Nuclear lipids: New functions for old molecules?

It is becoming increasingly evident that stimulation of nuclear lipid metabolism plays a central role in many signal transduction pathways that ultimately result in various cell responses including proliferation and differentiation. Nuclear lipid metabolism seems to be at least as complex as that existing at the plasma membrane. However, a distinctive feature of nuclear lipid biochemical pathways is their operational independence from their cell periphery counterparts. Although initially it was thought that nuclear lipids would serve as a source for second messengers, recent evidence points to the likelihood that lipids present in the nucleus also fulfil other roles. The aim of this review is to highlight the most intriguing advances made in the field over the last year, such as the production of new probes for the in situ mapping of nuclear phosphoinositides, the identification of two sources for nuclear diacylglycerol production, the emerging details about the peculiar regulation of nuclear phosphoinositide synthesizing enzymes, and the distinct possibility that nuclear lipids are involved in processes such as chromatin organization and pre-mRNA splicing.     

Reactive oxygen species in photosystem II: relevance for oxidative signaling

Reactive oxygen species (ROS) are formed in photosystem II (PSII) under various types of abiotic and biotic stresses. It is considered that ROS play a role in chloroplast-to-nucleus retrograde signaling, which changes the nuclear gene expression. However, as ROS lifetime and diffusion are restricted due to the high reactivity towards biomolecules (lipids, pigments, and proteins) and the spatial specificity of signal transduction is low, it is not entirely clear how ROS might transduce signal from the chloroplasts to the nucleus. Biomolecule oxidation was formerly connected solely with damage; nevertheless, the evidence appears that oxidatively modified lipids and pigments are be involved in chloroplast-to-nucleus retrograde signaling due to their long diffusion distance. Moreover, oxidatively modified proteins show high spatial specificity; however, their role in signal transduction from chloroplasts to the nucleus has not been proven yet. The review attempts to summarize and evaluate the evidence for the involvement of ROS in oxidative signaling in PSII.

     

Regulated nuclear targeting

Extracellular signals are transduced to the nucleus through respective signal transduction pathways. Evidence in animals and yeast indicates the importance of regulated nuclear targeting in these processes. Although little is known about plants in this regard, some plant signaling factors have recently been shown to translocate to the nucleus upon receipt of a signal.     

Signal transduction within the nucleus by mitogen-activated protein kinase.

The nucleus is an important target of signal transduction by growth factor receptors that stimulate mitogen-activated protein (MAP) kinases. We tested the hypothesis that MAP kinases have a signaling role within the nucleus by examining the effect of the expression of a human MAP kinase isoform (p41mapk) in tissue culture cells. The expressed p41mapk was found to be localized in both the cytoplasmic and nuclear compartments of the cells. Significantly, the expression of p41mapk caused an increase in the phosphorylation of a nuclear substrate: Ser62 of c-Myc. Phosphorylation at Ser62 stimulated the activity of the NH2-terminal transactivation domain of c-Myc. Thus, p41mapk causes the phosphorylation and regulation of a physiologically significant nuclear target of signal transduction. These data establish that at least one MAP kinase isoform has a nuclear role during signal transduction.     

Nuclear transportation of diacylglycerol kinase gamma and its possible function in the nucleus

Diacylglycerol kinases (DGKs) convert diacylglycerol (DG) to phosphatidic acid, and both lipids are known to play important roles in lipid signal transduction. Thereby, DGKs are considered to be a one of the key players in lipid signaling, but its physiological function remains to be solved. In an effort to investigate one of nine subtypes, we found that DGKgamma came to be localized in the nucleus with time in all cell lines tested while seen only in the cytoplasm at the early stage of culture, indicating that DGKgamma is transported from the cytoplasm to the nucleus. The nuclear transportation of DGKgamma didn't necessarily need DGK activity, but its C1 domain was indispensable, suggesting that the C1 domain of DGKgamma acts as a nuclear transport signal. Furthermore, to address the function of DGKgamma in the nucleus, we produced stable cell lines of wild-type DGKgamma and mutants, including kinase negative, and investigated their cell size, growth rate, and cell cycle. The cells expressing the kinase-negative mutant of DGKgamma were larger in size and showed slower growth rate, and the S phase of the cells was extended. These findings implicate that nuclear DGKgamma regulates cell cycle.     

Inner nuclear membrane and signal transduction

Recent research has shown that the inner nuclear membrane is a site for regulation of signal transduction from the plasma membrane to the nucleus. This has coincided with discoveries showing that mutations in extrinsic and intrinsic inner nuclear membrane proteins cause a variety of inherited diseases. In most instances, the mechanisms by which mutations in inner nuclear membrane proteins cause disease are not understood. In at least one case, however, an alteration in signal transduction appears to underlie disease pathogenesis.     

