POSTNATAL LIVER-SPECIFIC EXPRESSION OF HUMAN INSULIN-LIKE GROWTH FACTOR-II IS HIGHLY STIMULATED BY THE TRANSCRIPTIONAL ACTIVATORS LIVER-ENRICHED ACTIVATING PROTEIN AND CCAAT ENHANCER BINDING PROTEIN-ALPHA/

Transcription of the human gene encoding insulin-like growth factor II (IGF-II) is directed by four promoters (P1-P4), which are active in a tissue-and development-dependent manner. High levels of IGF-tl in pos tnatal serum are due to activation of P1 in the liver. Since fiver tis sue contains high levels of the bZIP factors, liver-enriched activatin g protein (LAP), the CCAAT/enhancer binding protein-alpha (C/EBP alpha ), and the D-element binding protein (DBP), and since P1 contains a fu nctional C/EBP alpha binding site (P1 CBS), we have examined the role of these transcription factors in the activation of IGF-II P1. Transie nt transfection experiments reveal that P1 can be activated up to 15-f old by LAP and up to B-fold by C/EBP alpha but can not be activated by DBP. Electrophoretic mobility shift assays with liver nuclear extract show that P1 CBS is predominantly bound by LAP and C/EBP alpha. Mutat ional analysis of Pi CBS indicates that DBP binding is prevented by on e distinct G-C base pair in the P1 CBS element. The P1 CBS element is a high affinity binding site, which was demonstrated by comparing P1 C BS with other LAP-C/EBP alpha binding sites employing quantitative ele ctrophoretic mobility shift assay. These results indicate that LAP and C/EBP alpha are major contributors to the postnatal liver-specific ac tivation of the human IGF-II promoter P1.