Phorbol diester-induced phosphorylation of nuclear matrix proteins in HL60 promyelocytes. Possible role in differentiation studied by cationic detergent gel electrophoresis

Two-Dimensional 16-BAC/SDS Polyacrylamide Gel Electrophoresis of Mitochondrial Membrane Proteins

The substitution of the reverse polarity benzyldimethyl-n-hexadecylammonium chloride (16-BAC) polyacrylamide gel electrophoresis (PAGE) for isoelectric focusing (IEF) in the first dimension of electrophoresis improves the solubility of extremely hydrophobic proteins and their recovery compared to conventional 2D IEF/SDS PAGE. The acidic environment of 16-BAC PAGE has also been shown to better preserve the labile methylation of basic proteins such as the histones. Several improvements of the 2D 16-BAC/SDS PAGE method are collectively described here with particular emphasis on the separation of mitochondrial membrane proteins of low molecular mass. Lowering the 16-BAC concentration 50-fold in the gel and buffers decreases the formation of mixed 16-BAC/SDS micelles, which otherwise interferes with the separation of very low molecular mass proteins in second dimension SDS PAGE, and consequently improved the resolution of mitochondrial membrane proteins in the 10-30 kDa range.

The synaptic vesicle proteome

Synaptic vesicles are key organelles in neurotransmission. Vesicle integral or membrane-associated proteins mediate the various functions the organelle fulfills during its life cycle. These include organelle transport, interaction with the nerve terminal cytoskeleton, uptake and storage of low molecular weight constituents, and the regulated interaction with the pre-synaptic plasma membrane during exo- and endocytosis. Within the past two decades, converging work from several laboratories resulted in the molecular and functional characterization of the proteinaceous inventory of the synaptic vesicle compartment. However, up until recently and due to technical difficulties, it was impossible to screen the entire organelle thoroughly. Recent advances in membrane protein identification and mass spectrometry (MS) have dramatically promoted this field. A comparison of different techniques for elucidating the proteinaceous composition of synaptic vesicles revealed numerous overlaps but also remarkable differences in the protein constituents of the synaptic vesicle compartment, indicating that several protein separation techniques in combination with differing MS approaches are required to identify and characterize the synaptic vesicle proteome. This review highlights the power of various gel separation techniques and MS analyses for the characterization of the proteome of highly purified synaptic vesicles. Furthermore, the newly detected protein assignments to synaptic vesicles, especially those proteins which are new to the inventory of the synaptic vesicle proteome, are critically discussed.

Characterization of a novel metabolic strategy used by drug-resistant tumor cells

Acquired or inherent drug resistance is the major problem in achieving successful cancer treatment. However, the mechanism(s) of pleiotropic drug resistance remains obscure. We have identified and characterized a cellular metabolic strategy that differentiates drug-resistant cells from drug-sensitive cells. This strategy may serve to protect drug-resistant cells from damage caused by chemotherapeutic agents and radiation. We show that drug-resistant cells have low mitochondrial membrane potential, use nonglucose carbon sources (fatty acids) for mitochondrial oxygen consumption when glucose becomes limited, and are protected from exogenous stress such as radiation. In addition, drug-resistant cells express high levels of mitochondrial uncoupling protein 2 (UCP2). The discovery of this metabolic strategy potentially facilitates the design of novel therapeutic approaches to drug resistance.

Immunocytochemical evaluation of protein kinase C translocation to the inner nuclear matrix in 3T3 mouse fibroblasts after IGF-I treatment

The complex pathway which links the agonist-cell membrane receptor binding to the response at the genome level involves, among other elements, protein kinase C (PKC). Agonists acting at the cell membrane can affect an autonomous nuclear polyphosphoinositide signaling system inducing an activation of nuclear phosphoinositidase activity and a subsequent translocation of PKC to the nuclear region. The fine localization of PKC has been investigated by means of electron microscopy quantitative immunogold labeling in 3T3 mouse fibroblasts, mitogenically stimulated by IGF-I. The enzyme, which in untreated cells is present in the cytoplasm, except for the organelles, and in the nucleoplasm, after IGF-I treatment is reduced in the cytoplasm and almost doubled in the nucleus. The PKC isoform translocated to the nucleus is the alpha isozyme, which is found not only associated with the nuclear envelope but mainly with the interchromatin domains. By using in situ matrix preparations, PKC appears to be retained at the nuclear matrix level, both at the nuclear lamina and at the inner nuclear matrix, suggesting a direct involvement in the phosphorylation of nuclear proteins which are responsible for the regulation of DNA replication.

