Although nematodes like Caenorhabditis elegans are incapable of de novo cholesterol biosynthesis, they can use 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 serious defects 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. 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), ß-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 other antifilarial drugs. In particular, reductions in the expression of lipoprotein family members 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 to 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 for these genes. In fact, multiple predicted SREs 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 highly fluctuated with 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.