Evidence that C4b-binding protein (proline-rich protein) is synthesized by hepatocytes

C4b-binding protein (C4bp), a glycoprotein involved in regulating the classical pathway of the complement system, binds the activated form of C4b and accelerates the decay rate of the C4b, C2a complex. Recently, sequence analysis of the cDNA for proline-rich protein (PRP) demonstrated that PRP is identical with C4bp. We measured the concentration of C4bp in serum by single radial immunodiffusion in patients with various liver diseases. Concentration of C4bp was significantly lower in hepatic cirrhosis (P = 0.001) and higher in fatty liver (P = 0.0002) than the control values, after adjusting for age, sex, and concentration of total cholesterol, triglyceride, and C-reactive protein. Significant positive correlations were observed between the concentration of C4bp in serum and total protein, albumin, cholinesterase level, and lecithin-cholesterol acyltransferase activity. Immunohistochemical analysis of human liver with specific antiserum to human C4bp demonstrated reaction endproducts in the hepatocytes around the central veins. These observations provide evidence that C4bp is synthesized by hepatocytes.     

Hemopexin is a developmentally regulated, acute-phase plasma protein in the chicken

Identity has been established between chicken hemopexin and alpha 1-globulin "M," a plasma known for the hormone responsiveness of its synthesis in monolayer cultures of embryonic chicken hepatocytes (Grieninger, G., Plant, P. W., Liang, T. J., Kalb, R. G., Amrani, D., Mosesson, M. W., Hertzberg, K. M., and Pindyk, J. (1983) Ann. N. Y. Acad. Sci. 408, 469-489). Identification was based on immunological cross-reactivity, electrophoretic behavior on sodium dodecyl sulfate-polyacrylamide gels, heme-binding capacity, and pattern of cleavage by proteolytic enzymes. Electroimmunoassays were used to investigate plasma protein levels, particularly those of hemopexin, in the acute-phase response and embryonic development. Acute-phase plasma protein production, elicited by injection of chickens with turpentine, bore many similarities to the pattern of hepatocellular plasma protein synthesis produced in response to the addition of specific hormones in culture. The response of the stressed chickens included elevated levels of hemopexin and fibrinogen (5- and 2-fold, respectively) accompanied by a 50% drop in albumin. Hemopexin levels of developing chick embryos were measured for several days before and after hatching. Onset of hemopexin production occurred around the time of hatching, and was followed by a steep increase (more than 1000-fold over 4 days). Similarly, it was not until the 12th h of culture that hepatocytes isolated from both early and late stage chicken embryos began to produce hemopexin, although, from their initiation in culture, they secreted a number of other plasma proteins in quantity. After 12 h, hepatocellular output of hemopexin rapidly accelerated. This precocious induction ex vivo required no hormonal or macromolecular medium supplements. These observations indicate that the embryonic chicken hepatocyte culture system will provide a useful model for studying the regulation of hemopexin biosynthesis in hepatic development and the acute-phase response.     

Evidence that the rate of phosphatidylcholine catabolism is regulated in cultured rat hepatocytes

The regulation of phosphatidylcholine (PC) catabolism has been studied in choline-deficient rat hepatocytes. Supplementation of choline-deficient hepatocytes, prelabeled with [3H]choline, with 100 microM choline increased the rate of PC catabolism by approx. 2-fold. The major product of PC degradation was glycerophosphocholine in both choline-deficient and choline-supplemented cells. Choline supplementation decreased the radioactivity recovered in lysoPC by 50%. This effect was accompanied by a 2-fold increase of labeled glycerophosphocholine. Comparable results were obtained when PC of the cells was prelabeled with [3H]methionine or [3H]glycerol. The activity of phospholipase A in cytosol, mitochondria and microsomes isolated from choline-deficient rat liver was similar to the activity in control liver, when determined with [3H]PC vesicles as the substrate. Measurement of the activity of phospholipase A with endogenously [3H]choline-labeled PC showed that the formation of lysoPC in mitochondria isolated form choline-supplemented cells was 40% lower than in choline-deficient cells. Alternatively, the formation of [3H]glycerophosphocholine and [3H]choline in microsomes from choline-supplemented cells was significantly higher (1.4-fold) than in microsomes from choline-deficient cells. These results suggest that the rate of PC catabolism is regulated in rat hepatocytes and that the concentration of PC might be an important regulatory factor.     

