Molecular characterization and in situ localization of murine endoglin reveal that it is a transforming growth factor-beta binding protein of endothelial and stromal cells.

Abstract

Endoglin is an integral membrane glycoprotein predominantly expressed on human endothelial cells and recently shown to bind transforming growth factor-beta 1 (TGF beta 1) with high affinity. We now report the cloning and sequencing of a full-length murine endoglin complementary DNA of 2902 base pairs which hybridizes specifically with a single messenger RNA (mRNA) species. The polypeptide of 653 amino acids has an overall identity of 72% with human and porcine endoglin. The transmembrane and cytoplasmic domains of all three proteins differ by two to four amino acids and are 70% identical to the corresponding regions of the TGF beta binding protein, betaglycan. Relative levels of murine endoglin mRNA were estimated by polymerase chain reaction and found to be high in ovary and uterus, intermediate in heart and muscle, and low in placenta and spleen. In situ hybridization and immunofluorescence confirmed that murine endoglin, like its human counterpart, is present in blood vessels and capillaries in all tissues examined. In addition, the stromal cells in the connective tissue of intestine, stomach, heart, muscle, uterus, ovary, and testis were strongly and specifically reactive with complementary RNA probes and with a polyclonal antibody to endoglin; epithelial cell layers were distinctly unreactive. This distribution is similar to that of extracellular TGF beta 1, particularly in heart and uterus, and suggests that endoglin on stromal fibroblast-like cells might be regulating access of TGF beta 1 to the signaling receptor complex. NCTC-2071 fibroblasts in culture were shown to express high levels of endoglin mRNA by polymerase chain reaction. After chemical cross-linking with [125I]TGF beta 1 and immunoprecipitation with the polyclonal antihuman endoglin serum, a radiolabeled band of mol wt 180,000 corresponding to dimeric endoglin was observed under nonreducing conditions, whereas a single band of mol wt 90,000 was seen under reducing conditions. Thus murine fibroblast endoglin is capable of binding TGF beta 1. Future studies should establish the specialized role of endoglin in the TGF beta receptor complex of endothelial and stromal cells.

Lack of specificity of endoglin expression for tumor blood vessels

“…This species with respect to humans shows an apparently higher expression of the antigen in normal organs. 14,43 Endoglin-specific MAbs, radiolabeled or conjugated with Ricin A chain, have already been used in tumor therapy animal models, with a strong antitumor efficacy without side effects being observed. 29,30,44 Since the MAb used is specific for human endoglin and gives a weaker reaction with mouse endoglin, the authors suggested that an even stronger antitumor effect can be expected in therapeutic applications on humans, however, a higher risk of side effects on normal tissues should also be taken into account.…”

“…11 It is transiently expressed as well during human heart development. 12,13 Recently, high expression in intestine, stomach and uterine stromal cells in mouse has been reported, 14,15 and an MAb raised against human mesenchymal stem cells has been shown to recognize an epitope on endoglin. 16 Two different isoforms have been identified, a full-length protein with a cytoplasmic domain of 47 amino acids, 17 and a splice variant with a truncated cytoplasmic tail of 14 amino acids.…”

Expression of normal and truncated forms of human endoglin

“…To test this possibility, we used the mouse fibroblast cell line NCTC, which expresses murine endoglin [7]. The existence of interspecies dimers between human and murine transmembrane endoglins could be demonstrated by using non-cross-reactive mAb against murine or human endoglin ( Figure 6A).…”

“…HHT is a highly penetrant autosomal dominant vascular dysplasia associated with frequent epistaxis, gastrointestinal bleeding, telangiectases and arteriovenous malformations in brain, lung and liver [3,4]. Endoglin (CD105) is a 180 kDa homodimeric membrane glycoprotein strongly expressed by human endothelial cells [5] ; it is also present in syncytiotrophoblasts of full-term placenta [6], stromal cells [7,8] and certain haemopoietic cells [9][10][11][12]. Two different isoforms, Lendoglin and S-endoglin, differing in the amino acid compositions of their cytoplasmic tails, have been characterized [13,14].…”

Expression of the TGF-β coreceptor endoglin in epidermal keratinocytes and its dual role in multistage mouse skin carcinogenesis

“…Endoglin plays an important role in mammalian physiology as demonstrated by genetic inactivation, since Endoglin null (Eng À/À ) murine embryos die of vascular defects at 10-10.5 days post coitum (Bourdeau et al, 1999;Li et al, 1999;Arthur et al, 2000). Endoglin is a disulfide-linked homodimer of 180 kDa expressed in endothelial cells (Gougos and Letarte, 1988Letarte, , 1990Li et al, 2000), macrophages (Lastres et al, 1992), stromal cells (St-Jacques et al, 1994;Robledo et al, 1996), and other cellular lineages within the vascular system and connective tissues (Fonsatti et al, 2001). Several tumor cells also express endoglin, including pre-B-cell and monocytic leukemia (Quackenbush and Letarte, 1985;Haruta and Seon, 1986;Lastres et al, 1992;Zhang et al, 1996), melanoma (Altomonte et al, 1996), choriocarcinoma (Letamendia et al, 1998b), and ovarian carcinoma (Jindal et al, 1995).…”