Apolipoprotein AI and high-density lipoprotein have anti-inflammatory effects on adipocytes via cholesterol transporters: ATP-binding cassette A-1, ATP-binding cassette G-1, and scavenger receptor B-1.

RATIONALE: Macrophage accumulation in adipose tissue associates with insulin resistance and increased cardiovascular disease risk. We previously have shown that generation of reactive oxygen species and monocyte chemotactic factors after exposure of adipocytes to saturated fatty acids, such as palmitate, occurs via translocation of NADPH oxidase 4 into lipid rafts (LRs). The anti-inflammatory effects of apolipoprotein AI (apoAI) and high-density lipoprotein (HDL) on macrophages and endothelial cells seem to occur via cholesterol depletion of LRs. However, little is known concerning anti-inflammatory effects of HDL and apoAI on adipocytes. OBJECTIVE: To determine whether apoAI and HDL inhibit inflammation in adipocytes and adipose tissue, and whether this is dependent on LRs. METHODS AND RESULTS: In 3T3L-1 adipocytes, apoAI, HDL, and methyl-ß-cyclodextrin inhibited chemotactic factor expression. ApoAI and HDL also disrupted LRs, reduced plasma membrane cholesterol content, inhibited NADPH oxidase 4 translocation into LRs, and reduced palmitate-induced reactive oxygen species generation and monocyte chemotactic factor expression. Silencing ATP-binding cassette A-1 abrogated the effect of apoAI, but not HDL, whereas silencing ATP-binding cassette G-1 or scavenger receptor B-1 abrogated the effect of HDL but not apoAI. In vivo, apoAI transgenic mice fed a high-fat, high-sucrose, cholesterol-containing diet showed reduced chemotactic factor and proinflammatory cytokine expression and reduced macrophage accumulation in adipose tissue. CONCLUSIONS: ApoAI and HDL have anti-inflammatory effects in adipocytes and adipose tissue similar to their effects in other cell types. These effects are consistent with disruption and removal of cholesterol from LRs, which are regulated by cholesterol transporters, such as ATP-binding cassette A-1, ATP-binding cassette G-1, and scavenger receptor B-1.

Apolipoprotein A-I attenuates palmitate-mediated NF-κB activation by reducing Toll-like receptor-4 recruitment into lipid rafts.

While high-density lipoprotein (HDL) is known to protect against a wide range of inflammatory stimuli, its anti-inflammatory mechanisms are not well understood. Furthermore, HDL's protective effects against saturated dietary fats have not been previously described. In this study, we used endothelial cells to demonstrate that while palmitic acid activates NF-κB signaling, apolipoprotein A-I, (apoA-I), the major protein component of HDL, attenuates palmitate-induced NF-κB activation. Further, vascular NF-κB signaling (IL-6, MCP-1, TNF-α) and macrophage markers (CD68, CD11c) induced by 24 weeks of a diabetogenic diet containing cholesterol (DDC) is reduced in human apoA-I overexpressing transgenic C57BL/6 mice compared to age-matched WT controls. Moreover, WT mice on DDC compared to a chow diet display increased gene expression of lipid raft markers such as Caveolin-1 and Flotillin-1, and inflammatory Toll-like receptors (TLRs) (TLR2, TLR4) in the vasculature. However apoA-I transgenic mice on DDC show markedly reduced expression of these genes. Finally, we show that in endothelial cells TLR4 is recruited into lipid rafts in response to palmitate, and that apoA-I prevents palmitate-induced TLR4 trafficking into lipid rafts, thereby blocking NF-κB activation. Thus, apoA-I overexpression might be a useful therapeutic tool against vascular inflammation.

HDL lipids and insulin resistance.

There is renewed interest in high-density lipoproteins (HDLs) due to recent findings linking atherosclerosis to the formation of dysfunctional HDL. This article focuses on the universe of HDL lipids and their potential protective or proinflammatory roles in vascular disease and insulin resistance. HDL carries a wide array of lipids including sterols, triglycerides, fat-soluble vitamins, and a large number of phospholipids, including phosphatidylcholine, sphingomyelin, and ceramide with many biological functions. Ceramide has been implicated in the pathogenesis of insulin resistance and has many proinflammatory properties. In contrast, sphingosine-1-phosphate, which is transported mainly in HDL, has anti-inflammatory properties that may be atheroprotective and may account for some of the beneficial effects of HDL. However, the complexity of the HDL lipidome is only beginning to reveal itself. The emergence of new analytical technologies should rapidly increase our understanding of the function of HDL lipids and their role in disease states.

Estradiol induces export of sphingosine 1-phosphate from breast cancer cells via ABCC1 and ABCG2.

Sphingosine 1-phosphate (S1P), a potent sphingolipid mediator produced by sphingosine kinase isoenzymes (SphK1 and SphK2), regulates diverse cellular processes important for breast cancer progression acting in an autocrine and/or paracrine manner. Here we show that SphK1, but not SphK2, increased S1P export from MCF-7 cells. Whereas for both estradiol (E(2)) and epidermal growth factor-activated SphK1 and production of S1P, only E(2) stimulated rapid release of S1P and dihydro-S1P from MCF-7 cells. E(2)-induced S1P and dihydro-S1P export required estrogen receptor-alpha, not GPR30, and was suppressed either by pharmacological inhibitors or gene silencing of ABCC1 (multidrug resistant protein 1) or ABCG2 (breast cancer resistance protein). Inhibiting these transporters also blocked E(2)-induced activation of ERK1/2, indicating that E(2) activates ERK via downstream signaling of S1P. Taken together, our findings suggest that E(2)-induced export of S1P mediated by ABCC1 and ABCG2 transporters and consequent activation of S1P receptors may contribute to nongenomic signaling of E(2) important for breast cancer pathophysiology.

