The novel findings of the present investigation are that dietary supplementation with EPA+DHA (i) causes a dose-dependent increase in plasma adiponectin and prevention of LV chamber enlargement; (ii) prevents the switch in MHC isoform from MHC-α to MHC-β; and (iii) prevents the increase in urinary thromboxane B2 and sharply decreases serum TNF-α. In contrast, ALA supplementation had minimal anti-inflammatory and cardioprotective effects, despite robust incorporation into cardiac membrane phospholipids. Taken together, these findings provide the first clear evidence that dietary supplementation with ω3 PUFA derived from fish, but not from vegetable sources, can prevent cardiac remodelling and dysfunction under pressure overload conditions.
The changes in myocardial phospholipid fatty acid composition observed here, although quantitatively different from what is seen in humans, were nevertheless qualitatively similar. For example, giving ~2.9% energy as EPA+DHA for 1 month to patients awaiting cardiac surgery raised the EPA+DHA content of right atrial appendage from 5.3 to 11.5% and lowered arachidonic acid from 21 to 16%.25
In rats given EPA+DHA at 2.3% energy, cardiac tissue EPA+DHA levels increased from 12 to 27%, whereas arachidonic acid fell from 24 to 9%. Thus the doses of fish oil employed in the present study resulted in changes in cardiac phospholipid composition similar to those observed in clinical studies. In addition, the changes in membrane phospholipid composition was similar between sham and AAB animals, as was the increase in adiponectin and the decrease in TNF-α, suggesting that these effects occur independent of pressure overload.
We observed a strong negative correlation between plasma adiponectin concentration with LV remodelling and contractile dysfunction, suggesting a causal role for EPA+DHA-induced elevation in adiponectin in the prevention of LV chamber enlargement. Previous studies showed that the elevation in plasma adiponectin with EPA+DHA is due to the activation of PPAR-γ in adipose tissue and up-regulation of expression and secretion of adiponectin.1,9
Adiponectin-deficient mice have enhanced LV hypertrophy and dysfunction in response to pressure overload, which can be rescued by adenovirus-mediated delivery of adiponectin.11
In the present study, EPA+DHA supplementation elevated plasma adiponectin in a dose-dependent manner. These protective effects of adiponectin have been attributed to the activation of AMPK11
and the inhibition of Akt;22
however, in the present study and our previously investigation,1
ω-3 PUFA supplementation did not affect the activation of AMPK or Akt, suggesting that any protective effect of adiponectin is mediated by other mechanisms.
It is well established that the inflammatory response contributes to the development of heart failure,26,27
and elevated adiponectin is associated with reduced inflammation,10
which could prevent LV remodelling and pathology. Adiponectin inhibits TNF-α-induced inflammation in human aortic endothelial cells.28
In the present study, pressure overload was associated with elevated serum TNF-α levels, and the attenuation of LV remodelling and cardiac dysfunction by EPA+DHA supplementation was accompanied by a decline in serum TNF-α concentration. This observation is consistent with previous studies in TNF-knockout mice that showed reduced inflammatory and fibrogenic responses, apoptosis, and cardiac remodelling in response to pressure overload.29
The suppression of serum TNF-α and the prevention of LV structural and molecular remodelling in the present study suggest a causal role for the TNF-α lowering effects of EPA+DHA in the prevention of heart failure. Additional studies in adiponectin-knockout mice are required to determine whether EPA+DHA-induced up-regulation of adiponectin is an essential component of the suppression of serum TNF-α and/or the prevention of LV dysfunction.
We found that pressure overload elevated urinary levels of thromboxane B2
and 6-keto prostaglandin F1
, which could be linked to shear stress-induced production of thromboxane A2
by platelets and prostacyclin by endothelial cells. In addition, cardiomyocytes can produce eicosanoids in response to inflammatory mediators such as TNF-α.30
Fish oil supplementation has previously been show to decrease myocardial thromboxane and prostaglandins,31
and we showed here that EPA+DHA supplementation decreases urinary levels of both thromboxane B2
and 6-keto prostaglandin F1
. In addition, this decrease corresponds with a parallel decline in the content of arachidonic acid in phospholipids, suggesting that decreased availability of the precursor could be responsible for this effect.
Supplementation with EPA+DHA could prevent LV remodelling and dysfunction through modification of mitochondrial membrane properties and ligand activation of PPARs and increased expression of key metabolic enzymes. LV hypertrophy and chamber enlargement induced by pressure overload are associated with a decrease in the activity of mitochondrial enzymes involved in the fatty acid oxidation and energy transduction.24
Both EPA and DHA are activators of PPAR-α in vitro
however, we did not observe a significant increase in the mRNA or activity of the PPAR-α-regulated fatty acid oxidation enzyme MCAD. In addition, there were no effects on the activities of the mitochondrial enzymes citrate synthase, isocitrate dehydrogenase, or aconitase, suggesting that supplementation with EPA+DHA did not increase the capacity of mitochondrial carbon substrate oxidation and NADH generation. Alternatively, supplementation with EPA+DHA could exert a protective effect through improvement in mitochondrial function and the efficiency of ATP generation. Pepe and McLennan32
showed that isolated perfused hearts from rats with fed fish oil had reduced myocardial oxygen consumption without a decrease in LV power generation, at low or high workload, and during ischaemia or reperfusion, resulting in greater LV mechanic efficiency. The mechanism for this effect is not clear, but could be due to improved mitochondrial coupling and/or a decrease in ATP hydrolysis by processes not directly related to force generation. Fish oil supplementation can alter the function of cardiac mitochondria by decreasing matrix Ca2+
and increasing state III respiration with succinate as the substrate.34
Additional studies are required to elucidate the diverse effects of EPA and DHA in the improved cardiac response to pressure overload. Specifically, proteomic analysis of plasma, tissue, and isolated cardiac mitochondria may prove fruitful in identifying novel effects of ω-3 PUFA, as recently suggested by the proteomic analysis of serum lipoproteins in healthy volunteers treated with fish oil.35
4.1. Conclusions and clinical implications
Patients with hypertension are at risk for developing heart failure36
and thus are treated with drugs that lower afterload and suppress the neurohormonal over-activation that drives the progression to failure. Nevertheless, many optimally treated patients develop heart failure, thus new cardioprotective therapies are required that act independent of mechanisms addressed by currently used drugs. Anti-inflammatory agents, such as pentoxifylline,37
or adiponectin mimetic agents,39
or drugs that modify myocardial energy metabolism, such as perhexiline,40,41
are promising therapeutic approaches for the prevention and treatment of heart failure. The results of the present study show that dietary supplementation with EPA+DHA, but not ALA, exerts similar effects, as seen in the prevention of LV remodelling and dysfunction, associated with elevated plasma adiponectin and reduced inflammation, as evidenced by decreases in urinary thromboxane B2
metabolites and serum TNF-α. Thus dietary supplementation with EPA+DHA acts on established therapeutic targets that are not addressed with currently approved medications and may be effective adjunctive therapy for the prevention and treatment of heart failure.