N-methyl-D-aspartate receptor-induced translocation of protein kinase C to the nucleus in rat cerebellar slices

Rat cerebellar slices were incubated in absence and presence of N-methyl-D-aspartate and then used to prepare a purified nuclear fraction. The purity of the nuclear fraction was assessed by electron microscopy and measurements of Na+, K(+)-ATPase activity. The presence of protein kinase C in nuclear fractions was measured by [3H]phorbol dibutyrate binding. Treatment of cerebellar slices with N-methyl-D-aspartate caused a significant, two-fold increase in the density of nuclear [3H]phorbol dibutyrate binding sites, indicating the translocation of protein kinase C to the nuclear fraction. The effect of N-methyl-D-aspartate was prevented by the presence of the N-methyl-D-aspartate receptor antagonist (+)5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate (MK-801). These results suggest a possible role for protein kinase C in mediating N-methyl-D-aspartate-induced nuclear events.

Alpha-thrombin-induced nuclear sn-1,2-diacylglycerols are derived from phosphatidylcholine hydrolysis in cultured fibroblasts

Diglycerides play an important role in a number of agonist-induced signal transduction pathways. We have recently demonstrated that alpha-thrombin induces a rapid increase in the level of diglyceride mass in the nucleus and a selective increase in nuclear PKC-alpha [Leach, K.L., Ruff, V.A., Jarpe, M.B., Fabbro, D., Adams, L.D., & Raben, D.M. (1992) J. Biol. Chem. 267, 21816-21822]. In the present report, we examined the potential source of the induced nuclear diglycerides by examining the molecular species profiles of both the induced diglycerides and nuclear phospholipids by capillary gas chromatography. The molecular species profiles of the nuclear diglycerides generated resemble the species profiles of PC, and not PI species, at all times. In addition, while our previous data indicated that the molecular species of whole-cell phospholipids did not change in response to alpha-thrombin, nuclear PE was altered in a dramatic and selective manner in response to this agonist. These results demonstrate that PC hydrolysis is the predominant, if not exclusive, source of the alpha-thrombin-induced nuclear diglycerides in these fibroblasts.

High-resolution two-dimensional electrophoresis of nuclear proteins: A comparison of HeLa nuclei prepared by three different methods

A comparative analysis of HeLa cell nuclear proteins is presented using Iso-Dalt methods of protein resolution in two dimensions. The nuclear proteins were prepared by (1) spin through glycerol cushion, (2) spin through sucrose cushion, or (3) Triton wash. Improved resolution of total nuclear proteins in the range of pH 4.5-6.0 was achieved by substituting longer isotubes in combination with broad-range ampholines during the isoelectric focusing step. An attempt to indicate silver stainable protein spots common to total cellular extracts and nuclear preparations has been made. Also, proteins that appear to be well represented in all three nuclear preparations and remain undetectable in the total cellular protein pattern have been marked as probably being enriched nuclear proteins. Such a comparative analysis of whole nuclear protein preparations made it possible to document that the different preparations preserved the same set of proteins. The Triton-wash method of obtaining nuclei was identified as the preferred choice. Coomassie-stained gels and blots of these nuclear proteins could serve as a guide for accessing relevant protein spots for further biochemical analysis such as immunoblotting.

Increased phosphorylation of nuclear substrates for rat brain protein kinase C in regenerating rat liver nuclei

Protein phosphorylation catalysed by rat brain protein kinase C (PKC) has been studied in nuclei isolated from normal and regenerating rat liver. Histone H1 and a 40,000 molecular weight protein were hyperphosphorylated at all the explored regeneration times, ranging from 3 to 22 h after partial hepatectomy. Phosphorylation of the two substrates was totally dependent on calcium and lipids and was abolished by low concentration of staurosporine. The observed early change of phosphate content of histone H1 and of the 40,000 molecular weight protein on the time scale of liver regeneration suggests that PKC might be involved in the initial nuclear events leading to cell proliferation.

