**6. Regulation of metabolism under fasting conditions by PPARα and FGF21**

PPARα is a nuclear receptor expressed in the liver, kidney, heart, and brown adipose tissue, which is activated by long-chain fatty acids and involved in fatty acid metabolism (**Figure 6**). PPARα forms a heterodimer with retinol X receptor (RXR) and regulates transcription by binding to the response sequence called PPAR response element (PPRE) in the gene regulatory regions. In the liver, PPARα promotes the expressions of a variety of genes related to β-oxidation, including FGF21, HMGCS2, CPT1a, and acyl-CoA oxidase (ACOX), which is the rate-limiting enzyme for peroxisomal β-oxidation (**Figure 6**).

FGF21, a target gene of PPARα, is a member of the FGF family, with 22 members in humans, and belongs to the same subfamily as FGF15/19 and FGF23 [7, 8]. FGFs in this subfamily characteristically function as hormones. FGF21 is expressed in the liver, adipose tissue, skeletal muscle, and pancreas, and the liver mainly secretes FGF21 as a hormone [9]. The expression of FGF21 is enhanced by fasting and ketogenic (or high-fat) diets [7, 10]. FGF21 levels in infants are higher than fasting FGF21 levels in adults, which is thought to be induced by milk-derived free fatty acids. In addition to PPARα, glucocorticoid receptors, activating transcription factor 4 (ATF4), cAMP response element binding protein *Metabolic Responses to Energy-Depleted Conditions DOI: http://dx.doi.org/10.5772/intechopen.100391*

**Figure 6.** *Regulation of energy metabolism by FGF21.*

H (CREBH), carbohydrate response element binding protein (ChREBP), PPARγ, farnesoid X receptor (FXR), and activin B induce the expression of FGF21 in the liver [11], while liver X receptor (LXR) inhibits the expression [12]. In the skeletal muscles, FGF21 is expressed through ATF4 under specific conditions such as metabolic stresses in mitochondria, and is also regulated by the phosphatidylinositol-3 kinase (PI3K) and Akt signals [12].

FGF21 plays an important role in the regulation of systemic energy metabolism during fasting. In the white adipose tissue, FGF21 induces lipolysis by enhancing the transcription of HSL and ATGL [7]. Free fatty acids produced by lipolysis promote fatty acid oxidation and the protein expression of HMGCS2, which induces the production of ketone bodies (**Figure 6**). FGF21 also enhances the expression of lipolytic enzymes in the liver, suppresses glycogenolysis, and promotes gluconeogenesis, but does not significantly affect glycolysis. FGF21 binds to the FGF receptor (FGFR) and its co-factor β-Klotho on the plasma membrane of target cells, and regulates transcription and translation of the target genes via phosphorylation cascades. FGF21 also regulates glucose metabolism in the liver partially via peroxisome proliferator-activated receptor γ coactivator 1-α (PGC-1α), which acts as a co-activator of transcriptional regulators and induces gene expression related to gluconeogenesis, fatty acid oxidation, and ketone body production during fasting [13]. In rodent models, pharmacological concentrations of FGF21 promote glucose uptake in adipocytes, increase insulin sensitivity, reduce blood triglyceride and hepatic fat, and suppress weight gain on a high-fat diet. In addition, it induces the expression of Glut1 in adipocytes [14], increases the number of pancreatic islets and the content of insulin per islet [15], and reduces blood glucagon levels by suppressing glucagon secretion [14]. In diabetic monkeys, FGF21 lowers blood glucose, serum insulin, triglyceride, and low density lipoprotein (LDL) cholesterol levels, increases high density lipoprotein (HDL) cholesterol levels in the blood, and induces weight loss without inducing hypoglycemia [16]. Human blood FGF21 levels were shown to be elevated in individuals with obesity, type 2 diabetes, and insulin resistance. It is also reported that blood FGF21 levels in hyperlipidemic patients are twice as high as in the normal group, and that administration of fibrate, a PPARα ligand, increases blood FGF21 levels.

In addition, FGF21 levels in humans are increased by prolonged fasting for 7 days [17]. Thus, even under feeding conditions, FGF21 induces a fasting-like metabolic states, such as gluconeogenesis, fatty acid oxidation, and ketone body production (**Figure 6**) [12].

Living organisms suppress GH and reproductive signals to reduce unnecessary energy consumption during fasting. Mice with excess FGF21 show reduced response to GH and suppressed reproductive signals (**Figure 6**) [18, 19]. In detail, FGF21 transgenic mice show reduced phosphorylation of signal transducer and activator of transcription 5 (STAT5) downstream of Janus kinase (JAK) 2 in response to GH in the liver and reduced blood IGF-1 levels [18]. In addition, FGF21 transgenic female mice have a suppressed luteinizing hormone (LH) surge due to inhibition of vasopressin signals in the hypothalamus [19]. The latest statistical analysis, which excluded the contribution of insulin resistance and body fat percentage, showed elevated blood FGF21 levels in human anorexia [20], suggesting that FGF21 may be involved in impaired GH signaling in anorexic patients. In addition, FGF21 plays a role in energy conservation by inducing a hibernation-like state (torpor) in mice [7]. During mouse torpor and squirrel hibernation, in addition to hypothermia and hypoactivity, it is known that pancreatic lipase is ectopically induced outside the pancreas and FGF21 induces the ectopic expression in the liver [7, 21]. The pancreatic lipase is capable of hydrolyzing TGs into glycerol and fatty acids over a wide temperature range, and thus may provide fatty acids as an energy source during torpor and hibernation.

