Abstract
Although nematodes like Caenorhabditis elegans are incapable of de novo cholesterol biosynthesis, they can utilize nonfunctional sterols by converting them into cholesterol and other sterols for cellular function. The results reported previously and presented here suggest that blocking of sterol conversion to cholesterol in C. elegans by 25-azacoprostane-HCl (azacoprostane) treatment causes a serious defect in germ cell development, growth, cuticle development, and motility behavior. To establish a biochemical basis for these physiological abnormalities, we performed proteomic analysis of mixed stage worms that had been treated with the drug. Our results from a differential display proteomic analysis revealed significant decreases in the levels of proteins involved in collagen and cytoskeleton organization such as protein disulfide isomerase (6.7-fold), beta-tubulin (5.41-fold), and NEX-1 protein (>30-fold). Also reduced were enzymes involved in energy production such as phosphoglycerate kinase (4.8-fold) and phosphoenolpyruvate carboxykinase (8.5-fold), a target for antifilarial drugs such as azacoprostane. In particular, reductions in the expression of lipoprotein families such as vitellogenin-2 (7.7-fold) and vitellogenin-6 (5.4-fold) were prominent in the drug-treated worms, indicating that sterol metabolism disturbance caused by azacoprostane treatment is tightly coupled with suppression of the lipid transfer-related proteins at the protein level. However, competitive quantitative reverse transcriptase polymerase chain reaction showed that the transcriptional levels of vit-2, vit-6, and their receptors (e.g. rme-2 and lrp-1) in drug-treated worms were 3- to 5-fold higher than those in the untreated group, suggesting a presence of a sterol regulatory element-binding protein (SREBP)-like pathway in these genes. In fact, multiple predicted sterol regulatory elements or related regulatory sequences responding to sterols were found to be located at the 5'-flanking regions in vit-2 and lrp-1 genes, and their transcriptional activities fluctuated highly in response to changes in sterol concentration. Thus, many physiological abnormalities caused by azacoprostane-mediated sterol metabolism disturbance appear to be exerted at least in part through SREBP pathway in C. elegans.
Highlights
Nematodes like Caenorhabditis elegans are incapable of de novo cholesterol biosynthesis, they can utilize nonfunctional sterols by converting them into cholesterol and other sterols for cellular function
This sequential reaction seems possible in C. elegans because it appears to have an uncharacterized 7-cholesterol desaturase by which cholesterol is readily converted to 7-DHC, a major sterol in C. elegans for cellular function [7, 8]
The initial purpose of our study was to identify proteins uniquely or differentially expressed before and after sterol metabolism was disturbed by treatment of C. elegans with azacoprostane
Summary
Nematodes like Caenorhabditis elegans are incapable of de novo cholesterol biosynthesis, they can utilize nonfunctional sterols by converting them into cholesterol and other sterols for cellular function. Nematodes like Caenorhabditis elegans are incapable of de novo cholesterol biosynthesis [5, 6], they can utilize a variety of nonpermissible (to nematode) sterol analogues (e.g. sitosterol, stigmasterol, and other phytosterols) taken from culture media or the environment, which are converted by nematodes into socalled permissible sterol analogues such as 7-dehydrocholesterol (7-DHC) following dealkylation and C-24 reduction [7]. When exogenous sterol supply is restricted, many physiological abnormalities including growth inhibition, brood size reduction, egg-laying defects, and endomitotic (emo) phenocopy are observed [8, 10] Most of these phenomena occur when worms are grown in the presence of sitosterol as a sterol nutrient and 25-azacoprostane-HCl (azacoprostane) [6, 11], an inhibitor of the sterol ⌬24-reductase (24-SR) that catalyzes conversion of desmosterol to cholesterol [12]. The abbreviations used are: 7-DHC, 7-dehydrocholesterol; 25azacoprostane-HCl, 25-aza-5-cholestane hydrochloride; CHAPS, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid; 2DE, two-dimensional electrophoresis; FBS, fetal bovine serum; LPDS, lipoprotein-deficient serum; MALDI-TOF, matrix-assisted laser desorption/ionization time-of-flight; SRE, sterol regulatory element; SREBP, sterol regulatory element-binding protein; 24-SR, sterol ⌬24reductase; IPG, immobilized pH gradient; RT, reverse transcriptase; PEPCK, phosphoenolpyruvate carboxykinase; RME, receptor-mediated endocytosis
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