Different adaptive responses to hypoxia in normal and multiple myeloma endothelial cells

Background/Aims: Hypoxia is a powerful stimulator of angiogenesis under physiological as well as pathological conditions. Normal endothelial cells (EC), such as human umbilical vein EC (HUVEC), are relatively affected by hypoxic insult in terms of cell survival. In contrast, EC from tumors are particularly resistant to hypoxia-induced cell death. Previous reports have shown that EC in bone marrow from multiple myeloma (MM) patients had a hypoxic phenotype, even under normoxic conditions. The aim of this study was to evaluate whether HUVEC and MMEC adapt differently to hypoxia. Methods: Cell proliferation was assessed by the CyQUANT assay. Cdc25A, p21, Bax, Bcl-xl, BNIP3, glucose transporter (GLUT)-1, monocarboxylate transporter (MCT)-4 and carbonic anhydrase (CA)IX mRNA expression was determined by qRT-PCR. HIF-1α, BNIP3, Beclin-1, LC3B, livin, Bax, Bcl-xl, p21, p62 and β-actin protein expression was analyzed by western blot. Apoptosis was determined by TUNEL assay. Silencing of BNIP3 was achieved by stealth RNA system technology. Results: While HUVEC survival was reduced after prolonged hypoxic exposure, MMEC were completely unaffected. This difference was also significant in terms of livin, cdc25A and p21 expression. Hypoxia induced apoptosis and inhibited autophagy in HUVEC, but not in MMEC, where hypoxic treatment resulted in a more sustained adaptive response. In fact, MMEC showed a more significant increase in the expression of genes regulated transcriptionally by hypoxia-inducible factor (HIF)-1α. Interestingly, they showed higher expression of BNIP3 than did HUVEC, indicating a more pronounced autophagic (and pro-survival) phenotype. The potential role of BNIP3 in EC survival was confirmed by BNIP3 siRNA experiments in HUVEC, where BNIP3 inhibition resulted in reduced cell survival and increased apoptosis. Conclusion: These findings provide further information on how hypoxia may affect EC survival and could be important for a better understanding of EC physiology under normal and pathological conditions, such as in multiple myeloma.