Inhibition of 15-lipoxygenase leads to delayed organelle degradation in the reticulocyte

Erythroid mitochondrial retention triggers myeloid-dependent type I interferon in human SLE

Emerging evidence supports that mitochondrial dysfunction contributes to systemic lupus erythematosus (SLE) pathogenesis. Here we show that programmed mitochondrial removal, a hallmark of mammalian erythropoiesis, is defective in SLE. Specifically, we demonstrate that during human erythroid cell maturation, a hypoxia-inducible factor (HIF)-mediated metabolic switch is responsible for the activation of the ubiquitin-proteasome system (UPS), which precedes and is necessary for the autophagic removal of mitochondria. A defect in this pathway leads to accumulation of red blood cells (RBCs) carrying mitochondria (Mito+ RBCs) in SLE patients and in correlation with disease activity. Antibody-mediated internalization of Mito+ RBCs induces type I interferon (IFN) production through activation of cGAS in macrophages. Accordingly, SLE patients carrying both Mito+ RBCs and opsonizing antibodies display the highest levels of blood IFN-stimulated gene (ISG) signatures, a distinctive feature of SLE.

Methylated HNRNPK acts on RPS19 to regulate ALOX15 synthesis in erythropoiesis

Post-transcriptional control is essential to safeguard structural and metabolic changes in enucleated reticulocytes during their terminal maturation to functional erythrocytes. The timely synthesis of arachidonate 15-lipoxygenase (ALOX15), which initiates mitochondria degradation at the final stage of reticulocyte maturation is regulated by the multifunctional protein HNRNPK. It constitutes a silencing complex at the ALOX15 mRNA 3′ untranslated region that inhibits translation initiation at the AUG by impeding the joining of ribosomal 60S subunits to 40S subunits. To elucidate how HNRNPK interferes with 80S ribosome assembly, three independent screens were applied. They consistently demonstrated a differential interaction of HNRNPK with RPS19, which is localized at the head of the 40S subunit and extends into its functional center. During induced erythroid maturation of K562 cells, decreasing arginine dimethylation of HNRNPK is linked to a reduced interaction with RPS19 in vitro and in vivo. Dimethylation of residues R256, R258 and R268 in HNRNPK affects its interaction with RPS19. In noninduced K562 cells, RPS19 depletion results in the induction of ALOX15 synthesis and mitochondria degradation. Interestingly, residue W52 in RPS19, which is frequently mutated in Diamond-Blackfan Anemia (DBA), participates in specific HNRNPK binding and is an integral part of a putative aromatic cage.

A Proposed Concept for Defective Mitophagy Leading to Late Stage Ineffective Erythropoiesis in Pyruvate Kinase Deficiency

Pyruvate kinase deficiency (PKD) is a rare congenital hemolytic anemia caused by mutations in the PKLR gene. Here, we review pathophysiological aspects of PKD, focusing on the interplay between pyruvate kinase (PK)-activity and reticulocyte maturation in the light of ferroptosis, an iron-dependent process of regulated cell death, and in particular its key player glutathione peroxidase 4 (GPX4). GPX4 plays an important role in mitophagy, the key step of peripheral reticulocyte maturation and GPX4 deficiency in reticulocytes results in a failure to fully mature. Mitophagy depends on lipid oxidation, which is under physiological conditions controlled by GPX4. Lack of GPX4 leads to uncontrolled auto-oxidation, which will disrupt autophagosome maturation and thereby perturb mitophagy. Based on our review, we propose a model for disturbed red cell maturation in PKD. A relative GPX4 deficiency occurs due to glutathione (GSH) depletion, as cytosolic L-glutamine is preferentially used in the form of α-ketoglutarate as fuel for the tricarboxylic acid (TCA) cycle at the expense of GSH production. The relative GPX4 deficiency will perturb mitophagy and, subsequently, results in failure of reticulocyte maturation, which can be defined as late stage ineffective erythropoiesis. Our hypothesis provides a starting point for future research into new therapeutic possibilities, which have the ability to correct the oxidative imbalance due to lack of GPX4.

Glutathione peroxidase 4 and vitamin E control reticulocyte maturation, stress erythropoiesis and iron homeostasis

Glutathione peroxidase 4 (GPX4) is unique as it is the only enzyme that can prevent detrimental lipid peroxidation in vivo by reducing lipid peroxides to the respective alcohols thereby stabilizing oxidation products of unsaturated fatty acids. During reticulocyte maturation, lipid peroxidation mediated by 15-lipoxygenase in humans and rabbits and by 12/15-lipoxygenase (ALOX15) in mice was considered the initiating event for the elimination of mitochondria but is now known to occur through mitophagy. Yet, genetic ablation of the Alox15 gene in mice failed to provide evidence for this hypothesis. We designed a different genetic approach to tackle this open conundrum. Since either other lipoxygenases or non-enzymatic autooxidative mechanisms may compensate for the loss of Alox15, we asked whether ablation of Gpx4 in the hematopoietic system would result in the perturbation of reticulocyte maturation. Quantitative assessment of erythropoiesis indices in the blood, bone marrow (BM) and spleen of chimeric mice with Gpx4 ablated in hematopoietic cells revealed anemia with an increase in the fraction of erythroid precursor cells and reticulocytes. Additional dietary vitamin E depletion strongly aggravated the anemic phenotype. Despite strong extramedullary erythropoiesis reticulocytes failed to mature and accumulated large autophagosomes with engulfed mitochondria. Gpx4-deficiency in hematopoietic cells led to systemic hepatic iron overload and simultaneous severe iron demand in the erythroid system. Despite extremely high erythropoietin and erythroferrone levels in the plasma, hepcidin expression remained unchanged. Conclusively, perturbed reticulocyte maturation in response to Gpx4 loss in hematopoietic cells thus causes ineffective erythropoiesis, a phenotype partially masked by dietary vitamin E supplementation.

Contributions of 12/15-Lipoxygenase to Bleeding in the Brain Following Ischemic Stroke

Ischemic strokes are caused by one or more blood clots that typically obstruct one of the major arteries in the brain, but frequently also result in leakage of the blood-brain barrier and subsequent hemorrhage. While it has long been known that the enzyme 12/15-lipoxygenase (12/15-LOX) is up-regulated following ischemic strokes and contributes to neuronal cell death, recent research has shown an additional major role for 12/15-LOX in causing this hemorrhagic transformation. These findings have important implications for the use of 12/15-LOX inhibitors in the treatment of stroke.

RNA Binding Proteins and Regulation of mRNA Translation in Erythropoiesis

Control of gene expression in erythropoiesis has to respond to signals that may emerge from intracellular processes or environmental factors. Control of mRNA translation allows for relatively rapid modulation of protein synthesis from the existing transcriptome. For instance, the protein synthesis rate needs to be reduced when reactive oxygen species or unfolded proteins accumulate in the cells, but also when iron supply is low or when growth factors are lacking in the environment. In addition, regulation of mRNA translation can be important as an additional layer of control on top of gene transcription, in which RNA binding proteins (RBPs) can modify translation of a set of transcripts to the cell’s actual protein requirement. The 5′ and 3′ untranslated regions of mRNA (5′UTR, 3′UTR) contain binding sites for general and sequence specific translation factors. They also contain secondary structures that may hamper scanning of the 5′UTR by translation complexes or may help to recruit translation factors. In addition, the term 5′UTR is not fully correct because many transcripts contain small open reading frames in their 5′UTR that are translated and contribute to regulation of mRNA translation. It is becoming increasingly clear that the transcriptome only partly predicts the proteome. The aim of this review is (i) to summarize how the availability of general translation initiation factors can selectively regulate transcripts because the 5′UTR contains secondary structures or short translated sequences, (ii) to discuss mechanisms that control the length of the mRNA poly(A) tail in relation to mRNA translation, and (iii) to give examples of sequence specific RBPs and their targets. We focused on transcripts and RBPs required for erythropoiesis. Whereas differentiation of erythroblasts to erythrocytes is orchestrated by erythroid transcription factors, the production of erythrocytes needs to respond to the availability of growth factors and nutrients, particularly the availability of iron.

