Mice lacking endogenous TRPV1 express reduced levels of thermogenic proteins and are susceptible to diet-induced obesity and metabolic dysfunction

Abstract

Transient receptor potential vanilloid subfamily 1 (TRPV1) is a non-selective cation channel protein expressed in neuronal and non-neuronal cells. Although TRPV1 is implicated in thermogenesis and diet-induced obesity (DIO), its precise role remains controversial. TRPV1^-/- mice are protected from DIO, while TRPV1 activation enhances thermogenesis to prevent obesity. To reconcile this, we fed wild-type and TRPV1^-/- mice for 32 weeks with normal chow or a high-fat diet and analyzed the weight gain, metabolic activities, and thermogenic protein expression in white and brown fats. TRPV1^-/- mice became obese, exhibited reduced locomotor activity, reduced energy expenditure, enhanced hepatic steatosis, and decreased thermogenic protein expression in adipose tissues. Our data reveal that lack of TRPV1 does not prevent obesity, but rather enhances metabolic dysfunction.

Keywords: TRPV1−/−; adipose tissue; adiposity; fatty liver; metabolism; obesity.

Figure 1.. Effects of NCD and HFD-feeding on weight gain, food/water intake, metabolic, and locomotor activities in NCD or HFD-fed WT and TRPV1^−/− mice.

A. Time courses of body weight gain (g) ± S.E.M. in NCD or HFD-fed WT and TRPV1^−/− mice. B and C. Mean energy/water intake ± S.E.M. intake per day in these mice. D. and E. Hematoxylin and Eosin staining for epididymal white (D) and brown (E) fats of WT and TRPV1^−/− mice. Time courses of VCO2, VO2, and RER in the dark and light cycle (black and white boxes, respectively) for WT (F, G, and H) and TRPV1^−/− (I, J, and K) mice at the end of 32 weeks of feeding NCD or HFD. L. Mean RER ± S.E.M. for these mice. Time courses of energy expenditure in the dark and light cycle for WT (M) and TRPV1^−/− (N) mice. O. Mean energy expenditure ± S.E.M. for these mice. Time courses of locomotor activity in the dark and light cycle for WT (P) and TRPV1^−/− (Q) mice. R. Mean locomotor activity ± S.E.M. for these mice. ** statistical significance at P < 0.05 for n = 12 per condition.

Figure 2.. mRNA levels of adipogenic and thermogenic genes in NCD or HFD-fed WT and TRPV1^−/− mice.

Mean mRNA level ± S.E.M. normalized to 18s in the EF and BF for TRPV1 (A and B), PPARs (C and D), UCP-1 and BMP8b (E and F), PGC-1α, PRDM-16 and SiRT-1 (G and H). Adipose tissue isolated from six mice per condition was pooled and used for the quantification of genes. Five independent experiments were performed for each condition. ** statistical significance at P < 0.05 for n = 6 independent experiments.

Figure 3.. Adipogenic and thermogenic proteins expression in the intrascapular BAT of NCD or HFD-fed WT and TRPV1^−/− mice.

Representative western blot showing the expression of PPARα (A) and PPARγ (B), PGC-1α (C), PRDM-16 (D), SiRT-1 (E), and UCP-1 (F) in the intrascapular BAT of NCD or HFD fed WT and TRPV1^−/− mice.

Figure 4.. Adipogenic and thermogenic protein expression in the subcutaneous (inguinal) WAT of NCD or HFD-fed WT and TRPV1^−/− mice.

Representative western blot showing the expression of PPARα (A) and PPARγ (B), PGC-1α (C), PRDM-16 (D), SiRT-1 (E), and UCP-1 (F) in the inguinal WAT of NCD or HFD fed WT and TRPV1^−/− mice.

Figure 5.. HFD-induced fatty liver is enhanced in TRPV1^−/− mice.

A. Hematoxylin and Eosin staining showing hepatic steatosis in NCD or HFD fed WT (top panel) and TRPV1^−/− (bottom panel) mice (n = 6 per condition). B. Oil red O staining in the liver sections of WT (top panel) and TRPV1^−/− mice (bottom panel) (n = 6 per condition). C. Steatosis and ballooning scores for the liver sections of WT and TRPV1^−/− mice. Scale bar represents 50 μm. ** statistical significance at P < 0.05 for n = 5 per condition.

Figure 6.. HFD feeding impairs glucose handling and causes glucose intolerance and hypercholesterolemia in WT and TRPV1^−/− mice.

Mean fasting blood glucose ± S.E.M (A; n = 6) mean fasting serum cholesterol ± S.E.M (B; n = 6) and mean plasma GLP-1 ± S.E.M (C; n = 6) in the WT and TRPV1 at the end of 32 weeks of feeding NCD or HFD. D. IPGTT in WT and TRPV1^−/− mice (n = 6). E. The area under the curve ± S.E.M for these mice. ** statistical significance at P < 0.05 for n = 6 per condition.