Inhibition of protein palmitoylation, raft localization, and T cell signaling by 2-bromopalmitate and polyunsaturated fatty acids

Inhibition of fatty acid uptake by TGR5 prevents diabetic cardiomyopathy

Diabetic cardiomyopathy is characterized by myocardial lipid accumulation and cardiac dysfunction. Bile acid metabolism is known to play a crucial role in cardiovascular and metabolic diseases. Takeda G-protein-coupled receptor 5 (TGR5), a major bile acid receptor, has been implicated in metabolic regulation and myocardial protection. However, the precise involvement of the bile acid-TGR5 pathway in maintaining cardiometabolic homeostasis remains unclear. Here we show decreased plasma bile acid levels in both male and female participants with diabetic myocardial injury. Additionally, we observe increased myocardial lipid accumulation and cardiac dysfunction in cardiomyocyte-specific TGR5-deleted mice (both male and female) subjected to a high-fat diet and streptozotocin treatment or bred on the diabetic db/db genetic background. Further investigation reveals that TGR5 deletion enhances cardiac fatty acid uptake, resulting in lipid accumulation. Mechanistically, TGR5 deletion promotes localization of CD36 on the plasma membrane through the upregulation of CD36 palmitoylation mediated by the palmitoyl acyltransferase DHHC4. Our findings indicate that the TGR5-DHHC4 pathway regulates cardiac fatty acid uptake, which highlights the therapeutic potential of targeting TGR5 in the management of diabetic cardiomyopathy.

Mechanisms and functions of protein S-acylation

Over the past two decades, protein S-acylation (often referred to as S-palmitoylation) has emerged as an important regulator of vital signalling pathways. S-Acylation is a reversible post-translational modification that involves the attachment of a fatty acid to a protein. Maintenance of the equilibrium between protein S-acylation and deacylation has demonstrated profound effects on various cellular processes, including innate immunity, inflammation, glucose metabolism and fat metabolism, as well as on brain and heart function. This Review provides an overview of current understanding of S-acylation and deacylation enzymes, their spatiotemporal regulation by sophisticated multilayered mechanisms, and their influence on protein function, cellular processes and physiological pathways. Furthermore, we examine how disruptions in protein S-acylation are associated with a broad spectrum of diseases from cancer to autoinflammatory disorders and neurological conditions.

Modulators for palmitoylation of proteins and small molecules

As an essential form of lipid modification for maintaining vital cellular functions, palmitoylation plays an important role in in the regulation of various physiological processes, serving as a promising therapeutic target for diseases like cancer and neurological disorders. Ongoing research has revealed that palmitoylation can be categorized into three distinct types: N-palmitoylation, O-palmitoylation and S-palmitoylation. Herein this paper provides an overview of the regulatory enzymes involved in palmitoylation, including palmitoyltransferases and depalmitoylases, and discusses the currently available broad-spectrum and selective inhibitors for these enzymes.

A palmitoylation-depalmitoylation relay spatiotemporally controls GSDMD activation in pyroptosis

Gasdermin D (GSDMD) is the executor of pyroptosis, which is important for host defence against pathogen infection. Following activation, caspase-mediated cleavage of GSDMD releases an amino-terminal fragment (GSDMD-NT), which oligomerizes and forms pores in the plasma membrane, leading to cell death and release of proinflammatory cytokines. The spatial and temporal regulation of this process in cells remains unclear. Here we identify GSDMD as a substrate for reversible S-palmitoylation on C192 during pyroptosis. The palmitoyl acyltransferase DHHC7 palmitoylates GSDMD to direct its cleavage by caspases. Subsequently, palmitoylation of GSDMD-NT promotes its translocation to the plasma membrane, where APT2 depalmitoylates GSDMD-NT to unmask the C192 residue and promote GSDMD-NT oligomerization. Perturbation of either palmitoylation or depalmitoylation suppresses pyroptosis, leading to increased survival of mice with lipopolysaccharide-induced lethal septic shock and increased sensitivity to bacterial infection. Our findings reveal a model through which a palmitoylation-depalmitoylation relay spatiotemporally controls GSDMD activation during pyroptosis.

Antiretroviral drugs from multiple classes induce loss of excitatory synapses between hippocampal neurons in culture

Introduction: Antiretroviral (ARV) drugs have improved prognoses for people living with HIV. However, HIV-associated neurocognitive disorders (HAND) persist despite undetectable viral loads. Some ARVs have been linked to neuropsychiatric effects that may contribute to HAND. Synapse loss correlates with cognitive decline in HAND and synaptic deficits may contribute to the neuropsychiatric effects of ARV drugs. Methods: Using an automated high content assay, rat hippocampal neurons in culture expressing PSD95-eGFP to label glutamatergic synapses and mCherry to fill neuronal structures were imaged before and after treatment with 25 clinically used ARVs. Results and Discussion: At a concentration of 10 μM the protease inhibitors nelfinavir and saquinavir, the non-nucleoside reverse transcriptase inhibitors etravirine and the 8-OH metabolite of efavirenz, the integrase inhibitor bictegravir, and the capsid inhibitor lenacapavir produced synaptic toxicity. Only lenacapavir produced synapse loss at the nanomolar concentrations estimated free in the plasma, although all 4 ARV drugs induced synapse loss at Cmax. Evaluation of combination therapies did not reveal synergistic synaptic toxicity. Synapse loss developed fully by 24 h and persisted for at least 3 days. Bictegravir-induced synapse loss required activation of voltage-gated Ca2+ channels and bictegravir, etravirine, and lenacapavir produced synapse loss by an excitotoxic mechanism. These results indicate that select ARV drugs might contribute to neuropsychiatric effects in combination with drugs that bind serum proteins or in disease states in which synaptic function is altered. The high content imaging assay used here provides an efficient means to evaluate new drugs and drug combinations for potential CNS toxicity.

Reduction of DHHC5-mediated beclin 1 S-palmitoylation underlies autophagy decline in aging

Autophagy is a lysosome-dependent degradation pathway essential for cellular homeostasis, which decreases with age. However, it is unclear how aging induces autophagy decline. Here we show the role of protein S-palmitoylation in autophagy. We identify the palmitoyl acyltransferase DHHC5 as a regulator of autophagy by mediating the palmitoylation of beclin 1, which in turn promotes the formation of ATG14L-containing class III phosphatidylinositol-3-kinase complex I and its lipid kinase activity by promoting the hydrophobic interactions between beclin 1 and adapter proteins ATG14L and VPS15. In aging brains of human and nonhuman primate, the levels of DHHC5 exhibit a marked decrease in expression. We show that DHHC5 deficiency in neurons leads to reduced cellular protein homeostasis in two established murine models of Alzheimer's disease, which exaggerates neurodegeneration in an autophagy-dependent manner. These findings identify reduction of DHHC5-mediated beclin 1 S-palmitoylation as an underlying mechanism by which aging induces autophagy decline.

Metabolic transitions regulate global protein fatty acylation

Intermediary metabolites and flux through various pathways have emerged as key determinants of post-translational modifications. Independently, dynamic fluctuations in their concentrations are known to drive cellular energetics in a bi-directional manner. Notably, intracellular fatty acid pools that drastically change during fed and fasted states act as precursors for both ATP production and fatty acylation of proteins. Protein fatty acylation is well regarded for its role in regulating structure and functions of diverse proteins; however, the effect of intracellular concentrations of fatty acids on protein modification is less understood. In this regard, we unequivocally demonstrate that metabolic contexts, viz. fed and fasted states, dictate the extent of global fatty acylation. Moreover, we show that presence or absence of glucose that influences cellular and mitochondrial uptake/utilization of fatty acids and affects palmitoylation and oleoylation, which is consistent with their intracellular abundance in fed and fasted states. Employing complementary approaches including click-chemistry, lipidomics, and imaging, we show the top-down control of cellular metabolic state. Importantly, our results establish the crucial role of mitochondria and retrograde signaling components like SIRT4, AMPK, and mTOR in orchestrating protein fatty acylation at a whole cell level. Specifically, pharmacogenetic perturbations that alter either mitochondrial functions and/or retrograde signaling affect protein fatty acylation. Besides illustrating the cross-talk between carbohydrate and lipid metabolism in mediating bulk post-translational modification, our findings also highlight the involvement of mitochondrial energetics.

