Metabolism and signaling activities of nuclear lipids

Apart from the lipids present in the nuclear envelope, the nucleus also contains lipids which are located further inside and are resistant to treatment with nonionic detergents. Evidence is being accumulated on the importance of internal nuclear lipid metabolism. Nuclear lipid metabolism gives rise to several lipid second messengers that function within the nucleus. Moreover, it is beginning to emerge that nuclear lipids not only act as precursors of bioactive second messengers but may be directly involved in regulation of nuclear structure and gene expression. Over the last 10years, especially the role of the inositol lipid cycle in nuclear signal transduction has been extensively studied. This cycle is activated following a variety of stimuli and is regulated independently from the inositide cycle located at the plasma membrane. However, the nucleus contain other lipids, such as phosphatidylcholine, sphingomyelin, fatty acids and eicosanoids. There are numerous reports which suggest that these classes of nuclear lipids may play roles in the nucleus as important as those of phosphoinositides. This review aims at highlighting the most important aspects regarding the metabolism and signaling activities of nuclear phosphatidylcholine, sphingomyelin, fatty acids and eicosanoids.Received 7 November 2003; received after revision 18 December 2003; accepted 29 December 2003     

Possible mechanisms and function of nuclear trafficking of the colony-stimulating factor-1 receptor

Receptor tyrosine kinases (RTK) have long being studied with respect to the “canonical” signaling. This includes ligand-induced activation of a receptor tyrosine kinase at the cell surface that leads to receptor dimerization, followed by its phosphorylation in the intracellular domain and activation. The activated receptor then recruits cytoplasmic signaling molecules including other kinases. Activation of the downstream signaling cascade frequently leads to changes in gene expression following nuclear translocation of downstream targets. However, RTK themselves may localize within the nucleus, as either full-length molecules or cleaved fragments, with or without their ligands. Significant differences in this mechanism have been reported depending on the individual RTK, cellular context or disease. Accumulating evidences indicate that the colony-stimulating factor-1 receptor (CSF-1R) may localize within the nucleus. To date, however, little is known about the mechanism of CSF-1R nuclear shuttling, as well as the functional role of nuclear CSF-1R.     

VRK1 and AURKB form a complex that cross inhibit their kinase activity and the phosphorylation of histone H3 in the progression of mitosis

Regulation of cell division requires the integration of signals implicated in chromatin reorganization and coordination of its sequential changes in mitosis. Vaccinia-related kinase 1 (VRK1) and Aurora B (AURKB) are two nuclear kinases involved in different steps of cell division. We have studied whether there is any functional connection between these two nuclear kinases, which phosphorylate histone H3 in Thr3 and Ser10, respectively. VRK1 and AURKB are able to form a stable protein complex, which represents only a minor subpopulation of each kinase within the cell and is detected following nocodazole release. Each kinase is able to inhibit the kinase activity of the other kinase, as well as inhibit their specific phosphorylation of histone H3. In locations where the two kinases interact, there is a different pattern of histone modifications, indicating that there is a local difference in chromatin during mitosis because of the local complexes formed by these kinases and their asymmetric intracellular distribution. Depletion of VRK1 downregulates the gene expression of BIRC5 (survivin) that recognizes H3-T3ph, both are dependent on the activity of VRK1, and is recovered with kinase active murine VRK1, but not with a kinase-dead protein. The H3–Thr3ph–survivin complex is required for AURB recruitment, and their loss prevents the localization of ACA and AURKB in centromeres. The cross inhibition of the kinases at the end of mitosis might facilitate the formation of daughter cells. A sequential role for VRK1, AURKB, and haspin in the progression of mitosis is proposed.     

