Secretion and degradation of glucagon by HIT cells

HIT cells have been widely used to study synthesis and secretion of insulin. It has been assumed that this cell line secretes no other islet hormones. To ascertain whether HIT cells synthesize, secrete, and degrade glucagon, we examined cell extracts for this peptide and compared secretion and degradation of glucagon and insulin during stimulation of the cells by arginine. Glucagon levels in acid extracts of HIT cells were found to be 0.72 +/- 0.15 pmol/mg protein. Both glucagon and insulin were maximally stimulated in a glucagon/insulin molar ratio of 0.029 by arginine concentrations of 25-50 nM, and the concentration of arginine that provided half-maximum responses for both hormones was approximately 3 mM. Diminution of arginine-induced glucagon secretion was caused by somatostatin, a physiological inhibitor of pancreatic islet alpha-cell function. HPLC was used to authenticate the glucagon levels stimulated by arginine for 60 min and measured by RIA. Thirty-six percent of immunoreactive glucagon was found in the fractions representing authentic glucagon, whereas the remaining 64% eluted earlier. Experiments examining the fate of radiolabeled glucagon exposed to HIT cells revealed time-dependent degradation of the radioisotope to earlier eluting forms, which accounted for approximately 50% of the radioactivity by 60 min and was complete by 18 h, indicating that the early peak detected by RIA represented a metabolite of glucagon. Radioisotopic insulin was degraded more slowly with an apparent half-life of approximately 36 h. We conclude that HIT cells are not only able to synthesize, secrete, and degrade insulin, but also much smaller amounts of glucagon.

The end1 gene of Schizosaccharomyces pombe coding for a DNase is identical with the pnu1 gene coding for an RNase

The DNA nuclease activity encoded by the end1 gene, and its inactivation by mutation, was described in connection with the characterization of DNA topoisomerases in the fission yeast Schizosaccharomyces pombe (Uemura and Yanagida, 1984). Subsequently, end1 mutant strains were used for the preparation of cell extracts for the study of enzymes and intermediates involved in DNA metabolism. The molecular identification of the end1 gene and its identity with the pnu1 gene is presented. The end1-458 mutation alters glycine to glutamate in the conserved motif TGPYLP. The pnu1 gene codes for an RNase that is induced by nitrogen starvation (Nakashima et al., 2002b). Thus, the End1/Pnu1 protein, like related mitochondrial proteins in other organisms, is an example of a sugar-non-specific nuclease. The analysis of strains carrying a pnu1 deletion revealed no defects in meiotic recombination and spore viability.

Pseudovacuoles--immobilized by high-pressure freezing--are associated with blebbing in walker carcinosarcoma cells

By applying high pressure freezing and freeze-substitution, we observed large inclusions of homogeneous appearance in the front of locomoting Walker carcinosarcoma cells that have not been described earlier. Live cell imaging revealed that these inclusions were poor in lipids and nucleic acids but had a high lysine (and hence protein) content. Usually one such structure 2-5 mum in size was present at the front of motile Walker cells, predominantly in the immediate vicinity of newly forming blebs. By correlating the lysine-rich areas in fixed and embedded cells with electron microscopic pictures, inclusions could be assigned to confined, faintly stained cytoplasmic areas that lacked a surrounding membrane; they were therefore called pseudovacuoles. After high-pressure freezing and freeze substitution, pseudovacuoles appeared to be filled with 20 nm large electron-transparent patches surrounded by 12 and 15 nm large particles. The heat shock protein Hsp90 was identified by peptide sequencing as a major fluorescent band on SDS-PAGE of lysine-labelled Walker cell extracts. By immunofluorescence, Hsp90 was found to be enriched in pseudovacuoles. Colocalization of the lysine with a potassium-specific dye in living cells revealed that pseudovacuoles act as K+ stores in the vicinity of forming blebs. We propose that pseudovacuoles might support blebbing by locally regulating the intracellular hydrostatic pressure.

