phytosterols and homobrassinolide

Oxysterols are natural ligands of certain nuclear receptors known as liver X receptors (LXR). LXRs are regulators of fatty acid, cholesterol, and glucose homeostasis. Dietary phyto-oxysterol 28-homobrassinolide (28-HB) has been demonstrated to transactivate rat LXR α and β. In this study we assessed the potential of 28-HB to effect such changes in - (1) human HepG2 cancer cell line, (2) isolated perfused goat liver, and (3) high-fat diet-fed C57BL/6J mice. Serum and perfusate marker levels along with hexokinase activity were determined through enzyme assays. Fat deposition was studied by Oil Red O staining, ATP-binding cassette transporter (ABCA1), and sterol regulatory element-binding transcription factor 2 (SREBP2) protein expression by Western blot and their mRNA expression through real-time PCR. In HepG2 cells, 28-HB (5-20 μM) treatment indicated a 2-fold increase in glucose utilization and ABCA1 and SREBP2 protein expression within 12 h. Tissue glucose and cholesterol levels decreased in 28-HB perfused goat liver within 2 h, whereas cholesterol level increased 54% in the perfusate (p < 0.05) and tissue hexokinase activity increased 23% (p < 0.05). Glucokinase, ABCA1, and SREBF1 gene expression increased 2.6, 5.37, and 2.85 fold respectively in the perfused tissue after 4 h. High-fat diet-fed C57BL/6J mice when treated with 28-HB (1-20 µg/day) for 6 weeks exhibited a marked decrease in aortic fat deposit and serum marker levels. Our study suggests that 28-HB modulates cholesterol and glucose homeostasis in animal cells through activation of LXR involving ABCA1 and SREBP-1 and 2 augmentations.

Topics: Animals; Cholestanones; Mice; Phytosterols; Sterol Regulatory Element Binding Protein 1

Although cell elongation is a basic function of plant morphogenesis, many of the molecular events involved in this process are still unknown. In this work an extremely dwarf mutant, originally named bul, was used to study one of the main processes of plant development, cell elongation. Genetic analyses revealed that the BUL locus was linked to the nga172 marker on chromosome 3. Recently, after mapping the new dwf7 mutation of Arabidopsis, which is allelic to ste1, it was reported that dwf7 is also linked to the same marker. Sterol analyses of the bull-1 mutant indicated that bul1-1 is defective in the delta 7-sterol-C5-desaturation step leading to brassinosteroid biosynthesis. Considering these findings, we designated our bul mutant as bul1-1/dwf7-3/ste1-4. The bul1-1 mutant was characterized by a very dwarf phenotype, with delayed development and reduced fertility. The mutant leaves had a dark-green colour, which was probably due to continuous stomatal closure. The bul1-1 mutant showed a partially de-etiolated phenotype in the dark. Cellular characterization and rescue experiments with brassinosteroids demonstrated the involvement of the BUL1-1 protein in brassinosteroid-dependent plant growth processes.

Topics: Amino Acid Sequence; Arabidopsis; Cell Wall; Chlorophyll; Cholestanones; Genes, Plant; Genetic Complementation Test; Light; Microtubules; Molecular Sequence Data; Mutation; Oxidoreductases; Phenotype; Phytosterols; Plant Growth Regulators; Plant Proteins; Plant Structures; Sequence Homology, Amino Acid; Signal Transduction

In order to elucidate the involvement of brassinosteroids in the cell elongation process leading to normal plant morphology, indirect immunofluorescence and molecular techniques were use to study the expression of tubulin genes in the bul1-1 dwarf mutant of Arabidopsis thaliana (L.) Heynh., the characteristics of which are reported in this issue (M. Catterou et al., 2001). Microtubules were studied specifically in the regions of the mutant plant where the elongation zone is suppressed (hypocotyls and petioles), making the reduction in cell elongation evident. Indirect immunofluorescence of alpha-tubulin revealed that very few microtubules were present in mutant cells, resulting in the total lack of the parallel microtubule organization that is typical of elongating cells in the wild type. After brassinosteroid treatment, microtubules reorganized and became correctly oriented, suggesting the involvement of brassinosteroids in microtubule organization. Molecular analyses showed that the microtubule reorganization observed in brassinosteroid-treated bul1-1 plants did not result either from an activation of tubulin gene expression, or from an increase in tubulin content, suggesting that a brassinosteroid-responsive pathway exists which allows microtubule nucleation/organization and cell elongation without activation of tubulin gene expression.

Topics: Arabidopsis; Cell Division; Cell Wall; Cholestanones; Genes, Plant; Hypocotyl; Immunohistochemistry; Microtubules; Mutation; Phytosterols; Plant Epidermis; Plant Growth Regulators; Plant Leaves; Plant Proteins; Tubulin