Hammerhead ribozyme targeting connective tissue growth factor mRNA blocks transforming growth factor-beta mediated cell proliferation

Purpose. Excessive scarring following trauma or surgery of cornea, conjunctiva or retina can greatly impair visual outcome. At present, no agents are clinically available that selectively reduce activity of genes that regulate fibrosis. Connective tissue growth factor (CTGF) has been linked to fibrosis in several tissues, including cornea and conjunctiva. In this study, hammerhead ribozymes targeting CTGF mRNA were synthesized, kinetic parameters were measured, and the effect on TGF-β-mediated cell proliferation was measured in cultured human fibroblasts. Methods. The mRNA sequence of human CTGF was scanned for potential hammerhead ribozyme cleavage sites, and predicted secondary folding structures around the sites were calculated. Synthetic 12mer ribozymes and 33mer oligonucleotide mRNA targets corresponding to two sites were synthesized, and kinetic constants calculated from Hanes–Wolff plots of in vitro cleavage reactions. The ribozyme with higher percentage cleavage and kinetic rate was cloned into an expression plasmid (pTR-UF21) and stably transfected into cultured human fibroblasts. An inactive ribozyme plasmid served as a negative control. The effects of the ribozyme on expression of TGF-β-induced CTGF mRNA and protein levels were measured using ELISA and real-time TaqMan quantitative RT-PCR. Finally, the effect of the CTGF ribozyme on TGF-β-mediated proliferation of fibroblasts was measured using a non-radioactive cell proliferation microtiter assay. Results. Of the eight potential hammerhead ribozyme cleavage sites in human CTGF mRNA, two sites (CHR 745, and CHR 859) were identified with optimal secondary folding. CHR 859 cleaved 94% of the target mRNA, compared to 46% cleavage for CHR 745 after 16 hr of reaction. CHR 859 had a Km of 1·56 μ m and a Kcat of 2·97 min−1, while CHR 745 had a Km of 7·80 μ m and a Kcat of 5·7 min−1. The turnover numbers (Kcat/Km) of CHR 859 and CHR 745 were 1·9×106 m min−1 and 7·4×105 m min−1, respectively, indicating CHR 859 is 2·6 times more efficient than CHR 745 in destroying CTGF mRNA. Stable transfection of CHR 859 into human fibroblasts reduced CTGF mRNA levels 55% and protein levels 72% compared to the inactive ribozyme control. Furthermore, TGF-β-induced cell proliferation was reduced 90% in fibroblasts stably transfected with CHR 859 compared to control cell groups. Conclusions. The CHR 859 hammerhead ribozyme cleaved human CTGF mRNA with high kinetic efficiency in vitro, effectively reduced levels of CTGF mRNA and protein in cultured human fibroblasts, and blocked TGF-β-induced cell proliferation without nonspecific toxicity. These data support the concept that CTGF mediates TGF-β-induced cell proliferation, and imply that regulating CTGF expression with ribozymes may be effective in reducing ocular scarring.