Dependence on heparin chain-length of the interaction of heparin with human plasma low density lipoproteins

Apolipoprotein A-I regulates lipid hydrolysis by hepatic lipase

Association of hepatic lipase (HL) with pure heparan sulfate proteoglycans (HSPG) has little effect on hydrolysis of high density lipoprotein (HDL) particles, but significantly inhibits (>80%) the hydrolysis of low (LDL) and very low density lipoproteins (VLDL). Lipolytic inhibition is associated with a differential ability of the lipoproteins to remove HL from the HSPG. LDL and VLDL are unable to displace HL, whereas HDL readily displaces HL from the HSPG. These data show that HSPG-bound HL is inactive. Purified apolipoprotein (apo) A-I is more efficient than HDL at liberating HL from HSPG, and HL displacement is associated with the direct binding of apoA-I to HSPG. However, displacement of HL by apoA-I does not enhance hydrolysis of VLDL particles. This appears due to the direct inhibition of HL by apoA-I. Both apoA-I and HDL are able to inhibit VLDL lipid hydrolysis by up to 60%. Inhibition of VLDL hydrolysis is associated with the binding of apoA-I to the surface of the VLDL particle and a concomitant decreased affinity for HL. These data show that apoA-I can regulate lipid hydrolysis by HL by liberating/activating the enzyme from cell surface proteoglycans and by directly modulating lipoprotein binding and hydrolysis.

The potential mechanism for the effect of heparin on tissue plasminogen activator-mediated plasminogen activation

The effects and possible role of heparin on tissue plasminogen activator-mediated plasminogen activation was thoroughly investigated. Direct analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated that heparin increased the conversion of plasminogen to plasmin. Experiments by fluorescence quenching suggested that the stimulation of tissue plasminogen activator activity probably was due to a direct binding of heparin to tissue plasminogen activator, causing a conformational change of tissue plasminogen activator and rendering it more accessible to plasminogen interaction. The absence of additive stimulation effects on tissue plasminogen activator-mediated plasminogen activation when both heparin and fibrinogen were present also implied that both compounds interacted with tissue plasminogen activator via the same domain; it appeared to be most likely via the kringle-2 domain in tissue plasminogen activator based on studies using epsilon-aminocaproic acid as an inhibitor. Unlike heparin-induced stimulation of antithrombin-thrombin interaction, the heparin-induced stimulation of tissue plasminogen activator did not seem to follow a template model. Only in the presence of a high plasminogen or a low tissue plasminogen activator concentration, massive stimulation of tissue plasminogen activator activity was observed via a pseudotemplate model. The results suggest that precautions concerning high heparin dose should be given during its conjunctive clinical use with tissue plasminogen activator in thrombolytic therapy to reduce the risk of hemorrhage.

Investigation of phospholipid area compression induced by calcium-mediated dextran sulfate interaction

The association of anionic polyelectrolytes such as dextran sulfate (DS) to zwitterionic phospholipid surfaces via Ca(2+) bridges results in a perturbation of lipid packing at physiologically relevant Ca(2+) concentrations. Lipid area compression was investigated in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) multilamellar bilayer dispersions by (2)H-NMR and in monolayer studies. Binding of DS to DMPC surfaces via Ca(2+) results in denser lipid packing, as indicated by higher lipid chain order. DMPC order parameters are homogeneously increased throughout the lipid bilayer. Higher order translates into more extended hydrocarbon chains and decreased average lipid area per molecule. Area compression is reported as a function of DS concentration and molecular weight. Altering the NaCl and Ca(2+) concentrations modified electrostatic interactions between DS and phospholipid. A maximal area reduction of DeltaA = 2.7 A(2) per DMPC molecule is observed. The lipid main-phase transition temperature increases upon formation of DMPC/Ca(2+)/DS-complexes. Lipid area compression after addition of DS and Ca(2+) to the subphase was also observed in monolayer experiments. A decrease in surface tension of up to 3.5 mN/m at constant molecular area was observed. DS binds to the lipid headgroups by formation of Ca(2+) bridges without penetrating the hydrophobic region. We suggest that area compression is the result of an attractive electrostatic interaction between neighboring lipid molecules induced by high local Ca(2+) concentration due to the presence of DS. X-ray diffraction experiments demonstrate that DS binding to apposing bilayers reduces bilayer separation. We speculate that DS binding alters the phase state of low-density lipoproteins that associate with polyelectrolytes of the arterial connective tissue in the early stages of arteriosclerosis.

Ca2+-mediated interaction between dextran sulfate and dimyristoyl-sn-glycero-3-phosphocholine surfaces studied by 2H nuclear magnetic resonance

The binding of dextran sulfates (DSs) with varying chain lengths to phosphatidylcholine multilamellar vesicles was investigated as a function of polyelectrolyte, NaCl, and Ca2+ concentration. Attractive forces between negatively charged polyelectrolytes and zwitterionic phospholipids arise from the assembly of calcium bridges. The formation of calcium bridges between the sulfate groups on the dextran sulfate and the phosphate group of the lipid results in increased calcium binding in mixtures of DS and 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). At high NaCl concentration, the plateau adsorption of DS 500 is increased. The strength of dextran sulfate binding to DMPC is reflected in the changes of the 2H NMR quadrupolar splittings of the headgroup methylenes. Association forces increase with the number of calcium bridges formed. Low-molecular-weight DS does not bind to DMPC surfaces whereas longer-chain DSs strongly influence headgroup structure as a result of strong association. DS binding increases with increasing concentration; however, further association of the polyelectrolyte can be promoted only if negative charges are sufficiently screened. DS binding to lipid bilayers is a complicated balance of calcium bridging and charge screening. From our data we postulate that the structure of the adsorbed layer resembles a lattice of DS strands sandwiched between the bilayer lamellae.

Retrograde axonal transport of the alpha-subunit of the GTP-binding protein GZ in mouse sciatic nerve: a potential pathway for signal transduction in neurons

We have utilized antibodies against the alpha subunit of GZ in fluorescence immunohistochemistry to determine whether this GTP-binding protein can translocate along nerves by intra-axonal transport. After ligation of the mouse sciatic nerve we found an increase in GZ-like immunoreactivity on the proximal and distal side with time, suggesting that the alpha subunit undergoes orthograde axonal transport and also returns to the cell body by retrograde axonal transport in the sciatic nerve. Unlike the retrograde transport of Gi alpha, shown in a previous study to be present in most sciatic axons, GZ alpha only accumulated in a subpopulation of axons, suggesting that different G-proteins could convey information specific to neuronal subtypes. These results support our proposal that GZ may play a second messenger role in communicating information from the terminals back to cell bodies. Gi alpha and GZ alpha may be representative of relatively stable signalling molecules by which the signal from some neurotrophic molecules can be translocated from the neuron periphery to the cell body without the need for the retrograde transport of the neurotrophic factor itself.