In the heart, FAs contribute approximately 70% of the ATP necessary for normal heart function. During diabetes, there is an increased FA supply to the diabetic heart to compensate for the diminished contribution of glucose as an energy source due to insulin deficiency. Because of the dramatic increase in FA influx compared to FA oxidation, FA and TG accumulation occurs in the heart[3, 23], leading to lipotoxicity. Consistent with these reports, we found that although the activity of total and peroxisomal fatty acid β-oxidation was increased, the accumulation of TG and total free fatty acid in the diabetic heart occurred in tandem with increases in serum TG and total free fatty acid.
Mitochondrial β-oxidation constitutes the major process by which most of the short- (<C8 ), medium- (<C14 ), and long-chain (<C22 ) saturated and unsaturated fatty acids are oxidized to generate energy. In the diabetic status, we found a slight increase in the fatty acids of C16:0, C18:0, C20:0, and C22:0 (0.39-fold, 0.65-fold, 0.31-fold, and 0.59-fold, respectively) and a significant increase in the fatty acids of C18:1n-9 and C22:4n-6 (2-fold and 6-fold, respectively). We also found decreases in the fatty acids of C18:2n-6 and C18:3n-3 (0.13-fold and 0.62-fold, respectively) in the serum with an increase in the fatty acids of C18:0, C18:1n-9, and C22:4n-6, (0.29-fold, 1.3-fold, and 1.5-fold, respectively) and a decrease in the fatty acids of C18:3n-3 (0.45-fold) in the heart. Considered together with the increase in total fatty acid oxidation activity and mRNA levels of the mCPT-1 gene, which encodes the rate-limiting enzyme in mitochondrial β-oxidation, it suggests that the type 2 diabetic heart removed more fatty acids from circulation to provide energy though mitochondrial β-oxidation; this supports the common view that in diabetes, the FA supply is in excesses, causing the increased cellular oxidative capacity, intracellular TG and FFA accumulation, which is associated with lipotoxicity.
It was reported that in STZ-induced insulin deficiency, because of the decrease in 6-desaturase activity, the key enzyme in the conversion of linoleic acid to long-chain PUFA, the increase in linoleic acid (C18:2n-6) was observed in both cardiac and mitochondrial phospholipids. Because the serum C18:2n-6 was decreased and the cardiac mitochondrial β-oxidation was increased in our results, the increase of cardiac C18:2n-6 may also be due to the decrease of 6-desaturase activity.
Peroxisomes are the major sites of degradation of very long-chain saturated fatty acids and polyunsaturated fatty acids by the sequential action of ACOX1 or ACOX3, DBP or LBP, and THLA/B or SCPx. In the present type 2 diabetic hearts, in addition to mitochondrial oxidation, peroxisomal β-oxidation was also observed to be enhanced; this was indicated by increases in activity and mRNA levels of genes involved in the peroxisomal pathway, together with the decrease in lignoceric acid (C24:0) levels that were β-oxidized only by the peroxisomal pathway. Regarding PUFA, peroxisomal β-oxidation takes a dual function: synthesis and degradation[27, 28]. Although peroxisomal β-oxidation participates in the synthesis of DHA in some organs, such as the liver and brain, it has been reported that the rat heart cannot synthesize DHA from circulating ALA because it lacks elongase-2. Thus, the content of PUFA in the heart depends on the transportation to the heart from circulation and degradation in the heart. In our study, the content of EPA, DPA and DHA were significantly decreased in the DM heart, but changes were slight in diabetic serum compared with CON serum. Moreover, the activity of peroxisomal β-oxidation in the diabetic heart correlated negatively with the content of EPA, DPA and DHA. Therefore, the decrease in cardiac long chain PUFAs, including EPA, DPA and DHA, might be related to the elevated peroxisomal β-oxidation activity observed in this study.
It is worth noting that all the observed n-3 PUFAs were significantly decreased in the type 2 diabetic hearts. Moreover, because of the significant decrease in n-3 PUFAs and the slight increase in n-6 PUFAs, the n-6/n-3 ratio was doubled in the diabetic heart. During the past 3 decades, thousands of epidemiologic, observational, experimental, and randomized controlled studies have been published regarding the cardiovascular protective effects of n-3 PUFAs, especially DHA and EPA. For example, DHA and EPA can ultimately increase arrhythmic thresholds, increase the threshold of ventricular fibrillation, increase heart rate variability, reduce ischemic damage, and favorably affect autonomic tone[30–32]. Furthermore, several authors tended to explain the PUFAs effects in terms of a balance between n-6 and n-3 FAs, rather than the absolute amount of each single molecule. In the most simplistic interpretation, a very high n-6/n-3 ratio is considered detrimental for human health. These data, therefore, showed that the reduction of the n-3 PUFA content (mainly EPA, DPA and DHA) and a high n-6/n-3 ratio might be another factor associated with type 2 diabetic hearts in addition to the accumulation of the ectopic free fatty acids and TG. This may also explain why a higher intake of dietary long chain n-3 PUFA, especially EPA and DHA, is required for heart diseases in type 2 diabetes.
In some studies, the activation of PPARα and subsequent enhanced expression of the PPARα target gene, which increases fatty acid oxidation, were observed during insulin resistance and diabetes[33, 34]. In our results, although the mRNA expression of PPARα had no changes, the enhanced mRNA expressions of ACOX1, 3 and mCPT-1 (PPARα targets) represented the activation of PPARα in the type 2 diabetic rat heart.
In conclusion, we found that, except for the increasing mitochondrial β-oxidation, the activity of peroxisomal β-oxidation was also significantly increased in type 2 diabetic rat hearts. The increasing activity was, at least partly, due to the elevated expression of genes involved in peroxisomal β-oxidation, especially ACOX1 and 3 (PPARα targets). The enhanced peroxisomal β-oxidation and reduced n-3 PUFA content might be another adverse factor associated with type 2 diabetic hearts.