Hypoxia induces apoptosis of HUVECs in an in vitro capillary model by activating proapoptotic signal p38 through suppression of ERK1/2.

We recently reported that hypoxia induces chromatin condensation and cell nuclear fragmentation, morphological markers of apoptosis, to tube-forming HUVECs in an in vitro blood vessel model by activating p38 MAPK. In this report, we further examined what role p38 plays and how it is activated during hypoxia-induced apoptosis. First, in order to confirm that p38 can indeed induce apoptosis, the cells were treated with anisomycin, a p38 activator, during normoxia. The activator treatment induced apoptosis and activation of p38 and caspase-3 in a very short time, which indicated that p38 activation alone was sufficient to trigger apoptosis in tube-forming HUVECs. We then observed hypoxia-induced changes in intracellular signals, ERK1/2 and Akt. ERK1/2 inactivation was shown to occur prior to p38 activation and caspase-3 cleavage during hypoxia. On the other hand, anisomycin had no inhibitory effect on ERK1/2 activation during normoxia. It was also shown that the amount of Akt protein slightly decreased by either hypoxia or anisomycin treatment. We then investigated how these two survival signals, ERK1/2 and Akt, are involved in p38 activation by using MEK inhibitor U0126 and PI3K inhibitor LY294002. When tube-forming HUVECs were treated with U0126 or LY294002 during normoxia, the two inhibitors were able to induce apoptosis and activation of p38 and caspase-3 in a relatively short time. U0126 was able to inhibit ERK1/2 activation, but had almost no effect on Akt activation. In contrast, LY294002 was able to inhibit Akt activation, but had very little effect on ERK1/2 activation. These results indicate that ERK1/2 inactivation, rather than Akt decrease, is responsible for hypoxia-induced p38 activation. Taken together, our results strongly suggest that hypoxia-induced apoptosis is regulated through signal transduction in which inactivation of ERK1/2 leads to activation of p38, which then triggers caspase cascade as an execution mechanism of apoptosis.