Integrin-actin interactions

The integrin family of extracellular matrix receptors regulates many aspects of cell life, in particular cell adhesion and migration. These two processes depend on organization of the actin cytoskeleton into adhesive and protrusive organelles in response to extracellular signals. Integrins are important switch points for the spatiotemporal control of actin-based motility in higher eukaryotes. Ligands of integrin cytoplasmic tails are central elements of signalling pathways involving small GTPases as well as protein and lipid kinases in the regulation of Factin crosslinking, actin treadmilling and de novo nucleation of actin filaments. We present an overview of common pathways and discuss recent evidence for their differential use by individual integrin receptors.Received 24 November 2004; received after revision 17 January 2005; accepted 19 January 2005     

Molecular aspects of membrane fission in the secretory pathway

Membrane fission is essential in various intracellular dissociative transport steps. The molecular mechanisms by which endocytic vesicles detach from the plasma membrane are being rapidly elucidated. Much less is known about the fission mechanisms operating at Golgi tubular networks; these include the Golgi transport and sorting stations, the trans-Golgi and cis-Golgi networks, where the geometry and physical properties of the membranes differ from those at the cell surface. Here we discuss the lipid and protein machineries that have so far been related to the fission process, with emphasis on those acting in the Golgi complex. Received 10 May 2002; received after revision 20 June 2002; accepted 26 June 2002 RID="*" ID="*"Corresponding author.     

Friedrich Miescher Prize awardee lecture review. A conserved family of nuclear export receptors mediates the exit of messenger RNA to the cytoplasm

The distinguishing feature of eukaryotic cells is the segregation of RNA biogenesis and DNA replication in the nucleus, separate from the cytoplasmic machinery for protein synthesis. As a consequence, messenger RNAs (mRNAs) and all cytoplasmic RNAs from nuclear origin need to be transported from their site of synthesis in the nucleus to their final cytoplasmic destination. Nuclear export occurs through nuclear pore complexes (NPCs) and is mediated by saturable transport receptors, which shuttle between the nucleus and cytoplasm. The past years have seen great progress in the characterization of the mRNA export pathway and the identification of proteins involved in this process. A novel family of nuclear export receptors (the NXF family), distinct from the well-characterized family of importin β-like proteins, has been implicated in the export of mRNA to the cytoplasm. Received 23 January 2001; received after revision 12 April 2001; accepted 12 April 2001     

Protein phosphatases and their potential implications in neuroprotective processes

Several neurological disorders such as stroke, amyotrophic lateral sclerosis and epilepsy result from excitotoxic events and are accompanied by neuronal cell death. These processes engage multiple signalling pathways and recruit numerous molecular components, in particular several families of protein kinases and protein phosphatases. While many investigations have examined the importance of protein kinases in excitotoxicity, protein phosphatases have not been well studied in this context. However, recent advances in understanding the functions of protein phosphatases have suggested that they may play a neuroprotective role. In this review, we summarize some of the recent findings that illustrate the pleiotropic and complex functions of tyrosine and serine/threonine protein phosphatases in the cascade of events leading to neuronal cell death, and highlight their potential intervention in limiting the extent of neuronal death.Received 8 January 2005; received after revision 3 March 2005; accepted 14 March 2005     

Regulation of SNARE fusion machinery by fatty acids

Vesicle fusion is a ubiquitous biological process involved in membrane trafficking and a variety of specialised events such as exocytosis and neurite outgrowth. The energy to drive biological membrane fusion is provided by fusion proteins called SNAREs. Indeed, SNARE proteins play critical roles in neuronal development as well as neurotransmitter and hormone release. SNARE proteins form a very tight alpha-helical bundle that can pull two membranes together, thereby initiating fusion. Whereas a great deal of attention has been paid to partner proteins that can affect SNARE function, recent genetic and biochemical evidence suggests that local lipid environment may be as important in SNARE regulation. Direct lipid modification of SNARE fusion proteins and their regulation by fatty acids following phospholipase action will be discussed here in detail. Our analysis highlights the fact that lipids are not a passive platform in vesicle fusion but intimately regulate SNARE function. Received 20 December 2006; received after revision 6 February 2007; accepted 15 March 2007     

Nibbling within the nucleus: turnover of nuclear contents

As the site of gene expression and regulation, the nucleus is the control center of the cell. It might be thought that degradation of nuclear contents is strictly ‘off-limits,’ given the importance of the genetic information contained within the nucleus, but it has recently been reported that partial degradation of the nucleus may occur in yeast. Here we summarize the evidence for the degradation and quality control of proteins found with the nucleus and its compartments, and of nucleic acids that may occur under certain specific conditions. Only under certain special conditions such as differentiation of the lens are the entire nuclear contents degraded. Received 6 September 2006; received after revision 25 October 2006; accepted 13 December 2006     

