Activation of cAMP Signaling Interferes with Stress-Induced p53 Accumulation in ALL-Derived Cells by Promoting the Interaction between p53 and HDM2 1 2
ABSTRACT The tumor suppressor p53 provides an important barrier to the initiation and maintenance of cancers. As a consequence, p53 function must be inactivated for a tumor to develop. This is achieved by mutation in approximately 50% of cases and probably by functional inactivation in the remaining cases. We have previously shown that the second messenger cAMP can inhibit DNA damage-induced wild-type p53 accumulation in acute lymphoblastic leukemia cells, leading to a profound reduction of their apoptotic response. In the present article, we provide a mechanistic insight into the regulation of p53 levels by cAMP. We show that increased levels of cAMP augment the binding of p53 to its negative regulator HDM2, overriding the DNA damage-induced dissociation of p53 from HDM2. This results in maintained levels of p53 ubiquitination and proteasomal degradation, which in turn counteracts the DNA damage-induced stabilization of the p53 protein. The apoptosis inhibitory effect of cAMP is further shown to depend on this effect on p53 levels. These findings potentially implicate deregulation of cAMP signaling as a candidate mechanism used by transformed cells to quench the p53 response while retaining wild-type p53.
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ABSTRACT: BackgroundB cell precursor acute lymphoblastic leukaemia (BCP-ALL) is the most common paediatric cancer. BCP-ALL blasts typically retain wild type p53, and are therefore assumed to rely on indirect measures to suppress transformation-induced p53 activity. We have recently demonstrated that the second messenger cyclic adenosine monophosphate (cAMP) through activation of protein kinase A (PKA) has the ability to inhibit DNA damage-induced p53 accumulation and thereby promote survival of the leukaemic blasts.Development of BCP-ALL in the bone marrow (BM) is supported by resident BM-derived mesenchymal stromal cells (MSCs). MSCs are known to produce prostaglandin E2 (PGE2) which upon binding to its receptors is able to elicit a cAMP response in target cells. We hypothesized that PGE2 produced by stromal cells in the BM microenvironment could stimulate cAMP production and PKA activation in BCP-ALL cells, thereby suppressing p53 accumulation and promoting survival of the malignant cells.Methods Primary BCP-ALL cells isolated from BM aspirates at diagnosis were cocultivated with BM-derived MSCs, and effects on DNA damage-induced p53 accumulation and cell death were monitored by SDS-PAGE/immunoblotting and flow cytometry-based methods, respectively. Effects of intervention of signalling along the PGE2-cAMP-PKA axis were assessed by inhibition of PGE2 production or PKA activity. Statistical significance was tested by Wilcoxon signed-rank test or paired samples t test.ResultsWe demonstrate that BM-derived MSCs produce PGE2 and protect primary BCP-ALL cells from p53 accumulation and apoptotic cell death. The MSC-mediated protection of DNA damage-mediated cell death is reversible upon inhibition of PGE2 synthesis or PKA activity. Furthermore our results indicate differences in the sensitivity to variations in p53 levels between common cytogenetic subgroups of BCP-ALL.Conclusions Our findings support our hypothesis that BM-derived PGE2, through activation of cAMP-PKA signalling in BCP-ALL blasts, can inhibit the tumour suppressive activity of wild type p53, thereby promoting leukaemogenesis and protecting against therapy-induced leukaemic cell death. These novel findings identify the PGE2-cAMP-PKA signalling pathway as a possible target for pharmacological intervention with potential relevance for treatment of BCP-ALL.Molecular Cancer 01/2015; 14(1):14. DOI:10.1186/s12943-014-0278-9 · 5.40 Impact Factor
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ABSTRACT: More recently, arsenic trioxide (ATO), was integrated into acute promyelocytic leukemia (APL) treatment, showing high efficacy and tolerability in patients with both ATRA-sensitive and ATRA-resistant APL. ATO could induce apoptosis at relatively high concentrations (0.5 to 2.0 micromol/L) and partial differentiation at low concentrations (0.1 to 0.5 micromol/L) in leukemic promyelocytes. It is known that cAMP agonists enhance low-dose ATO-induced APL cells differentiation. Less well appreciated was the possible interaction between relatively high-doses of ATO and enhanced levels of cAMP in APL cells. Here, we show that elevation of cAMP levels by forskolin inhibited ATO-mediated apoptosis in APL-derived NB4 cells, and this inhibition could be averted by cell permeable cAMP-dependent protein kinase inhibitor (14-22) amide. Inactivating phosphorylation of the proapoptotic protein Bad at Ser118 and phosphorylation of the CREB proto-oncogene at Ser133 were observed upon elevation of cAMP levels in NB4 cells. Phosphorylation of these PKA target proteins is known to promote cell survival in AML cells. The ability of cAMP to endow the APL cells with survival advantage is of particular importance when cAMP agonists may be considered as adjuncts to APL therapy.European journal of pharmacology 05/2014; DOI:10.1016/j.ejphar.2014.04.040 · 2.68 Impact Factor
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ABSTRACT: The rescue effect describes the phenomenon where irradiated cells or organisms derive benefits from the feedback signals sent from the bystander unirradiated cells or organisms. An example of the benefit is the mitigation of radiation-induced DNA damages in the irradiated cells. The rescue effect can compromise the efficacy of radioimmunotherapy (RIT) (and actually all radiotherapy). In this paper, the discovery and subsequent confirmation studies on the rescue effect were reviewed. The mechanisms and the chemical messengers responsible for the rescue effect studied to date were summarized. The rescue effect between irradiated and bystander unirradiated zebrafish embryos in vivo sharing the same medium was also described. In the discussion section, the mechanism proposed for the rescue effect involving activation of the nuclear factor κB (NF-κB) pathway was scrutinized. This mechanism could explain the promotion of cellular survival and correct repair of DNA damage, dependence on cyclic adenosine monophosphate (cAMP) and modulation of intracellular reactive oxygen species (ROS) level in irradiated cells. Exploitation of the NF-κB pathway to improve the effectiveness of RIT was proposed. Finally, the possibility of using zebrafish embryos as the model to study the efficacy of RIT in treating solid tumors was also discussed.International Journal of Molecular Sciences 02/2015; 16(2):2591-2609. DOI:10.3390/ijms16022591 · 2.34 Impact Factor