From Erythroblasts to Mature Red Blood Cells: Organelle Clearance in Mammals

Erythropoiesis occurs mostly in bone marrow and ends in blood stream. Mature red blood cells are generated from multipotent hematopoietic stem cells, through a complex maturation process involving several morphological changes to produce a highly functional specialized cells. In mammals, terminal steps involved expulsion of the nucleus from erythroblasts that leads to the formation of reticulocytes. In order to produce mature biconcave red blood cells, organelles and ribosomes are selectively eliminated from reticulocytes as well as the plasma membrane undergoes remodeling. The mechanisms involved in these last maturation steps are still under investigation. Enucleation involves dramatic chromatin condensation and establishment of the nuclear polarity, which is driven by a rearrangement of actin cytoskeleton and the clathrin-dependent generation of vacuoles at the nuclear-cytoplasmic junction. This process is favored by interaction between the erythroblasts and macrophages at the erythroblastic island. Mitochondria are eliminated by mitophagy. This is a macroautophagy pathway consisting in the engulfment of mitochondria into a double-membrane structure called autophagosome before degradation. Several mice knock-out models were developed to identify mitophagy-involved proteins during erythropoiesis, but whole mechanisms are not completely determined. Less is known concerning the clearance of other organelles, such as smooth and rough ER, Golgi apparatus and ribosomes. Understanding the modulators of organelles clearance in erythropoiesis may elucidate the pathogenesis of different dyserythropoietic diseases such as myelodysplastic syndrome, leukemia and anemia.

O-prenylated 3-carboxycoumarins as a novel class of 15-LOX-1 inhibitors

Allyloxy, Isopentenyloxy, geranyloxy and farnesyloxy derivatives of 3-carboxycoumarin, at position 5, 6, 7, and 8, were synthesized and their inhibitory potency against human 15-lipoxygenase-1 (human 15-LOX-1) were determined. Among the synthetic coumarins, O-allyl and O-isopentenyl derivatives demonstrated no considerable lipoxygenase inhibition while O-geranyl and O-farnesyl derivatives demonstrated potent inhibitory activity. 5-farnesyloxy-3-carboxycoumarin demonstrated the most potent inhibitory activity by IC50 = 0.74 μM while 6-farnesyloxy-3-carboxycoumarin was the weakest inhibitor among farnesyl analogs (IC50 = 10.4 μM). Bonding affinity of the designed molecular structures toward 15-LOX-1 3D structure complexed with RS75091, as potent 15-LOX-1 inhibitor, was studied by utilizing docking analysis. There was a direct relationship between lipoxygenase inhibitory potency and prenyl length chain. The ability of the prenyl portion to fill the lipophilic pocket which is formed by Ile663, Ala404, Arg403, Ile400, Ile173 and Phe167 side chains can explain the observed relationship. Similarity rate between the docked models and complexed form of RS75091, from point of view of configuration and conformation, could explain inhibitory potency variation between each prenyloxy substitution of 3-carboxycoumarins.

Translational control mediated by hnRNP K links NMHC IIA to erythroid enucleation

Post-transcriptional regulation is crucial for structural and functional alterations in erythropoiesis. Enucleation of erythroid progenitors precedes reticulocyte release into circulation. In enucleated cells, reticulocyte 15-lipoxygenase (r15-LOX, also known as ALOX15) initiates mitochondria degradation. Regulation of r15-LOX mRNA translation by hnRNP K determines timely r15-LOX synthesis in terminal maturation. K562 cells induced for erythroid maturation recapitulate enucleation and mitochondria degradation. HnRNP K depletion from maturing K562 cells results in enhanced enucleation, which even occurs independently of maturation. We performed RIP-Chip analysis to identify hnRNP K-interacting RNAs comprehensively. Non-muscle myosin heavy chain (NMHC) IIA (also known as MYH9) mRNA co-purified with hnRNP K from non-induced K562 cells, but not from mature cells. NMHC IIA protein increase in erythroid maturation at constant NMHC IIA mRNA levels indicates post-transcriptional regulation. We demonstrate that binding of hnRNP K KH domain 3 to a specific sequence element in the NMHC IIA mRNA 3'UTR mediates translation regulation in vitro Importantly, elevated NMHC IIA expression results in erythroid-maturation-independent enucleation as shown for hnRNP K depletion. Our data provide evidence that hnRNP-K-mediated regulation of NMHC IIA mRNA translation contributes to the control of enucleation in erythropoiesis.

Structural and functional biology of arachidonic acid 15-lipoxygenase-1 (ALOX15)

Lipoxygenases (LOX) form a family of lipid peroxidizing enzymes, which have been implicated in a number of physiological processes and in the pathogenesis of inflammatory, hyperproliferative and neurodegenerative diseases. They occur in two of the three domains of terrestrial life (bacteria, eucarya) and the human genome involves six functional LOX genes, which encode for six different LOX isoforms. One of these isoforms is ALOX15, which has first been described in rabbits in 1974 as enzyme capable of oxidizing membrane phospholipids during the maturational breakdown of mitochondria in immature red blood cells. During the following decades ALOX15 has extensively been characterized and its biological functions have been studied in a number of cellular in vitro systems as well as in various whole animal disease models. This review is aimed at summarizing the current knowledge on the protein-chemical, molecular biological and enzymatic properties of ALOX15 in various species (human, mouse, rabbit, rat) as well as its implication in cellular physiology and in the pathogenesis of various diseases.

Secreted lipoxygenase from Pseudomonas aeruginosa exhibits biomembrane oxygenase activity and induces hemolysis in human red blood cells

Pseudomonas aeruginosa (PA) expresses a secreted lipoxygenase (LOX), which oxygenates free arachidonic acid predominantly to 15S-H(p)ETE. The enzyme is capable of binding phospholipids at its active site and physically interacts with model membranes. However, its membrane oxygenase activity has not been quantified. To address this question, we overexpressed PA-LOX as intracellular his-tag fusion protein in Escherichia coli, purified it to electrophoretic homogeneity and compared its biomembrane oxygenase activity with that of rabbit ALOX15. We found that both enzymes were capable of oxygenating mitochondrial membranes to specific oxygenation products and 13S-H(p)ODE and 15S-H(p)ETE esterified to phosphatidylcholine and phosphatidylethanolamine were identified as major oxygenation products. When normalized to similar linoleic acid oxygenase activity, the rabbit enzyme exhibited a much more effective mitochondrial membrane oxygenase activity. In contrast, during long-term incubations (24 h) with red blood cells PA-LOX induced significant (50%) hemolysis whereas rabbit ALOX15 was more or less ineffective. These data indicate the principle capability of PA-LOX of oxygenating membrane bound phospholipids which is likely to alter the barrier function of the biomembranes. Although the membrane oxygenase activity was lower than the fatty acid oxygenase activity of PA-LOX red blood cell membrane oxygenation might be of biological relevance for P. aeruginosa septicemia.

