Mad1 expression in the absence of differentiation: effect of cAMP on the B-lymphoid cell line Reh

N-acetyl cysteine turns EPAC activators into potent killers of acute lymphoblastic leukemia cells

Today, the majority of patients with pediatric B cell precursor acute lymphoblastic leukemia (BCP-ALL, hereafter ALL) survive their disease, but many of the survivors suffer from life-limiting late effects of the treatment. ALL develops in the bone marrow, where the cells are exposed to cAMP-generating prostaglandin E2. We have previously identified the cAMP signaling pathway as a putative target for improved efficacy of ALL treatment, based on the ability of cAMP signaling to reduce apoptosis induced by DNA damaging agents. In the present study, we have identified the antioxidant N-acetyl cysteine (NAC) as a powerful modifier of critical events downstream of the cell-permeable cAMP analog 8-(4-chlorophenylthio) adenosine-3', 5'- cyclic monophosphate (8-CPT). Accordingly, we found NAC to turn 8-CPT into a potent killer of ALL cells in vitro both in the presence and absence of DNA damaging treatment. Furthermore, we revealed that NAC in combination with 8-CPT is able to delay the progression of ALL in a xenograft model in NOD-scid IL2Rγnull mice. NAC was shown to rely on the ability of 8-CPT to activate the guanine-nucleotide exchange factor EPAC, and we demonstrated that the ALL cells are killed by apoptosis involving sustained elevated levels of calcium imposed by the combination of the two drugs. Taken together, we propose that 8-CPT in the presence of NAC might be utilized as a novel strategy for treating pediatric ALL patients, and that this powerful combination might be exploited to enhance the therapeutic index of current ALL targeting therapies.

cAMP-Mediated Autophagy Promotes Cell Survival via ROS-Induced Activation of PARP1: Implications for Treatment of Acute Lymphoblastic Leukemia

DNA-damaging therapy is the basis for treatment of most cancers, including B-cell precursor acute lymphoblastic leukemia (BCP-ALL, hereafter ALL). We have previously shown that cAMP-activating factors present in the bone marrow render ALL cells less sensitive to DNA damage-induced apoptosis, by enhancing autophagy and suppressing p53. To sensitize ALL cells to DNA-damaging therapy, we have searched for novel targets that may counteract the effects induced by cAMP signaling. In the current study, we have identified PARP1 as a potential target. We show that the PARP1 inhibitors olaparib or PJ34 inhibit cAMP-mediated autophagy and thereby potentiate the DNA-damaging treatment. Furthermore, we reveal that cAMP-mediated PARP1 activation is preceded by induction of reactive oxygen species (ROS) and results in depletion of nicotinamide adenine dinucleotide (NAD), both of which are autophagy-promoting events. Accordingly, we demonstrate that scavenging ROS by N-acetylcysteine and repleting NAD independently reduce DNA damage-induced autophagy. In addition, olaparib augmented the effect of DNA-damaging treatment in a human xenograft model of ALL in NOD-scidIL2Rgamma^null mice. On the basis of the current findings, we suggest that PARP1 inhibitors may enhance the efficiency of conventional genotoxic therapies and thereby provide a novel treatment strategy for pediatric patients with ALL. IMPLICATIONS: PARP1 inhibitors augment the DNA damage-induced killing of ALL cells by limiting the opposing effects of cAMP-mediated autophagy, which involves ROS-induced PARP1 activation and depletion of cellular NAD levels.

cAMP-mediated inhibition of DNA replication and S phase progression: involvement of Rb, p21Cip1, and PCNA

cAMP exerts an antiproliferative effect on a number of cell types including lymphocytes. This effect of cAMP is proposed to be mediated by its ability to inhibit G1/S transition. In this report, we provide evidence for a new mechanism whereby cAMP might inhibit cellular proliferation. We show that elevation of intracellular levels of cAMP inhibits DNA replication and arrests the cells in S phase. The cAMP-induced inhibition of DNA synthesis was associated with the increased binding of p21Cip1 to Cdk2-cyclin complexes, inhibition of Cdk2 kinase activity, dephosphorylation of Rb, and dissociation of PCNA from chromatin in S phase cells. The ability of cAMP to inhibit DNA replication and trigger release of PCNA from chromatin required Rb and p21Cip1 proteins, since both processes were only marginally affected by increased levels of cAMP in Rb-/- and p21Cip1-/- 3T3 fibroblasts. Importantly, the implications of cAMP-induced inhibition of DNA synthesis in cancer treatment was demonstrated by the ability of cAMP to reduce apoptosis induced by S phase-specific cytotoxic drugs. Taken together, these results demonstrate a novel role for cAMP in regulation of DNA synthesis and support a model in which activation of cAMP-dependent signaling protects cells from the effect of S phase-specific antitumor agents.

