Ectopic β-Human Chorionic Gonadotropin Production by a Human Hepatoma Cell Line (FOCUS): Isolation and Immunochemical Characterization*

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Abstract
We have established a human hepatocellular carcinoma cell line designed FOCUS that produces and secretes the .beta.-subunit of hCG. In the study of .beta.hCG production by FOCUS cells, we have developed and employed a series of monoclonal immunoradiometric assays (IRMAs) that detect epitopes unique to .beta.hCG, .alpha.hCG, hCG, and sequence-specific regions of the carboxyl-terminal peptide of .beta.hCG. The cells secrete approximately 15 ng .beta.hCG/106 cells and per 24 h; however, we were unable to detect either hCG or the .alpha.-subunit. The ectopic .beta.hCG was subsequently affinity purified from the culture medium and partially characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. The mol wt of ectopic .beta.hCG was approximately 35,000 daltons. Immunochemical analysis of ectopic .beta.hCG by sequence-specific mnoclonal antibodies revealed that epitopes corresponding to amino acid sequnces 109-115, 121-145, 134-140, and 139-145 of the carboxyl-terminal peptide were present. FOCUS cells also demonstrate an intracytoplasmic localization of .beta.hCG by immunoperoxidase-staining techniques. Taken together, these findings suggest that FOCUS cells produce and secrete only .beta.hCG and that thus far, its physical properties appear indistinguishable from those of the native subunit. This unique cell line will be useful to study .beta.hCG gene(s) regulation as well as the mechanisms of ectopic .beta.hCG production and secretion.
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