To the extent possible under law, AOP-Wiki has waived all copyright and related or neighboring rights to KE:973

Event: 973

Key Event Title

The KE title should describe a discrete biological change that can be measured. It should generally define the biological object or process being measured and whether it is increased, decreased, or otherwise definably altered relative to a control state. For example “enzyme activity, decreased”, “hormone concentration, increased”, or “growth rate, decreased”, where the specific enzyme or hormone being measured is defined. More help

KE5 : Decrease, AKT/eNOS activity

Short name
The KE short name should be a reasonable abbreviation of the KE title and is used in labelling this object throughout the AOP-Wiki. The short name should be less than 80 characters in length. More help
Decrease, AKT/eNOS activity

Biological Context

Structured terms, selected from a drop-down menu, are used to identify the level of biological organization for each KE. Note, KEs should be defined within a particular level of biological organization. Only KERs should be used to transition from one level of organization to another. Selection of the level of biological organization defines which structured terms will be available to select when defining the Event Components (below). More help
Level of Biological Organization

Cell term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help
Cell term
endothelial cell of vascular tree

Organ term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help

Key Event Components

Further information on Event Components and Biological Context may be viewed on the attached pdf.Because one of the aims of the AOP-KB is to facilitate de facto construction of AOP networks through the use of shared KE and KER elements, authors are also asked to define their KEs using a set of structured ontology terms (Event Components). In the absence of structured terms, the same KE can readily be defined using a number of synonymous titles (read by a computer as character strings). In order to make these synonymous KEs more machine-readable, KEs should also be defined by one or more “event components” consisting of a biological process, object, and action with each term originating from one of 22 biological ontologies (Ives, et al., 2017; See List). Biological process describes dynamics of the underlying biological system (e.g., receptor signalling). The biological object is the subject of the perturbation (e.g., a specific biological receptor that is activated or inhibited). Action represents the direction of perturbation of this system (generally increased or decreased; e.g., ‘decreased’ in the case of a receptor that is inhibited to indicate a decrease in the signalling by that receptor).Note that when editing Event Components, clicking an existing Event Component from the Suggestions menu will autopopulate these fields, along with their source ID and description. To clear any fields before submitting the event component, use the 'Clear process,' 'Clear object,' or 'Clear action' buttons. If a desired term does not exist, a new term request may be made via Term Requests. Event components may not be edited; to edit an event component, remove the existing event component and create a new one using the terms that you wish to add. More help
Process Object Action
catalytic activity nitric oxide synthase, endothelial decreased
catalytic activity AKT kinase decreased

Key Event Overview

AOPs Including This Key Event

All of the AOPs that are linked to this KE will automatically be listed in this subsection. This table can be particularly useful for derivation of AOP networks including the KE. Clicking on the name of the AOP will bring you to the individual page for that AOP. More help
AOP Name Role of event in AOP Point of Contact Author Status OECD Status
Hypertension KeyEvent Frazer Lowe (send email) Not under active development Under Development


This is a structured field used to identify specific agents (generally chemicals) that can trigger the KE. Stressors identified in this field will be linked to the KE in a machine-readable manner, such that, for example, a stressor search would identify this as an event the stressor can trigger. NOTE: intermediate or downstream KEs in one AOP may function as MIEs in other AOPs, meaning that stressor information may be added to the KE description, even if it is a downstream KE in the pathway currently under development.Information concerning the stressors that may trigger an MIE can be defined using a combination of structured and unstructured (free-text) fields. For example, structured fields may be used to indicate specific chemicals for which there is evidence of an interaction relevant to this MIE. By linking the KE description to a structured chemical name, it will be increasingly possible to link the MIE to other sources of chemical data and information, enhancing searchability and inter-operability among different data-sources and knowledgebases. The free-text section “Evidence for perturbation of this MIE by stressor” can be used both to identify the supporting evidence for specific stressors triggering the MIE as well as to define broad chemical categories or other properties that classify the stressors able to trigger the MIE for which specific structured terms may not exist. More help

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected from an ontology. In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help
Term Scientific Term Evidence Link
Homo sapiens Homo sapiens High NCBI
Bos taurus Bos taurus High NCBI
Rattus norvegicus Rattus norvegicus High NCBI
Mus musculus Mus musculus High NCBI