Regulation of mitophagy in ischemic brain injury

The selective degradation of damaged or excessive mitochondria by autophagy is termed mitophagy. Mitophagy is crucial for mitochondrial quality control and has been implicated in several neurodegenerative disorders as well as in ischemic brain injury. Emerging evidence suggested that the role of mitophagy in cerebral ischemia may depend on different pathological processes. In particular, a neuroprotective role of mitophagy has been proposed, and the regulation of mitophagy seems to be important in cell survival. For these reasons, extensive investigations aimed to profile the mitophagy process and its underlying molecular mechanisms have been executed in recent years. In this review, we summarize the current knowledge regarding the mitophagy process and its role in cerebral ischemia, and focus on the pathological events and molecules that regulate mitophagy in ischemic brain injury.

Selective removal of mitochondria via mitophagy: distinct pathways for different mitochondrial stresses

The efficient and selective elimination of damaged or excessive mitochondria in response to bioenergetic and environmental cues is critical for maintaining a healthy and appropriate population of mitochondria. Mitophagy is considered to be the central mechanism of mitochondrial quality and quantity control. Atg32, a mitophagy receptor in yeast, recruits mitochondria targeted for degradation into the isolation membrane via both direct and indirect interactions with Atg8. In mammals, different mitophagy effectors, including the mitophagy receptors NIX, BNIP3 and FUDNC1 and the PINK1/Parkin pathway, have been identified to participate in the selective clearance of mitochondria. One common feature of mitophagy receptors is that they harbor an LC3-interacting region (LIR) that interacts with LC3, thus promoting the sequestration of mitochondria into the isolation membrane. Additionally, both receptor- and Parkin/PINK1-mediated mitophagy have been found to be regulated by reversible phosphorylation. Here, we review the recent progress in the understanding of the molecular mechanisms involved in selective mitophagy at multiple levels. We also discuss different mitophagy receptors from an evolutionary perspective and highlight the specific functions of and possible cooperation between distinct mechanisms of mitophagy.

MtDNA mutagenesis impairs elimination of mitochondria during erythroid maturation leading to enhanced erythrocyte destruction

Haematopoietic progenitor cells show special sensitivity to mitochondrial DNA (mtDNA) mutagenesis, which suggests that increased mtDNA mutagenesis could underlie anemias. Here we show that elevated mtDNA mutagenesis in mice with a proof-reading deficient mtDNA polymerase (PolG) leads to incomplete mitochondrial clearance, with asynchronized iron loading in erythroid precursors, and increased total and free cellular iron content. The resulting Fenton chemistry leads to oxidative damage and premature destruction of erythrocytes by splenic macrophages. Our data indicate that mitochondria actively contribute to their own elimination in reticulocytes and modulate iron loading. Asynchrony of this sequence of events causes severe mitochondrial anaemia by depleting the organism of red blood cells and the bone marrow of iron. Our findings account for the anaemia development in a progeroid mouse model and may have direct relevance to the anemias associated with human mitochondrial disease and ageing.

A novel role for 12/15-lipoxygenase in regulating autophagy

12/15-Lipoxygenase (LOX) enzymatically generates oxidized phospholipids in monocytes and macrophages. Herein, we show that cells deficient in 12/15-LOX contain defective mitochondria and numerous cytoplasmic vacuoles containing electron dense material, indicating defects in autophagy or membrane processing, However, both LC3 expression and lipidation were normal both basally and on chloroquine treatment. A LOX-derived oxidized phospholipid, 12-hydroxyeicosatetraenoic acid-phosphatidylethanolamine (12-HETE-PE) was found to be a preferred substrate for yeast Atg8 lipidation, versus native PE, while both native and oxidized PE were effective substrates for LC3 lipidation. Last, phospholipidomics demonstrated altered levels of several phospholipid classes. Thus, we show that oxidized phospholipids generated by 12/15-LOX can act as substrates for key proteins required for effective autophagy and that cells deficient in this enzyme show evidence of autophagic dysfunction. The data functionally link phospholipid oxidation with autophagy for the first time.

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12/15-Lipoxygenase-deficient macrophages show evidence of autophagic dysfunction.

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12-HETE-PE is a substrate for LC2 and Atg8 lipidation.

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Macrophages deficient in 12/15-lipoxygenase show altered phospholipid content.

Mammalian lipoxygenases and their biological relevance

Lipoxygenases (LOXs) form a heterogeneous class of lipid peroxidizing enzymes, which have been implicated not only in cell proliferation and differentiation but also in the pathogenesis of various diseases with major public health relevance. As other fatty acid dioxygenases LOXs oxidize polyunsaturated fatty acids to their corresponding hydroperoxy derivatives, which are further transformed to bioactive lipid mediators (eicosanoids and related substances). On the other hand, lipoxygenases are key players in the regulation of the cellular redox homeostasis, which is an important element in gene expression regulation. Although the first mammalian lipoxygenases were discovered 40 years ago and although the enzymes have been well characterized with respect to their structural and functional properties the biological roles of the different lipoxygenase isoforms are not completely understood. This review is aimed at summarizing the current knowledge on the physiological roles of different mammalian LOX-isoforms and their patho-physiological function in inflammatory, metabolic, hyperproliferative, neurodegenerative and infectious disorders. This article is part of a Special Issue entitled "Oxygenated metabolism of PUFA: analysis and biological relevance".

Translation control of TAK1 mRNA by hnRNP K modulates LPS-induced macrophage activation

Macrophage activation by bacterial lipopolysaccharides (LPS) is induced through Toll-like receptor 4 (TLR4). The synthesis and activity of TLR4 downstream signaling molecules modulates the expression of pro- and anti-inflammatory cytokines. To address the impact of post-transcriptional regulation on that process, we performed RIP-Chip analysis. Differential association of mRNAs with heterogeneous nuclear ribonucleoprotein K (hnRNP K), an mRNA-specific translational regulator in differentiating hematopoietic cells, was studied in noninduced and LPS-activated macrophages. Analysis of interactions affected by LPS revealed several mRNAs encoding TLR4 downstream kinases and their modulators. We focused on transforming growth factor-β-activated kinase 1 (TAK1) a central player in TLR4 signaling. HnRNP K interacts specifically with a sequence in the TAK1 mRNA 3' UTR in vitro. Silencing of hnRNP K does not affect TAK1 mRNA synthesis or stability but enhances TAK1 mRNA translation, resulting in elevated TNF-α, IL-1β, and IL-10 mRNA expression. Our data suggest that the hnRNP K-3' UTR complex inhibits TAK1 mRNA translation in noninduced macrophages. LPS-dependent TLR4 activation abrogates translational repression and newly synthesized TAK1 boosts macrophage inflammatory response.

Genetic ablation and short-duration inhibition of lipoxygenase results in increased macroautophagy

12/15-lipoxygenase (12/15-LOX) is involved in organelle homeostasis by degrading mitochondria in maturing red blood cells and by eliminating excess peroxisomes in liver. Furthermore, 12/15-LOX contributes to diseases by exacerbating oxidative stress-related injury, notably in stroke. Nonetheless, it is unclear what the consequences are of abolishing 12/15-LOX activity. Mice in which the alox15 gene has been ablated do not show an obvious phenotype, and LOX enzyme inhibition is not overtly detrimental. We show here that liver histology is also unremarkable. However, electron microscopy demonstrated that 12/15-LOX knockout surprisingly leads to increased macroautophagy in the liver. Not only macroautophagy but also mitophagy and pexophagy were increased in hepatocytes, which otherwise showed unaltered fine structure and organelle morphology. These findings were substantiated by immunofluorescence showing significantly increased number of LC3 puncta and by Western blotting demonstrating a significant increase for LC3-II protein in both liver and brain homogenates of 12/15-LOX knockout mice. Inhibition of 12/15-LOX activity by treatment with four structurally different inhibitors had similar effects in cultured HepG2 hepatoma cells and SH-SY5Y neuroblastoma cells with significantly increased autophagy discernable already after 2 hours. Hence, our study reveals a link between ablation or inhibition of 12/15-LOX and stimulation of macroautophagy. The enhanced macroautophagy may be related to the known tissue-protective effects of LOX ablation or inhibition under various diseased conditions caused by oxidative stress and ischemia. This could provide an important cleaning mechanism of cells and tissues to prevent accumulation of damaged mitochondria and other cellular components.