S-palmitoylation regulates innate immune signaling pathways: molecular mechanisms and targeted therapies

S-palmitoylation is a reversible posttranslational lipid modification that targets cysteine residues of proteins and plays critical roles in regulating the biological processes of substrate proteins. The innate immune system serves as the first line of defense against pathogenic invaders and participates in the maintenance of tissue homeostasis. Emerging studies have uncovered the functions of S-palmitoylation in modulating innate immune responses. In this review, we focus on the reversible palmitoylation of innate immune signaling proteins, with particular emphasis on its roles in the regulation of protein localization, protein stability, and protein-protein interactions. We also highlight the potential and challenge of developing therapies that target S-palmitoylation or de-palmitoylation for various diseases.

Regulation of ERK2 activity by dynamic S-acylation

Extracellular signal-regulated kinases (ERK1/2) are key effector proteins of the mitogen-activated protein kinase pathway, choreographing essential processes of cellular physiology. Here, we discover that ERK1/2 are subject to S-acylation, a reversible lipid modification of cysteine residues, at C271/C254. The levels of ERK1/2 S-acylation are modulated by epidermal growth factor (EGF) signaling, mirroring its phosphorylation dynamics, and acylation-deficient ERK2 displays altered phosphorylation patterns. We show that ERK1/2 S-acylation is mediated by "writer" protein acyl transferases (PATs) and "eraser" acyl protein thioesterases (APTs) and that chemical inhibition of either lipid addition or removal alters ERK1/2's EGF-triggered transcriptional program. Finally, in a mouse model of metabolic syndrome, we find that ERK1/2 lipidation levels correlate with alterations in ERK1/2 lipidation writer/eraser expression, solidifying a link between ERK1/2 activity, ERK1/2 lipidation, and organismal health. This study describes how lipidation regulates ERK1/2 and offers insight into the role of dynamic S-acylation in cell signaling more broadly.

Diverse Roles of Protein Palmitoylation in Cancer Progression, Immunity, Stemness, and Beyond

Protein S-palmitoylation, a type of post-translational modification, refers to the reversible process of attachment of a fatty acyl chain-a 16-carbon palmitate acid-to the specific cysteine residues on target proteins. By adding the lipid chain to proteins, it increases the hydrophobicity of proteins and modulates protein stability, interaction with effector proteins, subcellular localization, and membrane trafficking. Palmitoylation is catalyzed by a group of zinc finger DHHC-containing proteins (ZDHHCs), whereas depalmitoylation is catalyzed by a family of acyl-protein thioesterases. Increasing numbers of oncoproteins and tumor suppressors have been identified to be palmitoylated, and palmitoylation is essential for their functions. Understanding how palmitoylation influences the function of individual proteins, the physiological roles of palmitoylation, and how dysregulated palmitoylation leads to pathological consequences are important drivers of current research in this research field. Further, due to the critical roles in modifying functions of oncoproteins and tumor suppressors, targeting palmitoylation has been used as a candidate therapeutic strategy for cancer treatment. Here, based on recent literatures, we discuss the progress of investigating roles of palmitoylation in regulating cancer progression, immune responses against cancer, and cancer stem cell properties.

Src family kinases engage differential pathways for encapsulation into extracellular vesicles

Extracellular vesicles (EVs) are heterogeneous biological nanoparticles secreted by all cell types. Identifying the proteins preferentially encapsulated in secreted EVs will help understand their heterogeneity. Src family kinases including Src and Fyn are a group of tyrosine kinases with fatty acylation modifications and/or multiple lysine residues (contributing charge interaction) at their N-terminus. Here, we demonstrate that Src and Fyn kinases were preferentially encapsulated in EVs and fatty acylation including myristoylation and palmitoylation facilitated their encapsulation. Genetic loss or pharmacological inhibition of myristoylation suppressed Src and/or Fyn kinase levels in EVs. Similarly, loss of palmitoylation reduced Fyn levels in EVs. Additionally, mutation of lysine at sites 5, 7, and 9 of Src kinase also inhibited the encapsulation of myristoylated Src into EVs. Knockdown of TSG101, which is a protein involved in the endosomal sorting complexes required for transport (ESCRT) protein complex mediated EVs biogenesis and led to a reduction of Src levels in EVs. In contrast, filipin III treatment, which disturbed the lipid raft structure, reduced Fyn kinase levels, but not Src kinase levels in EVs. Finally, elevated levels of Src protein were detected in the serum EVs of host mice carrying constitutively active Src-mediated prostate tumors in vivo. Collectively, the data suggest that different EVs biogenesis pathways exist and can regulate the encapsulation of specific proteins into EVs. This study provides an understanding of the EVs heterogeneity created by different EVs biogenesis pathways.

Palmitoylation regulates neuropilin-2 localization and function in cortical neurons and conveys specificity to semaphorin signaling via palmitoyl acyltransferases

Secreted semaphorin 3F (Sema3F) and semaphorin 3A (Sema3A) exhibit remarkably distinct effects on deep layer excitatory cortical pyramidal neurons; Sema3F mediates dendritic spine pruning, whereas Sema3A promotes the elaboration of basal dendrites. Sema3F and Sema3A signal through distinct holoreceptors that include neuropilin-2 (Nrp2)/plexinA3 (PlexA3) and neuropilin-1 (Nrp1)/PlexA4, respectively. We find that Nrp2 and Nrp1 are S-palmitoylated in cortical neurons and that palmitoylation of select Nrp2 cysteines is required for its proper subcellular localization, cell surface clustering, and also for Sema3F/Nrp2-dependent dendritic spine pruning in cortical neurons, both in vitro and in vivo. Moreover, we show that the palmitoyl acyltransferase ZDHHC15 is required for Nrp2 palmitoylation and Sema3F/Nrp2-dependent dendritic spine pruning, but it is dispensable for Nrp1 palmitoylation and Sema3A/Nrp1-dependent basal dendritic elaboration. Therefore, palmitoyl acyltransferase-substrate specificity is essential for establishing compartmentalized neuronal structure and functional responses to extrinsic guidance cues.

Cell cycle-dependent palmitoylation of protocadherin 7 by ZDHHC5 promotes successful cytokinesis

Cell division requires dramatic reorganization of the cell cortex, which is primarily driven by the actomyosin network. We previously reported that protocadherin 7 (PCDH7) gets enriched at the cell surface during mitosis, which is required to build up the full mitotic rounding pressure. Here, we report that PCDH7 interacts with and is palmitoylated by the palmitoyltransferase, ZDHHC5. PCDH7 and ZDHHC5 colocalize at the mitotic cell surface and translocate to the cleavage furrow during cytokinesis. The localization of PCDH7 depends on the palmitoylation activity of ZDHHC5. Silencing PCDH7 increases the percentage of multinucleated cells and the duration of mitosis. Loss of PCDH7 expression correlates with reduced levels of active RhoA and phospho-myosin at the cleavage furrow. This work uncovers a palmitoylation-dependent translocation mechanism for PCDH7, which contributes to the reorganization of the cortical cytoskeleton during cell division.

