SR-BI deficiency disassociates obesity from hepatic steatosis and glucose intolerance development in high fat diet-fed mice
Scavenger receptor BI (SR-BI) has been suggested to modulate adipocyte function
- In this study, we compared the metabolic phenotype of wild-type and SR-BI deficient mice fed an obesogenic diet enriched in fat.
- Both males and females gained significantly more weight in response to 12 weeks high fat diet feeding compared to a normal diet, and plasma free cholesterol and triglyceride levels were 2-fold higher in both genders.- Interestingly, the exacerbated obesity was paralleled by better glucose handling in both sexes.
Scavenger receptor class B type I (SR-BI)
- SR-BI is a membrane-associated glycoprotein that can bind native lipoprotein species such as HDL, LDL, and VLDL
- It is highly expressed in hepatocytes and steroidogenic tissues, where it mediates the selective uptake of cholesteryl esters from HDL
- In addition to their function in adipocyte cholesterol homeostasis, HDL has also been implicated in glucose and triglyceride metabolism
- A defect in white adipocyte functionality can predispose to obesity and metabolic pathologies such as type 2 diabetes, nonalcoholic fatty liver disease, and cardiovascular disease
Mice
- Age-matched 8- to 11-week old male and female C57BL/6 wild-type mice (N = 10 and N = 9), both of which were provided ad libitum with high fat diet #12451 from Research Diet Service BV, The Netherlands containing the fat component lard, the protein components casein and cysteine, and the carbohydrate sources sucrose, lodex 10, and corn starch
- Body weight was monitored weekly
- No significant effect of the genotype on food intake was detected
- After 10 weeks of diet feeding, all mice were fasted overnight and subsequently given an oral 2 g/kg glucose bolus in water
Plasma measurements
- Concentrations of free cholesterol and cholesteryl esters were measured using standard colorimetric assays
- The cholesterol distribution over the different lipoproteins was analyzed by fractionation of 30 µl plasma using a Superose 6 column (3.2 × 30 mm, Smart-system, Pharmacia).
- Total cholesterol content of the effluent was determined using the same colorimetrical assays.
Lipid extraction and quantification
- Triglyceride content was determined by the use of a Nonidet P 40 Substitute-based extraction protocol, essentially as described by Nahon et al.
- Cholesterol was extracted from liver tissue using the Folch extraction method
- The concentration of free cholesterol and cholesteryl esters was determined using the enzymatic colorimetric assays
- Free cholesterol and cholesterol were corrected for the protein input
Histological analysis
- Gonadal white adipose tissue and livers were embedded in paraffin and sectioned at 5 µm thickness
- Paraffin sections were stained with hematoxylin and eosin for imaging on a Leica DMRE microscope
- Mean lesion area of each individual mouse was calculated from 5 cryosections
Gene expression analysis
- Total RNA was isolated by the acid guanidinium thiocyanate-phenol chloroform extraction method and reverse-transcribed using RevertAid reverse transcriptase.
Data analysis
- Statistical analysis was performed using Graphpad Instat software (San Diego, USA, http://www.graphpad.com).
Results
- Age-matched SR-BI knockout mice and C57BL/6 wild-type control mice of both genders were fed a commonly used high fat (45% of energy) lard diet for 12-13 weeks to stimulate the development of obesity
- Wild-type male mice gained 33 ± 5% of weight versus 24 ± 3% in wild type females
- Both groups of mice weighed more than their female counterparts at baseline, with no significant baseline effect of either genotype being present.
- SRBI deficiency was associated with an exacerbated body weight gain in both male and female mice, while the less extensive elevation in plasma cholesteryl esters in females (+39%) failed to reach significance (Fig. 2A).
- Plasma free cholesterol levels were increased to a similar extent (~2-fold) in both males and females, independent of the genotype and independent of sex (two-way ANOVA; P <.001 for genotype; P >.05 for interaction)
- A highly significant correlation between gonadal white adipose tissue weights and respective body weights was detected in both genotypes
Discussion
- SR-BI deficiency in high fat diet-fed mice is associated with a decrease in PPARgamma regulated genes in adipocytes, a higher predisposition to obesity and atherosclerotic lesion formation, and protection against the development of fatty liver disease and glucose intolerance.
- An interesting finding of our studies was that the reduced hepatic triglyceride content after overnight fasting was not secondary to de novo lipogenesis, but rather generated via the flux of fatty acids from adipose tissue to the liver upon liberation during adipocyte triglyceride hydrolysis (Figure 7).
- To our knowledge, no reports have described a difference in body weight development or glucose tolerance between low fat, nonadipogenic, and nonobesogenic (regular chow) diet fed mice.
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