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Relationship: 3368

Title

A descriptive phrase which clearly defines the two KEs being considered and the sequential relationship between them (i.e., which is upstream, and which is downstream). More help

Increased, cholesterol synthesis enzymes leads to Increased, plasma LDL cholesterol

Upstream event

The causing Key Event (KE) in a Key Event Relationship (KER). More help

Increased, cholesterol synthesis enzymes

Downstream event

The responding Key Event (KE) in a Key Event Relationship (KER). More help

Increased, plasma LDL cholesterol

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes.Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

AOP Name	Adjacency	Weight of Evidence	Quantitative Understanding	Point of Contact	Author Status	OECD Status
Activation, Pregnane-X receptor, NR1l2 leads to increased plasma low-density lipoprotein (LDL) cholesterol via increased cholesterol synthesis	adjacent	High		John Frisch (send email)	Under development: Not open for comment. Do not cite

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) that help to define the biological applicability domain of the KER.In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER. More help

Term	Scientific Term	Evidence	Link
mammals	mammals	High	NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help

Sex	Evidence
Unspecific	High

Life Stage Applicability

An indication of the the relevant life stage(s) for this KER. More help

Term	Evidence
All life stages	Moderate

Key Event Relationship Description

Provides a concise overview of the information given below as well as addressing details that aren’t inherent in the description of the KEs themselves. More help

Cholesterol has a variety of roles in organisms, including as a cellular membrane component that helps maintain structure and fluidity, and a precursor for steroid hormones (Sakakura et al. 2001; Horton et al. 2003; Howe et al. 2017). The cholesterol synthesis pathway involves a number of precursor molecules and enzymes (Sakakura et al. 2001; Itkonen et al. 2023), with high cholesterol levels acting as a negative feedback on additional cholesterol synthesis (Itkonen et al. 2023; MacFarlaine et al. 2014). Individuals with high levels of plasma low-density lipoprotein (LDL) cholesterol have departed from lipid homeostasis in maintaining cholesterol levels needed for typical function, and in displaying hypercholesterolemia are at greater risk of cardiovascular events (Lalanne et al. 2005; Lambert et al. 2006).

Evidence Collection Strategy

Include a description of the approach for identification and assembly of the evidence base for the KER. For evidence identification, include, for example, a description of the sources and dates of information consulted including expert knowledge, databases searched and associated search terms/strings. Include also a description of study screening criteria and methodology, study quality assessment considerations, the data extraction strategy and links to any repositories/databases of relevant references.Tabular summaries and links to relevant supporting documentation are encouraged, wherever possible. More help

This Key Event Relationship was developed as part of an Environmental Protection Agency effort to represent putative AOPs from peer-reviewed literature which were heretofore unrepresented in the AOP-Wiki. Itkonen et al. (2023) focused on identifying Adverse Outcome Pathways that linked PXR activation to increased level of plasma low-density lipoprotein (LDL) cholesterol through review of existing literature, and provided initial network analysis.

Cited empirical studies are focused on increased cholesterol synthesis enzymes and resulting increased plasma low-density lipoprotein (LDL) cholesterol levels in mammals, in support of development of AOP 545 for Itkonen et al. (2023) content.

Authors of KER 3368 did a further evaluation of published peer-reviewed literature to provide additional evidence in support of the key event relationship.

Evidence Supporting this KER

Addresses the scientific evidence supporting KERs in an AOP setting the stage for overall assessment of the AOP. More help

Biological Plausibility

Addresses the biological rationale for a connection between KEupstream and KEdownstream. This field can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured. More help

Cholesterol synthesis enzymes and plasma low-density lipoprotein (LDL) cholesterol levels have been studied in a variety of gene-knockout, gene transfection, and diet studies designed to disrupt maintenance of lipid homeostasis in laboratory mammals. Evidence from gene expression and protein expression studies show a consistent response in increase in cholesterol synthesis enzyme expression and plasma low-density lipoprotein (LDL) cholesterol levels, which reinforces the evidence for the enzymes role in biosynthesis of cholesterol. Cholesterol levels can also also be affected by changes in diet, uptake, and elimination (Duan et al. 2022).

Empirical Evidence

Provides specific (citable) evidence that supports the idea of a change in the upstream KE (KEupstream) leading to, or being associated with, a subsequent change in the downstream KE (KEdownstream), assuming the perturbation of KEupstream is sufficient. More help

Species	Duration	Dose	Increased cholesterol synthesis enzymes?	Increased plasma LDL cholesterol?	Summary	Citation
Mouse (Mus musculus)	2.5 days	Gene knockout study, high fat 2.0 % cholesterol diet.	yes	yes	Mice with knock-out INSIG regulator genes had increased gene and enzyme expression for loci involved in cholesterol synthesis versus wild-type mice (no statistical analysis), leading to statistically significant increased plasma cholesterol levels versus wild-type mice (type of cholesterol not measured but increased LDL is plausible).	Engleking et al. (2005)
Mouse (Mus musculus	20 weeks	Gene knockout study	yes	yes	Mice with knock-out INSIG regulator genes had statistically significant increased gene expression for cholesterol synthesis enzymes and lipid synthesis activity, leading to statistically significant increased plasma cholesterol levels versus wild-type mice (type of cholesterol not measured but increased LDL is plausible).	MacFarlaine et al. (2014)
Mouse (Mus musculus)	1 week	100 mg/kg body weight efavirenz daily.	yes	yes	8 week old C57BL/6 male mice exposed to efavirenz had statistically significant increased cholesterol biosynthesis enzyme squalene epoxidase (SQLE) gene expression and increased SQLE protein expression per immunoblot, leading to statistically significant increased cholesterol synthesis and statistically significant increased plasma LDL cholesterol levels versus control mice.	Gwag et al. (2019)

