Effects of dietary aflatoxin on the hepatic ... - Semantic Scholar

Published December 4, 2014

Effects of dietary aflatoxin on the hepatic expression of apoptosis genes in growing barrows1 S. M. Rustemeyer,* W. R. Lamberson,† D. R. Ledoux,† K. Wells,† K. J. Austin,* and K. M. Cammack*2 *Department of Animal Science, University of Wyoming, Laramie 82071; and †Department of Animal Sciences, University of Missouri, Columbia 65203

ABSTRACT: The most common and toxic form of aflatoxin, aflatoxin B1 (AFB1), is produced by molds growing on crops. Use of moldy corn can result in high concentrations of AFB1 in swine diets, which could potentially lead to an increased incidence of aflatoxicosis, a disease associated with decreased health and performance through reduced feed intake, reduced BW gain, and impaired liver function. The objective of this study was to determine the effects of AFB1 on the hepatic gene expression of growing barrows. Ninety Duroc × Yorkshire crossbred barrows (age = 35 ± 5 d; initial BW = 14.2 ± 3.0 kg) were allocated to 9 pens with 10 pigs per pen, and randomly assigned in a 3 × 3 factorial arrangements of treatments to receive diets containing 0 µg/kg of AFB1, 250 µg/kg of AFB1, or 500 µg/kg of AFB1 for 7, 28, or 70 d. Because performance was most affected in animals administered AFB1 for an extended period, liver samples from d 70 animals were used for RNA-sequencing analysis. Of 82,744 sequences probed, 179 had transcripts that were highly correlated (r ≥ |0.8|; P < 0.0001) with treatment. Of the 179 signifi-

cant transcripts, 46 sequences were negatively and 133 sequences positively related to treatment. Forty-three unique functional groups were identified. Genes within the apoptosis regulation functional group were selected for 1) confirmation of d 70 gene expression differences using real-time reverse-transcription (RT)-PCR (n = 4 genes), and 2) investigation of d 7 expression to identify early responses to AFB1 (n = 15 genes) using real-time RT-PCR. Expression of the 4 apoptosis genes selected for confirmation, cyclin-dependent kinase inhibitor 1A, zinc finger matrin type 3, kininogen 1, and pim-1 oncogene, was confirmed with real-time RT-PCR. Of the 15 genes tested in d 7 liver samples, 4 were differentially expressed: cyclin-dependent kinase inhibitor 1A; zinc finger matrin type 3; tyrosine 3-monooxygenase/ tryptophan 5-monooxygenase activation protein, zeta polypeptide; and apoptosis enhancing nuclease. Results from this study demonstrate that administration of an AFB1-contaminated diet to growing barrows alters hepatic gene expression, and in particular apoptosis genes.

Key words: aflatoxin, gene expression, RNA, sequencing, swine ©2011 American Society of Animal Science. All rights reserved.

INTRODUCTION

J. Anim. Sci. 2011. 89:916–925 doi:10.2527/jas.2010-3473

health and performance. Susceptibility varies with species, age, AFB1 concentration, and duration of exposure (CAST, 2003). Regulatory limits for total AFB1in swine feed are |0.80|; P < 0.0001) between transcript copy number and treatment. GTP = guanosine triphosphate.

correlation coefficient of 0.90, which is comparable with the Griffith et al. (2010) study.

Hepatic Gene Expression: d 7 All 15 apoptosis genes were tested for expression differences in d 7 liver samples using real-time RT-PCR (Table 5). There was an increase (P < 0.001) in expression of CDKN1A in LO and HI barrows compared with CON, but no expression difference (P = 0.057) in HI vs. LO barrows. Greater (P < 0.001) expression of ZMAT3 was observed in LO and HI barrows than in CON barrows; expression of ZMAT3 was additionally greater (P = 0.006) in HI vs. LO barrows. A decrease

(P = 0.038) in expression of YWHAZ was observed in HI barrows compared with CON barrows, but no expression differences (P > 0.282) were observed between CON and LO barrows or between HI and LO barrows. Finally, AEN expression was greater (P < 0.001) in LO and HI barrows compared with CON, and also greater (P = 0.026) in HI barrows than LO. No differences in expression of KNG1 (P ≥ 0.203), PIM1 (P ≥ 0.226), STAT1 (P ≥ 0.439), PP2CB (P ≥ 0.744), BNIP2 (P ≥ 0.130), FEM1B (P ≥ 0.364), CUL2 (P ≥ 0.133), IGF2 (P ≥ 0.091), NDUFS3 (P ≥ 0.167), COL2A1 (P ≥ 0.284), or NAE1 (P ≥ 0.195) genes were observed in d 7 liver samples.

