Calcium-dependent binding of phosphorylated human pre interleukin 1 alpha to phospholipids

Recruitment of pro-IL-1α to mitochondrial cardiolipin, via shared LC3 binding domain, inhibits mitophagy and drives maximal NLRP3 activation

The balance between NLRP3 inflammasome activation and mitophagy is essential for homeostasis and cellular health, but this relationship remains poorly understood. Here we found that interleukin-1α (IL-1α)-deficient macrophages have reduced caspase-1 activity and diminished IL-1β release, concurrent with reduced mitochondrial damage, suggesting a role for IL-1α in regulating this balance. LPS priming of macrophages induced pro-IL-1α translocation to mitochondria, where it directly interacted with mitochondrial cardiolipin (CL). Computational modeling revealed a likely CL binding motif in pro-IL-1α, similar to that found in LC3b. Thus, binding of pro-IL-1α to CL in activated macrophages may interrupt CL-LC3b-dependent mitophagy, leading to enhanced Nlrp3 inflammasome activation and more robust IL-1β production. Mutation of pro-IL-1α residues predicted to be involved in CL binding resulted in reduced pro-IL-1α-CL interaction, a reduction in NLRP3 inflammasome activity, and increased mitophagy. These data identify a function for pro-IL-1α in regulating mitophagy and the potency of NLRP3 inflammasome activation.

Co-localization of Interleukin-1α and Annexin A2 at the plasma membrane in response to oxidative stress

Interleukin-1α (IL-1α) and Annexin A2 (AnxA2) are pleiotropic molecules with both intracellular and extracellular roles. They share several characteristics including unconventional secretion aided by S100 proteins, anchoring of the externalized proteins at the outer surface of the plasma membrane and response to oxidative stress. Although IL-1α and AnxA2 have been implicated in a variety of biological processes, including cancer, little is known about the mechanisms of their cellular release. In the present study, employing the non-cancerous breast epithelial MCF10A cells, we demonstrate that IL-1α and AnxA2 establish a close association in response to oxidative stress. Stress conditions lead to translocation of both proteins towards lamellipodia rich in vimentin and association of full-length IL-1α and Tyr23 phosphorylated AnxA2 with the plasma membrane at peripheral sites depleted of F-actin. Notably, membrane-associated IL-1α and AnxA2 preferentially localize to the outer edges of the MCF10A cell islands, suggesting that the two proteins participate in the communication of these epithelial cells with their neighboring cells. Similarly, in U2OS osteosarcoma cell line both endogenous IL-1α and transiently produced IL-1α/EGFP associate with the plasma membrane. While benign MFC10A cells present membrane-associated IL-1α and AnxA2 at the edges of their cell islands, the aggressive cancerous U2OS cells communicate in such manner also with distant cells.

Transbilayer phospholipid movement facilitates the translocation of annexin across membranes

Annexins are cytosolic phospholipid-binding proteins that can be found on the outer leaflet of the plasma membrane. The extracellular functions of annexin include modulating fibrinolysis activity and cell migration. Despite having well-described extracellular functions, the mechanism of annexin transport from the cytoplasmic inner leaflet to the extracellular outer leaflet of the plasma membrane remains unclear. Here, we show that the transbilayer movement of phospholipids facilitates the transport of annexins A2 and A5 across membranes in cells and in liposomes. We identified TMEM16F (also known as anoctamin-6, ANO6) as a lipid scramblase required for transport of these annexins to the outer leaflet of the plasma membrane. This work reveals a mechanism for annexin translocation across membranes which depends on plasma membrane phospholipid remodelling.

Unconventional Pathways of Secretion Contribute to Inflammation

In the conventional pathway of protein secretion, leader sequence-containing proteins leave the cell following processing through the endoplasmic reticulum (ER) and Golgi body. However, leaderless proteins also enter the extracellular space through mechanisms collectively known as unconventional secretion. Unconventionally secreted proteins often have vital roles in cell and organism function such as inflammation. Amongst the best-studied inflammatory unconventionally secreted proteins are interleukin (IL)-1β, IL-1α, IL-33 and high-mobility group box 1 (HMGB1). In this review we discuss the current understanding of the unconventional secretion of these proteins and highlight future areas of research such as the role of nuclear localisation.

Targeting IL-1 in Sjögren's syndrome

INTRODUCTION

IL-1 plays key roles in the biological functions of various cells. In particular, many roles of IL-1 in the immune system have been discovered by numerous studies. This review focuses on the association of IL-1 with the pathogenesis of autoimmunity.

