High-fat feeding-induced hyperinsulinemia increases cardiac glucose uptake and mitochondrial function despite peripheral insulin resistance

Abstract

In obesity, reduced cardiac glucose uptake and mitochondrial abnormalities are putative causes of cardiac dysfunction. However, high-fat diet (HFD) does not consistently induce cardiac insulin resistance and mitochondrial damage, and recent studies suggest HFD may be cardioprotective. To determine cardiac responses to HFD, we investigated cardiac function, glucose uptake, and mitochondrial respiration in young (3-month-old) and middle-aged (MA) (12-month-old) male Ldlr(-/-) mice fed chow or 3 months HFD to induce obesity, systemic insulin resistance, and hyperinsulinemia. In MA Ldlr(-/-) mice, HFD induced accelerated atherosclerosis and nonalcoholic steatohepatitis, common complications of human obesity. Surprisingly, HFD-fed mice demonstrated increased cardiac glucose uptake, which was most prominent in MA mice, in the absence of cardiac contractile dysfunction or hypertrophy. Moreover, hearts of HFD-fed mice had enhanced mitochondrial oxidation of palmitoyl carnitine, glutamate, and succinate and greater basal insulin signaling compared with those of chow-fed mice, suggesting cardiac insulin sensitivity was maintained, despite systemic insulin resistance. Streptozotocin-induced ablation of insulin production markedly reduced cardiac glucose uptake and mitochondrial dysfunction in HFD-fed, but not in chow-fed, mice. Insulin injection reversed these effects, suggesting that insulin may protect cardiac mitochondria during HFD. These results have implications for cardiac metabolism and preservation of mitochondrial function in obesity.

Figure 1.

Ldlr^−/− mice get systemic insulin resistance with WHFD. A and B, Body weight (A) and percent body fat (B) increased in young and MA Ldlr^−/− mice after 3 months of WHFD. C–E, Fasting glucose (C) and insulin (D) levels and homeostatic model assessment of insulin resistance (HOMA-IR) (E) were measured before and after 3 months of WHFD in both age groups. F, WHFD reduced insulin tolerance in young and MA Ldlr^−/− mice in an insulin tolerance test. Results are shown as mean ± SEM. For A and B: *, P < .05; **, P < .01; ***, P < .005 vs age-matched chow; †††, P < .005 vs young for matched diet; n = 10 young, n = 7 MA-chow, n = 8 MA-WHFD; for C and D: *, P < .05 vs age-matched chow and matched diet duration, †, P < .05 vs young for matched diet and duration of diet, n = 5 per group; for F: *, P < .05 vs chow for young mice; †, P < .05 vs chow for MA mice by Mann-Whitney U test, n = 11 young-chow, n = 8 young-WHFD, n = 7 MA-chow, n = 13 MA-WHFD.

Figure 2.

WHFD increases FDG uptake in heart. A, Chow- and WHFD-fed young and MA Ldlr^−/− mice were imaged for basal FDG uptake by PET after 3 months of diets. B and C, Quantification of cardiac FDG uptake (B) and insulin-stimulated cardiac FDG uptake (C). Results are shown as mean ± SEM. For B: *, P < .05 vs age-matched chow, †, P < .05 vs young for matched diet, n = 4 young, 3 MA-chow, and 5 MA-WHFD; for C: *, P < .05 vs chow for matched treatment and age; †, P < .05 vs no insulin (−Ins) for matched diet by Mann-Whitney U test; n = 3 for all chow mice, n = 4 for all WHFD mice.

Figure 3.