Luminal H2 O2 promotes ER Ca2+ dysregulation and toxicity of palmitate in insulin-secreting INS-1E cells

The endoplasmic reticulum (ER) lumen is not only the major site for the assembly and folding of newly synthesized proteins but also the main intracellular Ca²⁺ store. Ca²⁺ ions are involved in versatile biochemical processes, including posttranslational processing and folding of nascent proteins. Disruption of ER Ca²⁺ homeostasis is usually accompanied by an ER stress response that can ultimately lead to apoptosis if unresolved. Abnormal ER Ca²⁺ depletion has been linked to pancreatic β-cell dysfunction and death under lipotoxic conditions. However, the underlying mechanisms how the β-cell toxic saturated free fatty acid palmitate perturbs ER Ca²⁺ homeostasis and its interplay with other organelles are not fully understood. In the present study, we demonstrate that treatment of insulin-secreting INS-1E cells with palmitate diminished ER Ca²⁺ levels, elevated cytosolic/mitochondrial Ca²⁺ content, lowered the mitochondrial membrane potential, and ATP content. In addition, palmitate-pretreated β-cells contained significantly less luminal Ca²⁺ , revealed a severely impaired ER Ca²⁺ reuptake rate, and substantially lower insulin content. Importantly, detoxification of luminal H₂ O₂ by expression of the ER-resident glutathione peroxidase 8 (GPx8) abrogated the lipotoxic effects of palmitate. Moreover, GPx8 supported oxidative protein folding and preserved insulin content under lipotoxic conditions. A direct involvement of luminal H₂ O₂ in palmitate-mediated ER Ca²⁺ depletion could be corroborated by the ectopic expression of an ER-luminal active catalase. Our data point to the critical role of luminal H₂ O₂ in palmitate-mediated depletion of ER Ca²⁺ through redox-dependent impairment of Ca²⁺ ATPase pump activity upstream of mitochondrial dysfunction in insulin-secreting INS-1E cells.

Trypanosoma brucei Acyl-Protein Thioesterase-like (TbAPT-L) Is a Lipase with Esterase Activity for Short and Medium-Chain Fatty Acids but Has No Depalmitoylation Activity

Dynamic post-translational modifications allow the rapid, specific, and tunable regulation of protein functions in eukaryotic cells. S-acylation is the only reversible lipid modification of proteins, in which a fatty acid, usually palmitate, is covalently attached to a cysteine residue of a protein by a zDHHC palmitoyl acyltransferase enzyme. Depalmitoylation is required for acylation homeostasis and is catalyzed by an enzyme from the alpha/beta hydrolase family of proteins usually acyl-protein thioesterase (APT1). The enzyme responsible for depalmitoylation in Trypanosoma brucei parasites is currently unknown. We demonstrate depalmitoylation activity in live bloodstream and procyclic form trypanosomes sensitive to dose-dependent inhibition with the depalmitoylation inhibitor, palmostatin B. We identified a homologue of human APT1 in Trypanosoma brucei which we named TbAPT-like (TbAPT-L). Epitope-tagging of TbAPT-L at N- and C- termini indicated a cytoplasmic localization. Knockdown or over-expression of TbAPT-L in bloodstream forms led to robust changes in TbAPT-L mRNA and protein expression but had no effect on parasite growth in vitro, or cellular depalmitoylation activity. Esterase activity in cell lysates was also unchanged when TbAPT-L was modulated. Unexpectedly, recombinant TbAPT-L possesses esterase activity with specificity for short- and medium-chain fatty acid substrates, leading to the conclusion, TbAPT-L is a lipase, not a depalmitoylase.

Protein palmitoylation-mediated palmitic acid sensing causes blood-testis barrier damage via inducing ER stress

Blood-testis barrier (BTB) damage promotes spermatogenesis dysfunction, which is a critical cause of male infertility. Dyslipidemia has been correlated with male infertility, but the major hazardous lipid and the underlying mechanism remains unclear. In this study, we firstly discovered an elevation of palmitic acid (PA) and a decrease of inhibin B in patients with severe dyszoospermia, which leaded us to explore the effects of PA on Sertoli cells. We observed a damage of BTB by PA. PA penetration to endoplasmic reticulum (ER) and its damage to ER structures were exhibited by microimaging and dynamic observation, and consequent ER stress was proved to mediate PA-induced Sertoli cell barrier disruption. Remarkably, we demonstrated a critical role of aberrant protein palmitoylation in PA-induced Sertoli cell barrier dysfunction. An ER protein, Calnexin, was screened out and was demonstrated to participate in this process, and suppression of its palmitoylation showed an ameliorating effect. We also found that ω-3 poly-unsaturated fatty acids down-regulated Calnexin palmitoylation, and alleviated BTB dysfunction. Our results indicate that dysregulated palmitoylation induced by PA plays a pivotal role in BTB disruption and subsequent spermatogenesis dysfunction, suggesting that protein palmitoylation might be therapeutically targetable in male infertility.

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An elevation of circulating PA was identified in patients with severe dyszoospermia.

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PA-induced over-palmitoylation in Sertoli cells leads to ER stress and BTB damage.

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The palmitoylation of the ER protein Calnexin regulates Sertoli cell barrier function.

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ω-3 PUFAs ameliorate PA-induced damage and over-palmitoylation in Sertoli cells.

Oxidized high-density lipoprotein promotes CD36 palmitoylation and increases lipid uptake in macrophages

Oxidized high-density lipoprotein (oxHDL) reduces the ability of cells to mediate reverse cholesterol transport and also shows atherogenic properties. Palmitoylation of cluster of differentiation 36 (CD36), an important receptor mediating lipoprotein uptake, is required for fatty acid endocytosis. However, the relationship between oxHDL and CD36 has not been described in mechanistic detail. Here, we demonstrate using acyl-biotin exchange analysis that oxHDL activates CD36 by increasing CD36 palmitoylation, which promotes efficient uptake in macrophages. This modification increased CD36 incorporation into plasma lipid rafts and activated downstream signaling mediators, such as Lyn, Fyn, and c-Jun N-terminal kinase, which elicited enhanced oxHDL uptake and foam cell formation. Furthermore, blocking CD36 palmitoylation with the pharmacological inhibitor 2-bromopalmitate decreased cell surface translocation and lowered oxHDL uptake in oxHDL-treated macrophages. We verified these results by transfecting oxHDL-induced macrophages with vectors expressing wildtype or mutant CD36 (mCD36) in which the cytoplasmic palmitoylated cysteine residues were replaced. We show that cells containing mCD36 exhibited less palmitoylated CD36, disrupted plasma membrane trafficking, and reduced protein stability. Moreover, in ApoE^−/−CD36^−/− mice, lipid accumulation at the aortic root in mice receiving the mCD36 vector was decreased, suggesting that CD36 palmitoylation is responsible for lipid uptake in vivo. Finally, our data indicated that palmitoylation of CD36 was dependent on DHHC6 (Asp-His-His-Cys) acyltransferase and its cofactor selenoprotein K, which increased the CD36/caveolin-1 interaction and membrane targeting in cells exposed to oxHDL. Altogether, our study uncovers a causal link between oxHDL and CD36 palmitoylation and provides insight into foam cell formation and atherogenesis.