Effect of testosterone on E1S-sulfatase activity in non-malignant and cancerous breast cells in vitro

INTRODUCTION: Testosterone (T) is a therapeutic option for women with hypoactive sexual desire disorder. T may have an impact on the mammary gland by altering local estrogen synthesis. The aim of the present study was to measure the effect of T on estrone-sulfate (E1S)-sulfatase (STS) expression, and activity using hormone-dependent BC cells with high and low aggressive potential (BT-474, MCF-7), and HBL-100 as a breast cell line of non-malignant origin. METHODS: Cells were incubated in RPMI 1640 medium containing 5% steroid-depleted fetal calf serum for 3d, and subsequently incubated in absence or presence of T alone, and combined with anastrozole (A) at 10(-8)M, and 10(-6)M at 37 degrees C for either 24h or directly in cell extracts ("direct"). STS protein expression was measured by dot-blot (immunoblotting), and STS, HSD17B1 and HSD17B2 mRNA levels by quantitative RT-PCR. STS activity was evaluated by incubating homogenized breast cells with [(3)H]-E1S and separating the products E1, and E2 by thin layer chromatography. RESULTS: Basal STS mRNA expression did not reveal group differences. However, STS mRNA was decreased by T+A in MCF-7 cells. 17HSDB1 expression was decreased by T+A in BT-474 cells, and 17HSDB2 expression was decreased by A and T+A treatment in MCF-7 cells. Basal and T treated STS protein expression was significantly higher in malignant compared to non-malignant breast cells. However, T did not induce significant intra-cell line differences. Similarly, basal and T treated STS activity was significantly higher in highly malignant compared to non-malignant breast cells. Regardless of cell lines, T slightly decreased STS activity after "direct" incubation, but led to an increase of local estrogen formation after 24h which was attenuated, and partly reversed by A, respectively. CONCLUSIONS: The more aggressive the breast cell line, the higher the local estrogen formation. The transition from normal to malignant seems to be accompanied by an altered autoregulation. The given local endocrine milieu seems to be essential for response to T.

Benzamidine derivatives inhibit the virulence of Porphyromonas gingivalis

We have previously shown that benzamidine-type compounds can inhibit the activity of arginine-specific cysteine proteinases (gingipains HRgpA and RgpB); well-known virulence factors of Porphyromonas gingivalis. They also hinder in vitro growth of this important periodontopathogenic bacterium. Apparently growth arrest is not associated with their ability to inhibit these proteases, because pentamidine, which is a 20-fold less efficient inhibitor of gingipain than 2,6-bis-(4-amidinobenzyl)-cyclohexanone (ACH), blocked P. gingivalis growth far more effectively. To identify targets for benzamidine-derived compounds other than Arg-gingipains, and to explain their bacteriostatic effects, P. gingivalis ATCC 33277 and P. gingivalis M5-1-2 (clinical isolate) cell extracts were subjected to affinity chromatography using a benzamidine-Sepharose column to identify proteins interacting with benzamidine. In addition to HRgpA and RgpB the analysis revealed heat-shock protein GroEL as another ligand for benzamidine. To better understand the effect of benzamidine-derived compounds on P. gingivalis, bacteria were exposed to benzamidine, pentamidine, ACH and heat, and the expression of gingipains and GroEL was determined. Exposure to heat and benzamidine-derived compounds caused significant increases in GroEL, at both the mRNA and protein levels. Interestingly, despite the fact that gingipains were shown to be the main virulence factors in a fertilized egg model of infection, mortality rates were strongly reduced, not only by ACH, but also by pentamidine, a relatively weak gingipain inhibitor. This effect may depend not only on gingipain inhibition but also on interaction of benzamidine derivatives with GroEL. Therefore these compounds may find use in supportive periodontitis treatment.

In vitro incorporation of molybdate into demolybdoproteins in Escherichia coli.