Evidence that adriamycin resistance in Chinese hamster lung cells is regulated by phosphorylation of a plasma membrane glycoprotein

Incubation of adriamycin resistant Chinese hamster lung cells with low levels of N-ethylmaleimide (NEM) results in a major increase in the cellular accumulation of drug. When resistant cells are prelabeled with [³²Pi] and thereafter treated with NEM there also occurs a selective superphosphorylation of an 180K plasma membrane glycoprotein (P-180). This phosphorylation reaction occurs at both serine and threonine residues. In similar experiments with drug sensitive cells only minor levels of this protein can be detected. Detailed studies have established that in cells which have reverted to drug sensitivity there is a parallel loss in the presence of phosphorylated P-180. Also in cells which have undergone partial reversion to drug sensitivity there is a correlation between levels of superphosphorylated P-180 and adriamycin resistance. These results provide evidence that adriamycin resistance is dependent on the presence of P-180. The results also suggest that the biological activity of this protein is highly regulated by phosphorylation and that in the superphosphorylated state P-180 is inactive and under these conditions the resistant cell is converted to a drug sensitive phenotype.     

MCL-1 is a stress sensor that regulates autophagy in a developmentally regulated manner

Apoptosis has an important role during development to regulate cell number. In differentiated cells, however, activation of autophagy has a critical role by enabling cells to remain functional following stress. In this study, we show that the antiapoptotic BCL-2 homologue MCL-1 has a key role in controlling both processes in a developmentally regulated manner. Specifically, MCL-1 degradation is an early event not only following induction of apoptosis, but also under nutrient deprivation conditions where MCL-1 levels regulate activation of autophagy. Furthermore, deletion of MCL-1 in cortical neurons of transgenic mice activates a robust autophagic response. This autophagic response can, however, be converted to apoptosis by either reducing the levels of the autophagy regulator Beclin-1, or by a concomitant activation of BAX. Our results define a pathway whereby MCL-1 has a key role in determining cell fate, by coordinately regulating apoptosis and autophagy.     

Developmentally regulated cell-cell adhesion in Dictyostelium purpureum is mediated by a glycoprotein synthesized in nonadhesive cells

Upon starvation the cellular slime mold, Dictyostelium purpureum, develops a form of cell-cell adhesion aiding in the formation of large multicellular aggregates, which are capable of further differentiation. The molecule that mediates this adhesion is a glycoprotein of Mr approximately 40,000. The protein shares a common carbohydrate epitope with another well-characterized cell adhesion molecule from Dictyostelium discoideum, contact sites A, but the polypeptides to which it is attached differ for each species. Although mediating a developmental form of adhesiveness, the protein is synthesized in vegetative cells at a time when they do not adhere. Most of the vegetative protein is associated with cell membranes and appears to be on the surface of these cells. The protein is compared to other cell adhesion molecules from other species of cellular slime molds, and possible explanations for its inability to function in vegetative cells are discussed.     

Evidence that noradrenergic transmitter release is regulated by presynaptic receptors

A review is provided of the evidence in support of the existence of prejunctional alpha adrenoceptors on noradrenergic nerve terminals as well as the evidence for their physiological importance. The use of alpha-adrenoceptor agonists and antagonists has provided convincing data in support of the presynaptic receptor hypothesis. Moreover, there is ample evidence for the location of alpha adrenoceptors on nerve terminals. This evidence has often been forgotten in arguments opposing the presynaptic alpha-adrenoceptor hypothesis. The precise physiological role of presynaptic alpha adrenoceptors is still an open question, but there is support from a wide range of experiments in favor of a physiological role. Although it is not known which of these functions is most important, presynaptic alpha adrenoceptors may: regulate the pulse-to-pulse regulation of norepinephrine release during nerve stimulation, prevent noise, and protect the neuroeffector cell from excessive activation by transmitter during periods of rest or as physiological antagonists to the facilitation of transmitter release. In summary, evidence reviewed here strongly supports the existence of presynaptic alpha adrenoceptors. These receptors are clearly important pharmacologically and may play a physiological role in noradrenergic transmission. The exact physiological function must await further experimentation.     

A teratocarcinoma glycoprotein carrying a developmentally regulated carbohydrate marker is a member of the immunoglobulin gene superfamily

Using the lambda gt11 expression library of murine teratocarcinoma cells, we isolated cDNA clones encoding a core protein carrying a developmentally regulated carbohydrate marker, namely the binding site for Dolichos biflorus agglutinin. The deduced amino acid sequence of the polypeptide (molecular weight, 37,102) revealed a leader sequence, nine potential asparagine glycosylation sites, and a transmembrane region. Sequence homology to variable domain of immunoglobulin kappa chain has been detected in a domain; homologous amino acids near cysteine residues are those conserved in many members of the immunoglobulin gene superfamily. In contrast to other members of the superfamily, the core protein gene was significantly expressed in embryonal carcinoma cells, which are similar to undifferentiated cells of early embryos.     

