CD13/APN is activated by angiogenic signals and is essential for capillary tube formation

Abstract

In the hematopoietic compartment, the CD13/APN metalloprotease is one of the earliest markers of cells committed to the myeloid lineage where it is expressed exclusively on the surface of myeloid progenitors and their differentiated progeny. CD13/APN is also found in nonhematopoietic tissues, and its novel expression on the endothelial cells of angiogenic, but not normal, vasculature was recently IntroductionAngiogenesis refers to the formation of new blood vessels from the existing vasculature and occurs at extremely low levels in the adult organism. One notable exception to this paradigm occurs in tumors that have undergone the "angiogenic switch" from a benign to a metastatic phenotype, in which new vessels are actively assembled and directly responsible for the sustained growth and dissemination of the tumor. 1 It is clear that the modulation of blood vessel growth is a remarkably effective means to limit or control tumor growth and spread, and therefore, the search for unique targets modulating angiogenesis is of significant importance.Recent studies designed to identify unique peptides that home specifically to solid tumors in murine breast carcinoma models revealed that the NGR motif binds strictly to the endothelium of angiogenic blood vessels. 2 Further investigation identified the CD13/APN cell-surface antigen as the principal receptor for this peptide motif and demonstrated that this protein is expressed exclusively on the endothelial cells of angiogenic but not normal vasculature, 3 thereby explaining the tumorspecific destination of the NGR peptide. CD13/APN was originally described in studies seeking to identify lineage-specific markers that would facilitate the classification of human leukemias. 4 In this regard, the appearance of CD13 coincides with commitment to the myeloid lineage and is exclusively expressed on the normal and leukemic progeny of myeloid cells within the hematopoietic compartment. The subsequent molecular cloning of the gene encoding this cell surface glycoprotein identified it as the membrane-bound metalloprotease, aminopeptidase N (APN, EC 3.4.11.2), thus extending its known range of expression beyond the hematopoietic system to include fibroblasts and epithelial cells in the liver, intestine, brain, and lung. 5 CD13/APN cleaves amino terminal residues from short peptides, and consequently, its specific function in individual tissues (primarily the activation or inactivation of small bioactive molecules) is mandated by available substrates (reviewed in Shipp and Look 6 ). Insights into the function of this unique vascular marker in angiogenesis were gained through experiments in which treatment of animals with CD13/APN functional inhibitors significantly arrested retinal neovascularization, chorioallantoic membrane angiogenesis, and xenograft tumor growth, indicating that CD13/ APN plays an important role in the progression of tumor vasculogenesis and identifying it as a critical regulator of angiogenesis. 3 Therefore, understanding the mechanisms regulating the expression o...

CD13/APN transcription is regulated by the proto-oncogene c-Maf via an atypical response element

“…1A]. We used the murine hemangioendothelioma cell line (EOMA) as our model system which faithfully recapitulates CD13 regulation in primary endothelial cells (Bhagwat et al, 2001. While the −153bp promoter fragment (153/luc) was marginally less active (81%) than the full-length promoter (1kb/luc), further deletion of the sequences upstream of either −115bp (115/luc) or −70bp (70/luc) significantly compromised promoter activity (29% and 3% respectively).…”

“…The CD13 or aminopeptidase N (CD13/APN) cell surface glycoprotein is not expressed on resting vessels but its expression is induced on activated endothelial cells in response to angiogenic growth factors Bhagwat et al, 2001). In vitro inhibition of CD13 with peptidase inhibitors or antibodies abrogates endothelial invasion and morphogenesis (Bhagwat et al, 2001;Bhagwat et al, 2003;Petrovic et al, 2003) and in vivo inhibition reduces angiogenesis and halts tumor growth .…”

“…In vitro inhibition of CD13 with peptidase inhibitors or antibodies abrogates endothelial invasion and morphogenesis (Bhagwat et al, 2001;Bhagwat et al, 2003;Petrovic et al, 2003) and in vivo inhibition reduces angiogenesis and halts tumor growth . Investigation into the pathways regulating the expression of CD13 in activated endothelium may identify novel factors and fundamental processes involved in the activation of endothelial cells.…”

“…CD13 expression in cells committed to the myeloid lineage is regulated by its distal promoter located eight kilobases upstream of the translational start codon, while transcription in endothelial cells and epithelial cells is directed by the proximal promoter immediately upstream of the start codon (Bhagwat et al, 2001). Through Ras-dependent mitogen-activated pathway kinase (MAPK) and phospatidylinositol-3 kinase signaling pathways, angiogenic factor such as bFGF and VEGF activate the transcription factor Ets-2-directed transcription of CD13 in endothelial cells (Bhagwat et al, 2001;Bhagwat et al, 2003;Petrovic et al, 2003). However, further experiments indicated that additional transcriptional mechanisms independent Ets-2 controlled CD13 expression.…”

cNGR: A Novel Homing Sequence for CD13/APN Targeted Molecular Imaging of Murine Cardiac Angiogenesis In Vivo

“…7 In accordance, CD13 is transcriptionally activated by multiple angiogenic signals and ap-pears to regulate angiogenesis in in vitro models. 8,9 The expression of CD13 in adult ischemic tissues undergoing active neovascularization has not been studied yet.…”