Dietary methylmercury and fatty acids affect the lipid metabolism of adipose tissue and liver in rainbow trout.

Methylmercury (MeHg) is a pervasive environmental contaminant in aquatic ecosystems that can reach elevated concentrations in fish of high trophic levels, such as salmonids. The present study aims at investigating the individual and combined impacts of dietary MeHg and fatty acids on lipid metabolism in juvenile rainbow trout (Oncorhynchus mykiss) with a focus on two key organs, adipose tissue and liver. MeHg and fatty acids are both known to act on energy homeostasis although little is known about their interplay on lipid metabolism in fish. Fish were fed diets enriched in linoleic acid (LA, 18:2 n-6), α-linolenic acid (ALA, 18:3 n-3), eicosapentaenoic acid (EPA, 20:5 n-3) or docosahexaenoic acid (DHA, 22:6 n-3) for ten weeks, with the addition of MeHg to the diets during the last six weeks (0, 2.4 or 5.5 mg MeHg/kg dry matter). LA and ALA are polyunsaturated fatty acids (PUFA) typical of plant-derived oils whereas EPA and DHA are n-3 long chain PUFA largely found in fish oil, all used in feed formulation in aquaculture. The results showed that the LA-enriched diet induced a higher whole-body lipid content compared to the three other diets. On the contrary, the addition of MeHg led to a significant reduction of the whole-body lipid content, regardless of the diet. Interestingly, the adipocytes were larger both in presence of LA, compared to EPA and DHA, or MeHg, indicating a lipogenic effect of these two compounds. No effect was, however, observed on lipid accumulation per gram of adipose tissue. The fatty acid composition of adipose tissue and liver was significantly modified by the dietary lipids, reflecting both the fatty acid composition of the diets and the high bioconversion capacity of the rainbow trout. Exposure to MeHg selectively led to a release of n-6 PUFA from the hepatic membranes of fish fed the LA-enriched diet, showing a disruption of the pathways using n-6 PUFA. This study highlights the significant impact of MeHg exposure and dietary fatty acids on lipid metabolism in fish. Further investigation is needed to elucidate the underlying mechanisms and to explore the potential involvement of other organs.

Methylmercury displays pro-adipogenic properties in rainbow trout preadipocytes.

Methylmercury (MeHg) is a ubiquitous contaminant largely found in aquatic environments, especially in species at high trophic level such as salmonids. The aim of this study was to evaluate the effects of MeHg on adipocyte differentiation and lipid metabolism in rainbow trout. Primary cultured preadipocytes were exposed to increasing concentrations of MeHg during six days with or without a hormonal cocktail. Main results showed a dose-dependent intracellular accumulation of neutral lipids with a preferential uptake of n-3 polyunsaturated fatty acids. Interestingly, this accumulation occurred after a fairly low uptake of MeHg by preadipocytes and was maintained after the cellular exposure to MeHg. In membrane phospholipids, arachidonic acid (20:4 n-6) was released in a dose-dependent manner. At the transcriptional level, the expression of several adipocyte-specific genes (perilipin 2 and apolipoprotein Eb) as well as lipid-related genes (fatty acid synthase and fatty acid binding protein 11a) was up-regulated in preadipocytes exposed to MeHg. These results highlight for the first time the disrupting effect of MeHg in trout adipocyte metabolism, providing new insights regarding the role of environmental pollutants in adipose tissue dysfunction and related pathologies.

In vitro Lipolysis and Leptin Production of Elephant Seal Blubber Using Precision-Cut Adipose Tissue Slices.

Adipose tissue plays key roles in energy homeostasis. Understanding its metabolism and regulation is essential to predict the impact of environmental changes on wildlife health, especially in fasting-adapted species. However, in vivo experimental work in wild vertebrates can be challenging. We have developed a novel in vitro approach of precision-cut adipose tissue slices from northern elephant seal (Mirounga angustirostris) as a complementary approach to whole animal models. Blubber biopsies were collected from 14 pups during early and late post-weaning fast (Año Nuevo, CA, United States), precision-cut into 1 mm thick slices and maintained in culture at 37°C for at least 63 h. The slices exhibited an efficient response to ß-adrenergic stimulation, even after 2 days of culture, revealing good in vitro tissue function. The response to lipolytic stimulus did not vary between regions of outer and inner blubber, but was higher at early than at late fast for inner blubber slices. At early fast, lipolysis significantly reduced leptin production. At this stage, inner blubber slices were also more efficient at producing leptin than outer blubber slices, especially in the non-lipolytic condition. This model will aid the study of adipose tissue metabolism and its response to environmental stressors in marine mammals.

