The role of transhepatic bile salt flux in the control of hepatic secretion of triacylglycerol-rich lipoproteins in vivo in rodents

Bile salts (BS) have been shown to suppress the secretion of very-low-density lipoprotein-triglyceride (VLDL-TG) in rat and human hepatocytes in vitro. In the present study, we investigated whether the transhepatic BS flux affects VLDL-TG concentration and hepatic VLDL-TG secretion in vivo. In rats, the transhepatic BS flux was quantitatively manipulated by 1-week chronic bile diversion (BD), followed by intraduodenal infusion with taurocholate (TC) or saline for 6 h. In mice, the transhepatic BS flux was manipulated by a 3-week dietary supplementation with TC (0.5 wt.%) or cholestyramine (2 wt.%). In rats, BD followed by saline or TC infusion did not affect plasma triacylglycerol (TG) concentration, hepatic TG production rate or VLDL lipid composition, compared to control rats. In mice supplemented for 3 weeks with TC or cholestyramine, the transhepatic BS flux was increased by 335% and decreased by 48%, respectively, compared to controls. Among the three experimental groups of mice, an inverse relationship between transhepatic BS flux and either plasma TG concentration (R(2)=0.89) or VLDL-TG production rate (R(2)=0.87) was observed, but differences were relatively small. Present data support the concept that BS can reduce VLDL-TG concentration and inhibit hepatic TG secretion in vivo; however, this occurs only at supraphysiological transhepatic BS fluxes in mice.     

Fat malabsorption in essential fatty acid-deficient mice is not due to impaired bile formation

Essential fatty acid (EFA) deficiency induces fat malabsorption, but the pathophysiological mechanism is unknown. Bile salts (BS) and EFA-rich biliary phospholipids affect dietary fat solubilization and chylomicron formation, respectively. We investigated whether altered biliary BS and/or phospholipid secretion mediate EFA deficiency-induced fat malabsorption in mice. Free virus breed (FVB) mice received EFA-containing (EFA(+)) or EFA-deficient (EFA(-)) chow for 8 wk. Subsequently, fat absorption, bile flow, and bile composition were determined. Identical dietary experiments were performed in multidrug resistance gene-2-deficient [Mdr2((-/-))] mice, secreting phospholipid-free bile. After 8 wk, EFA(-)-fed wild-type [Mdr2((+/+))] and Mdr2((-/-)) mice were markedly EFA deficient [plasma triene (20:3n-9)-to-tetraene (20:4n-6) ratio >0.2]. Fat absorption decreased (70.1 +/- 4.2 vs. 99.1 +/- 0.3%, P < 0.001), but bile flow and biliary BS secretion increased in EFA(-) mice compared with EFA(+) controls (4.87 +/- 0.36 vs. 2.87 +/- 0.29 microl x min(-1) x 100 g body wt(-1), P < 0.001, and 252 +/- 30 vs. 145 +/- 20 nmol x min(-1) x 100 g body wt(-1), P < 0.001, respectively). BS composition was similar in EFA(+)- and EFA(-)-fed mice. Similar to EFA(-) Mdr2((+/+)) mice, EFA(-) Mdr2((-/-)) mice developed fat malabsorption associated with twofold increase in bile flow and BS secretion. Fat malabsorption in EFA(-) mice is not due to impaired biliary BS or phospholipid secretion. We hypothesize that EFA deficiency affects intracellular processing of dietary fat by enterocytes.     

Inhibition of apolipoprotein B secretion by taurocholate is controlled by the N-terminal end of the protein in rat hepatoma McArdle-RH7777 cells

