induction using Semliki Forest virus recombinant particles. Gene Ther 7, 1477–1482 (2000). https://doi.org/10.1038/sj.gt.3301263
- cancer gene therapy
- lung carcinoma
- virus vector
- Semliki Forest virus
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