The cytotoxic activity of the bacteriophage λ-holin protein reduces tumour growth rates in mammary cancer cell xenograft models

Background: The potential use of gene therapy for cancer treatment is being intensively studied. One approach utilises the expression of genes encoding cytotoxic proteins. Such proteins can affect cellular viability, for example by inhibiting the translation machinery or disturbing membrane integrity. The bacteriophage Lambda (λ)-holin protein is known to form a lesion in the cytoplasmic membrane of E. coli, triggering bacterial cell lysis and thereby enabling the release of new bacteriophage particles. The aim of this study was to evaluate whether the λ-holin protein has a cytotoxic impact on eukaryotic cells and whether it holds potential as a new therapeutic protein for cancer gene therapy. Methods: To explore this possibility, stably transfected human cell lines were established that harbour a tetracycline (Tet)-inducibly system for controlled expression of the λ-holin gene. The effect of the λ-holin protein on eukaryotic cells was studied in vitro by applying several viability assays. We also investigated the effect of λ-holin gene expression in vivo using a human breast cancer cell tumour xenograft as well as a syngeneic mammary adenocarcinoma mouse model. Results: The λ-holin-encoding gene was inducibly expressed in eukaryotic cells in vitro. Expression led to a substantial reduction of cell viability of more than 98%. In mouse models, λ-holin-expressing tumour cell xenografts revealed significantly reduced growth rates in comparison to xenografts not expressing the λ-holin gene. Conclusions: The λ-holin protein is cytotoxic for eukaryotic cells in vitro and inhibits tumour growth in vivo suggesting potential therapeutic use in cancer gene therapy. Copyright © 2005 John Wiley & Sons, Ltd.