Protein kinase C isoforms in murine erythroleukemia cells and their involvement in the differentiation process

Hexamethylbisacetamide and disruption of human immunodeficiency virus type 1 latency in CD4(+) T cells

BACKGROUND

Novel therapeutic approaches are needed to attack persistent proviral human immunodeficiency type 1 (HIV-1) infection. Hexamethylbisacetamide (HMBA), a hybrid bipolar compound, induces expression of the HIV-1 promoter in the long terminal repeat (LTR) region in a Tat-independent manner but mimics the effect of Tat, overcoming barriers to LTR expression and increasing the processivity of LTR transcription complexes.

METHODS

We studied alterations in cellular factors and their LTR occupancy induced by HMBA in models of latent HIV-1 infection. We measured the induction of viral outgrowth by HMBA in resting CD4(+) T cells from aviremic HIV-1-infected donors.

RESULTS

HMBA induced outgrowth of HIV-1 from resting CD4(+) T cells recovered from aviremic patients treated with antiretroviral therapy (ART). HMBA triggered cyclin-dependent kinase 9 (CDK9) recruitment to the LTR, a key factor in the induction of efficient HIV-1 expression, via an unexpected interaction with the transcription factor Sp1. The availability of Sp1 and Sp1 DNA binding sites were necessary for HMBA-induced CDK9 recruitment and LTR expression. HMBA signaling via both protein kinase C mu and phosphatidylinositol 3-kinase appeared to contribute to LTR induction.

CONCLUSIONS

The novel mechanism through which HMBA disrupts latent HIV-1 infection involves 2 cellular kinases that may be therapeutically exploited to induce expression of persistent proviral HIV-1.

1,6-Diaminohexane contributes to the hexamethylene bisacetamide-induced erythroid differentiation pathway by stimulating Ca2+ release from inositol 1,4,5-trisphosphate-sensitive stores and promoting Ca2+ influx

Hexamethylene bisacetamide (HMBA) stimulates Ca(2+) signals in murine erythroleukemia (MEL) cells serving as an important component of the HMBA-induced pathway that promotes differentiation to the erythroid phenotype. We observed that 1,6-diaminohexane (DAH) triggered a more rapid and robust increase in MEL cell Ca(2+) levels compared to HMBA and the monodeacetylated N-acetyl-1,6-diaminohexane (NADAH), and that polyamine deacetylase inhibition completely abolished the ability of HMBA and NADAH to induce Ca(2+) signals in MEL cells. Our work indicates that DAH mediates Ca(2+) signal propagation via its ability to activate inositol 1,4,5-trisphosphate (IP(3)) receptors, as we observed similar Ca(2+) release characteristics and heparin sensitivity of DAH and IP(3) in permeabilized MEL cells. Finally, we observed that the DAH-induced Ca(2+) release pathway robustly coupled to a Ca(2+) influx pathway that could be distinguished from thapsigargin-induced Ca(2+) influx by its unusual insensitivity to 2-aminoethoxydiphenyl borate.

4-Hydroxynonenal-induced MEL cell differentiation involves PKC activity translocation

4-Hydroxynonenal (HNE) is a highly reactive aldehyde, produced by cellular lipid peroxidation, able to inhibit proliferation and to induce differentiation in MEL cells at concentrations similar to those detected in several normal tissues. Inducer-mediated differentiation of murine erythroleukemia (MEL) cells is a multiple step process characterized by modulation of several genes as well as by a transient increase in the amount of membrane-associated protein kinase C (PKC) activity. Here we demonstrate that a rapid translocation of PKC activity from cytosol to the membranes occurs during the differentiation induced by HNE. When PKC is completely translocated by phorbol-12-myristate-13-acetate (TPA), the degree of HNE-induced MEL cells differentiation is highly decreased. However, if TPA is washed out from the culture medium before the exposition to the aldehyde, HNE gradually resumes its differentiative ability. The incubation of cells with a selective inhibitor of PKC activity, bisindolylmaleimide GF 109203X, partially prevents the HNE-induced differentiation in MEL cells. In conclusion, our results demonstrate that HNE-induced MEL cell differentiation is preceded by a rapid translocation of PKC activity, and that the inhibition of this phenomenon prevents the onset of terminal differentiation.

