Paracrine effect of human chorionic gonadotropin ectopically produced from papillary thyroid cancer cells on growth and function of FRTL-5 rat thyroid cells

It is well known that human chorionic gonadotropin (hCG) is sometimes secreted from nontrophoblastic neoplasms. To elucidate the role of ectopic hCG, we investigated the effect of hCG produced from a papillary thyroid cancer cell line (B-CPAP cells) on stimulation and growth promotion of FRTL-5 rat thyroid cells. Ectopic hCG contained in the culture medium of B-CPAP cells was purified using gel filtration and bioassayed for thyrotropic activity in FRTL-5 cells. Addition of ectopic hCG (up to 5.2 x 10(4) IU/L) increased cyclic adenosine monophosphate (cAMP) accumulation and 3H-thymidine incorporation in FRTL-5 cells dose dependently. These effects were almost as potent as the stimulation induced by standard hCG CR-127. After the absorption of the ectopic hCG by anti-hCG-beta monoclonal antibody, the cAMP accumulation was significantly decreased. Analysis of ectopic hCG isoforms with different isoelectric points indicated the predominance of the acidic hCG isoform with isoelectric point (pI) 3.8-3.2 that is the major isoform of standard hCG. Basic isoforms (pI 5.7-5.3) with higher thyrotropic potency were also detected. These results indicate that the ectopic hCG secreted from papillary thyroid cancer cells possess intrinsic thyroid-stimulating and growth-promoting activity. The ectopic hCG may act as an autocrine-paracrine factor in nontrophoblastic neoplasms.