Importance of substrate conformation in the phosphorylation of chromatin-associated proteins by exogenous protein kinase C

Protein kinase C (PKC)-mediated phosphorylation of chromatin-associated proteins was studied in vitro. HL-60 and HeLa nuclear proteins were notably unresponsive to exogenously added brain PKC. In contrast, 3T3 fibroblasts and lymphocytes from primary cultures exhibited PKC-dependent phosphorylation of chromatin-associated proteins when chromatin was induced to expand. Unexpanded nuclei in all cell lines were unresponsive. Responsiveness was particularly obvious in the decondensed chromatin of primary lymphocytes, where a large number of proteins were phosphorylated in response to exogenous PKC. DNAase-I and micrococcal nuclease strongly modulated these phosphorylation patterns indicating that the substrates were DNA-associated. It was concluded that although substrate conformation, i.e. condensation state, was the primary determining factor in control of PKC-dependent nuclear protein phosphorylation, different cell lines greatly differ in their overall responsiveness to exogenous PKC.

Electrophoretic analysis of nuclear matrix proteins and the potential clinical applications

Nuclear matrix proteins form the skeleton of the nucleus and participate in the various cellular functions of the nucleus. These proteins have been demonstrated to be tissue-type specific and can potentially reflect changes in the state of differentiation of the cell. Elucidating nuclear matrix protein changes necessitates the use of high-resolution two-dimensional polyacrylamide gel electrophoresis. Separation of this complex mixture into its component parts resolves protein changes when comparing the normal state to a diseased state of a cell. Evidence has been reviewed which shows the potential use of nuclear matrix proteins and antibodies to nuclear matrix proteins as diagnostic tools for various cancers, autoimmune diseases, adenoviral infection, and other diseases. Consequently, the central functions of the nuclear matrix in the cell allow it to have significant potential as a diagnostic agent.

A vincristine-resistant murine erythroleukemia cell line secretes a differentiation enhancing factor

A clone of vincristine resistant murine erythroleukemia cells V3.17[44], characterized by high sensitivity to terminal erythroid differentiation induced by hexamethylene bisacetamide, secretes into the extracellular medium a protein factor which partially reduces the latent period before commitment and accelerates the expression of the terminal differentiated phenotype in a slow responding murine erythroleukemia N23 cell variant. This differentiation enhancing factor increases the rate of protein kinase C down-regulation which occurs at slower rate during cell differentiation. The activity of the factor is detected either by coculture of the two cell line variants or by addition of conditioned medium from V3.17[44] cells to a culture of N23 cells in the presence of the inducer. In addition to being secreted by V3.17[44] cells, this factor can also be detected in the cytoplasm of both V3.17[44] and N23 cells, associated with a particulate fraction from which it can be released by sonication.

zeta-Related protein kinase C in nuclei of nerve cells

To determine whether or not PKC is present in the nuclei of nerve tissue we made use of biochemical and immunocytochemical techniques. A 219-fold purification of rabbit brain nuclear protein kinase C was achieved by sequential steps of Triton X-100 extraction of isolated nuclei, DEAE-cellulose, Butyl-toyopearl and hydroxylapatite chromatography. The major peak of protein kinase C activity was eluted from the hydroxylapatite column at the KPO4 concentration of 0.3 M. Both Ca2+ and Ptd Ser were required for stimulation of the enzyme. Immunoblot analysis revealed that the kinase fraction was immunoreactive with a polyclonal antibody, PC-zeta, that had been raised against a peptide synthesized according to the deduced sequence of rat zeta protein kinase C. Light-microscopy revealed strong immunoreactivity in the nuclei of Purkinje cells in cerebellum and pyramidal cells in the rat cerebral cortex. These observations suggest that a zeta-related protein kinase C is present in the nuclei of nerve cells.

