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. Postnatally, the liver becomes the main endocrine source of this important growth factor. The hlGF-I gene contains two alternatively used leader 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 investigated the role of promoter 1 (P1), located upstream of leader exon 1, in the tissue-specific expression of the IGF-I gene in human adult liver. Factors involved in this process have not been described to date. In this report we show, employing transient transfection experiments in Hep3B cells, that two liver-enriched transcription factors, CCAAT/enhancer binding protein alpha (C/EBP alpha) and liver-enriched activating protein (LAP), enhance the activity of IGF-I Pi. DNase I footprinting experiments demonstrate that a C/EBP-LAP binding site is located 119 base pairs upstream of the major transcription start site in exon 1. Comparison with other C/EBP-LAP binding sites reveals that the binding site in Pi is a high affinity binding site. Mutations of the C/EBP-LAP binding site completely abolished the enhancing effect of C/EBP alpha and LAP, indicating that their activating signal is indeed conferred by this binding site. These results suggest that both C/EBP alpha and LAP play important roles in the liver-specific expression of the hlGF-I gene and provide the first clues in the elucidation of its complicated developmental stage- and tissue-specific expression pattern.