Protein kinase C-α isoform is involved in erythropoietin-induced erythroid differentiation of CD34+ progenitor cells from human bone marrow

Protein kinase C (PKC) is a family of serine/threonine protein kinases involved in many cellular responses. Although the analysis of PKC activity in many systems has provided crucial insights to its biologic function, the precise role of different isoforms on the differentiation of normal hematopoietic progenitor cells into the various lineages remains to be investigated. The authors have assessed the state of activation and protein expression of PKC isoforms after cytokine stimulation of CD34+ progenitor cells from human bone marrow. Freshly isolated CD34+ cells were found to express PKC-, PKC-β2, and PKC-ɛ, whereas PKC-δ, PKC-γ, and PKC-ζ were not detected. Treatment with erythropoietin (EPO) or with EPO and stem cell factor (SCF) induced a predominantly erythroid differentiation of CD34+ cells that was accompanied by the up-regulation of PKC- and PKC-β2 protein levels (11.8- and 2.5-fold, respectively) compared with cells cultured in medium. Stimulation with EPO also resulted in the nuclear translocation of PKC- and PKC-β2 isoforms. Notably, none of the PKC isoforms tested were detectable in CD34+ cells induced to myeloid differentiation by G-CSF and SCF stimulation. The PKC inhibitors staurosporine and calphostin C prevented EPO-induced erythroid differentiation. Down-regulation of the PKC-, PKC-β2, and PKC-ɛ expression by TPA pretreatment, or the down-regulation of PKC- with a specific ribozyme, also inhibited the EPO-induced erythroid differentiation of CD34+ cells. No effect was seen with PKC-β2–specific ribozymes. Taken together, these findings point to a novel role for the PKC- isoform in mediating EPO-induced erythroid differentiation of the CD34+ progenitor cells from human bone marrow.

Protein kinase C-theta is specifically activated in murine erythroleukaemia cells during mitosis

Protein kinase C-theta is a member of the n-protein kinase C subfamily that in mitotic cells translocates to centrosomes and kinetochores. Although this kinase is expressed in comparable amounts in murine erythroleukaemia cells during the interphase or metaphase, when localized in the mitotic structures, it selectively phosphorylates a 66 kDa protein, also associated to chromosomes. Moreover, protein kinase C-theta immunoprecipitated from cells at the metaphase results four times more active in the absence of lipid cofactors as compared with the kinase obtained from cells in the interphase. This activation is accomplished by interaction of protein kinase C-theta with a protein factor which also promotes an increased autophosphorylation of the kinase. These findings indicate that in the mitotic phase of the cell cycle, protein kinase C-theta recognizes a protein factor which operates as a positive modulator of the kinase activity in the absence lipids.

Lineage-Restricted Expression of Protein Kinase C Isoforms in Hematopoiesis

The pattern of expression of several protein kinase C (PKC) isoforms (, βΙ, δ, ɛ, η, and ζ) during the course of hematopoietic development was investigated using primary human CD34+ hematopoietic cells and stable cell lines subcloned from the growth factor-dependent 32D murine hematopoietic cell line. Each 32D cell clone shows the phenotype and growth factor dependence characteristics of the corresponding hematopoietic lineage. Clear-cut differences were noticed between erythroid and nonerythroid lineages. (1) The functional inhibition of PKC-ɛ in primary human CD34+ hematopoietic cells resulted in a twofold increase in the number of erythroid colonies. (2) Erythroid 32D Epo1 cells showed a lower level of bulk PKC catalytic activity, lacked the expression of ɛ and η PKC isoforms, and showed a weak or absent upregulation of the remaining isoforms, except βΙ, upon readdition of Epo to growth factor-starved cells. (3) 32D, 32D GM1, and 32D G1 cell lines with mast cell, granulo-macrophagic, and granulocytic phenotype, respectively, expressed all the PKC isoforms investigated, but showed distinct responses to growth factor readdition. (4) 32D Epo 1.1, a clone selected for interleukin-3 (IL-3) responsiveness from 32D Epo1, expressed the ɛ isoform only when cultured with IL-3. On the other hand, when cultured in Epo, 32D Epo1.1 cells lacked the expression of both ɛ and η PKC isoforms, similarly to 32D Epo1. (5) All 32D cell lines expressed the mRNA for PKC-ɛ, indicating that the downmodulation of the ɛ isoform occurred at a posttranscriptional level. In conclusion, the PKC isoform expression during hematopoiesis appears to be lineage-specific and, at least partially, related to the growth factor response.

