Palmitoylcarnitine modulates palmitoylation of proteins: implication for differentiation of neural cells

Molecular mechanism of palmitic acid and its derivatives in tumor progression

Palmitic acid (PA) is a saturated fatty acid commonly found in coconut oil and palm oil. It serves as an energy source for the body and plays a role in the structure and function of cell membranes. Beyond its industrial applications, PA has gained attention for its potential therapeutic properties. Modern pharmacological studies have demonstrated that PA exhibits anti-inflammatory, antioxidant, and immune-enhancing effects. In recent years, PA has emerged as a promising anti-tumor agent with demonstrated efficacy against various malignancies including gastric cancer, liver cancer, cervical cancer, breast cancer, and colorectal cancer. Its anti-tumor effects encompass inducing apoptosis in tumor cells, inhibiting tumor cell proliferation, suppressing metastasis and invasion, enhancing sensitivity to chemotherapy, and improving immune function. The main anticancer mechanism of palmitic acid (PA) involves the induction of cell apoptosis through the mitochondrial pathway, facilitated by the promotion of intracellular reactive oxygen species (ROS) generation. PA also exhibits interference with the cancer cell cycle, leading to cell cycle arrest predominantly in the G1 phase. Moreover, PA induces programmed cell autophagy death, inhibits cell migration, invasion, and angiogenesis, and synergistically enhances the efficacy of chemotherapy drugs while reducing adverse reactions. PA acts on various intracellular and extracellular targets, modulating tumor cell signaling pathways, including the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt), endoplasmic reticulum (ER), B Cell Lymphoma-2 (Bcl-2), P53, and other signaling pathways. Furthermore, derivatives of PA play a significant regulatory role in tumor resistance processes. This paper provides a comprehensive review of recent studies investigating the anti-tumor effects of PA. It summarizes the underlying mechanisms through which PA exerts its anti-tumor effects, aiming to inspire new perspectives for the treatment of malignant tumors in clinical settings and the development of novel anti-cancer drugs.

SARS-CoV-2 infection dysregulates NAD metabolism

Introduction

Severe COVID-19 results initially in pulmonary infection and inflammation. Symptoms can persist beyond the period of acute infection, and patients with Post-Acute Sequelae of COVID (PASC) often exhibit a variety of symptoms weeks or months following acute phase resolution including continued pulmonary dysfunction, fatigue, and neurocognitive abnormalities. We hypothesized that dysregulated NAD metabolism contributes to these abnormalities.

Methods

RNAsequencing of lungs from transgenic mice expressing human ACE2 (K18-hACE2) challenged with SARS-CoV-2 revealed upregulation of NAD biosynthetic enzymes, including NAPRT1, NMNAT1, NAMPT, and IDO1 6 days post-infection.

Results

Our data also demonstrate increased gene expression of NAD consuming enzymes: PARP 9,10,14 and CD38. At the same time, SIRT1, a protein deacetylase (requiring NAD as a cofactor and involved in control of inflammation) is downregulated. We confirmed our findings by mining sequencing data from lungs of patients that died from SARS-CoV-2 infection. Our validated findings demonstrating increased NAD turnover in SARS-CoV-2 infection suggested that modulating NAD pathways may alter disease progression and may offer therapeutic benefits. Specifically, we hypothesized that treating K18-hACE2 mice with nicotinamide riboside (NR), a potent NAD precursor, may mitigate lethality and improve recovery from SARS-CoV-2 infection. We also tested the therapeutic potential of an anti- monomeric NAMPT antibody using the same infection model. Treatment with high dose anti-NAMPT antibody resulted in significantly decreased body weight compared to control, which was mitigated by combining HD anti-NAMPT antibody with NR. We observed a significant increase in lipid metabolites, including eicosadienoic acid, oleic acid, and palmitoyl carnitine in the low dose antibody + NR group. We also observed significantly increased nicotinamide related metabolites in NR treated animals.

