EXPRESSION OF THE INSULIN-LIKE GROWTH-FACTOR-I GENE IS STIMULATED BY THE LIVER-ENRICHED TRANSCRIPTION FACTORS C EBP-ALPHA AND LAP/

The expression of the human insulin-like growth factor I(hlGF-I) gene is regulated in a developmental stage- and tissue-specific manner. Pos tnatally, the liver becomes the main endocrine source of this importan t growth factor. The hlGF-I gene contains two alternatively used leade r exons, exon 1 and exon 2. In human adult liver, exon 1 sequences are represented in about 80% of the transcripts. In this study we have in vestigated the role of promoter 1 (P1), located upstream of leader exo n 1, in the tissue-specific expression of the IGF-I gene in human adul t liver. Factors involved in this process have not been described to d ate. In this report we show, employing transient transfection experime nts in Hep3B cells, that two liver-enriched transcription factors, CCA AT/enhancer binding protein alpha (C/EBP alpha) and liver-enriched act ivating protein (LAP), enhance the activity of IGF-I Pi. DNase I footp rinting experiments demonstrate that a C/EBP-LAP binding site is locat ed 119 base pairs upstream of the major transcription start site in ex on 1. Comparison with other C/EBP-LAP binding sites reveals that the b inding site in Pi is a high affinity binding site. Mutations of the C/ EBP-LAP binding site completely abolished the enhancing effect of C/EB P alpha and LAP, indicating that their activating signal is indeed con ferred by this binding site. These results suggest that both C/EBP alp ha and LAP play important roles in the liver-specific expression of th e hlGF-I gene and provide the first clues in the elucidation of its co mplicated developmental stage- and tissue-specific expression pattern.