Identification of an octamer element required for in vivo expression of the TIE1 gene in endothelial cells

TIE1, an endothelial-cell-specific tyrosine kinase receptor, is required for the survival and growth of microvascular endothelial cells during the capillary sprouting phase of vascular development. To investigate the molecular mechanisms that regulate the expression of TIE1 in the endothelium, we analysed transgenic mouse embryos carrying wild-type or mutant TIE1 promoter/LacZ constructs. Our data indicate that an upstream DNA octamer element (5'-ATGCAAAT-3') is required for the in vivo expression of TIE1 in embryonic endothelial cells. Transgenic embryos carrying the wild-type TIE1 promoter (-466 to +78 bp) fused to LacZ and spanning the octamer element demonstrate endothelial-cell-specific expression of the reporter transgene. Point mutations introduced within the octamer element result in a significant decrease of endothelial LacZ expression, suggesting that the octamer site functions as a positive regulator for TIE1 gene expression in endothelial cells. DNA-protein binding studies show that the octamer element exhibits an endothelial-cell-specific pattern of binding via interaction with endothelial-cell-restricted factor(s). Our findings suggest an important role for the octamer element in regulating the expression of the TIE1 receptor in the embryonic endothelium and suggest a common mechanism for the regulation of the angiogenic and cell-specific TIE1 and TIE2 genes during vascular development.

Endothelial cell-specific regulation of the murine endothelin-1 gene

Endothelin-1 (ET-1) plays an important role in the development, physiology and pathophysiology of the cardiovascular system in mammals. ET-1 is mainly expressed in endothelial cells thus making it an attractive model for the study of transcriptional regulation in this cell type. We have previously reported that expression of the human ET-1 gene is positively regulated by a cooperative interaction between GATA-2 and AP-1 transcription factors in cultured endothelial cells, however these factors are not sufficient to mediate cell type-specific expression. In vivo transcription studies of the murine ET-1 gene have demonstrated the presence of important cell-specific DNA elements in the 5.9 kb region upstream of the transcription initiation site. Using reporter gene transfection, site-directed mutagenesis and DNA-protein binding studies of the 5.9 kb region, we have identified a tripartite DNA element that positively regulates the expression of ET-1 specifically in cultured endothelial cells. This complex enhancer element demonstrates an endothelial cell-specific pattern of binding, suggesting that it interacts with cell-restricted regulatory factors. These findings provide important insights into the mechanisms that mediate the expression of ET-1 in the endothelium and a basis for future transgenic and cloning studies aimed at identifying the endothelial cell-specific binding site and transcription factor(s).

Octamer-dependent in vivo expression of the endothelial cell-specific TIE2 gene

The TIE2 gene, also known as TEK, encodes a tyrosine kinase receptor that is required for the normal development of the vascular system during embryogenesis. TIE2 is specifically expressed in endothelial cells; however, the transcriptional mechanisms that regulate this highly restricted pattern of expression remain unknown. Here we demonstrate that a consensus octamer element located in the 5'-flanking region of TIE2 is required for normal expression in embryonic endothelial cells. Transgenic embryos carrying a TIE2/LacZ construct spanning 2.1 kilobases of upstream regulatory sequences exhibit expression of the reporter transgene specifically in endothelial cells. Site-directed mutagenesis of a consensus octamer element located in this region results in the loss of enhancer activity and significantly impairs the endothelial expression of the reporter transgene. Consistent with the in vivo data, in vitro DNA-protein binding studies show that the consensus octamer element displays an endothelial cell-specific pattern of binding, suggesting an interaction with a protein complex consisting of Oct1 and an endothelial cell-restricted cofactor. These data identify a novel role for the octamer element as an essential regulator of TIE2 expression, define the first known transcriptional pathway that mediates the expression of a developmental endothelial cell gene, and provide insights into the transcriptional mechanisms that regulate development of the vasculature during embryogenesis.

Functional analysis of the endothelial cell-specific Tie2/Tek promoter identifies unique protein-binding elements

To investigate the molecular basis of endothelial cell-specific gene expression, we have examined the DNA sequences and the cognate DNA-binding proteins that mediate transcription of the murine tie2/tek gene. Reporter transfection experiments conformed with earlier findings in transgenic mice, indicating that the upstream promoter of Tie2/Tek is capable of activating transcription in an endothelial cell-specific fashion. These experiments have also allowed the identification of a single upstream inhibitory region (region I) and two positive regulatory regions (regions U and A) in the proximal promoter. Electrophoretic mobility-shift assays have allowed further characterization of three novel DNA-binding sequences associated with these regions and have provided preliminary characterization of the protein factors binding to these elements. Two of the elements (U and A) confer increased transcription on a heterologous promoter, with element U functioning in an endothelial-cell-selective manner. By employing embryonic endothelial-like yolk sac cells in parallel with adult-derived endothelial cells, we have identified differences in functional activity and protein binding that may reflect mechanisms for specifying developmental regulation of tie2/tek expression. Further study of the DNA and protein elements characterized in these experiments is likely to provide new insight into the molecular basis of developmental- and cell-specific gene expression in the endothelium.