Aop: 212

AOP Title


Histone deacetylase inhibition leading to testicular atrophy

Short name:


Histone deacetylase inhibition leading to testicular atrophy

Graphical Representation


Click to download graphical representation template




Shihori Tanabe, Akihiko Hirose, Takashi Yamada

Division of Risk Assessment, Biological Safety Research Center, National Institute of Health Sciences

Point of Contact


Shihori Tanabe   (email point of contact)



  • Shihori Tanabe



Author status OECD status OECD project SAAOP status
Under development: Not open for comment. Do not cite EAGMST Under Review 1.52 Included in OECD Work Plan

This AOP was last modified on February 10, 2020 02:33


Revision dates for related pages

Page Revision Date/Time
Histone deacetylase inhibition February 10, 2020 03:09
Histone acetylation, increase February 10, 2020 03:15
Cell cycle, disrupted February 10, 2020 03:18
Apoptosis February 10, 2020 03:23
Testicular atrophy February 10, 2020 03:38
Spermatocyte depletion February 10, 2020 03:31
Histone deacetylase inhibition leads to Histone acetylation, increase February 10, 2020 03:50
Histone acetylation, increase leads to Cell cycle, disrupted February 10, 2020 04:04
Cell cycle, disrupted leads to Apoptosis February 10, 2020 04:42
Histone deacetylase inhibition leads to Cell cycle, disrupted February 10, 2020 04:27
Apoptosis leads to Spermatocyte depletion February 10, 2020 04:16
Histone deacetylase inhibition leads to Apoptosis February 10, 2020 04:31
Spermatocyte depletion leads to Testicular atrophy February 10, 2020 04:22
Histone deacetylase inhibition leads to Spermatocyte depletion February 10, 2020 04:34
Histone deacetylase inhibition leads to Testicular atrophy February 10, 2020 04:40
Methoxyacetic acid January 21, 2018 20:38
Butyrate January 21, 2018 20:39
Trichostatin A January 21, 2018 20:39
Valproate November 29, 2016 18:42



Testicular toxicity is of interest for human health risk assessment especially in terms of reproductive and developmental toxicity, however, the testicular toxicity has not fully elucidated. Histone deacetylase inhibitors (HDIs) are approved as anti-cancer drugs, since HDIs have apoptotic effect in cancer cells. HDIs includes the short chain fatty acids, hydroxamic acids, benzamides and epoxides. The intracellular mechanisms of induction of the spermatocyte apoptosis by HDIs are suggested as histone deacetylase (HDAC) inhibition as MIE, histone acetylation increase, disrupted cell cycle, apoptosis, and spermatocyte depletion as KEs. Adverse outcome has been defined as testicular atrophy. The HDIs inhibit deacetylation of the histone, leading to the increase in histone acetylation. The apoptosis induced by disrupted cell cycle leads to spermatocyte depletion and testis atrophy. This AOP may be one of the pathways induced by HDIs, which suggests the pathway networks of protein hyperacetylations.

Abbreviation: AOP: adverse outcome pathway, HDAC: histone deacetylase, HDI: HDAC inhibitor, KE: key event, MIE: molecular initiating event

Background (optional)


Summary of the AOP


Events: Molecular Initiating Events (MIE)


Key Events (KE)


Adverse Outcomes (AO)


Sequence Type Event ID Title Short name
1 MIE 1502 Histone deacetylase inhibition Histone deacetylase inhibition
2 KE 1503 Histone acetylation, increase Histone acetylation, increase
4 KE 1505 Cell cycle, disrupted Cell cycle, disrupted
5 KE 1262 Apoptosis Apoptosis
6 KE 1515 Spermatocyte depletion Spermatocyte depletion
7 AO 1506 Testicular atrophy Testicular atrophy

Relationships Between Two Key Events
(Including MIEs and AOs)