When Escherichia coli was grown in the presence of tungstate, inactive forms of two molybdoenzymes, nitrate reductase and formate dehydrogenase, accumulated and were converted to their active forms upon incubation of cell suspensions with molybdate and chloramphenicol. The conversion to the active enzymes did not occur in cell extracts. When incubated with [(99)Mo]molybdate and chloramphenicol, the tungstate-grown cells incorporated (99)Mo into protein components which were released from membranes by procedures used to release nitrate reductase and formate dehydrogenase and which migrated with these activities on polyacrylamide gels. Although neither activity was formed during incubation of the crude extract with molybdate, (99)Mo was incorporated into protein components which were released from the membrane fraction under the same conditions and were similar to the active enzymes in their electrophoretic properties. The in vitro incorporation of (99)Mo occurred specifically into these components and was equal to or greater than the amount incorporated in vivo under the same conditions. Molybdenum in preformed, active nitrate reductase and formate dehydrogenase did not exchange with [(99)Mo]molybdate, demonstrating that the observed incorporation depended on the demolybdo forms of the enzymes. We conclude that molybdate may be incorporated into the demolybdo forms both in vivo and in vitro; some unknown additional factor or step, required for active enzyme formation, occurs in vivo but not in vitro under the conditions employed.

U7 snRNP-specific Lsm11 protein: dual binding contacts with the 100 kDa zinc finger processing factor (ZFP100) and a ZFP100-independent function in histone RNA 3' end processing

The 3' cleavage generating non-polyadenylated animal histone mRNAs depends on the base pairing between U7 snRNA and a conserved histone pre-mRNA downstream element. This interaction is enhanced by a 100 kDa zinc finger protein (ZFP100) that forms a bridge between an RNA hairpin element upstream of the processing site and the U7 small nuclear ribonucleoprotein (snRNP). The N-terminus of Lsm11, a U7-specific Sm-like protein, was shown to be crucial for histone RNA processing and to bind ZFP100. By further analysing these two functions of Lsm11, we find that Lsm11 and ZFP100 can undergo two interactions, i.e. between the Lsm11 N-terminus and the zinc finger repeats of ZFP100, and between the N-terminus of ZFP100 and the Sm domain of Lsm11, respectively. Both interactions are not specific for the two proteins in vitro, but the second interaction is sufficient for a specific recognition of the U7 snRNP by ZFP100 in cell extracts. Furthermore, clustered point mutations in three phylogenetically conserved regions of the Lsm11 N-terminus impair or abolish histone RNA processing. As these mutations have no effect on the two interactions with ZFP100, these protein regions must play other roles in histone RNA processing, e.g. by contacting the pre-mRNA or additional processing factors.

STUDIES ON THE MECHANISM OF ASSEMBLY OF MURINE LEUKEMIA VIRUS

The purpose of this research was to elucidate the mechanism of assembly of retroviruses, specifically of murine leukemia virus, as studied through the treatment of virus-infected cells with interferon and through the use of temperature sensitive (ts) mutants. Our studies have shown a rapid and specific association of Rauscher murine leukemia virus (R-MuLV) precursor polyprotein Pr65('gag) with cytoskeletal elements in infected mouse fibroblasts. The Pr65('gag) associated with Nonidet P-40 (NP40)-insoluble cytoskeletal structures appeared to be subphosphorylated in comparison to NP40-soluble Pr65('gag). The association of Pr65('gag) with skeletal elements could be disrupted by extraction of the cytoskeleton with sodium deoxycholate, an ionic detergent. Both the skeleton-associated Pr65('gag) and its NP40-soluble counterpart were labeled with {('3)H}-palmitate, indicating their probable association with lipids presumably in the plasma membrane. Pr65('gag) molecules bound to skeletal elements in the infected cell appeared to be more stable to proteolytic processing than NP40-soluble Pr65('gag). Our studies with certain ts mutants of murine leukemia virus, defective in virus assembly, including Mo-MuLV ts3 and R-MuLV ts17, ts24, ts25 and ts26, have shown that virions released at 39(DEGREES)C (nonpermissive temperature) had high levels of uncleaved Pr65('gag) relative to that seen in virions released at 33(DEGREES)C (permissive temperature). Examination of cell extracts revealed that Pr54('gag) was more stable to processing at 39(DEGREES)C than at 33(DEGREES)C, whereas the 'env' and glycosylated 'gag' proteins were processed to the same extent at both temperatures. Detergent extraction of pulse-labeled cells to generate an NP40-insoluble cytoskeleton-enriched fraction showed that in ts3-, ts17- and ts24-infected cells, Pr65('gag) accumulated in the cytoskeleton-enriched fraction. In contrast, cells infected with ts25 or ts26 showed no preferential localization of Pr65('gag) in the cytoskeleton in a short pulse, but instead, Pr65('gag) accumulated in both the NP40-soluble and -insoluble fractions during a chase-incubation. The association of Pr65('gag) with cytoskeletal elements in the cell was neither increased nor decreased by blocking virus assembly and release with interferon. Based on these and other results, we have proposed a model for the active role of cytoskeleton-associated Pr65('gag) in retrovirus assembly.^