Bile salts (BS) inhibit the secretion of apolipoprotein B (apoB) and triacylglycerol (TG) in primary rat, mouse and human hepatocytes and in mice in vivo. We investigated whether lipidation of apoB into a lipoprotein particle is required for this inhibitory action of BS. The sodium/taurocholate co-transporting polypeptide (Ntcp) was co-expressed in McArdle-RH7777 (McA-RH7777) cells stably expressing the full-length human apoB100 (h-apoB100, secreted as TG-rich lipoprotein particles) or carboxyl-truncated human apoB18 (h-apoB18, secreted in lipid-free form). The doubly transfected cell lines (h-apoB/r-Ntcp) effectively accumulated taurocholic acid (TC). TC incubation decreased the secretion of endogenous rat apoB100 (-50%) and h-apoB18 (-35%), but did not affect secretion of rat apoA-I. Pulse-chase experiments (35S-methionine) indicated that the impaired secretion of radiolabeled h-apoB18 and h-apoB100 was associated with accelerated intracellular degradation. The calpain protease inhibitor N-acetyl-leucyl-leucyl-norleucinal (ALLN) partially inhibited intracellular apoB degradation but did not affect the amount of either h-apoB18 or h-apoB100 secreted into the medium, indicating that inhibition of apoB secretion by TC is not due to calpain-dependent proteasomal degradation. We conclude that TC does not inhibit apoB secretion by interference with its lipidation, but rather involves a mechanism dependent on the N-terminal end of apoB.     

The role of apolipoprotein E in the elimination of liposomes from blood by hepatocytes in the mouse

We evaluated the role of apolipoprotein E (apoE) in the clearance of neutral and negatively charged liposomes by hepatocytes in apoE-deficient mice. Negatively charged liposomes were cleared at identical rates in apoE-deficient and wild-type mice; neutral liposomes were cleared at a 3.6-fold slower rate in apoE-deficient mice. ApoE deficiency did not affect hepatic uptake of negatively charged liposomes but lowered that of neutral liposomes >5-fold. Hepatocyte uptake of neutral liposomes was reduced >20-fold in apoE-deficient mice; that of negatively charged liposomes remained unchanged. We conclude that uptake of neutral liposomes by hepatocytes is nearly exclusively apoE-mediated.     

β-Arrestin Recruitment and G Protein Signaling by the Atypical Human Chemokine Decoy Receptor CCX-CKR

Chemokine receptors form a large subfamily of G protein-coupled receptors that predominantly activate heterotrimeric G_i proteins and are involved in immune cell migration. CCX-CKR is an atypical chemokine receptor with high affinity for CCL19, CCL21, and CCL25 chemokines, but is not known to activate intracellular signaling pathways. However, CCX-CKR acts as decoy receptor and efficiently internalizes these chemokines, thereby preventing their interaction with other chemokine receptors, like CCR7 and CCR9. Internalization of fluorescently labeled CCL19 correlated with β-arrestin2-GFP translocation. Moreover, recruitment of β-arrestins to CCX-CKR in response to CCL19, CCL21, and CCL25 was demonstrated using enzyme-fragment complementation and bioluminescence resonance energy transfer methods. To unravel why CCX-CKR is unable to activate G_i signaling, CCX-CKR chimeras were constructed by substituting its intracellular loops with the corresponding CCR7 or CCR9 domains. The signaling properties of chimeric CCX-CKR receptors were characterized using a cAMP-responsive element (CRE)-driven reporter gene assay. Unexpectedly, wild type CCX-CKR and a subset of the chimeras induced an increase in CRE activity in response to CCL19, CCL21, and CCL25 in the presence of the G_i inhibitor pertussis toxin. CCX-CKR signaling to CRE required an intact DRY motif. These data suggest that inactive G_i proteins impair CCX-CKR signaling most likely by hindering the interaction of this receptor with pertussis toxin-insensitive G proteins that transduce signaling to CRE. On the other hand, recruitment of the putative signaling scaffold β-arrestin to CCX-CKR in response to chemokines might allow activation of yet to be identified signal transduction pathways.     

Epistatic effects of polymorphisms in genes from the renin-angiotensin, bradykinin, and fibrinolytic systems on plasma t-PA and PAI-1 levels

Tissue plasminogen activator (t-PA) and plasminogen activator inhibitor 1 (PAI-1) directly influence thrombus formation and degradation and thereby risk for arterial thrombosis. Activation of the renin-angiotensin system has been linked to the production of PAI-1 expression via the angiotensin II type 1 receptor (AT1R). In addition, bradykinin can induce the release of t-PA through a B2 receptor mechanism. In the present study, we aimed to investigate the epistatic effects of polymorphisms in genes from the renin-angiotensin, bradykinin, and fibrinolytic systems on plasma t-PA and PAI-1 levels in a large population-based sample (n=2527). We demonstrated a strong significant interaction within genetic variations of the bradykinin B2 gene (P=0.002) and between ACE and bradykinin B2 (p=0.003) polymorphisms on t-PA levels in females. In males, polymorphisms in the bradykinin B2 and AT1R gene showed the most strong effect on t-PA levels (P=0.006). In both females and males, the bradykinin B2 gene interacted with AT1R gene on plasma PAI-1 levels (P=0.026 and P=0.039, respectively). In addition, the current study found a borderline significant interaction between PAI 4G5G and ACE I/D on plasma t-PA and PAI-1 levels. These results support the idea that the interplay between the renin-angiotensin, bradykinin, and fibrinolytic systems might play an important role in t-PA and PAI-1 biology.     