S-acylation-dependent membrane microdomain localization of the regulatory Kvβ2.1 subunit

The voltage-dependent potassium (Kv) channel Kvβ family was the first identified group of modulators of Kv channels. Kvβ regulation of the α-subunits, in addition to their aldoketoreductase activity, has been under extensive study. However, scarce information about their specific α-subunit-independent biology is available. The expression of Kvβs is ubiquitous and, similar to Kv channels, is tightly regulated in leukocytes. Although Kvβ subunits exhibit cytosolic distribution, spatial localization, in close contact with plasma membrane Kv channels, is crucial for a proper immune response. Therefore, Kvβ2.1 is located near cell surface Kv1.3 channels within the immunological synapse during lymphocyte activation. The objective of this study was to analyze the structural elements that participate in the cellular distribution of Kvβs. It was demonstrated that Kvβ peptides, in addition to the cytoplasmic pattern, targeted the cell surface in the absence of Kv channels. Furthermore, Kvβ2.1, but not Kvβ1.1, targeted lipid raft microdomains in an S-acylation-dependent manner, which was concomitant with peptide localization within the immunological synapse. A pair of C-terminal cysteines (C301/C311) was mostly responsible for the specific palmitoylation of Kvβ2.1. Several insults altered Kvβ2.1 membrane localization. Therefore, growth factor-dependent proliferation enhanced surface targeting, whereas PKC activation impaired lipid raft expression. However, PSD95 stabilized Kvβ2.1 in these domains. This data shed light on the molecular mechanism by which Kvβ2.1 clusters into immunological synapses during leukocyte activation.

S-palmitoylation determines TMEM55B-dependent positioning of lysosomes

The spatiotemporal cellular distribution of lysosomes depends on active transport mainly driven by microtubule motors such as kinesins and dynein. Different protein complexes attach these molecular motors to their vesicular cargo. TMEM55B (also known as PIP4P1), as an integral lysosomal membrane protein, is a component of such a complex that mediates the retrograde transport of lysosomes by establishing interactions with the cytosolic scaffold protein JIP4 (also known as SPAG9) and dynein-dynactin. Here, we show that TMEM55B and its paralog TMEM55A (PIP4P2) are S-palmitoylated proteins that are lipidated at multiple cysteine residues. Mutation of all cysteines in TMEM55B prevents S-palmitoylation and causes retention of the mutated protein in the Golgi. Consequently, non-palmitoylated TMEM55B is no longer able to modulate lysosomal positioning and the perinuclear clustering of lysosomes. Additional mutagenesis of the dileucine-based lysosomal sorting motif in non-palmitoylated TMEM55B leads to partial missorting to the plasma membrane instead of retention in the Golgi, implicating a direct effect of S-palmitoylation on the adaptor protein-dependent sorting of TMEM55B. Our data suggest a critical role for S-palmitoylation in the trafficking of TMEM55B and TMEM55B-dependent lysosomal positioning.

Sodium hydrogen exchanger (NHE1) palmitoylation and potential functional regulation

AIMS

Determine the effect of palmitoylation on the sodium hydrogen exchanger isoform 1 (NHE1), a member of the SLC9 family.

MAIN METHODS

NHE1 expressed in native rat tissues or in heterologous cells was assessed for palmitoylation by acyl-biotinyl exchange (ABE) and metabolic labeling with [3H]palmitate. Cellular palmitoylation was inhibited using 2-bromopalmitate (2BP) followed by determination of NHE1 palmitoylation status, intracellular pH, stress fiber formation, and cell migration. In addition, NHE1 was activated with LPA treatment followed by determination of NHE1 palmitoylation status and LPA-induced change in intracellular pH was determined in the presence and absence of preincubation with 2BP.

KEY FINDINGS

In this study we demonstrate for the first time that NHE1 is palmitoylated in both cells and rat tissue, and that processes controlled by NHE1 including intracellular pH (pHi), stress fiber formation, and cell migration, are regulated in concert with NHE1 palmitoylation status. Importantly, LPA stimulates NHE1 palmitoylation, and 2BP pretreatment dampens LPA-induced increased pHi which is dependent on the presence of NHE1.

SIGNIFICANCE

Palmitoylation is a reversible lipid modification that regulates an array of critical protein functions including activity, trafficking, membrane microlocalization and protein-protein interactions. Our results suggest that palmitoylation of NHE1 and other control/signaling proteins play a major role in NHE1 regulation that could significantly impact multiple critical cellular functions.

Regulation of EGFR signalling by palmitoylation and its role in tumorigenesis

The epidermal growth factor receptor (EGFR) is an essential driver of oncogenic signalling, and EGFR inhibitors are some of the earliest examples of successful targeted therapies in multiple types of cancer. The tractability of EGFR as a therapeutic target is overshadowed by the inevitable drug resistance that develops. Overcoming resistance mechanisms requires a deeper understanding of EGFR regulation in cancer cells. In this review, we discuss our recent discovery that the palmitoyltransferase DHHC20 palmitoylates EGFR on the C-terminal domain and plays a critical role in signal regulation during oncogenesis. Inhibiting DHHC20 expression or mutating the palmitoylation site on EGFR alters the EGF-induced signalling kinetics from a transient signal to a sustained signal. The change in signalling is accompanied by a decrease in cell proliferation in multiple human cancer cell lines. Our in vivo studies demonstrate that ablating the gene Zdhhc20 by CRISPR/Cas9-mediated inhibition in a mouse model of oncogenic Kras-driven lung adenocarcinoma potently inhibits tumorigenesis. The negative effect on tumorigenesis is mediated by EGFR since the expression of a palmitoylation-resistant mutant form of EGFR also inhibits Kras-driven lung adenocarcinoma. Finally, reducing EGFR palmitoylation increases the sensitivity of multiple cancer cell lines to existing inhibitors of EGFR and downstream signalling effector pathways. We will discuss the implications of these effects and strategies for targeting these new vulnerabilities.

Synaptotagmin 7 is targeted to the axonal plasma membrane through γ-secretase processing to promote synaptic vesicle docking in mouse hippocampal neurons

Synaptotagmin 7 (SYT7) has emerged as a key regulator of presynaptic function, but its localization and precise role in the synaptic vesicle cycle remain the subject of debate. Here, we used iGluSnFR to optically interrogate glutamate release, at the single-bouton level, in SYT7KO-dissociated mouse hippocampal neurons. We analyzed asynchronous release, paired-pulse facilitation, and synaptic vesicle replenishment and found that SYT7 contributes to each of these processes to different degrees. 'Zap-and-freeze' electron microscopy revealed that a loss of SYT7 diminishes docking of synaptic vesicles after a stimulus and inhibits the recovery of depleted synaptic vesicles after a stimulus train. SYT7 supports these functions from the axonal plasma membrane, where its localization and stability require both γ-secretase-mediated cleavage and palmitoylation. In summary, SYT7 is a peripheral membrane protein that controls multiple modes of synaptic vesicle (SV) exocytosis and plasticity, in part, through enhancing activity-dependent docking of SVs.

Cysteine residues in the C-terminal tail of connexin32 regulate its trafficking

Gap junctions (GJs) are formed by the assembly of constituent transmembrane proteins called connexins (Cxs). Aberrations in this assembly of Cxs are observed in several genetic diseases as well as in cancers. Hence it becomes imperative to understand the molecular mechanisms underlying such assembly defect. The polarized cells in the epithelia express Connexin32 (Cx32). The C-terminal tail (CT) of Cx32 orchestrates several aspects of GJ dynamics, function and growth. The study here was aimed at determining if post-translational modifications, specifically, palmitoylation of cysteine residues, present in the CT of Cx32, has any effect on GJ assembly. The CT of Cx32 was found to harbor three cysteine residues, which are likely to be modified by palmitoylation. The study here has revealed for the first time that Cx32 is palmitoylated at cysteine 217 (C217) in cell line derived from prostate tumors. However, it was found that mutating C217 to alanine affected neither the trafficking nor the ability of Cx32 to assemble into GJs. Intriguingly, it was discovered that mutating cysteine 280 and 283, only in combination, blocked the trafficking of Cx32 from the trans-Golgi network to the cell surface. The mutants showed reduced stability due to enhanced lysosomal degradation. Overall, the findings reveal the importance of the two C-terminal cysteine residues of Cx32 in regulating its trafficking and stability and hence its ability to assemble into GJs.

