Overexpression of apolipoprotein A5 in mice is not protective against body weight gain and aberrant glucose homeostasis

Abstract

Apolipoprotein A5 (APOA5) is expressed primarily in the liver and modulates plasma triglyceride levels in mice and humans. Mice overexpressing APOA5 exhibit reduced plasma triglyceride levels. Because there is a tight association between plasma triglyceride concentration and traits of the metabolic syndrome, we used transgenic mice overexpressing human APOA5 to test the concept that these mice would be protected from diet-induced obesity and insulin resistance. Male and female transgenic and wild-type mice on the FVB/N genetic background were fed standard rodent chow or a diet rich in fat and sucrose for 18 weeks, during which time clinical phenotypes associated with obesity and glucose homeostasis were measured. We found that APOA5 transgenic (A5tg) mice were resistant to diet-induced changes in plasma triglyceride but not total cholesterol levels. Body weights were similar between the genotypes for females and males, although male A5tg mice showed a modest but significant increase in the relative size of inguinal fat pads. Although male A5tg mice showed a significantly increased ratio of plasma glucose to insulin, profiles of glucose clearance as evaluated after injections of glucose or insulin failed to reveal any differences between genotypes. Overall, our data showed that there was no advantage to responses to diet-induced obesity with chronic reduction of plasma triglyceride levels as mediated by overexpression of APOA5.

Figure 1

Plasma total triglyceride (A) and cholesterol (B) levels for mice fed rodent chow or the high fat/high sucrose (HFHS) diet for 18 weeks. Wild-type (open bars) and APOA5 transgenic (A5tg) (filled bars) mice were fasted for 4 hours in the morning prior to bleeding via the retro-orbital sinus and plasma lipids were quantified as described in Methods and Materials using colorimetric kits purchased from commercial vendors. Plasma triglyceride levels for A5tg mice were significantly lower than wild-type values for both sexes and diets. Male wild-types but not females showed increased triglyceride levels with the HFHS diet. Data are presented as Mean ± SE for 6–10 mice per group; between genotypes as *p<0.05, ***p<0.0001; between diets as †p<0.05, ††p<0.005, †††p<0.0001; between sexes as ‡p<0.03, ‡‡p<0.002.

Figure 2

Lipoprotein profiles for wild-type (WT) and APOA5 transgenic (A5tg) mice fed the HFHS diet for 18 weeks. Equal plasma aliquots were pooled from 4 mice in each sex and sex group and lipoproteins separated using fast-performance liquid chromatography as described in Methods and Materials. (A) Triglyceride and (B) cholesterol contents are presented for male (square) and female (circle) mice of strains A5tg (open symbols) and FVB (filled symbols) mice. VLVL, IDL, LDL and HDL elution profiles are indicated.

Figure 3

Fat depot weights as a percent of body weight (A) and lipase activity values (B) for male mice fed diets for 18 weeks. Wild-type (open bars) and A5tg (filled bars) mice were killed after 4 hours of fasting and individual fat depots were removed and weighed. Adipose tissue fat pads are given as a percent of total body weight. Adipose tissue lipase activity measurements were performed as described in Methods and Materials. Briefly, tissues were homogenized and neutral lipase activity was determined using radiolabeled triolein and using rat serum as a source of apoC-II as described in the text. Activity units are given as nmol free fatty acid hydrolyzed/min/ml of lysate (nmol/min/ml). Data are presented as Mean ± SE for 6–13 mice per group. Significance between genotypes is given as *p<0.05 and between diet as †p<0.025, ††p<0.002.

Figure 4

Glucose tolerance (A) and insulin sensitivity (B) curves for mice fed the HFHS diet for 16 or 14 weeks, respectively. For glucose tolerance, mice were fasted overnight, gAavaged with a 25% glucose solution (in sterile PBS) at a dose of 2 g glucose/kg body weight and blood glucose monitored over 120 minutes. For insulin sensitivity, mice were fasted overnight, injected intraperitoneally with 0.1 U/ml Humulin R insulin in sterile PBS at a dose of 0.5 units insulin/kg body weight. Wild-type (WT) (open symbols) and APOA5 transgenic (A5tg) (filled symbols) mice show comparable responses between genotypes for both male (square) and female (circle) mice. Values are presented as percent changes with respect to starting plasma glucose levels.