Differentiation and growth modulation of myeloid leukemia cells by the protein kinase C activating agent bryostatin-1

Bryostatin-1 enhances the proliferation and functionality of exhausted CD8+ T cells by upregulating MAP Kinase 11

Introduction

Bryostatin-1, a potent agonist of the protein kinase C, has been studied for HIV and cancer therapies. In HIV research, it has shown anti-HIV effects during acute infection and reactivation of latent HIV in chronic infection. As effective CD8+ T cell responses are essential for eliminating reactivated virus and achieving a cure, it is important to investigate how bryostatin-1 affects HIV-specific CD8+ T cells. HIV-specific CD8+ T cells often become exhausted, showing reduced proliferative potential and impaired cytokine production, a dysfunction also observed in cancer. Therefore, we further investigated how bryostatin-1 directly impacts exhausted CD8+ T cells.

Methods

PBMCs from people with HIV (PWH) were treated with bryostatin-1 and tracked with proliferation dye for cell expansion. One day 6, HIV-specific CD8+ T cells were detected by tetramers staining and examined by flow cytometry. By utilizing an established in vitro murine T cell exhaustion system, changes in inhibitory receptors, transcription factors, cytokine production and killing capacity of bryostatin-1 treated exhausted CD8+ T cells were determined by flow cytometry. RNA-seq analysis was performed to study transcriptional changes in these cells.

Results

We found that bryostatin-1 improved the expansion and decreased PD-1 expression of HIV-specific CD8+ T cells. Bryostatin-1 enhanced the functionality and proliferation while decreasing inhibitory receptor expression of in vitro generated exhausted CD8+ T cells. Bryostatin-1 upregulated TCF-1 and decreased TOX expression. These changes were confirmed through RNA-seq analysis. RNA-seq revealed that mitogen-activated protein kinases (MAPK) 11 was significantly downregulated in exhausted CD8+ T cells, however, it greatly upregulated after bryostatin-1 treatment. Inhibition of MAPK11 in bryostatin-1-treated cells blocked the increased proliferation and IFN-γ production induced by bryostatin-1, but did not affect other bryostatin-1 induced effects, such as the reduction of inhibitory receptors.

Discussion

Our data demonstrate that bryostatin-1 induces a MAPK 11-dependent improvement in the proliferative and functional capacity of exhausted T cells. This study provides a rationale for bryostatin-1's potential to help eradicate the HIV reservoir during treatment, and it may also contribute to cancer immunotherapy by functionally improving exhausted CD8+ T cells.

Structural insights into C1-ligand interactions: Filling the gaps by in silico methods

Protein Kinase C isoenzymes (PKCs) are the key mediators of the phosphoinositide signaling pathway, which involves regulated hydrolysis of phosphatidylinositol (4,5)-bisphosphate to diacylglycerol (DAG) and inositol-1,4,5-trisphosphate. Dysregulation of PKCs is implicated in many human diseases making this class of enzymes an important therapeutic target. Specifically, the DAG-sensing cysteine-rich conserved homology-1 (C1) domains of PKCs have emerged as promising targets for pharmaceutical modulation. Despite significant progress, the rational design of the C1 modulators remains challenging due to difficulties associated with structure determination of the C1-ligand complexes. Given the dearth of experimental structural data, computationally derived models have been instrumental in providing atomistic insight into the interactions of the C1 domains with PKC agonists. In this review, we provide an overview of the in silico approaches for seven classes of C1 modulators and outline promising future directions.

Recent Advances in the Development of Anticancer Drugs that Act against Signalling Pathways

Cancer can be considered a disease of deranged intracellular signalling. The intracellular signalling pathways that mediate the effects of oncogenes on cell growth and transformation present attractive targets for the development of new classes of drugs for the prevention and treatment of cancer. This is a new approach to developing anticancer drugs and the potential, as well as some of the problems, inherent in the approach are discussed. Anticancer drugs that produce their effects by disrupting signalling pathways are already in clinical trial. Some properties of these drugs, as well as other inhibitors of signalling pathways under development as potential anticancer drugs, are reviewed.