WHFD enhances mitochondrial function in heart. Panels A–C, Respiration of isolated heart mitochondria (panel A), RCRs (panel B), and calculated P to O ratios (panel C) for PC substrate. Panels D–G, Respiration rates of isolated heart mitochondria in response to GM (panel D), succinate (panel E), rotenone after GM plus succinate (panel F), and coupled respiration rates (panel G) derived from oligomycin treatment: O₂ flux per mass ([GM + succinate + ADP] − oligomycin). For detailed protocols, see Supplemental Figure 1. Panels H and I, Mitochondrial DNA content of heart (panel H) and liver (panel I). Results are shown as mean ± SEM. For panels A, D, and E: *, P < .05 vs age-matched chow for a given state of respiration; †, P < .05; ††, P < .01 vs state 2 within same age group for matching diet; for panels B, C, and F–I: *, P < .05; **, P < .01 vs age-matched chow; †, P < .05; ††, P < .01 vs young for matched diet by Mann-Whitney U test. For panels A–G: n = 4 young-chow, n = 5 young-WHFD, n = 4 MA-chow, n = 5 MA-WHFD; for panels H–I, n = 5 per group. Abbreviations: C, chow; HF, HFD.

Figure 4.

Mitochondrial protein expression is maintained in WHFD despite increased oxidative stress and inflammation. A, Protein expression of NDUFB8, SDHB, UQCRC2, ATP5a, and HADHA. B and C, Mitochondrial protein carbonyls (B) and expression of inflammatory gene Cd68 (C) were measured in chow and WHFD-fed MA Ldlr^−/− mice. Results are shown as mean ± SEM. For A, n = 5 per group; for B, n = 4 chow and 7 WHFD; and for C, n = 5 chow and 8 WHFD. *, P < .05; ***, P < .005 vs chow by Mann-Whitney U test.

Figure 5.

Hearts of WHFD-fed mice are insulin-sensitive. A–C, Protein expression of phospho-Tyr612-IRS1 (A), phospho-Ser473-AKT (B), and phospho-GSK3β (C) increased with HFD in hearts from overnight-fasted MA Ldlr^−/− mice. Results are shown as mean ± SEM; n = 5 chow and 7 WHFD. *, P < .05 vs chow by Mann-Whitney U test.

Figure 6.

Insulin is important for cardiac glucose uptake and mitochondrial function in WHFD-fed mice. A–C, Blood glucose (A), insulin (B), and cardiac FDG uptake (C) were determined after PBS or STZ treatment to Ldlr^−/− mice given chow or WHFD. D–F, Mitochondrial respiration in response to PC (D), GM (E), and succinate (F) substrates were suppressed by STZ but restored with insulin in WHFD-fed mice. Respiration values are normalized to PBS in WHFD samples. G and H, Protein expression of UQCRC2 and ATP5a (G) and gene expression of Glut1, Glut4, and Pdk4 (H). Results are shown as mean ± SEM. For A–C: *, P < .05; **, P < .01 vs PBS for matched diet; †, P < .05; ††, P < .01 vs chow for matched treatment (n = 4 chow-PBS, 5 chow-STZ, 5 WHFD-PBS, and 6 WHFD-STZ); for D–F: *, P < .05 vs PBS for matched diet; †, P < .05 vs STZ for matched diet (n = 3 STZ and 4 for all other groups; for G and H: *, P < .05 vs PBS; †, P < .05; ††, P < .01 vs STZ by Mann-Whitney U test (n = 4 per group). P = .057 for PC STZ vs STZ plus insulin (STZ+Ins); P = .057 for succinate PBS vs STZ for WHFD and STZ vs STZ+Ins for WHFD.

Figure 7.

Increased cardiac glucose uptake and mitochondrial respiration are not unique to Ldlr^−/− mice. Young (3-month-old) C57BL6 mice were given either a chow or 60% LHFD (Research Diets D12492) for 3 months. A–F, LHFD-induced changes in body weight (A), body fat (B), glucose (C), insulin (D), cholesterol (E), and FDG uptake (F) in heart measured by PET imaging. G–I, Cardiac mitochondrial function using substrates PC (G), pyruvate malate (H), and succinate (I) were measured in isolated mitochondria. Results are shown as mean ± SEM. For A–F, n = 5 per group; for G–I, n = 7 chow and 8 LHFD. *, P < .05; **, P < .01 vs chow (A–F) and vs chow for same state (G–I); †††, P < .005 vs state 2 for matched diet (G–I) by Mann-Whitney U test.