To study the effect of Angptl4 on the metabolic response to dietary fat, we performed the oral fat load with C18:3 triglyceride in wild-type, Angptl4−/− and Angptl4 transgenic (Angptl4-Tg) mice. In agreement with inhibition of LPL by Angptl4, the postprandial increase in plasma triglyceride was dramatically increased in Angptl4-Tg mice, whereas it was entirely blunted in Angptl4−/− mice (Figure 4A). Consistent with LPL inhibition, Angptl4 overexpression markedly reduced cardiac fatty acid uptake from [3H]triolein-labeled very-low- density lipoprotein (VLDL)-like particles (Figure 4B). These results suggest that upregulation of Angptl4 by dietary fatty acids will lead to reduced cardiac uptake of fatty acids via inhibition of LPL, thereby suppressing the stimulus that led to induction of Angptl4 expression.
Figure 4: Angptl4 overexpression raises postprandial plasma triglyceride levels and decreases cardiac fatty acid uptake. A. Wild-type (gray squares), Angptl4−/− (open squares), and Angptl4-Tg (black squares) mice were given a single oral gavage of synthetic triglyceride (TG) composed entirely of C18:3. Plasma triglycerides were determined in blood collected via the tail vein. B. [3H]-labeled triolein was incorporated into VLDL-like emulsion particles and directly injected into the tail vein of wild-type and Angptl4-Tg mice. The heart was collected 30 minutes after the injection for determination of radioactivity. *Significantly different between wild-type and Angptl4- Tg mice according to Student’s t test (P<0.05). Error bars represent SEM.
To examine whether the inhibitory effect of Angptl4 on cardiac fatty acid uptake is associated with reduced fatty acid-induced oxidative stress, expression of Fkbp5, Lcn2, and Gsta3 was determined 6 hours after oral gavage with either control treatment (CMC) or C18:3 triglyceride in wild-type, Angptl4−/− and Angptl4-Tg mice. All three genes represent markers of oxidative stress [28–30]. Consistent with a protective role of Angptl4 against fatty acid-induced oxidative stress, the magnitude of induction of Fkbp5, Lcn2, and Gsta3 by C18:3 was dependent on Angptl4 genotype and inversely correlated with Angptl4 expression (Figure 5A and 5B). Expression of Fkbp5, Lcn2, and Gsta3 after the oral fat load was not related to plasma free fatty acid (FFA) levels, which
Figure 5: Markers of oxidative stress are inversely correlated with Angptl4 expression after oral fat load. Wild-type, Angptl4−/−, and Angptl4-Tg mice were given a single oral gavage of 0.5% CMC (open bars) or synthetic triglyceride composed entirely of C18:3 (closed bars). Mice were euthanized 6 hours later. A. mRNA expression levels of oxidative stress genes Fkbp5, Lcn2, and Gsta3 and endoplasmic reticulum stress marker gene Herpud1, as determined by real-time PCR. B. mRNA expression levels of Angptl4. C. Plasma levels of FFA and TG. Differences were evaluated statistically using two-way ANOVA. Significance (probability value) of effect of genotype (G), treatment (T), and interaction (I) between genotype and treatment is indicated in each graph. n.s. indicates non significant. Error bars represent SEM.
Finally, we examined whether Angptl4 may exert a similar effect in the context of a chronic fat overload. To that end, we measured expression of the oxidative stress markers and performed immunohistochemical and quantitative analysis of 4-HNE protein adducts in wild-type and Angptl4−/− mice fed a HFD for 8 weeks. 4-HNE is one of the major biologically active aldehydes formed during inflammation and oxidative stress. Formation of 4-HNE protein adducts is a marker for lipid peroxidation.
Although high fat feeding did not influence expression levels of Fkbp5, Lcn2 and Gsta3, expression was significantly higher in Angptl4−/− mice fed HFD compared to wild-type mice fed HFD (Figure 6A). A similar tendency was observed for Herpud1. Furthermore, lipid peroxidation was increased in Angptl4−/− mice fed HFD, as shown by enhanced 4-HNE staining (Figure 6B). These results were supported by quantitative analysis of 4HNE protein adducts (Figure 6C) and MDA adducts (Figure 6D), which were significantly increased in Angptl4−/− mice fed HFD compared to wild-type mice fed HFD. These data indicate that Angptl4 protects against oxidative stress in the context of a chronic fat overload. No differences in cardiac triglyceride levels were observed between wild-type and Angptl4−/− mice (Figure 6E).
