Alterations in gene expression associated with changes in the state of endothelial differentiation

The endothelium maintains a developmental plasticity which allows rapid phenotypic change in response to extracellular signals during normal processes, such as corpus luteum formation and wound healing, and in pathologic processes, such as tumor angiogenesis. Endothelial cells (EC) in culture have been very useful for investigating various aspects of endothelial growth and behavior. In spite of documented similarities between EC in vitro and the endothelium in vivo, many characteristics of the vessel endothelium are lost when the cells are placed into culture. We have undertaken to identify differences in gene expression between differentiated vessel endothelium and dedifferentiated EC. We utilized a new technique called differential display which compares polymerase chain reaction (PCR)-amplified mRNA from two (or more) cell populations. Endothelium scraped directly from freshly obtained aortas, and demonstrated to be free of contaminants, were used as the source of differentiated RNA, whereas proliferating, primary explanted EC grown for five days in the presence of basic fibroblast growth factor (bFGF) provided a pool of 'dedifferentiated' RNA. Using differential display, we have observed numerous reproducible differences in gene expression. To confirm that the expression differences visualized by differential display represented actual differences in gene expression, we isolated vessel-specific and culture-specific cDNA tags for additional analysis. Three cDNA tags specific to vessel endothelium were cloned and sequenced, and compared to nucleotide and protein databases. Two of the clones (A1 and 2.5) displayed no significant sequence similarity, whereas a third clone (A2) is nearly identical to a human expressed sequence tag (EST) and has significant sequence similarities to a plant and Xenopus ubiquitin-like protein. Northern and/or in situ hybridization analysis of the A1 and A2 genes confirmed their restricted expression to the vessel endothelium. The expression of A1 by the endothelium in vivo is not simply a function of growth state, as cultured cells did not express A1 even when grown to postconfluence. One other cDNA fragment, selected as a culture-induced gene, was identified by sequence analysis as the bovine homologue of laminin B1, and Northern analysis confirmed that expression was induced upon culturing of EC. Use of differential display to study endothelial gene expression will allow us to investigate the molecular mechanisms that underlie initiation and maintenance of endothelial differentiation.