API

Relationship: 396

Title

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reduction in ovarian granulosa cells, Aromatase (Cyp19a1) leads to Reduction, 17beta-estradiol synthesis by ovarian granulosa cells

Upstream event

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reduction in ovarian granulosa cells, Aromatase (Cyp19a1)

Downstream event

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Reduction, 17beta-estradiol synthesis by ovarian granulosa cells

Key Event Relationship Overview

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AOPs Referencing Relationship

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AOP Name Directness Weight of Evidence Quantitative Understanding
Aromatase (Cyp19a1) reduction leading to impaired fertility in adult female directly leads to Moderate

Taxonomic Applicability

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Sex Applicability

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Life Stage Applicability

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How Does This Key Event Relationship Work

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Aromatase is the cytochrome P450 enzyme complex responsible for the conversion of androgens to estrogens during steroidogenesis [reviewed by (Simpson et al., 1994)], which is a key reaction in the sex differentiation in vertebrates. Reduction in level of aromatase or in the catalytic activity of the aromatase itself will reduce the levels of estrogens in tissues and dramatically disrupt estrogen (E2) hormone action.

Weight of Evidence

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Biological Plausibility

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Aromatase in the specialized cells of the ovary, hypothalamus, and placenta clearly serves crucial role in reproduction for mammalian and other vertebrates by converting the androgens to estrogens. Therefore, the coordinated and cell-specific expression of the aromatase (Cyp19a1) gene in the ovary plays a key role in the 17beta-estradiol (E2) synthesis. Within the ovary, aromatase expression and activity is primarily localized in the granulosa cells (reviewed in (Havelock, Rainey, & Carr, 2004). C-19 androgens diffuse from the theca cells into granulosa cells where aromatase can catalyze their conversion to C-18 estrogens. Therefore, inhibition, decrease of level or activity of ovarian aromatase can generally be assumed to directly impact E2 synthesis by the granulosa cells.

Empirical Support for Linkage

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Environmental agents, toxicants, and various natural products can impact on aromatase activity and/or alteration in protein levels to result in reduced levels of estrogen.


Studies providing evidence for the linkage of aromatase decrease and decreased E2 production include: Bisphenol A: in vitro, human: significant reduction of aromatase from 40μM and decreased E2 production from 80 μM (Kwintkiewicz, Nishi, Yanase, & Giudice, 2010) at the same time point. MEHP, in vitro:

• human, from 10μM decreased aromatase activity (dose dependent), at 167 μM decrease in mRNA levels of aromatase and from 10μM decrease of estradiol production (dose dependent), measured at the same time point (48h) (Reinsberg, Wegener-Toper, van der Ven, van der Ven, & Klingmueller, 2009)

• rat, dose response decrease in aromatase levels from 50μM, dose dependent decrease of E2 production from 100μM, at the same (48 h) (Lovekamp & Davis, 2001).

• rat, decrease in aromatase levels at 100μg/ml DEHP, 10μg/ml MEHP and dose dependent decrease of E2 production from 10μg/ml DEHP, 0.1μg/ml MEHP and at the same time point (96 h) (Gupta et al., 2010).

Table 1 summarises available empirical evidence.


Compound

Species
Study type
comments
Aromatase decrease levels/activity
Reference

MEHP

rat
in vitro
decrease activity of aromatase and dose and time dependent decrease of E2 production
decrease activity of aromatase 100 µM
E2 production 50-100 µM
(Davis, Weaver, Gaines, & Heindel, 1994),

MEHP

rat
ex vivo
decrease aromatase level and dose decrease of E2 production
aromatase level 50µM
E2 production 100-200 µM
(Lovekamp & Davis, 2001)

DEHP

rat
in vivo
Does dependent reduction of E2 levels, and Does dependent reduction decrease aromatase expression
300-600mg/kg/day
(Xu et al., 2010),

MEHP

human
in vitro
Dose dependent reduction E2 production and reduction aromatase of expression
reduction E2 levels IC(50)= 49- 138 µM, at 167µM decrease aromatase
(Reinsberg et al., 2009)

MEHP/DEHP


mice
ex vivo
dose dependent E2 production, and reduction of aromatase levels
E2 production at DEHP (10 -100 μg/ml);MEHP (0.1 and 10 μg/ml)
Aromatase levels DEHP (100 μg/ml); MEHP 0.1 μg/ml
(Gupta et al., 2010)

This KE describes decreased levels and/or availability of aromatase different from aromatase inhibition.

Uncertainties or Inconsistencies

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Upstream events An upstream event has been postulated to involve PPARγ activation, however the studies confirming its role in the reduction of aromatase levels are incomplete. The mechanisms involving Peroxisome Proliferator Activated receptor γ activation leading to aromatase (Cyp19a1) reduction relating to the pathway are described in greater detail in the page Peroxisome Proliferator Activated receptor γ activation indirectly leads to aromatase (Cyp19a1) reduction .

