TRANSCRIPTIONAL AND POSTTRANSCRIPTIONAL MECHANISMS IN EPIDERMAL GROWTH-FACTOR REGULATION OF HUMAN CHORIONIC-GONADOTROPIN (HCG) SUBUNITS ANDHCG RECEPTOR GENE-EXPRESSION IN HUMAN CHORIOCARCINOMA CELLS

Epidermal growth factor (EGF) stimulates the secretion of hCG in chori ocarcinoma cells. However, the molecular mechanisms involved in this E GF action have never previously been investigated. The present study i nvestigated them as well as EGF regulation of the hCG/LH (LH) receptor gene in JEG-3 human choriocarcinoma cells. The JEG-3 cells contain mu ltiple EGF receptor messenger RNA (mRNA) transcripts and a single 170- kilodalton immunoreactive receptor protein. The human EGF can bind to the receptor protein and stimulate the receptor autophosphorylation as well as the phosphorylation of four other membrane proteins. Culturin g JEG-3 cells with recombinant human EGF resulted in a dose- and time- dependent increase in hCG secretion. The maximal effect was seen at 10 0 ng/ml EGF, with a time lag of about 5 h. Tyrosine kinase, but not pr otein kinase-C or protein kinase-A, signaling was involved in the EGF action to increase hCG secretion. The EGF-induced increase in hCG secr etion was not due to an increase in cell number or differentiation int o multinuclear syncytia. EGF treatment resulted in a dose- and time-de pendent increase in steady state levels of hCG alpha and hCG beta mRNA s. This increase was due to the stabilization of subunit mRNA transcri pts. The increase in subunit mRNAs preceded the increase in hCG secret ion. The EGF treatment resulted in a dose- and time-dependent decrease in steady state levels of the hCG/LH receptor mRNA transcripts. The d ecrease was due to a transcriptional inhibition of receptor gene. EGF treatment paradoxically stabilized hCG/LH receptor protein. In summary , EGF treatment up-regulates hCG subunits gene expression and down-reg ulates hCG/LH receptor mRNAs involving transcriptional and posttranscr iptional mechanisms in JEG-3 human choriocarcinoma cells.