Nuclear expression of diacylglycerol kinases: possible involvement in DNA replication

The existence of intranuclear lipid-dependent signal transduction systems has been demonstrated by several independent groups. Remarkably, intranuclear lipid-dependent signal transduction pathways are regulated independently from their membrane/cytosolic counterparts. A sizable body of evidence suggests that nuclear lipid signaling controls critical biological functions such as cell proliferation, differentiation, and apoptosis. Diacylglycerol (DG) is a fundamental lipid second messenger which is produced in the nucleus. Since the levels of nuclear DG fluctuate during the cell cycle progression, it has been suggested that this lipid second messenger has important regulatory roles. Most likely, nuclear DG serves as a chemoattractant for some isoforms of protein kinase C that migrate to the nucleus in response to a variety of agonists. The nucleus also contains diacylglycerol kinases (DGKs), i.e. the enzymes that, by converting DG into phosphatidic acid (PA), terminate DG-dependent events. This review aims at highlighting the different isozymes of DGKs present within the nucleus as well as at discussing their potential functions with particular emphasis placed on DNA replication.     

Intranuclear 3′-phosphoinositide metabolism and Akt signaling: New mechanisms for tumorigenesis and protection against apoptosis?

Lipid second messengers, particularly those derived from the polyphosphoinositide metabolism, play a pivotal role in multiple cell signaling networks. Phosphoinositide 3-kinase (PI3K) generate 3'-phosphorylated inositol lipids that are key players in a multitude of cell functions. One of the best characterized targets of PI3K lipid products is the serine/threonine protein kinase Akt (protein kinase B, PKB). Recent findings have implicated the PI3K/Akt pathway in tumorigenesis because it stimulates cell proliferation and suppresses apoptosis. However, it was thought that this signal transduction network would exert its carcinogenetic effects mainly by operating in the cytoplasm. Evidence accumulated over the past 15 years has highlighted the presence of an autonomous nuclear inositol lipid cycle, and strongly suggests that lipid molecules are important components of signaling pathways operating at the nuclear level. PI3K, its lipid product phosphatidylinositol (3,4,5) trisphosphate (PtdIns(3,4,5)P3), and Akt have been identified within the nucleus and recent data suggest that they counteract apoptosis also by operating in this cell compartment through a block of caspase-activated DNase and inhibition of chromatin condensation. In this review, we shall summarize the most updated and intriguing findings about nuclear PI3K/PtdIns(3,4,5)P3/Akt in relationship with tumorigenesis and suppression of apoptotic stimuli.     

The membrane and lipids as integral participants in signal transduction: lipid signal transduction for the non-lipid biochemist

Reviews of signal transduction have often focused on the cascades of protein kinases and protein phosphatases and their cytoplasmic substrates that become activated in response to extracellular signals. Lipids, lipid kinases, and lipid phosphatases have not received the same amount of attention as proteins in studies of signal transduction. However, lipids serve a variety of roles in signal transduction. They act as ligands that activate signal transduction pathways as well as mediators of signaling pathways, and lipids are the substrates of lipid kinases and lipid phosphatases. Cell membranes are the source of the lipids involved in signal transduction, but membranes also constitute lipid barriers that must be traversed by signal transduction pathways. The purpose of this review is to explore the magnitude and diversity of the roles of the cell membrane and lipids in signal transduction and to highlight the interrelatedness of families of lipid mediators in signal transduction.     

In situ mechanical properties of the chondrocyte cytoplasm and nucleus

The way in which the nucleus experiences mechanical forces has important implications for understanding mechanotransduction. Knowledge of nuclear material properties and, specifically, their relationship to the properties of the bulk cell can help determine if the nucleus directly experiences mechanical loads, or if it is a signal transduction mechanism secondary to cell membrane deformation that leads to altered gene expression. Prior work measuring nuclear material properties using micropipette aspiration suggests that the nucleus is substantially stiffer than the bulk cell [Guilak, F., Tedrow, J.R., Burgkart, R., 2000. Viscoelastic properties of the cell nucleus. Biochem. Biophys. Res. Commun. 269, 781–786], whereas recent work with unconfined compression of single chondrocytes showed a nearly one-to-one correlation between cellular and nuclear strains [Leipzig, N.D., Athanasiou, K.A., 2008. Static compression of single chondrocytes catabolically modifies single-cell gene expression. Biophys. J. 94, 2412–2422]. In this study, a linearly elastic finite element model of the cell with a nuclear inclusion was used to simulate the unconfined compression data. Cytoplasmic and nuclear stiffnesses were varied from 1 to 7 kPa for several combinations of cytoplasmic and nuclear Poisson's ratios. It was found that the experimental data were best fit when the ratio of cytoplasmic to nuclear stiffness was 1.4, and both cytoplasm and nucleus were modeled as incompressible. The cytoplasmic to nuclear stiffness ratio is significantly lower than prior reports for isolated nuclei. These results suggest that the nucleus may behave mechanically different in situ than when isolated.     