Roles of mismatch repair proteins in the recognition and processing of DNA interstrand crosslinks in human cells

DNA interstrand crosslinks (ICLs) are among the most toxic type of damage to a cell. Many ICL-inducing agents are widely used as therapeutic agents, e.g. cisplatin, psoralen. A bettor understanding of the cellular mechanism that eliminates ICLs is important for the improvement of human health. However, ICL repair is still poorly understood in mammals. Using a triplex-directed site-specific ICL model, we studied the roles of mismatch repair (MMR) proteins in ICL repair in human cells. We are also interested in using psoralen-conjugated triplex-forming oligonucleotides (TFOs) to direct ICLs to a specific site in targeted DNA and in the mammalian genomes. ^ MSH2 protein is the common subunit of two MMR recognition complexes, and MutSα and MutSβ. We showed that MSH2 deficiency renders human cell hypersensitive to psoralen ICLs. MMR recognition complexes bind specifically to triplex-directed psoralen ICLs in vitro. Together with the fact that psoralen ICL-induced repair synthesis is dramatically decreased in MSH2 deficient cell extracts, we demonstrated that MSH2 function is critical for the recognition and processing of psoralen ICLs in human cells. Interestingly, lack of MSH2 does not reduce the level of psoralen ICL-induced mutagenesis in human cells, suggesting that MSH2 does not contribute to error-generating repair of psoralen ICLs, and therefore, may represent a novel error-free mechanism for repairing ICLs. We also studied the role of MLH1, anther key protein in MMR, in the processing of psoralen ICLs. MLH1-deficient human cells are more resistant to psoralen plus UVA treatment. Importantly, MLH1 function is not required for the mutagenic repair of psoralen ICLs, suggesting that it is not involved in the error-generating repair of this type of DNA damage in human cells. ^ These are the first data indicating mismatch repair proteins may participate in a relatively error-free mechanism for processing psoralen ICL in human cells. Enhancement of MMR protein function relative to nucleotide excision repair proteins may reduce the mutagenesis caused by DNA ICLs in humans. ^ In order to specifically target ICLs to mammalian genes, we identified novel TFO target sequences in mouse and human genomes. Using this information, many critical mammalian genes can now be targeted by TFOs.^

The yeast STM1 protein binds G*G multiplex nucleic acids in vitro and associates with ribosomes in vivo