Discussion

Our data suggest that infection perturbs NAD pathways, identify novel mechanisms that may explain some pathophysiology of CoVID-19 and suggest novel strategies for both treatment and prevention.

Determining the Bioenergetic Capacity for Fatty Acid Oxidation in the Mammalian Nervous System

The metabolic state of the brain can greatly impact neurologic function. Evidence of this includes the therapeutic benefit of a ketogenic diet in neurologic diseases, including epilepsy. However, brain lipid bioenergetics remain largely uncharacterized. The existence, capacity, and relevance of mitochondrial fatty acid β-oxidation (FAO) in the brain are highly controversial, with few genetic tools available to evaluate the question. We have provided evidence for the capacity of brain FAO using a pan-brain-specific conditional knockout (KO) mouse incapable of FAO due to the loss of carnitine palmitoyltransferase 2, the product of an obligate gene for FAO (CPT2B-/-). Loss of central nervous system (CNS) FAO did not result in gross neuroanatomical changes or systemic differences in metabolism. Loss of CPT2 in the brain did not result in robustly impaired behavior. We demonstrate by unbiased and targeted metabolomics that the mammalian brain oxidizes a substantial quantity of long-chain fatty acids in vitro and in vivo Loss of CNS FAO results in robust accumulation of long-chain acylcarnitines in the brain, suggesting that the mammalian brain mobilizes fatty acids for their oxidation, irrespective of diet or metabolic state. Together, these data demonstrate that the mammalian brain oxidizes fatty acids under normal circumstances with little influence from or on peripheral tissues.

Enteric neuropathy can be induced by high fat diet in vivo and palmitic acid exposure in vitro

Objective

Obese and/or diabetic patients have elevated levels of free fatty acids and increased susceptibility to gastrointestinal symptoms. Since the enteric nervous system is pivotal in regulating gastrointestinal functions alterations or neuropathy in the enteric neurons are suspected to occur in these conditions. Lipid induced intestinal changes, in particular on enteric neurons, were investigated in vitro and in vivo using primary cell culture and a high fat diet (HFD) mouse model.

Design

Mice were fed normal or HFD for 6 months. Intestines were analyzed for neuronal numbers, remodeling and lipid accumulation. Co-cultures of myenteric neurons, glia and muscle cells from rat small intestine, were treated with palmitic acid (PA) (0 – 10⁻³ M) and / or oleic acid (OA) (0 – 10⁻³ M), with or without modulators of intracellular lipid metabolism. Analyses were by immunocyto- and histochemistry.

Results

HFD caused substantial loss of myenteric neurons, leaving submucous neurons unaffected, and intramuscular lipid accumulation in ileum and colon. PA exposure in vitro resulted in neuronal shrinkage, chromatin condensation and a significant and concentration-dependent decrease in neuronal survival; OA exposure was neuroprotective. Carnitine palmitoyltransferase 1 inhibition, L-carnitine- or alpha lipoic acid supplementation all counteracted PA-induced neuronal loss. PA or OA alone both caused a significant and concentration-dependent loss of muscle cells in vitro. Simultaneous exposure of PA and OA promoted survival of muscle cells and increased intramuscular lipid droplet accumulation. PA exposure transformed glia from a stellate to a rounded phenotype but had no effect on their survival.

Conclusions

HFD and PA exposure are detrimental to myenteric neurons. Present results indicate excessive palmitoylcarnitine formation and exhausted L-carnitine stores leading to energy depletion, attenuated acetylcholine synthesis and oxidative stress to be main mechanisms behind PA-induced neuronal loss.High PA exposure is suggested to be a factor in causing diabetic neuropathy and gastrointestinal dysregulation.

