Understanding lipotoxicity in NAFLD pathogenesis: is CD36 a key driver?

Abstract

Non-alcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease worldwide. NAFLD stages range from simple steatosis (NAFL) to non-alcoholic steatohepatitis (NASH) which can progress to cirrhosis and hepatocellular carcinoma. One of the crucial events clearly involved in NAFLD progression is the lipotoxicity resulting from an excessive fatty acid (FFA) influx to hepatocytes. Hepatic lipotoxicity occurs when the capacity of the hepatocyte to manage and export FFAs as triglycerides (TGs) is overwhelmed. This review provides succinct insights into the molecular mechanisms responsible for lipotoxicity in NAFLD, including ER and oxidative stress, autophagy, lipoapotosis and inflammation. In addition, we highlight the role of CD36/FAT fatty acid translocase in NAFLD pathogenesis. Up-to-date, it is well known that CD36 increases FFA uptake and, in the liver, it drives hepatosteatosis onset and might contribute to its progression to NASH. Clinical studies have reinforced the significance of CD36 by showing increased content in the liver of NAFLD patients. Interestingly, circulating levels of a soluble form of CD36 (sCD36) are abnormally elevated in NAFLD patients and positively correlate with the histological grade of hepatic steatosis. In fact, the induction of CD36 translocation to the plasma membrane of the hepatocytes may be a determining factor in the physiopathology of hepatic steatosis in NAFLD patients. Given all these data, targeting the fatty acid translocase CD36 or some of its functional regulators may be a promising therapeutic approach for the prevention and treatment of NAFLD.

Fig. 1. Lipotoxic effects mediated by FFAs contribute to NAFLD progression.

FFAs-induced lipotoxicity promotes ER and oxidative stress, insulin resistance and impairs autophagy. As a consequence, FFAs activate apoptotic cascades thus promoting tissue damage and inflammation. Altogether, these molecular events contribute to NAFLD progression.

Fig. 2. Endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR) signalling.

ER stress signalling involves three main protein sensors, PRKR-like endoplasmic reticulum kinase (PERK), inositol-requiring enzyme-1α (IRE1α) and activating transcription factor-6 (ATF6). These proteins remain inactive while are bound to the intraluminal chaperone glucose-regulated protein 78 (GRP78), also named Binding Protein (BiP). In response to ER stress, these mediators become activated and released, thereby triggering molecular cascades that activate the unfolded protein response (UPR). Overall, activation of each sensor promotes ATF4, XBP1s and ATF6 translocation to the nucleus to induce the expression of their relevant target genes associated with apoptosis, inflammation, antioxidant response and protein folding mechanisms, among others, to restore ER homeostasis.

Fig. 3. FFAs-mediated ER stress signalling contributes to hepatic insulin resistance during NAFLD.

a JNK1/2, activated by IRE1α, promotes inhibitory serine phosphorylation of the insulin receptor substrate 1 (IRS1), which interrupts insulin-dependent signalling. The key role of IRE1α–JNK1/2 was evidenced by the fact that the chaperone TUDCA, an ER stress inhibitor, blocked the activation of JNK1/2 and IRS1 serine phosphorylation, thereby preserving the integrity of the insulin signalling cascade. b S6K1 is phosphorylated at Thr389 in response to PA^,, a mechanism described earlier to mediate IRS1 phosphorylation at Ser307. In line with these findings, S6K1 deficiency in hepatocytes blocked the effects of tunicamycin, a classical ER stressor, and also ameliorated PA-induced insulin resistance, suggesting that targeting S6K1 is an attractive strategy against ER-mediated lipotoxicity and insulin resistance.

Fig. 4. FFAs activate apoptosis via intrinsic or extrinsic pathways.

Whereas the intrinsic apoptotic mechanism is initiated by intracellular stimuli such as oxidative stress, ER stress or organelle dysfunction, the extrinsic pathway is activated in response to external stimuli, namely by binding of death ligands, such as TNF-related apoptosis-inducing ligand (TRAIL), TNFα or Fas (CD95/APO-1), to their respective death receptors (DR) in the cell surface. In hepatocytes, the apoptotic signals from DR are not robust enough to trigger the effector caspase cascade, so the intrinsic pathway is also activated to boost the apoptotic response^,. The induction of the intrinsic pathway involves a decrease of anti-apoptotic proteins such as Bcl2 and the translocation of pro-apoptotic members (Bax, Bak) to the mitochondria triggering cytochrome c release and other apoptosis-inducing factors to the cytosol, thereby activating procaspase-9 and downstream apoptotic effectors.

Fig. 5. Intercellular communication between liver cells is altered in NASH livers.

In healthy liver, different hepatic cell types such as hepatocytes, KCs, HSCs and LSECs, communicate and regulate each other by secreting signalling mediators. Upon NAFLD condition, after FFAs overload, damaged cells release higher amounts of pro-inflammatory cytokines, damage-associated patterns (DAMPs), extracellular vesicles (EVs) and other molecules that can activate HSCs and KCs and promote LSECs to lose their fenestrations, likely contributing in a coordinated manner to the progression of NASH to more severe stages of NAFLD.

Fig. 6. Modulation of CD36 expression in the liver directly affects NAFLD.

CD36 not only acts as a FFA transporter but also regulates β-oxidation and autophagy among others lipid metabolism pathways in liver cells. Overexpression of hepatic CD36 concurs with a marked elevation of hepatic FFAs uptake and decreased β-oxidation and autophagy, thus contributing to hepatosteatosis. Conversely, downregulation of hepatic CD36 diminishes FFAs uptake and increases β-oxidation and autophagy protecting against hepatosteatosis.