Mitochondrial Oxidative Stress Alters a Pathway in Caenorhabditis elegans Strongly Resembling That of Bile Acid Biosynthesis and Secretion in Vertebrates

摘要

Mammalian bile acids (BAs) are oxidized metabolites of cholesterol whose amphiphilic properties serve in lipid and cholesterol uptake. BAs also act as hormone-like substances that regulate metabolism. The Caenorhabditis elegans clk-1 mutants sustain elevated mitochondrial oxidative stress and display a slow defecation phenotype that is sensitive to the level of dietary cholesterol. We found that: 1) The defecation phenotype of clk-1 mutants is suppressed by mutations in tat-2 identified in a previous unbiased screen for suppressors of clk-1 . TAT-2 is homologous to ATP8B1, a flippase required for normal BA secretion in mammals. 2) The phenotype is suppressed by cholestyramine, a resin that binds BAs. 3) The phenotype is suppressed by the knock-down of C. elegans homologues of BA–biosynthetic enzymes. 4) The phenotype is enhanced by treatment with BAs. 5) Lipid extracts from C. elegans contain an activity that mimics the effect of BAs on clk-1 , and the activity is more abundant in clk-1 extracts. 6) clk-1 and clk-1;tat-2 double mutants show altered cholesterol content. 7) The clk-1 phenotype is enhanced by high dietary cholesterol and this requires TAT-2. 8) Suppression of clk-1 by tat-2 is rescued by BAs, and this requires dietary cholesterol. 9) The clk-1 phenotype, including the level of activity in lipid extracts, is suppressed by antioxidants and enhanced by depletion of mitochondrial superoxide dismutases. These observations suggest that C. elegans synthesizes and secretes molecules with properties and functions resembling those of BAs. These molecules act in cholesterol uptake, and their level of synthesis is up-regulated by mitochondrial oxidative stress. Future investigations should reveal whether these molecules are in fact BAs, which would suggest the unexplored possibility that the elevated oxidative stress that characterizes the metabolic syndrome might participate in disease processes by affecting the regulation of metabolism by BAs. Author Summary Cholesterol metabolism, in particular the transport of cholesterol in the blood by lipoproteins, is an important determinant of human cardiovascular health. Bile acids are breakdown products of cholesterol that have detergent properties and are secreted into the gut by the liver. Bile acids carry out three distinct roles in cholesterol metabolism: 1) Their synthesis from cholesterol participates in cholesterol elimination. 2) They act as detergents in the uptake of dietary cholesterol from the gut. 3) They regulate many aspects of metabolism, including cholesterol metabolism, by molecular mechanisms similar to that of steroid hormones. We have found that cholesterol uptake and lipoprotein metabolism in the nematode Caenorhabditis elegans are regulated by molecules whose activities, biosynthesis, and secretion strongly resemble that of bile acids and which might be bile acids. Most importantly we have found that oxidative stress upsets the regulation of the synthesis of these molecules. The metabolic syndrome is a set of cardiovascular risk factors that include obesity, high blood cholesterol, hypertension, and insulin resistance. Given the function of bile acids as metabolic regulators, our findings with C. elegans suggest the unexplored possibility that the elevated oxidative stress that characterizes the metabolic syndrome may participate in mammalian disease processes by affecting the regulation of bile acid synthesis.