Apoptosis induces expression of sphingosine kinase 1 to release sphingosine-1-phosphate as a "come-and-get-me" signal.

Sphingosine-1-phosphate (S1P) is a bioactive lipid that regulates myriad important cellular processes, including growth, survival, cytoskeleton rearrangements, motility, and immunity. Here we report that treatment of Jurkat and U937 leukemia cells with the pan-sphingosine kinase (SphK) inhibitor N,N-dimethylsphingosine to block S1P formation surprisingly caused a large increase in expression of SphK1 concomitant with induction of apoptosis. Another SphK inhibitor, D,L-threo-dihydrosphingosine, also induced apoptosis and produced dramatic increases in SphK1 expression. However, up-regulation of SphK1 was not a specific effect of its inhibition but rather was a consequence of apoptotic stress. The chemotherapeutic drug doxorubicin, a potent inducer of apoptosis in these cells, also stimulated SphK1 expression and activity and promoted S1P secretion. The caspase inhibitor ZVAD reduced not only doxorubicin-induced lethality but also the increased expression of SphK1 and secretion of S1P. Apoptotic cells secrete chemotactic factors to attract phagocytic cells, and we found that S1P potently stimulated chemotaxis of monocytic THP-1 and U937 cells and primary monocytes and macrophages. Collectively, our data suggest that apoptotic cells may up-regulate SphK1 to produce and secrete S1P that serves as a "come-and-get-me" signal for scavenger cells to engulf them in order to prevent necrosis.

A rapid and sensitive method to measure secretion of sphingosine-1-phosphate.

The serum-borne, bioactive sphingolipid mediator, sphingosine-1-phosphate (S1P), regulates numerous important physiological and pathological processes, mainly acting through specific cell surface G-protein-coupled receptors. Although many mammalian cells can produce S1P, there is little information as to how it is secreted to reach its receptors. Progress in elucidating this mechanism has been hampered by the difficulty of measuring very low levels of S1P. This chapter describes a simple, rapid method to measure S1P export from cells. It also discusses the current knowledge of how S1P is exported out of cells and its physiological significance.

Ceramide kinase regulates growth and survival of A549 human lung adenocarcinoma cells.

Ceramide-1-phosphate (C1P) is emerging as a new addition to the family of bioactive sphingolipid metabolites. At low concentrations, C1P enhanced survival of NIH 3T3 fibroblasts and A549 lung cancer cells, while at high concentrations, it reduced survival and induced apoptosis. Apoptosis correlated with degradation of C1P to pro-apoptotic ceramide. To examine the role of endogenous C1P, expression of ceramide kinase, the enzyme that produces C1P, was downregulated, which reduced cellular proliferation, progression into S phase and enhanced apoptosis induced by serum starvation. Our results suggest that ceramide kinase determines the balance between pro-apoptotic ceramide and anti-apoptotic C1P to regulate cell fate, reminiscent of its function in plants.

Role of ABCC1 in export of sphingosine-1-phosphate from mast cells.

Mast cells play a pivotal role in inflammatory and immediate-type allergic reactions by secreting a variety of potent inflammatory mediators, including sphingosine-1-phosphate (S1P). However, it is not known how S1P is released from cells. Here, we report that S1P is exported from mast cells independently of their degranulation and demonstrate that it is mediated by ATP binding cassette (ABC) transporters. Constitutive and antigen-stimulated S1P release was inhibited by MK571, an inhibitor of ABCC1 (MRP1), but not by inhibitors of ABCB1 (MDR-1, P-glycoprotein). Moreover, down-regulation of ABCC1 with small interfering RNA, which decreased its cell surface expression, markedly reduced S1P export from both rat RBL-2H3 and human LAD2 mast cells. Transport of S1P by ABCC1 influenced migration of mast cells toward antigen but not degranulation. These findings have important implications for S1P functions in mast cell-mediated immune responses.

Bacillus anthracis and Bacillus cereus PcrA helicases can support DNA unwinding and in vitro rolling-circle replication of plasmid pT181 of Staphylococcus aureus.

Replication of rolling-circle replicating (RCR) plasmids in gram-positive bacteria requires the unwinding of initiator protein-nicked plasmid DNA by the PcrA helicase. In this report, we demonstrate that heterologous PcrA helicases from Bacillus anthracis and Bacillus cereus are capable of unwinding Staphylococcus aureus plasmid pT181 from the initiator-generated nick and promoting in vitro replication of the plasmid. These helicases also physically interact with the RepC initiator protein of pT181. The ability of PcrA helicases to unwind noncognate RCR plasmids may contribute to the broad-host-range replication and dissemination of RCR plasmids in gram-positive bacteria.