Furthermore, FGF21 transgenic mice exhibit a long lifespan (**Figure 6**) [22]. It is reported that the median lifespan of wild-type mice was 28 months, while that of FGF21 transgenic mice was 38 months. Interestingly, the longevity of the FGF21 transgenic mice did not require restriction of food intake, and insulin sensitivity was maintained even when food intake was increased. This suggests that FGF21 increases lifespan by shifting systemic metabolism to a fasting-like state regardless of changes in food intake. These phenotypes are attributed to the suppression of GH/IGF-1 signaling accompanied by decreased IGF-1 production, but do not involve mTOR signaling, AMPK signaling, and NAD+ metabolism. In the signaling of the endocrine FGF subfamily (FGF15/19, FGF21, and FGF23), β-Klotho on the plasma membrane is required for FGF15/19 and FGF21, and α-Klotho for FGF23. α-Klotho was originally reported as a longevity gene [23], and it inhibits insulin/ IGF-1 signaling by its truncated extracellular region circulating in the bloodstream. Thus, it is possible that α-Klotho causes longevity through a similar mechanism to FGF21, but α-Klotho is different from FGF21 in that it causes insulin resistance [22]. FGF21 also decreases preference for sweetness and alcohol via the central nervous system [24–27], which may contribute to the regulation of eating behavior in response to the energy states.

#### **7. Regulation of metabolism during fasting by insulin and glucagon**

Low blood insulin levels play an important role in the regulation of energy metabolism during fasting. Insulin activates PI3K through phosphorylation of insulin receptor substrate (IRS), and subsequent activation of Akt leads to phosphorylation and translocation of FOXO1 out of the nucleus, resulting in the suppression of the expression of ATGL, the rate-limiting enzyme in lipolysis. In addition, insulin signaling phosphorylates FOXA2 and excludes it from the nucleus, thereby suppressing the expression of HMGCS2, the rate-limiting enzyme for ketone body production. Therefore, the decrease in insulin signaling during fasting is involved in the expression of ATGL, gluconeogenesis-related genes, and ketone

#### *Metabolic Responses to Energy-Depleted Conditions DOI: http://dx.doi.org/10.5772/intechopen.100391*

body production-related genes. Fasting also regulates the expression of IRSs and the PI3K activity. The expression of IRS-2 and the PI3K activity are elevated during fasting and decrease immediately after food intake in the liver [28]. In this context, IRS-2 appears to act for a short time after food intake so that it can respond again to the next coming dietary stimuli, whereas the expression of IRS-1 is relatively constant regardless of feeding conditions [28]. Therefore, it is considered that while both IRS-1 and IRS-2 are involved in inhibition of gluconeogenesis immediately after food intake, IRS-1 is mainly involved in glycogen production that is initiated at an interval after that. IRS-1 and IRS-2 differ not only in their expression patterns but also in their functions. IRS-1 deficient mice do not show strong diabetic symptoms, because IRS-2 can compensate for glucose intolerance by promoting the proliferation of pancreatic β cells [29]. The increased transcription of the IRS-2 gene during fasting and its repression after refeeding are regulated by the glucagon receptor-PKA-CREB-regulated transcription coactivator 2 (CRTC2)-CREB pathway and the insulin receptor signaling in the liver, while the regulation mechanism in other tissues remains unclear [30]. Refeeding induces insulin binding to the insulin receptor and downstream PI3K activation, which represses IRS-2 transcription via FOXO1 phosphorylation by Akt.

During fasting, glucagon is secreted from pancreatic α cells in response to hypoglycemia and stimulates glucose production in the liver through gluconeogenesis and glycogenolysis. The glucagon receptor-cAMP-PKA pathway promotes gluconeogenesis by inducing the expression of the catalytic subunits of G6Pase and PEPCK in the liver. PKA-dependent phosphorylation of CREB and dephosphorylation of CRTC2 forms CREB-CRTC2 complex that recruits a histone acetyltransferase, CREB binding protein (CBP) and promotes transcription of the gene encoding PGC-1α. Furthermore, PGC-1α, which is activated by SIRT1-mediated deacetylation and inactivated by general control nonderepressible-5 (GCN5)-mediated acetylation, promotes gluconeogenesis together with FOXO1 and hepatocyte nuclear factor 4α (HNF4α) [30].
