Phosphatidylcholine signaling in response to CSF-1

Accelerated phosphatidylcholine turnover in macrophages promotes adipose tissue inflammation in obesity

White adipose tissue (WAT) inflammation contributes to the development of insulin resistance in obesity. While the role of adipose tissue macrophage (ATM) pro-inflammatory signalling in the development of insulin resistance has been established, it is less clear how WAT inflammation is initiated. Here, we show that ATMs isolated from obese mice and humans exhibit markers of increased rate of de novo phosphatidylcholine (PC) biosynthesis. Macrophage-specific knockout of phosphocholine cytidylyltransferase A (CCTα), the rate-limiting enzyme of de novo PC biosynthesis pathway, alleviated obesity-induced WAT inflammation and insulin resistance. Mechanistically, CCTα-deficient macrophages showed reduced ER stress and inflammation in response to palmitate. Surprisingly, this was not due to lower exogenous palmitate incorporation into cellular PCs. Instead, CCTα-null macrophages had lower membrane PC turnover, leading to elevated membrane polyunsaturated fatty acid levels that negated the pro-inflammatory effects of palmitate. Our results reveal a causal link between obesity-associated increase in de novo PC synthesis, accelerated PC turnover and pro-inflammatory activation of ATMs.

Disabled-2 is a negative immune regulator of lipopolysaccharide-stimulated Toll-like receptor 4 internalization and signaling

Toll-like receptor 4 (TLR4) plays a pivotal role in the host response to lipopolysaccharide (LPS), a major cell wall component of Gram-negative bacteria. Here, we elucidated whether the endocytic adaptor protein Disabled-2 (Dab2), which is abundantly expressed in macrophages, plays a role in LPS-stimulated TLR4 signaling and trafficking. Molecular analysis and transcriptome profiling of RAW264.7 macrophage-like cells expressing short-hairpin RNA of Dab2 revealed that Dab2 regulated the TLR4/TRIF pathway upon LPS stimulation. Knockdown of Dab2 augmented TRIF-dependent interferon regulatory factor 3 activation and the expression of subsets of inflammatory cytokines and interferon-inducible genes. Dab2 acted as a clathrin sponge and sequestered clathrin from TLR4 in the resting stage of macrophages. Upon LPS stimulation, clathrin was released from Dab2 to facilitate endocytosis of TLR4 for triggering the TRIF-mediated pathway. Dab2 functions as a negative immune regulator of TLR4 endocytosis and signaling, supporting a novel role for a Dab2-associated regulatory circuit in controlling the inflammatory response of macrophages to endotoxin.

Dynamic expression of Dab2 in the mouse embryonic central nervous system

BACKGROUND

Dab2, one of two mammalian orthologs of Drosophila Disabled, has been shown to be involved in cell positioning and formation of visceral endoderm during mouse embryogenesis, but its role in neuronal development is not yet fully understood. In this report, we have examined the localization of the Dab2 protein in the mouse embryonic central nervous system (CNS) at different developmental stages.

RESULTS

Dab2 protein was transiently expressed in rhombomeres 5 and 6 of the developing hindbrain between E8.5 and E11.5, and in the floor plate of the neural tube from E9.5 to E12.5, following which it was no longer detectable within these regions. Dab2 protein was also identified within circumventricular organs including the choroid plexus, subcommissural organ and pineal gland during their early development. While Dab2 was still strongly expressed in the adult choroid plexus, immunoreactivity within the subcommissural organ and pineal gland was lost after birth. In addition, Dab2 was transiently expressed within a subpopulation of Iba1-positive mononuclear phagocytes (including presumed microglial progenitors) within the neural tube from E10.0 and was lost by E14.5. Dab2 was separately localized to Iba1 positive cells from E9.5 and subsequently to F4/80 positive cells (mature macrophage/myeloid-derived dendritic cells) positioned outside the neural tube from E12.5 onwards, implicating Dab2 expression in early cells of the mononuclear phagocyte lineage. Dab2 did not co-localize with the pan-neuronal marker PGP9.5 at any developmental stage, suggesting that Dab2 positive cells in the developing CNS are unlikely to be differentiating neurons.

CONCLUSION

This is the first study to demonstrate the dynamic spatiotemporal expression of Dab2 protein within the CNS during development.

