Characterization of murine S-endoglin isoform and its effects on tumor development

Abstract

Endoglin is a transmembrane glycoprotein that acts as an auxiliary receptor for transforming growth factor-b (TGF-b) and modulates cellular responses to this pleiotropic cytokine. Endoglin is strongly expressed in endothelial cells, where it appears to exert a crucial role in vascular development and angiogenesis. Two endoglin isoforms (L and S), differing in their cytoplasmic domains, have been previously characterized in human tissues. We now demonstrate the existence of similar L-and Sendoglin variants in murine tissues with 47 and 35 amino acids, respectively, in their cytoplasmic tail. RT-PCR analysis showed that L is the predominant endoglin isoform expressed in mouse tissues, although S-endoglin mRNA is significantly expressed in liver and lung, as well as in endothelial cell lines. Furthermore, a protein of size equivalent to recombinant S-endoglin expressed in mammalian cells was detected in mouse endothelial cells by Western blot analysis. L-and S-endoglin isoforms can form disulfide-linked heterodimers, as demonstrated by cotransfection of L-and S-endoglin constructs. To address the role of S-endoglin in vivo, an S-Eng þ transgenic mouse model that targets S-endoglin expression to the endothelium was generated. The lethal phenotype of endoglin-null (Eng À/À ) mice was not rescued by breeding S-Eng þ transgenic mice into the endoglin-null background. S-Eng þ mice exhibited reduced tumor growth and neovascularization after transplantation of Lewis lung carcinoma cells. In addition, S-Eng þ mice showed a drastic inhibition of benign papilloma formation when subjected to two-stage chemical skin carcinogenesis. These results point to S-endoglin as an antiangiogenic molecule, in contrast to L-endoglin which is proangiogenic.

Dynamic expression of Endoglin, a TGF-beta co-receptor, during pre-circulation vascular development in chick

“…Searches of EST databases did not indicate the existence of the alternatively spliced S-Endoglin in chick, an isoform up-regulated in senescent endothelial cells in mammals (Bellon et al, 1993;Blanco et al, 2008;Perez-Gomez et al, 2005;Velasco et al, 2008). A high degree of identity between chick and mammalian Endoglins was found in the transmembrane and short cytoplasmic tail domains, with 100% conservation in the last 24 amino acid residues containing the sites that are known to be phosphorylated by activated TβRII, Alk1 and Alk5 and a PDZ motif (Koleva et al, 2006).…”

Novel biochemical pathways of endoglin in vascular cell physiology

“…Both cytosolic domains can be phosphorylated by serine and threonine kinases [Lastres et al, 1994], including the TGFβ type I and II receptors Koleva et al, 2006]. Recently, a short endoglin isoform was characterized in mice [Perez-Gomez et al, 2005]. Although the L-endoglin isoform and betaglycan contain a consensus PDZ-binding motif (SerSerMetAla) present at the carboxyl terminus, the S-endoglin isoform lacks this motif.…”

TGF-β signaling in vascular biology and dysfunction

“…Elevated endoglin expression levels are used as a marker of tumor angiogenesis and endoglin antibodies coupled with toxin or radioactivity have been successfully used to target ECs in anti-angiogenic therapy [55,56]. Two endoglin splice forms have been reported, termed long (L) and short (S)-endoglin with pro-and anti-angiogenic activity, respectively [57]. While the L-form is most abundantly expressed and promotes TGF-β/ALK1 signaling, S-endoglin expression was recently shown to be specifically high in senescent ECs and it preferentially promotes TGF-β/ALK5 signaling [58,59].…”