Figure 6: Angptl4 protects against oxidative stress in the context of chronic fat overload. A. Wild-type and Angptl4−/− mice were fed a LFD or HFD for 8 weeks. Expression levels of Fkbp5, Lcn2, Gsta3, and Herpud1 in the heart were measured by real-time PCR. Results are expressed as fold change compared to wild-type mice on LFD. B. Immunohistochemistry of 4-HNE protein adducts in mouse cardiac tissue from mice fed HFD for 8 weeks. Representative sections are shown. Magnification, ×400. Right, Negative control obtained using rabbit serum. Quantitative measurement of 4HNE protein adducts (C) and MDA adducts (D). E. Cardiac triglyceride content. Gray bars indicate wild-type mice; black bars, Angptl4−/− mice. *Significantly different between wild-type and Angptl4−/− mice according to Student’s t test (P<0.05). Error bars represent SEM.
Discussion
In the present article we show that the gene most significantly and consistently upregulated by short term treatment with dietary fatty acids is Angptl4. Induction of Angptl4 by dietary fatty acids is mediated by PPARβ/δ and confers a protective effect against fatty acid-induced oxidative stress by restricting cardiac fatty acid uptake via inhibition of LPL. Overall, our data suggest that upregulation of Angptl4 by fatty acids is part of a feedback mechanism aimed at preventing myocardial fatty acid accumulation, thereby minimizing lipid-induced oxidative stress and lipotoxicity (Figure 7). Although our follow-up studies only included linolenic acid, the results are likely generalizable to other dietary fatty acids.
Lipotoxicity describes the untoward consequences of fat overload in a particular tissue and may be related to fatty acid-induced oxidative stress, accumulation of lipotoxic intermediates such as ceramides and fatty acyl-CoA, and excess storage of triglycerides [31]. Chronic lipotoxicity in the heart has been shown to promote cardiomyopathy in several animal models [12,32–34]. Most of these models are characterized by a mismatch between myocardial fatty acid uptake and utilization, as in mice with heart-specific overexpression of acyl-CoA synthetase, fatty acid transport protein 1, or lipoprotein lipase [32– 34]. Although triglycerides are unlikely to be the actual culprit in cardiac lipotoxicity, they may be guilty by association as its levels may be positively correlated with lipotoxic intermediates. However, we did not see increased cardiac triglyceride levels in Angptl4−/− mice compared to wild-type mice after 8 weeks of high fat feeding, suggesting that the increase in fatty acid uptake is limited or that the incoming fatty acids are efficiently oxidized. In the present study, it was not possible to investigate the effect of Angptl4 deletion on cardiac lipid storage, oxidative stress, and parameters of cardiac dysfunction after a more prolonged period of HFD, as a cachectic phenotype progressively emerges after 12 weeks of HFD [53].
body Angptl4 overexpression and deletion models, strictly we cannot rule out that the observed effects may be related to changes in extracardiac Angptl4 expression. However, the current literature mainly supports a paracrine function of Angptl4, and the role of Angptl4 as endocrine factor remains somewhat uncertain.
Angptl4 was discovered by screening for target genes of PPARα and PPARγ in liver and adipose tissue, respectively [35,36]. It is member of a family of angiopoietins and angiopoietin-like proteins and is produced by a variety of organs. Numerous studies using Angptl4 transgenic or knock-out mice have invariably shown a stimulatory effect of Angptl4 on plasma triglyceride levels, which is achieved by inhibiting LPL activity [13,14,19,22,37–40]. These data have established Angptl4 as an important regulator of plasma triglyceride levels. The present data suggest that Angptl4 is upregulated by dietary fatty acids to inhibit local LPL activity and consequently reduce fatty acid uptake and lipid-induced oxidative stress.