PCB-153 shows different dynamics of mobilisation from differentiated rat adipocytes during lipolysis in comparison with PCB-28 and PCB-118.

BACKGROUND: Polychlorinated biphenyls (PCBs) are persistent organic pollutants. Due to their lipophilic character, they are preferentially stored within the adipose tissue. During the mobilisation of lipids, PCBs might be released from adipocytes into the bloodstream. However, the mechanisms associated with the release of PCBs have been poorly studied. Several in vivo studies followed their dynamics of release but the complexity of the in vivo situation, which is characterised by a large range of pollutants, does not allow understanding precisely the behaviour of individual congeners. The present in vitro experiment studied the impact of (i) the number and position of chlorine atoms of PCBs on their release from adipocytes and (ii) the presence of other PCB congeners on the mobilisation rate of such molecules. METHODOLOGY/PRINCIPAL FINDINGS: Differentiated rat adipocytes were used to compare the behaviour of PCB-28, -118 and -153. Cells were contaminated with the three congeners, alone or in cocktail, and a lipolysis was then induced with isoproterenol during 12 hours. Our data indicate that the three congeners were efficiently released from adipocytes and accumulated in the medium during the lipolysis. Interestingly, for a same level of cell lipids, PCB-153, a hexa-CB with two chlorine atoms in ortho-position, was mobilised slower than PCB-28, a tri-CB, and PCB-118, a penta-CB, which are both characterised by one chlorine atom in ortho-position. It suggests an impact of the chemical properties of pollutants on their mobilisation during periods of negative energy balance. Moreover, the mobilisation of PCB congeners, taken individually, did not seem to be influenced by the presence of other congeners within adipocytes. CONCLUSION/SIGNIFICANCE: These results not only highlight the obvious mobilisation of PCBs from adipocytes during lipolysis, in parallel to lipids, but also demonstrate that the structure of congeners defines their rate of release from adipocytes.

Efficient in vitro adipocyte model of long-term lipolysis: a tool to study the behavior of lipophilic compounds.

The triglycerides (TGs) stored in the white adipose tissue are mobilized during periods of negative energy balance. To date, there is no in vitro model of adipocytes imitating a long period of negative energy balance in which triglycerides are highly mobilized. Such model would allow studying the mobilization of TGs and lipophilic compounds trapped within the adipose tissue (e.g., pollutants and vitamins). The present study aims at developing a performing long-term in vitro lipolysis in adipocytes, resulting in a significant decrease of TG stores. Lipolysis was induced on differentiated rat adipocytes by a lipolytic medium with or without isoproterenol for 12 h. The condition with isoproterenol was duplicated, once with medium renewal every 3 h and once without medium renewal. Adding isoproterenol efficiently triggered lipolysis in a short time (3 h). However, a single stimulation by isoproterenol, without medium renewal, was not sufficient to reduce the TG content during a longer term (12 h). A reesterification of fatty acids occurred after a few hours of lipolysis, resulting in a novel increase of cellular lipids. Regular medium renewal combined with repeated isoproterenol stimulations led to almost emptied cells after 12 h. However, medium renewal without isoproterenol stimulation for 12 h was as efficient in terms of lipid mobilization. Our study demonstrates that, over a short-term period, isoproterenol is required to exert a significant lipolytic effect on adipocytes. During a long-term period, the presence of isoproterenol is no longer essential. Instead, medium renewal becomes the main factor involved in cell emptying. The efficiency of this protocol was demonstrated by visual tracking of the cells and by monitoring the dynamics of release of a lipophilic compound, PCB-153, from adipocytes during lipolysis.