Protein kinase C-mediated phosphorylation of the leukemia-associated HOXA9 protein impairs its DNA binding ability and induces myeloid differentiation

HOXA9 expression is a common feature of acute myeloid leukemia, and high-level expression is correlated with poor prognosis. Moreover, HOXA9 overexpression immortalizes murine marrow progenitors that are arrested at a promyelocytic stage of differentiation when cultured and causes leukemia in recipient mice following transplantation of HOXA9 expressing bone marrow. The molecular mechanisms underlying the physiologic functions and transforming properties of HOXA9 are poorly understood. This study demonstrates that HOXA9 is phosphorylated by protein kinase C (PKC) and casein kinase II and that PKC mediates phosphorylation of purified HOXA9 on S204 as well as on T205, within a highly conserved consensus sequence, in the N-terminal region of the homeodomain. S204 in the endogenous HOXA9 protein was phosphorylated in PLB985 myeloid cells, as well as in HOXA9-immortalized murine marrow cells. This phosphorylation was enhanced by phorbol ester, a known inducer of PKC, and was inhibited by a specific PKC inhibitor. PKC-mediated phosphorylation of S204 decreased HOXA9 DNA binding affinity in vitro and the ability of the endogenous HOXA9 to form cooperative DNA binding complexes with PBX. PKC inhibition significantly reduced the phorbol-ester induced differentiation of the PLB985 hematopoietic cell line as well as HOXA9-immortalized murine bone marrow cells. These data suggest that phorbol ester-induced myeloid differentiation is in part due to PKC-mediated phosphorylation of HOXA9, which decreases the DNA binding of the homeoprotein.

Primary acute myeloid leukaemia blasts resistant to cytokine-induced differentiation to dendritic-like leukaemia cells can be forced to differentiate by the addition of bryostatin-1

Primary acute myeloid leukaemia (AML) blasts can be induced to differentiate into dendritic-like leukaemia cells (DLLC) by culture with certain cytokine combinations. DLLC offer potential for use as autologous vaccines based on their ability to present putative leukaemia-specific antigens to T cells. It has been reported, however, that in around 30-50% of AML cases the leukaemia cells are not capable of undergoing DLLC differentiation. The purpose of this study was to identify the features that represent successful DLLC differentiation and, for those cases shown to be resistant to cytokine-induced differentiation, to use differentiating agents in an attempt to overcome the differentiation block. Leukaemia cells derived from 42 patients with AML were cultured in vitro with cytokines GM-CSF, IL-4 and TNFalpha/CD40L. In 22 cases the leukaemic cells underwent DLLC differentiation based on characteristic morphological changes and expression of costimulatory and dendritic cell-associated molecules. Four cases were not evaluable because of poor viability over the culture period. The remaining 16 cases failed to show evidence of DLLC differentiation. Many of these differentiation resistant cases were associated with poor risk karyotypic features. Nine of the resistant cases were selected for further study. Differentiating agents trichostatin (TSA), azacytidine (AZA) and bryostatin (BRYO) were used in combination with cytokines for the first 96 h of the culture period. Bryostatin (BRYO) alone was shown to be capable of overcoming differentiation resistance and allowing DLLC differentiation to proceed.

Evidence for the biosynthesis of bryostatins by the bacterial symbiont "Candidatus Endobugula sertula" of the bryozoan Bugula neritina

The marine bryozoan, Bugula neritina, is the source of the bryostatins, a family of macrocyclic lactones with anticancer activity. Bryostatins have long been suspected to be bacterial products. B. neritina harbors the uncultivated gamma proteobacterial symbiont "Candidatus Endobugula sertula." In this work several lines of evidence are presented that show that the symbiont is the most likely source of bryostatins. Bryostatins are complex polyketides similar to bacterial secondary metabolites synthesized by modular type I polyketide synthases (PKS-I). PKS-I gene fragments were cloned from DNA extracted from the B. neritina-"E. sertula" association, and then primers specific to one of these clones, KSa, were shown to amplify the KSa gene specifically and universally from total B. neritina DNA. In addition, a KSa RNA probe was shown to bind specifically to the symbiotic bacteria located in the pallial sinus of the larvae of B. neritina and not to B. neritina cells or to other bacteria. Finally, B. neritina colonies grown in the laboratory were treated with antibiotics to reduce the numbers of bacterial symbionts. Decreased symbiont levels resulted in the reduction of the KSa signal as well as the bryostatin content. These data provide evidence that the symbiont E. sertula has the genetic potential to make bryostatins and is necessary in full complement for the host bryozoan to produce normal levels of bryostatins. This study demonstrates that it may be possible to clone bryostatin genes from B. neritina directly and use these to produce bryostatins in heterologous host bacteria.

