MED13-dependent signaling from the heart confers leanness by enhancing metabolism in adipose tissue and liver

Abstract

The heart requires a continuous supply of energy but has little capacity for energy storage and thus relies on exogenous metabolic sources. We previously showed that cardiac MED13 modulates systemic energy homeostasis in mice. Here, we sought to define the extra-cardiac tissue(s) that respond to cardiac MED13 signaling. We show that cardiac overexpression of MED13 in transgenic (MED13cTg) mice confers a lean phenotype that is associated with increased lipid uptake, beta-oxidation and mitochondrial content in white adipose tissue (WAT) and liver. Cardiac expression of MED13 decreases metabolic gene expression in the heart but enhances them in WAT. Although exhibiting increased energy expenditure in the fed state, MED13cTg mice metabolically adapt to fasting. Furthermore, MED13cTg hearts oxidize fuel that is readily available, rendering them more efficient in the fed state. Parabiosis experiments in which circulations of wild-type and MED13cTg mice are joined, reveal that circulating factor(s) in MED13cTg mice promote enhanced metabolism and leanness. These findings demonstrate that MED13 acts within the heart to promote systemic energy expenditure in extra-cardiac energy depots and point to an unexplored metabolic communication system between the heart and other tissues.

Keywords: energy homeostasis; mediator complex; metabolic flexibility; metabolic gene expression; metabolism.

Figure 1. Cardiac overexpression of MED13 increases lipid metabolism in white adipose tissue

1. Increased whole-body [³H]-triolein clearance in MED13cTg mice after a 3-h fast.

2. Increased lipid ([³H]-triolein) uptake in MED13cTg white adipose tissue (WAT) after a 3-h fast.

3. Increased lipid ([³H]-triolein) oxidation in MED13cTg WAT and liver after a 3-h fast.

Data information: Data are mean ± SEM, t-test, n = 8–12.

Figure 2. Cardiac overexpression of MED13 increases metabolic gene expression and mitochondria content in white adipose tissue

1. Enhanced expression of genes involved in fatty acid uptake and oxidation and Krebs cycle in MED13cTg WAT determined by RNA deep sequencing, n = 3.

2. Increased mitochondrial protein expression detected by Western blot analysis in MED13cTg WAT, n = 3. Data are mean ± SEM, t-test.

3. Mitochondrial content demonstrated by transmission electron microscopy in MED13cTg WAT. Top scale bars, 5 μm; bottom scale bars, 1 μm.

Source data are available online for this figure.

Figure 3. Overexpression of cardiac MED13 alters liver fatty acid metabolism

1. A

   Liver from fed MED13cTg mice on normal chow accumulate less lipids than WT, indicated by decreased oil red O staining. Top scale bars, 100 μm; bottom scale bars, 20 μm.

2. B

   MED13cTg liver mitochondria have higher basal oxygen consumption rates, measured using the XF24 Extracellular Flux Analyzer. Data are mean ± SEM, t-test, n = 5.

3. C–D

   MED13cTg liver mitochondria have (C) decreased steady-state levels of short, medium, and long-chain acylcarnitine, and (D) decreased levels of Acyl-CoA species, measured by metabolomics.

Figure 4. Overexpression of cardiac MED13 alters cardiac metabolism

1. A

   Expression of fatty acid oxidation and Krebs cycle genes in MED13cTg heart determined by RNA deep sequencing, n = 3.

2. B–C

   MED13cTg hearts have decreased (B) acylcarnitines and (C) long-chain acyl-CoA species in the fed state, as measured by metabolomics, n = 5.

3. D–F

   In conditions simulating the fed state, MED13cTg hearts displayed (D) normal coronary flow rates, (E) decreased oxygen consumption, and (F) decreased long-chain fatty acid (LCFA) substrate oxidation rates, as determined by Langendorff heart perfusions and ¹³C NMR analysis, n = 4–6.

4. G–I

   In conditions simulating the fasted state, MED13cTg hearts displayed (G) normal coronary flow rates, (H) increased oxygen consumption, and (I) increased endogenous substrate oxidation, as determined by Langendorff heart perfusions and ¹³C NMR analysis.

5. J

   In conditions simulating the fed state, when glucose is readily available, the oxidation ratio shifts towards a preference for glucose in MED13cTg hearts. In conditions simulating the fasted state, when LCFAs are more readily available, oxidation ratios are similar in WT and MED13cTg hearts, n = 4–6.

Data information: Data are mean ± SEM, t-test.

Figure 5. MED13cTg mice maintain the ability to adapt metabolically to fasting

1. A–C

   Non-esterified free fatty acids (NEFA) (A), triglycerides (B), and glucose levels (C) in serum from fed and fasted mice.

2. D–F

   Acylcarnitine species in (D) heart, (E) liver, and (F) serum from fed and fasted mice.

3. G–I

   Amino acids in (G) serum, (H) liver, and (I) heart from fed and fasted mice, n = 5.

Data information: Data are mean ± SEM, two-way ANOVA followed by Tukey's test, *P < 0.05 TG fed versus WT fed, ^#P < 0.05 fed versus fasted.

Figure 6. Circulating factor(s) regulate enhanced WAT and liver metabolism and contribute to the lean phenotype of MED13cTg mice

1. A

   Heterotypic parabiotic pairs gain less weight than isotypic WT parabiotic pairs.

2. B

   Heterotypic TG parabiots weigh the same as isotypic TG parabiots, while heterotypic WT parabiots weigh significantly less than isotypic WT parabiots.

3. C

   Schematic of isotypic and heterotypic parabiosis.

4. D,E

   Expression of (D) fatty acid oxidation genes and (E) Krebs cycle genes in WAT of parabiots as determined by qPCR. 18S rRNA was used for normalization.

5. F,G

   Oxygen consumption is significantly increased in mitochondria isolated from (F) WAT and (G) liver of isotypic TG and heterotypic TG and WT parabiots, n = 6 pair (12 mice) per group.

6. H

   Cardiac MED13 increases lipid uptake in adipose tissue, and increases lipid oxidation and decreases lipid storage in both adipose tissue and liver. Inter-organ communication in MED13cTg mice is controlled by circulating factors that enhance WAT and liver metabolism resulting in a lean phenotype.

Data information: Data are mean ± SEM, two-way ANOVA followed by Dunnett's test, *P < 0.05 versus isotypic WT-WT parabiots.