Identification of Tumor Endothelial Cells with High Aldehyde Dehydrogenase Activity and a Highly Angiogenic Phenotype

Abstract

Tumor blood vessels play an important role in tumor progression and metastasis. It has been reported that tumor endothelial cells (TECs) exhibit highly angiogenic phenotypes compared with those of normal endothelial cells (NECs). TECs show higher proliferative and migratory abilities than those NECs, together with upregulation of vascular endothelial growth factor (VEGF) and VEGF receptor 2 (VEGFR2). Furthermore, compared with NECs, stem cell markers such as Sca-1, CD90, and multidrug resistance 1 are upregulated in TECs, suggesting that stem-like cells exist in tumor blood vessels. In this study, to reveal the biological role of stem-like TECs, we analyzed expression of the stem cell marker aldehyde dehydrogenase (ALDH) in TECs and characterized ALDHhigh TECs. TECs and NECs were isolated from melanoma-xenografted nude mice and normal dermis, respectively. ALDH mRNA expression and activity were higher in TECs than those in NECs. Next, ALDHhigh/low TECs were isolated by fluorescence-activated cell sorting to compare their characteristics. Compared with ALDHlow TECs, ALDHhigh TECs formed more tubes on Matrigel-coated plates and sustained the tubular networks longer. Furthermore, VEGFR2 expression was higher in ALDHhigh TECs than that in ALDHlow TECs. In addition, ALDH was expressed in the tumor blood vessels of in vivo mouse models of melanoma and oral carcinoma, but not in normal blood vessels. These findings indicate that ALDHhigh TECs exhibit an angiogenic phenotype. Stem-like TECs may have an essential role in tumor angiogenesis.

Tumor endothelial cells with high aldehyde dehydrogenase activity show drug resistance

“…(22) We previously reported that TEC expressed higher ALDH levels compared with NEC. (17) Consistent with the previous report, ALDH mRNA expression levels in TEC were significantly higher than those in NEC (Fig. 1a).…”

“…Furthermore, HM‐TEC showed a more stem cell‐like phenotype and a more resistant profile to anticancer drugs (paclitaxel) with ATP‐binding cassette subfamily B member 1 (ABCB1) upregulation compared with LM‐TEC. In our previous study, no difference in ABCB1 expression levels was observed between ALDH high and ALDH low TEC and, therefore, ABCB1 and ALDH may have mutually exclusive roles in drug resistance development.…”

“…We previously demonstrated that ALDH high TEC have a lower proliferation rate compared with that of the ALDH low TEC. (17) However, when the cell cycle was analyzed using PI, a higher percentage of ALDH high TEC (47.4%) was shown to be in G2/ M phase, whereas 18.9% of ALDH low TEC were in G2/M phase (Fig. 5a).…”

“…We demonstrated that ALDH high TEC possess a more proangiogenic phenotype and are more resistant to serum starvation compared with ALDH low TEC. (17) EC, unlike tumor cells, have the advantages of genetic stability and absence of drug resistance development. Hence, targeting EC has been an important concept in antiangiogenic therapy.…”

“…We found that ALDH was upregulated in TEC compared with that in NEC. (17) We isolated ALDH high TEC and ALDH low TEC and reported that ALDH high TEC are more proangiogenic compared with ALDH low TEC. In addition, a side population of EC with ABC transporter gene upregulation is present in blood vessels.…”