Purpose: To determine the important transcriptional control elements and sites of expression of fibulin-3 in mammalian retina.
Methods: Sequencing and 5' rapid amplification of cDNA ends (RACE) were undertaken to characterize the genomic sequence upstream of the FBLN3 coding sequence. Reporter deletion-mutation constructs were used in luciferase transfection assays to determine the important regulatory motifs. Fibulin-3 expression in mouse and human retina was studied by in situ hybridization and RT-PCR. The effect of 17beta-estradiol on fibulin-3 production was studied in COS-7 and ARPE-19 cells.
Results: Two untranslated exons were fully sequenced completing the characterization of FBLN3 that comprises 12 exons. Reporter assays suggest that the FBLN3 proximal promoter is contained within 425 bp upstream of exon 1. Important regulatory elements include three Sp1-binding sites, a Tant motif (trans-activating) and an estrogen response element (ERE) binding site (trans-repressing). No TATA or CAAT regulatory boxes were identified. RT-PCR suggests that the fibulin-3 gene is expressed in murine and human RPE, and in situ studies confirm that Fbln3 is expressed in the outer and inner nuclear layers, but strikingly not in the ganglion cell layer. Fibulin-3 expression in ARPE-19 cells could be modified by varying the amount of estrogen in the cell culture medium.
Conclusions: The 5' end of the FBLN3 gene has been characterized, and the important upstream motifs regulating its transcription have been identified. Fibulin-3 is expressed in adult retina and at early stages in human and mouse development. Estrogen may be an important regulator of fibulin-3 expression, and this highlights a novel mechanism by which circulating estrogen may control the composition of the retinal extracellular matrix.