The Raf signal transduction cascade as a target for chemotherapeutic intervention in growth factor-responsive tumors

This review focuses on the Ras-Raf-mitogen-activated protein kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) signal transduction pathway and the consequences of its unregulation in the development of cancer. The roles of some of the cell membrane receptors involved in the activation of this pathway, the G-protein Ras, the Raf, MEK and ERK kinases, the phosphatases that regulate these kinases, as well as the downstream transcription factors that become activated, are discussed. The roles of the Ras-Raf-MEK-ERK pathway in the regulation of apoptosis and cell cycle progression are also analyzed. In addition, potential targets for pharmacological intervention in growth factor-responsive cells are evaluated.

Pharmacology and clinical experience with bryostatin 1: a novel anticancer drug

Bryostatin 1 (bryo 1) is an example of a novel class of anticancer drug which modulates protein kinase C (PKC) activity. It has varied biological effects mediated largely by the initial activation of PKC, followed by its rapid downregulation. Bryo 1 stimulates in vitro and in vivo haematopoietic progenitor cell growth in a concentration-dependent and lineage-specific fashion. Granulocytes, lymphocytes, monocytes and platelets are all functionally stimulated by bryo 1. Stimulation of cytotoxic T-cell activity by bryo 1 has led to research utilising bryo 1 as an immunotherapeutic agent in mouse tumour xenograft models. The clinical development of bryo 1 followed the demonstration of direct in vitro activity against various tumour cell lines. Multiple Phase I trials have shown muscle pain and flu-like symptoms are the most common toxicities associated with administration of bryo 1. There is particular interest in the role of bryo 1 in haematologic malignancies because of its capacity to induce leukaemic cell differentiation. There is ample in vitro data demonstrating that bryo 1 can sensitise tumour cells to cytotoxic agents. Recent clinical work has focused on combining bryo 1 with traditional chemotherapeutic agents for both haematologic and non-haematologic cancers.

Effects of bryostatin-1 on chronic myeloid leukaemia-derived haematopoietic progenitors

Bryostatin-1 belongs to the family of macrocyclic lactones isolated from the marine bryozoan Bugula neritina and is a potent activator of protein kinase C (PKC). Bryostatin has been demonstrated to possess both in vivo and in vitro anti-leukaemic potential. In samples derived from chronic myeloid leukaemia (CML) patients, it has been demonstrated that bryostatin-1 induces a macrophage differentiation, suppresses colony growth in vitro and promotes cytokine secretion from accessory cells. We investigated the effect of bryostatin-1 treatment on colony-forming unit-granulocyte macrophage (CFU-GM) capacity in the presence of accessory cells, using mononuclear cells, as well as in the absence of accessory cells using purified CD34-positive cells. Cells were obtained from 14 CML patients as well as from nine controls. Moreover, CD34-positive cells derived from CML samples and controls were analysed for stem cell frequency and ability using the long-term culture initiating cell (LTCIC) assay at limiting dilution. Individual colonies derived from both the CFU-GM and LTCIC assays were analysed for the presence of the bcr-abl gene with fluorescence in situ hybridization (FISH) to evaluate inhibition of malignant colony growth. The results show that at the CFU-GM level bryostatin-1 treatment resulted in only a 1.4-fold higher reduction of CML colony growth as compared to the control samples, both in the presence and in the absence of accessory cells. However, at the LTCIC level a sixfold higher reduction of CML growth was observed as compared to the control samples. Analysis of the LTCICs at limiting dilution indicates that this purging effect is caused by a decrease in output per malignant LTCIC combined with an increase in the normal stem cell frequency. It is concluded that bryostatin-1 selectively inhibits CML growth at the LTCIC level and should be explored as a purging modality in CML.

The inhibitory effects of bryostatin 1 administration on the growth of rabbit papillomas

Bryostatin 1 is a protein kinase C modulator that shows antineoplastic activity in a variety of tumor systems. This study examined the effects of bryostatin 1 administration on papilloma growth in rabbits. Investigations of optimal route, dose, and schedule were performed. Several groups of rabbits were inoculated with cottontail rabbit papillomavirus (CRPV) DNA. Bryostatin 1 was administered i.v., both daily and weekly, and intralesionally both weekly and bi-weekly. Intralesionally dosed papillomas were examined histologically for immune cell infiltration. In weekly and daily i.v. trials, 2.5 and 1.0 microg/kg, respectively, showed the greatest overall reduction in tumor size. Bryostatin 1 administered intralesionally also slowed papilloma growth. Treated lesions had significantly higher numbers of heterophils and eosinophils.