Structural and functional aspects of nuclear lipids in normal and tumor cells

This review summarizes available data on the structural and functional role of neutral lipids and phospholipids in normal and tumor eukaryotic cells. The role of acidic phospholipids (cardiolipin, phosphatidylinositol, and phosphatidylglycerol) in regulation of activities of DNA- and RNA-polymerases, DNA-topoisomerases I and II, DNA-methylases, and replication initiation proteins (dnaA and T-antigen) is discussed. The role of sphingolipids is emphasized considering, on one hand, the involvement of sphingosines in signal transduction, chromatin association-dissociation, and regulation of DNA and RNA synthesis and protein kinase C and, on the other hand, participation of ceramides and dihydroceramides in apoptosis. The possible role of sphingomyelin, sphingosine, cardiolipin, and diglycerides in the contacts of DNA loops with nuclear matrix is analyzed. Lipid hormones indirectly influence supercoiled DNA conformation; the effect of hormones on metabolism of phospholipids and neutral lipids in chromatin and nuclear matrix is reviewed. Characteristics of lipid composition in chromatin and nuclear matrix of the tumor cells are discussed.     

Preferential utilization of Imp7/8 in nuclear import of Smads

Trafficking of Smad proteins between the cytoplasm and nucleus is a critical component of transforming growth factor beta (TGF-beta) signal transduction. Smad4 translocates into the nucleus either in response to TGF-beta stimulation or when its nuclear export is blocked by leptomycin B (LMB). We demonstrate that both TGF-beta-induced and basal state spontaneous nuclear import of Smad4 require importin 7 and 8 (Imp7,8). Our data suggest that in the nuclear import of Smad4, the role of Imp8 is irreplaceable by Imp7, and that Smads preferentially bind Imp8. Interestingly, in contrast to its mammalian counterpart Smad4, Drosophila Medea appears to utilize different mechanisms for TGF-beta-induced or basal state nuclear accumulation, with the latter independent of Msk (Drosophila Imp7/8) function. In addition, overexpression of Imp8 alone was sufficient to cause an increased concentration of Smad1, 3 and 4 in the nucleus, but had very limited effects on Smad2. These observations suggest selective involvement of Imp8/Msk in nuclear import of different Smads under different conditions.     

Nuclear phosphatidylinositol 3,4,5-trisphosphate, phosphatidylinositol 3-kinase, Akt, and PTen: emerging key regulators of anti-apoptotic signaling and carcinogenesis

Inositol lipid-derived second messengers have long been known to have an important regulatory role in cell physiology. Phosphatidylinositol 3-kinase (PI3K) synthesizes the second messenger 3,4,5'-phosphatidylinositol trisphosphate (Ptdlns 3,4,5P3) which controls a multitude of cell functions. Down-stream of PI3K/PtdIns 3,4,5P3 is the serine/threonine protein kinase Akt (protein kinase B, PKB). Since the PI3K/ PtdIns 3,4,5P3 /Akt pathway stimulates cell proliferation and suppresses apoptosis, it has been implicated in carcinogenesis. The lipid phosphatase PTEN is a negative regulator of this signaling network. Until recently, it was thought that this signal transduction cascade would promote its anti-apoptotic effects when activated in the cytoplasm. Several lines of evidence gathered over the past 20 years, have highlighted the existence of an autonomous nuclear inositol lipid cycle, strongly suggesting that lipids are important components of signaling pathways operating at the nuclear level. PI3K, PtdIns(3,4,5)P3, Akt, and PTEN have been identified within the nucleus and recent findings suggest that they are involved in cell survival also by operating in this organelle, through a block of caspase-activated DNase and inhibition of chromatin condensation. Here, we shall summarize the most updated and intriguing findings about nuclear PI3K/ PtdIns(3,4,5)P3/Akt/PTEN in relationship with carcinogenesis and suppression of apoptosis.     

Subcellular localization of Hairless protein shows a major focus of activity within the nucleus

Hairless, a major antagonist of the Notch signaling-pathway in Drosophila (Bang and Posakony, 1992; Maier et al., 1992), associates with Suppressor of Hairless [Su(H)], thereby inhibiting trans-activation of Notch target genes (Brou et al., 1994). These molecular interactions could occur either at the step of signal transduction in the cytoplasm or during implementation of the signal within the nucleus. We examined the subcellular distribution of Hairless, showing that it is a low abundant, ubiquitous protein that is cytosolic as well as nuclear. High levels of Hairless cause nuclear retention of Su(H), loss of Hairless reduces the amount of Su(H) in the nucleus.