Lipoxygenase: an emerging target for stroke therapy

Neuroprotection as approach to stroke therapy has recently seen a revival of sorts, fueled in part by the continuing necessity to improve acute stroke care, and in part by the identification of novel drug targets. 12/15- Lipoxygenase (12/15-LOX), one of the key enzymes of the arachidonic acid cascade, contributes to both neuronal cell death and vascular injury. Inhibition of 12/15-LOX may thus provide multifactorial protection against ischemic injury. Targeting 12/15-LOX and related eicosanoid pathways is the subject of this brief review.

Regulation and function of mitophagy in development and cancer

Beyond its role in recycling intracellular components nonselectively to sustain survival in response to metabolic stresses, autophagy can also selectively degrade specific cargoes such as damaged or dysfunctional organelles to maintain cellular homeostasis. Mitochondria, known as the power plant of cells, are the critical and dynamic organelles playing a fundamental role in cellular metabolism. Mitophagy, the selective autophagic elimination of mitochondria, has been identified both in yeast and in mammalian cells. Moreover, defects in mitophagy may contribute to a variety of human disorders such as neurodegeneration and myopathies. However, the role of mitophagy in development and cancer remains largely unclear. In this review, we summarize our current knowledge of the regulation and function of mitophagy in development and cancer.

Regulation of mouse lens maturation and gene expression by Krüppel-like factor 4

Conditional disruption of Klf4 in the surface ectoderm-derived tissues of the eye results in defective cornea, conjunctiva and the lens. This report describes the effects of disruption of Klf4 in the lens in greater detail. Expression of Klf4, first detected in the embryonic day-12 (E12) mouse lens, peaked at E16 and was decreased in later stages. Early embryonic disruption of Klf4 resulted in a smaller lens with cortical vacuolation and nuclear opacity. Microarray comparison of Klf4CN and WT lens transcriptomes revealed fewer changes in the E16.5 (59 increases, 20 decreases of >1.5-fold) than the PN56 Klf4CN lens (239 increases, 182 decreases of >2-fold). Klf4-target genes in the lens were distinct from those previously identified in the cornea, suggesting disparate functions for Klf4 in these functionally related tissues. Transcripts encoding different crystallins were down-regulated in the Klf4CN lens. Shsp/αB-crystallin promoter activity was stimulated upon co-transfection with pCI-Klf4. Mitochondrial density was significantly higher in the Klf4CN lens epithelial cells, consistent with mitochondrial dysfunction being the most significantly affected pathway within the PN56 Klf4CN lens. The Klf4CN lens contained elevated levels of Alox12 and Alox15 transcripts, less reduced glutathione (GSH) and more oxidized glutathione (GSSG) than the WT, suggesting that it is oxidatively stressed. Although the expression of 2087 genes was modulated during WT lens maturation, transcripts encoding crystallins were abundant at E16.5 and remained stable at PN56. Among the 1065 genes whose expression increased during WT lens maturation, there were 104 Klf4-target genes (9.8%) with decreased expression in the PN56 Klf4CN lens. Taken together, these results demonstrate that Klf4 expression is developmentally regulated in the mouse lens, where it controls the expression of genes associated with lens maturation and redox homeostasis.

Functions of heterogeneous nuclear ribonucleoproteins in stem cell potency and differentiation

Stem cells possess huge importance in developmental biology, disease modelling, cell replacement therapy, and tissue engineering in regenerative medicine because they have the remarkable potential for self-renewal and to differentiate into almost all the cell types in the human body. Elucidation of molecular mechanisms regulating stem cell potency and differentiation is essential and critical for extensive application. Heterogeneous nuclear ribonucleoproteins (hnRNPs) are modular proteins consisting of RNA-binding motifs and auxiliary domains characterized by extensive and divergent functions in nucleic acid metabolism. Multiple roles of hnRNPs in transcriptional and posttranscriptional regulation enable them to be effective gene expression regulators. More recent findings show that hnRNP proteins are crucial factors implicated in maintenance of stem cell self-renewal and pluripotency and cell differentiation. The hnRNPs interact with certain sequences in target gene promoter regions to initiate transcription. In addition, they recognize 3′UTR or 5′UTR of specific gene mRNA forming mRNP complex to regulate mRNA stability and translation. Both of these regulatory pathways lead to modulation of gene expression that is associated with stem cell proliferation, cell cycle control, pluripotency, and committed differentiation.

Role of 15-lipoxygenase/15-hydroxyeicosatetraenoic acid in hypoxia-induced pulmonary hypertension

Pulmonary arterial hypertension (PAH) is a rare disease with a complex aetiology characterized by elevated pulmonary artery resistance, which leads to right heart ventricular afterload and ultimately progressing to right ventricular failure and often death. In addition to other factors, metabolites of arachidonic acid cascade play an important role in the pulmonary vasculature, and disruption of signaling pathways of arachidonic acid plays a central role in the pathogenesis of PAH. 15-Lipoxygenase (15-LO) is upregulated in pulmonary artery endothelial cells and smooth muscle cells of PAH patients, and its metabolite 15-hydroxyeicosatetraenoic acid (15-HETE) in particular seems to play a central role in the contractile machinery, and in the initiation and propagation of cell proliferation via its effects on signal pathways, mitogens, and cell cycle components. Here, we focus on our important research into the role played by 15-LO/15-HETE, which promotes a proliferative, antiapoptotic, and vasoconstrictive physiological milieu leading to hypoxic pulmonary hypertension.

15-Lipoxygenase-1 as a tumor suppressor gene in colon cancer: is the verdict in?

15-Lipoxygenase-1 (15-LOX-1) is an inducible and highly regulated enzyme in normal human cells that plays a key role in the production of lipid signaling mediators, such as 13-hydroxyoctadecadienoic acid (13-HODE) from linoleic acid. 15-LOX-1 significantly contributes to the resolution of inflammation and to the terminal differentiation of normal cells. 15-LOX-1 is downregulated in human colorectal polyps and cancers. Emerging data support a tumor suppressor role for 15-LOX-1, especially in colon cancer. These data indicate that 15-LOX-1 promotes various anti-tumorigenic events, including cell differentiation and apoptosis, and inhibits chronic inflammation, angiogenesis, and metastasis. The transcriptional repression of 15-LOX-1 in colon cancer cells is complex and involves multiple mechanisms (e.g., histone methylation, transcriptional repressor binding). Re-expression of 15-LOX-1 in colon cancer cells can function as an important therapeutic mechanism and could be further exploited to develop novel treatment approaches for this common cancer.

Normal and disordered reticulocyte maturation

PURPOSE OF REVIEW

Reticulocyte remodeling has emerged as an important model for the understanding of vesicular trafficking and selective autophagy in mammalian cells. This review covers recent advances in our understanding of these processes in reticulocytes and the role of these processes in erythroid development.

