Mitochondria as signaling organelles in the vascular endothelium

Abstract

Vascular endothelial cells are highly glycolytic and consume relatively low amounts of oxygen (O2) compared with other cells. We have confirmed that oxidative phosphorylation is not the main source of ATP generation in these cells. We also show that at a low O2 concentration (<1%) endogenous NO plays a key role in preventing the accumulation of the ␣-subunit of hypoxia-inducible factor 1. At higher O2 concentrations (1-3%) NO facilitates the production of mitochondrial reactive oxygen species. This production activates the AMP-activated protein kinase by a mechanism independent of nucleotide concentrations. Thus, the primary role of mitochondria in vascular endothelial cells may not be to generate ATP but, under the control of NO, to act as signaling organelles using either O2 or O2-derived species as signaling molecules. Diversion of O2 away from endothelial cell mitochondria by NO might also facilitate oxygenation of vascular smooth muscle cells.AMP-activated protein kinase ͉ hypoxia-inducible factor 1␣ ͉ hypoxia ͉ nitric oxide

Unforeseen decreases in dissolved oxygen levels affect tube formation kinetics in collagen gels

“…Using DPI to inhibit ROS production in HUVECs precluded the rise of intracellular ROS levels observed after 48 h and resulted in the complete inhibition of the tube formation. A cell viability assay proved that the cells remain viable during their culture in media containing DPI, suggesting that the inhibition of tubulogenesis is not due to cell death, which agrees with other studies demonstrating the effects of DPI on angiogenesis (34,45). Treatment with DPI was also previously shown to inhibit mitochondrial respiration (28), which may in turn increase the DO levels available in the gel.…”

“…The direction of the change in ROS production due to changes in DO levels depends on both the cell type and the exposure time to various DO levels. However, the vast majority of the results obtained on ECs agree on the fact that hypoxia (34) and reoxygenation (46,47) increase the levels of intracellular ROS. Previous studies have shown that an optimal level of NOXderived ROS is required for EC proliferation, migration, and capillary formation (2,36).…”

“…Previous studies have demonstrated the effect of hypoxia (34) and reoxygenation (46,47) on the ROS production of ECs. Therefore, we investigated the effects of varying DO levels on intracellular ROS levels during twoday cultures of HUVEC in 3D collagen gel using H 2 -DCFDA dye (10,36).…”

“…Research has demonstrated the involvement of ROS in angiogenesis through HIF␣-dependent, via inhibition of prolyl hydroxylases (33), and -independent mechanisms (5,10,21,33,49). Interestingly, one study suggested that increased ROS levels in ECs, unlike in other cell types (9,33), did not affect the stabilization of HIF1␣ (34).…”

“…Also, a growing body of evidence suggests that the two isoforms of HIF␣ differ in terms of their stabilization dependence on the available DO (6,44,52). HIF1␣, considered the master regulator of angiogenesis under hypoxic conditions, only stabilizes at very low levels of oxygen (Ͻ1%) in ECs-at levels lower than in other cell types (1,34). By contrast, HIF2␣ can be stabilized under normoxia or mild hypoxia (1 to 3% O 2 ) in different cell types, including ECs (44,52).…”