Palmitoylcarnitine affects localization of growth associated protein GAP-43 in plasma membrane subdomains and its interaction with Gα(o) in neuroblastoma NB-2a cells

Palmitoylcarnitine was observed previously to promote differentiation of neuroblastoma NB-2a cells, and to affect protein kinase C (PKC). Palmitoylcarnitine was also observed to increase palmitoylation of several proteins, including a PKC substrate, whose expression augments during differentiation of neural cells-a growth associated protein GAP-43, known to bind phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)]. Since palmitoylated proteins are preferentially localized in sphingolipid- and cholesterol-rich microdomains of plasma membrane, the present study has been focused on a possible effect of palmitoylcarnitine on GAP-43 localization in these microdomains. Palmitoylcarnitine treatment resulted in GAP-43 appearance in floating fractions (rafts) in sucrose gradient and increased co-localization with cholesterol and with PI(4,5)P(2), although co-localization of both lipids decreased. GAP-43 disappeared from raft fraction upon treatment with 2-bromopalmitate (an inhibitor of palmitoylating enzymes) and after treatment with etomoxir (carnitine palmitoyltransferase I inhibitor). Raft localization of GAP-43 was completely abolished by treatment with methyl-β-cyclodextrin, a cholesterol binding agent, while there was no change upon sequestration of PI(4,5)P(2) with neomycin. GAP-43 co-precipitated with a monomeric form of Gα(o), a phenomenon diminished after palmitoylcarnitine treatment and paralleled by a decrease of Gα(o) in the raft fraction. These observations point to palmitoylation of GAP-43 as a mechanism leading to an increased localization of this protein in microdomains of plasma membrane rich in cholesterol, in majority different, however, from microdomains in which PI(4,5)P(2) is present. This localization correlates with decreased interaction with Gα(o) and suppression of its activity-an important step regulating neural cell differentiation.

Palmitoylcarnitine regulates estrification of lipids and promotes palmitoylation of GAP-43

Palmitoylcarnitine was previously shown to promote differentiation of neuroblastoma NB-2a cells. It was also observed to increase palmitoylation of several proteins and to diminish incorporation of palmitic acid to phospholipids, as well as to affect growth associated protein GAP-43 by decreasing its phosphorylation and interaction with protein kinase C. The present study was focused on influence of palmitoylcarnitine on palmitoylation of GAP-43 and lipid metabolism. Althought palmitoylcarnitine did not significantly affect the total phospholipids and fatty acid content, it increased incorporation of palmitate moiety to triacylglicerides and cholesterol esters, with a decrease of free cholesterol content. The presence of palmitoylcarnitine significantly increased the amount of covalently bound palmitate to GAP-43, which can regulate the signal transduction pathways.

Palmitoylcarnitine modulates interaction between protein kinase C betaII and its receptor RACK1

Palmitoylcarnitine, known to promote differentiation of neuroblastoma NB-2a cells as well as to inhibit protein kinase C (PKC) activity and to decrease phorbol ester binding, was shown previously to diminish the amount of complex formed between PKCdelta and its substrate GAP-43. In the present work we studied the effect of palmitoylcarnitine on the interaction between PKCbetaII and its receptor RACK1. Palmitoylcarnitine was found to decrease autophosphorylation of PKCbetaII on serine in a concentration-dependent manner and to decrease the amount of PKCbetaII/RACK1 complex. The effect of palmitoylcarnitine on cellular localization was found to be dependent on the presence of ATP; palmitoylcarnitine lowered the amount of PKCbetaII in cytosol and decreased the amount of PKCbetaII-RACK1 complex in membrane in the absence of ATP. Palmitoylcarnitine also reversed the effect of phorbol ester on the increase in the amount of PKCbetaII in membrane. Palmitoylcarnitine binds to PKCbetaII through hydrophobic interactions, although acylation of PKCbetaII by the palmitate moiety has been excluded. The presence of palmitoylcarnitine did not have any additive effect on the diminution of PKCbetaII-RACK1 complex formation in the presence of a RACK1-binding peptide from within the C2 region of PKCbetaII. These results rather exclude a possibility of interaction of palmitoylcarnitine with the C2 domain and suggest a possible interaction with the V5 domain and a conformational change affecting the C1 region.