Interaction of phosphatidylinositol-4-monophosphate with a low activity form of DNA polymerase alpha: a potential mechanism for enzyme activation

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Phosphatidylinositol 4-kinases: hostages harnessed to build panviral replication platforms

Several RNA viruses have recently been shown to hijack members of the host phosphatidylinositol (PtdIns) 4-kinase (PI4K) family of enzymes. They use PI4K to generate membranes enriched in phosphatidylinositide 4-phosphate (PtdIns4P or PI4P) lipids, which can be used as replication platforms. Viral replication machinery is assembled on these platforms as a supramolecular complex and PtdIns4P lipids regulate viral RNA synthesis. This article highlights these recent studies on the regulation of viral RNA synthesis by PtdIns4P lipids. It explores the potential mechanisms by which PtdIns4P lipids can contribute to viral replication and discusses the therapeutic potential of developing antiviral molecules that target host PI4Ks as a form of panviral therapy.

Nuclear phosphoinositides: location, regulation and function

Lipid signalling in human disease is an important field of investigation and stems from the fact that phosphoinositide signalling has been implicated in the control of nearly all the important cellular pathways including metabolism, cell cycle control, membrane trafficking, apoptosis and neuronal conduction. A distinct nuclear inositide signalling metabolism has been identified, thus defining a new role for inositides in the nucleus, which are now considered essential co-factors for several nuclear processes, including DNA repair, transcription regulation, and RNA dynamics. Deregulation of phoshoinositide metabolism within the nuclear compartment may contribute to disease progression in several disorders, such as chronic inflammation, cancer, metabolic, and degenerative syndromes. In order to utilize these very druggable pathways for human benefit there is a need to identify how nuclear inositides are regulated specifically within this compartment and what downstream nuclear effectors process and integrate inositide signalling cascades in order to specifically control nuclear function. Here we describe some of the facets of nuclear inositide metabolism with a focus on their relationship to cell cycle control and differentiation.

Nuclear and chromatin lipids: metabolism in normal and gamma-irradiated rats

The data on nuclear and chromatin lipid metabolism are reviewed. The amount of neutral lipids and phospholipids in nuclei of rat thymus, liver and neocortex neuron as well as the amount of lipids in rat thymus and liver chromatin are described. The metabolic responses of nuclear and chromatin lipids from thymus to different doses and dose rates of gamma-irradiation of rats are discussed. In most cases, the nuclear and chromatin lipid responses are distinct. Changes in nuclear and chromatin lipid metabolism in response to gamma-irradiation are suggested to connect with the signal transduction pathway and the regulation of the transcriptional and replicative chromatin activity. The influence of beta-carotene and picrotoxin on rat liver nuclear lipids and neocortex neuronal nuclear lipids, respectively, was analyzed. The possible involvement of the lipid traffic in the chromatin lipid responses to gamma-irradiation and other agents is suggested.

Phospholipid signalling in the nucleus. Een DAG uit het leven van de inositide signalering in de nucleus

Diverse methodologies, ranging from activity measurements in various nuclear subfractions to electron microscopy, have been used to demonstrate and establish that many of the key lipids and enzymes responsible for the metabolism of inositol lipids are resident in nuclei. PtdIns(4)P, PtdIns(4,5)P2 and PtdOH are all present in nuclei, as well as the corresponding enzyme activities required to synthesise and metabolise these compounds. In addition other non-inositol containing phospholipids such as phosphatidylcholine constitute a significant percentage of the total nuclear phospholipid content. We feel that it is pertinent to include this lipid in our discussion as it provides an alternative source of 1, 2-diacylglycerol (DAG) in addition to the hydrolysis of PtdIns(4, 5)P2. We discuss at length data related to the sources and possible consequences of nuclear DAG production as this lipid appears to be increasingly central to a number of general physiological functions. Data relating to the existence of alternative pathways of inositol phospholipid synthesis, the role of 3-phosphorylated inositol lipids and lipid compartmentalisation and transport are reviewed. The field has also expanded to a point where we can now also begin to address what role these lipids play in cellular proliferation and differentiation and hopefully provide avenues for further research.

Effects of aging and dietary restriction on DNA polymerases: gene expression, enzyme fidelity, and DNA excision repair

Hepatic DNA polymerases isolated from young and old C57BL/6N mice fed ad libitum or calorically restricted differed in chromatographic characteristics, binding affinity for DNA template-primer, specific activity, and fidelity of synthesis. DNA polymerase alpha total and specific activity declined slightly, while the nucleotide misincorporation frequency increased dramatically, with increased age of the donor animals. A positive correlation was observed between polymerase alpha specific activity and the affinity of enzyme binding to activated DNA template-primer. Both the age-associated decline in enzyme activity and the decrease in fidelity of synthesis were modified by dietary restriction, with higher specific activity levels and lower misincorporation frequencies for DNA polymerases from dietarily restricted animals compared with ad libitum animals of all ages. Fidelity of both DNA polymerase alpha and beta increased following treatment with the phosphoinositide hydrolysis product inositol-1,4-bisphosphate. The data suggest that dietary restriction could play an important role in decreasing the age-associated decline in function of physiological systems sensitive to decreased or defective DNA synthesis.

