Changes of nuclear membrane lipid composition affect RNA nucleocytoplasmic transport

The expanding organelle lipidomes: current knowledge and challenges

Lipids in cell membranes and subcellular compartments play essential roles in numerous cellular processes, such as energy production, cell signaling and inflammation. A specific organelle lipidome is characterized by lipid synthesis and metabolism, intracellular trafficking, and lipid homeostasis in the organelle. Over the years, considerable effort has been directed to the identification of the lipid fingerprints of cellular organelles. However, these fingerprints are not fully characterized due to the large variety and structural complexity of lipids and the great variability in the abundance of different lipid species. The process becomes even more challenging when considering that the lipidome differs in health and disease contexts. This review summarizes the information available on the lipid composition of mammalian cell organelles, particularly the lipidome of the nucleus, mitochondrion, endoplasmic reticulum, Golgi apparatus, plasma membrane and organelles in the endocytic pathway. The lipid compositions of extracellular vesicles and lamellar bodies are also described. In addition, several examples of subcellular lipidome dynamics under physiological and pathological conditions are presented. Finally, challenges in mapping organelle lipidomes are discussed.

Modulation of DNA Damage Response by Sphingolipid Signaling: An Interplay that Shapes Cell Fate

Although once considered as structural components of eukaryotic biological membranes, research in the past few decades hints at a major role of bioactive sphingolipids in mediating an array of physiological processes including cell survival, proliferation, inflammation, senescence, and death. A large body of evidence points to a fundamental role for the sphingolipid metabolic pathway in modulating the DNA damage response (DDR). The interplay between these two elements of cell signaling determines cell fate when cells are exposed to metabolic stress or ionizing radiation among other genotoxic agents. In this review, we aim to dissect the mediators of the DDR and how these interact with the different sphingolipid metabolites to mount various cellular responses.

Nuclear lipid microdomain as place of interaction between sphingomyelin and DNA during liver regeneration

Nuclear sphingomyelin is a key molecule for cell proliferation. This molecule is organized with cholesterol and proteins to form specific lipid microdomains bound to the inner nuclear membrane where RNA is synthesized. Here, we have reported the ability of the sphingomyelin present in the nuclear microdomain to bind DNA and regulate its synthesis, and to highlight its role in cell proliferation induced by partial hepatectomy. During G1/S transition of the cell cycle, sphingomyelin and DNA content is very high and it is strongly reduced after exogenous sphingomyelinase treatment. During the S-phase of the cell cycle, the stimulation of sphingomyelinase and inhibition of sphingomyelin-synthase are accompanied by the DNA synthesis start. To assess the specificity of the results, experiments were repeated with trifluoperazine, a drug known to affect the synthesis of lipids and DNA and to stimulate sphingomyelinase activity. The activity of sphingomyelinase is stimulated in the first hour after hepatectomy and sphingomyelin-DNA synthesis is strongly attenuated. It may be hypothesized that the nuclear microdomain represents a specific area of the inner nuclear membrane that acts as an active site of chromatin anchorage thanks to the stabilizing action of sphingomyelin. Thus, sphingomyelin metabolism in nuclear lipid microdomains is suggested to regulate cell proliferation.

The natural antioxidant rosmarinic acid spontaneously penetrates membranes to inhibit lipid peroxidation in situ

Exogenous molecules from dietary sources such as polyphenols are very efficient in preventing the alteration of lipid membranes by oxidative stress. Among the polyphenols, we have chosen to study rosmarinic acid (RA). We investigated the efficiency of RA in preventing lipid peroxidation and in interacting with lipids. We used liposomes of 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLPC) to show that RA was an efficient antioxidant. By HPLC, we determined that the maximum amount of RA associated with the lipids was ~1 mol%. Moreover, by using Langmuir monolayers, we evidenced that cholesterol decreases the penetration of RA. The investigation of transferred lipid/RA monolayers by atomic force microscopy revealed that 1 mol% of RA in the membrane was not sufficient to alter the membrane structure at the nanoscale. By fluorescence, we observed no significant modification of membrane permeability and fluidity caused by the interaction with RA. We also deduced that RA molecules were mainly located among the polar headgroups of the lipids. Finally, we prepared DLPC/RA vesicles to evidence for the first time that up to 1 mol% of RA inserts spontaneously in the membrane, which is high enough to fully prevent lipid peroxidation without any noticeable alteration of the membrane structure due to RA insertion.

