Purpose: Luteinizing hormone receptor (LHR) is found in abundance on human ovarian, breast, endometrial and prostate carcinomas but at only low levels on non-gonadal tissues. To selectively kill LHR-expressing tumors, granzyme B (GrB) was linked to a protein in which both chains of human chorionic gonadotropin were yoked together (YCG).
Methods: GrB-YCG was expressed and secreted from insect Sf9 cells. Its GrB enzymatic activity and binding affinity for hLHR were then characterized. The differential cytotoxicity of GrB-YCG versus GrB alone was tested in a panel of LHR-expressing tumor cells by SRB assay, and the mechanisms involved in the cell death were investigated by confocal fluorescence microscopy, flow cytometry, and western blot analysis.
Results: GrB-YCG was successfully expressed and secreted from Sf9 insect cells and purified from cell culture supernatants. The serine protease activity of GrB-YCG was equivalent to that of human recombinant GrB. An in vitro hormone binding assay revealed that the GrB-YCG molecule also retained the ability to bind to the LHR receptor with an affinity similar to that of native hCG. Upon cell binding, GrB-YCG was rapidly internalized into LHR-expressing human ovarian cancer cells and produced selective and potent tumor cell killing by inducing apoptosis through activation of caspase-3.
Conclusions: These results validate LHR as a therapeutic target and indicate that delivery of the human pro-apoptotic enzyme GrB to tumor cells by yoked hCG has substantial selectivity and therapeutic potential for human tumors that express high levels of LHR such as ovarian carcinomas.