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Relationship: 436

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

Decrease, Steroidogenic acute regulatory protein (STAR) leads to Reduction, Cholesterol transport in mitochondria

Upstream event

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

Decrease, Steroidogenic acute regulatory protein (STAR)

Downstream event

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

Reduction, Cholesterol transport in mitochondria

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
PPARα activation in utero leading to impaired fertility in males	adjacent	Moderate		Elise Grignard (send email)	Open for citation & comment	Under Review

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
Mixed	High

Life Stage Applicability

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

Term	Evidence
All life stages	High

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

Steroidogenic acute regulatory protein (StAR) mediates the cholesterol transport from the outer to the inner mitochondrial membrane, where it undergoes side chain cleavage by cytochrome P-450 enzyme (P450scc) that yields the steroid precursor, pregnenolone (Besman et al. 1989). The cholesterol transfer within the mitochondria is the rate-limiting step in the production of steroid hormones. Therefore reduced amount/activity of the StAR impairs the cholesterol delivery that is necessary for the hormone biosynthesis.

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

The KER describes a generally recognized and understood process, i.e. canonical knowledge. The aim of the literature search was therefore to identify review articles and book chapters that summarise the canonical knowledge. PubMed was searched using key words related to steroidogenesis. The search was restricted to reviews from the last 10 years.

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

The first step in steroidogenesis takes place within mitochondria. Cholesterol, the precursor molecule of steroids, is stored in either the plasma membrane or lipid droplets primarily of steroidogenic cells. These include theca cells and luteal cells in the ovary, Leydig cells in the testis and cells in the adrenal cortex, brain and placenta. The cholesterol delivery to the inner mitochondrial membrane (IMM), which contains insignificant cholesterol amounts, is accomplished either by ATP-dependent vesicular transport or through non-vesicular transport through protein carriers (Aghazadeh et al., 2015).

The non-vesicular transport of the hydrophobic cholesterol through the aqueous intermembrane space of the mitochondria is regulated by the transduceosome protein complex. The complex is assembled upon hormonal stimulation and consists of cytosolic proteins, including StAR and outer mitochondrial membrane ones like voltage-dependent anion channel 1 (VDAC1) and translocator protein (TSPO) (Aghazadeh et al., 2015). It is hypothesized that kinases, with the most prominent being protein kinase A (PKA), activate transcription factors that trigger StAR transcription and also activate StAR protein (Tugaeva & Sluchanko, 2019). Subsequently, StAR binds cholesterol and in response TSPO and VDAC1 shuttle cholesterol to P450scc. The cholesterol transfer through the tranduceosome accounts for more than 70% of the cholesterol transport to the mitochondria, therefore, any decrease on StAR levels would result in a decrease of cholesterol transport (Miller, 2017). This estimation refers to non-steroidogenic cells, and non-vesicular transport is estimated to be higher in steroidogenic cells, as it is more efficient.

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

The role of StAR in cholesterol transport becomes evident in individuals with StAR deficiency. These individuals have a condition called congenital lipoid adrenal hyperplasia (LCAH), characterized by an inability to synthesize steroids and accumulated fatty deposits in steroidogenic cells, attributed to the failure of cholesterol transport to mitochondria (Stocco, 2002). These individuals can have underdeveloped genitalia and may require lifelong hormone replacement therapy. Additionally, mouse StAR knock-out studies treated with corticosteroid replacement demonstrate the same phenotype (Caron et al., 1997; Hasegawa et al., 2000).

Down-regulation of StAR and impaired steroidogenesis was reported upon exposure to phthalates i.a. (Barlow et al. 2003), (Borch et al. 2006), for details see Table 1.

			KE: StAR, decrease	KE: Cholesterol transport, decrease
Compound	Species	Effect level			Details	References
Phthalate (DBP)	rat	LOEL=500 mg/kg/day	mRNA StAR decrease (by ~34%)	reduced Leydig cell lipid content		(Barlow et al. 2003)
Phthalate (DBP)	rat	LOEL=500 mg/kg/day (GD12-19)		decrease uptake of cholesterol Leydig cell mitochondria	decreased testosterone, decreased expression of scavenger receptor B1, P450(SCC), steroidogenic acute regulatory protein, and cytochrome p450c17	(Thompson, Ross, and Gaido 2004)
Phthalate (DBP)	rat	LOEL=500 mg/kg	mRNA and protein StAR decrease		1 dose, Time course analysis (0,5,1,2,3,6,12,18, 24h killed at GD), decreased testosterone in foetal testis	(Thompson et al. 2005)
Phthalate (DEHP)	rat	LOEL=300 mg/ kg/day	mRNA StAR decrease		dose-dependently reduced StAR, TSOP mRNA (GD 21 testes), also on protein levels in Leydig cells	(Borch et al. 2006)
Phthalate (DBP)	rat	LOEL=500 mg/kg/day, (GD12 to 21)	mRNA StAR decrease		Testes examined GD 16, 19, and 21, cytochrome P450 side chain cleavage, cytochrome P450c17, decrease. Testicular testosterone and androstenedione decreased (GD 19 and 21)	(Shultz 2001)
Phthalate (MEHP)	rat	LOEC=250 μM	protein StAR decrease (immature and adult Leydig cells)	cholesterol transport, decrease (into the mitochondria of immature and adult Leydig cells	decreased testosterone by approximately 60%, in vitro ( immature and adult Leydig cells)	(Svechnikov, Svechnikova, and Söder 2008)
Phthalate (DBP)	rat	LOEL=500 mg/kg/day	mRNA StAR decrease		GD 12 -20, examinations on GD20	(Johnson et al. 2011)