Micro-Raman detection of nuclear membrane lipid fluctuations in senescent epithelial breast cancer cells

Originally identified in cultured cells, oncogenic cellular senescence is a growth-arrest mechanism which may inhibit tumor development by limiting the ability of cells to divide. However, literature shows that these cells secrete tumor-inducing and tumor-suppressing proteins leading to poor prognosis. Understanding the progression of oncogenic cellular senescence and associated mechanisms provides important implications for improving tumorigenesis therapeutic treatments. Micro-Raman spectroscopic imaging has grown in popularity as an imaging technique compared to the standard imaging predecessors and can be attributed to its numerous benefits such as no sample perturbation and the provision of direct chemical information. Through the use of label-free micro-Raman spectroscopy, control and senescent cells were noninvasively imaged. Resulting spectral images were processed using chemometric techniques, and average nuclei spectra from each sample set were compared. In turn, changes in the -cis and -trans unsaturated lipid isomer content were found to differ among proliferating and senescent cells. This may lead to increased nuclear fluidity and may contribute to the inability of senescent cells to complete the cell cycle. In the tumor environment, this detected increase in nuclear envelope fluidity could lead to downstream gene expression modifications and increased nucleo-cytoplasmic RNA translocation. Understanding nuclear envelope fluidity could provide insight into secretory profiles of senescent cells and their role in carcinogenesis, meriting further investigation into novel therapeutic technique development for oncogenic cellular senescence.

Are the olive oil and other dietary lipids related to cancer? Experimental evidence

There is a wealth of evidence supporting the relationship between dietary lipids and cancer, particularly those of the breast, colon and rectum and prostate. The main support comes from the international correlational studies and, especially, from experimental ones. The evidence from human analytical studies is less consistent because of several conflicting findings, probably due to methodological issues. Experimentally, it has been clearly demonstrated that quantity and type of dietary lipids as well as the particular critical phases of the carcinogenesis in which they act, are the essential factors in this relationship. Thus, whereas high dietary intake of n-6 polyunsaturated fatty acids (PUFA), primarily LA, and saturated fat has tumor-enhancing effects, long chain n-3 PUFA, CLA and GLA have inhibitory effects. Monounsaturated fatty acids (MUFA), mainly OA, present in high quantities in olive oil, seem to be protective although some inconsistent results have been reported. Bioactive compounds of virgin olive oil may also account for the protective effect of this oil, which is the main source of fat in the Mediterranean diet. Experimental studies have also provided evidence of several putative mechanisms of action of dietary lipids on cancer. Lipids can influence the hormonal status, modify cell membranes structure and function, cell signalling transduction pathways and gene expression, and modulate the function of the immune system. Although further studies are needed to evaluate and verify these mechanisms in humans, based on the multiple ways dietary lipids can act, they may have an important influence on tumorigenesis.

Cholinergic and glutamatergic alterations beginning at the early stages of Alzheimer disease: participation of the phospholipase A2 enzyme

RATIONALE

Alzheimer disease (AD), a progressive neurodegenerative disorder, is the leading cause of dementia in the elderly. A combination of cholinergic and glutamatergic dysfunction appears to underlie the symptomatology of AD, and thus, treatment strategies should address impairments in both systems. Evidence suggests the involvement of phospholipase A(2) (PLA(2)) enzyme in memory impairment and neurodegeneration in AD via actions on both cholinergic and glutamatergic systems.

OBJECTIVES

To review cholinergic and glutamatergic alterations underlying cognitive impairment and neuropathology in AD and attempt to link PLA(2) with such alterations.