Lineage-Restricted Expression of Protein Kinase C Isoforms in Hematopoiesis

The pattern of expression of several protein kinase C (PKC) isoforms (, βΙ, δ, ɛ, η, and ζ) during the course of hematopoietic development was investigated using primary human CD34+ hematopoietic cells and stable cell lines subcloned from the growth factor-dependent 32D murine hematopoietic cell line. Each 32D cell clone shows the phenotype and growth factor dependence characteristics of the corresponding hematopoietic lineage. Clear-cut differences were noticed between erythroid and nonerythroid lineages. (1) The functional inhibition of PKC-ɛ in primary human CD34+ hematopoietic cells resulted in a twofold increase in the number of erythroid colonies. (2) Erythroid 32D Epo1 cells showed a lower level of bulk PKC catalytic activity, lacked the expression of ɛ and η PKC isoforms, and showed a weak or absent upregulation of the remaining isoforms, except βΙ, upon readdition of Epo to growth factor-starved cells. (3) 32D, 32D GM1, and 32D G1 cell lines with mast cell, granulo-macrophagic, and granulocytic phenotype, respectively, expressed all the PKC isoforms investigated, but showed distinct responses to growth factor readdition. (4) 32D Epo 1.1, a clone selected for interleukin-3 (IL-3) responsiveness from 32D Epo1, expressed the ɛ isoform only when cultured with IL-3. On the other hand, when cultured in Epo, 32D Epo1.1 cells lacked the expression of both ɛ and η PKC isoforms, similarly to 32D Epo1. (5) All 32D cell lines expressed the mRNA for PKC-ɛ, indicating that the downmodulation of the ɛ isoform occurred at a posttranscriptional level. In conclusion, the PKC isoform expression during hematopoiesis appears to be lineage-specific and, at least partially, related to the growth factor response.

Effect of polyisobutylcyanoacrylate nanoparticles and lipofectin loaded with oligonucleotides on cell viability and PKC alpha neosynthesis in HepG2 cells

The aim of the present study was to evaluate the inhibitory effect on protein kinase C alpha (PKC alpha) neosynthesis of antisense oligonucleotides delivered by two types of carriers. First, PKC alpha antisense oligonucleotides were associated with polyisobutylcyanoacrylate (PIBCA) nanoparticles pre-coated with cetyltrimethyl ammonium bromide (CTAB), a hydrophobic cation. Adsorption of oligonucleotides onto PIBCA nanoparticles was shown to be a saturating process. From these studies, it was possible to identify two types of particles: positively and negatively charged. Secondly, Lipofectin was used as another carrier system. These systems were incubated with HepG2 cells. Toxicity was evaluated by the MTT assay, and PKC alpha neosynthesis was determined by Western blots in conditions where nanoparticles and Lipofectin were not inducing cytotoxicity. It was observed that both mismatch and antisense oligonucleotides induced an inhibition of PKC alpha neosynthesis when loaded onto cationic or anionic nanoparticles as well as when complexed to cationic liposomes (Lipofectin). This non-specific effect was only observed in the phase of PKC alpha neosynthesis when the cells were first depleted in PKC alpha by phorbol 12-myristate beta-acetate (12-PMA) and in the absence of serum. These results strongly suggest that delivery systems, PIBCA nanoparticles or Lipofectin, containing a positively charged component (CTAB or cationic lipids), are able to induce a perturbation in the intracellular metabolic activity. In conclusion, it was shown that the commonly used strategy of oligonucleotides targeting with cationic non-viral vectors may display non-specific effects which can lead to artifactual results.

G-protein-coupled receptors in normal human erythroid progenitor cells

Human erythroid progenitor cells were isolated from peripheral blood of healthy donors and amplified in a suspension culture system using recombinant growth factors (stem cell factor, interleukin-3, granulocyte-macrophage colony-stimulating factor and erythropoietin) as well as conditioned medium from a human bone marrow stroma cell line to support cell proliferation. After 6-8 days of culture, the cell population consisted mainly of erythroid colony-forming cells (burst-forming units, BFU-Es and colony-forming units, CFU-Es). In these cells, we studied ligand-induced changes in intracellular Ca2+ concentration ([Ca2+]i) and cAMP formation as the primary effector systems of guanine nucleotide-binding protein (G protein)-coupled receptors. The results confirmed the functional expression of receptors for adenosine (type A2B), prostaglandin E1 and isoprenaline (beta-adrenoceptor), all of which stimulated adenylyl cyclase, as well as for ADP (purinoceptor types P2T and P2U), platelet-activating factor and thrombin all of which caused a transient increase in [Ca2+]i. The efficacy of adenosine and prostaglandin E1 in stimulating cAMP formation was more than 5 times higher than that of isoprenaline, suggesting a low beta-adrenoceptor density. The response to adenosine and isoprenaline decreased by 80 and 55% respectively during maturation into the proerythroblast stage. Similarly, thapsigargin-sensitive intracellular Ca2+ stores and ligand-induced Ca2+ release declined by about 60% during the CFU-E-to-erythroblast transition. The overall functional expression pattern of G protein-coupled receptors differed from that in human erythroleukaemia cell lines or from that in platelets. Primary culture systems for nontransformed cells, such as the one presented here, thus will be indispensable for the study of the functional role of G protein-dependent signalling during haematopoiesis.

