Hypertrophic cardiomyopathy and dysregulation of cardiac energetics in a mouse model of biliary fibrosis.

UNLABELLED: Cardiac dysfunction is a major cause of morbidity and mortality in patients with end-stage liver disease; yet the mechanisms remain largely unknown. We hypothesized that the complex interrelated impairments in cardiac structure and function secondary to progression of liver diseases involve alterations in signaling pathways engaged in cardiac energy metabolism and hypertrophy, augmented by direct effects of high circulating levels of bile acids. Biliary fibrosis was induced in male C57BL/6J mice by feeding a 0.1% 3,5-diethoxycarbonyl-1,4-dihydroxychollidine (DDC) supplemented diet. After 3 weeks, mice underwent live imaging (dual energy x-ray absorptiometry [DEXA] scanning, two-dimensional echocardiography [2DE], electrocardiography, cardiac magnetic resonance imaging), exercise treadmill testing, and histological and biochemical analyses of livers and hearts. Compared with chow-fed mice, DDC-fed mice fatigued earlier on the treadmill, with reduced VO(2). Marked changes were identified electrophysiologically (bradycardia and prolonged QT interval) and functionally (hyperdynamic left ventricular [LV] contractility along with increased LV thickness). Hearts of DDC-fed mice showed hypertrophic signaling (activation of v-akt murine thymoma viral oncogene/protein kinase B [AKT], inhibition of glycogen synthase kinase-3beta [GSK3beta], a 20-fold up-regulation of beta myosin heavy chain RNA and elevated G(s)alpha/G(i)alpha ratio. Genes regulating cardiac fatty acid oxidation pathways were suppressed, along with a threefold increase in myocardial glycogen content. Treatment of mouse cardiomyocytes (which express the membrane bile acid receptor TGR5) with potent natural TGR5 agonists, taurochenodeoxycholic acid and lithocholic acid, activated AKT and inhibited GSK3beta, similar to the changes seen in DDC-fed mouse hearts. This provides support for a novel mechanism whereby circulating natural bile acids can induce signaling pathways in heart associated with hypertrophy. CONCLUSION: Three weeks of DDC feeding-induced biliary fibrosis leads to multiple functional, metabolic, electrophysiological, and hypertrophic adaptations in the mouse heart, recapitulating some of the features of human cirrhotic cardiomyopathy.

Regulation of the lifespan in dendritic cell subsets.

The lifespan of dendritic cells (DCs) can potentially influence immune responses by affecting the duration of DCs in stimulating lymphocytes. Significant differences in the lifespan have been reported for various DC subsets, however, the molecular mechanisms for regulating such differences between DC subsets remain unclear. In this study, we compared the apoptosis signaling molecules in two major DC subjects, the myeloid DCs (mDCs) and plasmacytoid DCs (pDCs). We observed a lower ratio between anti-apoptotic Bcl-2/Bcl-xL and pro-apoptotic Bax/Bak in shorter-lived myeloid DCs (mDCs) than in longer-lived plasmacytoid DCs (pDCs) or T cells. Transfection with Bcl-2 or Bcl-xL prolonged the survival of mouse primary mDCs in vitro, while deletion of Bcl-2 accelerated DC turnover in vivo. In addition, the ratios between anti-apoptotic Bcl-2/Bcl-xL and pro-apoptotic Bax/Bak could be regulated in DCs. Signaling from toll-like receptors (TLRs) up-regulated Bcl-xL and improved DC survival. Our data suggest that differential expression of apoptosis signaling molecules regulates the lifespan of different DC subsets.

Caspase-9-induced mitochondrial disruption through cleavage of anti-apoptotic BCL-2 family members.

Mitochondrial disruption during apoptosis results in the release of cytochrome c that forms apoptosomes with Apaf-1 and caspase-9. Activation of caspase-9 by dimerization in apoptosomes then triggers a caspase signaling cascade. In addition, other apoptosis signaling molecules released from the mitochondrion, such as apoptosis-inducing factor and endonuclease G, may induce caspase-9-independent apoptosis. To determine the signaling events induced by caspase-9, we used chemically induced dimerization for specific activation of caspase-9. We observed that caspase-9 dimerization resulted in the loss of mitochondrial membrane potential and the cleavage of anti-apoptotic Bcl-2, Bcl-xL, and Mcl-1. Moreover, cleavage-resistant Bcl-2, Bcl-xL, or Mcl-1 potently inhibited caspase-9-dependent loss of mitochondrial membrane potential and the release of cytochrome c. Our data suggest that a caspase-9 signaling cascade induces feedback disruption of the mitochondrion through cleavage of anti-apoptotic Bcl-2, Bcl-xL, and Mcl-1.