Inhibitors of DHHC family proteins

Protein S-acylation is a prevalent post-translational protein lipidation that is dynamically regulated by 'writer' protein S-acyltransferases and 'eraser' acylprotein thioesterases. The protein S-acyltransferases comprise 23 aspartate-histidine-histidine-cysteine (DHHC)-containing proteins, which transfer fatty acid acyl groups from acyl-coenzyme A onto protein substrates. DHHC proteins are increasingly recognized as critical regulators of S-acylation-mediated cellular processes and pathology. As our understanding of the importance and breadth of DHHC-mediated biology and pathology expands, so too does the need for chemical inhibitors of this class of proteins. In this review, we discuss the challenges and progress in DHHC inhibitor development, focusing on 2-bromopalmitate, the most commonly used inhibitor in the field, and N-cyanomethyl-N-myracrylamide, a new broad-spectrum DHHC inhibitor. We believe that current and ongoing advances in structure elucidation, mechanistic interrogation, and novel inhibitor design around DHHC proteins will spark innovative strategies to modulate these critical proteins in living systems.

Lipid anchoring and electrostatic interactions target NOT-LIKE-DAD to pollen endo-plasma membrane

Phospholipases cleave phospholipids, major membrane constituents. They are thus essential for many developmental processes, including male gamete development. In flowering plants, mutation of phospholipase NOT-LIKE-DAD (NLD, also known as MTL or ZmPLA1) leads to peculiar defects in sexual reproduction, notably the induction of maternal haploid embryos. Contrary to previous reports, NLD does not localize to cytosol and plasma membrane of sperm cells but to the pollen endo-plasma membrane (endo-PM), a specific membrane derived from the PM of the pollen vegetative cell that encircles the two sperm cells. After pollen tube burst, NLD localizes at the apical region of the egg apparatus. Pharmacological approaches coupled with targeted mutagenesis revealed that lipid anchoring together with electrostatic interactions are involved in the attachment of NLD to this atypical endo-PM. Membrane surface-charge and lipid biosensors indicated that phosphatidylinositol-4,5-bisphosphate is enriched in the endo-PM, uncovering a unique example of how membrane electrostatic properties can define a specific polar domain (i.e., endo-PM), which is critical for plant reproduction and gamete formation.

Defining raft domains in the plasma membrane

Many plasma membrane (PM) functions depend on the cholesterol concentration in the PM in strikingly nonlinear, cooperative ways: fully functional in the presence of physiological cholesterol levels (35~45 mol%), and nonfunctional below 25 mol% cholesterol; namely, still in the presence of high concentrations of cholesterol. This suggests the involvement of cholesterol-based complexes/domains formed cooperatively. In this review, by examining the results obtained by using fluorescent lipid analogs and avoiding the trap of circular logic, often found in the raft literature, we point out the fundamental similarities of liquid-ordered (Lo)-phase domains in giant unilamellar vesicles, Lo-phase-like domains formed at lower temperatures in giant PM vesicles, and detergent-resistant membranes: these domains are formed by cooperative interactions of cholesterol, saturated acyl chains, and unsaturated acyl chains, in the presence of >25 mol% cholesterol. The literature contains evidence, indicating that the domains formed by the same basic cooperative molecular interactions exist and play essential roles in signal transduction in the PM. Therefore, as a working definition, we propose that raft domains in the PM are liquid-like molecular complexes/domains formed by cooperative interactions of cholesterol with saturated acyl chains as well as unsaturated acyl chains, due to saturated acyl chains' weak multiple accommodating interactions with cholesterol and cholesterol's low miscibility with unsaturated acyl chains and TM proteins. Molecules move within raft domains and exchange with those in the bulk PM. We provide a logically established collection of fluorescent lipid probes that preferentially partition into raft and non-raft domains, as defined here, in the PM.

Regulation of Dynamic Protein S-Acylation

Protein S-acylation is the reversible addition of fatty acids to the cysteine residues of target proteins. It regulates multiple aspects of protein function, including the localization to membranes, intracellular trafficking, protein interactions, protein stability, and protein conformation. This process is regulated by palmitoyl acyltransferases that have the conserved amino acid sequence DHHC at their active site. Although they have conserved catalytic cores, DHHC enzymes vary in their protein substrate selection, lipid substrate preference, and regulatory mechanisms. Alterations in DHHC enzyme function are associated with many human diseases, including cancers and neurological conditions. The removal of fatty acids from acylated cysteine residues is catalyzed by acyl protein thioesterases. Notably, S-acylation is now known to be a highly dynamic process, and plays crucial roles in signaling transduction in various cell types. In this review, we will explore the recent findings on protein S-acylation, the enzymatic regulation of this process, and discuss examples of dynamic S-acylation.

Regulation and function of the palmitoyl-acyltransferase ZDHHC5

Protein palmitoylation (S-acylation) has emerged as an important player in a range of cellular processes, and as a result, the palmitoyl-acyltransferase (PAT) enzymes which mediate this modification have entered into the spotlight. Palmitoyltransferase ZDHHC5 (ZDHHC5) is among the more unique members of the PAT family as it is mainly localised to the plasma membrane and contains an extended cytoplasmic domain with several regulatory features. ZDHHC5 plays a vital role in a wide range of processes in different cell types. In this review, we offer a summary of the functions of ZDHHC5 in synaptic plasticity, cardiac function, cell adhesion and fatty acid uptake, among other processes. We also explore recent work has revealed several mechanisms to control the activity, localisation and function of ZDHHC5.

Extracellular vesicles derived from macrophages display glycyl-tRNA synthetase 1 and exhibit anti-cancer activity

Glycyl-tRNA synthetase 1 (GARS1), a cytosolic enzyme secreted from macrophages, promotes apoptosis in cancer cells. However, the mechanism underlying GARS1 secretion has not been elucidated. Here, we report that GARS1 is secreted through unique extracellular vesicles (EVs) with a hydrodynamic diameter of 20-58 nm (mean diameter: 36.9 nm) and a buoyant density of 1.13-1.17 g/ml. GARS1 was anchored to the surface of these EVs through palmitoylated C390 residue. Proteomic analysis identified 164 proteins that were uniquely enriched in the GARS1-containing EVs (GARS1-EVs). Among the identified factors, insulin-like growth factor II receptor, and vimentin also contributed to the anti-cancer activity of GARS1-EVs. This study identified the unique secretory vesicles containing GARS1 and various intracellular factors that are involved in the immunological defence response against tumorigenesis.

Protein Palmitoylation in Leukocyte Signaling and Function

Palmitoylation is a post-translational modification (PTM) based on thioester-linkage between palmitic acid and the cysteine residue of a protein. This covalent attachment of palmitate is reversibly and dynamically regulated by two opposing sets of enzymes: palmitoyl acyltransferases containing a zinc finger aspartate-histidine-histidine-cysteine motif (PAT-DHHCs) and thioesterases. The reversible nature of palmitoylation enables fine-tuned regulation of protein conformation, stability, and ability to interact with other proteins. More importantly, the proper function of many surface receptors and signaling proteins requires palmitoylation-meditated partitioning into lipid rafts. A growing number of leukocyte proteins have been reported to undergo palmitoylation, including cytokine/chemokine receptors, adhesion molecules, pattern recognition receptors, scavenger receptors, T cell co-receptors, transmembrane adaptor proteins, and signaling effectors including the Src family of protein kinases. This review provides the latest findings of palmitoylated proteins in leukocytes and focuses on the functional impact of palmitoylation in leukocyte function related to adhesion, transmigration, chemotaxis, phagocytosis, pathogen recognition, signaling activation, cytotoxicity, and cytokine production.

