Cell metabolism is regulated by several molecules in many signaling pathways and consists of biochemical stimuli transmitted to effector molecules, by the inhibition or activation of downstream molecules, to elicit an intended response.

Lipids, particularly fatty acids and cholesterol, constitute an important part of cell membranes and act as signaling factors or metabolic intermediates in many molecular pathways (Shimano & Sato, 2017). A summary of the cellular signaling interactions connected to lipid metabolism in the liver is shown in Table 1.

Table 1.
Summary of signaling pathways involved in hepatic lipid processing and resulting metabolic effects.

Key: ↑ increased/stimulation; ↓ decreased/inhibition; → result in.

5’-adenosine monophosphate-activated protein kinase (AMPK) is a major energy sensor. It regulates metabolism and is especially associated with glycolysis, which is activated when there is depletion of stored ATP and therefore an increase in the AMP/ATP ratio due to higher demand for energy or caloric restriction (Musso et al., 2016; Navarro-Yepes et al., 2014). AMPK regulates autophagy, which is a way to increase the nutrients available to cells under starvation conditions, by the regulation of Unc-51-like autophagy activating kinase 1 (ULK1) and mTORC1 complexes. AMPK can also be activated via mitochondrial reactive oxygen species (ROS) formation as a result of hypoxia (Navarro-Yepes et al., 2014). Similarly, the inactivation of AMPK induced by reduced insulin sensitivity not only results in defective autophagy but also leads to ROS generation (Yang et al., 2018).

AKT signaling and the AMPK pathway have several shared regulatory steps. AKT is associated with glucose uptake and glycolysis resulting in ATP synthesis, which in turn causes AMPK to remain inactive. AKT can also directly block AMPK activation through the phosphorylation of a C-terminal residue on AMPK, which blocks AMPK phosphorylation by liver kinase B1 (LKB1). These two signaling pathways sometimes have equivalent roles and other times have opposing roles; for example, both pathways can increase glucose metabolism in response to different stimuli and AKT stimulates anabolic activities for ATP consumption while AMPK halts anabolism to produce ATP. AKT and AMPK also have opposing roles in protein synthesis, lipogenesis, glycogenesis, mTORC1 signaling, and autophagy (Manning & Toker, 2017).

AKT and AMPK pathways affect autophagy in an opposed manner as well. The inhibition of the PI3K/AKT/mTORC1 complex pathway, as a result of growth factor stimulation, leads to the phosphorylation of autophagy-related protein 13 (ATG13), which prevents ATG13 from entering the ULK1 complex and consequently blocks the recruitment of ATG13 to the pre-autophagosomal complex, subsequently inhibiting autophagy. Antagonistically, under conditions of low energy, such as glucose or amino acid starvation, the AMP/ATP ratio increases, leading to AMPK activation, mTOR inhibition, and autophagy activation (Musso et al., 2016; Navarro-Yepes et al., 2014; Park et al., 2016; Sinha et al., 2018).

Silent information regulator T1 (SIRT1) is a protein involved in the activity of proteins and enzymes that are important regulators of glucose and lipid metabolism, as well as energy levels. SIRT1 acetylation simulates a calorie depletion state, resulting in AMPK activation. This effect leads to reduced de novo lipogenesis and increased insulin sensitivity, lipolysis, and mitochondrial fatty acid oxidation (Li, Lian, Liu, Hu, & Wang, 2015; Musso et al., 2016).

Hepatic lipid metabolism is also affected by thyroid hormones when the cAMP-protein kinase A (PKA) and Ca²⁺/AMPK pathways are activated. Thyroid stimulating hormone (TSH) can increase the phosphorylation of the 3-hydroxy-3-methylglutaryl-CoA reductase enzyme (HMG-CoAR), mediated by AMPK, to inhibit HMG-CoAR activity, which in turn results in reduced cholesterol synthesis (Sinha et al., 2018).