AREAS COVERED

An overview of the biological functions of the IL-1 family and the IL-1 receptors (IL-1Rs), including the maintenance of systemic or local homeostasis, and the signaling pathway through IL-1/IL-1R in various immune systems are described. Several functions of IL-1 in the pathogenesis of Sjögren's syndrome (SS) have been demonstrated with a focus on the immune responses and target tissues in SS. In addition to the role of IL-1 in the immune responses in SS, the function of IL-1 in ocular mucosa lesions in SS has been described. Lastly, there is an overview of possible therapeutic strategies for IL-1 inhibition in SS.

EXPERT OPINION

IL-1 plays critical roles in the onset and development of SS by controlling systemic or local immune responses and maintaining the survival and mucosal defense of target epithelial cells. The inhibition of the pathogenic functions of IL-1 may be beneficial for treating SS.

Presence of a cytoplasmic retention sequence within the human interleukin-1alpha precursor

Interleukin (IL)-1alpha is primarily translated as a 33 kDa molecule (IL-1alpha1-271), and then processed into a 17 kDa molecule (IL-1alpha119-271) by calpain. The precursor region of IL-1alpha (IL-1 alpha1-118) contains a nuclear localization signal (KVLKKRRL, residues 79-86). We investigated the intracellular localization of IL-1alpha fused with green fluorescent protein or beta-galactosidase. IL-1alpha1-118 was localized exclusively in the nucleus, but IL-1 alpha1-271 in both the nucleus and the cytoplasm, suggesting the presence of a cytoplasmic retention signal within the mature region of IL-1alpha. Furthermore, the intracellular localization of IL-1alpha with deletions from the C terminus, internal deletions and point mutations suggested that the cytoplasmic retention signal is located within residues 168-201.

The non-classical export routes: FGF1 and IL-1alpha point the way

Non-classical protein release independent of the ER-Golgi pathway has been reported for an increasing number of proteins lacking an N-terminal signal sequence. The export of FGF1 and IL-1alpha, two pro-angiogenic polypeptides, provides two such examples. In both cases, export is based on the Cu2+-dependent formation of multiprotein complexes containing the S100A13 protein and might involve translocation of the protein across the membrane as a 'molten globule'. FGF1 and IL-1alpha are involved in pathological processes such as restenosis and tumor formation. Inhibition of their export by Cu2+ chelators is thus an effective strategy for treatment of several diseases.

The Calpain Hypothesis of Neurodegeneration: Evidence for a Common Cytotoxic Pathway

Calpain's general function and pathogenic role in the CNS are reviewed. Collectively, the literature indicates that calpain proteolysis plays a common and important role in a variety of acute neurodegenerative conditions, including focal ischemia (stroke), global ischemia, traumatic brain injury, and spinal cord injury. This evidence indicates that 1) calpain is activated in an abnormally sustained fashion during cellular events commonly associated with neurodegeneration (e.g., excessive interstitial glutamate and cytosolic calcium); 2) many of calpain's preferred substrates are degraded as important components in these neurodegenerative conditions; 3) calpain activation occurs early in the pathogenic cascade of each, prior to onset of substantial cell death; and 4) calpain inhibitors can effectively reduce the severity of neuronal damage and loss of function normally associated with these acute neurodegenerative perturbations. Calpain proteolysis is also implicated in chronic neurodegenerative diseases, with the strength of current evidence varying among specific diseases. The evidence accumulated for a plausible role in Alzheimer's disease (AD) is currently the strongest. For example, empirical links have been established between abnormal calpain proteolysis and 1) the cellular formation of classic Alzheimer's pathology, such as β-amyloid plaques, neurofibrillary tangles, and Alz-50 immunoreactivity; 2) the brain regions with greatest concentrations of AD-related pathology; and 3) the degeneration of key brain pathways vulnerable in the early stages of the disease. Similar, though less extensive, evidence exists for a potential role of abnormal calpain proteolysis in Parkinson's disease. Finally, for several other chronic neurodegenerative conditions (e.g., Huntington's disease and amyotrophic lateral sclerosis), early evidence is emerging that calpain may also play some pathogenic role. Thus, these data support the possibility that uncontrolled calpain proteolysis may contribute to and/or accelerate the loss of neurons associated with a wide range of neurodegenerative conditions and may, therefore, represent an important, final common cytotoxic pathway for many diverse forms of neurodegeneration. NEUROSCIENTIST 3:314–327, 1997