Nuclear inositides: inconsistent consistencies

It is now clear that phosphoinositides, which play a major role in the regulation of a variety of cellular processes in the cytoplasm, are found within the nucleus. Their role in this subcellular compartment is still contentious: however, data has suggested that nuclear inositides generate substrates, such as PtdIns(4,5)P2, utilised by a number of nuclear signalling pathways: for example, nuclear phospholipase C and the PtdIns 3-kinase cascade. There is also evidence that PtdIns(4,5)P2 may play a role in the localisation and regulation of a number of nuclear proteins such as the BAF complex, which is involved in the regulation of chromatin structure. Although the presence of nuclear inositides has been demonstrated in a number of different cell types, suggesting that it is ubiquitous, there are many inconsistencies within the literature concerning the locations and isotypes of enzymes that are involved in their regulation and in the potential second messengers which are generated by them. This review aims to highlight some of these inconsistencies in order to focus on areas that need further characterisation.     

Expression of nerve growth factor-like polypeptides and immunoreactivity related to the two types of neurotrophin receptors in earthworm tissues

Nerve growth factor (NGF) belongs by sequence homology to the neurotrophins, a family of proteins binding the same p75 receptor and closely related members of the Trk family of receptor tyrosine kinases. Fundamental in the vertebrate nervous system, neurotrophin signals have also been suggested as essential for relatively complex nervous systems occurring in invertebrate species that live longer than Caenorhabditis elegans and Drosophila melanogaster. Mammalian neurotrophins have been found to influence invertebrate neuronal growth. However, there are only a few data on the presence of molecules related to neurotrophin signalling components in invertebrates. Our studies provide evidence that analogues of neurotrophins and neurotrophin receptors are expressed in Eisenia foetida earthworms. In particular, NGF-like and Trk-like immunoreactive proteins are both expressed in the nervous system, whereas p75-like positivity identifies tubular structures associated with dorsal pores that are involved in the earthworm response to mechanical irritation or stress. Received 12 November 2001; received after revision 8 January 2002; accepted 8 January 2002     

Thyroid hormone controls carnitine status through modifications of γ-butyrobetaine hydroxylase activity and gene expression

The carnitine system plays a key role in β-oxidation of long-chain fatty acids by permitting their transport into the mitochondrial matrix. The effects of hypothyroidism and hyperthyroidism were studied on γ-butyrobetaine hydroxylase (BBH), the enzyme responsible for carnitine biosynthesis in the rat. In rat liver, BBH activity was decreased in the hypothyroid state and increased in hyperthyroid animals. The modifications in BBH activity correlated with changes in the enzyme Vmax values. These changes were shown to be related to hepatic BBH mRNA abundance. Thyroid hormones are known to interact with lipid metabolism, in particular by increasing long-chain fatty acid oxidation through activation of carnitine-dependent fatty acid import into mitochondria. Our study showed that thyroid hormones also increased carnitine bioavailability. Received 23 October 2001; received after revision 11 January 2002; accepted 15 January 2002     

Gold nanoparticles induce nuclear damage in breast cancer cells,which is further amplified by hyperthermia

Gold nanoparticles have emerged as promising tools for cancer research and therapy, where they can promote thermal killing. The molecular mechanisms underlying these events are not fully understood. The geometry and size of gold nanoparticles can determine the severity of cellular damage. Therefore, small and big gold nanospheres as well as gold nanoflowers were evaluated side-by-side. To obtain quantitative data at the subcellular and molecular level, we assessed how gold nanoparticles, either alone or in combination with mild hyperthermia, altered the physiology of cultured human breast cancer cells. Our analyses focused on the nucleus, because this organelle is essential for cell survival. We showed that all the examined gold nanoparticles associated with nuclei. However, their biological effects were quantitatively different. Thus, depending on the shape and size, gold nanoparticles changed multiple nuclear parameters. They redistributed stress-sensitive regulators of nuclear biology, altered the nuclear morphology, reorganized nuclear laminae and envelopes, and inhibited nucleolar functions. In particular, gold nanoparticles reduced the de novo biosynthesis of RNA in nucleoli, the subnuclear compartments that produce ribosomes. While small gold nanospheres and nanoflowers, but not big gold nanospheres, damaged the nucleus at normal growth temperature, several of these defects were further exacerbated by mild hyperthermia. Taken together, the toxicity of gold nanoparticles correlated with changes in nuclear organization and function. These results emphasize that the cell nucleus is a prominent target for gold nanoparticles of different morphologies. Moreover, we demonstrated that RNA synthesis in nucleoli provides quantitative information on nuclear damage and cancer cell survival.     

