Increased BAT Thermogenesis in Male Mouse Apolipoprotein A4 Transgenic Mice

Abstract

Dietary lipids induce apolipoprotein A4 (APOA4) production and brown adipose tissue (BAT) thermogenesis. Administration of exogenous APOA4 elevates BAT thermogenesis in chow-fed mice, but not high-fat diet (HFD)-fed mice. Chronic feeding of HFD attenuates plasma APOA4 production and BAT thermogenesis in wildtype (WT) mice. In light of these observations, we sought to determine whether steady production of APOA4 could keep BAT thermogenesis elevated, even in the presence of HFD consumption, with an aim toward eventual reduction of body weight, fat mass and plasma lipid levels. Transgenic mice with overexpression of mouse APOA4 in the small intestine (APOA4-Tg mice) produce greater plasma APOA4 than their WT controls, even when fed an atherogenic diet. Thus, we used these mice to investigate the correlation of levels of APOA4 and BAT thermogenesis during HFD consumption. The hypothesis of this study was that overexpression of mouse APOA4 in the small intestine and increased plasma APOA4 production would increase BAT thermogenesis and consequently reduce fat mass and plasma lipids of HFD-fed obese mice. To test this hypothesis, BAT thermogenic proteins, body weight, fat mass, caloric intake, and plasma lipids in male APOA4-Tg mice and WT mice fed either a chow diet or a HFD were measured. When fed a chow diet, APOA4 levels were elevated, plasma triglyceride (TG) levels were reduced, and BAT levels of UCP1 trended upward, while body weight, fat mass, caloric intake, and plasma lipids were comparable between APOA4-Tg and WT mice. After a four-week feeding of HFD, APOA4-Tg mice maintained elevated plasma APOA4 and reduced plasma TG, but UCP1 levels in BAT were significantly elevated in comparison to WT controls; body weight, fat mass and caloric intake were still comparable. After 10-week consumption of HFD, however, while APOA4-Tg mice still exhibited increased plasma APOA4, UCP1 levels and reduced TG levels, a reduction in body weight, fat mass and levels of plasma lipids and leptin were finally observed in comparison to their WT controls and independent of caloric intake. Additionally, APOA4-Tg mice exhibited increased energy expenditure at several time points when measured during the 10-week HFD feeding. Thus, overexpression of APOA4 in the small intestine and maintenance of elevated levels of plasma APOA4 appear to correlate with elevation of UCP1-dependent BAT thermogenesis and subsequent protection against HFD-induced obesity in mice.

Apolipoprotein A4 Elevates Sympathetic Activity and Thermogenesis in Male Mice

“…The current experiments showed that APOA4 infusion did not raise whole-body energy expenditure in lean mice during LFD feeding although it increased BAT thermogenesis and did not enhance IWAT thermogenesis. Consistently with previous findings [22][23][24]52], the present study indicated that APOA4 infusion did not reduce daily caloric intake in the LFD-fed mice. When fed a standard diet, APOA4-KO or APOA4-Tg mice and their controls utilize the same energy substrates for heat production [23,24].…”

“…IWAT contains brown adipocytes with thermogenic capability [51]. Consistently with previous findings in APOA4-KO or transgenic mice with an overexpression of mouse APOA4 in the small intestine (APOA4-Tg mice) when maintained on a chow diet [23,24], the present experiments demonstrated that APOA4 infusion did not alter IWAT mass. In addition, APOA4 infusion was not able to enhance UCP1 or ATGL levels in IWAT through the induction of sympathetic activity.…”

“…Whole-body energy expenditure in lean APOA4-KO mice or lean APOA4-Tg mice was comparable to that of their wildtype controls during a standard diet feeding [23,24]. The current experiments showed that APOA4 infusion did not raise whole-body energy expenditure in lean mice during LFD feeding although it increased BAT thermogenesis and did not enhance IWAT thermogenesis.…”

“…The membrane was incubated in a 5% blotting-grade blocker solution (Bio-Rad Laboratories) and then incubated with one of the following antibodies diluted in 5% bovine serum albumin: beta-tubulin (1:1000), UCP1 (1:1000), ATGL (1:1000), CD36 (1:1000), CPT1 (#15184-1-AP, 1:5000, Proteintech Group, Inc., Rosemont, IL, USA), CPT2 (#26555-1-AP, 1:1000, Proteintech group, Inc.), beta-actin (#66009-lg, 1:10,000, Proteintech Group, Inc.), polyclonal APOA4 (#PA5-14554, 1:1000, Invitrogen, Waltham, MA, USA) or APOA1 (#PA5-29557, 1:1000, Invitrogen) antibody. After incubation with the primary antibody overnight at 4 • C, the immunoblots were washed and then incubated with horseradish peroxidase (HRP)-conjugated secondary antibody (#p044901-2, 1:6000, Dako Cytomation, Santa Clara, CA, USA) for 1 h at 25 • C. Detection of the proteins involved visualizing them with the enhanced chemiluminescence system (Immobilon western chemiluminescent HRP substrate, EMD Millipore Corporation, Billerica, MA, USA) and a C-Digit blot scanner (Li-Cor Biosciences, Lincoln, NE, USA) according to our published protocols [20,23,24]. For tissue proteins, the average band intensity of each protein was measured.…”

“…APOA4 regulates BAT thermogenesis, lipid transport, glucose metabolism, cholesterol efflux, inflammation, and thrombosis, and protects against the development of atherosclerosis and obesity [13,[18][19][20][21]. APOA4 also serves as a short-term satiating protein that reduces meal size and increases meal frequency but does not alter daily caloric intake [22,23]. In the presence of dietary lipids, reduced norepinephrine (NE) synthesis, impaired UCP1dependent BAT thermogenesis, and decreased energy expenditure are found in mice with a global knockout of APOA4 (APOA4-KO mice) [24].…”