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With cAMP signaling having a profound inhibitory effect on DNA damage-induced apoptosis in B-cell precursor acute lymphoblastic leukemia (BCP-ALL) cells, understanding how this signaling pathway affects the survival capacity of the cell has important implications for cancer therapy. We have recently shown that p53 is critical for the inhibitory effect of cAMP on genotoxic agents-mediated apoptosis in BCP-ALLs. Here, we show that elevation of cAMP levels in cells exposed to DNA damage enhances the nuclear translocation and DNA binding of NF-κB by accelerating the phosphorylation of IKKβ and thereby phosphorylation and degradation of IκBα. Furthermore, we show that the ability of cAMP to potentiate the ionizing radiation-induced activation of NF-κB requires the activity of MEK. Importantly, pharmacological or genetic ablation of NF-κB reversed the inhibitory effect of cAMP on DNA damage-induced apoptosis, demonstrating that, in addition to p53, cAMP relies on the activity of NF-κB to provide cells with a survival advantage in the face of DNA damage. Collectively, our results uncover a novel and important interaction between the cAMP and NF-κB pathways that may have implications for the targeted treatment of lymphoid malignancies, such as BCP-ALL, in which aberrant NF-κB activity functions as a driving force for treatment resistance.

Abstract. Activation of cAMP signalling potently inhibits DNA damage-induced apoptosis in acute lymphoblastic leukemia cells by promoting the turnover of p53 protein. Recently, we showed that the cAMP-induced destabilization of p53 in DNA-damaged cells occurs as a result of enhanced interaction between p53 and HDM2. In this report, we present results showing that increased levels of cAMP in cells with DNA damage enhances the deacetylation of p53, an event that facilitates the interaction of p53 with HDM2, thus annulling the stabilizing effect of DNA damage on p53. The combined inhibition of the HDAC and SIRT1 deacetylases abolished the cAMP-mediated deacetylation of p53, implying that cAMP-mediated deacetylation of p53 is dependent on the activity of these two classes of histone deacetylases. Importantly, diminishing the activity of HDACs and SIRT1 was also found to reverse the inhibitory effect of cAMP on the DNA damage-induced p53 stabilization and apoptosis, suggesting the involvement of the p53 acetylation pathway in the anti-apoptotic effect of cAMP signalling.

Cyclin D3 has been shown to play a major role in the regulation of cell cycle progression in lymphocytes. It is therefore important to understand the mechanisms involved in the regulation of this protein. We have previously shown that both basal and cAMP-induced degradation of cyclin D3 in Reh cells is dependent on Thr-283 phosphorylation by glycogen synthase kinase-3b (GSK-3b). We now provide evidence of an alternative mechanism being involved in the regulation of cyclin D3 degradation. Treatment of lymphoid cells with okadaic acid (OA), an inhibitor of protein phosphatases 1 and 2A (PP1 and PP2A), induces rapid phosphorylation and proteasomal degradation of cyclin D3. This degradation is not inhibited by the GSK-3b inhibitors lithium or Kenpaullone, or by substitution of Thr-283 with Ala on cyclin D3, indicating that cyclin D3 can be degraded independently of Thr-283 phosphorylation and GSK-3b activity. Interestingly, in vitro experiments revealed that PP1, but not PP2A, was able to dephosphorylate cyclin D3 efficiently, and PP1 was found to associate with His-tagged cyclin D3. These results support the hypothesis that PP1 constitutively keeps cyclin D3 in a stable, dephosphorylated state, and that treatment of cells with OA leads to phosphorylation and degradation of cyclin D3 through inhibition of PP1.

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