Glucosylceramide is synthesized at the cytosolic surface of various Golgi subfractions

In our attempt to assess the topology of glucosylceramide biosynthesis, we have employed a truncated ceramide analogue that permeates cell membranes and is converted into water soluble sphingolipid analogues both in living and in fractionated cells. Truncated sphingomyelin is synthesized in the lumen of the Golgi, whereas glucosylceramide is synthesized at the cytosolic surface of the Golgi as shown by (a) the insensitivity of truncated sphingomyelin synthesis and the sensitivity of truncated glucosylceramide synthesis in intact Golgi membranes from rabbit liver to treatment with protease or the chemical reagent DIDS; and (b) sensitivity of truncated sphingomyelin export and insensitivity of truncated glucosylceramide export to decreased temperature and the presence of GTP-gamma-S in semiintact CHO cells. Moreover, subfractionation of rat liver Golgi demonstrated that the sphingomyelin synthase activity was restricted to fractions containing marker enzymes for the proximal Golgi, whereas the capacity to synthesize truncated glucosylceramide was also found in fractions containing distal Golgi markers. A similar distribution of glucosylceramide synthesizing activity was observed in the Golgi of the human liver derived HepG2 cells. The cytosolic orientation of the reaction in HepG2 cells was confirmed by complete extractability of newly formed NBD-glucosylceramide from isolated Golgi membranes or semiintact cells by serum albumin, whereas NBD-sphingomyelin remained protected against such extraction.     

Divergent effects of cycloheximide on the induction of class II and class III cytochrome P450 mRNAs in cultures of adult rat hepatocytes

We have previously reported that when hepatocytes isolated from adult male rats are cultured in serum-free medium on matrigel, a reconstituted basement membrane gel, it is possible to elicit a stimulation of gene expression for both Class II cytochrome P450b/e and Class III cytochrome P450p by phenobarbital treatment (E.G. Schuetz et al., 1990 J. Biol. Chem. 265, 1188-1192). In the present study, an investigation of the requirement of protein synthesis for the rise in mRNAs for these cytochromes, pretreatment of the cells with cycloheximide prior to adding phenobarbital or "phenobarbital-like" inducers to the culture medium inhibited induction of P450b/e mRNA (46-90%), whereas the accumulation of P450p mRNA was enhanced (2- to 19-fold). Heme depletion did not appear to explain these observations because the inhibitory effects of cycloheximide on the induction of P450b/e mRNA were not overcome by supplementation of the medium with exogenous heme or with delta-aminolevulinic acid. Because Class IIIA P450s are regulated by gender as well as by phenobarbital, we examined the basal expression of P450p mRNA in cultures of hepatocytes derived from male rats and found that cycloheximide treatment was without effect. However, in cultures of hepatocytes isolated from female rats, where P450p mRNA is barely detectable, cycloheximide treatment greatly enhanced expression of P450p mRNA. As was observed in the cultured cells, the treatment of living female rats with cycloheximide also increased the amounts of P450p mRNA to levels comparable to those found in livers of untreated male rats. Analysis of Northern blots hybridized with oligonucleotides specific for P450PCN1(IIIA1) and P450PCN2(IIIA2), respectively, revealed that untreated male rat liver and cultures of hepatocytes prepared from these animals expressed readily detectable amounts of P450PCN1(IIIA1) mRNA. Such analyses confirmed that cycloheximide treatment selectively increased P450PCN1(IIIA1) mRNA in female rat liver, whereas the amount of mRNA for P450PCN2(IIIA2), a closely related male-specific family member, was unaffected. We conclude that the pathways for the induction of P450b/e and P450p by phenobarbital, and the pathways for the gender-specific basal expression of P450PCN1(IIIA1) and P450PCN2(IIIA2) are not the same and can be distinguished by their differential response to inhibition of ongoing protein synthesis.     

Gramicidin A induced fusion of large unilamellar dioleoylphosphatidylcholine vesicles and its relation to the induction of type II nonbilayer structures.

The fusogenic properties of gramicidin were investigated by using large unilamellar dioleoylphosphatidylcholine vesicles. It is shown that gramicidin induces aggregation and fusion of these vesicles at peptide to lipid molar ratios exceeding 1/100. Both intervesicle lipid mixing and mixing of aqueous contents were demonstrated. Furthermore, increased static and dynamic light scattering and a broadening of 31P NMR signals occurred concomitant with lipid mixing. Freeze-fracture electron microscopy revealed a moderate vesicle size increase. Lipid mixing is paralleled by changes in membrane permeability: small solutes like carboxyfluorescein and smaller dextrans, FD-4(Mr approximately 4000), rapidly (1-2 min) leak out of the vesicles. However, larger molecules like FD-10 and FD-17 (Mr approximately 9400 and 17,200) are retained in the vesicles for greater than 10 min after addition of gramicidin, thereby making detection of contents mixing during lipid mixing possible. At low lipid concentrations (5 microM), lipid mixing and leakage are time resolved: leakage of CF shows a lag phase of 1-3 min, whereas lipid mixing is immediate and almost reaches completion during this lag phase. It is therefore concluded that leakage, just as contents mixing, occurs subsequent to aggregation and lipid mixing. Although addition of gramicidin at a peptide/lipid molar ratio exceeding 1/50 eventually leads to hexagonal HII phase formation and a loss of vesicle contents, it is concluded that leakage during fusion (1-2 min) is not the result of HII phase formation but is due to local changes in lipid structure caused by precursors of this phase. By making use of gramicidin derivatives and different solvent conformations, it is shown that there is a close parallel between the ability of the peptide to induce the HII phase and its ability to induce intervesicle lipid mixing and leakage. It is suggested that gramicidin-induced fusion and HII phase formation share common intermediates.     

