The Virtual Free Radical School

Cell Signaling by Oxidants: Mitogen-Activated Protein Kinases (MAPK) and Activator Protein – 1 (AP-1)

Brooke T. Mossman* and Maria Stern

Environmental Pathology ProgramDepartment of Pathology

College of MedicineUniversity of VermontBurlington, VT 05405

Tel: 802.656.0382Brooke.Mossman@uvm.edu

Cell Signaling by Oxidants

Society For Free Radical Biology and Medicine Mossman & Stern 1

Overview

Oxidants can modulate cell signaling events by modifying cell surface receptors, phosphatases and protein phosphorylation, etc. These phenomena are important in transactivation of transcription factors and activation/inactivation of gene transcription that may regulate steps in the development of disease. At least two classical signaling pathways, the Mitogen-Activated Protein Kinases (MAPK) and signaling leading to activation of NF-B, are activated by oxidants. Physiological oxidant stresses, such as asbestos, induce primarily the extracellular signal-regulated kinases (ERKs) whereas H2O2 causes activation of all three MAPK cascades. A consequence of MAPK activation is formation of Activator Protein–1 (AP-1), which binds to the promoter regions of intermediate response genes governing cell proliferation, differentiation, etc.

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Cell Signaling (In)Activation ofGene Transcription

Gene Product Biological Effect Disease•Cell Proliferation•Cell Transformation•Cell Death/Apoptosis

Importance of Cell Signaling in the Development of Proliferative Diseases such as Cancer or Fibrosis

•Protein•Transcription Factor

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Two Classical Signaling Pathways/Transcription Factors are Associated with Exposure to Oxidants:

• MAPK/AP-1 (Activator Protein-1) • NF-B * (Nuclear Factor – B)

Proliferation

Survival

Apoptosis/Death

Inflammation

Survival

Cell Cycle Control

* See Janssen-Heininger et al., Free Rad. Biol. Med. 28: 1317 – 27, 2000.

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* ERK=Extracellular Signal-Regulated Kinase* JNK=c-Jun N-terminal Kinase* p38

Stimulus

MAPK Kinase Kinase

MAPK Kinase

MAPK

Response

Growth Factors ROS / RNS

Raf

MEK1/2

ERK1/2 *

Mitogenicgrowth, differentiation

CytokinesCellular Stress

(Osmotic stress, ROS)

MEKK

MEK3 MEK4

p38 * JNK1/2 *

Stress Responses

General Schema for Mitogen-Activated Protein Kinase (MAPK) Cascade

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ROS - Induced Mitogen-Activated Protein Kinase (MAPK) Cascade

Asbestos, H2O2, & Silica *

Raf

MEK1 & MEK2

ERK1/2

ApoptosisDNA Damage

MEKK HOMOLOG

MEK3 MEK4

JNK1/2

Proliferation

p38

c-fos/c-jun

??

? ?

* See Ramos et al., Molec. Cell. Biochem. 234/235: 111-115, 2002

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ROS-Induced Stimulation of Extracellular Signal Regulated Kinases (ERK1/2)

AsbestosFibers

EGF-R

RAS

P

RAF P

MEK1/2 P

ERK ERK P

ERK P

Injury/Cell Proliferation/Survival Disease?

Cytoplasm

NucleusP

O2.-

HO.

TGACTCAACTGAGT

X CatalaseNACDeferoxamine Mesylate

SRE

The ERK family (at least 8 isoforms) are typically activated in a series of protein phosphorylation events after phosphorylation of cell surface receptors (i.e. the epidermal growth factor receptor (EGFR) or other extracellular signals. Phosphorylated members of the ERK family then function to transcriptionally regulate specific subsets of genes. For example, phosphorylated ERK2 translocates to the nucleus to phosphorylate ternary complex factor (TCF) which finds to the serum response element (SRE) of c-fos. Mossman & Stern 7

Relationships between MAPK Activation and AP-1

Stress, growth factors, cytokines, oxidants

MAPK

mRNA induction: c-fos, c-jun

Protein synthesis: c-Fos, c-Jun

AP-1 formation: Fos/Jun, Jun/Jun

OutcomeMossman & Stern 8

Formation of Activator Protein–1 (AP-1)

stimulus

early response genes

c-fos

c-jun

mRNA

mRNA

intermediate response gene

Cell Proliferation, etc.

AP-1

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Summary• Oxidants can induce MAPK.

• The MAPKs family includes extracellular signal-regulated kinases (ERKs), which are generally activated by mitogens, and c-Jun NH2 – terminal kinase (JNKs) and p38 MAPKs, both activated by cytokines and cellular stresses.

• Upon activation, JNKs and ERKs phosphorylate Jun and Fos proteins, i.e. AP-1 family members. Although p38 MAPKs do not activate AP-1 proteins directly, they can regulate jun and fos transcription by phosphorylating enhancer binding proteins (C/EBPs) binding to their promoter elements.

• By selective dimerization of AP-1 family members (Jun/Jun or Fos/Jun partners) and diverse binding specificities with the promoter regions of genes, the AP-1 transcription factor regulates gene expression important in cell injury, repair, proliferation, and differentiation.

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