The N-terminal propiece of interleukin 1 alpha is a transforming nuclear oncoprotein

Interleukin 1 alpha (IL-1 alpha) is a pleiotropic cytokine involved in the immune response, inflammatory processes, and hematopoiesis, and acts as a mitogen for several malignant cell types, including acute leukemia and Kaposi sarcoma cells. These diverse activities have been exclusively attributed to the plasma membrane receptor-binding, 17-kDa C-terminal component (mature IL-1 alpha) that results from proteolytic processing of the 31- to 33-kDa precursor protein. No biologic function has been ascribed to the unusually large, 16-kDa N-terminal propiece formed as a result of proteolytic processing of IL-1 alpha. We report that the IL-1 alpha N-terminal propiece is concentrated by means of a nuclear localization sequence within the nuclei of both transfected and leukemic cell lines. Overexpression of this component in glomerular mesangial cells, a model perivascular myofibroblast cell type capable of IL-1 alpha synthesis and processing, results in malignant transformation to a spindle cell-type tumor. The functionally bipartite nature of the IL-1 alpha precursor represents a unique combination of the C-terminal, classical cytokine and an N-terminal nuclear oncoprotein. These findings suggest that nuclear transport of the IL-1 alpha N-terminal component may represent a critical component in the transformation of IL-1 alpha-producing cells in the bone marrow or the perivascular area to a malignant phenotype.

Roles of the phosphorylation of human interleukin 1 alpha in proteolytic processing

To investigate the role of phosphorylation of a precursor form of interleukin-1 alpha (pre-IL-1 alpha), we obtained cells producing either phosphorylated or unphosphorylated pre-IL-1 alpha. Although calcium-dependent proteolytic processing of unphosphorylated pre-IL-1 alpha could be observed in cell lysates, proteolytic processing was not induced by treatment with calcium ionophore in intact cells producing the unphosphorylated pre-IL-1 alpha. Further, unphosphorylated pre-IL-1 alpha showed no calcium-dependent binding to acidic phospholipids at concentrations below 5 x 10(-6) M. These results suggested that phosphorylated pre-IL-1 alpha became susceptible to proteolytic processing by association with the cell membrane in a calcium-dependent manner.

Selective release of a processed form of interleukin 1 alpha

Interleukin 1 alpha (IL-1 alpha) is synthesized as a 33 kDa form and proteolytically processed into a 17 kDa form. Although IL-1 alpha has no signal peptide, it is released from cells. To investigate the relationship between the processing and release of IL-1 alpha, human bladder carcinoma cells (HTB9 5637) which express IL-1 alpha constitutively, were treated with calcium ionophore (A23187). A23187 induced the processing of 33 kDa IL-1 alpha and selectively released processed 17 kDa IL-1 alpha, without any change in the release of 33 kDa IL-1 alpha. When extracellular calcium was chelated by EGTA, or when intracellular calpain was inhibited by the cell-permeable cysteine-protease inhibitor, E64d, the processing of 33 kDa IL-1 alpha was significantly blocked, the release of 33 kDa IL-1 alpha being unchanged. These results indicate that the release of IL-1 alpha was accompanied by the processing of 33 kDa IL-1 alpha.

Lipid dynamics and peripheral interactions of proteins with membrane surfaces

A large body of evidence strongly indicates biomembranes to be organized into compositionally and functionally specialized domains, supramolecular assemblies, existing on different time and length scales. For these domains and intimate coupling between their chemical composition, physical state, organization, and functions has been postulated. One important constituent of biomembranes are peripheral proteins whose activity can be controlled by non-covalent binding to lipids. Importantly, the physical chemistry of the lipid interface allows for a rapid and reversible control of peripheral interactions. In this review examples are provided on how membrane lipid (i) composition (i.e., specific lipid structures), (ii) organization, and (iii) physical state can each regulate peripheral binding of proteins to the lipid surface. In addition, a novel and efficient mechanism for the control of the lipid surface association of peripheral proteins by [Ca2+], lipid composition, and phase state is proposed. The phase state is, in turn, also dependent on factors such as temperature, lateral packing, presence of ions, metabolites and drugs. Confining reactions to interfaces allows for facile and cooperative large scale integration and control of metabolic pathways due to mechanisms which are not possible in bulk systems.