Surface exposure of phosphatidylserine in pathological cells

The asymmetric phospholipid distribution in plasma membranes is normally maintained by energy-dependent lipid transporters that translocate different phospholipids from one monolayer to the other against their respective concentration gradients. When cells are activated, or enter apoptosis, lipid asymmetry can be perturbed by other lipid transporters (scramblases) that shuttle phospholipids non-specifically between the two monolayers. This exposes phosphatidylserine (PS) at the cells outer surface. Since PS promotes blood coagulation, defective scramblase activity upon platelet stimulation causes a bleeding disorder (Scott syndrome). PS exposure also plays a pivotal role in the recognition and removal of apoptotic cells via a PS-recognizing receptor on phagocytic cells. Furthermore, expression of PS at the cell surface can occur in a wide variety of disorders. This review aims at highlighting how PS expression in different cells may complicate a variety of pathological conditions, including those that promote thromboembolic complications or produce aberrations in apoptotic cell removal.Received 26 November 2004; received after revision 3 January 2005; accepted 10 January 2005 Available online 09 March 2005     

Biochemical effects and growth inhibition in MCF-7 cells caused by novel sulphonamido oxa-polyamine derivatives

The novel polyamine derivatives sulphonamido oxa-spermine (oxa-Spm) and sulphonamido oxa-spermidine (oxa-Spd) exhibited rapid cytotoxic action towards MCF-7 human breast cancer cells with IC₅₀ values of 4.35 and 6.47 μM, respectively, after 24-h drug exposure. Neither compound is a substrate of serum amine oxidase. Both oxa-Spm and oxa-Spd caused cell shrinkage, as determined by phase-contrast microscopy. After incubation with 10 μM of either compound for 8 h, the cells underwent chromatin condensation and nuclear fragmentation. However, no clear DNA ladder was obtained by electrophoresis. The sulphonamido oxa-polyamine derivatives and especially oxa-Spd enhanced the activity of polyamine oxidase (PAO), an enzyme capable of oxidising N¹-acetylated spermine and spermidine to spermidine and putrescine, respectively, generating cytotoxic H₂O₂ and 3-acetamidopropanal as by-products. The intracellular polyamine content was only marginally reduced in response to drug treatment. In conclusion, our data show that these novel sulphonamido oxa-polyamine derivatives possess high cytotoxic activity against MCF-7 cells and indicate that induction of PAO may mediate their cytotoxicity via apoptosis. Received 17 January 2002; received after revision 22 February 2002; accepted 22 February 2002     

ABCA2: a candidate regulator of neural transmembrane lipid transport

Studies in the past years have implicated multispan transmembrane transport molecules of the ATP binding cassette (ABC) transporter family in cellular lipid export processes. The prototypic ABC transporter ABCA1 has recently been demonstrated to act as a major facilitator of cellular cholesterol and phospholipid export. Moreover, the transporter ABCA4 (ABCR) plays a pivotal role in retinaldehyde processing, and ABCA3 has recently implicated in lung surfactant processing. These pioneering observations have directed considerable attention to the A subfamily of ABC proteins. ABCA2 is the codefining member of the ABC A-transporter subclass. Although known for some time, it was not until recently that its complete molecular structure was established. Unlike other ABC A-subfamily members, ABCA2 is predominantly expressed in the brain and neural tissues. The unique expression profile together with available structural data suggest roles for this largest known ABC protein in neural transmembrane lipid export. Received 31 January 2002; received after revision 11 March 2002; accepted 11 March 2002