METHODS

Medline databases were searched (no date restrictions) for published articles with links among the terms Alzheimer disease (mild, moderate, severe), mild cognitive impairment, choline acetyltransferase, acetylcholinesterase, NGF, NGF receptor, muscarinic receptor, nicotinic receptor, NMDA, AMPA, metabotropic glutamate receptor, atrophy, glucose metabolism, phospholipid metabolism, sphingolipid, membrane fluidity, phospholipase A(2), arachidonic acid, attention, memory, long-term potentiation, beta-amyloid, tau, inflammation, and reactive species. Reference lists of the identified articles were checked to identify additional studies of interest.

RESULTS

Overall, results suggest the hypothesis that persistent inhibition of cPLA(2) and iPLA(2) isoforms at early stages of AD may play a central role in memory deficits and beta-amyloid production through down-regulation of cholinergic and glutamate receptors. As the disease progresses, beta-amyloid induced up-regulation of cPLA(2) and sPLA(2) isoforms may play critical roles in inflammation and oxidative stress, thus participating in the neurodegenerative process.

CONCLUSION

Activation and inhibition of specific PLA(2) isoforms at different stages of AD could be of therapeutic importance and delay cognitive dysfunction and neurodegeneration.

Phosphatidylcholine/sphingomyelin metabolism crosstalk inside the nucleus

It is known that phospholipids represent a minor component of chromatin. It has been highlighted recently that these lipids are metabolized directly inside the nucleus, thanks to the presence of enzymes related to their metabolism, such as neutral sphingomyelinase, sphingomyelin synthase, reverse sphingomyelin synthase and phosphatidylcholine-specific phospholipase C. The chromatin enzymatic activities change during cell proliferation, differentiation and/or apoptosis, independently from the enzyme activities present in nuclear membrane, microsomes or cell membranes. This present study aimed to investigate crosstalk in lipid metabolism in nuclear membrane and chromatin isolated from rat liver in vitro and in vivo. The effect of neutral sphingomyelinase activity on phosphatidylcholine-specific phospholipase C and sphingomyelin synthase, which enrich the intranuclear diacylglycerol pool, and the effect of phosphatidylcholine-specific phospholipase C activity on neutral sphingomyelinase and reverse sphingomyelin synthase, which enrich the intranuclear ceramide pool, was investigated. The results show that in chromatin, there exists a phosphatidylcholine/sphingomyelin metabolism crosstalk which regulates the intranuclear ceramide/diacylglycerol pool. The enzyme activities were inhibited by D609, which demonstrated the specificity of this crosstalk. Chromatin lipid metabolism is activated in vivo during cell proliferation, indicating that it could play a role in cell function. The possible mechanism of crosstalk is discussed here, with consideration to recent advances in the field.

Molecular mechanisms of the effects of olive oil and other dietary lipids on cancer

Cancer is one of the main causes of mortality worldwide. Geographical differences in incidence rates suggest a key effect of environmental factors, especially diet, in its aetiology. Epidemiologic and experimental studies have found a role of dietary lipids in cancer, particularly breast, colorectal, and prostate cancers. Their incidence in the Mediterranean countries, where the main source of fat is olive oil, is lower than in other areas of the world. Human studies about the effects of dietary lipids are little conclusive, probably due to methodological issues. On the other hand, experimental data have clearly demonstrated that the influence of dietary fats on cancer depends on the quantity and the type of lipids. Whereas a high intake of n-6 PUFA and saturated fat has tumor-enhancing effects, n-3 PUFA, conjugated linoleic acid and gamma-linolenic acid have inhibitory effects. Data regarding MUFA have not always been conclusive, but high olive oil diets seem to have protective effects. Such effects can be due to oleic acid, the main MUFA in olive oil, and to certain minor compounds such as squalene and phenolic compounds. This work aims to review the current knowledge about the relationship between dietary lipids and cancer, with a special emphasis on olive oil, and the molecular mechanisms underlying these effects: modifications on the carcinogenesis stages, hormonal status, cell membrane structure and function, signal transduction pathways, gene expression, and immune system.