Title Adjacency Evidence Quantitative Understanding
Histone deacetylase inhibition leads to Histone acetylation, increase adjacent High Moderate
Histone acetylation, increase leads to Cell cycle, disrupted adjacent Moderate Moderate
Cell cycle, disrupted leads to Apoptosis adjacent Moderate Moderate
Apoptosis leads to Spermatocyte depletion adjacent High Not Specified
Spermatocyte depletion leads to Testicular atrophy adjacent High Not Specified
Histone deacetylase inhibition leads to Cell cycle, disrupted non-adjacent High Moderate
Histone deacetylase inhibition leads to Apoptosis non-adjacent Moderate Moderate
Histone deacetylase inhibition leads to Spermatocyte depletion non-adjacent Moderate Moderate
Histone deacetylase inhibition leads to Testicular atrophy non-adjacent Moderate Moderate

Network View





Name Evidence Term
Methoxyacetic acid High
Butyrate High
Trichostatin A High
Valproate Moderate

Life Stage Applicability


Life stage Evidence
Adult, reproductively mature High

Taxonomic Applicability


Term Scientific Term Evidence Link
human Homo sapiens Moderate NCBI
mouse Mus musculus Moderate NCBI
rat Rattus norvegicus High NCBI

Sex Applicability


Sex Evidence
Male High

Overall Assessment of the AOP


Attached file: Overallassessmentaop212rev 2 10 20

1. Support for Biological Plausibility of KERs

MIE => KE1: Histone deacetylase inhibition leads to histone acetylation increase

Biological Plausibility of the MIE => KE1 is high.
Rationale: Upon the inhibition of HDAC by HDIs, the acetylation of lysine in histone remains and it leads to transcriptional activation or repression, changes in DNA replication and DNA damage repair. Activity of histone acetyltransferase (HAT) in testis nuclear protein was increased with MAA addition [Wade et al., 2008].

KE1 => KE2: Histone acetylation, increase leads to cell cycle, disrupted

Biological Plausibility of the KE1 => KE2 is moderate.
Rationale: Gene transcription is regulated by histone acetylation [Struhl, 1998]. Acetylation of histones neutralizes the positive charge of the histones. Thus, less compacted DNA can be bound more easily by transcription factors and transcribed. In the models proposed for the relationship between histone acetylation and transcription, histone acetylation can be untargeted and occur at both promoter and nonpromoter regions, targeted generally to promoter regions, or targeted to specific promoters by gene-specific activator proteins [Richon et al., 2000; Struhl, 1998].

KE2 => KE3: Cell cycle, disrupted leads to apoptosis

Biological Plausibility of the KE2 => KE3 is moderate.
Rationale: Prolonged cell cycle arrest will lead to either senescence or apoptosis. Especially for fast dividing and still differentiating cells, such an arrest will most certainly induce apoptosis as the normal cellular program cannot be followed.

KE3 => KE4: Apoptosis leads to spermatocyte depletion

Biological Plausibility of the KE3 => KE4 is moderate.
Rationale: During development and in tissue homeostasis, apoptosis is needed to control organ size. If apoptosis is induced at a higher rate, one can assume it leading to atrophy of the target organ. Especially when target organ / target cells are fast replicating, abnormal levels of apoptosis will lead to depletion.

KE4 => AO: Spermatocyte depletion leads to testicular atrophy

Biological Plausibility of the KE4 => AO is moderate.
Rationale: Spermatocyte depletion is one of the main characteristics of testicular atrophy.

2. Support for essentiality of KEs

KE2: Cell cycle, disrupted

Essentiality of the KE2 is moderate.
Rationale for Essentiality of KEs in the AOP: HDAC1-defecient embryonic stem cells showed reduced proliferation rates, which correlates with decreased cyclin-associated kinase activities and elevated levels of the cyclin-dependent kinase inhibitor 1A, a cell cycle regulator p21 [Lagger et al., 2002]. Loss of HDAC1 leads to significantly reduced overall deacetylase activity, hyperacetylation of a subset of histones H3 and H4 [Lagger et al., 2002].

