Cyclic GMP signaling in cardiomyocytes modulates fatty acid trafficking and prevents triglyceride accumulation

Abstract

While the balance between carbohydrates and fatty acids for energy production appears to be crucial for cardiac homeostasis, much remains to be learned about the molecular mechanisms underlying this relationship. Given the reported benefits of cGMP signaling on the myocardium, we investigated the impact of its chronic activation on cardiac energy metabolism using mice overexpressing a constitutively active cytoplasmic guanylate cyclase (GC +/0 ) in cardiomyocytes. Ex vivo working GC +/0 heart perfusions with 13 C-labeled substrates revealed an altered pattern of exogenous substrate fuel selection compared to controls, namely a 38±9% lower contribution of exogenous fatty acids to acetyl-CoA formation, while that of carbohydrates remains unchanged despite a two-fold increase in glycolysis. The lower contribution of exogenous fatty acids to energy production is not associated with changes in energy demand or supply (contractile function, oxygen consumption, tissue acetyl-CoA or CoA levels, citric acid cycle flux rate) or in the regulation of β-oxidation (acetyl-CoA carboxylase activity, tissue malonyl-CoA levels).However, GC +/0 hearts show a two-fold increase in the incorporation of exogenous oleate into triglycerides. Furthermore, the following molecular data are consistent with a concomitant increase in triglyceride hydrolysis: (i) increased abundance of hormone sensitive lipase (HSL) protein (24±11%) and mRNA (22±4%) as well as (ii) several phosphorylation events related to HSL inhibitory (AMPK) and activation (ERK 1/2) sites, which should contribute to enhance its activity. These changes in exogenous fatty acid trafficking in GC +/0 hearts appear to be functionally relevant, as demonstrated by their resistance to fasting-induced triglyceride accumulation. While the documented metabolic profile of GC +/0 mouse hearts is partly reminiscent of hypertrophied hearts, the observed changes in lipid trafficking have not been

Mouse strain differences in metabolic fluxes and function of ex vivo working hearts

“…All mice had similar body weight and heart wet weight (in grams, heart wet weight: 129S6/SvEvTac, 0.28 Ϯ 0.03; SJL/JCrNTac, 0.25 Ϯ 0.03; C57Bl/6J, 0.28 Ϯ 0.03; and C57Bl/6NCrl, 0.28 Ϯ 0.03; body weight: 129S6/SvEvTac, 25.58 Ϯ 0.40; SJL/JCrNTac, 25.61 Ϯ 0.44; C57Bl/6J, 25.54 Ϯ 0.37; and C57Bl/6NCrl, 25.56 Ϯ 0.54). It is noteworthy that the measured heart wet weights are consistent with our previously published data (26,27,30), but because oedema occurs following ex vivo heart perfusion with a crystalloid perfusion buffer, a factor of 8.9 Ϯ 0.2 (n ϭ 29) had previously been determined for conversion of measured gram wet weight to gram dry weight (41).…”

“…For our comparison, we have used our well-established model of normoxic working mouse hearts perfused ex vivo with 13 C-labeled substrates under physiologically relevant conditions of substrate supply and energy demand (18,26,27,30,41). The main advantages of this model are that it allows for detailed and simultaneous measurements of 1) the dynamics of cardiac energy substrate metabolism, information that is not accessible from static measurements of mRNA, protein expression or even enzymatic activity and 2) contractile function.…”

“…Fatty acids (FA) from heart tissue triglycerides (TG) were analyzed according to a previously described, modified gas chromatography-mass spectrometry method, which included lipid extraction and TG isolation, the addition of mixed internal and external standards for quantification ( ]docosanoic acid), and FA transmethylation before sample injection into a Agilent 6890 N gas chromatograph equipped with a Varian CP7420 polar capillary column (100 m; 0.25-mm internal diameter; 0.23-m thickness) coupled with a 5973 detector operated in positive chemical ionization mode with ammonia as reagent gas (18,27). 13 C-enrichment of OLE in TG was assessed by monitoring at mass-over-charge (m/z) 314 (for unlabeled OLE) and 315 (for [1-13 C]OLE).…”

“…Our previous work with various transgenic mouse models does, however, provide some evidence of differences among commonly used control mouse strains, particularly in the partitioning of exogenous long-chain fatty acids (LCFA) for oxidation and storage between hearts from 129S6/SvEvTac versus various C57Bl/6 or C57Bl/10 substrains (17,18,26,27,30). These results were all obtained in our validated experimental paradigm, namely, ex vivo perfusion in working mode, with concomitant evaluation of myocardial contractility and metabolic fluxes with 13 C-labeled substrates.…”

Oxytocin as an Inducer of Cardiomyogenesis

“…Recent reports indicate that BNP and ANP activity is associated with lipolysis and postprandial lipid oxidation (Birkenfeld et al, 2008). Both hormones modulate fatty acid trafficking and prevent triglyceride accumulation in CMs via cGMP signaling (Khairallah et al, 2008). The different cardioprotective actions of OT were recently demonstrated in animal models of myocardial infarction (MI).…”