Regarding lipid metabolism, phosphorylated AMPK can also improve high-fat-diet-induced NASH through the suppression of several key lipogenic factors in the liver related to cholesterol and fatty acid synthesis, such as sterol regulatory element-binding protein (SREBP-1), which results in reduced triglyceride synthesis, HMG-CoAR levels, cholesterol synthesis, and ACC enzyme activation, which in turn decreases malonyl-CoA levels and reduces free fatty acids and VLDL synthesis (Musso et al., 2016; Su et al., 2017).

SREBPs are nuclear transcription factors that control the expression of enzymes involved in fatty acid biosynthesis. SREBPs are synthesized as precursors bound to the endoplasmic reticulum (ER) membrane and are complexed with SREBP cleavage-activating protein (SCAP), which promotes the translocation of SREBPs to the Golgi, where they are cleaved to then move into the nucleus. SREBPs present three active isoforms after cleavage: SREBP-1a, SREBP-1c, and SREBP-2. SREBP-2 is the main regulator of intracellular cholesterol homeostasis, while SREBP-1c phosphorylates genes involved in fatty acid synthesis (McRae et al., 2016; Musso et al., 2016).

SREBP1c is expressed in most tissues, controlling lipogenic gene expression, and acting in the nutritional regulation of fatty acids and triglycerides in organs such as the liver, while SREBP-1a is mostly expressed in the heart, intestinal epithelium, macrophages and bone marrow dendritic cells, where it activates global lipid synthesis in rapidly growing cells. SREBP-2 regulates the transcription of genes related to cholesterol metabolism in every tissue (Shimano & Sato, 2017). An abundance of cholesterol in the ER membrane causes SCAP to bind to the products of insulin-induced genes (INSIGs), causing the SREBP-SCAP complex to be confined to the ER. The membrane-bound transcription factor site-1 protease (MBTPS1) and MBTPS2 are transferred from the Golgi to the ER membrane and activate SREBP-1, as a consequence of reduced phosphatidylcholine levels (Röhrig & Schulze, 2016).

The cleavage of SREBPs has been shown to be stimulated by the PI3K/AKT/mTOR pathway (Boyd & Ye, 2018). The activation and nuclear accumulation of SREBP-1c by the insulin and AKT/mTORC1 signaling pathways in the liver enhances de novo lipid synthesis and is also related to protein synthesis throughout cell growth processes (Manning & Toker, 2017; Röhrig & Schulze, 2016). mTORC1 regulates anabolism by inducing lipid, nucleotide and protein synthesis, therefore hindering catabolic processes such as autophagy. mTORC1 activation also increases cholesterol synthesis and uptake through the regulation of SREBP-2, causing cholesterol accumulation (Musso et al., 2016).

After the activation of the mTORC1 pathway, SREBP is regulated by phosphatidate phosphatase lipin 1 (LPIN1), an enzyme that converts phosphatidic acid into diacylglycerol and is involved in the synthesis of phospholipids, such as phosphatidylcholine, or triglycerides. The AKT phosphorylation pathway inhibits the glycogen synthase kinase 3β (GSK3β), further inducing SREBP processing and improving its stability to promote the expression of lipid biosynthesis enzymes (Röhrig & Schulze, 2016).

Cholesterol and fatty acid homeostasis vary in different nutritional states; SREBP-1 and SREBP-2 are activated under conditions of energy abundance, whereas in energy depleted states, they are inhibited. ER stress may be initiated by SREBP-1 activation as well as protein synthesis. To reestablish homeostasis, the unfolded protein response (UPR) is activated; however, in addition to the existing ER stress, SREBP-1c continues to be activated, establishing the conditions for cellular stress, inflammation response and ultimately steatosis. The resulting lipotoxicity is a feature that characterizes obesity, diabetes mellitus, atherosclerosis, non-alcoholic fatty liver disease (NAFLD), NASH, and other metabolic disorders (Shimano & Sato, 2017). In NAFLD, SREBP-2 might be atypically activated, stimulating excessive cholesterol accumulation (Zhang et al., 2018).

