The carnitine shuttle links mitochondrial metabolism to histone acetylation and lipogenesis

Abstract

Acetyl-CoA is a central metabolite used for lipid synthesis in the cytosol and histone acetylation in the nucleus, among other pathways. The two major precursors to acetyl-CoA in the nuclear-cytoplasmic compartment are citrate and acetate, which are processed to acetyl-CoA by ATP-citrate lyase (ACLY) and acyl-CoA synthetase short-chain 2 (ACSS2), respectively. While some evidence has suggested the existence of additional routes to nuclear-cytosolic acetyl-CoA, such pathways remain poorly defined. To investigate this, we generated cancer cell lines lacking both ACLY and ACSS2. Unexpectedly, and in contrast to observations in fibroblasts, ACLY and ACSS2 double knockout (DKO) cancer cells remain viable and proliferate, maintain pools of cytosolic acetyl-CoA, and are competent to acetylate proteins in both cytosolic and nuclear compartments. Using stable isotope tracing, we show that both glucose and fatty acids feed acetyl-CoA pools and histone acetylation in DKO cells. Moreover, we provide evidence for the carnitine shuttle and carnitine acetyltransferase (CrAT) as a substantial pathway to transfer two-carbon units from mitochondria to cytosol independent of ACLY. Indeed, in the absence of ACLY, glucose can feed fatty acid synthesis in a carnitine responsive and CrAT-dependent manner. This work defines a carnitine-facilitated route to produce nuclear-cytosolic acetyl-CoA, shedding light on the intricate regulation and compartmentalization of acetyl-CoA metabolism.

Inter-organelle crosstalk supports acetyl-coenzyme A homeostasis and lipogenesis under metabolic stress

“…While these KO systems lack some physiology in the acute nature of pathway targeting, they also allow one to focus biochemical studies on these alternate pathways when substrate levels are high. However, we and Luke Izzo et al( 49 ). in this issue also note key questions that remain such as the relative contribution of mitochondrial versus peroxisomal oxidation and the dependence on carnitine for transport since both organelles express CRAT ( 64 , 65 ).…”

“…However, based on the previous [U- 13 C5]glutamine tracing results (Fig. 5D) mitochondrial citrate minimally contributes to fatty acid synthesis in the ACLY-KO or ACLY/ACSS2-DKO cells, and no carnitine was added to cultures (49), suggesting that ACLY-deficient cells use peroxisomal metabolism to supply acetyl-CoA for biosynthesis (Fig 5G ).…”

Carnitine octanoyltransferase is important for the assimilation of exogenous acetyl-L-carnitine into acetyl-CoA in mammalian cells

“…The role of LAC in the context of the carnitine shuttle has been extensively studied ( 1 , 2 , 3 , 4 , 7 ). Here, we characterize the cellular metabolism of exogenous LAC in the context of glucose limitation.…”

“…In eukaryotes, the zwitterionic metabolite carnitine serves a critical role in energy homeostasis by transporting esterified fatty acids across the inner mitochondrial membrane for β-oxidation ( 1 , 2 , 3 , 4 , 5 , 6 ). At the same time, it also returns some of the acetyl-CoA products of β-oxidation to the cytosol in the form of acetyl-L-carnitine (LAC) ( 3 , 7 ). These CoA:carnitine transacylation reactions are catalyzed by the well-known enzymes carnitine acetyltransferase (CRAT) and carnitine palmitoyltransferase (CPT) ( 1 , 4 , 5 , 6 ) ( Fig.…”

Dynamic protein deacetylation is a limited carbon source for acetyl-CoA-dependent metabolism

“…Despite contributing to acetyl-CoA pools, deacetylation did not affect the total cellular concentration of acetyl-CoA upon acetate removal. Our data thus suggest that even though 50% of the acetyl-CoA pool is sustained for at least an hour in the absence of acetate and ACLY (Fig 1F), this is a result of decreased turnover (S1 Fig) and possible contribution from alternative metabolic source(s) other than deacetylation [50]. The functional consequence of deacetylation as an acute carbon source of acetyl-CoA is unclear.…”