RECENT FINDINGS

Enucleation is caused by the coalescence of vesicles at the nuclear-cytoplasmic junction and microfilament contraction. Mitochondrial elimination is achieved through selective autophagy, in which mitochondria are targeted to autophagosomes, and undergo subsequent degradation and exocytosis. The mechanism involves an integral mitochondrial outer membrane protein and general autophagy pathways. Plasma membrane remodeling, and the elimination of certain intracellular organelles occur through the exosomal pathway.

SUMMARY

Vesicular trafficking and selective autophagy have emerged as central processes in cellular remodeling. In reticulocytes, this includes enucleation and the elimination of all membrane-bound organelles and ribosomes. Ubiquitin-like conjugation pathways, which are required for autophagy in yeast, are not essential for mitochondrial clearance in reticulocytes. Thus, in higher eukaryotes, there appears to be redundancy between these pathways and other processes, such as vesicular nucleation. Future studies will address the relationship between autophagy and vesicular trafficking, and the significance of both for cellular remodeling.

DDX6 recruits translational silenced human reticulocyte 15-lipoxygenase mRNA to RNP granules

Erythroid precursor cells lose the capacity for mRNA synthesis due to exclusion of the nucleus during maturation. Therefore, the stability and translation of mRNAs that code for specific proteins, which function in late stages of maturation when reticulocytes become erythrocytes, are controlled tightly. Reticulocyte 15-lipoxygenase (r15-LOX) initiates the breakdown of mitochondria in mature reticulocytes. Through the temporal restriction of mRNA translation, the synthesis of r15-LOX is prevented in premature cells. The enzyme is synthesized only in mature reticulocytes, although r15-LOX mRNA is already present in erythroid precursor cells. Translation of r15-LOX mRNA is inhibited by hnRNP K and hnRNP E1, which bind to the differentiation control element (DICE) in its 3' untranslated region (3'UTR). The hnRNP K/E1-DICE complex interferes with the joining of the 60S ribosomal subunit to the 40S subunit at the AUG. We took advantage of the inducible human erythroid K562 cell system that fully recapitulates this process to identify so far unknown factors, which are critical for DICE-dependent translational regulation. Applying RNA chromatography with the DICE as bait combined with hnRNP K immunoprecipitation, we specifically purified the DEAD-box RNA helicase 6 (DDX6) that interacts with hnRNP K and hnRNP E1 in a DICE-dependent manner. Employing RNA interference and fluorescence in situ hybridization, we show that DDX6 colocalizes with endogenous human (h)r15-LOX mRNA to P-body-like RNP granules, from which 60S ribosomal subunits are excluded. Our data suggest that in premature erythroid cells translational silencing of hr15-LOX mRNA is maintained by DDX6 mediated storage in these RNP granules.

Autophagy in mammalian development and differentiation

It has been known for many decades that autophagy, a conserved lysosomal degradation pathway, is highly active during differentiation and development. However, until the discovery of the autophagy-related (ATG) genes in the 1990s, the functional significance of this activity was unknown. Initially, genetic knockout studies of ATG genes in lower eukaryotes revealed an essential role for the autophagy pathway in differentiation and development. In recent years, the analyses of systemic and tissue-specific knockout models of ATG genes in mice has led to an explosion of knowledge about the functions of autophagy in mammalian development and differentiation. Here we review the main advances in our understanding of these functions.

A novel role for iron in modulating the activity and membrane-binding ability of a trimmed soybean lipoxygenase-1

Lipoxygenases (LOXs) are iron-containing enzymes that play critical roles in plants and animals. As yet, metal atom extraction, reconstitution, and substitution have not been successfully applied to soybean LOX-1 [Glycine max (L.) Merrill], a prototype member of the LOX family that is widely used in structural and kinetic studies. Here, tryptic digestion of native LOX-1, used as a control, allowed us to isolate the 60-kDa C-terminal region (termed miniLOX), that retains the catalytically active iron in a more accessible position. Then, iron was removed to obtain an unprecedented apo-miniLOX, which was reconstituted and substituted with different metal ions. These forms of miniLOX were characterized vs. native LOX-1 by kinetic analysis, near UV circular dichroism, steady-state fluorescence, and fluorescence resonance energy transfer. MiniLOX showed a 2-fold increase in the membrane-binding affinity compared with native LOX-1 and a remarkable 4-fold increase compared with apo-miniLOX (K(d)=9.2+/-1.0, 17.9+/-2.0, and 45.4+/-4.3 microM, respectively). Furthermore, miniLOX reconstituted with Fe(II) or Fe(III) partially recovered its membrane-binding ability (K(d)=21.4+/-2.4 and 18.9+/-5.5 microM, respectively), overall supporting a novel noncatalytic role for iron in the LOX family.

12/15-Lipoxygenase targets neuronal mitochondria under oxidative stress

12/15-Lipoxygenase (12/15-LOX) is an important mediator of brain injury following experimental stroke in rodents. It contributes to neuronal death, but the underlying mechanism remains unclear. We demonstrate here that in neuronal HT22 cells subjected to glutamate-induced oxidative stress, 12/15-LOX damages mitochondria, and this represents the committed step that condemns the cell to die. Importantly these events, including breakdown of the mitochondrial membrane potential, the production of reactive oxygen species, and cytochrome c release, can all be replicated by incubation of 12/15-LOX with mitochondria in vitro, without the need to add other cytosolic factors. Proteasome activity is required downstream of mitochondrial damage to complete the cell death cascade, but proteasome inhibition is only partially protective. These findings position 12/15-LOX as the central executioner in an oxidative stress-related neuronal death program.

Mitochondrial clearance is regulated by Atg7-dependent and -independent mechanisms during reticulocyte maturation

Mitochondrial clearance is a well recognized but poorly understood biologic process, and reticulocytes, which undergo programmed mitochondrial clearance, provide a useful model to study this phenomenon. At the ultrastructural level, mitochondrial clearance resembles an autophagy-related process; however, the role of autophagy in mitochondrial clearance has not been established. Here we provide genetic evidence that autophagy pathways, initially identified in yeast, are involved in mitochondrial clearance from reticulocytes. Atg7 is an autophagy protein and an E1-like enzyme, which is required for the activity of dual ubiquitin-like conjugation pathways. Atg7 is required for the conjugation of Atg12 to Atg5, and Atg8 to phosphatidylethanolamine (PE), and is essential for autophagosome formation. In the absence of Atg7, mitochondrial clearance from reticulocytes is diminished but not completely blocked. Mammalian homologs of Atg8 are unmodified in Atg7(-/-) erythroid cells, indicating that canonical autophagy pathways are inactive. Thus, mitochondrial clearance is regulated by both autophagy-dependent and -independent mechanisms. In addition, mitochondria, which depolarize in wild-type cells before elimination, remain polarized in Atg7(-/-) reticulocytes in culture. This suggests that mitochondrial depolarization is a consequence rather than a cause of autophagosome formation in reticulocytes.