Relationship between thallium(I)-mediated plasma membrane fluidification and cell oxidants production in Jurkat T cells

The effects of thallous cation (Tl(+)) on: (a) the production of oxidant species and (b) membrane fluidity were evaluated in human leukemia T cells (Jurkat). After 72 h of incubation in the presence of Tl(+) (5-100 microM), no significant changes in cell viability were observed, although the average cell size was decreased as evaluated by steady-state light scattering. Tl(+) (5-100 microM) caused a significant increase in the concentration of cellular oxidants as measured with the probe 5(6)-carboxy-2',7'-dichlorodihydrofluorescein diacetate (DCDCDHF). Similarly, a higher lipid oxidation products release was observed as measured by TBARS production. Both Tl(+)-mediated DCDCDHF oxidation and TBARS production were prevented when cells were supplemented with 2mM Trolox. Tl(+) (5-100 microM) also induced a concentration-dependent increase in plasma membrane fluidity, evaluated with the probe 6-(9-anthroyloxy)stearic acid (6-AS). This effect of Tl(+) was neither associated to the externalization of phosphatidylserine, nor observed in Trolox-supplemented cells. Significant correlations were found between the increase in plasma membrane fluidity and TBARS production and DCDCDHF oxidation. Together, the present results suggest that the increase in cellular oxidants caused by Tl(+) could oxidize membrane fatty acids, resulting in an increase in membrane fluidity. These effects could underlie the pathology associated with Tl(+) toxicity.

Antioxidant and membrane effects of procyanidin dimers and trimers isolated from peanut and cocoa

The antioxidant and membrane effects of dimer (Dim) and trimer (Trim) procyanidins isolated from cocoa (Theobroma cacao) (B- and C-bonded) and peanut (Arachis hypogea L.) skin (A-bonded) were evaluated in phosphatidyl choline liposomes. When liposomes were oxidized with a steady source of oxidants, the above dimers and trimers inhibited to a similar extent lipid oxidation in a concentration (0.33-5 microM)-dependent manner. With respect to membrane effects, Dim A1, Dim B, Trim A, and Trim C increased (Dim A1 = Dim B and Trim A = Trim C), while Dim A2 decreased, membrane surface potential. All of the procyanidins tested decreased membrane fluidity as determined by fluorescent probes at the water-lipid interface, an effect that extended into the hydrophobic region of the bilayer. Both dimers and trimers protected the lipid bilayer from disruption by Triton X-100. The magnitude of the protection was Dim A1 > Dim A2 > Dim B and Trim C > Trim A. Thus, dimers and trimers can interact with membrane phospholipids, presumably with their polar headgroup. As a consequence of this interaction, they can provide protection against the attack of oxidants and other molecules that challenge the integrity of the bilayer.

Relevance of lipid polar headgroups on boron-mediated changes in membrane physical properties

Using liposomes composed of either brain phosphatidylcholine (PC), or binary mixtures of PC and phosphatidylserine (PS), galactolipids (GL), phosphatidylinositol (PI), cardiolipin (CL), phosphatidic acid (PA), or phosphatidylethanolamine (PE), we investigated the effects of graded amounts of boric acid (B, 0.5-1000 microM) on the following membrane physical properties: (a) surface potential, (b) lipid rearrangement through lateral phase separation, (c) fluidity, and (d) hydration. Incubation of the different populations of vesicles with B was associated with a small, but statistically significant, increase in membrane surface potential in PC, PC:PS, PC:GL, PC:PI, PC:PA, and PC:PE liposomes. B-induced lipid lateral rearrangement through lateral phase separation in PC, PC:PA, and PC:PE liposomes; but had no effects on PC:PS, PC:GL, and PC:PI liposomes. In PC liposomes B affected membrane fluidity at the water-lipid interface without affecting the hydrophobic core of the bilayer. In all the other binary liposomes studied, B increased membrane fluidity in both, the hydrophobic portion of the membrane and in the anionic domains. The above was associated with a decrease in the fluidity of the cationic domains. B (10-1000 microM) decreased membrane hydration regardless the composition of the liposomes. The obtained results demonstrate the ability of B to interact with membranes, and induce changes in membrane physical properties. Importantly, the extent of B-membrane interactions and the consequent effects were dependent on the nature of the lipid molecule; as such, B had greater affinity with lipids containing polyhydroxylated moieties such as GL and PI. These differential interactions may result in different B-induced modulations of membrane-associated processes in cells.