3. Empirical support for KERs

MIE => KE1: Histone deacetylase inhibition leads to histone acetylation, increase

Empirical Support of the MIE => KE1 is high.
Rationale: HDAC inhibitors increase histone acetylation in brain [Schroeder et al., 2013]. The major empirical evidence came from the incubation of cell culture cells with small molecule compounds that inhibit HDAC enzymes followed by western blots or acid urea gel analysis. The first evidence was shown by Riggs et al. who showed that incubation of HeLa cells with n-butyrate leads to an accumulation of acetylated histone proteins [Riggs et al., 1977]. Later, it was shown that n-butyrate specifically increases the acetylation of histone by the incorporation of radioactive [3H]acetate and analysis of the histones on acid urea gels that allow the detection of acetylated histones [Cousens et al., 1979]. TSA was shown to be an HDAC inhibitor by acid urea gel analysis in 1990 [Yoshida et al., 1990] and good evidence for VPA as an HDAC inhibitor in vitro and in vivo was shown using acetyl-specific antibodies and western blot [Gottlicher et al., 2001].

KE1 => KE2: Histone acetylation, increase leads to cell cycle, disrupted

Empirical Support of the KE1 => KE2 is moderate.
Rationale: Increase in histone acetylation by HDAC inhibition induces the cell cycle regulator expression and inhibits progression through the cell cycle. Histone acetylation regulates the gene transcriptional mechanism [Struhl, 1998]. Acetylation of histones promotes the RNA polymerase reaction [Allfrey et al., 1964; Pogo et al., 1966]. Since several results supported a model in which increased histone acetylation is targeted to specific gene and occurs throughout the entire genome, not just the promoter regions, histone acetylation may lead to gene transcription of the cell cycle regulator [Richon et al., 2000].

KE2 => KE3: Cell cycle, disrupted leads to apoptosis

Empirical Support of the KE2 => KE3 is moderate.
Rationale: Cell cycle arrest such as G1 arrest and G1/S arrest are observed in apoptosis [Li et al., 2012; Dong et al., 2010]. microRNA-1 and microRNA-206 represses CCND2, while microRNA-29 represses CCND2 and induces G1 arrest and apoptosis in rhabdomyosarcoma [Li et al., 2012].

KE3 => KE4: Apoptosis leads to spermatocyte depletion

Empirical Support of the KE3 => KE4 is high.
Rationale: MicroRNA-21 regulates the spermatogonial stem cell homeostasis, in which suppression of microRNA-21 with anti-miR-21 oligonucleotides led to apoptosis of spermatogonial stem cell-enriched germ cell cultures and the decrease in the number of spermatogonial stem cells [Niu et al., 2011].

KE4 => AO: Spermatocyte depletion leads to testicular atrophy

Empirical Support of the KE4 => AO is high.
Rationale: The testicular atrophy seen in 2-methoxyethanol (2-ME), or its major metabolite MAA, treated rats in vivo and in human, and rat in vitro culture was associated with spermatocyte depletion [Beattie et al., 1984].

Domain of Applicability


The AOP is applicable to the reproductively mature males in rats, mice and humans. The administration route or doses of HDAC inhibitors may affect the intensity of the toxicity.

Essentiality of the Key Events


Key Event Direct/Indirect Evidence
MIE: Histone deacetylase inhibition HDAC inhibition induced testicular toxicity including testis atrophy [Miller et al., 1982]. HDAC inhibition in cell culture resulted in the testicular toxicity including germ cell apoptosis and cell morphology change [Li et al., 1996].
KE1: Histone acetylation, increase The HDAC inhibition induced cell death in spermatocytes in both rat and human seminiferous tubules [Li et al., 1996].
KE2: Cell cycle, disrupted In HDAC1-/- fibroblast lines, increase in the amount of cells in G1 phase and decrease in the amount of cells in S phase were observed, which indicates the importance of HDAC inhibition in cell cycle regulation [Zupkovitz et al., 2010].
KE3: Apoptosis HDAC inhibition leads to cell death through the apoptotic pathways [Falkenberg et al., 2014].
KE4: Spermatocyte depletion The HDAC inhibition induced cell death in spermatocytes in both rat and human seminiferous tubules [Li et al., 1996]. The HDAC inhibitor treatment resulted in degeneration in spermatocytes in rat seminiferous tubules [Li et al., 1996]. The HDAC inhibition induced the germ cell apoptosis in human testicular tissues [Li et al., 1996].