The Relative Expression of ERα Isoforms ERα66 and ERα36 Controls the Cellular Response to 24R,25-Dihydroxyvitamin D3 in Breast Cancer

Vitamin D3 and its metabolites have antitumorigenic properties in vitro and in vivo; however, clinical trials and retrospective studies on the effectiveness of vitamin D3 oral supplementation against cancer have been inconclusive. One reason for this may be that clinical trials ignore the complex vitamin D metabolome and the many active vitamin D3 metabolites present in the body. Recent work by our lab showed that 24R,25(OH)₂D₃, a vitamin D₃ metabolite that is active in chondrocyte proliferation and differentiation, has antitumorigenic properties in estrogen receptor alpha-66 (ERα66)-positive (ER⁺) breast cancer, but not in ERα66-negative (ER^-) breast cancer. Here we show that 24R,25(OH)₂D₃ is protumorigenic in an in vivo mouse model (NOD.Cg-Prkdc^scidIl2rg^tm1Wjl /SzJ (NSG) mice) of ER^- breast cancer, causing greater tumor growth than in mice treated with vehicle alone. In vitro results indicate that the effect of 24R,25(OH)₂D₃ is via a membrane-associated mechanism involving ERs and phospholipase D. 24R,25(OH)₂D₃ increased proliferation and reduced apoptosis in ERα66-negative HCC38 breast cancer cells, and stimulated expression of metastatic markers. Overexpressing ESRI, which encodes ERα66, ERα46, and ERα36, reduced the proapoptotic response of ERα66^- cells to 24R,25(OH)₂D₃, possibly by upregulating ERα66. Silencing ESR1 in ERα66⁺ cells increased apoptosis. This suggests 24R,25(OH)₂D₃ is differentially tumorigenic in cancers with different ERα isoform profiles. Antiapoptotic actions of 24R,25(OH)₂D₃ require ERα36 and proapoptotic actions require ERα66. IMPLICATIONS: These results suggest that 24R,25(OH)₂D₃, which is a major circulating metabolite of vitamin D, is functionally active in breast cancer and that the regulatory properties of 24R,25(OH)₂D₃ are dependent upon the relative expression of ERα66 and ERα36.

The promoting role of lysosome-localized c-Src in autophagosome-lysosome fusion

Src-family kinases (SFKs), such as c-Src, Lyn and Fyn, belong to non-receptor-type tyrosine kinases and play key roles in cell proliferation, adhesion, and migration. SFKs are anchored to the plasma membrane, Golgi membranes and lysosomal membranes through lipid modifications. Although the functions of SFKs being localized to the plasma membrane are intensively studied, those of SFKs being localized to organelle membranes are poorly understood. Here, we show that, among SFKs, c-Src in particular is involved in a decrease in the amount of LC3-II. c-Src and non-palmitoylated Lyn [Lyn(C3S) (cysteine-3 → serine-3)], which are localized onto lysosomes, decrease the amount of LC3-II and treatment with SFK inhibitors increases the amount of LC3-II, suggesting the importance of SFKs' lysosomal localization for a change of autophagic flux in a kinase activity-dependent manner. Colocalization of LC3-II with the lysosome-associated membrane protein LAMP1 shows that lysosome-localized SFKs promote the fusion of autophagosomes with lysosomes. Lysosome-localized SFKs play a positive role in the maintenance of cell viability under starvation conditions, which is further supported by knockdown of c-Src. Therefore, our results suggest that autophagosome-lysosome fusion is promoted by lysosome-localized c-Src, leading to cell survival under starvation conditions.

Inhibition of NRAS Signaling in Melanoma through Direct Depalmitoylation Using Amphiphilic Nucleophiles

Activating mutations in the small GTPase NRAS are responsible for driving tumor growth in several cancers. Unfortunately, the development of NRAS inhibitors has proven difficult due to the lack of hydrophobic binding pockets on the protein's surface. To overcome this limitation, we chose to target the post-translational S-palmitoyl modification of NRAS, which is required for its signaling activity. Utilizing an amphiphile-mediated depalmitoylation (AMD) strategy, we demonstrate the ability to directly cleave S-palmitoyl groups from NRAS and inhibit its function. C8 alkyl cysteine causes a dose-dependent decrease in NRAS palmitoylation and inhibits downstream signaling in melanoma cells with an activating mutation in NRAS. This compound reduces cell growth in NRAS-driven versus non-NRAS-driven melanoma lines and inhibits tumor progression in an NRAS-mutated melanoma xenograft mouse model. Our work demonstrates that AMD can effectively suppress NRAS activity and could represent a promising new avenue for discovering lead compounds for treatment of NRAS-driven cancers.

Detection of Protein S-Acylation using Acyl-Resin Assisted Capture

Protein S-acylation, also referred to as S-palmitoylation, is a reversible post-translational modification of cysteine residues with long-chain fatty acids via a labile thioester bond. S-acylation, which is emerging as a widespread regulatory mechanism, can modulate almost all aspects of the biological activity of proteins, from complex formation to protein trafficking and protein stability. The recent progress in understanding of the biological function of protein S-acylation was achieved largely due to the development of novel biochemical tools allowing robust and sensitive detection of protein S-acylation in a variety of biological samples. Here, we describe acyl resin-assisted capture (Acyl-RAC), a recently developed method based on selective capture of endogenously S-acylated proteins by thiol-reactive Sepharose beads. Compared to existing approaches, Acyl-RAC requires fewer steps and can yield more reliable results when coupled with mass spectrometry for identification of novel S-acylation targets. A major limitation in this technique is the lack of ability to discriminate between fatty acid species attached to cysteines via the same thioester bond.

Estrogen signaling and estrogen receptors as prognostic indicators in laryngeal cancer

Laryngeal squamous cell carcinoma (LSCC) has been shown to respond to 17β-estradiol. However, the presence and characterization of estrogen receptors (ER) and other sex hormone receptors in LSCC are still being determined. Sex hormone receptors and the way sex hormones impact LSCC tumors are important for understanding which patients would benefit from hormone therapies, such as anti-estrogen therapies. This information also has prognostic value, as there may be a correlation between ER profiles and LSCC aggression. Recent work by our team and others has shown that the canonical ER, estrogen receptor α (ERα), and its splice variant ERα36, are important modulators of estrogen signaling in LSCC. This review describes some common 17β-estradiol signaling pathways, and explains how these signaling pathways might control LSCC tumor growth. We also show that loss of ERα, but not ERα36, imbues LSCC with enhanced aggression, a pattern which has previously only been observed in breast cancer. We make a case for using ERα as a tumorigenic modulator and pathogenic marker in LSCC on par with the use of ERα as a prognostic marker in breast cancer.