To study the role of PPARs in cardiac gene regulation by dietary fatty acids, we ideally should have used cardiomyocyte-specific PPAR−/− mice but unfortunately we did not have access to these animals. The PPAR-dependent upregulation of Angptl4 and Ucp3 by dietary linolenic acid underscores the importance of TG-rich lipoproteins as source of PPAR ligands in the heart, which are liberated via LPL [41,42] Remarkably, upregulation of Angptl4 by C18:3 in intact heart and cultured cardiomyocytes was entirely mediated by PPARβ/δ and not PPARα. These data were supported by ChIP data showing C18:3-induced binding of PPARβ/δ and not PPARα to the Angptl4 gene in the intact heart. Because we did not study PPARγ, strictly we cannot rule out a role for PPARγ in mediating the effect of (dietary) fatty acids on cardiac Angptl4 expression. However, as similar results were obtained in cultured cardiomyocytes, which express little PPARγ, the data favor a specific role of PPARβ/δ in the regulation of cardiac Angptl4 by C18:3. It is unclear what mechanism may underlie the differential role of PPARβ/δ versus PPARα in mediating induction of Angptl4 and Ucp3 by C18:3, respectively. Induction of Ucp3 by linoleic acid via PPARα rules out a PPARβ/δ-specific ligand activity that is generated by linolenic acid in heart. Gel shift and transactivation studies have failed to provide convincing evidence for the existence of response elements or promoters that are specifically or selectively bound or regulated by a particular PPAR isotype [43,44]. Indeed, in vitro experiments have revealed
that all three PPARs are intrinsically able to (trans)activate the human and mouse Angptl4 gene [25]. However, the situation may be different in vivo in the absence of PPAR overexpression or when PPARs are activated via endogenous ligands rather than via high-affinity synthetic agonists. Thus, the dominant receptor in the regulation of a particular PPAR target is likely context- and tissue-dependent and additionally depends on whether PPAR is activated via endogenous or synthetic agonists. When as in cardiomyocytes two or more PPARs are expressed in the same cell and are simultaneously activated, it is possible that specific binding of one PPAR isotype to a particular PPRE is promoted via interactions with another protein that binds adjacent to the PPRE and is expressed in a tissue-specific manner. In this context, it is interesting to mention that recent genome wide profiling of PPARα and PPARγ binding sites revealed colocalization of PPAR binding with other transcription factor binding sites and demonstrated interplay between PPARs and other transcription factors in PPAR-mediated gene regulation [45–47].
Recently, targeted PPARβ/δ overexpression in the heart was shown to have a clear differential effect on cardiac metabolism compared to PPARα overexpression [48]. In contrast to PPARα, PPARβ/δ overexpression did not impact fatty acid transport and failed to induce myocardial lipid accumulation. Based on the data presented here it can be hypothesized that PPARβ/δ is neutral toward cardiac lipid storage by inducing Angptl4 expression, which in turn feeds back on fatty acid uptake.
Multiple studies support an effect of Angptl4 on endothelial function, mostly pointing to an antiangiogenic activity of Angptl4 [49–51]. Our immunohistochemical results indicate that Angptl4 is absent from vascular endothelial cells in the heart, whereas it is abundantly present in cardiomyocytes. These data are in line with previous studies showing that Angptl4 is absent from a number of different endothelial cells, yet is dramatically induced under hypoxic conditions [50]. Hypoxia also upregulates Angptl4 in cardiomyocytes [52]. Induction of Angptl4 by hypoxia and the associated inhibition of fatty acid uptake may be an adaptive mechanism to shift
overload and consequently fatty acid–induced oxidative stress, one of the hallmarks of lipotoxic cardiomyopathy.
Figure 7: Model of the role of Angptl4 in the heart. Triglycerides arrive at the heart packaged into chylomicrons or VLDL particles. Plasma triglycerides are hydrolyzed by lipoprotein lipase to release fatty acids, that are taken up by the cardiomyocyte. Excess uptake of fatty acids gives rise to oxidative stress and leads to induction of Angptl4 expression via PPARβ/δ. Angptl4 will feed back on fatty acid uptake by inhibiting lipoprotein lipase.
Sources of Funding
This study was supported by the Nutrigenomics Consortium, TI Food and Nutrition, Marie Curie Research Training Network NucSys, and the Netherlands Heart Foundation (2007B046).
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