Tumor modulation of autoreactivity: decreased macrophage and autoreactive T cell interactions

The autologous mixed lymphocyte reaction (AMLR) is an in vitro measure of autoreactivity, a key mechanism in immune homeostasis. In this system, macrophages (M phi) act as accessory cells to autoreactive L3T4+ T cells by presenting self-Ia and releasing soluble modulators. During tumor growth, changes occur in M phi and T cells. Tumor-bearing host (TBH) M phi have a reduced ability to act as accessory cells. In fact, TBH M phi suppressed autoreactivity by 60-70%. The decrease in TBH M phi or T-cell abilities was not due to differences in cell numbers or incubation time. Because tumor growth causes increased prostaglandin E2 (PGE2) production by M phi, indomethacin was used to assess the contribution of prostaglandins. Normal and TBH T-cell reactivity increased nearly 50% when stimulated by normal host M phi, while normal and TBH T-cell reactivity increased nearly 100% when stimulated by TBH M phi. Thus increased prostaglandin production is partly responsible for the increased TBH suppressor M phi activity and in the normal host, suppressor M phi may be responsible for maintaining immune regulation. To assess the direct role of prostaglandins in T-cell hyporesponsiveness, PGE2 was titrated into the cultures. PGE2 suppressed normal and TBH T-cell responsiveness in a dose-dependent manner. Normal host T cells were suppressed to a greater extent than TBH T cells by PGE2 (66% versus 42% suppression, respectively). Reduced Ia expression and active suppressor mechanisms are not the only mechanisms mediating hypoautoreactivity during tumor growth. TBH autoreactive L3T4+ T cells were less responsive to self-Ia; they were only 60-80% as reactive as their normal counterparts. To address whether the helper T (TH)-cell defect involved cytokines, T cells were treated with interleukin (IL)-1, IL-2, and IL-4. In all cases, the TBH T-cell response to the factors was decreased (only 60-75% as reactive as normal T cells). Because TBH M phi-mediated suppression can override the addition of IL-1, IL-2, and IL-4, indomethacin was also added with the exogenous interleukins. This coaddition significantly enhanced normal host autoreactivity above control levels while TBH autoreactivity (the combination of TBH T cells and TBH M phi) only returned to normal host unstimulated levels. Tumor growth modulates the immune response at least by (i) decreasing the accessory cell abilities of TBH M phi through decreased Ia expression and increased production of suppressive molecules such as prostaglandins; and (ii) decreasing the responsiveness to immune enhancing factors by TH cells.     

Hereditary motor endplate disease (med) of the mouse: Observations on dissociated myogenic cells and their development in culture

Summary Myogenic cells from mice homozygous for the lethal mutation motor endplate disease (med/med) were grown in culture. Like muscle cells taken from wild type (+/?) litter mates they fused to form myotubes which contracted, developed cross striations, and exposed acetylcholine receptors (AChR) on their surface. However, a decrease of 30% in the number of mononucleated cells per unit fresh weight of muscle was observed as early as 2–3 days postnatal, i.e., at least one week prior to the onset of physiological symptoms. Hence, in addition to influencing the functional maintenance of motor endplates, the med gene seems to control early events in muscle development.     

Human transfer factor: fractionation and biologic activity.

Human transfer factor (TF) was fractionated by exclusion chromatography and the fractions were tested for biologic activity in vivo and in vitro. Specific TF activity in vivo was found to reside in the major UV-absorbing peak (Fraction III). Fraction III eluted at 2.7 X V(O) and transferred tuberculin, candida, or KLH-reactivity to previously negative recipients. Fraction III from nonreactive donors was ineffective. When the fractions were tested in vitro, we found that both the mitogenic activity of whole TF and the suppressive activity to mitogen activation when present in TF was found in Fraction I. Fraction III contained components responsible for augmentation of PHA and PWM responses. In addition, Fraction III contained the component responsible for antigen-dependent augmentation of lymphocyte transformation. Fraction IV was suppressive to antigen-induced lymphocyte transformation. These data suggest that TF preparations contain components which can affect immune reactions in both specific and nonspecific ways.     

D-alanyl-D-alanine carboxypeptidase in the bacterial form and L-form of Proteus mirabilis.

