Expression of alpha- and beta-human chorionic gonadotropin subunits in cultured human cells

We surveyed several human cell lines for production of alpha- and beta-human chorionic gonadotropin (hCG) under a variety of conditions known to induce gene expression. alpha- and beta-hCG subunits were monitored in culture media by specific radioimmunoassays and were shown to be quite sensitive to serum refeeding and growth state of all cell types studied. The permanent line JEG-3 secreted both alpha- and beta-subunits whereas HeLa cells secreted only the alpha-subunit. Production of both subunits was augmented in these permanent cell lines, for each growth state, by pretreating cells with 5-azacytidine; in contrast, spontaneous beta-hCG production by normal human fibroblasts (four of six strains) was only rarely increased after 5-azacytidine treatment, and more often was suppressed by 30 to 40%. Three of five strains from inherited chromosomal breakage syndromes produced immunoassayable beta-hCG spontaneously, two of which increased secretion upon treatment with either UV or mitomycin C. Surprisingly, one normal cell strain of fetal origin was induced to secrete alpha-hCG, but not beta-hCG, after UV irradiation. JEG-3 and HeLa cells produced detectable cognate mRNA for alpha- or beta-hCG subunits or both by Northern and S1 nuclease protection analyses, whereas such transcripts from untransformed human fibroblasts were consistently below detectable levels. Quantitation of beta-hCG mRNA by RNA:RNA annealing kinetics indicates that even the fibroblast strain producing the highest secreted beta-hCG levels contained cognate mRNAs at only approximately 0.1 per cell. We conclude that hCG expression in human fibroblasts is strongly repressed at the transcriptional level, although a variety of conditions (growth state, serum refeeding, cell senescence, or DNA damage) can affect the level of "leaky" expression, at least in some responding fraction of cells.