Modulation of B lymphocyte function by an aqueous fraction of the ethanol extract of Cissampelos sympodialis Eichl (Menispermaceae)

Cissampelos sympodialis Eichl species are used in folk medicine for the treatment of asthma, arthritis and rheumatism. In the present study, we investigated the immunomodulatory effect of an aqueous fraction of a 70% (v/v) ethanol extract of C. sympodialis leaves on B lymphocyte function. The hydroalcoholic extract inhibited the in vitro proliferative response of resting B cells induced by LPS (IC50 = 17.2 g/ml), anti-delta-dextran (IC50 = 13.9 g/ml) and anti-IgM (IC50 = 24.3 g/ml) but did not affect the anti-MHC class II antibody-stimulated proliferative response of B cell blasts obtained by stimulation with IL-4 and anti-IgM. Incubation with the hydroalcoholic extract used at 50 g/ml induced a 700% increase in intracellular cAMP levels. IgM secretion by resting B cells (obtained from normal mice) and polyclonally activated B cells (obtained from Trypanosoma cruzi-infected animals) was inhibited by the hydroalcoholic extract. The latter were more sensitive to the hydroalcoholic extract since 6.5 g/ml induced a 20% inhibition in the response of cells from normal mice while it inhibited the response of B cells from infected animals by 75%. The present data indicate that the alcoholic extract of C. sympodialis inhibited B cell function through an increase in intracellular cAMP levels. The finding that the hydroalcoholic extract inhibited immunoglobulin secretion suggests a therapeutic use for the extract from C. sympodialis in conditions associated with unregulated B cell function and enhanced immunoglobulin secretion. Finally, the inhibitory effect of the hydroalcoholic extract on B cells may indicate an anti-inflammatory effect of this extract.

Cyclic AMP inhibits translation of cyclin D3 in T lymphocytes at the level of elongation by inducing eEF2-phosphorylation

The purpose of the present study was to understand the mechanism by which activated protein kinase A (PKA) leads to down-regulation of cyclin D3 in lymphocytes. By using Jurkat cells as a model system, we have been able to demonstrate that cyclin D3 is reduced at the level of translation by inhibition of elongation. One of the important factors involved in translational elongation is the eukaryotic elongation factor 2 (eEF2). eEF2 promotes translation in its unphosphorylated form, and we observed a rapid phosphorylation of the eEF2-protein upon forskolin treatment. When using specific inhibitors of the eEF2-kinase prior to forskolin treatment, we were able to inhibit the increased phosphorylation of eEF2. Furthermore, inhibition of eEF2-kinase prevented the forskolin-mediated down-regulation of cyclin D3. Taken together, it appears that activation of PKA in Jurkat cells reduces the expression of cyclin D3 at the level of translational elongation by increasing the phosphorylation of eEF2 and thereby inhibiting its activity.

Forskolin-mediated G1 arrest in acute lymphoblastic leukaemia cells: phosphorylated pRB sequesters E2Fs

Increased intracellular levels of cAMP, induced by forskolin, lead to permanent G1 arrest of Reh cells. As expected, we observed a rapid dephosphorylation of the retinoblastoma protein (pRB) within 2 hours of forskolin treatment concomitant with reduced activity of the pRB-specific kinases. Interestingly, however, the dephosphorylation of pRB, as well as the inhibition of the kinase activities, was only transient, despite the permanent arrest of cells in G1. Importantly, although the pRB-specific kinases were fully active after 48 hours, pRB became only partially rephosphorylated.

The transient dephosphorylation of pRB could be explained by the transient decrease in the activities of the pRB-specific kinases, but to understand why pRB became only partially rephosphorylated, despite fully activated kinases,we postulated that cAMP could activate a pRB-directed phosphatase. It was therefore interesting to find that the phosphatase inhibitor, tautomycin, was able to abolish the forskolin-mediated dephosphorylation of pRB, without increasing the activities of the pRB-specific kinases.

To understand how Reh cells expressing hyperphosphorylated forms of pRB can remain arrested in G1, we used three different methods to test for the ability of pRB to form functional complexes with the family of E2F transcription factors. As expected, we observed an increased complex formation between E2F-1, E2F-4 and pRB after 2 hours when pRB was in its most dephosphorylated state. Suprisingly, however, prolonged treatment with forskolin, which induced partial rephosphorylation of pRB, in fact further increased the complex formation between the E2Fs and pRB, and this also resulted in reduced E2F-promoter activity in vivo. These data imply that in Reh cells, partially phosphorylated forms of pRB retain the ability to inhibit E2F-promoter activity, and thereby prevent cells from entering into S-phase.

Co-induction of Mad1 and c-Myc in activated normal B lymphocytes

The purpose of the present study was to examine the expression of the Myc network proteins c-Myc, Mad1 and Max in normal cells under different growth and differentiation conditions. A dominant view has been that Mad1 as a c-Myc antagonist plays a role in growth inhibition linked to differentiation. Of particular interest to us was therefore to study the regulation of Mad1 in cells undergoing differentiation in the absence of growth cessation. To do so we utilized normal B lymphocytes isolated from peripheral blood. The cells were induced to concomitant proliferation and differentiation by stimulation with a combination of anti-IgM antibodies (anti-mu) and the phorbol ester TPA. Thus, by 72 h of stimulation the percentage of plasmablasts increased from 3 to 17%, and the percentage of lymphocytes decreased from 89 to 27%. The most intriguing observation we made using this cell system was a pronounced coinduction of Mad1 and c-Myc. The levels of c-Myc and Mad1 mRNAs and proteins increased within 3 h of anti-mu stimulation, and the levels were further enhanced by TPA. Furthermore, the expressions of both c-Myc and Mad1 were reduced by forskolin, which also inhibited the anti-mu + TPA driven growth and differentiation of the B lymphocytes. The level of Max remained virtually unchanged. Taken together, our results indicate that a high level of Mad1 in normal human B cells is linked to differentiation and not to growth inhibition. Furthermore, the results demonstrate that Mad1 and c-Myc are not necessarily expressed in a reciprocal manner, which underlines an independent role of Mad1 unrelated to its function as a c-Myc antagonist.