Major attention has been devoted to the role played by neuronal cells in AD development. Most of the amyloid found in AD subjects is produced in the brain and contained in plaques with a high content of highly aggregated amyloids. However, cognitive function does not generally correlate with the level of cerebral plaques. It was only in recent years that the contribution of the cerebrovascular system has been appreciated 
. In this respect, it should be noted that a large proportion of AD patients have cerebral amyloid angiopathy (CAA) involving vascular amyloidosis within the cerebral circulation. It has in fact been proposed that vascular pathology and the resultant impairment of oxygen delivery to the brain may play a primary role in the development of AD 
. In this study, we have delineated a new role for the cerebrovascular system involving Aβ1–42
fibril induced APP synthesis in the human endothelial cell line Hep-1.
We first demonstrate that incubation of endothelial cells with fibrillar Aβ1–42 results in elevated levels of the APP (). Using an antibody specific for Aβ1–40, we also report an increase in the concentration of Aβ1–40 peptides. This finding is indicated both by flow cytometry ( and ) and confocal microscopy (). The increased levels of APP together with increased Aβ1–40 cannot be explained just by altered processing of APP and requires increased synthesis of APP by the endothelial cells.
Such an effect would seem to require that the Aβ1–42
fibrils are endocytosed by cells. Evidence for amyloid uptake and transcytosis in endothelial cells was provided more than a decade ago 
. Among the potentially physiologically relevant amyloid receptors that can be involved in amyloid uptake, the RAGE (receptor for advanced glycation end products) 
was shown to bind Aβ1–40
even in the fibrillar form with a high affinity 
Support, from our data, for a contribution of stress to the observed increase in APP and Aβ comes from the augmentation of the effects under serum-deprived conditions 
. An analogous stimulation of amyloid production has been reported 
in serum-deprived human primary neuron cultures.
Altered cellular function can be triggered by the aggregation state of the amyloids in addition to stress. Our observation that only Aβ1–42
fibrils but not Aβ1–40
fibrils induce cellular changes implies that this phenomenon is triggered by the highly aggregated fibrillar form that is favored by the Aβ peptide ending at residue 42 
. Additional studies will be required to explain how the cellular changes produced by stress and/or amyloid aggregation induce APP synthesis and its processing. Two processes induced by exogenous Aβ fibrils are likely to be involved: inflammatory response and oxidative stress. As for increased APP synthesis, we have previously shown that cellular uptake of fibrillar Aβ induces interleukin-1α (IL-1α) expression 
. The link between IL-1α and APP can be found in an earlier study which showed an IL-1α mediated upregulation of APP at the translational level 
. Interestingly, the APP mRNA level was also unchanged after the Hep-1 cells were treated with Aβ1–42
(Rajadas unpublished data) indicating the increase of APP could also be translational in our study. As for increased Aβ production, there is ample evidence that in neuronal cells, Aβ accumulation and oxidative stress, each accelerating the other, generate a vicious circle of more Aβ production and oxidation 
. Based on our observation, it is possible that the similar process is also taking place in endothelial cells. This notion is supported by an Aβ immunotherapy study which indicated that Aβ depositions in the brain parenchyma and blood vessels occur independently 
Once inside the cell, the amyloid can affect many other cellular responses. One of the cellular responses we observed is the upregulation of eNOS () which is consistent with a previous study showing a Ca2+
dependent induction of eNOS by hydrogen peroxide in endothelial cells. This upregulation has been attributed to a compensatory mechanism for increased superoxide formation 
. Elevated eNOS expression has been observed in cardiovascular and other vascular pathologies, wherein increased levels of ROS have been detected 
. Another cellular response we observed is increased autophagy (). Since autophagy is involved in APP processing as a protective mechanism 
, this response is expected and also explains the increased Aβ1–40
production. The intricate relations between oxidative stress and autophagy and its implication in AD vascular pathogenesis awaits further elucidation.
The increased cellular granulation indicated both by changes in side-scattering in the flow cytometry experiment () as well as by microscopy () seem to indicate damage to the cells. Thus, coupled with the altered cellular function, and the new synthesis of APP and its metabolism, amyloid toxicity results in damage to the endothelial cells.
In conclusion, our observation of increased APP and amyloid in ECs incubated with Aβ1–42 fibrils establishes, for the first time, that the exposure of endothelial cells to Aβ1–42 fibrils results in an elevated amyloid load in endothelial cells. This process provides a new potential pathway for amyloidosis with these newly formed amyloids in the endothelial cells able to contribute to both vascular amyloidosis and/or parenchyma amyloidosis.