The formation of triple helical, or triplex DNA has been suggested to occur in several cellular processes such as transcription, replication, and recombination. Our laboratory previously found proteins in HeLa nuclear extracts and in S. cerevisiae whole cell extracts that avidly bound a Purine-motif (Pu) triplex probe in gel shift assays, or EMSA. In order to identify a triplex DNA-binding protein, we used conventional and affinity chromatography to purify the major Pu triplex-binding protein in yeast. Peptide microsequencing and data base searches identified this protein as the product of the STM1 gene. Confirmation that Stm1p is a Pu triplex-binding protein was obtained by EMSA using both recombinant Stm1p and whole cell extracts from stm1Δ yeast. Stm1p had previously been identified as G4p2, a G-quartet DNA- and RNA-binding protein. To study the cellular role and identify the nucleic acid ligand of Stm1p in vivo, we introduced an HA epitope at either the N- or C-terminus of Stm1p and performed immunoprecipitations with the HA.11 mAb. Using peptide microsequencing and Northern analysis, we positively identified a subset of both large and small subunit ribosomal proteins and all four rRNAs as associating with Stm1p. DNase I treatment did not affect the association of Stm1p with ribosomal components, but RNase A treatment abolished the association with all ribosomal proteins and RNA, suggesting this association is RNA-dependent. Sucrose gradient fractionation followed by Western and EMSA analysis confirmed that Stm1p associates with intact 80S monosomes, but not polysomes. The presence of additional, unidentified RNA in the Stm1p-immunoprecipitate, and the absence of tRNAs and elongation factors suggests that Stm1p binds RNA and could be involved in the regulation of translation. Immunofluorescence microscopy data showed Stm1p to be located throughout the cytoplasm, with a specific movement to the bud during the G2 phase of the cell cycle. A dramatically flocculent, large cell phenotype is observed when Stm1p has a C-terminal HA tag in a protease-deficient strain background. When STM1 is deleted in this background, the same phenotype is not observed and the deletion yeast grow very slowly compared to the wild-type. These data suggest that STM1 is not essential, but plays a role in cell growth by interacting with an RNP complex that may contain G*G multiplex RNA. ^

Organic matter and sediment properties of samples from site GeoB15105 from the Black Sea

Dissolved organic matter (DOM) in marine sediments is a complex mixture of thousands of individual constituents that participate in biogeochemical reactions and serve as substrates for benthic microbes. Knowledge of the molecular composition of DOM is a prerequisite for a comprehensive understanding of the biogeochemical processes in sediments. In this study, interstitial water DOM was extracted with Rhizon samplers from a sediment core from the Black Sea and compared to the corresponding water-extractable organic matter fraction (<0.4 µm) obtained by Soxhlet extraction, which mobilizes labile particulate organic matter and DOM. After solid phase extraction (SPE) of DOM, samples were analyzed for the molecular composition by Fourier Transform Ion-Cyclotron Resonance Mass Spectrometry (FT-ICR MS) with electrospray ionization in negative ion mode. The average SPE extraction yield of the dissolved organic carbon (DOC) in interstitial water was 63%, whereas less than 30% of the DOC in Soxhlet-extracted organic matter was recovered. Nevertheless, Soxhlet extraction yielded up to 4.35% of the total sedimentary organic carbon, which is more than 30-times the organic carbon content of the interstitial water. While interstitial water DOM consisted primarily of carbon-, hydrogen- and oxygen-bearing compounds, Soxhlet extracts yielded more complex FT-ICR mass spectra with more peaks and higher abundances of nitrogen- and sulfur-bearing compounds. The molecular composition of both sample types was affected by the geochemical conditions in the sediment; elevated concentrations of HS- promoted the early diagenetic sulfurization of organic matter. The Soxhlet extracts from shallow sediment contained specific three- and four-nitrogen-bearing molecular formulas that were also detected in bacterial cell extracts and presumably represent proteinaceous molecules. These compounds decreased with increasing sediment depth while one- and two-nitrogen-bearing molecules increased, resulting in a higher similarity of both sample types in the deep sediment. In summary, Soxhlet extraction of sediments accessed a larger and more complex pool of organic matter than present in interstitial water DOM.

Src interacts with dynamin and synapsin in neuronal cells

The nonreceptor tyrosine kinase Src is expressed at a high level in cells that are specialized for regulated secretion, such as the neuron, and is concentrated on secretory vesicles or at the site of exocytosis. To investigate the possibility that Src may play a role in regulating membrane traffic, we searched for neuronal proteins that will interact with Src. The SH3 domain of Src, but not that of the splice variant N-Src, bound to three proteins from mouse synaptosomes or PC12 cells: dynamin, synapsin Ia, and synapsin Ib. Dynamin and the synapsins coprecipitated with Src from PC12 cell extracts, and they colocalized with a subset of Src in the PC12 cell by immunofluorescence. Neither dynamin nor the synapsins were phosphorylated by Src, suggesting that the interaction of these proteins serves to direct the kinase activity of Src toward other proteins in the vesicle population. In immunoprecipitates containing Src and dynamin, the clathrin adaptor protein α-adaptin was also found. The association of Src and synapsin suggests a role for Src in the life cycle of the synaptic vesicle. The identification of a complex containing Src, dynamin, and α-adaptin indicates that Src may play a more general role in membrane traffic as well.