The 31-kDa precursor of interleukin 1 alpha is myristoylated on specific lysines within the 16-kDa N-terminal propiece

The cytokine interleukin 1 alpha (IL-1 alpha) is a critical mediator of the immune and inflammatory responses. A unique determinant of its activity as compared with IL-1 beta may be its association with the plasma membrane. While the biologic activity of "membrane IL-1" has been extensively reported, the mechanism of membrane binding remains unclear. We report that the N terminus of the 31-kDa IL-1 alpha precursor is myristoylated on specific internal lysine residues. Immunoprecipitation of [3H]myristic acid-radiolabeled human monocyte lysates with IgG antibodies to the 31-kDa IL-1 alpha precursor recovered a protein with the physicochemical properties of the IL-1 alpha N-terminal propiece (16 kDa, pI 4.45). Glycyl N-myristoylation of this protein is precluded by the absence of a glycine residue at position 2, suggesting that the propiece is myristoylated on epsilon-amino groups of lysine. To determine which lysine(s) are acylated, a series of synthetic peptides containing all lysines found in the IL-1 alpha N-terminal propiece were used in an in vitro myristoylation assay containing peptide, myristoyl-CoA, and monocyte lysate as enzyme source. Analysis of the reaction products by reverse-phase HPLC and gas-phase sequencing demonstrated the specific myristoylation of Lys-82 and Lys-83, yielding predominantly monoacylated product. A conserved sequence in the IL-1 beta propiece was myristoylated with at least 8-fold less efficiency. Acylation of the IL-1 alpha precursor by a previously unrecognized lysyl epsilon-amino N-myristoyl-transferase activity may facilitate its specific membrane targeting.

Interleukin 1: the patterns of translation and intracellular distribution support alternative secretory mechanisms

Interleukin-1 (IL-1) is synthesized as a 31 kDa precursor protein, whose multiple extracellular activities are attributed to receptor binding of a processed, carboxy-terminal 17 kDa peptide. Unlike other secreted proteins, the IL-1 precursor lacks a hydrophobic leader sequence and is not found in organelles composing the classical secretory pathway. In order to further clarify the intracellular processing of IL-1, we studied its site of synthesis in human monocytes. Secreted and integral membrane proteins are translated on membrane-bound polyribosomes, while intracellular proteins are translated on free polyribosomes. Free and membrane-bound polysomes were isolated from Lipid A-stimulated monocyte lysates and immunoblotted using antibodies specific to the N-terminal regions of the IL-1 alpha and beta precursors. Free polysome fractions showed multiple small bands consistent with nascent peptide chains; membrane-bound polysomes yielded no detectable IL-1. Polysome fractions were then analyzed by immunoelectron microscopy; nascent IL-1 alpha and beta peptide chains were readily seen emerging from cytoskeletal-associated free polyribosomes, but not membrane-bound polyribosomes. Electron microscopic in situ hybridization revealed IL-1 mRNA chains attached to cytoskeletal-associated free, but not membrane-bound polyribosomes. The intracellular distribution of the fully synthesized IL-1 beta precursor was studied in human mesangial cells (HMC), whose cytoskeletal organization is more readily evaluated than that of monocytes. Dual immunofluorescence microscopy of these cells revealed a complex intracellular distribution of the fully synthesized 31 kDa IL-1 precursors. IL-1 was asymmetrically distributed between cytosolic, microtubule, and nuclear compartments, without association with actin or intermediate filaments. This demonstration of the sites of IL-1 synthesis and patterns of intracellular distribution provide further evidence for an extracellular release mechanism which is clearly distinct from the classical secretory pathway.

Interaction of phospholipids with proteins and peptides. New advances 1990

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.

Purification of recombinant human tumor necrosis factor precursor from Escherichia coli

To study its biological functions, tumor necrosis factor precursor (proTNF) with a molecular size of 26-KDa was obtained as a recombinant protein from Escherichia coli. The recombinant proTNF was successfully accumulated in the insoluble form, corresponding to about 10-15% of total E. coli proteins. Solubilization, gel filtration and anion exchange chromatography were performed under denatured conditions followed by dialysis in phosphate-buffered saline. These processes removed most of the contaminating bacterial proteins, yielding proTNF with a purity of about 70-80%. This recombinant proTNF is expected to be useful for functional studies on activated macrophages with membrane integrated proTNF.