Cholesterol metabolism is tightly regulated at the cellular level. Here we show that miR-33, an intronic microRNA (miRNA) located within the gene encoding sterol-regulatory element–binding factor–2 (SREBF-2), a transcriptional regulator of cholesterol synthesis, modulates the expression of genes involved in cellular cholesterol transport. In mouse and human cells, miR-33 inhibits the expression of the adenosine triphosphate–binding cassette (ABC) transporter, ABCA1, thereby attenuating cholesterol efflux to apolipoprotein A1. In mouse macrophages, miR-33 also targets ABCG1, reducing cholesterol efflux to nascent high-density lipoprotein (HDL). Lentiviral delivery of miR-33 to mice represses ABCA1 expression in the liver, reducing circulating HDL levels. Conversely, silencing of miR-33 in vivo increases hepatic expression of ABCA1 and plasma HDL levels. Thus, miR-33 appears to regulate both HDL biogenesis in the liver and cellular cholesterol efflux.

Atherosclerosis, driven by inflamed lipid-laden lesions, can occlude the coronary arteries and lead to myocardial infarction. This chronic disease is a major and expensive health burden. However, the body is able to mobilize and excrete cholesterol and other lipids, thus preventing atherosclerosis by a process termed reverse cholesterol transport (RCT). Insight into the mechanism of RCT has been gained by the study of two rare syndromes caused by the mutation of ABC transporter loci. In Tangier Disease, loss of ABCA1 prevents cells from exporting cholesterol and phospholipid, thus resulting in the build-up of cholesterol in the peripheral tissues and a loss of circulating HDL. Consistent with HDL being an athero-protective particle, Tangier patients are more prone to develop atherosclerosis. Likewise, sitosterolemia is another inherited syndrome associated with premature atherosclerosis. Here mutations in either the ABCG5 or G8 loci, prevents hepatocytes and enterocytes from excreting cholesterol and plant sterols, including sitosterol, into the bile and intestinal lumen. Thus, ABCG5 and G8, which from a heterodimer, constitute a transporter that excretes cholesterol and dietary sterols back into the gut, while ABCA1 functions to export excess cell cholesterol and phospholipid during the biogenesis of HDL. Interestingly, a third protein, ABCG1, that has been shown to have anti-atherosclerotic activity in mice, may also act to transfer cholesterol to mature HDL particles. Here we review the relationship between the lipid transport activities of these proteins and their anti-atherosclerotic effect, particularly how they may reduce inflammatory signaling pathways. Of particular interest are recent reports that indicate both ABCA1 and ABCG1 modulate cell surface cholesterol levels and inhibit its partitioning into lipid rafts. Given lipid rafts may provide platforms for innate immune receptors to respond to inflammatory signals, it follows that loss of ABCA1 and ABCG1 by increasing raft content will increase signaling through these receptors, as has been experimentally demonstrated. Moreover, additional reports indicate ABCA1, and possibly SR-BI, another HDL receptor, may directly act as anti-inflammatory receptors independent of their lipid transport activities. Finally, we give an update on the progress and pitfalls of therapeutic approaches that seek to stimulate the flux of lipids through the RCT pathway.

We have found a novel phospholipid antibiotic (named bacilysocin) which accumulates within (or associates with) the cells of Bacillus subtilis 168 and determined the structure by nuclear magnetic resonance and mass spectrometry analyses. The structure of bacilysocin elucidated was 1-(12-methyltetradecanoyl)-3-phosphoglyceroglycerol. Bacilysocin demonstrated antimicrobial activity, especially against certain fungi. Production of bacilysocin commenced immediately after growth ceased and before the formation of heat-resistant spores. The production of bacilysocin was completely blocked when the ytpA gene, which encodes a protein homologous to lysophospholipase, was disrupted, but blockage of the ytpA gene did not significantly affect growth. Sporulation was also impaired, with a 10-fold reduction in heat-resistant spore titers being detected. Since the ytpA disruptant actually lacked phospholipase activity, we propose that the YtpA protein functions as an enzyme for the biosynthesis of bacilysocin.The gram-positive bacterium Bacillus subtilis produces a large number of antibiotics, which are classified as ribosomal or nonribosomal. Nonribosomally synthesized circular oligopeptides that contain a fatty acid chain exhibit potent antibacterial or antifungal activity, as represented by surfactin, the iturinic group, and fengycin (16). B. subtilis 168 is the beststudied strain in the genus Bacillus, the genome of which was completely sequenced in 1997. Strain 168 is known to produce three ribosomal antibiotics, TasA (12), subtilosin (1), sublancin (10), and two nonribosomal antibiotics, surfactin (14) and bacilysin (9). The production of other antibiotics by strain 168 has also been predicted on the basis of genome sequence analysis, as exemplified by plipastatin (13). The ribosomal peptide antibiotics are synthesized during active growth, while nonribosomal ones are synthesized after growth has ceased. The role of antibiotic production for the producing organism is still under speculation. The best-accepted theory is that nonribosomal antibiotics may play a role in competition with other microorganisms during spore germination (for a review, see references 5 and 16). The detection of novel antibiotics produced by B. subtilis 168 would therefore be helpful in providing an understanding of the intrinsic (if any) role of antibiotics in the life cycle of this organism. In the present paper, we describe the isolation and identification of a new antibiotic (named bacilysocin) produced by B. subtilis 168. MATERIALS AND METHODSStrain, media, and culture conditions. B. subtilis strain 168 (trpC2) was precultured at 30°C for 24 h in NG medium (2) supplemented with 50 g of tryptophan per ml, and then 0.1 ml of the resulting culture was inoculated into 10 ml of fresh NG medium, followed by incubation under shaking at 30°C. The titers of the heat-resistant spores were determined by heating the cultures for 10 min at 80°C and then plating them onto an NG agar plate.Detection and bioassay of bacilysocin. Bacilysocin was extracted wit...