DISCUSSION Apoptosis is a complex process that is necessary for regulating cell survival through removal of diseased or damaged cells. Because of the liver damage, especially DNA damage, caused by AFB1 in swine, changes in activity of genes involved in the apoptosis process would be anticipated.

CDKN1A Upon DNA damage, CDKN1A regulates cell cycle arrest in the p53 checkpoint pathway (Bendjennat et al., 2003). The p53 tumor suppressor is regulated by many apoptosis genes. This transcription factor triggers cell cycle arrest, senescence, and apoptosis; too much can lead to cell death and too little can lead to tumor development (Vilborg et al., 2009). Cyclin-dependent kinase inhibitor 1A binds and inhibits cyclin-dependent kinase activity, preventing phosphorylation of substrates and blocking cell cycle progression. Tumor suppressor p53 can activate CDKN1A, and CDKN1A can be a negative regulator of p53 stability (Broude et al., 2007). Overexpression of CDKN1A causes cell cycle arrest. Damage of DNA and oxidative stress induce CDKN1A expression, and loss of CDKN1A may contribute to tumor suppression through sensitized apoptotic response (Wang et al., 1997). Additionally, phosphorylation of STAT1 leads to enhanced gene expression of CDKN1A (Hikasa et al., 2003). An increase in expression of CDKN1A in LO and HI compared with CON barrows was observed in both d 7 and 70 samples, suggesting that this gene may be an early and constant indicator of apoptosis activity due to AFB1 administration.

ZMAT3 Zinc finger matrin type 3 is regulated by p53 and positively regulates p53 mRNA by binding and stabilizing the mRNA to prevent its deadenylation. This act forms a positive feedback loop and increases p53 protein levels, allowing enhanced p53 response to DNA damage (Vilborg et al., 2009, 2010). The increase in ZMAT3 expression in LO and HI compared with CON barrows at both d 7 and 70 may indicate that, similar

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Apoptosis genes in aflatoxin-treated barrows

Table 2. Expression levels determined by RNA-sequencing of apoptosis-regulating genes with treatment differences (P < 0.05) in transcript copy number1,2,3 Gene4

CON

CDKN1A BNIP2 ZMAT3 AEN IGF2 STAT1 PP2CB CUL2 NAE1 COL2A1 NDUFS3 PIM1 YWHAZ KNG1 FEM1B

187.62 12.22 6.04 109.53 105.57 75.65 192.69 76.04 26.39 67.37 153.80 67.95 258.67 589.33 47.02

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

LO a

1,117.23 22.15 76.72 408.22 42.73 192.26 309.36 135.22 51.76 94.42 212.02 36.50 540.10 303.43 81.88

77.81 1.48a 9.69a 29.94a 6.76a 17.83a 19.50a 8.71a 3.33a 4.55a 10.95a 4.27a 47.76a 27.04a 7.94a

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

HI b

95.30 1.82b 11.86b 36.67b 8.28b 21.84b 23.88b 10.67b 4.08b 5.57b 13.41b 5.23b 58.50b 33.11b 9.72b

1,481.85 31.24 129.43 507.84 28.70 251.85 380.87 159.77 59.38 108.93 252.01 27.26 687.45 292.79 115.99

