Activation of mitochondrial energy metabolism protects against cardiac failure

Abstract

Cardiac failure is the most prevalent cause of death at higher age, and is commonly associated with impaired energy homeostasis in the heart. Mitochondrial metabolism appears critical to sustain cardiac function to counteract aging. In this study, we generated mice transgenically over-expressing the mitochondrial protein frataxin, which promotes mitochondrial energy conversion by controlling iron-sulfur-cluster biogenesis and hereby mitochondrial electron flux. Hearts of transgenic mice displayed increased mitochondrial energy metabolism and induced stress defense mechanisms, while overall oxidative stress was decreased. Following standardized exposure to doxorubicin to induce experimental cardiomyopathy, cardiac function and survival was significantly improved in the transgenic mice. The insulin/IGF-1 signaling cascade is an important pathway that regulates survival following cytotoxic stress through the downstream targets protein kinase B, Akt, and glycogen synthase kinase 3. Activation of this cascade is markedly inhibited in the hearts of wild-type mice following induction of cardiomyopathy. By contrast, transgenic overexpression of frataxin rescues impaired insulin/IGF-1 signaling and provides a mechanism to explain enhanced cardiac stress resistance in transgenic mice. Taken together, these findings suggest that increased mitochondrial metabolism elicits an adaptive response due to mildly increased oxidative stress as a consequence of increased oxidative energy conversion, previously named mitohormesis. This in turn activates protective mechanisms which counteract cardiotoxic stress and promote survival in states of experimental cardiomyopathy. Thus, induction of mitochondrial metabolism may be considered part of a generally protective mechanism to prevent cardiomyopathy and cardiac failure.

Figure 1.. Over-expression of frataxin induces mito-chondrial metabolism and ROS defense in the heart.

(A) Representative anti-hemagglutinin immunoblot showing several tissues from a transgene-negative littermate (“-”) and a transgenic (“+”) animal each. “Control” is a previously published cell line over-expressing frataxin [20]. (B) Aconitase activity measured in murine heart samples. Grey bars indicate wild-type (WT) and white bars indicate frataxin-transgenic (FX) animals (also applies to subsequent figures). (C) ATP, (D) NADH, (E)) NADPH, (F) reduced glutathione (GSH) and (G) thiobarbituric acid reactive substances (TBARS) contents in the hearts of wild-type and frataxin-transgenic animals. Error bars represent S.E.M., *p < 0.05, **p < 0.01, ***p < 0.001 (applies to this and all subsequent figures) (n=4).

Figure 2.. Activation of mitochondrial metabolism improves cardiac function following doxorubicin-induced cardio-myopathy.

(A) Maximum rate of pressure development in the left ventricle (dP/dtmax), (B) left-ventricular maximum rate of pressure decrease (dP/dtmin), (C) end-systolic pressure (Pes), (D) end-systolic volume (Ves), (E) end-diastolic pressure (Ped), (F) end-diastolic volume (Ved), (G) time constant of isovolumetric left ventricular pressure decline (tau, τ). (H) stroke work, (I) stroke volume, (J) cardiac output, (K) ejection fraction, and (L) heart rate in wild-type vs. frataxin-transgenic animals following administration of doxorubicin. Grey bars depict wild-type litter mates, white bars frataxin-transgenic mice (n=6 each).

Figure 3.. Activation of mitochondrial energy metabolism increases survival rates following doxorubicin-induced cardiomyopathy.

Survival plot of animals exposed to doxorubicin. Triangles depict frataxin-transgenic (FX) animals; squares depict wild-type (WT) animals (n=24 per genotype).

Figure 4.. Activation of mitochondrial metabolism sustains activation of the insulin signaling cascade following doxorubicin-induced cardiomyopathy.

Three animals were analyzed by western blotting for each group. Lanes 1-3: WT untreated; lanes 4-6: FX untreated; lanes 7-9: WT doxorubicin-treated; lanes 10-12: FX doxorubicin-treated. Membranes were probed with the indicated antibody, then stripped and re-probed with α-tubulin as loading control. Approximate protein band size to the left as indicated by arrows.

Figure 5.. Activation of mitochondrial metabolism induces mitohormesis, hereby increasing stress resistance and survival of cardiomyopathy.