Life Stages

The structured ontology terms for life-stage are more comprehensive than those for taxa, but may still require further description/development and explanation in the free text section. More help
Life stage Evidence
All life stages Not Specified

Sex Applicability

The authors must select from one of the following: Male, female, mixed, asexual, third gender, hermaphrodite, or unspecific. More help
Term Evidence
Unspecific Not Specified

Key Event Description

A description of the biological state being observed or measured, the biological compartment in which it is measured, and its general role in the biology should be provided. For example, the biological state being measured could be the activity of an enzyme, the expression of a gene or abundance of an mRNA transcript, the concentration of a hormone or protein, neuronal activity, heart rate, etc. The biological compartment may be a particular cell type, tissue, organ, fluid (e.g., plasma, cerebrospinal fluid), etc. The role in the biology could describe the reaction that an enzyme catalyses and the role of that reaction within a given metabolic pathway; the protein that a gene or mRNA transcript codes for and the function of that protein; the function of a hormone in a given target tissue, physiological function of an organ, etc. Careful attention should be taken to avoid reference to other KEs, KERs or AOPs. Only describe this KE as a single isolated measurable event/state. This will ensure that the KE is modular and can be used by other AOPs, thereby facilitating construction of AOP networks. More help

Endothelial nitric oxide synthase (eNOS) is responsible for the generation of nitric oxide (NO), which is an important regulator of vascular homeostasis. The activity of eNOS can be regulated through calmodulin-mediated dimerization, tetrahydrobiopterin-mediated conversion of L-arginine to L-citrulline, protein-protein interactions with heat shock protein 90 and caveolin, S-nitrosylation, acetylation and phosphorylation (Atochin et al., 2007; Qian and Fulton, 2013). eNOS has been shown to be phosphorylated at multiple sites including tyrosine (Y), serine (Ser) and threonine (Thr) residues. Serine-threonine protein kinase AKT, a multifunctional regulator of cellular processes like glucose metabolism and proliferation, can directly phosphorylate eNOS at Ser1177/Ser1179, leading to increased eNOS enzymatic activity and subsequent NO production (Dimmeler et al., 1999; Fulton et al., 1999). Inhibition of AKT or a mutation of the AKT phosphorylation site on eNOS attenuates eNOS phosphorylation and its activity, resulting in decreased NO bioavailability and endothelial dysfunction (Dimmler et al. 1999)

How It Is Measured or Detected

One of the primary considerations in evaluating AOPs is the relevance and reliability of the methods with which the KEs can be measured. The aim of this section of the KE description is not to provide detailed protocols, but rather to capture, in a sentence or two, per method, the type(s) of measurements that can be employed to evaluate the KE and the relative level of scientific confidence in those measurements. Methods that can be used to detect or measure the biological state represented in the KE should be briefly described and/or cited. These can range from citation of specific validated test guidelines, citation of specific methods published in the peer reviewed literature, or outlines of a general protocol or approach (e.g., a protein may be measured by ELISA).Key considerations regarding scientific confidence in the measurement approach include whether the assay is fit for purpose, whether it provides a direct or indirect measure of the biological state in question, whether it is repeatable and reproducible, and the extent to which it is accepted in the scientific and/or regulatory community. Information can be obtained from the OECD Test Guidelines website and the EURL ECVAM Database Service on Alternative Methods to Animal Experimentation (DB-ALM). ?

Western blot analysis can be performed to determine the expression levels of phosphorylated eNOS, phosphorylated Akt, total Akt and total eNOS proteins using the appropriate anti-phospho-eNOS, anti-phospho-Akt, anti-eNOS, and anti-Akt antibodies. Alternatively, eNOS activity can be measured using the conversion of L-arginine to L-citrulline assay.

ELISA kits for AKT/eNOS and phospho AKT/eNOS expression are commercially available.

Domain of Applicability

This free text section should be used to elaborate on the scientific basis for the indicated domains of applicability and the WoE calls (if provided). While structured terms may be selected to define the taxonomic, life stage and sex applicability (see structured applicability terms, above) of the KE, the structured terms may not adequately reflect or capture the overall biological applicability domain (particularly with regard to taxa). Likewise, the structured terms do not provide an explanation or rationale for the selection. The free-text section on evidence for taxonomic, life stage, and sex applicability can be used to elaborate on why the specific structured terms were selected, and provide supporting references and background information.  More help

Decreased Akt and eNOS activity was observed in humans, cows, mice and rats following exposure to stressors.