Membranes of the bacterial form and the stable and unstable L-forms of Proteus mirabilis contain LD and DD-carboxypeptidase. The DD-carboxypeptidase is inhibited non-competitively by penicillin G. The enzyme of the bacterial form is highly penicillin-sensitive (Ki - 4 X 10(-9) M penicillin G). Inhibition is only partly reversible by treatment with penicillinase or by dialysis against buffer. In contrast, the DD-carboxypeptidase of the unstable L-form, grown in the presence of penicillin, is 175-fold less penicillin-sensitive (Ki = 7 X 10(7) M penicillin G). Inhibition is completely reversed by penicillinase or dialysis. After inhibition by penicillin and subsequent reactivation the penicillin sensitivity of the bacterial DD-carboxtpeptidase is similar to the sensitivity of the enzyme of the unstable L-form. The hypothesis is proposed that P. mirabilis contains two DD-carboxypeptidases of different penicillin sensitivity and with different mechanisms of penicillin binding. Peptidoglycan synthesis in the cell walls of the unstable L-form is probably carried out with the help of only one DD-carboxypeptidase, viz. the completely reactivatable enzyme with the lower penicillin sensitivity.     

Human plasma R-type vitamin B12-binding proteins. II. The role of transcobalamin I, transcobalamin III, and the normal granulocyte vitamin B12-binding protein in the plasma transport of vitamin B12.

The normal human granulocyte vitamin B12-binding protein, transcobalamin I, and transcobalamin III, have been labeled with 125I-labeled N-succinimidyl 3-(4-hydroxyphenyl)propionate and utilized for plasma clearance studies performed with rabbits. Both moieties of 125I-labeled granulocyte vitamin B12-binding protein-[57Co]vitamin B12 were cleared rapidly from the plasma (is less than 90% by 5 min) by the liver. After 30 min, the bulk of the 125I reappeared in the plasma in small molecular weight (less than 1000) form and was rapidly excreted in the urine. After 60 min the bulk of the [57Co]vitamin B12 reappeared in the plasma bound to rabbit transcobalamin II and was subsequently taken up by a variety of tissues. Approximately 15% of the 125I-labeled granulocyte vitamin B12-binding protein-[57Co-a1vitamin B12 was excreted intact into the bile during the period from 10 to 80 min after injection. The hepatic uptake of the protein-vitamin B12 complex was blocked by the prior injection of desialyzed fetuin but not by native fetuin. Similar results were obtained with 125I-labeled transcobalamin III-[57Co]vitamin B12. Approximately 90% of both moieties of 125I-labeled transcobalamin I-[57Co]vitamin B12 had prolonged plasma survivals similar to that of 125I-labeled bovine serum albumin. After treatment with neuraminadase, both moieties of the 125I-labeled transcobalamin I-[57Co]vitamin B12 complex were cleared rapidly from the plasma by the liver in a manner that was indistinguishable from that observed in the case of untreated granulocyte vitamin B12-binding protein and transcobalamin III. These observations indicate that desialyzed transcobalamin I and the native forms of the granulocyte vitamin B12-binding protein and transcobalamin III are cleared from plasma by the mechanism elucidated by Ashwell and Morell (Ashwell, G., and Morell A. G. (1974) Adv. Enzymol. 41, 99-128) that is capable of clearing a wide variety of asialoglycoproteins. These observations have implications concerning the function of the human R-type vitamin B12-binding proteins, the nature of the enterohepatic circulation of vitamin B12, the biological significance of the mechanism described by Ashwell and Morell, and the etiology of the increased plasma concentration of human R-type protein that occurs frequently in chronic myelogenous leukemia and occasionally in hepatocellular carcinoma and other solid tumors.     

Characterization of the binding of human growth hormone to microsomal membranes from rat liver.

The binding of 125I-labelled human growth hormone to the 100000g microsomal membrane fraction prepared from the livers of normal female rats was dependent on time, temperature, pH, membrane concentration and concentration of 125I-labelled human growth hormone. At 22 degrees C binding reached a steady state after 16h, with the mean maximal specific binding being 20% of the tracer initially added. Dissociation of 125I-labelled human growth hormone from the membranes, after addition of excess of unlabelled hormone, was relatively slow with a half-time greater than 24h. Only minor degradation of the 125I-labelled human growth hormone was observed during incubation with membranes for 16 or 25h at 22 degrees C. Similarly, no significant change in the ability of membranes to bind human growth hormone was evident after preincubation of the membranes for 16 or 25h. Specificity studies showed that up to 90% of the 125I-labelled human growth hormone bound could be displaced by 1 mug of unlabelled hormone. Ovine prolactin also showed considerable competition for the binding site. Non-primate growth-hormone preparations (ovine, bovine, porcine and rat) and non-related hormones (insulin, thyrotropin, lutropin and follitropin) all showed negligible competition. Scatchard analysis of the binding data was consistent with two classes of binding site with binding affinities of 0.64 X 10(10) +/- 0.2 X 10(10)M-1 and 0.03 X 10(10) +/- 0.007 X 10(10)M-1 and corresponding binding capacities of 98.4 +/- 10 fmol/mg of protein and 314.6 +/- 46.3 fmol/mg of protein. These studies provide data which, in general, are consistent with the criteria required for hormone-receptor interaction. However, proof of the thesis that the human-growth-hormone-binding sites in female rat liver represent physiological receptors must await the demonstration of a correlation between hormone binding and a biological response.