Uncertainties and Inconsistencies

Addresses inconsistencies or uncertainties in the relationship including the identification of experimental details that may explain apparent deviations from the expected patterns of concordance. More help

Known modulating factors

This table captures specific information on the MF, its properties, how it affects the KER and respective references.1.) What is the modulating factor? Name the factor for which solid evidence exists that it influences this KER. Examples: age, sex, genotype, diet 2.) Details of this modulating factor. Specify which features of this MF are relevant for this KER. Examples: a specific age range or a specific biological age (defined by...); a specific gene mutation or variant, a specific nutrient (deficit or surplus); a sex-specific homone; a certain threshold value (e.g. serum levels of a chemical above...) 3.) Description of how this modulating factor affects this KER. Describe the provable modification of the KER (also quantitatively, if known). Examples: increase or decrease of the magnitude of effect (by a factor of...); change of the time-course of the effect (onset delay by...); alteration of the probability of the effect; increase or decrease of the sensitivity of the downstream effect (by a factor of...) 4.) Provision of supporting scientific evidence for an effect of this MF on this KER. Give a list of references. More help

Quantitative Understanding of the Linkage

Captures information that helps to define how much change in the upstream KE, and/or for how long, is needed to elicit a detectable and defined change in the downstream KE. More help

Response-response Relationship

Provides sources of data that define the response-response relationships between the KEs. More help

Time-scale

Information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). More help

Known Feedforward/Feedback loops influencing this KER

Define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits. More help

Domain of Applicability

A free-text section of the KER description that the developers can use to explain their rationale for the taxonomic, life stage, or sex applicability structured terms. More help

Life Stage: All life stages.

Sex: Applies to both males and females.

Taxonomic: Primarily studied in humans and laboratory rodents.

References

List of the literature that was cited for this KER description. More help

Duan, Y., Gong, K., Xu, S., Zhang, F., Meng, X., and Han, J. 2022. Regulation of cholesterol homeostasis in health and diseases: from mechanisms to targeted therapeutics. Signal Transduction and Targeted Therapy 7: 265.

Gwag, T., Meng, Z., Sui, Y., Helsley, R.N., Park, S.-H., Wang, S., Greenberg, R.N., and Zhou, C. 2019. Non-nucleoside reverse transcriptase inhibitor efavirenz activates PXR to induce hypercholesterolemia and hepatic steatosis. Journal of Hepatology 70: 930–940.

Horton, J.D., Shah, N.A., Warrington, J.A., Anderson, N.N., Park, S.W., Brown, M.S., and Goldstein, J.L. 2003. Combined analysis of oligonucleotide microarray data from transgenic and knockout mice identifies direct SREBP target genes. Proceedings of the National Academy of Sciences 100(21): 12027–12032.

Howe, V., Sharpe, L.J., Prabhu, A.V., and Brown, A.J. 2017. New insights into cellular cholesterol acquisition: promoter analysis of human HMGCR and SQLE, two key control enzymes in cholesterol synthesis. Biochim Biophys Acta 1862: 647–657.

Itkonen, A., Hakkola, J., and Rysa, J. 2023. Adverse outcome pathway for pregnane X receptor‑induced hypercholesterolemia. Archives of Toxicology 97: 2861–2877.

Lalanne, F., Lambert, G., Amar, M.J.A., Chetiveaux, M., Zair, Y., Jarnoux, A.-L., Ouguerram, K., Friburg, J., Seidah, N.G., Brewer, Jr., H.B., Krempf, M., and Costet, P. 2005. Wild-type PCSK9 inhibits LDL clearance but does not affect apoB-containing lipoprotein production in mouse and cultured cells. Journal of Lipid Research 46: 1312–1319.

Lambert, G., Jarnoux, A.-J., Pineau, T., Pape, O., Chetiveaux, M., Laboisse, C., Krempf, M., and Costet, P. 2006. Fasting induces hyperlipidemia in mice overexpressing Proprotein Convertase Subtilisin Kexin Type 9: Lack of modulation of very-low-density lipoprotein hepatic output by the low-density lipoprotein receptor. Endocrinology 147(10): 4985–4995.

MacFarlaine, M.R., Liang, G., and Engelking, L.J. 2014. Insig proteins mediate feedback inhibition of cholesterol synthesis in the intestine. The Journal of Biological Chemistry 289(4): 2148-2156.

Sakakura, Y., Shimano, H., Sone, H., Takahashi, A., Inoue, N., Toyshima, H., Suzuki, S. and Yamada, N. 2001. Sterol regulatory element-binding proteins induce an entire pathway of cholesterol synthesis. Biochemical and Biophysical Research Communications 286: 176–183.

NOTE: Italics indicate edits from John Frisch October 2024.