Serum levels of HDL are inversely correlated with the risk of coronary heart disease. The anti-atherogenic effect of HDL is partially mediated by its major protein constituent apoA-I. In this study, we identify bile acids that are activators of the nuclear receptor farnesoid X receptor (FXR) as negative regulators of human apoA-I expression. Intrahepatocellular accumulation of bile acids, as seen in patients with progressive familial intrahepatic cholestasis and biliary atresia, was associated with diminished apoA-I serum levels. In human apoA-I transgenic mice, treatment with the FXR agonist taurocholic acid strongly decreased serum concentrations and liver mRNA levels of human apoA-I, which was associated with reduced serum HDL levels. Incubation of human primary hepatocytes and hepatoblastoma HepG2 cells with bile acids resulted in a dose-dependent downregulation of apoA-I expression. Promoter mutation analysis and gel-shift experiments in HepG2 cells demonstrated that bile acid–activated FXR decreases human apoA-I promoter activity by a negative FXR response element mapped to the C site. FXR bound this site and repressed transcription in a manner independent of retinoid X receptor. The nonsteroidal synthetic FXR agonist GW4064 likewise decreased apoA-I mRNA levels and promoter activity in HepG2 cells.
Thierry Claudel, Ekkehard Sturm, Hélène Duez, Inés Pineda Torra, Audrey Sirvent, Vladimir Kosykh, Jean-Charles Fruchart, Jean Dallongeville, Dean W. Hum, Folkert Kuipers, Bart Staels
The access of testosterone and estradiol to target tissues is regulated by sex hormone–binding globulin (SHBG) in human blood. Serum SHBG levels are low in patients with hyperandrogenism, especially in association with polycystic ovarian syndrome (PCOS) and in individuals at risk for diabetes and heart disease. Here, we identify SHBG coding region variations from a compound heterozygous patient who presented with severe hyperandrogenism during pregnancy. Serum SHBG levels in this patient measured 2 years after her pregnancy were exceptionally low, and her non–protein-bound testosterone concentrations greatly exceeded the normal reference range. A single-nucleotide polymorphism within the proband’s maternally derived SHBG allele encodes a missense mutation, P156L, which allows for normal steroid ligand binding but causes abnormal glycosylation and inefficient secretion of SHBG. This polymorphism was identified in four other patients with either PCOS, ioiopathic hirsutism, or ovarian failure. The proband’s paternal SHBG allele carries a single-nucleotide deletion within exon 8, producing a reading-frame shift within the codon for E326 and a premature termination codon. CHO cells transfected with a SHBG cDNA carrying this mutation fail to secrete the predicted truncated form of SHBG. To our knowledge, these are the first examples of human SHBG variants linked to hyperandrogenism and ovarian dysfunction.
Kevin N. Hogeveen, Patrice Cousin, Michel Pugeat, Didier Dewailly, Benoît Soudan, Geoffrey L. Hammond
Myostatin is a TGF-β family member that acts as a negative regulator of muscle growth. Mice lacking the myostatin gene (Mstn) have a widespread increase in skeletal muscle mass resulting from a combination of muscle fiber hypertrophy and hyperplasia. Here we show that Mstn-null mice have a significant reduction in fat accumulation with increasing age compared with wild-type littermates, even in the setting of normal food intake (relative to body weight), normal body temperature, and a slightly decreased resting metabolic rate. To investigate whether myostatin might be an effective target for suppressing the development of obesity in settings of abnormal fat accumulation, we analyzed the effect of the Mstn mutation in two genetic models of obesity, agouti lethal yellow (Ay) and obese (Lepob/ob). In each case, loss of Mstn led to a partial suppression of fat accumulation and of abnormal glucose metabolism. Our findings raise the possibility that pharmacological agents that block myostatin function may be useful not only for enhancing muscle growth, but also for slowing or preventing the development of obesity and type 2 diabetes.
Alexandra C. McPherron, Se-Jin Lee
To investigate the role of insulin signaling on postnatal cardiac development, physiology, and cardiac metabolism, we generated mice with a cardiomyocyte-selective insulin receptor knockout (CIRKO) using cre/loxP recombination. Hearts of CIRKO mice were reduced in size by 20–30% due to reduced cardiomyocyte size and had persistent expression of the fetal β-myosin heavy chain isoform. In CIRKO hearts, glucose transporter 1 (GLUT1) expression was reduced by about 50%, but there was a twofold increase in GLUT4 expression as well as increased rates of cardiac glucose uptake in vivo and increased glycolysis in isolated working hearts. Fatty acid oxidation rates were diminished as a result of reduced expression of enzymes that catalyze mitochondrial β-oxidation. Although basal rates of glucose oxidation were reduced, insulin unexpectedly stimulated glucose oxidation and glycogenolysis in CIRKO hearts. Cardiac performance in vivo and in isolated hearts was mildly impaired. Thus, insulin signaling plays an important developmental role in regulating postnatal cardiac size, myosin isoform expression, and the switching of cardiac substrate utilization from glucose to fatty acids. Insulin may also modulate cardiac myocyte metabolism through paracrine mechanisms by activating insulin receptors in other cell types within the heart.
Darrell D. Belke, Sandrine Betuing, Martin J. Tuttle, Christophe Graveleau, Martin E. Young, Mark Pham, Dongfang Zhang, Robert C. Cooksey, Donald A. McClain, Sheldon E. Litwin, Heinrich Taegtmeyer, David Severson, C. Ronald Kahn, E. Dale Abel