Evidence Assessment


Biological plausibility, coherence, and consistency of the experimental evidence

The available data supporting the AOP are logic, coherent and consistent with established biological knowledge, whereas there are possibilities for alternative pathways.


Alternative mechanism(s) that logically present themselves and the extent to which they may distract from the postulated AOP


There are some other important apoptotic pathways that are involved in cell death, as well as other important spermatocyte signaling or mechanism influences testicular toxicity.


p53 pathway

The study in which in vivo administration of trichostatin A (TSA), a HDI, in mice resulted in male meiosis impairment showed the involvement of p53-noxa-caspase-3 apoptotic pathway in TSA-induced spermatocyte apoptosis [Fenic et al., 2008]. Other study showed that MAA induced up-regulation of p21 expression is mediated through histone hyperacetylation and independent of p53/p63/p73 [Parajuli et al., 2014].


      NF-kappaB pathway

The present AOP focuses on p21 pathway leading to apoptosis, however, the alternative pathway such as NF-kappaB signaling pathways may be involved in apoptosis of spermatocytes [Wang et al., 2017].


Communication with Sertoli cells

The present AOP focuses on testicular atrophy by HDAC inhibition-induced apoptosis in spermatocytes, however, the signaling in Setoli cells may be involved in testicular atrophy. Sertoli cell secretes GDNF, FGF2, CXCL12 or Ccl9 molecules, which results in the activation of RET, FGFR, CXCR4 or CCR1 signaling in spermatogonial stem cells, respectively [Chen and Liu, 2015].


Decrease in deoxynucleotide pool by MAA

MAA induces decrease in deoxynucleotide pool, resulting apoptosis, which may be an alternative pathway other than p21-mediated pathway [Yamazoe et al., 2015]. Inhibition of 5,10-CH2-THF production by MAA may decreases deoxynucleotide pool in spermatocytes [Yamazoe et al., 2015].

Spermatocyte depletion by necrosis

Spermatocyte may be decreased by necrosis. Cell death mechanism other than apoptosis, such as necrosis, may be considered for spermatocyte depletion.

Quantitative Understanding


Concordance of dose-response relationships

This is a quantitative description on dose-response relationships from MIE to AOP. But some KE relationships individually are not fully supported with dose-response relationships, while there is empirical evidence to support that a change in KEup leads to an appropriate change in the respective KEdown.


Temporal concordance among the key events and adverse outcome

Temporal concordance between MIE and AOP has been described with in vivo experimental data. Empirical evidences show temporal concordance between MIE and the individual KEs, however, the temporal concordance among the individual KEs and AO is not fully elucidated.


Strength, consistency, and specificity of association of adverse outcome and initiating event

The scientific evidence on the linkage between MIE and AO has been described.


The quantitative understanding of the AOP in terms of indirect relations between HDAC inhibition and testicular atrophy was examined in in vivo experiments [Foster et al., 1983; Miller et al., 1982].


Considerations for Potential Applications of the AOP (optional)


The present AOP can be used in risk assessment of several types of the molecules including HDAC inhibitors as the anti-cancer drugs, as well as another types of the chemicals, in terms of testicular toxicity. HDAC inhibitors nowadays have been utilized as therapeutics for cancer or neurology disease, and the adverse effects of HDAC inhibitors should be evaluated. This AOP elucidating the pathway from HDAC inhibition to testicular atrophy may provides important insights for potential toxicity of HDAC inhibitors. It also provides a basis for the HDAC inhibition-induced epigenetic alteration and cell death. HDAC inhibitors such as rocilinostat are clinically evaluated as anti-cancer drugs in clinical trial.



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