We and others have previously
We and others have previously found that supplementation of old rats with ALCAR remediates the age-related decay in mitochondrial bioenergetics in liver (Hagen et al., 1998a, Hagen et al., 1998b), heart (Paradies et al., 1994, Paradies et al., 1999), muscle (Pesce et al., 2010) and iCRT 14 mg (Liu et al., 2002). ALCAR has been postulated to mediate this improvement in a straightforward manner, namely, by replenishing l-carnitine levels which otherwise decline with age (Costell and Grisolia, 1993, Costell et al., 1989, Maccari et al., 1990, Tanaka et al., 2004). However, this report shows that even though ALCAR supplementation remediated overall IFM CPT1 activity loss, its mechanism(s) of action may be distinct from its essential role involving mitochondrial fatty acyl-CoA import. This concept is consistent with our current results showing no role for l-carnitine on the age-associated decline in CPT1 catalytic efficiency, and also by the observation that IFM CPT1 catalysis improved slowly over the month-long ALCAR supplementation period. These results suggest a far more complex mechanism of action for this metabolite, which is not consistent with an immediate, direct replenishment of myocardial carnitine levels. Precisely how ALCAR improves IFM CPT1 activity in aged rat hearts is not currently known; however, a significant body of evidence indicates that it facilitates a number of metabolic changes (Hagen et al., 1998a, Hagen et al., 2002, Musicco et al., 2009, Musicco et al., 2011, Pesce et al., 2010), which may ultimately improve CPT1 activity specifically in IFM. Even though ALCAR is not a classical free radical terminating molecule, nevertheless, it decreases the formation of nitro-tyrosine protein adducts in alcohol-induced brain damage (Rump et al., 2010), and limits oxidative damage in the heart following ischemia/reperfusion injury (Calvani et al., 2000, Cui et al., 2003). Also, proteomic analysis of aging rat brain shows that ALCAR lowers levels of protein carbonylation (Poon et al., 2006). Furthermore, Gadaleta and coworkers revealed that the mitochondrial isoform of peroxiredoxin III was less oxidatively modified and more active in livers from old rats supplemented with ALCAR (Musicco et al., 2009). Additionally, ALCAR initiates increased mitochondrial biogenesis and turnover (Cassano et al., 2006, Pesce et al., 2010), which would promote clearance of damaged IFM CPT1 proteins. Finally, ALCAR rebalances membrane phospholipid content, and for mitochondria, reverses the age-related decline in cardiolipin levels, a key phospholipid necessary for proper electron transport chain function (Hagen et al., 1998a, Hagen et al., 1998b, Paradies and Ruggiero, 1990, Paradies et al., 1995). Taken together, ALCAR may indirectly improve IFM CPT1 activity by maintaining protein integrity through membrane restructuring or limiting oxidative damage, which otherwise specifically increases in the IFM subpopulation with age. This hypothesis is supported by evidence that ALCAR increases cellular stress response, through the regulation of gene expression and the synthesis of important proteins that sustain the oxidative stress defense (Calabrese et al., 2010, Calabrese et al., 2011). In particular, ALCAR stimulates expression of heme oxygenase-1 (HO-1) and endothelial NO synthase (eNOS) in human endothelial cells during H2O2-induced oxidative stress (Calo et al., 2006). Also, ALCAR decreases amyloid beta-mediated oxidative damage in primary cortical neuron cultures by again increasing expression of both HO-1 and heat shock protein 70 (Hsp70) (Calabrese et al., 2010). Furthermore, in rats exposed to γ-radiation, ALCAR reverses the loss of superoxide dismutase (SOD) and glutathione peroxidase (GSHPx), restores GSH levels, and prevents the accumulation of malondialdehyde (MDA) in the liver and the lungs (Mansour, 2006). Thus, it is conceivable that ALCAR improves IFM CPT1 activity by preventing oxidative damage to the protein.