Proteomic Analysis of Cattle Tick Rhipicephalus (Boophilus) microplus Saliva: A Comparison between Partially and Fully Engorged Females

The cattle tick Rhipicephalus (Boophilus) microplus is one of the most harmful parasites affecting bovines. Similarly to other hematophagous ectoparasites, R. microplus saliva contains a collection of bioactive compounds that inhibit host defenses against tick feeding activity. Thus, the study of tick salivary components offers opportunities for the development of immunological based tick control methods and medicinal applications. So far, only a few proteins have been identified in cattle tick saliva. The aim of this work was to identify proteins present in R. microplus female tick saliva at different feeding stages. Proteomic analysis of R. microplus saliva allowed identifying peptides corresponding to 187 and 68 tick and bovine proteins, respectively. Our data confirm that (i) R. microplus saliva is complex, and (ii) that there are remarkable differences in saliva composition between partially engorged and fully engorged female ticks. R. microplus saliva is rich mainly in (i) hemelipoproteins and other transporter proteins, (ii) secreted cross-tick species conserved proteins, (iii) lipocalins, (iv) peptidase inhibitors, (v) antimicrobial peptides, (vii) glycine-rich proteins, (viii) housekeeping proteins and (ix) host proteins. This investigation represents the first proteomic study about R. microplus saliva, and reports the most comprehensive Ixodidae tick saliva proteome published to date. Our results improve the understanding of tick salivary modulators of host defense to tick feeding, and provide novel information on the tick-host relationship.     

Damage of <Emphasis Type="Italic">Streptococcus mutans</Emphasis> biofilms by carolacton,a secondary metabolite from the myxobacterium <Emphasis Type="Italic">Sorangium cellulosum</Emphasis>

Background  

Streptococcus mutans is a major pathogen in human dental caries. One of its important virulence properties is the ability to form biofilms (dental plaque) on tooth surfaces. Eradication of such biofilms is extremely difficult. We therefore screened a library of secondary metabolites from myxobacteria for their ability to damage biofilms of S. mutans.     

Regional Calcium Accumulation and Cation Shifts in Rat Brain by Kainate

Abstract: Following local application of kainic acid, changes in the contents of Na⁺, K⁺, Ca²⁺, and Mg²⁺ of the striatum, cerebellum, and hippocampus of the rat were observed at various times after surgery. Within 1 h the levels of K⁺ decreased 20% whereas the levels of Na⁺ and Ca²⁺ increased at least 50% and 20%, respectively. These changes persisted for more than 8 weeks. Ca²⁺ levels rose further, to more than 10-fold during 8 weeks. The Mg²⁺ levels were slightly and only transiently decreased. Unilateral injections of kainate into the striatum affected the contents of these cations not only in this area, but also in the overlying cerebral cortex, the olfactory tubercle, and the ipsilateral substantia nigra. The Ca²⁺ increases were less when rats were kept on a diet deficient in Ca²⁺ and vitamin D. ⁴⁵Ca²⁺, intravenously administered, accumulated significantly more in the kainate-lesioned striatum and substantia nigra than in the homotopic contralateral areas. Electron microscopic examination of the localization of Ca²⁺ with the oxalate-pyroantimonate technique showed the appearance of extracellularly located deposits and the accumulation of Ca²⁺ in (possibly degenerating) myelinated axons in kainate-lesioned striata. This study provides evidence that calcification of cerebral tissue is closely associated with neurodegenerative processes and shows that kainate may serve as a tool to elucidate the mechanism of brain calcification. The results are discussed in relation to idiopathic calcinosis (striopallidodentate calcinosis, Fahr's disease).