Hepatosteatosis may also be caused by the stimulation of TSH receptors expressed in hepatocytes via SREBP-1c. Thyroid hormone induces the expression of HMG-CoAR and farnesyl pyrophosphate synthetase (FDPS) to stimulate hepatic cholesterol synthesis, upregulates LDL receptor levels to increase cholesterol efflux from peripheral tissues, and intensifies HDL metabolism by stimulating cholesteryl ester transfer protein (CETP) activity (Sinha et al., 2018). Figure 1 schematizes the studied pathways association with hepatosteatosis.

Figure 1.
The role of cell signaling molecules regarding steatosis

The transcription of genes responsible for metabolic regulation is controlled by various transcription factors that are susceptible to altered nutrient conditions, including peroxisome proliferator-activated receptors (PPARs), which can be classified as PPAR-α, PPAR-δ, and PPAR-γ (Kersten & Stienstra, 2017; McMullen et al., 2014; Welty et al., 2016).

PPARs are intrinsically important regulators of lipid metabolism. They are transcription factors that shift to the nucleus following activation by ligands, forming heterodimers with binding partners such as retinoid X receptors (RXR), and then binding to specific DNA sequences known as PPAR response elements (PPREs) in the promoters of target genes, where they substitute corepressors with coactivators to finally activate the expression of the target gene. These genes are essential to various metabolic processes, especially lipid and glucose homeostasis, and can play a role in cell differentiation (Janani & Kumari, 2015; Lodhi & Semenkovich, 2014; Régnier et al., 2018).

PPAR-α is mainly expressed in the liver where it regulates lipid metabolism, especially during fasting. It is also expressed in the large and small intestines, heart, kidneys, spleen and in white and brown adipose tissues, where it regulates fatty acid oxidation, ketogenesis, adipogenesis, energy balance, and triglyceride metabolism and storage (Feng et al., 2016; Jahansouz et al., 2018; McMullen et al., 2014).

PPAR-α is a fatty acid receptor and a major regulator of hepatocyte and fatty acid homeostasis. It acts as a nutritional sensor, activated by high levels of circulating triglycerides in the liver, which in turn activates the expression of genes involved in fatty acid β-oxidation and inhibits lipid accumulation. PPAR-α is also a transcriptional regulator of genes involved in fatty acid transport and in peroxisomal and mitochondrial β–oxidation. PPAR-α phosphorylation reduces triglyceride levels by increasing lipolysis, inducing LPL to catalyze the hydrolysis of triglycerides into free fatty acids and monoacylglycerol; PPAR-α inhibits VLDL production and stimulates the production of ApoA-I and ApoA-II, which are associated with HDL (Musso et al., 2016; Pawlak et al., 2015; Su et al., 2017).

During fasting, fatty acid oxidation in the hepatic and peripheral tissues increases, producing acetyl-CoA, which is converted into ketone bodies to provide energy for peripheral tissues. Then, PPAR-α upregulates mitochondrial HMG-CoA synthetase, which in turn catalyzes the reaction of acetyl-CoA and acetoacetyl-CoA to produce HMG-CoA and CoA (Pawlak et al., 2015). Additionally, glucagon stimulates PPAR-α activity. Insulin, along with the mTORC1/AKT/PKB pathway and its downstream effector S6K2, inhibits PPAR-α through the recruitment of nuclear receptor corepressor 1 (NCOR1), hence increasing ketogenesis. Hepatic PPAR-α activity can also be stimulated by AMPK during fasting. Fatty acid oxidation is increased by adiponectin through AMPK-dependent PPAR-α activation (Pawlak et al., 2015; Régnier et al., 2018).

PPAR-α agonists increase human SREBP-1c transcriptional activity through interacting with a direct repeat-1 element in the SREBP-1c promoter, and it also acts on SREBP-1c transcription through cross-regulation with the LXR signaling pathway (Pawlak et al., 2015). Finally, SIRT1 connects cell metabolism to PPAR-α activity (Régnier et al., 2018).

PPAR-α agonists and omega-3 reduce plasma triglyceride levels and LDL particles, and synthetic PPAR-α agonists may be very useful in the treatment of NAFLD and cardiovascular diseases (Pawlak et al., 2015; Welty et al., 2016). In previous studies, dietary abundance and PPAR-α activation reversed the normal suppression of autophagy. This resulted in increased autophagic lipid degradation and represents a potential pharmacological target to inhibit several chronic liver diseases related to lipid accumulation (Zhang et al., 2018).

