Identification of diphtheria toxin via screening as a potent cell cycle and p53-independent cytotoxin for human prostate cancer therapeutics.

BACKGROUND: Metastatic human prostate cancer requires novel therapeutic strategies in order to overcome its low proliferative rate and its resistance to conventional chemotherapeutic agents. To identify potential cytotoxin gene products for use in experimental therapeutics such as in vivo gene therapy, an in vitro screen was designed. METHODS: Eight recombinant cellular toxins were tested for activity against a spectrum of metastatic human prostate cancer cell phenotypes. Pseudomonas exotoxin A, ricin, tumor necrosis factor alpha (TNF-alpha), diphtheria toxin (DT), Crotalus durissus terrificus toxin, crotalus adamenteus toxin, Naja naja toxin, and Naja mocambique toxin were evaluated. Comparative survival distinguished the relative potencies of these cytotoxins for irreparable prostate cancer cell death. RESULTS: Of the phospholipase A2 toxins, Crotalus durissus terrificus and Naja mocambique are active against the PSA secreting LNCaP cell line; however, the effect is reversible, and no other hormone refractory prostate cell line tested is sensitive. Screening identified toxin-specific differences: dose-dependent cytotoxic activity against all human prostate cancer cell lines tested was only identified for ricin and diphtheria toxin (DT) as highly potent. DT has an IC50 in the range of 20-00 pM by clonogenic survival and kills irreversibly by both apoptosis as well as nonapototic pathways. Acquisition of p53 mutant status conferred no reduction in sensitivity to DT cytotoxicity. Cell cycle arrest by aphidicolin did not protect human prostate cells from irreversible DT-induced cell death. TNF-alpha had modest cytostatic activity in the screen; however, the combination of TNF-alpha and DT resulted in marked acceleration of the time to prostate cancer cell death. CONCLUSIONS: The rational screening of cytotoxins allows the identification of cell cycle-independent agents of variable potency against human prostate cancer. DT-mediated cell death is cell cycle independent, and p53 independent, making it particularly attractive for application to cytoreductive gene therapy, targeted monoclonal antibodies, and prodrug delivery of toxins applied to human prostate cancer therapeutics.

Gene targeting of a CFTR allele in HT29 human epithelial cells.

HT29 cells endogenously express the cystic fibrosis transmembrane conductance regulator (CFTR) and have been used previously as a model to examine cellular regulation of CFTR expression and chloride secretory function. Homologous recombination has been used to specifically disrupt CFTR transcription in the HT29-18-C1 subclone. Experiments demonstrate successful disruption of a CFTR allele by DNA constructs, which target insertion of the neomycin phosphotransferase gene into CFTR exon 1 via homologous recombination. The mutation of one allele is a partial knockout because this cell line has multiple CFTR alleles. The mutation is confirmed by polymerase chain reaction (PCR) and genomic Southern blot analysis. A 52-68% reduction in CFTR mRNA levels is observed in the mutant cell line by both Northern and PCR analysis. However, Western blots show no decrease in total CFTR protein levels. Consistent with the lack of reduction in CFTR protein, the partial knockout mutant does not demonstrate alterations in cyclic AMP or calcium stimulation of chloride efflux or net osmolyte loss. Results suggest that posttranscriptional regulation of CFTR levels may contribute to maintenance of cellular chloride transport function.