Chronic consumption of farmed salmon containing persistent organic pollutants causes insulin resistance and obesity in mice

Abstract

Background: Dietary interventions are critical in the prevention of metabolic diseases. Yet, the effects of fatty fish consumption on type 2 diabetes remain unclear. The aim of this study was to investigate whether a diet containing farmed salmon prevents or contributes to insulin resistance in mice.

Methodology/principal findings: Adult male C57BL/6J mice were fed control diet (C), a very high-fat diet without or with farmed Atlantic salmon fillet (VHF and VHF/S, respectively), and Western diet without or with farmed Atlantic salmon fillet (WD and WD/S, respectively). Other mice were fed VHF containing farmed salmon fillet with reduced concentrations of persistent organic pollutants (VHF/S(-POPs)). We assessed body weight gain, fat mass, insulin sensitivity, glucose tolerance, ex vivo muscle glucose uptake, performed histology and immunohistochemistry analysis, and investigated gene and protein expression. In comparison with animals fed VHF and WD, consumption of both VHF/S and WD/S exaggerated insulin resistance, visceral obesity, and glucose intolerance. In addition, the ability of insulin to stimulate Akt phosphorylation and muscle glucose uptake was impaired in mice fed farmed salmon. Relative to VHF/S-fed mice, animals fed VHF/S(-POPs) had less body burdens of POPs, accumulated less visceral fat, and had reduced mRNA levels of TNFα as well as macrophage infiltration in adipose tissue. VHF/S(-POPs)-fed mice further exhibited better insulin sensitivity and glucose tolerance than mice fed VHF/S.

Conclusions/significance: Our data indicate that intake of farmed salmon fillet contributes to several metabolic disorders linked to type 2 diabetes and obesity, and suggest a role of POPs in these deleterious effects. Overall, these findings may participate to improve nutritional strategies for the prevention and therapy of insulin resistance.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Mice fed VHF/S developed obesity and insulin resistance.

In two separate studies, mice fed C (n = 14), VHF (n = 43) or VHF/S diet (n = 38) were monitored for 8 weeks and assayed for various metabolic parameters. (A) Body weight gain (14–43 mice per group). (B) Energy intake (14–43 mice per group). (C) Quantification of adipose tissue. Total fat pad includes epididymal, retroperitoneal and inguinal fat pad (7–16 mice per group). (D) H&E staining showing representative morphology of adipocyte in epididymal fat of animals (4–5 mice per group). (E) Glucose tolerance test. Glucose was injected and blood glucose was assessed at indicated time points (7–13 mice per group). (F) Glucose-stimulated insulin release. Plasma insulin levels were measured before and 15 min after injection of glucose in mice (8–12 mice per group). (G) Insulin tolerance test. Random-fed mice were injected with insulin and blood glucose assessed at indicated time points (7–13 mice per group). (H) Blood glucose (4–7 mice per group) (I) Plasma insulin (4–6 mice per group). (J) Muscle glucose uptake. Ex vivo soleus muscles were incubated without or with insulin and glucose uptake assessed (7–12 mice per group). (K) In vivo insulin signaling. Overnight fasted animals (n = 4–5 per group) were injected with insulin or saline and expression of Akt and pAkt in gastrocnemius muscles was assessed. Graphic depicts densitometric analysis of normalization of pAkt/Akt protein. Representative western blots of muscle lysates are shown for phosphorylated Akt (Ser473) without or with insulin stimulation, and for total Akt expression after saline injection. Western blot analyses were repeated at least three times. (L) Triacylglyceride (TAG) concentrations in gastrocnemius muscles (6–12 mice per group). *p<0.05 vs. C. **p<0.03 vs. VHF.

Figure 2. Intake of WD/S exacerbated obesity and insulin resistance.

In two separate studies, mice fed C (n = 8), WD (n = 15) or WD/S (n = 15) diet were monitored for 6 weeks. (A) Body weight gain (8–15 mice per group). (G) Energy intake (8–15 mice per group). (C) Quantification of adipose tissue. Total fat pad includes epididymal, retroperitoneal and inguinal fat pad (4–6 mice per group). (D) Representative H&E staining (upper panel) and immunohistochemical detection of the macrophage-specific antibody F4/80 (lower panel) in epididymal fat (4–5 mice per group). Note the abundance of macrophages (arrows) surrounding adipocytes, crown-like structures, in epididymal fat of WD/S-fed animals. (E) Blood glucose and (F) plasma insulin was determined in random-fed and fasted mice (4–7 mice per group). (G) Glucose tolerance test. Glucose tolerance test was performed by injection of glucose in fasted mice and blood glucose was assessed at indicated time points (4–7 mice per group). (H) Insulin tolerance test. Insulin tolerance test was performed by injection of insulin in random-fed mice and blood glucose was assessed at indicated time points (4–7 mice per group). (I) Muscle glucose uptake. Glucose uptake was assessed in ex vivo soleus muscles incubated without or with insulin (4–6 mice per group). (J) TAG concentrations in gastrocnemius muscles (4-6 mice per group). *p<0.05 vs. C. **p<0.04 compared with WD.

Figure 3. POPs modulated the outcomes of farmed salmon intake.

In three separate studies, mice fed VHF/S (n = 31) and VHF/S-POPs (n = 25) for 8 weeks were screened for insulin resistance-induced metabolic disorders. (A) Concentrations of 7PCBs and DDTs in epididymal fat of animals (5 mice per group). (B) Body weight gain (25–31 mice per group). (C) Quantification of adipose tissue. Total fat pad includes epididymal, retroperitoneal and inguinal fat pad (8–11 mice per group). (D) Representative H&E staining (upper panel) and immunohistochemical detection of the macrophage-specific antibody F4/80 (lower panel) in epididymal fat (4–5 mice per group). Note the important infiltration of macrophages in epididymal fat (arrows) of mice fed VHF/S compared with VHF/S-POPs . (E) Real-time PCR determination of mRNA expression of Mac-2a, iNOS, TNFα and IL-6 in epididymal fat (5 mice per group). (F) Glucose tolerance test. Mice were injected with glucose and blood glucose assessed at indicated time points (8–13 mice per group). (G) Glucose-stimulated insulin release. Plasma insulin levels were measured before and 15 min after glucose injection (6–10 mice per group). (H) Insulin tolerance test. Random-fed mice were injected with insulin and blood glucose assessed at indicated time points (8–16 mice per group). (I) Muscle glucose uptake. Ex vivo soleus muscles were incubated without or with insulin, and glucose uptake assessed (6–12 mice per group). (J) TAG concentrations in gastrocnemius muscles (6–10 mice per group). (K) 3T3-L1 preadipocytes were treated with a weak differentiation cocktail containing cortisone and exposed to the organochlorine pesticide pp′-DDE. Graphic shows fold stimulation of lipid accumulation quantified by Oil red O staining. Results are expressed relative to vehicle-treated cells for three independent experiments. *p<0.05 vs. VHF/S or vehicle-treated cells.