Table 1 Summary table of empirical support for this KER. LOEC-lowest effect concentration, LOEL- lowest observed effect level, Dibutyl phthalate (DBP), Di-2-ethylhexyl phthalate (DEHP), mono(2-ethylhexyl) phthalate (MEHP).

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

Some steroidogenesis is independent of StAR; when nonsteroidogenic cells are transfected with the P450scc system, they convert cholesterol to pregnenolone at about 14% of the StAR-induced rate (Lin et al. 1995). The mechanism of StAR-independent steroidogenesis is unclear (Miller and Auchus 2011). Johnson et al proposed the involvment of sterol regulatory element–binding protein (SREBP) in phthalate mediated disruption of steroidogenesis. Their study showed lipid metabolism pathways transcriptionally regulated by SREBP were inhibited in the rat but induced in the mouse, and this differential species response corresponded with repression of the steroidogenic pathway. In rats exposed to 100 or 500 mg/kg DBP from gestational days (GD) 16 to 20, a correlation was observed between GD20 testis steroidogenic inhibition and reductions of testis cholesterol synthesis endpoints including testis total cholesterol levels (Johnson et al. 2011).

Additionally, the non-StAR-dependent transport can occur through vesicular transport or through non-specific lipid transport proteins called sterol carrier protein 2 or x (SCP2/SCPx) (Galano et al., 2022). These transport proteins are hypothesized to be a supplementary mechanism to StAR dependent cholesterol transport.

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

Mutations affecting S194 and S187 phosphorylation sites of StAR lead to LCAH (Aghazadeh et al., 2015). Phosphorylation of S194 can induce StAR activity by two-fold.

Modulating Factor (MF)	MF Specification	Effect(s) on the KER	Reference(s)

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

It is estimated that one molecule of StAR protein can transport 400 molecules of cholesterol per minute (Elustondo et al., 2017).

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

Taxonomic applicability.

The steroidogenic acute regulatory protein (StAR or STARD1) belongs to the larger family of START proteins, which all include a steroidogenic acute regulatory domain (STARD) (Tugaeva & Sluchanko, 2019). This domain can be found in genomes from plants, bacteria, protists, and animals, but not in archaea or yeast (Tugaeva & Sluchanko, 2019). However, the STARD1 subfamily is found in vertebrates.

Life stage applicability

StAR expression starts during fetal life (Men et al., 2017).

Sex applicability

This KER is applicable to both sexes as the role of StAR is essential for both (Lin et al., 1995; Stocco 2002; Miller, 2011).

References

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

Aghazadeh, Y., Zirkin, B. R., & Papadopoulos, V. (2015). Pharmacological Regulation of the Cholesterol Transport Machinery in Steroidogenic Cells of the Testis. In Vitamins and Hormones (Vol. 98, pp. 189–227). Academic Press Inc. https://doi.org/10.1016/bs.vh.2014.12.006 Barlow, Norman J, Suzanne L Phillips, Duncan G Wallace, Madhabananda Sar, Kevin W Gaido, and Paul M D Foster. 2003. “Quantitative Changes in Gene Expression in Fetal Rat Testes Following Exposure to Di(n-Butyl) Phthalate.” Toxicological Sciences : An Official Journal of the Society of Toxicology 73 (2) (June): 431–41. doi:10.1093/toxsci/kfg087.

Besman, M J, K Yanagibashi, T D Lee, M Kawamura, P F Hall, and J E Shively. 1989. “Identification of Des-(Gly-Ile)-Endozepine as an Effector of Corticotropin-Dependent Adrenal Steroidogenesis: Stimulation of Cholesterol Delivery Is Mediated by the Peripheral Benzodiazepine Receptor.” Proceedings of the National Academy of Sciences of the United States of America 86 (13) (July): 4897–901.

Borch, Julie, Stine Broeng Metzdorff, Anne Marie Vinggaard, Leon Brokken, and Majken Dalgaard. 2006. “Mechanisms Underlying the Anti-Androgenic Effects of Diethylhexyl Phthalate in Fetal Rat Testis.” Toxicology 223 (1-2) (June 1): 144–55. doi:10.1016/j.tox.2006.03.015.

