Toxicity of cationic lipid-ribozyme complexes in human prostate tumor cells can mimic ribozyme activity

Prostate tumor cell lines have been shown to both produce interleukin-6 (IL-6) and express the IL-6 receptor, suggesting a potential autocrine growth regulatory role for IL-6. We explored the role of IL-6 in the proliferation of the human prostatic carcinoma cell line, DU145, using ribozymes to inhibit IL-6 expression. Hammerhead-type ribozymes targeted against IL-6 mRNA sequences were prepared, and in vitro analyses were used to demonstrate that these molecules catalyzed the cleavage of IL-6 mRNA poly- nucleotide fragments. To test in situ activity, these ribozymes were transfected into DU145 cells using cationic transfection lipids, cytofectins. Treatment of cultured cells with ribozyme/cationic lipid complexes resulted in a reduction of IL-6 protein levels in the supernatant and reduced numbers of DU145 cells 48 h after treatment. However, similar results were also seen following treatment with control RNA/lipid complexes. This reduction in IL-6 levels and cell numbers was a function of the RNA/lipid complexes and was not seen with either lipid or RNA alone. Therefore, the reductions in IL-6 levels and cell numbers observed were not due to ribozyme-mediated cleavage of IL-6 mRNA, but rather reflected a dose-dependent, nonspecific toxic effect of the treatment with ribozyme/cytofectin complexes. This effect can resemble functional ribozyme activity, complicating analysis of the activity of synthetic ribozymes after transfection into cultured cells.