PPAR-δ (or PPAR-β) is expressed in most cells and tissues, particularly in adipose tissue, where it promotes lipid usage by stimulating genes involved in fatty acid oxidation (Jahansouz et al., 2018; Janani & Kumari, 2015;McMullen et al., 2014).

PPAR-δ activation lowers LDL and triglyceride levels, mitigating triglyceride accumulation in the liver and adipose tissue, increasing lipid catabolism and consequently decreasing weight gain. Exercise upregulates PPAR-δ, which in turn helps with fatty acid and lipolytic activity. Currently, there are no pharmacological agonists for PPAR-δ (Welty et al., 2016). The simultaneous activation of PPAR-α and PPAR-δ initiates weight gain and fat accumulation due to PPAR-γ agonism. The combination of PPAR-δ and PPAR-γ agonists, in turn, can alleviate insulin resistance and regulate glucose metabolism (La Rosa Rodriguez & Kersten, 2017; Feng et al., 2016).

PPAR-γ is only expressed in a limited number of cells and tissues. It is highly abundant in adipose tissue, but it is also found in the liver, muscle, colon, and macrophages. PPAR-γ regulates adipogenesis and the balance between glucose and lipid oxidation (La Rosa Rodriguez & Kersten, 2017).

Adipose PPAR-γ protects the liver and skeletal muscle tissues against excess lipids and maintains normal organ function (Janani & Kumari, 2015).

PPAR-γ is involved in the regulation of insulin sensitivity via stimulating the secretion of adipocytokines, adiponectin, and leptin in adipocytes, which are mediators of insulin action in peripheral tissues (Janani & Kumari, 2015; Lodhi & Semenkovich, 2014). PPAR-γ is also involved in lipid metabolism, lipogenesis, fat storage, adipokine production, thermogenesis, and adipocyte survival, differentiation and function (Lodhi & Semenkovich, 2014).

Due to its adipogenic function, increasing PPAR-γ activity causes weight gain; however, caloric restriction does not induce changes in the activity of PPAR-γ or PPAR-δ (Jahansouz et al., 2018; Janani & Kumari, 2015).

The activation of the Jun N-terminal kinase (JNK) signaling pathway is crucial for the autophagic process, contributing to Beclin-1 expression, which modulates autophagy dysregulation and p53 phosphorylation. The inhibition of JNK suppresses autophagy and attenuates insulin resistance. Autophagic dysfunction may lead to excessive lipid accumulation in the liver, contributing to the progression of NAFLD (Zhang et al., 2018).

JNK signaling inhibits AKT in the insulin cascade. In this case, insulin resistance may occur when the membrane distribution of proto-oncogene tyrosine-protein kinase SRC is altered by saturated fatty acids, which causes the stimulation of JNK signaling (Welty et al., 2016; Yang et al., 2018). TNF-α signaling activates intracellular JNK and inhibits nuclear factor kappa-B kinase subunit β (IKK-β), which leads to increased activating phosphorylation of serine on IRS-1 instead of normal tyrosine phosphorylation. Thus, an adipocyte product can cause insulin resistance and metabolic syndrome (Welty et al., 2016). The autophagy and JNK/IKK-β signaling pathways are inhibited when the interaction between B-cell lymphoma 2 (Bcl-2) and Beclin-1 is increased, and AMPK activation is decreased. Moreover, when JNK phosphorylates Bcl-2, autophagy is stimulated (Navarro-Yepes et al., 2014).

The JNK pathway is presumed to be involved in the impaired activity of oxidative phosphorylation in advanced steatosis and NASH. JNK activation during steatosis-associated ER stress can modulate NCOR-1 activity, inhibiting PPAR-α and subsequently causing decreased mitochondrial activity and facilitating fat accumulation (Baiceanu et al., 2016). Figure 2 summarizes the cell signaling pathways involved in lipid metabolism, as previously described.

Figure 2
Cell signaling pathways involved in lipid metabolism.