2.2.3. Oxidative Phosphorylation
The main source of cellular energy is the electron transport chain (ETC) and oxidative phosphorylation through critical activities of protein complexes in the inner mitochondrial membrane. High-energy electrons released during the citric acid cycle and β-oxidation are captured by nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), resulting in NADH and FADH
2, respectively
[47][28]. NADH and FADH
2 molecules donate these high-energy electrons to the ETC
[48][29]. The transfer of electrons to O
2 is an energy-yielding reaction by the passage of electrons through a series of carriers, which constitute the ETC. These carriers include four complexes (complex I, II, III, IV) in the inner mitochondrial membrane. A fifth protein complex (complex V), also in the inner mitochondrial membrane, then serves to couple the energy-yielding reactions of electron transport to ATP synthesis. Complex I receives electrons from NADH, while complex II receives electrons from FADH
2. Complexes I and II provide electrons to Coenzyme Q (CoQ). CoQ (also called ubiquinone) is a small, lipid-soluble molecule that carries electrons through the MIM to complex III. Electrons are then transferred from complex III to cytochrome
c, which then carries electrons to complex IV (cytochrome oxidase), where they are finally transferred to O
2. Water is formed as a result of electron transfer from Complex IV to oxygen. At complexes I, III, and IV, free energy is released as electrons pass along the chain, which is utilized to pump protons from the mitochondrial matrix to the intermembranous region, forming a proton gradient. The potential energy stored in this gradient is then used by a fifth protein complex (complex V), which couples the flow of protons along the electrochemical gradient back across the MIM to the synthesis of ATP.
2.3. Regulation of Mitochondrial Fatty-Acid Oxidation and Reactive Oxygen Species Formation
CPT1 is inhibited by malonyl-CoA, which is formed during the first step of the synthesis of FFAs from acetyl-CoA by acetyl-CoA carboxylase
[32,35][21][24]. Insulin has been shown to increase malonyl-CoA synthesis, which inhibits CPTI. Glucagon, on the other hand, decreases malonyl-CoA synthesis, leading to an increase in β-oxidation
[49][30]. FFAs are degraded into acetyl-CoA molecules, which can either be fully degraded to CO
2 by the Krebs cycle or condensed into ketone bodies, which are re-oxidized in peripheral tissues during fasting
[30,32][19][21]. Under normal circumstances, this process carefully controls energy storage and disposal; however, it is hampered in patients with fatty liver disease, causing oxidative stress
[30,50,51][19][31][32]. Increased oxidative stress causes inflammation directly by activating a number of inflammatory-signaling pathways, including the NF-κB and JNK pathways, as well as indirectly by increasing the gene expression of inflammatory cytokines including TNF-α, TGF-β, and Fas ligand
[52][33]. Reduced mitophagy leads to an accumulation of significantly damaged mitochondria, which causes cell necrosis and the release of mitochondrial damage-associated molecular patterns (DAMPs), which may promote liver inflammation
[53][34].
ROS such as superoxide anions, peroxides, and others are generated in the cytosol by enzymes such as amino acid oxidases, cyclooxygenases, lipoxygenases, nitric oxide (NO) synthase, and xanthine oxidase
[54,55,56][35][36][37]. By transferring a single electron from NADPH to molecular oxygen, it becomes NADPH oxidase, which is the key source of ROS in liver diseases and produces superoxide anions in the mitochondria
[57,58,59][38][39][40].
Mitochondrial ROS activate AMPK
[33,35][22][24] and mitogen-activated protein kinases (MAPKs), including c-Jun N-terminal kinase (JNK)
[66][41]. AMPK induces PGC-1α and promotes glucose and fatty-acid oxidation. PGC-1α interacts with the peroxisome proliferator–activated receptor (PPAR) to increase mitochondrial fatty acid β-oxidation by inducing the expression of multiple fatty acid-metabolizing enzymes, such as CPT1 and acyl-CoA dehydrogenases
[67][42]. By activating NRF2, H
2O
2 production by mitochondria activates AMPK, which regulates antioxidant enzyme expression
[35][24]. Proinflammatory cytokines such as interleukin 6 (IL-6), tumor necrosis factor (TNF-α), and interleukin 1β (IL-1β) are also stimulated by ROS development. The presence of oxidative stress in cells may set off a chain reaction that contributes to increased mtDNA damage and increased mitochondrial dysfunction
[68][43].
