The energy metabolic alterations associated with diabetic cardiomyopathy include excessive rates of myocardial fatty acid uptake and oxidation coupled with reduced glucose utilization. In an attempt to model the metabolic derangements of the diabetic heart, we have generated mice with cardiac-specific overexpression of the nuclear receptor PPARα. This strategy was employed so that the impact of altered myocardial energy metabolism on cardiac function could be assessed in the absence of the systemic diabetic state. We found that the MHC-PPAR mice exhibit a cardiac metabolic phenotype that is strikingly similar to that of the diabetic heart. Several lines of evidence support this conclusion. First, as in the diabetic heart, the expression of genes involved in the myocardial fatty acid utilization pathway is activated in MHC-PPAR hearts. Second, a reciprocal downregulation of the myocardial glucose transport, glycolytic, and glucose oxidation pathways occurs in the transgenic mice. Third, fatty acid uptake and oxidation rates are increased whereas glucose uptake and oxidation is abnormally suppressed in the hearts of MHC-PPAR mice. Lastly, the transgenic mice exhibit evidence of altered cardiac myocyte lipid balance as has been described for the diabetic heart.
The cardiac function of MHC-PPAR mice was evaluated to determine whether, as speculated, energy metabolic derangements lead to cardiomyopathy. We found that the MHC-PPAR hearts exhibit ventricular hypertrophy in association with activated expression of hypertrophic gene markers. Moreover, as occurs in severe forms of diabetic cardiomyopathy, the MHC-PPAR mouse lines with the highest level of transgene expression developed LV chamber dilatation and systolic ventricular dysfunction. Taken together, these results strongly suggest that the energy metabolic shifts of the diabetic heart become maladaptive and contribute to the development of cardiac dysfunction.
An important question raised by our results relates to the mechanistic link between altered myocardial energy metabolism and cardiac dysfunction in the diabetic heart. It is possible that in the context of high-level fatty acid uptake and mitochondrial β-oxidation, toxic lipid intermediates accumulate within cardiac myocytes. For example, a mismatch between the rates of fatty acid import and oxidation may result in cardiac myocyte lipid accumulation such as that observed in MHC-PPAR mice, which could cause contractile dysfunction or cell death. Indeed, previous studies have shown that lipid often accumulates in the myocardium of diabetic animals (7
). A recent study by Chiu et al. (10
) has shown that myocardial lipid accretion due to increased fatty acid uptake leads to myocyte apoptosis and cardiomyopathy — a “lipotoxic” effect. However, we did not find evidence for increased myocardial apoptosis in the MHC-PPAR mice (data not shown). Altered mitochondrial function could also play a role in the cardiac dysfunction observed in the MHC-PPAR mice. For example, increased mitochondrial flux may lead to the generation of harmful reactive oxygen species. The observed increase in UCP3 gene expression in the MHC-PPAR and diabetic heart (25
) may also relate to the development of cardiac dysfunction. However, the role of UCP3 as a bona fide mitochondrial respiratory uncoupler in heart remains unclear. Increased respiratory uncoupling could actually be adaptive in the setting of increased mitochondrial flux, such as occurs in the diabetic and MHC-PPAR heart. Lastly, reliance on FAO for ATP production, which results in higher mitochondrial oxygen consumption costs compared with glycolysis and glucose oxidation, could also contribute to ventricular dysfunction.
Reduced myocardial glucose utilization may also account for the observed cardiac dysfunction in the MHC-PPAR mice. Previous studies of the ischemic and reperfused heart indicate that a reduction in glycolysis and glucose oxidation is associated with diminished ventricular function (30
). In addition, the hearts of GLUT4-deficient mice develop profound dysfunction during ischemia (33
). The mechanisms for these observations are unknown but could involve distinct intracellular channeling routes of ATP produced by glucose oxidation or effects related to oxygen consumption efficiency.
The MHC-PPAR hearts exhibit several interesting lipid metabolic alterations in addition to the expected increase in FAO rates. First, the microPET studies demonstrated increased myocardial palmitate uptake. Secondly, TAGs accumulate in MHC-PPAR cardiac myocytes following a short-term fast, which serves to acutely increase circulating NEFA levels. This latter observation is interesting because TAG accumulates in the diabetic heart (7
). The results of the PET and fasting studies suggest that increased myocardial fatty acid uptake leads to a positive myocyte lipid balance in the MHC-PPAR heart. However, the mechanism involved in the increased myocardial fatty acid uptake remains unknown, because the fasting-induced increase in the expression of proteins involved in fatty acid import was not significantly greater in MHC-PPAR hearts. In addition, we cannot exclude the possibility that lipid synthesis pathways, including GPAT and DGAT, are activated directly by PPARα in the MHC-PPAR heart. However, we have not found evidence that GPAT or DGAT is a PPARα target gene based on expression levels in fasted PPARα–/–
mice (B.N. Finck and D.P. Kelly, unpublished data).
A surprising observation in this study relates to the dramatically altered expression of genes involved in glucose transport, glycolysis, and glucose oxidation in the hearts of MHC-PPAR mice. The inhibitory effects of FAO on glucose utilization in heart were first described as the “glucose fatty-acid cycle” by Randle et al. (34
). This effect involves inhibition of the pyruvate dehydrogenase complex by elevated acetyl-CoA/CoA and NADH/NAD+
ratios generated by increased β-oxidative flux. However, the reduction in myocardial glucose utilization observed in MHC-PPAR mice likely involves mechanisms distinct from that proposed for the Randle cycle, because it includes a gene regulatory component and involves effects on glucose transport. Fatty acid–induced insulin resistance has been well described and has been proposed to play a role in the development of type 2 diabetes (35
). Recent evidence indicates that fatty acid–induced insulin resistance involves alterations at the level of glucose transport secondary to derangements in insulin signaling (37
). Our results suggest that glucose transport can be inhibited in heart as a result of primary alterations in cellular lipid metabolism driven by PPARα. Consistent with the notion that increased cellular lipid uptake can serve as the initial trigger for alterations in insulin sensitivity, Kim et al. (39
) recently demonstrated that transgenic overexpression of lipoprotein lipase in skeletal muscle caused insulin resistance in nondiabetic mice.
The derangements in cardiac glucose utilization we observed in MHC-PPAR mice include a gene regulatory component. It is unlikely that all of the observed gene regulatory effects occur directly via PPARα, because the GLUT4 and PFK genes are not known PPARα targets. However, expression of PDK4 has been shown to be activated by PPARα ligands (40
). It is likely that GLUT4 and PFK gene expression is repressed via PPARα-independent transcriptional regulatory pathways linked to alterations in cellular energy metabolism induced by PPARα. The observation that glucose uptake and utilization can be altered in heart secondary to PPARα-mediated increases in fatty acid utilization suggests the intriguing possibility that some forms of insulin resistance or type 2 diabetes could be due to alterations in components of the PPARα regulatory complex or downstream genes involved in cellular fatty acid utilization.