Availability or reduced aromatase levels


Studies by Davis et al showed that MEHP impacts on availability (degradation) of aromatase as the decrease in E2 production is evident after the treatment with transcription and translation blockers (actinomycin D or cycloheximide). MEHP was further decreased E2 production independently of the presence of inhibitors pointing out at mechanisms of degradation rather than aromatase synthesis (Davis et al., 1994). MEHP can indirectly impact on aromatase rates by decreasing necessary cofactors (availability) or activation of aromatase inhibitors. Treinin et al showed in vitro dose dependent inhibition of progesterone production by MEHP in granulosa cells and reduced FSH-stimulated cAMP accumulation in granulosa cells implicating a direct or indirect effect of MEHP on FSH receptor (Treinen, Dodson, & Heindel, 1990). Similar effects of cAMP accumulation were seen in Sertoli cells (Lloyd & Foster, 1988), (Heindel & Chapin, 1989), (Heindel & Powell, 1992). Since granulosa and Sertoli cells share several structural and functional characteristics this mechanism is plausible. Study by Ma et al showed that inhaled DEHP (5 and 25 mg/m3) increased levels of LH and E2 in serum of prepubertal rats, and it increased ovarian Cyp19a1 expression (Ma et al., 2006), which is in disagreement with the key event relationship. This difference might be due to measurements of hormones during different phases of the estrous cycle, alterations in the experimental approaches used (in vivo versus in vitro) as well as exposure routes and doses given.

Quantitative Understanding of the Linkage

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Is it known how much change in the first event is needed to impact the second? Are there known modulators of the response-response relationships? Are there models or extrapolation approaches that help describe those relationships?


Several mechanistically-based models of ovarian steroidogenesis have been developed (Breen et al. 2013; Breen et al. 2007; Shoemaker et al. 2010; Quignot and Bois 2013). These may be adaptable to predict in vitro E2 production and/or plasma E2 concentrations from in vitro or in vivo measurements of changes of aromatase expression/availability.

Evidence Supporting Taxonomic Applicability

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See table 1.

References

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Davis, B. J., Weaver, R., Gaines, L. J., & Heindel, J. J. (1994). Mono-(2-ethylhexyl) phthalate suppresses estradiol production independent of FSH-cAMP stimulation in rat granulosa cells. Toxicology and Applied Pharmacology, 128(2), 224–8. doi:10.1006/taap.1994.1201

Gupta, R. K., Singh, J. M., Leslie, T. C., Meachum, S., Flaws, J. a, & Yao, H. H.-C. (2010). Di-(2-ethylhexyl) phthalate and mono-(2-ethylhexyl) phthalate inhibit growth and reduce estradiol levels of antral follicles in vitro. Toxicology and Applied Pharmacology, 242(2), 224–30. doi:10.1016/j.taap.2009.10.011

Havelock, J. C., Rainey, W. E., & Carr, B. R. (2004). Ovarian granulosa cell lines. Molecular and Cellular Endocrinology, 228(1-2), 67–78. doi:10.1016/j.mce.2004.04.018

Heindel, J. J., & Chapin, R. E. (1989). Inhibition of FSH-stimulated cAMP accumulation by mono(2-ethylhexyl) phthalate in primary rat Sertoli cell cultures. Toxicology and Applied Pharmacology, 97(2), 377–85.

Heindel, J. J., & Powell, C. J. (1992). Phthalate ester effects on rat Sertoli cell function in vitro: effects of phthalate side chain and age of animal. Toxicology and Applied Pharmacology, 115(1), 116–23.

Kwintkiewicz, J., Nishi, Y., Yanase, T., & Giudice, L. C. (2010). Peroxisome proliferator-activated receptor-gamma mediates bisphenol A inhibition of FSH-stimulated IGF-1, aromatase, and estradiol in human granulosa cells. Environmental Health Perspectives, 118(3), 400–6. doi:10.1289/ehp.0901161

Lloyd, S. C., & Foster, P. M. (1988). Effect of mono-(2-ethylhexyl)phthalate on follicle-stimulating hormone responsiveness of cultured rat Sertoli cells. Toxicology and Applied Pharmacology, 95(3), 484–9.

Lovekamp, T. N., & Davis, B. J. (2001). Mono-(2-ethylhexyl) phthalate suppresses aromatase transcript levels and estradiol production in cultured rat granulosa cells. Toxicology and Applied Pharmacology, 172(3), 217–24. doi:10.1006/taap.2001.9156

Ma, M., Kondo, T., Ban, S., Umemura, T., Kurahashi, N., Takeda, M., & Kishi, R. (2006). Exposure of prepubertal female rats to inhaled di(2-ethylhexyl)phthalate affects the onset of puberty and postpubertal reproductive functions. Toxicological Sciences : An Official Journal of the Society of Toxicology, 93(1), 164–71. doi:10.1093/toxsci/kfl036

Reinsberg, J., Wegener-Toper, P., van der Ven, K., van der Ven, H., & Klingmueller, D. (2009). Effect of mono-(2-ethylhexyl) phthalate on steroid production of human granulosa cells. Toxicology and Applied Pharmacology, 239(1), 116–23. doi:10.1016/j.taap.2009.05.022

Simpson, E. R., Mahendroo, M. S., Means, G. D., Kilgore, M. W., Hinshelwood, M. M., Graham-Lorence, S., … Michael, M. D. (1994). Aromatase cytochrome P450, the enzyme responsible for estrogen biosynthesis. Endocrine Reviews, 15(3), 342–55. doi:10.1210/edrv-15-3-342

Treinen, K. A., Dodson, W. C., & Heindel, J. J. (1990). Inhibition of FSH-stimulated cAMP accumulation and progesterone production by mono(2-ethylhexyl) phthalate in rat granulosa cell cultures. Toxicology and Applied Pharmacology, 106(2), 334–40.

Xu, C., Chen, J.-A., Qiu, Z., Zhao, Q., Luo, J., Yang, L., … Shu, W. (2010). Ovotoxicity and PPAR-mediated aromatase downregulation in female Sprague-Dawley rats following combined oral exposure to benzo[a]pyrene and di-(2-ethylhexyl) phthalate. Toxicology Letters, 199(3), 323–32. doi:10.1016/j.toxlet.2010.09.015