Phospholipids in the nucleus--metabolism and possible functions

Most of the phospholipids in the nuclear envelope are contained in the double nuclear membrane, and this has an active lipid metabolism consistent with its origins as a component of the endoplasmic reticular system. However, even after removal of the nuclear membrane with detergents, some phospholipids, mostly of unknown location and function, remain. Amongst these are all of the components of what appears to be a nuclear polyphosphoinositide signalling system, distinct from the well-established inositide pathway found in the plasma membrane. The consequences for nuclear function of the activation of these two inositide pathways are discussed, with a detailed consideration of proposed intranuclear functions for protein kinase C, and the maintenance of nuclear Ca2+ homoeostasis.

Effect of dietary restriction on the fidelity of DNA polymerases in aging mice

DNA polymerases purified from hepatic tissues of C57BL/6 mice showed an age-related decrease in both specific activity and fidelity of the various enzyme forms. Polymerases from dietary restricted mice exhibited less of a decline in specific activity and copied synthetic DNA templates with relatively higher fidelity than did enzymes from animals fed ad libitum. Polymerases treated with inositol-1,4-bisphosphate [I(1,4)P2] showed varying levels of increased activity, with fidelity increases up to 3-fold. These data indicate that aging is associated with decreases in both specific activity and fidelity of DNA polymerases isolated from a nondividing tissue, and that dietary restriction impedes the age-related decline in both specific activity and fidelity of these polymerases. The data further indicate that DNA polymerases may interact with phosphoinositide hydrolysis products resulting in increased specific activity and fidelity of the enzymes. Phosphoinositide interactions with polymerases could constitute an important mechanism moderating the age-related decrease in function and accuracy of DNA polymerases.

Effects of aging and dietary restriction on DNA polymerase expression in mice

DNA polymerase alpha was isolated from livers of 6-month-, 16-month-, or 26-month-old mice fed ad libitum, or calorically restricted. The enzymes differed in chromatographic characteristics, binding affinity for DNA, and activity, with both total activity and specific activity of DNA polymerase alpha decreasing as a function of age. A positive correlation was observed between polymerase alpha specific activity and the affinity of enzyme binding to activated DNA template-primer. The age-associated decline in enzyme activity was modified by dietary restriction, with measurably higher activity seen for polymerases from dietary restricted animals compared with ad libitum animals of all ages. The data suggest that dietary restriction could act to delay the age-associated decrease in cellular capacity for DNA synthesis, which may play a significant role in prolonging the onset of age-related diseases in which decreased DNA synthesis is a potential component.

Age-related changes in DNA polymerase alpha expression

DNA polymerase alpha isozymes differing in specific activity and affinity of binding to DNA were purified from human fibroblasts derived from donors of different ages. Fetal-derived fibroblasts expressed a single, high-activity enzyme (A2), with high affinity of binding to DNA. Adult-derived fibroblasts exhibited two forms of DNA polymerase alpha, one identical to the fetal enzyme, and a second with about tenfold less activity showing low affinity of binding to DNA (A1). The ratio of DNA polymerase A2/A1 decreased dramatically with age, from 100% A2 in fetal-derived fibroblasts to about 94% A1 in fibroblasts derived from a 66-year-old donor. The DNA binding affinity of polymerase alpha A1 from adult-derived fibroblasts increased concomitant with a significant increase in activity when the enzyme was treated with phosphatidylinositol-4-monophosphate (PIP), or with inositol-1, 4-bisphosphate (I(1,4)P2). The enzyme reverted back to a less active form, with loss of the noncovalently bound I(1,4)P2, as a function of time. When permeabilized human fibroblasts with low DNA excision repair capacity were treated with 7,8-dihydrodiol-9,10-epoxybenzo(a)-pyrene (BPDE) in the presence of 32P-ATP, phosphatidylinositol, and cycloheximide, excision repair was initiated and 32P-labeled DNA polymerase alpha was recovered in the absence of de novo protein synthesis. DNA synthesis associated with either scheduled DNA synthesis or BPDE-initiated excision repair declined as a function of increased age in human cells. The data suggest that the decline in both DNA excision repair-associated and mitogen-activated DNA synthesis may be correlated with decreased total intracellular levels of DNA polymerase and with the decline in polymerase alpha activity as a function of age, that DNA repair-associated initiation of DNA synthesis in adult-derived cells may increase with activation of a pool of low activity DNA polymerase alpha, and that DNA polymerase alpha activity increases as a function of enzyme interaction with a component of the PI phosphorylation cascade.