Heterogenous effects of bryostatin on human myeloid leukemia clonogenicity: dose and time scheduling dependency

The potent anti-neoplastic actions displayed in vitro by bryostatins have led to the introduction of short-term bryostatin-1 infusions in phase I clinical trials. Because bryostatin (bryo) undergoes a rapid clearance in vivo, we were interested in its scheduling effects on acute myeloid leukemia (AML) clonogenicity. Therefore, we assessed the primary plating efficiency (PE1) of AML samples in response to several bryo concentrations after various preincubation periods in a semi-solid colony forming assay. Whereas continuous exposure to 10 nM bryo during the assay period reduced the PE1 in nearly all samples tested, preincubation of eight AML patients' specimens for 1, 2, 3 or 4 days with bryo in a dose range of 0.1-10 nM showed a heterogenous PE1 response. Stimulatory as well as inhibitory effects on leukemic clonogenic growth were seen within individual specimens depending on dose and preincubation time with no single incubation time or concentration that caused unequivocal common overall inhibition of clonogenic growth in most or all of the samples. Higher doses of bryo also failed to result in specific inhibition of leukemic cells: 4/8 samples showed an increased clonogenic response to 250 nM bryo whereas normal bone marrow progenitor cells were consistently inhibited in their clonogenicity at this dose. A marked similarity, i.e. undulatory effects with increasing bryo concentrations, was found for HL60 leukemic cells. In conclusion, the effects of bryo on clonogenic leukemia cell growth are strongly dependent on scheduling and dose varying between and within individual AML samples. These results caution against in vivo bryo pulse therapy, as currently applied, for treatment of AML.

The clonogenic precursor cell in multiple myeloma

Multiple myeloma is characterized by the monoclonal expansion of plasma cells in the bone marrow. Although the predominant cell type is the plasma cell, the initial oncogenic transformation is considered to take place in a more immature B cell. There is still much controversy about this precursor cell type. Phenotypic analysis of bone marrow and peripheral blood revealed that in multiple myeloma a great diversity exists in the phenotype of the cells considered to be involved. Because of the lack of a myeloma specific genetic lesion it is very difficult to trace back the cell in which the transforming event, leading to multiple myeloma, took place. The only real clonal marker is the idiotype of the immunoglobulin molecule expressed by the myeloma cells. With recombinant DNA technology it is now possible to produce clonal markers for each individual myeloma patient which recognize only the immunoglobulin genes expressed by the myeloma cell and its precursors. The sequences of these myeloma immunoglobulin genes do reveal a lot of information about the stage in the B-cell differentiation pathway in which the oncogenic event might have taken place. The presence of somatic mutations in a non-random fashion without intraclonal variation leads to the conclusion that the precursor myeloma cell could not possibly be a pre-B cell or stem cell but has to be a mature B cell that has been in contact with antigen and has past through the phase of somatic mutation, like a memory B cell or plasmablast.(ABSTRACT TRUNCATED AT 250 WORDS)

Bryostatin-5 stimulates normal human hematopoiesis and inhibits proliferation of HL60 leukemic cells

In this study we explored the effects of bryostatin-5 on the clonogenic response of normal bone marrow mononuclear (BM) cells and HL60 myeloid leukemia cells. Leukemic HL60 colony formation was strongly inhibited by bryostatin-5 depending on dose and schedule. An inhibitory effect on HL60 colony formation was readily demonstrated after 1 h of exposure, reaching a maximal inhibitory effect at 96 h. Normal BM cells differed in their clonogenic response: short-term exposure to bryostatin-5 resulted in increased clonogenicity while longstanding exposure to bryostatin-5 permitted the survival of a substantial fraction of committed progenitors. This differential modulation of normal and leukemic myeloid clonogenicity by bryostatin-5 suggests a possible role for bryostatin-5 in the treatment of acute myeloid leukemia.

