Suicide effect of herpesvirus thymidine kinase gene delivered into human hepatoma cells in complex with oligopeptides

The delivery of the suicide herpes simplex virus type 1 thymidine kinase (TK) gene into tumor cells followed by therapy with synthetic guanylic nucleotide analogues, such as gancyclovir and acyclovir, is one approach to cancer treatment. The most effective strategy for suicide gene therapy is virus-mediated TK gene transfer, using mainly recombinant adenoviruses or retroviruses. The main disadvantages of this strategy include immunogenicity, toxicity (for adenoviral vectors) and the risk of side effects (for retroviral vectors). An alternative approach involves the use of nonviral gene delivery systems, such as molecular conjugates, cationic oligopeptides and cationic lipids. Although these are less effective than viral vectors, they lack the above-mentioned disadvantages and may be used repeatedly. To evaluate the possibility of in vivo nonviral delivery of the herpesvirus TK gene into mammalian tumors, we have studied the antitumor effect of this gene after its transfer into human hepatoma (HepG2) cells in complex with cationic oligopeptides, followed by acyclovir treatment. The herpesvirus TK gene was under the control of the mouse ribosomal L32 protein promoter, which is active only in proliferating cells. We used a gene transfer system based on the cationic peptide YKAK8WK, which is capable of forming compact complexes with plasmid DNA, and the amphipatic peptide JTS-1, a pH-dependent, endosome-destabilizing agent. TK-gene-transduced cells were cultivated in growth medium containing acyclovir (50 μg ml⁻¹). The percentage of dead cells was estimated after 48 h by propidium iodine staining. The apoptotic death of cells was simultaneously confirmed by gel electrophoresis detection of oligonucleosomic DNA fragments. The amount of dead cells among the HepG2 cells transfected by suicide TK genes was four- to fivefold more than the amount among control HepG2 cells grown at the same concentration of acyclovir in medium. These results demonstrate the possibility of using cationic oligopeptides as suicide TK gene carriers in gene therapeutic treatments of human tumors.