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

82.53c 1.57c 10.27c 31.75b 7.17b 18.91b 20.68b 9.24b 3.53b 4.82b 11.61b 4.53b 50.66b 28.68b 8.42c

a–c

Within a row, means without a common superscript letter differ (P < 0.05). CON = 0 µg/kg of aflatoxin B1; LO = 250 µg/kg of aflatoxin B1; HI = 500 µg/kg of aflatoxin B1. 2 Least squares means ± SE, units in copy number. 3 n = 9 for each of the CON and HI treatments; n = 6 for LO treatment. 4 Cyclin-dependent kinase inhibitor 1A (CDKN1A), B-cell lymphoma-2/adenovirus E1B 19 kDa interacting protein 2 (BNIP2), zinc finger matrin type 3 (ZMAT3), apoptosis enhancing nuclease (AEN), signal transducer and activator of transcription 1, 91 kDa (STAT1), protein phosphatase 2 (catalytic subunit, β isoform; PP2CB), cullin 2 (CUL2), neural precursor cell-expressed developmentally downregulated 8 activating enzyme E1 subunit 1 (NAE1), collagen (type II, α 1; COL2A1), NADH dehydrogenase (ubiquinone) Fe-S protein 3, 30 kDa (NADH-coenzyme Q reductase; NDUFS3), pim-1 oncogene (PIM1), tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein, zeta polypeptide (YWHAZ), kininogen 1 (KNG1), fem-1 homolog b (FEM1B). 1

to CDKN1A, changes in ZMAT3 expression may be an early sign of apoptotic response to AFB1.

KNG1 Kininogen 1 (high-molecular-weight kininogen) releases nonapeptide bradykinin, released from the interaction with plasma kallikrein. This release causes the generation of 2-chain high-molecular-weight kininogen (HKa), which can induce apoptosis of proliferating endothelial cells and inhibit angiogenesis (Merkulov et al., 2008). Expression of KNG1 was less in HI and LO Table 3. Relative expression of d 70 apoptosis genes selected for confirmation determined by real-time reverse-transcription PCR1,2,3 Gene4 CDKN1A ZMAT3 KNG1 PIM1

CON 1.68 1.75 6.55 9.80

± ± ± ±

LO a

0.42 1.12a 0.51a 1.31a

6.96 7.59 3.08 3.70

± ± ± ±

HI b

0.51 1.37b 0.63b 1.60b

7.50 10.57 2.65 2.64

± ± ± ±

0.42b 1.12b 0.51b 1.31b

a,b Within a row, means without a common superscript letter differ (P < 0.05). 1 CON = 0 µg/kg of aflatoxin B1; LO = 250 µg/kg of aflatoxin B1; HI = 500 µg/kg of aflatoxin B1. 2 Least squares means ± SE, units in relative expression to glyceraldehyde 3-phosphate dehydrogenase (housekeeping gene). 3 n = 9 for each of the CON and HI treatments; n = 6 for LO treatment. 4 Cyclin-dependent kinase inhibitor 1A (CDKN1A), zinc finger matrin type 3 (ZMAT3), kininogen 1 (KNG1), pim-1 oncogene (PIM1).

barrows compared with CON barrows at d 70 only, suggesting that KNG1 apoptotic activity is induced after a more chronic AFB1 exposure.

PIM1 Pim-1 oncogene is a serine/threonine kinase that is involved in cell survival, proliferation, differentiation, apoptosis, and tumorigenesis (Hu et al., 2009). Multiple cytokines activate PIM1 through the JAK/STAT pleiotropic signaling pathway, and PIM1 can also negatively regulate the JAK/STAT pathway. Pim-1 oncogene helps regulate cell apoptosis and anti-apoptotic activity by phosphorylating Bcl-xL/BCL2-associated death promoter (BAD), which is a pro-apoptotic member of the BCL2 family (Hu et al., 2009). Zhang et al. (2007) reported that PIM1 phosphorylates and stabilizes CDKN1A, promoting cell proliferation and contributing to tumorigenesis. In this way, PIM1 is important to antiapoptosis signaling, which may lead to increased tumor growth. In agreement with this, PIM1 expression was decreased and CDKN1A expression increased in LO and HI barrows on d 70, indicating increased apoptosis and decreased cell survival in barrows administered AFB1.

