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PI3 kinase is important for Ras, MEK and Erk activation of Epo-stimulated human erythroid progenitors

Cancer Research UK Cell Signalling Group, Weatherall Institute of Molecular Medicine, Oxford OX3 9DS, UK. <>
BMC Biology (Impact Factor: 7.98). 02/2004; 2:7. DOI: 10.1186/1741-7007-2-7
Source: PubMed

ABSTRACT Erythropoietin is a multifunctional cytokine which regulates the number of erythrocytes circulating in mammalian blood. This is crucial in order to maintain an appropriate oxygen supply throughout the body. Stimulation of primary human erythroid progenitors (PEPs) with erythropoietin (Epo) leads to the activation of the mitogenic kinases (MEKs and Erks). How this is accomplished mechanistically remained unclear.
Biochemical studies with human cord blood-derived PEPs now show that Ras and the class Ib enzyme of the phosphatidylinositol-3 kinase (PI3K) family, PI3K gamma, are activated in response to minimal Epo concentrations. Surprisingly, three structurally different PI3K inhibitors block Ras, MEK and Erk activation in PEPs by Epo. Furthermore, Erk activation in PEPs is insensitive to the inhibition of Raf kinases but suppressed upon PKC inhibition. In contrast, Erk activation induced by stem cell factor, which activates c-Kit in the same cells, is sensitive to Raf inhibition and insensitive to PI3K and PKC inhibitors.
These unexpected findings contrast with previous results in human primary cells using Epo at supraphysiological concentrations and open new doors to eventually understanding how low Epo concentrations mediate the moderate proliferation of erythroid progenitors under homeostatic blood oxygen levels. They indicate that the basal activation of MEKs and Erks in PEPs by minimal concentrations of Epo does not occur through the classical cascade Shc/Grb2/Sos/Ras/Raf/MEK/Erk. Instead, MEKs and Erks are signal mediators of PI3K, probably the recently described PI3K gamma, through a Raf-independent signaling pathway which requires PKC activity. It is likely that higher concentrations of Epo that are induced by hypoxia, for example, following blood loss, lead to additional mitogenic signals which greatly accelerate erythroid progenitor proliferation.

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    • "Both the MEK/ERK and PI3 Kinase/Akt pathways are required for EPO-induced pMF since the MEK inhibitor (U0126) and the PI3 Kinase inhibitor (LY294002) abolish EPO-induced pMF. However, since pre-treatment with LY294002 inhibits pERK expression in phrenic motor neurons, there may be complex interactions between these pathways that are unique to the EPO signaling cascade (Schmidt et al., 2004). Although mechanisms downstream from ERK and Akt are unknown, EPO may increase glutamate receptor insertion on the post-synaptic membrane between pre-motor and phrenic motor neurons, thereby amplifying synaptic inputs and eliciting pMF. "
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    • "The essentially normal response of upstream signal transduc - tion proteins and the essentially normal expression of MKP3 suggest that alternative signalling pathways modulate the level of phosphorylation of ERK1 / 2 . As it is known that ERKs activation can be modulated by cAMP / PKA signalling either in a c - Raf independent ( Lee , 1999 ) or c - Raf / B - Raf dependent manner ( Erhardt et al , 1995 ; Kolch , 2000 ) and moreover , that certain PKC isotypes activated by Ca 2+ have also been implicated in c - Raf ( Kolch et al , 1993 ) , MEK and ERK1 / 2 activation ( Kolch et al , 1993 ; Schonwasser et al , 1998 ; Schmidt et al , 2004 ) intracellular levels of both Ca 2+ and cAMP were determined in normal control and b - thalassaemia / Hb E proerythroblasts . Results showed that cAMP levels in b - thalassaemia / Hb E proerythroblasts were significantly increased as compared to normal controls ( P = 0AE003 ) ( Table II ) . "
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