Low-density lipoprotein receptor binding determinants switch from apolipoprotein E to apolipoprotein B during conversion of hypertriglyceridemic very-low-density lipoprotein to low-density lipoproteins

Using thrombin and trypsin as probes, we determined: first, that low-density lipoprotein (LDL) receptor binding determinants switch from apolipoprotein (apo) E to apo-B within the very-low-density lipoprotein (VLDL) Sf 20-60 region of the metabolic cascade from VLDL1 (Sf 100-400) of hypertriglyceridemic (HTG) human subjects to LDL. Second, two different conformations of apo-E exist in HTG-VLDL Sf greater than 60, one accessible (greater than or equal to 1 mol/mol of particle) and one inaccessible (1-2 mol/mol) to both thrombin and the LDL receptor; normal VLDL (Sf greater than 60) have only the inaccessible conformation and therefore do not bind to the LDL receptor. Third, thrombin degrades apo-B into large fragments, three of which have electrophoretic mobilities similar to B-48, B-74, and B-26; this, however, has no effect on apo-B-mediated receptor binding. Fibroblast studies showed that thrombin could abolish receptor uptake of HTG-VLDL1 and HTG-VLDL2 (Sf 60-100), had little or no effect on HTG-VLDL3 (Sf 20-60), and no effect on uptake of intermediate-density lipoprotein (IDL) or LDL. Trypsin abolished the binding of HTG-VLDL1 and HTG-VLDL2, reduced that of HTG-VLDL3, but had little to no effect on IDL or LDL binding. Immunochemical techniques revealed that thrombin cleaved some apo-E into the E-22 and E-12 fragments; after trypsin treatment no apo-E was detected in any HTG-lipoprotein. Normal VLDL subclasses contained less apo-E than the corresponding HTG-VLDL subclasses and it was not cleaved by thrombin. Apo-B immunoreactivities of VLDL subclasses were not significantly changed after treatment with thrombin, although thrombin cleaved some of the B-100 of each VLDL subclass, and all apo-B in IDL and LDL, into 4-6 major large fragments. Trypsin converted all of the apo-B of each lipoprotein into smaller fragments (Mr less than 100,000). We conclude that apo-E of the thrombin-accessible conformation mediates uptake of HTG-VLDL1 and HTG-VLDL2 but that apo-B alone is sufficient to mediate receptor binding of IDL and LDL; the switch from apo-E to apo-B as the primary or sufficient binding determinant occurs within the VLDL3 (Sf 20-60) region of the metabolic cascade, where receptor binding first appears in VLDL subclasses from normal subjects.

Leukotriene B4 binding to human neutrophils

[3H] Leukotriene B4 (LTB4) binds concentration dependently to intact human polymorphonuclear leukocytes (PMN's). The binding is saturable, reaches equilibrium in 10 min at 4 degrees C, and is readily reversible. Mathematical modeling analysis reveals biphasic binding of [3H] LTB4 indicating two discrete populations of binding sites. The high affinity binding sites have a dissociation constant of 0.46 X 10(-9)M and Bmax of 1.96 X 10(4) sites per neutrophil; the low affinity binding sites have a dissociation constant of 541 X 10(-9)M and a Bmax of 45.16 X 10(4) sites per neutrophil. Competitive binding experiments with structural analogues of LTB4 demonstrate that the interaction between LTB4 and the binding site is stereospecific, and correlates with the relative biological activity of the analogs. At 25 degrees C [3H] LTB4 is rapidly dissociated from the binding site and metabolized to 20-OH and 20-COOH-LTB4. Purification of neutrophils in the presence of 5-lipoxygenase inhibitors significantly increases specific [3H] LTB4 binding, suggesting that LTB4 is biosynthesized during the purification procedure. These data suggest that stereospecific binding and metabolism of LTB4 in neutrophils are tightly coupled processes.

Evidence for mediation of acetyl glyceryl ether phosphorylcholine stimulation of adenosine 3',5'-(cyclic)monophosphate levels in human polymorphonuclear leukocytes by leukotriene B4

Acetyl glyceryl ether phosphorylcholine induces human neutrophil aggregation. Incubation of neutrophils with either prostaglandin I2, or the cyclic AMP-dependent phosphodiesterase inhibitor, RO 20-1724 before the addition of PAF-acether attenuates subsequent aggregation. Paradoxically, a small elevation in cyclic AMP is observed coincident with the initiation of PAF-acether-stimulated aggregation. The elevation in cyclic AMP in response to PAF-acether is amplified by RO 20-1724, and the magnitude of the response is dependent upon the concentration of PAF-acether. The elevation in cyclic AMP is not due to prostaglandins, because indomethacin actually enhances the elevation in cyclic AMP induced by PAF-acether. The involvement of the neutrophil 5-lipoxygenase, and subsequent leukotriene B4 synthesis, is suggested by the observation that 5-lipoxygenase inhibitors limit both the elevation in cyclic AMP induced by PAF-acether, and the indomethacin enhancement. This indirect evidence is supported by the fact that leukotriene B4 itself elevates neutrophil cyclic AMP levels in intact cells, and stimulates the adenylate cyclase in broken cell preparations. Although the elevation in cyclic AMP induced by either PAF-acether or leukotriene B4 is coincident with the onset of neutrophil aggregation, it is not obligatory for aggregation. The adenylate cyclase inhibitor 2',5'-dideoxyadenosine blocks the PAF-acether-stimulated increase in cyclic AMP, and actually enhances aggregation. It is suggested that the increase in cyclic AMP observed after the addition of PAF-acether is due to concomitant leukotriene B4 synthesis, and is not obligatory for neutrophil aggregation, but is actually part of a feed-back regulatory system through which PAF-acether and leukotriene B4 can limit their own activity in neutrophils.