Activity-Based Sensing of S-Depalmitoylases: Chemical Technologies and Biological Discovery

While lipids were first appreciated as a critical hydrophobic barrier, our understanding of their roles at the cellular and organismal levels continues to grow. Not only are they important independent operators, providing a platform for both static and dynamic organization and communication within the cell, they also exert significant effects via the chemical modification of proteins. Addition of a lipid post-translational modification (PTM) alters protein hydrophobicity and behavior, with distinct consequences for subcellular trafficking, localization, intra- and intermolecular interactions, and stability. One of the most abundant and widespread protein lipidation events is S-acylation, installation of a long-chain lipid to the thiol of a cysteine side chain through a thioester linkage. S-Acylation is often referred to as S-palmitoylation, due to the prevalence of palmitate as the lipid modification. Unlike many lipid PTMs, S-acylation is enzymatically reversible, enabling the cell to tune proteome-wide properties through dynamic alterations in protein lipidation status. While much has been uncovered about the molecular effects of S-acylation and its implications for physiology, current biochemical and chemical methods only assess substrate lipidation levels or steady-state levels of enzyme activity. Yet, the writer protein acyl transferases (PATs) and eraser acyl protein thioesterases (APTs) are dynamically active, responsible for sometimes-rapid changes in S-palmitoylation status of target proteins. Thus, to understand the full scope, significance, and subtlety of S-deacylation and its regulation in the cell, it is necessary to observe the timing and cellular geography of regulatory enzyme activities. In this Account, we review the chemical tools developed by our group to selectively visualize and perturb the activity of APTs in live cells, highlighting the biological insights gained from their application. To visualize APT activity, we masked fluorogenic molecules with thioacylated, peptide-based APT substrate mimetics; APT activity and thus thiol deprotection releases a fluorescent product in the turn-on depalmitoylation probes (DPPs), while in ratiometric depalmitoylation probes (RDPs) the emission of the parent fluorophore is altered. Application of these probes in live cells reveals that APT activity is sensitive to cell signaling events and metabolic disturbances. Additionally, as indicated above, the location of regulatory enzymes is critical in lipid signaling, and one organelle of particular interest, due to its role in maintaining cellular homeostasis and its legion of lipidated proteins, is the mitochondria. Therefore, we developed a class of spatially constrained mitoDPPs to visualize mitochondrial APT activity as well as a selective inhibitor of mitochondrial deacylation activity, mitoFP. With these tools, we identify two mitochondrial S-depalmitoylases and connect mitochondrial S-depalmitoylation to redox buffering capacity. Moreover, some of the changes in activity observed are specific to the mitochondria, confirming spatial as well as temporal regulation of eraser protein activity. Overall, this chemical toolkit for S-depalmitoylase activity, imaging reagents and a targeted inhibitor, will continue to illuminate the regulatory mechanisms and roles of S-depalmitoylation within the complex homeostatic networks of the cell.

Depalmitoylation by Palmitoyl-Protein Thioesterase 1 in Neuronal Health and Degeneration

Protein palmitoylation is the post-translational, reversible addition of a 16-carbon fatty acid, palmitate, to proteins. Protein palmitoylation has recently garnered much attention, as it robustly modifies the localization and function of canonical signaling molecules and receptors. Protein depalmitoylation, on the other hand, is the process by which palmitic acid is removed from modified proteins and contributes, therefore, comparably to palmitoylated-protein dynamics. Palmitoylated proteins also require depalmitoylation prior to lysosomal degradation, demonstrating the significance of this process in protein sorting and turnover. Palmitoylation and depalmitoylation serve as particularly crucial regulators of protein function in neurons, where a specialized molecular architecture and cholesterol-rich membrane microdomains contribute to synaptic transmission. Three classes of depalmitoylating enzymes are currently recognized, the acyl protein thioesterases, α/β hydrolase domain-containing 17 proteins (ABHD17s), and the palmitoyl-protein thioesterases (PPTs). However, a clear picture of depalmitoylation has not yet emerged, in part because the enzyme-substrate relationships and specific functions of depalmitoylation are only beginning to be uncovered. Further, despite the finding that loss-of-function mutations affecting palmitoyl-protein thioesterase 1 (PPT1) function cause a severe pediatric neurodegenerative disease, the role of PPT1 as a depalmitoylase has attracted relatively little attention. Understanding the role of depalmitoylation by PPT1 in neuronal function is a fertile area for ongoing basic science and translational research that may have broader therapeutic implications for neurodegeneration. Here, we will briefly introduce the rapidly growing field surrounding protein palmitoylation and depalmitoylation, then will focus on the role of PPT1 in development, health, and neurological disease.

Glycoprotein Ib clustering in platelets can be inhibited by α-linolenic acid as revealed by cryo-electron tomography

Platelet adhesion to the sub-endothelial matrix and damaged endothelium occurs through a multi-step process mediated in the initial phase by glycoprotein Ib binding to von Willebrand factor (vWF), which leads to the subsequent formation of a platelet plug. The plant-derived ω-3 fatty acid α-linolenic acid is an abundant alternative to fish-derived n-3 fatty acids and has anti-inflammatory and antithrombotic properties. In this study, we investigated the impact of α-linolenic acid on human platelet binding to vWF under high-shear flow conditions (mimicking blood flow in stenosed arteries). Pre-incubation of fresh human blood from healthy donors with α-linolenic acid at dietary relevant concentrations reduced platelet binding and rolling on vWF-coated microchannels at a shear rate of 100 dyn/cm². Depletion of membrane cholesterol by incubation of platelet-rich plasma with methyl-β cyclodextrin abrogated platelet rolling on vWF. Analysis of glycoprotein Ib by applying cryo-electron tomography to intact platelets revealed local clusters of glycoprotein Ib complexes upon exposure to shear force: the formation of these complexes could be prevented by treatment with α-linolenic acid. This study provides novel findings on the rapid local rearrangement of glycoprotein Ib complexes in response to high-shear flow and highlights the mechanism of in vitro inhibition of platelet binding to and rolling on vWF by α-linolenic acid.

24R,25-Dihydroxyvitamin D3 regulates breast cancer cells in vitro and in vivo

BACKGROUND

Epidemiological studies indicate high serum 25(OH)D₃ is associated with increased survival in breast cancer patients. Pre-clinical studies attributed this to anti-tumorigenic properties of its metabolite 1α,25(OH)₂D₃. However, 1α,25(OH)₂D₃ is highly calcemic and thus has a narrow therapeutic window. Here we propose another metabolite, 24R,25(OH)₂D₃, as an alternative non-calcemic vitamin D₃ supplement.

METHODS

NOD-SCID-IL2γR null female mice with MCF7 breast cancer xenografts in the mammary fat pad were treated with 24R,25(OH)₂D₃ and changes in tumor burden and metastases were assessed. ERα66+ MCF7 and T47D cells, and ERα66- HCC38 cells were treated with 24R,25(OH)₂D₃in vitro to assess effects on proliferation and apoptosis. Effects on migration and metastatic markers were assessed in MCF7.

RESULTS

24R,25(OH)₂D₃ reduced MCF7 tumor growth and metastasis in vivo. In vitro results indicate that this was not due to an anti-proliferative effect; 24R,25(OH)₂D₃ stimulated DNA synthesis in MCF7 and T47D. In contrast, markers of invasion and metastasis were decreased. 24R,25(OH)₂D₃ caused dose-dependent increases in apoptosis in MCF7 and T47D, but not HCC38 cells. Inhibitors to palmitoylation, caveolae integrity, phospholipase-D, and estrogen receptors (ER) demonstrate that 24R,25(OH)₂D₃ acts on MCF7 cells through caveolae-associated, phospholipase D-dependent mechanisms via cross-talk with ERs.

CONCLUSION

These results indicate that 24R,25(OH)₂D₃ shows promise in treatment of breast cancer by stimulating tumor apoptosis and reducing metastasis.

GENERAL SIGNIFICANCE

24R,25(OH)₂D₃ regulates breast cancer cell survival through ER-associated mechanisms similar to 24R,25(OH)₂D₃ effects on chondrocytes. Thus, 24R,25(OH)₂D₃ may modulate cell survival in other estrogen-responsive cell types, and its therapeutic potential should be investigated in ER-associated pathologies.

Stearate-Induced Apoptosis in Human Pancreatic β-Cells is Associated with Changes in Membrane Protein Expression and These Changes are Inhibited by Oleate

PURPOSE

Lipotoxicity is implicated in type 2 diabetes pathogenesis. Its molecular mechanisms are not completely understood. The aim of this study is to identify new suspect proteins involved in pancreatic β-cell death induction by saturated fatty acids and its inhibition by unsaturated fatty acids.