2.4. Mitochondrial Fatty-Acid Oxidation Defects
The last three steps of long-chain fatty-acid oxidation are catabolized by MTP, a heterooctamer of 4 α- and 4 β- subunits associated with the inner mitochondrial membrane
[69][44]. The long-chain 3-enoyl-CoA hydratase enzymatic activity resides in the α-subunit amino-terminal domain while the carboxy-terminal domain contains the LCHAD enzymatic activity. The long-chain 3-ketoacyl-CoA thiolase enzymatic activity resides in the β-subunit. Both MTP subunit genes,
HADHA and
HADHB, are localized to chromosome 2p23
[70][45], and share a bidirectional promoter
[71][46]. MTP defects are recessively inherited and can manifest as either an isolated LCHAD deficiency or complete MTP deficiency, in which all three enzymes are deficient
[72][47]. Infants born with these recessively inherited disorders typically present with nonketotic hypoglycemia and hepatic encephalopathy, which may progress to coma and death
[73][48]. They can also present as unexpected death, cardiomyopathy, or slowly progressive myopathy and peripheral neuropathy
[74,75][49][50]. A common mutation in exon 15 of the α-subunit, G1528C, which causes an amino acid change at position 474 in the LCHAD catalytic site from glutamic acid to glutamine (E474Q)
[76,77][51][52].
3. Mechanism of the Association between Fetal LCHAD Deficiency and AFLP
The precise mechanism for the association between fetal LCHAD-deficiency and maternal AFLP is not fully elucidated.
Figure 3 depicts the likely mechanisms underlying the association between fetal LCHAD and maternal AFLP. Mitochondrial dysfunction and damage have been documented in children with LCHAD deficiency
[89,90,91][53][54][55]. It is likely that hepatotoxic long-chain 3-hydroxyacyl fatty acid intermediates produced in the fetus due to blockages in the mitochondrial β-oxidation caused by the fetal LCHAD deficiency will accumulate in the maternal circulation causing liver injury and AFLP. It is also highly likely that the placenta is the major source for the 3-hydroxy fatty acid metabolites.
Figure 3. A schematic representation of possible mechanisms leading to the development of maternal AFLP associated with fetal LCHAD deficiency. MTP: Mitochondrial Trifunctional Protein; LCHAD: Long chain acyl-CoA dehydrogenase.
The accumulation of cytotoxic 3-hydroxy fatty acids in maternal circulation is likely to cause microvesicular steatosis in maternal liver with disruption of β-oxidation and oxidative phosphorylation processes in the liver, causing decreased ATP production and increased ROS, leading to further mitochondrial damage
[97,98,99][56][57][58]. Damage to the mitochondrial membrane in the liver was reported in a rat model of microvesicular steatosis
[100,101][59][60]. Increased superoxide generation associated with reduced respiration and alterations to mitochondrial calcium homeostasis were also reported in placenta isolated from patients with AFLP.
4. Conclusions
In conclusion, AFLP is a rare but life-threatening complication of pregnancy with serious fetal and maternal consequences. The pathogenesis of AFLP is strongly linked to mitochondrial dysfunction associated with fetal LCHAD deficiency. Current evidence supports an important role for placental injury and oxidative stress causing subcellular damage and mitochondrial dysfunction. The release of toxic 3-hydroxy intermediate metabolites from the LCHAD-deficient placenta and fetus into the maternal circulation is likely to be a culprit in inducing AFLP in the pregnant mother. Further research is needed to explore the role of 3-hydroxy fatty acid metabolites in the pathogenesis of AFLP.