Influence of zinc deficiency on cell-membrane fluidity in Jurkat, 3T3 and IMR-32 cells

We investigated whether zinc deficiency can affect plasma membrane rheology. Three cell lines, human leukaemia T-cells (Jurkat), rat fibroblasts (3T3) and human neuroblastoma cells (IMR-32), were cultured for 48 h in control medium, in zinc-deficient medium (1.5 microM zinc; 1.5 Zn), or in the zinc-deficient medium supplemented with 15 microM zinc (15 Zn). The number of viable cells was lower in the 1.5 Zn group than in the control and 15 Zn groups. The frequency of apoptosis was higher in the 1.5 Zn group than in the control and 15 Zn groups. Membrane fluidity was evaluated using the 6-(9-anthroyloxy)stearic acid and 16-(9-anthroyloxy)palmitic acid probes. Membrane fluidity was higher in 1.5 Zn cells than in the control cells; no differences were observed between control cells and 15 Zn cells. The effect of zinc deficiency on membrane fluidity at the water/lipid interface was associated with a higher phosphatidylserine externalization. The higher membrane fluidity in the hydrophobic region of the bilayer was correlated with a lower content of arachidonic acid. We suggest that the increased fluidity of the membrane secondary to zinc deficiency is in part due to a decrease in arachidonic acid content and the apoptosis-related changes in phosphatidylserine distribution.

Protective effect of taurine on hypochlorous acid toxicity to nuclear nucleoside triphosphatase in isolated nuclei from rat liver

AIM: Taurine has been shown to be an effective scavenger of hypochlorous acid (HOCl). The role of HOCl is well established in tissue damage associated with inflammation and injury. In the present study, the effect of HOCl on nuclear nucleoside triphosphatase of hepatocytes and the ability of taurine to prevent this effect were investigated.

METHODS: Isolated hepatic nuclei from rat liver were exposed to HOCl with or without taurine. The NTPase activity on nuclear envelope was assayed using ATP and GTP as substrates, respectively.

RESULTS: The first series of experiments evaluated the toxicity of HOCl and the efficacy of taurine to protect NTPase. HOCl at 10^-9-5 × 10^-6 mol/L reduced nuclear NTPase activities in a concentration dependent manner (ATP and GTP as substrates) (P < 0.01). HOCl at 10^-6 mol/L reduced the NTPase activity by 65% (ATP as substrate) and 76% (GTP as substrate). Taurine (10^-7 to 10^-4 mol/L) was tested for protection against HOCl at 10^-6 mol/L and the nuclei treated with 5 × 10^{- 4} mol/L taurine exhibited only 20% and 12% reduction in NTPase activities compared to untreated controls. A second study was performed comparing taurine to glutathione (GSH). GSH and HOCl at 10^-6 mol/L exhibited 46% and 67.4% reduction in NTPase activities compared with control. GSH (10^-4 mol/L) which was incubated with the nuclei and HOCl still exhibited 44.2% and 44.8% reduction in NTPase activities of untreated control. Taurine with HOCl only exhibited 15.2% and 17.1% reduction in NTPase activities, which provided more powerful protection against HOCl than GSH. The third experiment was undertaken to evaluate the specificity of taurine against HOCl. Incubation of rat hepatic nuclei with Fe³⁺/H₂O₂ (1 mmol/L vs 5 µmol/L) resulted in a decrease in nuclear NTPase activities (P < 0.01). When hepatic nuclei were incubated with Tau (10^-4 mol/L) and Fe³⁺/H₂O₂ (1mmol/L vs 5 µmol/L), nuclear NTPase activities were only slightly increased as compared with that of incubation with Fe³⁺/H₂O₂ alone. However, GSH failed to alter the NTPase activities induced by Fe³⁺/H₂O₂.

CONCLUSION: The present findings indicate that HOCl can act as an inhibitor of nuclear NTPase. Taurine can antagonistically reduce the toxicity of HOCl to NTPase.