Mechanisms of indomethacin-induced alterations in the choline phospholipid metabolism of breast cancer cells

Human mammary epithelial cells (HMECs) exhibit an increase in phosphocholine (PC) and total choline-containing compounds, as well as a switch from high glycerophosphocholine (GPC)/low PC to low GPC/high PC, with progression to malignant phenotype. The treatment of human breast cancer cells with a nonsteroidal anti-inflammatory agent, indomethacin, reverted the high PC/low GPC pattern to a low PC/high GPC pattern indicative of a less malignant phenotype, supported by decreased invasion. Here, we have characterized mechanisms underlying indomethacin-induced alterations in choline membrane metabolism in malignant breast cancer cells and nonmalignant HMECs labeled with [1,2-13C]choline using 1H and 13C magnetic resonance spectroscopy. Microarray gene expression analysis was performed to understand the molecular mechanisms underlying these changes. In breast cancer cells, indomethacin treatment activated phospholipases that, combined with an increased choline phospholipid biosynthesis, led to increased GPC and decreased PC levels. However, in nonmalignant HMECs, activation of the anabolic pathway alone was detected following indomethacin treatment. Following indomethacin treatment in breast cancer cells, several candidate genes, such as interleukin 8, NGFB, CSF2, RHOB, EDN1, and JUNB, were differentially expressed, which may have contributed to changes in choline metabolism through secondary effects or signaling cascades leading to changes in enzyme activity.

Phospholipids are synthesized in the G2/M phase of the cell cycle

Very little is known about the metabolism of phospholipids in the G2 and M phases of the cell cycle, but limited studies have led to the postulation that phospholipid synthesis ceases during this period. To investigate whether phospholipids are synthesized in the G2/M phase of the cell cycle, protocols were developed to produce synchronized MCF-7 cell populations with greater than 80% of the cells in G1/S or G2/M phases that moved in synchrony following removal of the blocking agent. Analysis of the activities of key phosphatidylcholine and phosphatidylethanolamine biosynthetic enzymes in subcellular fractions obtained from MCF-7 cells at different cell cycle phases revealed that there was robust activity of key enzymes in the fractions prepared from MCF-7 cells in G2/M phase. Radiolabeled choline and ethanolamine were rapidly incorporated into cells maintained at G2/M phase with nocodazole, and the rates of incorporation were similar to those obtained in cells allowed to progress into the G1 phase. Furthermore, radiolabeled glycerol was incorporated into phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine and phosphatidic acid in MCF-7 cells maintained at G2/M phase with nocodazole. Similar results were obtained in CHO cells. These results demonstrate that glycerophospholipid synthesis is very active in the G2/M phase of these cells. Therefore, the postulated cessation of phospholipid synthesis in G2/M phases is not applicable to all cell types.

Macrophage Colony-Stimulating Factor Induces the Expression of Mitogen-Activated Protein Kinase Phosphatase-1 Through a Protein Kinase C-Dependent Pathway

M-CSF triggers the activation of extracellular signal-regulated protein kinases (ERK)-1/2. We show that inhibition of this pathway leads to the arrest of bone marrow macrophages at the G0/G1 phase of the cell cycle without inducing apoptosis. M-CSF induces the transient expression of mitogen-activated protein kinase phosphatase-1 (MKP-1), which correlates with the inactivation of ERK-1/2. Because the time course of ERK activation must be finely controlled to induce cell proliferation, we studied the mechanisms involved in the induction of MKP-1 by M-CSF. Activation of ERK-1/2 is not required for this event. Therefore, M-CSF activates ERK-1/2 and induces MKP-1 expression through different pathways. The use of two protein kinase C (PKC) inhibitors (GF109203X and calphostin C) revealed that M-CSF induces MKP-1 expression through a PKC-dependent pathway. We analyzed the expression of different PKC isoforms in bone marrow macrophages, and we only detected PKCβI, PKCε, and PKCζ. PKCζ is not inhibited by GF109203X/calphostin C. Of the other two isoforms, PKCε is the best candidate to mediate MKP-1 induction. Prolonged exposure to PMA slightly inhibits MKP-1 expression in response to M-CSF. In bone marrow macrophages, this treatment leads to a complete depletion of PKCβI, but only a partial down-regulation of PKCε. Moreover, no translocation of PKCβI or PKCζ from the cytosol to particulate fractions was detected in response to M-CSF, whereas PKCε was constitutively present at the membrane and underwent significant activation in M-CSF-stimulated macrophages. In conclusion, we remark the role of PKC, probably isoform ε, in the negative control of ERK-1/2 through the induction of their specific phosphatase.

