Human chorionic gonadotropin - a well-known hormone with unknown functions

Human chorionic gonadotropin (CG) belongs to the glycoprotein family consisting of LH, FSH and TSH. All of these hormones are composed of two subunits: common to the whole family alpha subunit and hormone-specific beta subunit. CG has paracrine effects on several processes such as placentation, implantation, angiogenesis and delaying the apoptosis of corpus luteum. Serum level of CG is used to monitor pregnancy and pregnancy disorders. Recent studies have shown that the synthesis of CG is a characteristic feature of a wide variety of malignant and non-malignant tumors. The role of CG in cancerogensis remains unclear, but the main hypothesis concerns its antiapoptotic impact of the hormone on the neoplastic cells. The synthesis of functional CG requires the activity of separate genes encoding both hormone's subunits, but it is the beta subunit accessibility which controls the process. The protein synthesis must be followed by proper folding and posttranslational modifications of the molecule. Particularly glycosylation of human chorionic gonadotropin was shown to have an impact on the hormone's function. The amount and the structure of carbohydrate residuals attached to CG may be different and lead to the formation of hormone variants, which vary in molecular mass. Normal CG with a molecular mass of about 37.5 kDa is produced by the syncytiotrophoblast, while the variant with higher molecular mass - 38.5-40 kDa, described as hyperglicosylated CG, is secreted by undifferentiated trophoblast cells and some cancers. It is suggested that those forms have different but complementary biological functions. However, the mechanism of the action of particular variants and signaling pathways activated by those forms are still obscure.