Cigarette smoke exposure was shown to inhibit the phosphorylation of AKT and eNOS in VEGF-stimulated human umbilical vein endothelial cells (HUVECs), resulting in decreased NO levels (Michaud et al. 2006).

In rat aortic rings, exposure to methylglyoxal and high concentrations of glucose decreased endothelium-dependent relaxation.  Further experiments in rat endothelial cells and HUVECs demonstrated a reduction in eNOS phosphorylation and activity, and reduced NO levels in response to the same stressors (Dhar et al. 2010).

In bovine aortic endothelial cells, AKT and eNOS phosphorylation were decreased following exposure to the peroxynitrite source; SIN-1, with an associated reduction in NO bioavailability.  These effects were ameliorated by treatment with the ROS scavenger DMPO (Das et al. 2014).

eNOS knockout mice are routinely used as models of hypertension.  Such mice display reduced bioavailability of NO and impaired vasodilation (Huang et al. 1995).

Reduced AKT/eNOS phosphorylation was reported under conditions of hyperglycaemia (in mice) and in HUVECs following treatment with high concentrations of glucose.  Aortic rings from hyperglycaemic mice demonstrated impaired vasodilation.  Resveratrol treatment was shown to improve vasodilation and eNOS phosphorylation in wild-type mice, but not AKT knockout mice.  Transfection of HUVECs with AKT siRNA abolished resveratrol-enhanced eNOS phosphorylation and NO release (Li et al. 2017),

Evidence for Perturbation by Stressor


List of the literature that was cited for this KE description. Ideally, the list of references, should conform, to the extent possible, with the OECD Style Guide ( (OECD, 2015). More help

Atochin, D.N., Wang, A., Liu, V.W.T., Critchlow, J.D., Dantas, A.P.V., Looft-Wilson, R., Murata, T., Salomone, S., Shin, H.K., Ayata, C., et al. (2007). The phosphorylation state of eNOS modulates vascular reactivity and outcome of cerebral ischemia in vivo. J. Clin. Invest. 117, 1961–1967.

Das A, Gopalakrishnan B, Druhan LJ, Wang TY, De Pascali F, Rockenbauer A, Racoma I, Varadharaj S, Zweier JL, Cardounel AJ, Villamena FA.  Reversal of SIN-1-induced eNOS dysfunction by the spin trap, DMPO, in bovine aortic endothelialcells via eNOS phosphorylation.  Br J Pharmacol. 2014, 171(9):2321-34. doi: 10.1111/bph.12572.

Dhar A, Dhar I, Desai KM, Wu L.  Methylglyoxal scavengers attenuate endothelial dysfunction induced by methylglyoxal and high concentrations of glucose.  Br J Pharmacol. 2010, 161(8):1843-56.

Dimmeler, S., Fleming, I., Fisslthaler, B., Hermann, C., Busse, R., and Zeiher, A.M. (1999). Activation of nitric oxide synthase in endothelial cells by Akt-dependent phosphorylation. Nature 399, 601–605.

Fulton, D., Gratton, J.P., McCabe, T.J., Fontana, J., Fujio, Y., Walsh, K., Franke, T.F., Papapetropoulos, A., and Sessa, W.C. (1999). Regulation of endothelium-derived nitric oxide production by the protein kinase AKT. Nature 399, 597–601.

Huang PL, Huang Z, Mashimo H, Bloch KD, Moskowitz MA, Bevan JA, Fishman MC.    Hypertension in mice lacking the gene for endothelial nitric oxide synthase.  Nature. 1995, 377(6546):239-42.

Li JY, Huang WQ, Tu RH, Zhong GQ, Luo BB, He Y.  Resveratrol rescues hyperglycemia-induced endothelial dysfunction via activation of Akt.  Acta Pharmacol Sin. 2017, 38(2):182-191.

Michaud SE, Dussault S, Groleau J, Haddad P, Rivard A.J. Cigarette smoke exposure impairs VEGF-induced endothelial cell migration: role of NO and reactive oxygen species.  Mol Cell Cardiol. 2006 Aug;41(2):275-84.

Qian, J., and Fulton, D. (2013). Post-translational regulation of endothelial nitric oxide synthase in vascular endothelium. Oxid. Physiol. 4, 347.