A central metabolic function of the liver is to maintain plasma glucose levels regardless of the nutritional state of the animal. In the setting of energy excess, glucose is converted to fatty acids via the conversion of glucose to pyruvate, which enters the Krebs cycle in the mitochondria (Figure ). Citrate formed in the Krebs cycle is shuttled to the cytosol where it is converted to acetyl-CoA by ATP citrate lyase. Acetyl-CoA carboxylase 1 (ACC1) then converts acetyl-CoA to malonyl-CoA, which is used by fatty acid synthase to form palmitic acid (C16:0). Palmitic acid is then either desaturated by stearoyl-CoA desaturase (SCD) to palmitoleic acid, or further elongated by the long chain fatty acyl elongase to form stearic acid (C18:0), which also can be desaturated to form oleic acid (C18:1) (15
). These fatty acids are used to synthesize triglycerides — the primary source of energy storage and transport. Humans (16
) and mice (17
) with hepatic steatosis accumulate excess oleic acid, the end-product of de novo fatty acid synthesis. This suggests that fatty acid synthetic rates are increased in the insulin-resistant liver.
De novo synthesis of fatty acids in liver is regulated independently by insulin and glucose (18
). Insulin’s ability to activate lipogenesis is transcriptionally mediated by the membrane-bound transcription factor, sterol regulatory element–binding protein-1c (SREBP-1c) (20
). SREBP-1c is one of three SREBP isoforms that belong to the basic helix-loop-helix-leucine zipper (bHLH-Zip) family of transcription factors (22
). In the nucleus, SREBP-1c transcriptionally activates all genes required for lipogenesis (15
). Importantly, the overexpression of SREBP-1c in transgenic mouse livers leads to the development of a classic fatty liver due to increased lipogenesis (24
). We (25
), and others (26
) have demonstrated that increased rates of hepatic fatty acid synthesis contribute to the development of fatty livers in rodent models of insulin-resistant diabetes and obesity.
Hyperinsulinemia and elevated hepatic glucose production are hallmarks of insulin resistance (28
). It might be anticipated that SREBP-1c would not be activated in states of insulin resistance. Surprisingly, even in the presence of profound insulin resistance, insulin stimulates hepatic SREBP-1c transcription, resulting in increased rates of de novo fatty acid biosynthesis (25
). The contribution SREBP-1c makes to triglyceride accumulation in insulin-resistant livers has been explored in ob/ob
mice are severely obese and insulin resistant due to a mutation in the leptin gene and, as a consequence, these mice have hepatic steatosis (29
). Inactivation of the Srebp-1
gene in the livers of ob/ob
mice results in an approximately 50% reduction in hepatic triglycerides (30
). Thus, SREBP-1 plays a significant role in the development of hepatic steatosis in this animal model of insulin resistance.
SREBP-1c also activates ACC2 (23
), an isoform of ACC that produces malonyl-CoA at the mitochondrial membrane (31
). Increases in malonyl-CoA result in decreased oxidation of fatty acids due to inhibition of carnitine palmitoyl transferase-1 (CPT-1), which shuttles fatty acids into mitochondria (32
). The critical role of ACC2 in hepatic fatty acid metabolism was revealed in mice that harbored the genetic deletion of the Acc2
gene. The Acc2
knockout mice were resistant to obesity, owing to increased activity of CPT-1, resulting in an increased rate of fatty acid oxidation (33
). Adenoviral-mediated expression of malonyl-CoA decarboxylase, an enzyme that degrades malonyl-CoA, also results in increased fatty acid β oxidation and reduced hepatic triglyceride stores (35
Carbohydrate (glucose)-mediated stimulation of lipogenesis is transcriptionally mediated by a second bHLH-Zip transcription factor, designated carbohydrate response element binding protein (ChREBP) (36
). Glucose activates ChREBP by regulating the entry of ChREBP from the cytosol into the nucleus and by activating the binding of the transcription factor to DNA (37
). Glucose stimulates ChREBP to bind to an E-box motif in the promoter of liver-type pyruvate kinase (L-PK), a key regulatory enzyme in glycolysis. L-PK catalyzes the conversion of phosphoenolpyruvate to pyruvate, which enters the Krebs cycle to generate citrate, the principal source of acetyl-CoA used for fatty acid synthesis. Recently, ChREBP knockout mice have been developed and characterized (38
). As predicted from in vitro studies, the expression of L-PK was reduced by approximately 90% in livers of ChREBP knockout mice. An unexpected finding was that the mRNA levels of all fatty acid synthesis enzymes also were reduced by approximately 50% (38
). This suggests that ChREBP can independently stimulate the transcription of all lipogenic genes. Thus, activation of L-PK stimulates both glycolysis and lipogenesis, thereby facilitating the conversion of glucose to fatty acids under conditions of energy excess. Whether inactivation of ChREBP will attenuate the development of fatty livers in insulin-resistant states is currently under investigation; however, it seems likely that excessive stimulation of lipogenesis by ChREBP stimulation would be important only after the development of hyperglycemia.
A third transcription factor that participates in the development of hepatic steatosis in rodents is PPAR-γ. PPAR-γ is a member of the nuclear hormone receptor superfamily that is required for normal adipocyte differentiation (39
). Normally, PPAR-γ is expressed at very low levels in the liver; however, in animal models with insulin resistance and fatty livers, the expression of PPAR-γ is markedly increased (40
). Previous studies have demonstrated that SREBP-1c can transcriptionally activate PPAR-γ, and it has been suggested that SREBP-1c may activate PPAR-γ by stimulating production of an activating ligand for the nuclear receptor (42
The importance of PPAR-γ expression in the development of fatty livers has been demonstrated by the development of liver-specific gene deletions of Ppar-
γ in two different insulin-resistant mouse models, the ob/ob
mouse and the lipodystrophic transgenic mouse, named AZIP/F-1. AZIP-F-1 mice are insulin resistant due to a near absence of white adipose tissue and leptin deficiency (41
). The genetic deletion of hepatic PPAR-γ in livers of either ob/ob
) or AZIP-F-1 (45
) mice markedly attenuates the development of hepatic steatosis, independent of the presence of hyperinsulinemia or hyperglycemia.
The precise molecular events mediated by PPAR-γ that promote triglyceride deposition in the liver have not been fully defined. It is also not known whether PPAR-γ expression is increased in human livers with steatosis.