Independent promoters regulate the expression of two amino terminally distinct forms of latent transforming growth factor-beta binding protein-1 (LTBP-1) in a cell type-specific manner

Latent transforming growth factor-beta (TGF-beta)-binding proteins (LTBPs) are components of the extracellular matrix and large latent TGF-beta comple xes are secreted by various cells. Human LTBP-1 is known to exist in differ ent forms. LTBP-1L (long) has an amino-terminal extension, which is not fou nd in the smaller LTBP-1S isoform. To study the formation and transcription al regulation of LTBP-1S and LTBP-1L isoforms, we determined the nucleotide sequences of their 5'-flanking regions. The upstream regions of both isofo rms are devoid of TATA boxes but contain other putative binding sites for s everal transcription factors. Genomic sequencing revealed that LTBP-1L tran script is alternatively spliced to an internal splice acceptor inside exon 1 of LTBP-1S and thus defined the genomic organization of the isoforms. Rep orter gene analysis of upstream regions indicated the presence of independe nt, functional promoters, which regulate the transcription of the isoforms by cell-specific manner. Deletion analyses of the promoter regions revealed specific elements modulating their basal and cell type-specific expression . In SV-40 virus-transformed WI-38 lung fibroblasts a regulatory element re pressed the transcription of LTBP-1S by a cell-specific manner. In amniotic epithelial cells, transcription of the LTBP-1S reporter gene construct was downregulated by a distal upstream element. mRNA levels of the isoforms of LTBP-1 were stimulated in response to TGF-beta 1 in WI-38 cells. However, since TGF-beta 1 failed to stimulate the transcription of LTBP-1 reporter g ene constructs, TGF-beta 1 may mediate the induction of the isoforms by pos t-transcriptional mechanisms. Chromosomal localization of the LTBP-1 gene w as refined to 2p22-24.