Chromatin modification requirements for 15-lipoxygenase-1 transcriptional reactivation in colon cancer cells

15-Lipoxygenase-1 (15-LOX-1) contributes significantly to inflammation regulation and terminal cell differentiation. 15-LOX-1 is transcriptionally silenced in cancer cells, and its transcriptional reactivation (e.g. via histone deacetylase inhibitors (HDACIs)) is essential for restoring terminal cell differentiation to cancer cells. STAT-6 acetylation via the histone acetyltransferase KAT3B has been proposed to be necessary for 15-LOX-1 transcriptional activation. However, the exact mechanism underlying 15-LOX-1 transcriptional reactivation in cancer cells is still undefined, especially in regard to the contribution of 15-LOX-1 promoter histone modifications. We therefore examined the relative mechanistic contributions of 15-LOX-1 promoter histone modifications and STAT-6 to 15-LOX-1 transcriptional reactivation by HDACIs in colon cancer cells. We found that: 1) histone H3 and H4 acetylation in the 15-LOX-1 promoter through KAT3B was critical to 15-LOX-1 transcriptional activation; 2) 15-LOX-1 transcription was activated independently from STAT-6; and 3) dimethyl-histone H3 lysine 9 (H3K9me2) demethylation in the 15-LOX-1 promoter via the histone lysine demethylase KDM3A was an early and specific histone modification and was necessary for activation of transcription. These findings demonstrate that histone modification in the 15-LOX-1 promoter is important to 15-LOX-1 transcriptional silencing in colon cancer cells and that HDACIs can activate gene transcription via KDM3A demethylation of H3K9me2.

On the mechanism of organelle degradation in the vertebrate lens

The programmed elimination of cytoplasmic organelles occurs during terminal differentiation of erythrocytes, keratinocytes and lens fiber cells. In each case, the process is relatively well understood phenomenologically, but the underlying molecular mechanisms have been surprisingly slow to emerge. This brief review considers the particular case of the lens where, in addition to their specialized physiological roles, organelles represent potential sources of light scattering. The article describes how the elimination of organelles from lens cells located on the visual axis contributes to the transparency of lens tissue. Classic anatomical studies of lens organelle degradation are discussed, along with more contemporary work utilizing confocal microscopy and other imaging modalities. Finally, recent data on the biochemistry of organelle degradation are reviewed. Several review articles on lens organelle degradation are available [Wride, M.A., 1996. Cellular and molecular features of lens differentiation: a review of recent advances. Differentiation 61, 77-93; Wride, M.A., 2000. Minireview: apoptosis as seen through a lens. Apoptosis 5, 203-209; Bassnett, S., 2002. Lens organelle degradation. Exp. Eye Res. 74, 1-6; Dahm, R., 2004. Dying to see. Sci. Am. 291, 82-89] and readers are directed to these for a comprehensive discussion of the earlier literature on this topic.

Protecting against cerebrovascular injury: contributions of 12/15-lipoxygenase to edema formation after transient focal ischemia

BACKGROUND AND PURPOSE

The concept of the neurovascular unit suggests that effects on brain vasculature must be considered if neuroprotection is to be achieved in stroke. We previously reported that 12/15-lipoxygenase (12/15-LOX) is upregulated in the peri-infarct area after middle cerebral artery occlusion in mice, and 12/15-LOX contributes to brain damage after ischemia-reperfusion. The current study was designed to investigate 12/15-LOX involvement in vascular injury in the ischemic brain.

METHODS

In cell culture, a human brain microvascular endothelial cell line was subjected to either hypoxia or H(2)O(2)-induced oxidative stress with or without lipoxygenase inhibitors. For in vivo studies, mice were subjected to 90 minutes middle cerebral artery occlusion, and the effects of either 12/15-LOX gene knockout or treatment with lipoxygenase inhibitors were compared. Expression of 12/15-LOX and claudin-5 as well as extravasation of immunoglobulin G were detected by immunohistochemistry. Edema was measured as water content of brain hemispheres according to the wet-dry weight method.

RESULTS

Brain endothelial cells were protected against hypoxia and H(2)O(2) by the lipoxygenase inhibitor baicalein. After focal ischemia, 12/15-LOX was increased in neurons and endothelial cells. The vascular tight junction protein claudin-5 underwent extensive degradation in the peri-infarct area, which was partially prevented by the lipoxygenase inhibitor baicalein. Leakage of immunoglobulin G into the brain parenchyma was significantly reduced in 12/15-LOX knockout mice as well as wild-type mice treated with baicalein. Likewise, brain edema was significantly ameliorated.

CONCLUSIONS

12/15-LOX may contribute to ischemic brain damage not just by causing neuronal cell death, but also by detrimental effects on the brain microvasculature. 12/15-LOX inhibitors may thus be effective as both neuroprotectants and vasculoprotectants.

Ulk1 plays a critical role in the autophagic clearance of mitochondria and ribosomes during reticulocyte maturation

Production of a red blood cell's hemoglobin depends on mitochondrial heme synthesis. However, mature red blood cells are devoid of mitochondria and rely on glycolysis for ATP production. The molecular basis for the selective elimination of mitochondria from mature red blood cells remains controversial. Recent evidence suggests that clearance of both mitochondria and ribosomes, which occurs in reticulocytes following nuclear extrusion, depends on autophagy. Here, we demonstrate that Ulk1, a serine threonine kinase with homology to yeast atg1p, is a critical regulator of mitochondrial and ribosomal clearance during the final stages of erythroid maturation. However, in contrast to the core autophagy genes such as atg5 and atg7, expression of ulk1 is not essential for induction of macroautophagy in response to nutrient deprivation or for survival of newborn mice. Together, these data suggest that the ATG1 homologue, Ulk1, is a component of the selective autophagy machinery that leads to the elimination of organelles in erythroid cells rather that an essential mechanistic component of autophagy.

mRNA silencing in human erythroid cell maturation: heterogeneous nuclear ribonucleoprotein K controls the expression of its regulator c-Src

Erythroid precursor cells undergo nuclear extrusion and degradation of mitochondria when they mature to erythrocytes. It has been suggested before that the reticulocyte 15-lipoxygenase (r15-LOX) plays an important role in initiating the breakdown of mitochondria in rabbit reticulocytes. The expression of rabbit r15-LOX is regulated by the heterogeneous nuclear ribonucleoproteins (hnRNP) K and E1 at the translational level. However, this mechanism has never been confirmed in human erythropoiesis. Based on K562 cells we have set up an inducible human erythroid cell system. We show that, during induction, K562 cells exhibit changes in morphology and protein expression that are characteristic for terminal erythroid maturation: nuclear exclusion, expression of endogenous human r15-LOX regulated by hnRNP K and hnRNP E1, and loss of mitochondria. Importantly, induction of terminal erythroid maturation in primary human CD34(+) cells recapitulated the results obtained in K562 cells. Employing the physiologically relevant K562 cell system we uncovered a new mechanism of interdependent post-transcriptional regulation of gene expression. The timely expression of the tyrosine kinase c-Src, which phosphorylates hnRNP K in later stages, is controlled by hnRNP K in early stages of erythroid maturation. hnRNP K binds to the 3'-untranslated region of the c-Src mRNA and inhibits its translation by blocking 80 S ribosome formation. In premature erythroid cells, small interfering RNA-mediated knockdown of hnRNP K, but not of hnRNP E1, leads to the de-repression of c-Src synthesis.

