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Phosphatidylinositol 3-oh kinase: an important element in survival signalling Scheid, Michael

Abstract

Homeostasis of blood cells is maintained by a tighdy controlled system of cell division and cell death. Disruption of this homeostasis may have profound implications in the development of cancer. This disruption can be a result of uncontrolled cell division, and/or resistance of the cells to programmed cell death, termed apoptosis. Hemopoietic cytokines are integral in controlling both of these processes, and while the signalling pathways that control cell division are quickly being elucidated, less is known about how these cytokines promote survival. One family of enzymes regulated by cytokine signalling are the phosphatidylinositol 3-kinases. B y using pharmacological inhibitors of these enzymes, it was determined that some, but not all cytokines require PI 3-kinase to prevent apoptosis. A well known downstream target of PI 3-kinase is the p70 S6 kinase. However, we showed that p70 S6 kinase could be dissociated from this survival function. The extracellular regulated kinases (termed p42erk2 and p44erk1) are MAPKs that may require a component of PI 3-kinase activity for full activation. Therefore the Erks could also be a component of the PI 3-kinase mediated survival pathway. We showed that inhibition of PI 3-kinase by wortmannin resulted in a 50% reduction in the activity of p44erk1 following cytokine stimulation, consistent with published reports. However, full inhibition of PI 3-kinase by a structurally unrelated inhibitor, LY-294002, did not reduce the activation of p44erk1, implicating targets other than PI 3-kinase in the action of wortmannin. These results agree with the observation that IL-4 can activate PI 3-kinase, and maintain survival dependent upon this activity, but IL-4 does not activate p42erk2, p44erk1, or other MAPK family members such as p38 or SAPK. Furthermore, inhibition of MEK, the upstream activator of the Erks, prevented the ability of cytokines to activate p44erk1, but it did not have any effect on cell survival. Thus, PI 3-kinase probably does not mediate survival via Erks or other MAPKs. Recently, a Bcl-2 family member, Bad, has been shown to undergo phosphorylation in response to cytokine stimulation. Phosphorylation of Bad may be crucial for the ability of cytokines to prevent Bad-induced apoptosis. Since PI 3-kinase activates PKB, a kinase that phosphorylates sequences similar to one found in Bad, we examined whether cytokines induce Bad phosphorylation and whether this was dependent on PI 3-kinase. Cytokine-induced phosphorylation of Bad was partially blocked by PI 3- kinase inhibitors. However phosphorylation of Bad induced by GM-CSF was unaffected by PI 3-kinase inhibitors. Conversely, IL-4 was found to stimulate PI 3-kinase, PKB, and promote cell survival, but was unable to induce Bad phosphorylation. These results suggest that other pathways besides PI 3-kinase lead to Bad phosphorylation and that phosphorylation of Bad is not required for cytokines to prevent apoptosis. Thus, the PI 3- kinase/PKB pathway may promote survival by as yet uncharacterized pathways that do not involve Bad phosphorylation. [Scientific formulae used in this abstract could not be reproduced.]

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