Non-p53 p53RE binding protein, a human transcription factor functionally analogous to P53

The transactivation activity of the p53 tumor suppressor protein is critical for regulating cell growth and apoptosis. We describe the identification of a transcription factor that is functionally similar to p53 and contains the same DNA binding and transcription activities specific for the p53 responsive DNA element (p53RE). This protein was highly purified through chromatography from HeLa cell extracts. The purified protein was able to bind specifically to the p53RE derived from a p21waf1 promoter and to stimulate p53RE-dependent transcription but not basal transcription in vitro. Its DNA-binding activity was inhibited by the wild type but not mutant p53RE-containing DNA oligomers. Also, this p53RE-binding activity was found in human p53 null Saos-2 osteosarcoma and H1299 small cell lung carcinoma cells. Interestingly, this activity exhibited a p53RE sequence preference that was distinct from the p53 protein. The activity is neither p53 nor p73, because anti-p53 or anti-73 antibodies were unable to detect this purified protein nor were the antibodies able to alter the p53-like activity, the p53RE-protein complex. These results demonstrate that, besides p73, an additional p53-like protein exists in cells, which is named NBP for non-p53, p53RE binding protein.

Deposition-related sites K5/K12 in histone H4 are not required for nucleosome deposition in yeast

Histone H4 can be acetylated at N-terminal lysines K5, K8, K12, and K16, but newly synthesized H4 is diacetylated at K5/K12 in diverse organisms. This pattern is widely thought to be important for histone deposition onto replicating DNA. To investigate the importance of K5/K12 we have mutagenized these lysines in yeast and assayed for nucleosome assembly. Assaying was done in the absence of the histone H3 N terminus, which has functions redundant with those of H4 in histone deposition. Nucleosome assembly was assayed by three methods. Because nucleosome depletion may be lethal, we examined cell viability. We also analyzed nucleosome assembly in vivo and in vitro by examining plasmid superhelicity density in whole cells and supercoiling in yeast cell extracts. All three approaches demonstrate that mutagenizing K5 and K12 together does not prevent cell growth and histone deposition in vivo or in vitro. Therefore, K5/K12 cannot be required for nucleosome assembly in yeast. It is only when the first three sites of acetylation—K5, K8, and K12—are mutagenized simultaneously that lethality occurs and assembly is most strongly decreased both in vivo and in vitro. These data argue for the redundancy of sites K5, K8, and K12 in the deposition of yeast histone H4.

Human CUL1 forms an evolutionarily conserved ubiquitin ligase complex (SCF) with SKP1 and an F-box protein

The SCF ubiquitin ligase complex of budding yeast triggers DNA replication by catalyzing ubiquitination of the S phase cyclin-dependent kinase inhibitor SIC1. SCF is composed of three proteins—ySKP1, CDC53 (Cullin), and the F-box protein CDC4—that are conserved from yeast to humans. As part of an effort to identify components and substrates of a putative human SCF complex, we isolated hSKP1 in a two-hybrid screen with hCUL1, the closest human homologue of CDC53. Here, we show that hCUL1 associates with hSKP1 in vivo and directly interacts with both hSKP1 and the human F-box protein SKP2 in vitro, forming an SCF-like particle. Moreover, hCUL1 complements the growth defect of yeast cdc53ts mutants, associates with ubiquitination-promoting activity in human cell extracts, and can assemble into functional, chimeric ubiquitin ligase complexes with yeast SCF components. Taken together, these data suggest that hCUL1 functions as part of an SCF ubiquitin ligase complex in human cells. Further application of biochemical assays similar to those described here can now be used to identify regulators/components of hCUL1-based SCF complexes, to determine whether the hCUL2–hCUL5 proteins also are components of ubiquitin ligase complexes in human cells, and to screen for chemical compounds that modulate the activities of the hSKP1 and hCUL1 proteins.