Proteasome inhibitor-induced apoptosis is mediated by positive feedback amplification of PKCdelta proteolytic activation and mitochondrial translocation

Emerging evidence implicates impaired protein degradation by the ubiquitin proteasome system (UPS) in Parkinson's disease; however cellular mechanisms underlying dopaminergic degeneration during proteasomal dysfunction are yet to be characterized. In the present study, we identified that the novel PKC isoform PKCδ plays a central role in mediating apoptotic cell death following UPS dysfunction in dopaminergic neuronal cells. Inhibition of proteasome function by MG-132 in dopaminergic neuronal cell model (N27 cells) rapidly depolarized mitochondria independent of ROS generation to activate the apoptotic cascade involving cytochrome c release, and caspase-9 and caspase-3 activation. PKCδ was a key downstream effector of caspase-3 because the kinase was proteolytically cleaved by caspase-3 following exposure to proteasome inhibitors MG-132 or lactacystin, resulting in a persistent increase in the kinase activity. Notably MG-132 treatment resulted in translocation of proteolytically cleaved PKCδ fragments to mitochondria in a time-dependent fashion, and the PKCδ inhibition effectively blocked the activation of caspase-9 and caspase-3, indicating that the accumulation of the PKCδ catalytic fragment in the mitochondrial fraction possibly amplifies mitochondria-mediated apoptosis. Overexpression of the kinase active catalytic fragment of PKCδ (PKCδ-CF) but not the regulatory fragment (RF), or mitochondria-targeted expression of PKCδ-CF triggers caspase-3 activation and apoptosis. Furthermore, inhibition of PKCδ proteolytic cleavage by a caspase-3 cleavage-resistant mutant (PKCδ-CRM) or suppression of PKCδ expression by siRNA significantly attenuated MG-132-induced caspase-9 and -3 activation and DNA fragmentation. Collectively, these results demonstrate that proteolytically activated PKCδ has a significant feedback regulatory role in amplification of the mitochondria-mediated apoptotic cascade during proteasome dysfunction in dopaminergic neuronal cells.

The transcription factor GATA-6 is overexpressed in vivo and contributes to silencing 15-LOX-1 in vitro in human colon cancer

Transcriptional suppression of 15-lipoxygenase (LOX)-1 (15-LOX-1) helps enable human colorectal cancer cells escape apoptosis, a critical mechanism for colonic tumorigenesis. GATA-6 is strongly expressed in vitro in cancer cells; its down-regulation by pharmaceuticals is associated with reversal of 15-LOX-1 transcriptional suppression. The mechanistic contribution of GATA-6 overexpression to colonic tumorigenesis, especially concerning 15-LOX-1 transcriptional suppression, remains unknown. We tested whether GATA-6 is differentially overexpressed in human colorectal cancers and whether reversing GATA-6 overexpression in colon cancer cells is sufficient to restore 15-LOX-1 expression and influence cell proliferation or apoptosis. The expression of GATA-6 RNA and protein was measured in paired human colorectal cancer and normal tissues from two separate patient groups. We used GATA-6 small interfering RNA transfection to down-regulate GATA-6 expression and examine the effects of this down-regulation on 15-LOX-1 expression, cell proliferation, and apoptosis in Caco-2 and HCT-116 colon cancer cells with and without the nonsteroidal antiinflammatory drug NS-398 or the histone deacetylase inhibitor sodium butyrate. GATA-6 mRNA and protein expressions were higher in cancer than normal epithelia of the colon. GATA-6 knockdown was insufficient by itself but contributed significantly to restoring 15-LOX-1 expression and inducing apoptosis by NS-398 or sodium butyrate. Maintaining 15-LOX-1 transcriptional silencing in cancer cells is a multifactorial process involving GATA-6 overexpression and other regulatory events.

A G316A Mutation of Manganese Lipoxygenase Augments Hydroperoxide Isomerase Activity

Lipoxygenases with R stereospecificity have a conserved Gly residue, whereas (S)-lipoxygenases have an Ala residue. Site-directed mutagenesis has shown that these residues control position and S/R stereospecificity of oxygenation. Recombinant Mn-LO was expressed in Pichia pastoris, and its conserved Gly-316 residue was mutated to Ala, Ser, Val, and Thr. The G316A mutant was catalytically active. We compared the catalytic properties of Mn-LO and the G316A mutant with 17:3n-3, 18:2n-6, 18:3n-3, and 19:3n-3 as substrates. Increasing the fatty acid chain length from C17 to C19 shifted the oxygenation by Mn-LO from the n-6 toward the n-8 carbon. The G316A mutant increased the oxygenation at the n-8 carbon of 17:3n-3 and at the n-10 carbon of the C17 and C18 fatty acids (from 1-2% to 7-11%). The most striking effect of the G316A mutant was a 2-, 7-, and 15-fold increase in transformation of the n-6 hydroperoxides of 19:3n-3, 18:3n-3, and 17:3n-3, respectively, to keto fatty acids and epoxyalcohols. The n-3 double bond was essential. An experiment under an oxygen-18 atmosphere showed that both oxygen atoms were retained in the epoxyalcohols. (R)-Hydroperoxides at n-6 of C17:3, 18:3, and 19:3 were transformed 5 times faster than S stereoisomers. The G316A mutant converted (13R)-hydroperoxylinolenic acid to 13-ketolinolenic acid (with an apparent K(m) of 0.01 mm) and to epoxyalcohols (viz. erythro- and threo-11-hydroxy-(12R,13R)-epoxy-(9Z,15Z)-octadecadienoic acids and one of the corresponding cis-epoxides as major products). A reducing lipoxygenase inhibitor stimulated the hydroperoxide isomerase activity, whereas a suicide-type lipoxygenase inhibitor reduced this activity. The n-3 double bond also appeared to influence the anaerobic formation of epoxyalcohols by Mn-LO, since 18:2n-6 and 18:3n-3 yielded different profiles of epoxyalcohols. Our results suggest that the G316A mutant augmented the hydroperoxide isomerase activity by positioning the hydroperoxy group at the n-6 carbon of n-3 fatty acids closer to the reduced catalytic metal.

15-Lipoxygenase is a component of the mammalian sperm cytoplasmic droplet

Lipoxygenases (LOXs) are a family of enzymes capable of peroxidizing phospholipids. A member of the LOX family of enzymes, 15-LOX, participates in the degradation of mitochondria and other organelles within differentiating red blood cells, the reticulocytes. The present study provides biochemical and immunocytochemical evidence for the presence of 15-LOX in the sperm cytoplasmic droplet (CD). Testicular, epididymal and ejaculated spermatozoa were evaluated for the presence of 15-LOX using an affinity-purified immune serum raised against a synthetic peptide corresponding to the C-terminal sequence of rabbit reticulocyte 15-LOX. Western blotting revealed an appropriate single band of ~81 kDa in boar spermatozoa but not in boar seminal plasma. When ejaculated boar spermatozoa were subjected to separation on a 45/90% Percoll gradient, 15-LOX co-migrated with the immotile sperm and cellular debris/CD fractions, but not with the motile sperm fraction containing morphologically normal spermatozoa without CDs. Varied levels of 15-LOX were expressed in ejaculated sperm samples from boars with varied semen quality. By immunofluorescence, prominent 15-LOX immunoreactivity was found within the residual body in the testis and within the CDs from caput, corpus and cauda epididymal and ejaculated spermatozoa. Components of the ubiquitin-dependent proteolytic pathway, which is thought to facilitate both spermiogenesis and reticulocyte organelle degradation, were also detected in the sperm CD. These included ubiquitin, the ubiquitin-conjugating enzyme E2, the ubiquitin C-terminal hydrolase PGP 9.5, and various 20S proteasomal core subunits of the α- and β-type. The 15-LOX and various components of the ubiquitin–proteasome pathway were also detected in sperm CDs of other mammalian species, including the human, mouse, stallion and wild babirusa boar. We conclude that 15-LOX is prominently present in the mammalian sperm CD and thus may contribute to spermiogenesis, CD function or CD removal.

