Molecular mechanisms of alpha-fetoprotein gene expression.

Alpha-fetoprotein (AFP) is the main component of mammalian fetal serum. It is synthesized by visceral endoderm of the yolk sac and by fetal liver. Immediately after birth AFP level in blood decreases dramatically. AFP synthesis is reactivated in liver tumors and germinogeneous teratoblastomas, in a lesser degree after chemical and mechanical liver injuries followed by regeneration (i.e., acute viral hepatitis). AFP blood level change is an important marker for liver tumors that is widely used in clinical practice. Therefore, the study of the molecular and cellular mechanisms participating in regulation of the oncoembryonal protein AFP is an important task. On various experimental models it has been shown that the expression is regulated mainly on the transcriptional level, the AFP gene having a 7 kb regulatory region upstream. Within this region a tissue-specific promoter, three independent enhancers, and a silencer that is at least partially responsible for AFP gene expression decrease in adult liver have been defined. Some ubiquitous and some tissue-specific transcription factors, including hepatocyte nuclear factors (HNFs), which mediate the transcription of most of the liver-specific genes, have been shown to bind to the promoter. However, the mechanisms determining drastic changes of AFP synthesis level in the course of ontogenesis and carcinogenesis remain incompletely clarified. Also, little is known about negative regulators of AFP gene expression in cells of non-hepatic origin and in adult liver.