New diagnostic modalities in soft tissue sarcoma

Molecular and cytogenetic analysis of soft tissue sarcoma has provided a vast amount of new genetic information over the past 10 years. Recent advances in genetic technology, such as fluorescence in situ hybridization (FISH), reverse transcriptase-polymerase chain reaction (RT-PCR), and positional cloning techniques have greatly increased the rate of new discoveries and soon may bring cytogenetic and molecular analysis to standard pathology laboratories. Karotypic analysis of soft tissue tumors have demonstrated specific cytogenetic aberrations which have proved to be extremely useful diagnostically and have solidified and improved soft tissue tumor classification systems. Objective and reproducible prognostication in soft tissue sarcoma remains problematic. Presently, the grade and size of the sarcoma are the most important factors used to estimate risk of relapse and overall survival. Assigning a pathologic grade to an individual sarcoma as a means of predicting clinical behavior is often difficult with a 40% discordance rate even between expert sarcoma pathologists. There is mounting evidence that the composition of membrane phospholipid in tumor tissue is an important indicator of a tumor's cellularity, proliferative capacity, and differentiation state. However, there is a lack of information on the biochemical determinants of sarcoma proliferation and differentiation. To address these problems, novel quantitative ex vivo nuclear magnetic resonance (NMR) methods have been applied to determine the biochemical changes in tissue lipid for soft tissue sarcoma. The biochemical changes in tissue lipid have been found to correlate with sarcoma cellularity, growth rate, and differentiation. Continued prospective NMR analysis of tissue lipid biochemistry in soft tissue tumors will permit the development of a clinically relevant biochemical system of prognostic determinants for soft tissue sarcoma in the future.

Classification of human liposarcoma and lipoma using ex vivo proton NMR spectroscopy

Prognostication in patients with liposarcoma is a complex and controversial subject based on recognition of lipoblasts, adipocyte nuclear atypia, and qualitative estimations of cellularity and cell size. We show here that for 30 patients with liposarcoma and 5 patients with lipoma, spectral differences on high-resolution, magic angle spinning proton nuclear magnetic resonance (hr-MAS 1H-NMR) spectroscopy relate to known biochemical changes and correlate with adipocyte tissue differentiation, histologic cell type, and cellularity. The NMR-visible level of triglyceride is shown to correlate with liposarcoma differentiation, since the triglyceride level in well-differentiated liposarcoma is 33-fold higher on average than for myxoid/round cell liposarcoma, which in turn is 6-fold higher than the dedifferentiated and/or pleomorphic subtypes. The NMR-visible phosphatidylcholine level serves as an estimate of total tissue cell membrane phospholipid mass and was found to correlate with liposarcoma subtype. Pleomorphic liposarcoma, the most aggressive and metastatic subtype, was found to have a threefold increase in NMR-visible phosphatidylcholine level compared with dedifferentiated liposarcoma. The level of NMR-visible phosphatidylcholine was twofold greater in well-differentiated liposarcoma compared with lipoma and was threefold larger for the hypercellular myxoid/round cell subtype compared with the pure myxoid histology. Thus, NMR-derived parameters of tissue lipid may be used for objective distinction of liposarcoma histologic subtype/grade and lipoma from liposarcoma. These biochemical parameters may ultimately improve prognostication in patients with liposarcoma.

Classification of human liposarcoma and lipoma using ex vivo proton NMR spectroscopy

Prognostication in patients with liposarcoma is a complex and controversial subject based on recognition of lipoblasts, adipocyte nuclear atypia, and qualitative estimations of cellularity and cell size. We show here that for 30 patients with liposarcoma and 5 patients with lipoma, spectral differences on high-resolution, magic angle spinning proton nuclear magnetic resonance (hr-MAS 1H-NMR) spectroscopy relate to known biochemical changes and correlate with adipocyte tissue differentiation, histologic cell type, and cellularity. The NMR-visible level of triglyceride is shown to correlate with liposarcoma differentiation, since the triglyceride level in well-differentiated liposarcoma is 33-fold higher on average than for myxoid/round cell liposarcoma, which in turn is 6-fold higher than the dedifferentiated and/or pleomorphic subtypes. The NMR-visible phosphatidylcholine level serves as an estimate of total tissue cell membrane phospholipid mass and was found to correlate with liposarcoma subtype. Pleomorphic liposarcoma, the most aggressive and metastatic subtype, was found to have a threefold increase in NMR-visible phosphatidylcholine level compared with dedifferentiated liposarcoma. The level of NMR-visible phosphatidylcholine was twofold greater in well-differentiated liposarcoma compared with lipoma and was threefold larger for the hypercellular myxoid/round cell subtype compared with the pure myxoid histology. Thus, NMR-derived parameters of tissue lipid may be used for objective distinction of liposarcoma histologic subtype/grade and lipoma from liposarcoma. These biochemical parameters may ultimately improve prognostication in patients with liposarcoma.