Characterization of the mouse insulin-like growth factor binding protein 4 gene regulatory region and expression studies

Insulin-like growth factor binding protein 4 (IGFBP-4) is known as a potent inhibitor of IGFs action in various cell types. In this study, the mouse IGFBP-4 gene 5' flanking region, the IGFBP-4 mRNA expression, and the IGFBP-4s intracellular transport were investigated. The regulatory region exhibits all elements typical for an eukaryotic TATA element containing promoter and was found to also contain functional elements to direct transcriptional activation of a luc reporter gene construct that gradually decreased by 5' unidirectional deletions. Responsiveness of the IGFBP-4 promoter activity was tested with thyroid hormone and found only within extended constructs but not when a potential TRalpha1-binding site had been deleted. By using exon specific probes, we observed a varying expression pattern of IGFBP-4 transcripts in three rodent cell lines. Surprisingly, mouse fibroblastic NIH/3T3 cells displayed exclusively about a 2.0-kb transcript apparently lacking the IGFBP-4 mRNA 5' region. Studies on the intracellular transport by establishment of an IGFBP4/green fluorescent protein (GFP) fusion protein clearly demonstrate that IGFBP-4 is transported continuously along the intracellular secretory pathway and is excluded from other intracellular compartments. The description of the genomic IGFBP-4 region in the mouse now opens new perspectives for further clarification of the role of IGFBP-4 in growth and development.