Antisense oligodeoxynucleotide inhibition of delta protein kinase C expression accelerates induced differentiation of murine erythroleukaemia cells

The potential regulatory role of delta protein kinase C (delta PKC) in murine erythroleukaemia cell differentiation was studied by using antisense oligodeoxynucleotides targeting the translation initiation region of mouse delta PKC mRNA. Cell treatment with antisense oligonucleotides, at a concentration of 20 microM, followed by hexamethylenebisacetamide induction, produced a specific 2-fold increase in the differentiation rate of both slowly and rapidly differentiating murine erythroleukaemia cell clones. Cell permeabilization by a cationic lipid resulted in a decrease of one order of magnitude in the amounts of antisense oligonucleotides necessary to elicit the maximal response, and accelerated the kinetics of the stimulatory effect. These changes in murine erythroleukaemia cell differentiation rates, observed in both cell clones, were associated with 60% and 50% decreases, respectively, in delta PKC immunoreactive protein in slowly and rapidly differentiating cells. The present results indicate strongly that basal levels of delta PKC in murine erythroleukaemia cells are essential in regulating the initial differentiation rate of these cells in response to chemical induction, and provide further evidence that this PKC isoform plays a fundamental role in maintaining the undifferentiated phenotype of murine erythroleukaemia cells.

Extracellular release of the 'differentiation enhancing factor', a HMG1 protein type, is an early step in murine erythroleukemia cell differentiation

Differentiation enhancing factor (DEF) is a 29 kDa protein expressed in murine erythroleukemia (MEL) cells and active in promoting a significant increase in the rate of hexamethylenebisacetamide induced differentiation of these cells. The factor was recently shown to possess an amino acid sequence identical to that reported for one of the HMG1 proteins, designated as 'amphoterin' on the basis of its highly dipolar sequence. In the present study, we have expressed DEF cDNA in an E. coli strain and found that the recombinant protein has functional properties identical to those observed with native DEF. Furthermore, we demonstrate that, following MEL cell stimulation with the chemical inducer, DEF is secreted in large amounts in the extracellular medium. In fact, the N-terminal sequence and the partial amino acid sequence of tryptic peptides from the secreted protein correspond to those of DEF isolated from the soluble fraction of resting MEL cells. These results are indicative for an extracellular localization as the site of action of DEF and suggest a novel function for proteins belonging to the HMG1 family. Finally, the early decay of DEF mRNA, in chemical induced MEL cells, support the hypothesis that the involvement of the enhancing factor occurs and is completed in the early phases of cell differentiation.

Changes in calcium influx affect the differentiation of murine erythroleukaemia cells

As indicated by direct evidence, obtained by altering the cell-membrane permeability for Ca2+ in murine erythroleukaemia (MEL) cells, calpain is the triggering factor which connects fluctuations of the intracellular Ca2+ concentrations to the decay of protein kinase C (PKC), as well as to the kinetics of cell differentiation induced by hexamethylenebisacetamide. Cell exposure to verapamil caused a profound decrease in the rate of PKC down-regulation and a slower initial rate of accumulation of mature erythroid cells, whereas addition of the Ca2+ ionophore A23187 produced opposite effects. The high susceptibility of PKC-delta to calpain degradation, at concentrations of Ca2+ much lower than those required for degradation of the other PKC isoforms, may be explained by the finding that this kinase isoform is predominantly associated with the cell membrane. The different cellular localizations, as well as the different susceptibilities to calpain digestion, further support the hypothesis that in MEL cells the various PKC isoforms play distinct biological functions that are critical for the maintenance of the undifferentiated state of the cell and for its commitment to terminal erythroid differentiation.

Modulation of the intracellular Ca(2+)-dependent proteolytic system is critically correlated with the kinetics of differentiation of murine erythroleukemia cells

Calpain has been identified as the intracellular proteinase that catalyzes the selective down-regulation of protein kinase C (PKC) isoforms, occurring in the early stages of commitment to terminal erythroid differentiation of murine erythroleukemia (MEL) cells induced by hexamethylenebisacetamide. This conclusion has been reached through direct experiments performed with two MEL cell clones, one characterized by a high and the other by a low rate of differentiation. In both cell types, introduction of an anti-calpain antibody resulted in a significant delay in the onset of down-regulation of PKC isoforms, and in an increase in the latent period that precedes differentiation. Both cell lines also displayed reduced rates of PKC decay and accumulation of mature erythroid cells. Furthermore, in the fast-responding clone, calpastatin, the natural calpain-inhibitor protein, was found to be almost completely absent, resulting in activation and expression of proteolytic activity of calpain even at micromolar concentrations of Ca2+, a condition not sufficient to trigger calpain activation in the slowly responding clone which contains high levels of calpastatin. The fast-responding MEL cell clone, enriched with calpastatin, displayed a lower rate of cell differentiation, with a kinetics almost identical to that observed following introduction of the anti-calpain antibody. It is proposed that Ca(2+)-dependent proteolysis plays a crucial role for the progress of MEL cell differentiation through the specific degradation of PKC isozymes.