Effects of thallium(I) and thallium(III) on liposome membrane physical properties

The hypothesis that thallium (Tl) interaction with membrane phospholipids could result in the alteration of membrane physical properties was investigated. Working with liposomes composed of brain phosphatidylcholine and phosphatidylserine, we found that Tl(+), Tl(3+), and Tl(OH)(3) (0.5-25 microM): (a) increased membrane surface potential, (b) decreased the fluidity of the anionic regions of the membrane, in association with an increased fluidity in the cationic regions, and (c) promoted the rearrangement of lipids through lateral phase separation. The magnitude of these effects followed the order Tl(3+), Tl(OH)(3)>Tl(+). In addition, Tl(3+) also decreased the hydration of phospholipid polar headgroups and induced membrane permeabilization. The present results show that Tl interacts with membranes inducing major alterations in the rheology of the bilayer, which could be partially responsible for the neurotoxic effects of this metal.

Phospholipase A2 inhibits nuclear nucleoside triphosphatase activity and mRNA export in isolated nuclei from rat liver

The present study is undertaken to investigate whether the phospholipase A(2) (PLA(2)) influences mRNA nucleocytoplasmic transport evaluated by nucleoside triphosphatase (NTPase) activity and mRNA export in isolated hepatic nuclear envelope. Isolated hepatic nuclei from rat liver were exposed to PLA(2) (10(-5) approximately 10(-2)/ml) with or without incorporation of nuclei with phosphatidylcholine (PC) liposome. Messenger RNA exports and NTPase activities of nuclear membrane were assayed using ATP and GTP as substrates. We found that the RNA efflux, evaluated by [3H] uridine, was potently decreased in a concentration-dependent manner, by incubation of hepatic nuclei with PLA(2), regardless using ATP or GTP as substrates. The PC content in nuclear membrane was also decreased by PLA(2)-treatment. The PC was incorporated into the nuclear membrane by addition of phospholipid liposomes into the incubation mixture. PC incorporation into the nuclear membrane did not alter mRNA export. However this resulted in a significant increase in mRNA export rate in PLA(2)-treated group. Messenger RNA export rate in PLA(2) (10(-3) unit/mL)- treated nuclear membrane was positively correlated with level of PC incorporation, both using ATP and GTP as substrates. The activity of nucleoside triphosphatase, a nuclear membrane-associated enzyme, showed parallel variations with mRNA transport. It is concluded that nuclear PLA(2) plays a regulatory role in RNA transport, which can be antagonized by exogenous PC. These might be pathophysiologically significance, although the mechanisms by which this effect takes place remain to be clarified.

Al(3+)-mediated changes in membrane physical properties participate in the inhibition of polyphosphoinositide hydrolysis

We investigated the possible involvement of Al(3+)-induced alterations in membrane physical properties in Al(3+)-mediated inhibition of polyphosphoinositide (PPI) hydrolysis by the enzyme phosphatidylinositol-specific phospholipase C (PI-PLC). Liposomes composed of brain phosphatidylcholine (PC) or of PC and a mixture of brain PPI (PC:PPI) were incubated in the presence of Al(3+) (1-100 microM). We evaluated: (1) the amount of membrane-bound Al(3+), (2) the effects of Al(3+) on key membrane physical properties (surface potential, lipid fluidity, and lipid arrangement), and (3) the hydrolysis of PPI. Al(3+) binding to PC:PPI (60:40 mol/mol) liposomes was 1.3 times higher than to PC:PPI (90:10 mol/mol) liposomes and did not change after treatment with Triton X-100. Al(3+) increased membrane surface potential, promoted the loss of membrane fluidity, and caused lateral phase separation in PC:PPI liposomes. Phosphatidylinositol and phosphatidylinositol monophosphate hydrolysis in the presence of PI-PLC was not affected by Al(3+), but a significant and concentration-dependent inhibition of PIP(2) hydrolysis was observed, an effect that was prevented by previous bilayer disruption with Triton X-100. The obtained results support the hypothesis that Al(3+) binding to liposomes promotes the formation of rigid clusters enriched in PPI, restricting the accessibility of the enzyme to the substrate and subsequently inhibiting PIP(2) hydrolysis by PI-PLC.

