Dissociation of ERK and Akt signaling in endothelial cell angiogenic responses to β-amyloid

Cerebrovascular deposits of β-amyloid (Aβ) peptides are found in Alzheimer's disease and cerebral amyloid angiopathy with stroke or dementia. Dysregulations of angiogenesis, the blood–brain barrier and other critical endothelial cell (EC) functions have been implicated in aggravating chronic hypoperfusion in AD brain. We have used cultured ECs to model the effects of β-amyloid on the activated phosphorylation states of multifunctional serine/threonine kinases since these are differentially involved in the survival, proliferation and migration aspects of angiogenesis. Serum-starved EC cultures containing amyloid-β peptides underwent a 2- to 3-fold increase in nuclear pyknosis. Under growth conditions with sublethal doses of β-amyloid, loss of cell membrane integrity and inhibition of cell proliferation were observed. By contrast, cell migration was the most sensitive to Aβ since inhibition was significant already at 1 μM (P = 0.01, migration vs. proliferation). In previous work, intracellular Aβ accumulation was shown toxic to ECs and Akt function. Here, extracellular Aβ peptides do not alter Akt activation, resulting instead in proportionate decreases in the phosphorylations of the MAPKs: ERK1/2 and p38 (starting at 1 μM). This inhibitory action occurs proximal to MEK1/2 activation, possibly through interference with growth factor receptor coupling. Levels of phospho-JNK remained unchanged. Addition of PD98059, but not LY294002, resulted in a similar decrease in activated ERK1/2 levels and inhibition of EC migration. Transfection of ERK1/2 into Aβ-poisoned ECs functionally rescued migration. The marked effect of extracellular Aβ on the migration component of angiogenesis is associated with inhibition of MAPK signaling, while Akt-dependent cell survival appears more affected by cellular Aβ.