Related Experiment Videos
Rsk1 mediates a MEK-MAP kinase cell survival signal
A Shimamura1, B A Ballif, S A Richards
1Department of Cell Biology, Department of Pediatric Hematology and Oncology, Harvard Medical School, Dana Farber Cancer Institute, Boston 02115, USA.
Background:
Growth factors activate an array of cell survival signaling pathways. Mitogen-activated protein (MAP) kinases transduce signals emanating from their upstream activators MAP kinase kinases (MEKs). The MEK-MAP kinase signaling cassette is a key regulatory pathway promoting cell survival. The downstream effectors of the mammalian MEK-MAP kinase cell survival signal have not been previously described.
Results:
We identify here a pro-survival role for the serine/threonine kinase Rsk1, a downstream target of the MEK-MAP kinase signaling pathway. In cells that are dependent on interleukin-3 (IL-3) for survival, pharmacological inhibition of MEKs antagonized the IL-3 survival signal. In the absence of IL-3, a kinase-dead Rsk1 mutant eliminated the survival effect afforded by activated MEK. Conversely, a novel constitutively active Rsk1 allele restored the MEK-MAP kinase survival signal. Experiments in vitro and in vivo demonstrated that Rsk1 directly phosphorylated the pro-apoptotic protein Bad at the serine residues that, when phosphorylated, abrogate Bad's pro-apoptotic function. Constitutively active Rsk1 caused constitutive Bad phosphorylation and protection from Bad-modulated cell death. Kinase-inactive Rsk1 mutants antagonize Bad phosphorylation. Bad mutations that prevented phosphorylation by Rsk1 also inhibited Rsk1-mediated cell survival.
Conclusions:
These data support a model in which Rsk1 transduces the mammalian MEK-MAP kinase signal in part by phosphorylating Bad.
Insights
The serine/threonine kinase Rsk1, a downstream target of the MEK-MAP kinase pathway, promotes cell survival. Rsk1 directly phosphorylates the pro-apoptotic protein Bad, inhibiting its function and preventing cell death.
Area of Science:
- Cellular signaling
- Molecular biology
- Cancer research
Background:
- Growth factors activate cell survival pathways, including the MEK-MAP kinase cascade.
- The downstream components of the MEK-MAP kinase survival pathway in mammals were previously unknown.
Purpose of the Study:
- To identify downstream effectors of the MEK-MAP kinase cell survival pathway.
- To elucidate the role of Rsk1 in mediating cell survival signals.
Main Methods:
- Pharmacological inhibition of MEK (MAP kinase kinase).
- Utilized kinase-dead and constitutively active Rsk1 mutants.
- In vitro and in vivo phosphorylation assays.
- Site-directed mutagenesis of the pro-apoptotic protein Bad.
Main Results:
- Rsk1, a downstream target of MEK-MAP kinase, plays a pro-survival role.
- Rsk1 directly phosphorylates the pro-apoptotic protein Bad at specific serine residues.
- Phosphorylation of Bad by Rsk1 abrogates its pro-apoptotic function, conferring cell survival.
- Constitutively active Rsk1 promotes Bad phosphorylation and cell survival, while kinase-inactive mutants antagonize this effect.
Conclusions:
- Rsk1 acts as a crucial transducer of the MEK-MAP kinase survival signal.
- Rsk1 mediates cell survival, in part, through the direct phosphorylation of Bad.