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

Abstract

Latent transforming growth factor-β (TGF-β)-binding proteins (LTBPs) are components of the extracellular matrix and large latent TGF-β complexes are secreted by various cells. Human LTBP-1 is known to exist in different forms. LTBP-1L (long) has an amino-terminal extension, which is not found in the smaller LTBP-1S isoform. To study the formation and transcriptional regulation of LTBP-1S and LTBP-1L isoforms, we determined the nucleotide sequences of their 5'-flanking regions. The upstream regions of both isoforms are devoid of TATA boxes but contain other putative binding sites for several transcription factors. Genomic sequencing revealed that LTBP-1L transcript is alternatively spliced to an internal splice acceptor inside exon 1 of LTBP-1S and thus defined the genomic organization of the isoforms. Reporter gene analysis of upstream regions indicated the presence of independent, 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 repressed 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-β1 in WI-38 cells. However, since TGF-β1 failed to stimulate the transcription of LTBP-1 reporter gene constructs, TGF-β1 may mediate the induction of the isoforms by post- transcriptional mechanisms. Chromosomal localization of the LTBP-1 gene was refined to 2p22-24.