P47 phosphoprotein of blood platelets (pleckstrin) is a major target for phorbol ester-induced protein phosphorylation in intact platelets, granulocytes, lymphocytes, monocytes and cultured leukaemic cells: absence of P47 in non-haematopoietic cells

Aggregating agents including phorbol esters which activate protein kinase C induce the rapid phosphorylation of a Mr = 47,000 cytosolic protein in blood platelets (P47 or pleckstrin). This protein is well resolved by analytical 16-BAC----SDS two-dimensional PAGE and was purified from platelets by preparative 16-BAC----SDS PAGE. Polyclonal antibodies were raised to the protein in mice and rabbits. These antisera detected a single protein with the migration of P47 on Western blots of platelet extracts, and the rabbit antisera immunoprecipitated 32P-labelled P47 from platelet cytosol. The presence of P47 in other haematopoietic cells was determined by prelabelling them with 32P and observing increased 32P incorporation into the location of P47 on autoradiographs of 16-BAC----SDS analytical PAGE of cells exposed to phorbol ester. The identity of the phosphoprotein found in this location was further established by probing Western blots of SDS PAGE gels of cultured cell lines with the P47 antisera. P47 was detected in peripheral blood lymphocytes, monocytes and granulocytes (including the granulocytes of two unrelated patients with X-linked chronic granulomatous disease). P47 was also found in HL-60 promyelocytes (especially after differentiation with retinoic acid), U937 histiocytes, HEL leukaemia cells, and Raji 'B' lymphoblasts. It was not detected in normal erythrocytes, K562 leukaemic cells, MOLT-3 'T' lymphoblasts, or in wide range of non-haematopoietic cell lines. We conclude that P47 is a major target for the action of phorbol ester induced phosphorylation in platelets, normal leucocytes and some haematopoietic cell lines. These cells have as their common feature the ability when stimulated to develop adhesive functions on their plasma membranes.

A procedure for the extraction and high resolution two-dimensional gel electrophoresis of total nuclear phosphoproteins from isotonically purified nuclei

Methods are described for the extraction and preparation of total nuclear proteins for high resolution two-dimensional polyacrylamide gel electrophoresis (2-D PAGE). The conditions for protein extraction and preparation limit both protease and phosphatase activity, allowing application of this technique to the reliable analysis of changes in nuclear protein composition and nuclear protein phosphorylation as well as other forms of post-translational modifications. Unlike other procedures for 2-D PAGE analysis of nuclear proteins the technique allows solubilization and extraction of all nuclear proteins along with removal of nucleic acids which interfere with isoelectric focusing and autoradiography of 32Pi-labeled proteins. It avoids lengthy dialysis in which precipitation of nuclear proteins often occurs and does not require precipitation and resolubilization of nuclear proteins to obtain sufficient protein concentrations for 2-D PAGE analysis; often impractical steps in which complete resolubilization of all proteins is not possible. It produces high resolution 2-D PAGE analysis in which identification of even low abundance proteins can be made, based on isoelectric point and molecular weight, allowing comparison with other studies.

Bovine pancreatic DNase I binds very tightly to DNA fragments and may be mistaken for putative endogenous nuclear proteins covalently bound to DNA

Using published methods for the isolation of nuclear proteins tightly bound to DNA, and resistant to removal by SDS or 16-BAC detergent and urea, several new protein bands in the region of 55 kd and 62 kd on SDS gel and 43 kd and 70 kd on 16--BAC gel electrophoresis were identified in extracts of avian erythroid nuclei. These bands were radiolabelled by subjecting the DNA--protein complexes to nick--translation in the presence of [32P]--dCTP, followed by prolonged digestion with excess bovine DNase I. Amino acid sequence analysis shows that these bands contain DNase I. These results indicate that DNase I can form stable complexes with DNA, and suggest that DNase I--DNA complexes may be mistakenly identified as nuclear proteins covalently bound to DNA.

Protein kinase C phosphorylates DNA topoisomerase I

The induction of mammalian cell proliferation requires the expression of a specific set of genes. Tumor promoters stimulate cell growth by activating the Ca2+ and phospholipid-dependent protein kinase, protein kinase C (PKC). DNA topoisomerase I, a nuclear enzyme involved in transcription, was phosphorylated by activated PKC in vitro. Phosphorylation by PKC stimulated the DNA relaxation activity of topoisomerase I two- to three-fold. Therefore, DNA topoisomerase I is a substrate for PKC-mediated activation by phosphorylation and may serve as a nuclear target of mitogenic signals generated by tumor promoters in vivo.