Exosome release by reticulocytes—An integral part of the red blood cell differentiation system

Reticulocyte maturation into erythrocytes is the final step of erythropoiesis that occurs in the blood circulation. This terminal differentiation period corresponds to a cellular remodeling phase following expulsion of the nucleus into the bone marrow. Among other events, this remodeling leads to the disappearance of intracellular organelles and acquisition of the typical cellular biconcave form. Here, we propose that exosome biogenesis and secretion, which contributes to net loss of the cell surface membrane via selective vesicular membrane secretion, is also closely interconnected with upstream (nucleus expulsion), accompanying (mitoptosis) and downstream (vesicle clearance) events.

Localization of ferrochelatase in Plasmodium falciparum

Our previous studies have demonstrated de novo haem biosynthesis in the malarial parasite (Plasmodium falciparum and P. berghei). It has also been shown that the first enzyme of the pathway is the parasite genome-coded ALA (delta-aminolaevulinate) synthase localized in the parasite mitochondrion, whereas the second enzyme, ALAD (ALA dehydratase), is accounted for by two species: one species imported from the host red blood cell into the parasite cytosol and another parasite genome-coded species in the apicoplast. In the present study, specific antibodies have been raised to PfFC (parasite genome-coded ferrochelatase), the terminal enzyme of the haem-biosynthetic pathway, using recombinant truncated protein. With the use of these antibodies as well as those against the hFC (host red cell ferrochelatase) and other marker proteins, immunofluorescence studies were performed. The results reveal that P. falciparum in culture manifests a broad distribution of hFC and a localized distribution of PfFC in the parasite. However, PfFC is not localized to the parasite mitochondrion. Immunoelectron-microscopy studies reveal that PfFC is indeed localized to the apicoplast, whereas hFC is distributed in the parasite cytoplasm. These results on the localization of PfFC are unexpected and are at variance with theoretical predictions based on leader sequence analysis. Biochemical studies using the parasite cytosolic and organellar fractions reveal that the cytosol containing hFC accounts for 80% of FC enzymic activity, whereas the organellar fraction containing PfFC accounts for the remaining 20%. Interestingly, both the isolated cytosolic and organellar fractions are capable of independent haem synthesis in vitro from [4-14C]ALA, with the cytosol being three times more efficient compared with the organellar fraction. With [2-14C]glycine, most of the haem is synthesized in the organellar fraction. Thus haem is synthesized in two independent compartments: in the cytosol, using the imported host enzymes, and in the organellar fractions, using the parasite genome-coded enzymes.

In vitro maturation of nascent reticulocytes to erythrocytes

Most studies of mammalian reticulocyte maturation have used blood reticulocytes.Nascent reticulocytes, as found in bone marrow, have not been available in developmentally synchronized populations. Nascent murine reticulocytes formed in vitro by enucleation of Friend virus–infected erythroblasts were purified and recultured for 110 hours. At 0 hours, all recultured cells were lobulated and contained dense, centralized reticulin. By 110 hours, about 20% to 25% of the cells became biconcave erythrocytes. Most ribosomes and cellular RNAs were degraded within 20 hours, and during that period, heme synthesis declined from a rate equal to that of late erythroblasts to less than 10% of that rate. Many mitochondria appeared normal until they showed outer membrane swelling, degradation, and apparent fusion with intracellular vacuoles at 40 hours of culture. During the period of mitochondrial loss, Bcl-XL, an antiapoptotic protein that accumulates during erythroblast differentiation and maintains mitochondrial membrane integrity, demonstrated progressive decreases and changes consistent with deamidation. Nevertheless, the reticulocytes did not undergo apoptosis, because their apoptotic machinery was degraded. This experimental system that provides a developmentally synchronized population of nascent murine reticulocytes that mature into biconcave erythrocytes in vitro should be useful in further investigations of the cellular events involved in reticulocyte maturation.

Clues for new therapeutics in osteoporosis and periodontal disease: new roles for lipoxygenases?

Lipoxygenase (LOX) pathways are well appreciated for their ability to regulate key events contributing to the cardinal signs of inflammation. Recent evidence indicates that LOX genes are associated with osteoporosis. Also, overexpression of the 15-LOX Type 1 in transgenic rabbits leads to a reduced inflammatory phenotype and protection from periodontal disease, as well as atherosclerosis. Osteoporosis and inflammation-associated bone degradation, such as periodontitis, affect many individuals worldwide and are known to have pathogenesis that involves local mediators via communication between osteoclasts and osteoblasts during osteogenesis. Evidence has emerged indicating that LOX gene expression is associated with reduced bone strength in murine models of osteoporosis. Overexpression of the 15-LOX gene and its products, such as lipoxins, confers endogenous anti-inflammation. This article discusses the recent findings that may link aberrant LOX pathway expression in these diseases, suggesting new avenues for therapeutic approaches via activation of endogenous pathways for resolution of local inflammation.

Expression of manganese lipoxygenase in Pichia pastoris and site-directed mutagenesis of putative metal ligands

Manganese lipoxygenase is secreted by the fungus Gaeumannomyces graminis. We expressed the enzyme in Pichia pastoris, which secreted approximately 30 mg Mn-lipoxygenase/L culture medium in fermentor. The recombinant lipoxygenase was N- and O-glycosylated (80-100 kDa), contained approximately 1 mol Mn/mol protein, and had similar kinetic properties (K(m) approximately 7.1 microM alpha-linolenic acid and V(max) 18 nmol/min/microg) as the native Mn-lipoxygenase. Mn-lipoxygenase could be quantitatively converted, presumably by secreted Pichia proteases, to a smaller protein (approximately 67 kDa) with retention of lipoxygenase activity (K(m) approximately 6.4 microM alpha-linolenic acid and V(max) approximately 12 nmol/min/microg). Putative manganese ligands were investigated by site-directed mutagenesis. The iron ligands of soybean lipoxygenase-1 are two His residues in the sequence HWLNTH, one His residue and a distant Asn residue in the sequence HAAVNFGQ, and the C-terminal Ile residue. The homologous sequences of Mn-lipoxygenase are H274VLFH278 and H462HVMN466QGS, respectively, and the C-terminal amino acid is Val-602. The His274Gln, His278Glu, His462Glu, and the Val-602 deletion mutants of Mn-lipoxygenase were inactive, and had lost >95% of the manganese content. His-463, Asn-466, and Gln-467 did not appear to be critical for Mn-lipoxygenase activity, as His463Gln, Asn466Gln, Asn466Leu, and Gln467Asn mutants metabolized alpha-linolenic acid to 11- and 13-hydroperoxylinolenic acids. We conclude that His-274, His-278, His-462, and Val-602 likely coordinate manganese.

High-level expression of rabbit 15-lipoxygenase induces collapse of the mitochondrial pH gradient in cell culture

A critical step in the development of mammalian erythroblasts into mature red blood cells is the extrusion of the nucleus, followed by intracellular degradation of the remaining organelles. It has been hypothesized that the breakdown of cellular organelles in rabbit reticulocytes is initiated by 15-lipoxygenase. In vitro, the purified rabbit reticulocyte 15-lipoxygenase binds and permeabilizes organellar membranes, thereby releasing the lumenal contents of the organelle. Here, we demonstrate that ectopic expression of 15-lipoxygenase leads to the collapse of the mitochondrial pH gradient in nonerythroid cells, using a novel reporter of mitochondrial pH, mito-pHluorin. No change in mitochondrial pH was observed with a mutant of 15-lipoxygenase that lacks enzymatic activity. These data demonstrate that 15-lipoxygenase is capable of disrupting the pH gradient maintained by mitochondria in living cells without additional factors specific for red blood cell development.