Different biological effects of the two protein kinase C activators bryostatin-1 and TPA on human carcinoma cell lines

Bryostatin 1 (Bryo) is a naturally occurring macrocyclic lactone with antineoplastic activity. Like the phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) it directly activates the calcium- and phospholipid-dependent protein kinase C (PKC), thus generating a number of different cellular responses. We investigated the effects of Bryo and TPA on DNA synthesis, proliferation, viability and c-myc protooncogene expression of the human carcinoma cell lines COLO-320, MEL-HO, and KB-3-1. TPA inhibited [3H]-thymidine incorporation in all three cell lines in a dose-dependent manner, whereas Bryo only inhibited the DNA synthesis in MEL-HO, but not in KB-3-1 and COLO-320 cells. Within the concentration ranges used, TPA and Bryo were found to have a low toxicity. Counting of the cells confirmed the observed inhibition of cell proliferation. However, the enzymatic conversion of MTT, applied as a colorimetric proliferation assay, was not significantly affected by both biomodulators. Time-course experiments revealed a rapid onset of the inhibitory effect on DNA synthesis. Bryo was further able to antagonize the TPA-mediated effects on proliferation suggesting an (at least partially) different mode of action of these PKC activators. Incubation of MEL-HO and COLO-320 cells with either of the two biomodulators resulted in a rapid and strong increase of c-myc mRNA. The present study emphasizes Bryo as an interesting, natural substance for the study of PKC-mediated biological effects.

Expression and modulation of annexin VIII in human leukemia-lymphoma cell lines

Annexin VIII is a calcium- and phospholipid-binding protein with anticoagulant activity. Annexin VIII mRNA was found to be specifically expressed in acute promyelocytic leukemia (APL) cells; it was not found in other types of acute myeloid leukemia (AML) nor in lymphoid malignancies. Using Northern blot analysis we investigated annexin VIII expression in 142 continuous human leukemia and lymphoma cell lines at the mRNA level. While the only APL cell line, NB-4, was indeed positive, other cell lines also displayed annexin VIII mRNA: 4/22 myeloid cell lines, 8/23 monocytic cell lines, 2/8 megakaryoblastic cell lines, 5/26 lymphoma-derived cell lines, 2/10 myeloma cell lines and 1/44 lymphoid leukemia cell lines. The strongest expression was seen in NB-4 and in the Hodgkin's disease derived cell line HDLM-2. Treatment of NB-4 cells with all-trans retinoic acid (ATRA) or the phorbol ester TPA induced terminal differentiation and down-regulated annexin VIII mRNA expression rapidly within a few hours; vitamin D3 was ineffective in this regard; the protein kinase C activator Bryostatin 1 up-regulated the expression. A panel of initially negative cell lines could not be induced by any of these biomodulators to transcribe annexin VIII. The half-life (T1/2) of annexin VIII mRNA was about 3-4 h using actinomycin D as transcription inhibitor. Treatment with ATRA or TPA prior to exposure to actinomycin shortened the T1/2 to 2 h while Bryostatin 1 extended it to 6h. As 21/141 non-APL cell lines were positive, annexin VIII cannot be used as a marker gene for APL cells; however, it might be associated with myelomonocytic or erythro-megakaryoblastic precursor cells. Annexin VIII gene expression might play a unique role in the proliferation and/or differentiation of leukemic cells and could be associated with the particular abnormal hemostasis of some leukemias.

Different effects of the two protein kinase C activators bryostatin-1 and TPA on growth and differentiation of human monocytic leukemia cell lines

The effects of bryostatin-1 (Bryo) and 12-O-tetradecanoyl-phorbol 13-acetate (TPA), both activators of protein kinase C (PKC), on proliferation and differentiation of two monocytic leukemia cell lines, JOSK-I and JOSK-M, were investigated. Treatment with TPA or Bryo inhibited cellular proliferation in a dose-dependent manner. Both drugs induced distinct phenotypic changes associated with monocytic differentiation. Although c-myc mRNA is often found to be down-regulated during biomodulator-triggered in vitro myelomonocytic differentiation, however, here the modulation of c-myc expression was less pronounced. All parameters studied were more prominently altered in TPA- than in Bryo-treated cells, and, were more distinct in JOSK-I than in JOSK-M. Since Bryo was able to antagonize the TPA-mediated effects on proliferation and morphological alterations, an (at least partially) different mode of action of these PKC activators on monocytic cell lines may be suggested.