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The Ras/phosphatidylinositol 3-kinase and Ras/ERK pathways function as independent survival modules each of which
L Xue1, J H Murray, A M Tolkovsky
1Department Biochemistry, University of Cambridge, Tennis Court Road, Cambridge, CB2 1QW, United Kingdom.
Abstract:
Ras promotes robust survival of many cell systems by activating the phosphatidylinositol 3-kinase (PI3-kinase)/Akt pathway, but little is understood about the survival functions of the Ras/ERK pathway. We have used three different effector-loop mutant forms of Ras, each of which activates a single downstream effector pathway, to dissect their individual contributions to survival of nerve growth factor (NGF)-dependent sympathetic neurons. The PI3-kinase pathway-selective protein Ras(Val-12)Y40C was as powerful as oncogenic Ras(Val-12) in preventing apoptosis induced by NGF deprivation but conferred no protection against apoptosis induced by cytosine arabinoside. Identical results were obtained with transfected Akt. In contrast, the ERK pathway-selective protein Ras(Val-12)T35S had no protective effects on NGF-deprived neurons but was almost as strongly protective as Ras(Val-12) against cytosine arabinoside-induced apoptosis. The protective effects of Ras(Val-12)T35S against cytosine arabinoside were completely abolished by the ERK pathway inhibitor PD98059. Ras(Val-12)E37G, an activator of RalGDS, had no survival effect on either death pathway, similar to RasS17N, the full survival antagonist. Thus, Ras provides two independent survival pathways each of which inhibits a distinct apoptotic mechanism. Our study presents one of the few clear-cut cases where only the Ras/ERK, but not the Ras/PI3K/Akt pathway, plays a dominant survival signaling role.
Insights
Ras signaling activates two distinct survival pathways. The Ras/ERK pathway protects against cytosine arabinoside-induced apoptosis, while the Ras/PI3K/Akt pathway is crucial for nerve growth factor-dependent neuronal survival.
Area of Science:
- Cellular signaling and survival pathways
- Molecular mechanisms of apoptosis regulation
Background:
- Ras proteins are key regulators of cell survival, primarily through the phosphatidylinositol 3-kinase (PI3K)/Akt pathway.
- The specific survival functions mediated by the Ras/ERK pathway remain less understood.
- Dissecting individual Ras effector pathways is crucial for understanding differential survival mechanisms.
Purpose of the Study:
- To investigate the distinct roles of Ras effector pathways in neuronal survival.
- To elucidate the contribution of the Ras/ERK pathway to cell survival.
- To differentiate the apoptotic mechanisms inhibited by Ras/ERK versus Ras/PI3K/Akt signaling.
Main Methods:
- Utilized effector-loop mutant forms of Ras to selectively activate downstream pathways (PI3K, ERK, RalGDS).
- Assessed neuronal survival in response to nerve growth factor (NGF) deprivation and cytosine arabinoside treatment.
- Employed the ERK pathway inhibitor PD98059 to confirm pathway specificity.
Main Results:
- Ras(Val-12)Y40C, activating the PI3K pathway, protected against NGF deprivation-induced apoptosis but not cytosine arabinoside-induced apoptosis.
- Ras(Val-12)T35S, activating the ERK pathway, showed no protection against NGF deprivation but significant protection against cytosine arabinoside-induced apoptosis.
- ERK pathway inhibition abolished the protective effects of Ras(Val-12)T35S against cytosine arabinoside.
Conclusions:
- Ras activates two independent survival pathways, each targeting distinct apoptotic mechanisms.
- The Ras/ERK pathway plays a dominant role in protecting against cytosine arabinoside-induced apoptosis.
- This study highlights a rare instance where the Ras/ERK pathway, not Ras/PI3K/Akt, is the primary mediator of survival signaling.