The concept of ‘‘metabolic inflexibility’’ was first introduced
to describe the failure of insulin-resistant
humansubjects to appropriately adjust mitochondrial
fuel selection in response to nutritional cues. This
phenomenon has since gained increasing recognition
as a core component of the metabolic syndrome, but
the underlying mechanisms have remained elusive.
Here, we identify an essential role for the mitochondrial
matrix enzyme, carnitine acetyltransferase
(CrAT), in regulating substrate switching and glucose
tolerance. By converting acetyl-CoA to its membrane
permeant acetylcarnitine ester, CrAT regulates mitochondrial
and intracellular carbon trafficking. Studies
in muscle-specific Crat knockout mice, primary
human skeletal myocytes, and human subjects
undergoing L-carnitine supplementation support a
model wherein CrAT combats nutrient stress, promotes
metabolic flexibility, and enhances insulin
action by permitting mitochondrial efflux of excess
acetyl moieties that otherwise inhibit key regulatory
enzymes such as pyruvate dehydrogenase. These
findings offer therapeutically relevant insights into
the molecular basis of metabolic inflexibility.
The concept of ‘‘metabolic inflexibility’’ was first introduced
to describe the failure of insulin-resistant
humansubjects to appropriately adjust mitochondrial
fuel selection in response to nutritional cues. This
phenomenon has since gained increasing recognition
as a core component of the metabolic syndrome, but
the underlying mechanisms have remained elusive.
Here, we identify an essential role for the mitochondrial
matrix enzyme, carnitine acetyltransferase
(CrAT), in regulating substrate switching and glucose
tolerance. By converting acetyl-CoA to its membrane
permeant acetylcarnitine ester, CrAT regulates mitochondrial
and intracellular carbon trafficking. Studies
in muscle-specific Crat knockout mice, primary
human skeletal myocytes, and human subjects
undergoing L-carnitine supplementation support a
model wherein CrAT combats nutrient stress, promotes
metabolic flexibility, and enhances insulin
action by permitting mitochondrial efflux of excess
acetyl moieties that otherwise inhibit key regulatory
enzymes such as pyruvate dehydrogenase. These
findings offer therapeutically relevant insights into
the molecular basis of metabolic inflexibility.
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