Evidence that somatomedin is synthesized by multiple tissues in the fetus

Production of somatomedin-C, a growth hormone-dependent peptide believed to mediate the growth-promoting actions of growth hormone, has been assessed using explants of fetal mouse tissues. Quantitation of this peptide in media of explants cultured for 3 days has been accomplished with a membrane receptor assay for somatomedin and a specific radioimmunoassay for somatomedin-C. Somatomedin-C is produced by the 11-day-gestation fetal mouse liver, increases exponentially in parallel with liver growth until the 16th day of gestation, and falls postnatally. Media somatomedin is believed to be derived by de novo synthesis since saline extracts of liver and most other fetal tissues contain only a small fraction of the activity in culture media. The immunoreactive material secreted into media appears to be closely related to human somatomedin-C since it produces dilution curves which are parallel to those of pure hormone, migrates on Sephacryl 200 at a size similar to that of one of the components of human serum somatomedin-C, dissociates into small molecular weight material with acid treatment, and isofocuses in a range comparable with that of somatomedin-C purified from human serum. Eleven-day limb bud mesenchymal micromass cultures and 17-day-gestation intestine, heart, brain, kidney, and lung also synthesize immunoreactive somatomedin-C in serum-free medium. For these tissues, the media activity was far in excess of the tissue extractable activity. Somatomedin activity in excess of the tissue extractable activity, however, was not found in media from 17-day-gestation placenta. The finding that multiple tissues synthesize somatomedin-C raises the possibility that the primary biological actions of this hormone are exerted locally at its sites of origin. Although a function of this type by a peptide has not been widely suspected, it seems plausible that the cells of fetal tissues are capable of producing local mitogens in much the same manner as the postulated inducers of tissue differentiation.     

Evidence that mammalian ribonucleotide reductase is a nuclear membrane associated glycoprotein

Epitope-specific antibodies to the M1 and M2 subunits of mammalian ribonucleotide reductase were prepared using peptides predicted to have a high antigenic index. Western blotting demonstrated that the anti-M1 antibody was specific for the 89-kilodalton M1 subunit (and its degradation fragments) and the anti-M2 antibody specifically recognized the 45-kilodalton M2 subunit. Both antibodies inhibited the CDP-reductase activity of the holoenzyme. Using these antibodies, both the M1 and M2 subunits were shown to be localized in the cytoplasm and in the nuclear regions of a number of cell types, including B77 avian sarcoma virus transformed NRK cells, T51B rat liver cells, 5123tc hepatoma cells, and rat liver cells in vivo. In addition, the M1 subunit was found to be localized as a halo around isolated rat liver nuclei. Biochemical analysis of the cytoplasmic fraction of liver cells and a Triton X-100 wash of nuclei from these cells confirmed the location of the enzyme activity in these cellular compartments. The M1 subunit appears to be glycosylated, as indicated by its retention on a Affi-Gel-concanavalin A affinity column. Therefore, in mammalian cells ribonucleotide reductase appears to be not only in the cytoplasm, but is also associated with the nuclear membrane or nuclear lamina. The activity of the enzyme in the membrane fraction changes dynamically during the cell cycle.     

A plasmodium protein kinase that is developmentally regulated, stimulated by spermine, and inhibited by quercetin

Plasmodium berghei-infected murine red cells possess protein kinase activity that is associated with the isolated parasites. Schizonts contain significantly higher levels of this protein kinase than the more immature forms, suggesting a relationship between this enzyme activity and parasite development. Partially purified protein kinase has a Km for ATP of approximately 30 microMs, whereas the Km for GTP is approximately 300 microMs and the substrate preference is phosvitin greater than casein much greater than histone greater than protamine. The Mg2+ optimum is 10-20 mM, and the protein kinase activity is stimulated by the polyamines spermine and spermidine. The flavone, quercetin, inhibits the protein kinase activity in a competitive manner with respect to ATP (Ki approximately 3 microMs), and P chabaudi also has a very similarly regulated protein kinase. Protein kinases from both species are very similar to the type I casein kinase.     

A new negatively regulated acute-phase phosphoprotein synthesized by rat hepatocytes.

The effect of acute inflammation on the production of the major phosphorylated protein (PP63) excreted by rat hepatocytes was investigated. Both intra- and extracellular forms of the protein labelled with [32P]Pi, [3H]fucose and [35S]methionine were immunoprecipitated with monospecific polyclonal antibodies, and relative rates of PP63 synthesis were measured. The hepatocytes of acutely inflamed rats produced and excreted 85% less 32P- and 3H-labelled PP63 than did control cells. This decreased amount of PP63 did not result from an impairment in the phosphorylation or glycosylation processes or from a blockade in excretion, but rather was found to be due to extensive shut-off in biosynthesis of the protein as measured by [35S]methionine incorporation. Thus PP63 would appear to represent a new negatively regulated acute-phase protein.