Cardiac Fatty Acid Oxidation and Cardiolipin Homeostasis
Summary¶
Cardiomyocytes normally obtain most of their energy from fatty acid oxidation (FAO), but unrestrained FAO can disturb lipid and energy homeostasis. In a mouse model, cardiomyocyte-specific loss of ACC1 and ACC2 depleted cardiolipin, impaired mitochondrial electron transport and caused dilated cardiomyopathy.
Experimental Findings¶
The balance of fatty acid oxidation (FAO) in the myocardium is vital for preserving both energy production and structural lipid integrity [1].
ACC1/ACC2 as the Metabolic Brake¶
- Mechanism: Acetyl-CoA carboxylase 1 (ACC1) and 2 (ACC2) synthesize malonyl-CoA, which acts as a potent endogenous inhibitor of carnitine palmitoyltransferase 1 (CPT1). CPT1 is the rate-limiting enzyme that facilitates the transport of long-chain fatty acids into the mitochondria for beta-oxidation.
- Knockout Phenotype: Cardiomyocyte-specific double-knockout (ACC dHKO) mice lack the ability to synthesize malonyl-CoA in the heart, removing the inhibitory brake on CPT1 and leading to constitutively elevated, unrestrained myocardial FAO.
Cardiolipin Depletion and Mitochondrial Dysfunction¶
- Linoleic Acid Depletion: The excessively high rates of FAO in ACC dHKO mice lead to the rapid consumption and depletion of intracellular linoleic acid (C18:2).
- Cardiolipin Loss: Linoleic acid is a crucial substrate for the remodeling of cardiolipin (a unique phospholipid of the inner mitochondrial membrane) into its mature, functional tetralinoleoyl form ($L_4CL$). The lack of linoleic acid leads to severe cardiolipin depletion.
- Structural Integrity Loss: The loss of cardiolipin destabilizes the mitochondrial electron transport chain (ETC) supercomplexes, leading to impaired oxidative phosphorylation, elevated reactive oxygen species (ROS) production, and progressive cardiomyocyte death.
- Cardiomyopathy: The dHKO mice consequently develop progressive dilated cardiomyopathy and heart failure.
Pharmacological Rescue¶
- CPT1 Inhibitors: Treatment of ACC dHKO mice with pharmacological CPT1 inhibitors, such as etomoxir or oxfenicine, limits fatty acid import and oxidation.
- Phenotypic Rescue: This inhibition restores linoleic acid levels, replenishes the mitochondrial cardiolipin pool, normalizes electron-transport activity, and prevents the onset of dilated cardiomyopathy.
Clinical Translation Considerations¶
These findings challenge the assumption that increasing myocardial FAO is universally beneficial in heart failure. Because this causal evidence is derived from a genetically engineered mouse model, the confidence level is set to medium for clinical translation in humans.