Acetyl glyceryl ether phosphorylcholine stimulates leukotriene B4 synthesis and cyclic AMP accumulation in human polymorphonuclear leukocytes

Both LTB4 and AGEPC stimulate PMN aggregation in a concentration-dependent manner. The lipoxygenase inhibitors ETYA and NDGA block AGEPC-induced but not LTB4-induced PMN aggregation. Agents that elevate PMN cyclic AMP levels block both LTB4-and AGEPC-induced aggregation. Paradoxically, shortly after aggregation is initiated by either AGEPC or LTB4, a transient spike in cyclic AMP levels is observed. Subsequent experiments show that LTB4 can directly elevate cyclic AMP in PMNs. Reverse-phase high-pressure liquid chromatography, coupled with selective ion gas chromatography/mass spectrometry, shows that AGEPC stimulates neutrophils to synthesize LTB4. These data suggest that AGEPC-induced neutrophil aggregation is mediated by LTB4.

Acetyl glyceryl ether phosphorylcholine stimulates leukotriene B4 synthesis in human polymorphonuclear leukocytes

Acetyl glyceryl ether phosphorylcholine (AGEPC) and leukotriene B4 (LTB4) induce concentration-dependent neutrophil aggregation. On a molar basis, LTB4 is approximately 10 to 100 times more potent than AGEPC. AGEPC-induced aggregation is attenuated by two inhibitors of arachidonate lipoxygenation, eicosatetraynoic acid and nordihydroguaiaretic acid, and to a lesser extent by the cyclooxygenase inhibitor, indomethacin. LTB4-induced aggregation is not readily reduced by the above inhibitors of arachidonic acid metabolism. Reverse phase high performance liquid chromatography, coupled with selective ion gas chromatography/mass spectrometry, shows that AGEPC stimulates neutrophils to synthesize sufficient LTB4 to account for the AGEPC response. In addition, the rate of LTB4 biosynthesis in response to AGEPC correlates well with the rate of AGEPC- and/or LTB4-induced neutrophils aggregation, and desensitization experiments indicate that AGEPC and LTB4 cross-desensitize. These data suggest that AGEPC-induced neutrophil aggregation may be mediated by LTB4.

Mechanism of interaction of CC-1065 (NSC 298223) with DNA

CC-1065 (NSC 298223), a potent new antitumor antibiotic produced by Streptomyces zelensis, interacts strongly with double-stranded DNA and appears to exert its cytotoxic effects through disruption of DNA synthesis. We undertook this study to elucidate the sites and mechanisms of CC-1065 interaction with DNA. The binding of CC-1065 to synthetic and native DNA was examined by differential circular dichroism or by Sephadex chromatography with photometric detection. The binding of CC-1065 with calf thymus DNA was rapid, being complete within 2 hr, and saturated at 1 drug per 7 to 11 base pairs. The interaction of CC-1065 with synthetic DNA polymers indicated a specificity for adenine- and thymine-rich sites. Agarose gel electrophoresis of CC-1065-treated supercoiled DNA showed that CC-1065 did not intercalate. Site exclusion studies using substitutions in the DNA grooves showed CC-1065 to bind primarily in the minor groove. CC-1065 did not cause DNA breaks; it inhibited susceptibility of DNA to nuclease S1 digestion. It raised the thermal melting temperature of DNA, and it inhibited the ethidium-induced unwinding of DNA. Thus, in contrast to many antitumor agents, CC-1065 stabilized the DNA helix. DNA helix overstabilization may be relevant to the mechanism of action of CC-1065.

Evaluating statistical analyses and reproducibility of microbial mutagenicity assays

The NCI/NTP has completed the first phase of a 4-laboratory study on the reproducibility of testing chemicals for mutagenicity in the Salmonella/microsome assay. This paper is a report of the statistical analysis of some of that data. This analysis involved (1) identifying and removing spurious data; (2) determining the adequacy of the remaining data in making a decision on the mutagenicity of the test chemical; (3) performing the statistical tests; and (4) interpreting the results. Using this procedure, 7 approaches were used to determine the mutagenicity of a test. These approaches were the (1) 2-fold rule, (20 modified 2-fold rule, (3) one-way analysis of variance (homogeneity test), (4) test for linear trend, (5) combination of 3 and 4, (6) 97.5th percentile threshold rule and (7) confidence interval threshold rule. The conclusions drawn by each rule were compared to the microbiologists' interpretation, and the results of these comparisons were presented. In addition, the strengths and weakness of each rule were discussed. The reproducibility of the assay in this study was examined, and a discussion of the significance of these results was presented.