G1 events and regulation of cell proliferation

Cells prepare for S phase during the G1 phase of the cell cycle. Cell biological methods have provided knowledge of cycle kinetics and of substages of G1 that are determined by extracellular signals. Through the use of biochemical and molecular biological techniques to study effects of growth factors, oncogenes, and inhibitors, intracellular events during G1 that lead to DNA synthesis are rapidly being discovered. Many cells in vivo are in a quiescent state (G0), with unduplicated DNA. Cells can be activated to reenter the cycle during G1. Similarly, cells in culture can be shifted between G0 and G1. These switches in and out of G1 are the main determinants of post-embryonic cell proliferation rate and are defectively controlled in cancer cells.

Two dimensional benzyldimethyl-n-hexadecylammonium chloride----sodium dodecyl sulfate preparative polyacrylamide gel electrophoresis: a high capacity high resolution technique for the purification of proteins from complex mixtures

A two dimensional preparative polyacrylamide gel electrophoresis system is described which has both high resolving power and high loading capacity (approximately 100 mg protein). The first dimension is electrophoresis toward the cathode at acid pH in the presence of the cationic detergent benzyldimethyl-n-hexadecylammonium chloride (16-BAC), and the second dimension is electrophoresis toward the anode in the presence of sodium dodecyl sulfate. Proteins purified in this way have been sequenced and used successfully as antigens.

A phorbol ester tolerant (PET) variant of HL-60 promyelocytes

The promyelocytic leukaemia cell line HL-60 differentiates to a macrophage-like cell when exposed to the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) and other agents which activate protein kinase C. To investigate this phenomenon we developed an HL-60 variant which does not differentiate when exposed to TPA. HL-60 cells were exposed to the mutagen ethyl methanesulphonate and were cloned in soft agar in the presence of a normally lethal concentration of TPA. One colony of cells that proliferated in TPA was obtained. The cells of this phorbol ester tolerant (PET) line have retained their resistance to TPA for several years without selective pressure. They are somewhat larger than their phorbol ester sensitive (S) parent, but they are otherwise morphologically similar. When PET-cells are exposed to TPA their growth is arrested for approximately 48 h. Thereafter, they resume their original rate of replication at all concentrations of TPA tested. S-cells undergo changes typical of HL-60 when exposed to TPA; they aggregate, stop growing, adhere to the flask and die. The PET-cells appeared to be as sensitive as S-cells to other agents which differentiate HL-60 such as retinoic acid, dimethysulphoxide, and 1,25-dihydroxyvitamin D3, as determined by rate of proliferation in culture, Wright's stain, nitroblue tetrazolium reduction, and induction of the ectoenzyme NAD-glycohydrolase. TPA-induced protein phosphorylation was studied using one- and two-dimensional polyacrylamide gel electrophoresis. Several proteins increased their incorporation of 32P when S- and PET-cells were exposed to TPA, the most prominent of which were the two previously described nuclear matrix proteins of 80 kd and 33 kd. There was no difference in the protein phosphorylation pattern in S- and PET-cells, nor in how this pattern changed on TPA exposure. Fluorescent activated cell sorting and karyotypic analysis revealed PET-cells to be a hypotetraploid variant of S-cells, with approximately 80 chromosomes, including a marker chromosome iso(1p) not found in the S-cells. Identification of the biochemical lesion responsible for this TPA resistance in PET cells will provide clues concerning the mechanism of this important pathway for the induction of cell differentiation.

Activation of double-stranded RNA dependent protein kinase by ribosomal RNA precursors

Two major changes in RNA metabolism occured in macrophages expressing tumoricidal activity: a down regulation of total RNA synthesis and an imbalanced accumulation of ribosomal RNA precursors with double stranded secondary structure. The aim of this study was to investigate a possible role of endogenous ds rRNA precursors in the activation of the dsRNA dependent protein kinase. Using a cell free transcription system purified rRNA precursors were obtained from the murine rRNA gene. The results indicate that rRNA precursors are potent activators of the dsPK. The effects are dose dependent and are not affected by treatment of rRNA precursors with proteinase K or RNase A. The treatment with RNase V(1), specific for dsRNA, completely abrogates the activity of transcripts. The results suggest that endogenous RNA, namely rRNA, could control dsPK activation and it can be speculated that dsPK activation may be involved in the control of tumoricidal activity by macrophages.

