Bcr-Abl-mediated redox regulation of the PI3K/AKT pathway.

Bcr-Abl causes chronic myelogenous leukemia, a myeloproliferative disorder characterized by clonal expansion of hematopoietic progenitor cells. In this study, inducible expression of Bcr-Abl in TonB.210 cells is associated with increased production of intracellular reactive oxygen species (ROS), which is thought to play a role in survival signaling when generated at specific levels. Elevated ROS in Bcr-Abl-expressing cells were found to activate PI3k/Akt pathway members such as Akt and GSK3beta as well as downstream targets beta-catenin and Mcl-1. The activation of these proteins was inhibited by the flavoprotein inhibitor diphenyleneiodonium, which is commonly used to inhibit NADPH oxidase (Nox). This indicated that increased ROS might be related to increased activity of one member of the Nox family. Knock-down experiments using siRNA suggest that Nox-4 is the main source of increased ROS following Bcr-Abl expression. We showed that Bcr-Abl-induced ROS could also increase survival pathway signaling through redox inhibition of PP1alpha, a serine threonine phosphatase that negatively regulates the PI3k/Akt pathway. Overall our results demonstrate that Bcr-Abl expression increases Nox-4-generated ROS, which in turn increases survival signaling through PI3k/Akt pathway by inhibition of PP1alpha, thus contributing to the high level of resistance to apoptosis seen in these Bcr-Abl-expressing cells.

Mechanisms of ROS modulated cell survival during carcinogenesis.

There is increasing evidence within the literature that the decreased susceptibility of tumour cells to stimuli that induce apoptosis can be linked to their inherently increased redox potential. The review primarily focuses on the PI3-kinase/Akt pathway, and the multiple points along this signalling pathway that may be redox regulated. The PI3-kinase/Akt pathway can influence a cells' sensitivity to death inducing signals, through direct manipulation of apoptosis regulating molecules or by regulating the activity of key transcription factors. Proteins involved in the control of apoptosis that are directly regulated by the PI3-kinase/Akt pathway include caspase-9, Bad and the transcription factor GSK-3beta. Lately, it is becoming increasingly obvious that phosphatases are a major counter balance to the PI3-kinase/Akt pathway. Phosphatases such as PP2A and PP1alpha can dephosphorylate signalling molecules within the PI3-kinase/Akt pathway, blocking their activity. It is the balance between the kinase activity and the phosphatase activity that determines the presence and strength of the PI3-kinase/Akt signal. This is why any protein modifications that hinder dephosphorylation can increase the tumours survival advantage. One such modification is the oxidation of the sulphydryl group in key cysteine residues present within the active site of the phosphatases. This highlights the link between the increased redox stress in tumours with the PI3-kinase/Akt pathway. This review will discuss the various sources of reactive oxygen species within a tumour and the effect of these radicals on the PI3-kinase/Akt pathway.

Hyperforin, a new lead compound against the progression of cancer and leukemia?

Extracts of the plant St John's wort, Hyperforin perforatum L., have been used for centuries in traditional medicine, notably for the treatment of depression. One of their main lipophilic components, a natural prenylated phloroglucinol termed hyperforin (HF), has been identified as the major molecule responsible for the antidepressant effects of this plant. Within the last few years, a number of studies have demonstrated that HF displays, in addition, several other biological properties of potential pharmacological interest. They include an antibacterial capacity and inhibitory effects on inflammatory mediators. It is worth noting that HF also promotes apoptosis of various cancer cells from solid tumors and hematological malignancies, including B-cell chronic lymphocytic leukemia. In addition, HF inhibits the capacity of migration and invasion of different tumor cells, as well as exhibiting antiangiogenic effects. Altogether, these properties qualify HF as a lead structure for the development of new therapeutic molecules in the treatment of various diseases, including some malignant tumors.

Hyperforin inhibits MMP-9 secretion by B-CLL cells and microtubule formation by endothelial cells.

We previously reported that hyperforin (HF), a natural phloroglucinol purified from Saint John's wort, can induce the apoptosis of leukemic cells from patients with B-cell lymphocytic leukemia (B-CLL) ex vivo. We show here that treatment of cultured B-CLL patients' cells with HF results in a marked inhibition of their capacity to secrete matrix metalloproteinase-9, an essential component in neo-angiogenesis through degradation of the extracellular matrix process. The phloroglucinol acts by decreasing the production of the latent 92 kDa pro-enzyme. The inhibitory effect of HF is associated with a decrease in VEGF release by the leukemic cells. Moreover, HF is found to prevent the formation of microtubules by human bone marrow endothelial cells cultured on Matrigel, evidencing its capacity to inhibit vessel formation. Our results show the antiangiogenesis activity of HF and strengthen its potential interest in the therapy of B-CLL.

Pro-apoptotic properties of hyperforin in leukemic cells from patients with B-cell chronic lymphocytic leukemia.

The effects of the hyperforin (HF), a natural phloroglucinol purified from Hypericum perforatum, were investigated ex vivo on leukemic cells from patients with B-cell chronic lymphocytic leukemia (B-CLL). HF was found to promote apoptosis of B-CLL cells, as shown by time- and dose-dependent stimulation of phosphatidylserine externalization and DNA fragmentation, by disruption of the mitochondrial transmembrane potential, caspase-3 activation and cleavage of the caspase substrate PARP-1. Moreover, HF-induced downregulation of Bcl-2 and Mcl-1, two antiapoptotic proteins that control mitochondrial permeability. HF also downregulated two proteins which are overexpressed by B-CLL patients' cells, the cell cycle inhibitor p27kip1 through caspase-dependent cleavage into a p23 form, and the nitric oxid (NO) synthase of type 2 (inducible NO synthase). This latter was accompanied by reduction in the production of NO known to be antiapoptotic in B-CLL cells. Preventing effects of the general caspase inhibitor z-VAD-fmk indicated that HF-promoted apoptosis of B-CLL cells was mostly caspase dependent. Furthermore, normal B lymphocytes purified from healthy donors appeared less sensitive to HF-induced apoptosis than B-CLL cells. These results indicate that HF may be of interest in the development of new therapies for B-CLL based on the induction of apoptosis and combination with cell cycle-dependent antitumor drugs.