Caron, K. M., Soo, S.-C., Wetsel, W. C., Stocco, D. M., Clark, B. J., & Parker, K. L. (1997). Targeted disruption of the mouse gene encoding steroidogenic acute regulatory protein provides insights into congenital lipoid adrenal hyperplasia. In Medical Sciences (Vol. 94). www.pnas.org.

Elustondo, P., Martin, L. A., & Karten, B. (2017). Mitochondrial cholesterol import. Biochimica et Biophysica Acta - Molecular and Cell Biology of Lipids, 1862(1), 90–101. https://doi.org/10.1016/j.bbalip.2016.08.012

Galano, M., Venugopal, S., & Papadopoulos, V. (2022). Role of STAR and SCP2/SCPx in the Transport of Cholesterol and Other Lipids. In International Journal of Molecular Sciences (Vol. 23, Issue 20). MDPI. https://doi.org/10.3390/ijms232012115

Hasegawa, T., Zhao, L., Caron, K. M., Majdic, G., Suzuki, T., Shizawa, S., Sasano, H., & Parker, K. L. (2000). Developmental Roles of the Steroidogenic Acute Regulatory Protein (StAR) as Revealed by StAR Knockout Mice. In Molecular Endocrinology (Vol. 14). https://academic.oup.com/mend/article/14/9/1462/2751100 Johnson, Kamin J, Erin N McDowell, Megan P Viereck, and Jessie Q Xia. 2011. “Species-Specific Dibutyl Phthalate Fetal Testis Endocrine Disruption Correlates with Inhibition of SREBP2-Dependent Gene Expression Pathways.” Toxicological Sciences : An Official Journal of the Society of Toxicology 120 (2) (April): 460–74. doi:10.1093/toxsci/kfr020.

Lin, D, T Sugawara, J F Strauss, B J Clark, D M Stocco, P Saenger, A Rogol, and W L Miller. 1995. “Role of Steroidogenic Acute Regulatory Protein in Adrenal and Gonadal Steroidogenesis.” Science (New York, N.Y.) 267 (5205) (March 24): 1828–31.

Men, Y., Fan, Y., Shen, Y., Lu, L., & Kallen, A. N. (2017). The steroidogenic acute regulatory protein (StAR) is regulated by the H19/let-7 axis. Endocrinology, 158(2), 402–409. https://doi.org/10.1210/en.2016-1340

Miller, W. L. (2017). Steroidogenesis: Unanswered Questions. In Trends in Endocrinology and Metabolism (Vol. 28, Issue 11, pp. 771–793). Elsevier Inc. https://doi.org/10.1016/j.tem.2017.09.002 Miller, Walter L, and Richard J Auchus. 2011. “The Molecular Biology, Biochemistry, and Physiology of Human Steroidogenesis and Its Disorders.” Endocrine Reviews 32 (1) (February): 81–151. doi:10.1210/er.2010-0013.

Shultz, V. D. 2001. “Altered Gene Profiles in Fetal Rat Testes after in Utero Exposure to Di(n-Butyl) Phthalate.” Toxicological Sciences 64 (2) (December 1): 233–242. doi:10.1093/toxsci/64.2.233.

Stocco, D. M. (2002). Clinical disorders associated with abnormal cholesterol transport: mutations in the steroidogenic acute regulatory protein. www.elsevier.com/locate/mce Svechnikov, Konstantin, Irina Svechnikova, and Olle Söder. 2008. “Inhibitory Effects of Mono-Ethylhexyl Phthalate on Steroidogenesis in Immature and Adult Rat Leydig Cells in Vitro.” Reproductive Toxicology (Elmsford, N.Y.) 25 (4) (August): 485–90. doi:10.1016/j.reprotox.2008.05.057.

Thompson, Christopher J, Susan M Ross, and Kevin W Gaido. 2004. “Di(n-Butyl) Phthalate Impairs Cholesterol Transport and Steroidogenesis in the Fetal Rat Testis through a Rapid and Reversible Mechanism.” Endocrinology 145 (3) (March): 1227–37. doi:10.1210/en.2003-1475.

Thompson, Christopher J, Susan M Ross, Janan Hensley, Kejun Liu, Susanna C Heinze, S Stanley Young, and Kevin W Gaido. 2005. “Differential Steroidogenic Gene Expression in the Fetal Adrenal Gland versus the Testis and Rapid and Dynamic Response of the Fetal Testis to Di(n-Butyl) Phthalate.” Biology of Reproduction 73 (5) (November): 908–17. doi:10.1095/biolreprod.105.042382.

Tugaeva, K. V., & Sluchanko, N. N. (2019). Steroidogenic Acute Regulatory Protein: Structure, Functioning, and Regulation. In Biochemistry (Moscow) (Vol. 84, pp. 233–253). Pleiades journals. https://doi.org/10.1134/S0006297919140141