Detection of transiently phosphorylated membrane proteins by protein blotting through a nonionic detergent layer

A rapid approach for detecting tentative membrane proteins which are transiently phosphorylated/dephosphorylated is described. Cell fractionation is unnecessary, as are other manipulations of sample preparation during which artifactual modifications or sample loss might occur. The method is shown to be useful for the detection of such phosphorylation during cellular response to the binding of specific ligand. Two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis was performed successively through gels of different sieving sizes. These "primary" gels were then subjected to "detergent blotting," a variation of electroblotting in which polyacrylamide gel containing the nonionic detergent Nonidet-P40 (secondary gel) was inserted between the primary gel and a Zeta-Probe membrane. Phosphorylated interleukin 2 receptors were selectively retained in the secondary gel. Upon stimulation of human platelets with thrombin, at least 11 polypeptides were found to be rapidly phosphorylated/dephosphorylated using the method. Among them, five phosphorylated polypeptides were trapped in the secondary gel, suggesting that they might be membrane proteins. This technique should be useful to rapidly screen transiently phosphorylated/dephosphorylated membrane proteins which might be involved in membrane transductional signaling.

Protein kinase C binding to isolated nuclei and its activation by a Ca2+/phospholipid-independent mechanism

The direct interaction of protein kinase C with the nucleus was examined utilizing endogenous protein phosphorylation and [3H]PDBu binding to detect the enzyme. Rat brain nuclei were relatively rich in phorbol ester receptors whereas liver nuclei contained less than 10% of their brain counterpart. Purified protein kinase C from rat brain could bind to purified rat liver nuclei at 4 degrees C or at 24 degrees C reaching apparent equilibrium by 20 min. The binding was linearly dependent on protein kinase C concentration and required free Ca2+ with an EC50 of 0.5 microM. Chelation of Ca2+ with EGTA resulted in rapid loss of phorbol ester receptors from nuclei. Differential extraction experiments with Triton X-100 and NaCl suggested that about 50% of the acquired phorbol ester receptors were bound to chromatin and 25% were associated with the nuclear matrix. Protein Kinase C bound to nuclei was also able to phosphorylate several endogenous nuclear substrates in a Ca2+/phospholipid-independent reaction. These data suggest that protein kinase C can associate with nuclear components leading to the phosphorylation of nuclear substrates.

Specific binding of phorbol esters to nuclei of human promyelocytic leukemia cells

In this report, we demonstrate that HL-60 nuclei isolated in calcium but not EGTA containing buffers specifically bind PE and express approximately 37,000 receptor sites/nucleus. Nuclear phorbol ester binding is lost by isolation in the absence of calcium, but can be repleted by the addition of partially purified protein kinase C and calcium. When HL-60 cells are treated with bryostatin 1, a compound which activates protein kinase C in a similar fashion to phorbol esters but does not induce differentiation of HL-60 cells, and nuclei are isolated in the presence of EGTA, these nuclei continue to bind phorbol esters. These experiments suggest that HL-60 nuclei bind PE in vitro, and that compounds that activate protein kinase C may increase nuclear binding of PE in situ.

Immunochemical characterization of protein kinase C in rat liver nuclei and subnuclear fractions

A doublet of immunoreactive bands has been identified in rat liver nuclei, nuclear matrix and lamina by means of a polyclonal antibody against protein kinase C. The two polypeptides show an apparent molecular weight of 77 and 74 kDa on SDS-polyacrylamide gels, and appear to be tightly bound nuclear components, resistant to detergent and high salt extraction. Given the complexity of the genes encoding for protein kinase C, these two forms of the enzyme might be translational products specifically located in the nucleus, involved in the transduction to the genomic apparatus of regulatory signals generated by growth factors and tumor promoters.