EXPERIMENTAL DESIGN

Employing 2DE analysis and subsequent western blot confirmation, the differences in membrane/membrane-associated protein expression in human β-cell line NES2Y are assessed during cell death induction by stearate and its inhibition by oleate.

RESULTS

Induction of apoptosis by stearate is associated with significantly increased levels of Hsp90β, peroxiredoxin-1, and 14-3-3γ in the membrane fraction of NES2Y cells and significantly decreased levels of annexin A2, annexin A4, and reticulocalbin-2. All these changes are significantly inhibited by oleate co-application. No expression changes are detected after application of stearate together with oleate. Furthermore, the expression of reticulocalbin-2 is significantly decreased after stearate application also in the whole cell lysate.

CONCLUSIONS AND CLINICAL RELEVANCE

Several membrane-associated proteins that could be related to pro- and anti-apoptotic signaling initiated by fatty acids in human pancreatic β-cells are identified. As far as we know, annexin A4, reticulocalbin-2, and 14-3-3γ represent novel molecules related to the effect of fatty acids on β-cell viability.

Sensitive FRET Biosensor Reveals Fyn Kinase Regulation by Submembrane Localization

Fyn kinase plays crucial roles in hematology and T cell signaling; however, there are currently limited tools to visualize the dynamic Fyn activity in live cells. Here we developed and characterized a highly sensitive Fyn biosensor based on fluorescence resonance energy transfer (FRET) to monitor Fyn kinase activity in live cells. Our results show that Fyn kinase activity can be induced in both mouse embryonic fibroblasts (MEFs) and T cells by ligand engagement. Two different motifs were further introduced to target the biosensor at the cellular membrane microdomains in MEFs, revealing that the Fyn-tagged biosensor had 70% greater response to growth factor stimulation than the Lyn-tagged version. This suggests that the plasma membrane microdomains can be categorized into different functional subdomains. Further experiments show that while the membrane accessibility is necessary for Fyn activation, the localization of Fyn outside of its microdomains causes its hyperactivity, indicating that membrane microdomains provide a suppressive microenvironment for Fyn regulation in MEFs. Interestingly, a relatively high Fyn activity can be observed at perinuclear regions, further supporting the notion that the membrane microenvironment has a significant impact on the local molecular functions. Our work hence highlights a novel Fyn FRET biosensor for live cell imaging and its application in revealing an intricate submembrane regulation of Fyn in live MEFs.

Fatty Acid Synthase Promotes the Palmitoylation of Chikungunya Virus nsP1

Chikungunya virus (CHIKV) is transmitted to people by mosquitoes, and CHIKV infection causes fever and joint pain. Fatty acid synthase (FASN) has been identified as a proviral factor for CHIKV. How FASN participates in CHIKV replication remains to be elucidated. In this study, we demonstrated that palmitic acid (PA) can restore the suppression of CHIKV replication by FASN inhibitors. The palmitoylation and plasma membrane localization of CHIKV nsP1 were reduced by FASN inhibitors. Triple mutation of Cys417, Cys418, and Cys419 in nsP1 blocked its palmitoylation and severely disrupted CHIKV replication. Furthermore, two zinc finger DHHC domain-containing palmitoyltransferases (ZDHHCs), ZDHHC2 and ZDHHC19, promoted nsP1 palmitoylation and CHIKV replication. Our results not only identified the key enzymes for the palmitoylation of nsP1 but also provided mechanistic insights into the roles of FASN in CHIKV replication.IMPORTANCE S-palmitoylation is an important form of lipid posttranslational modification, which affects the function of proteins by regulating their transport, stability, and localization. Previous studies have shown that FASN is critical for CHIKV replication; however, the mechanism for this function of FASN remains unknown. The key zinc finger DHHC domain-containing palmitoyltransferases involved in the palmitoylation of nsP1 are not clear. We demonstrated that FASN promoted CHIKV replication through nsP1 palmitoylation. ZDHHC2 and ZDHHC19 were identified as the major enzymes for nsP1 palmitoylation. Since nsP1 proteins are conserved in alphaviruses, our results highlight the mechanisms by which alphavirus nsP1 is palmitoylated.

SREBP-dependent lipidomic reprogramming as a broad-spectrum antiviral target

Viruses are obligate intracellular microbes that exploit the host metabolic machineries to meet their biosynthetic demands, making these host pathways potential therapeutic targets. Here, by exploring a lipid library, we show that AM580, a retinoid derivative and RAR-α agonist, is highly potent in interrupting the life cycle of diverse viruses including Middle East respiratory syndrome coronavirus and influenza A virus. Using click chemistry, the overexpressed sterol regulatory element binding protein (SREBP) is shown to interact with AM580, which accounts for its broad-spectrum antiviral activity. Mechanistic studies pinpoint multiple SREBP proteolytic processes and SREBP-regulated lipid biosynthesis pathways, including the downstream viral protein palmitoylation and double-membrane vesicles formation, that are indispensable for virus replication. Collectively, our study identifies a basic lipogenic transactivation event with broad relevance to human viral infections and represents SREBP as a potential target for the development of broad-spectrum antiviral strategies.

Viruses rely on host cell metabolism for replication, making these pathways potential therapeutic targets. Here, the authors show that AM580, a retinoid derivative and RAR-α agonist, affects replication of several RNA viruses by interfering with the activity of SREBP.

Systematic Screening of Depalmitoylating Enzymes and Evaluation of Their Activities by the Acyl-PEGyl Exchange Gel-Shift (APEGS) Assay

Palmitoylation is a reversible posttranslational lipid modification of proteins involved in a wide range of cellular functions. More than a thousand proteins are estimated to be palmitoylated. In neurons, PSD-95, a major postsynaptic scaffold protein, requires palmitoylation for its specific accumulation at the synapse and dynamically cycles between palmitoylated and depalmitoylated states. Although palmitoylating enzymes of PSD-95 have been well characterized, little is known about the depalmitoylating enzymes (e.g., thioesterases for palmitoylated PSD-95). An elegant pharmacological analysis has suggested that subsets of α/β hydrolase domain (ABHD)-containing proteins of the metabolic serine hydrolase superfamily involve thioesterases for palmitoylated proteins. Here, we describe a systematic method to screen the ABHD serine hydrolase genes, which unveiled ABHD17 as the depalmitoylating enzyme for PSD-95. Furthermore, we introduce the acyl-PEGyl exchange gel-shift (APEGS) method that enables quantification of palmitoylation levels/stoichiometries on proteins in various biological samples and can be used to monitor the dynamic depalmitoylation process of proteins.

Advances and challenges in understanding the role of the lipid raft proteome in human health

INTRODUCTION

Phase separation as a biophysical principle drives the formation of liquid-ordered 'lipid raft' membrane microdomains in cellular membranes, including organelles. Given the critical role of cellular membranes in both compartmentalization and signaling, clarifying the roles of membrane microdomains and their mutual regulation of/by membrane proteins is important in understanding the fundamentals of biology, and has implications for health. Areas covered: This article will consider the evidence for lateral membrane phase separation in model membranes and organellar membranes, critically evaluate the current methods for lipid raft proteomics and discuss the biomedical implications of lipid rafts. Expert commentary: Lipid raft homeostasis is perturbed in numerous chronic conditions; hence, understanding the precise roles and regulation of the lipid raft proteome is important for health and medicine. The current technical challenges in performing lipid raft proteomics can be overcome through well-controlled experimental design and careful interpretation. Together with technical developments in mass spectrometry and microscopy, our understanding of lipid raft biology and function will improve through recognition of the similarity between organelle and plasma membrane lipid rafts and considered integration of published lipid raft proteomics data.