Fig 1.
Schematic overview of the selected lipogenic genes and their function.
Adipogenesis is characterized by the expression of PPARγ, C/EBP-family members and SREBP. Insulin, glucocorticoids or IGF stimulate the expression of C/EBPβ, which in turn stimulates the expression of EBPα and PPARγ. Upon stimulation, PPARγ forms a heterodimer with RXR and together with EBPα regulate the expression of genes involved in adipogenesis and lipogenesis [26]. C/EBPα and C/EBPβ also regulate the expression of 11β-HSD1 and 11β-HSD2 (which catalyse the activation and deactivation, respectively, of cortisol and regulate glucocorticoid metabolism) [40] and of IGF-I and IGF-II (which regulate growth, cell proliferation and development) [41, 42]. SREBP1 and ChREBP, are stimulated upon insulin and glucose secretion, respectively, and regulate synergistically the expression of the de novo lipogenesis enzymes FASn and ACCα [43] that catalyse fatty acid synthesis. SREBP1 also regulates the expression of PPARγ [44]. Fatty acid catabolism through β-oxidation in peroxisomes is catalysed by ACOX1 [45]. Excess fatty acids derived from food, break-down from stored fat or synthesized through de novo lipogenesis are converted to triglycerides through a pathway the last step of which is catalysed by the DGAT2 enzyme. The transcriptional regulation of DGAT2 is also regulated by EBPβ and to a lesser extent by EBPα [46].
Fig 2.
Simplified schematic overview of the function of the major organs involved in lipid homeostasis.
The liver has a central role in the synthesis, metabolism and distribution of glucose and fatty acids. The adipose tissue is the principal site of excess energy storage in the form of fat (triglycerides) and liberates fatty acids upon demand. The muscle is the major site of lipid oxidation and energy expenditure. The brain acts as a lipid sensor; it receives signals from the other organs to adjust energy homeostasis by changing behaviour (e.g. food intake and expenditure). When energy consumption exceeds expenditure excess triglycerides are stored not only in adipose tissue but in muscle and liver as well, leading to metabolic disorders [48, 49].
Table 1.
Morphological parameters of male and female zebrafish following chronic waterborne exposure (9 months) to tributyltin (TBT).
Values are mean±SEM (n = 15); CF and HSI are the condition factor and hepatosomatic index, respectively; **p<0.05 and *<0.1 (compared to solvent control; one-way ANOVA followed by Dunnett's test)
Fig 3.
TBT effects on hepatic triglycerides.
The effect of TBT on hepatic triglyceride levels in male and female zebrafish following chronic exposure (9 months) to waterborne 10 and 50 ng/L of TBT (as Sn). Values as mean ± SEM (n = 4). * *p<0.05, *p<0.1 compared to control (one-way ANOVA, followed by Dunnett’s test).
Fig 4.
TBT in vivo induction of lipogenic genes in the liver.
qRT-PCR analysis of selected lipogenic transcription factors and metabolizing enzymes in the liver of (A) male, and (B) female zebrafish following chronic exposure (9 months) to waterborne 10 and 50 ng/L of TBT (as Sn). Values were normalized to rpl8 and expressed as the average fold changes ± SEM (n = 8) of the solvent control group. **p<0.05 and *p<0.1 compared to solvent control (one-way ANOVA, followed by Dunnett’ test).
Fig 5.
TBT in vivo induction of lipogenic genes in the brain.
qRT-PCR analysis of selected lipogenic transcription factors and metabolizing enzymes in the brain of (A) male, and (B) female zebrafish following chronic exposure (9 months) to waterborne 10 and 50 ng/L of TBT (as Sn). Values were normalized to β-actin and expressed as the average fold changes ± SEM (n = 7) of the solvent control group. **p<0.05 and *p<0.1 compared to solvent control (one-way ANOVA, followed by Dunnett’s test).
Fig 6.
Principal Component Analysis (PCA) in the liver.
PCA applied to the gene expression data of the lipogenic transcription factors and metabolizing enzymes in the liver of male and female zebrafish exposed to waterborne 10 and 50 ng/L of TBT (as Sn) for 9 months. Data are presented against PC1 and PC2 as (A) a loading plot of the 14 studied genes and (B) a scores’ plot of each treatment group of males and females separately.
Fig 7.
Principal Component Analysis (PCA) in the brain.
PCA applied to the gene expression data of the lipogenic transcription factors and metabolizing enzymes in the brain of male and female zebrafish exposed to waterborne 10 and 50 ng/L of TBT (as Sn) for 9 months. Data are presented against PC1 and PC2 as (A) a loadings plot of the 14 studied genes and (B) a scores’ plot of each treatment group of males and females separately.
Fig 8.
Summary of the TBT-altered genes in the liver and brain of zebrafish.
Schematic representation summary of genes that were either up- or down-regulated (p<0.1) in males (blue), females (pink), both genders (yellow) or neither (blank) in (A) the liver and (B) the brain of zebrafish following chronic waterborne exposure (9 months) to either 10 or 50 ng/L TBT (as Sn). The connectors indicate the genes that are related.