Aluminum affects membrane physical properties in human neuroblastoma (IMR-32) cells both before and after differentiation

The capacity of Al(3+) to induce changes in the physical properties of plasma membrane from human neuroblastoma cells (IMR-32) was investigated, and the magnitude of the changes was compared with that obtained after cell differentiation to a neuronal phenotype. Similarly to our previous results in liposomes, Al(3+) (10 to 100 microM) caused a significant loss of membrane fluidity, being the differentiated cells more affected than the nondifferentiated cells. Al(3+) also increased the relative content of lipids in gel phase and promoted lipid rearrangement through lateral phase separation, with the magnitude of this effect being similar in nondifferentiated and differentiated cells. Since membrane physical properties depend on bilayer composition, we characterized the content of proteins, phospholipids, cholesterol, and fatty acids in the IMR-32 cells before and after differentiation. Differentiated cells had a significantly higher content of unsaturated fatty acids, creating an environment that favors Al(3+)-mediated effects on the bilayer fluidity. The neurotoxic effects of Al(3+) may be, at least in part, due to alterations of neuronal membrane physical properties, with potential consequences on the normal functioning of membrane-related cellular processes.

The presence and the role of chromatin cholesterol in rat liver regeneration

BACKGROUND/AIMS

It has been shown that cholesterol is necessary in early G1 phase during cell duplication. In the present research we have studied the presence of cholesterol in the hepatocyte chromatin lipid fraction and its behaviour in liver regeneration.

METHODS

Hepatocyte nuclei and chromatin were isolated from normal and regenerating rat liver. The lipid fraction was extracted and analysed by chromatography. The activity of sphingomyelin-synthase in the chromatin was evaluated using labelled phosphatidylcholine.

RESULTS

In the chromatin, the amount of cholesterol is similar to that of sphingomyelin, and it increases in chromatin digested with exogenous sphingomyelinase or proteinase K. It may be concluded that a complex, formed by cholesterol, sphingomyelin and proteins, is present in the chromatin. The particular affinity between sphingomyelin and cholesterol in chromatin with respect the nuclear membrane may be tentatively explained as due to the enrichment in saturated fatty-acids of the chromatin sphingomyelin. Moreover the cholesterol inhibits the chromatin sphingomyelin-synthase activity. During liver regeneration, an increase in chromatin cholesterol is observed between 6 and 18 h after hepatectomy, when the neutral-sphingomyelinase activity increases and the sphingomyelin-synthase is inhibited.

CONCLUSIONS

The cholesterol is present in the chromatin and its amount changes in relation to cell proliferation in regenerating liver.

Nuclear membrane sphingomyelin-cholesterol changes in rat liver after hepatectomy

Sphingomyelin and cholesterol play an important role in stabilising the plasma membranes architecture and in many physiological process such as cell growth and differentiation. Degradation of sphingomyelin by exogenous sphingomyelinase induces a decrease of cholesterol due either to an increase of esterification or to a reduced biosynthesis. Variations of sphingomyelin due to the presence of a neutral-sphingomyelinase and of sphingomyelin-synthase have been recently shown in rat liver nuclear membranes. The aim of this research is to study the relation between sphingomyelin and cholesterol in the nuclear membranes following sphingomyelinase activation and during cell proliferation. The nuclear membranes, isolated from liver nuclei, were analysed for their content in protein, nucleic acids, and lipids (sphingomyelin and cholesterol) before and after sphingomyelinase activation and during hepatic regeneration. The activities of nuclear membrane SM-syntase and sphingomyelinase were also determined. The results confirmed that also in the nuclear membranes sphingomyelinase, especially exogenous, causes a strong decrease in cholesterol. The increase observed of sphingomyelin during the first 18 h after hepatectomy followed by a decrease at 24 h, due to the different activity of the enzymes, is accompanied by similar behaviour of cholesterol. This confirms the effect of neutral-sphingomyelinase on cholesterol, due to an increase of esterification process. Changes in cholesterol content modify the nuclear membranes fluidity and, as consequence, mRNA transport as previously shown. It can therefore be concluded that the neutral sphingomyelinase, present in the nuclei, may, across this mechanism, regulate the cell function.