STAT1 Signal transducer and activator of transcription 1 induces apoptosis and is considered a tumor suppressor (Regis et al., 2008). Apoptosis can be regulated

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Table 4. Comparison of fold changes (FC) estimated from RNA-sequencing and real-time reverse-transcription PCR results of 4 apoptosis genes chosen for confirmation RNA-sequencing2 Gene1

Real-time reverse-transcription PCR3

FC4 (LO/CON)

FC5 (HI/CON)

FC6 (HI/LO)

FC4 (LO/CON)

FC5 (HI/CON)

FC6 (HI/LO)

5.95 12.70 −1.94 −1.86

7.90 21.43 −2.02 −2.49

1.33 1.69 −1.04 −1.34

4.14 4.35 −2.13 −2.65

4.47 6.05 −2.47 −3.71

1.08 1.39 −1.16 −1.40

CDKN1A ZMAT3 KNG1 PIM1 1

Cyclin-dependent kinase inhibitor 1A (CDKN1A), zinc finger matrin type 3 (ZMAT3), kininogen 1 (KNG1), pim-1 oncogene (PIM1). n = 9 for CON (0 µg/kg of aflatoxin B1) and HI (500 µg/kg of aflatoxin B1) treatments; n = 6 for LO (250 µg/kg of aflatoxin B1) treatment. 3 n = 9 for each of the CON and HI treatments; n = 6 for LO treatment. 4 Fold change values expressed as a ratio between LO and CON groups, where FC >1.00 indicates an increase in gene expression and FC 1.00 indicates an increase in gene expression and FC 1.00 indicates an increase in gene expression and FC 1.00 indicates an increase in gene expression and FC 1.00 indicates an increase in gene expression and FC 1.00 indicates an increase in gene expression and FC 7 d) may be required for FEM1B activation similar to BNIP2.

Apoptosis enhancing nuclease encodes a DNase that enhances apoptosis by breaking DNA strands (Lee et al., 2005). The p53 gene phosphorylation status regulates AEN expression, and AEN also regulates p53 expression (Kawase et al., 2008). Expression of AEN was greater in LO and HI samples on d 70 and 7, suggesting this gene is important to both the short and long-term response to AFB1.

CUL2

NAE1

Cullin 2 has been predicted to function as a tumor suppressor, but little research investigating the specific role of CUL2 has been conducted. Cullin 2 binds to form a complex that is a subunit on the RNA polymerase II transcriptional machinery, so CUL2 could potentially influence RNA expression (Maeda et al., 2008). In d 70 samples, CUL2 was greater in LO and HI compared with CON samples, but no differences were found in d 7 samples. Expression of CUL2 may require a longer exposure period (>7 d) for activation.

Neural precursor cell-expressed developmentally downregulated 8 activating enzyme E1 subunit 1 is a regulatory subunit for the NEDD8 pathway, which regulates cullin activity and has a role in degradation of proteins important for cell-cycle progression, DNA damage, and stress response (Swords et al., 2010). Overexpression of NAE1 leads to apoptosis due to NEDD8 conjugation deregulation (Soucy et al., 2009). Expression of NAE1 was greater in HI and LO than CON in d 70 samples only, suggesting that activation

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of the NEDD8 pathway may require a more chronic AFB1 exposure.

Conclusions These results demonstrate that hepatic genes are differentially expressed due to consumption of an AFB1contaminated feed. In this study, genes associated with 43 unique functions were responsive to the AFB1 administration. Of specific interest, 15 genes with apoptotic roles, CDKN1A, KNG1, ZMAT3, PIM1, STAT1, PP2CB, YWHAZ, BNIP2, FEM1B, CUL2, IGF2, NDUFS3, COL2A1, AEN, and NAE1, were differentially expressed after long-term AFB1 exposure (70 d). Of those 15 genes, 4 (CDKN1A, ZMAT3, YWHAZ, and AEN) were also differentially expressed after only a short-term exposure (7 d), indicating those genes may be early indicators of an apoptotic response to AFB1. Further study of these differentially expressed genes may lead to prevention strategies and treatments for aflatoxicosis. Additionally, early response genes may prove to be useful for early diagnosis of aflatoxicosis, or even indicate differences in tolerance to dietary AFB1.

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