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. Author manuscript; available in PMC: 2008 Oct 28.
Published in final edited form as: Cell Cycle. 2008 Sep 2;7(17):2640–2642. doi: 10.4161/cc.7.17.6534

Protecting the stress response, guarding the MKP-1 mRNA

Yuki Kuwano 1, Myriam Gorospe 1,*
PMCID: PMC2574915  NIHMSID: NIHMS68153  PMID: 18728392

Abstract

The RNA-binding protein (RBP) HuR plays a vital role in the mammalian stress response, effecting changes in the proliferation and survival of damaged cells. HuR prominently influences the stress response by regulating the stability and translation of mRNAs encoding stress-response proteins. Recently, HuR was found to affect mitogen-activated protein kinase (MAPK) signaling, at least in part by post-transcriptionally promoting the expression of MAPK phosphatase-1 (MKP-1). As anticipated for a pivotal regulator of the MAPKs c-jun N-terminal kinase (JNK) and p38, MKP-1 expression is tightly regulated transcriptionally, post-transcriptionally and post-translationally. HuR’s influence on MKP-1 expression helps to ensure the appropriate abundance of MKP-1 and consequently the appropriate cellular response to stress stimuli.

Keywords: mRNA-binding protein, ribonucleoprotein complex, mRNA turnover, translational control, ELAV, MAP kinase, ERK, JNK, p38

HuR and the Mammalian Stress Response

In mammalian cells, HuR has been implicated in the response to many different types of damage. Initial evidence that HuR might be involved in the stress response came from correlative observations that exposure to toxic agents led HuR, a predominantly nuclear RBP, to accumulate in the cytoplasm. This cytoplasmic mobilization was observed in response to harmful stimuli such as oxidants [e.g., hydrogen peroxide (H2O2), arsenite], chemotherapeutic agents (e.g., prostaglandin A2), irradiation with short-wavelength ultraviolet light (UVC), nutrient depletion (e.g., polyamines), and inhibitors of transcription (e.g., actinomycin D).1–3 Given that specific machineries to degrade and translate mRNAs reside in the cytoplasm, the enhanced presence of HuR in this compartment was proposed as a mechanism whereby HuR could stabilize and translate specific subsets of target mRNAs under conditions of stress.4–9 Further evidence linking HuR to the stress response came from studies in which HuR levels were altered in cultured cells by either ectopic HuR overexpression or reduction of HuR levels. These perturbations revealed that elevating HuR abundance generally enhanced the cell’s ability to survive the damaging insult, while its reduction was often detrimental for the cell’s outcome.8,10–12 More recently, HuR’s role in the stress response was further tied to its post-translational modification. Phosphorylation of HuR at a region spanning RNA recognition motifs (RRMs) 1 and 2 by the checkpoint kinase Chk2 affected HuR’s ability to bind to target mRNAs, in turn affecting its post-transcriptional fate.10 In light of the influence of Chk2 on HuR binding to target mRNAs, the Chk2-mediated phosphorylation of HuR was proposed to modulate cell survival in response to stress conditions; however, this hypothesis is awaiting experimental testing.

HuR Influences the Expression of Stress-Response Proteins

HuR levels, cytoplasmic abundance and ability to bind target mRNAs together impact on the composition and concentration of HuR-mRNA ribonucleoprotein (RNP) complexes. As mentioned above, HuR’s stabilizing influence on target mRNAs, many of which encode stress-response proteins, has been extensively documented.3,4,13 Additionally, HuR can increase the translation of several target mRNAs under conditions of stress,7,8,12,14–17 although under non-stressed conditions, HuR can also function as a translational repressor.18–20.

Through its influence on gene expression patterns, HuR RNP complexes have been shown to modulate two major components of the stress response: cell proliferation and apoptosis. HuR can modify cell proliferation rates following damage by changing the levels of proteins that control the cell division cycle, including cyclins D1, A2 and B1, cyclin-dependent kinase inhibitors p21 and p27, and transcription factors c-Fos, c-Jun, HIF-1α, ATF-2 and c-Myc.2,18,21–25 Similarly, HuR can modulate apoptosis through its influence on the expression of pro- and anti-apoptotic proteins such as prothymosin-α, p53, nucleophosmin, Bcl-2, Mcl-1, SIRT1, cyclooxygenase-2, cytochrome c and VEGF.8,10,11,14,26,27 Collections of HuR-regulated proteins which alter cell proliferation and survival in response to stress, as well as the regulatory mechanisms involved are reported elsewhere.11,13,28,29 In addition to influencing cell proliferation and apoptosis, new evidence suggests that HuR could directly influence a third major aspect of the stress response: signaling through mitogen-activated protein kinases (MAPKs), as discussed next.

HuR Regulates MKP-1 Levels, MAPK Activity

Recently, HuR was also found to increase the levels of the stress-response protein MKP-1 [MAPK phosphatase-1, also named DUSP1, dual-specificity phosphatase 1], a critical regulator of MAPKs. MKP-1 specifically dephosphorylates and thereby inactivates MAPKs ERK (extracellular signal regulated kinase), JNK (c-Jun N-terminal kinase) and p38. Through its phosphatase action, MKP-1 regulates the magnitude and duration of MAPK signaling. As other immediate-early genes, the short-lived MKP-1 mRNA is rapidly induced by different stresses. Treatment with the oxidant increased HuR levels in the cytoplasm and its association H2O2 with MKP-1 mRNA, in turn elevating the MKP-1 mRNA half-life and promoting its recruitment to the translation machinery. Conversely, HuR silencing diminished the H2O2-stimulated MKP-1 mRNA stability and lowered MKP-1 translation, while ectopic reintroduction of HuR rescued these effects.12 The decreased levels of MKP-1 in HuR-silenced cultures significantly enhanced the phos- porylation of JNK and p38 after H2O2 treatment.

These findings are particularly significant as they reveal an additional layer of influence by HuR during the stress response. Besides its direct post-transcriptional effect on stress mRNAs, HuR directly affects MKP-1 expression, thereby controlling the strength and timing of MAPK signaling cascades. Moreover, many stress-response genes are transcriptionally induced via MAPK-activated transcription factors. Together, a regulatory paradigm can be proposed in which stress signals activate both MAPKs and HuR; MAPKs carry out stress response functions, including the activation of transcription factors (TFs) which transcriptionally induce stress-response genes, while HuR post-transcriptionally increases the stability and/or translation of mRNAs encoding stress-response proteins. The synergistic action of MAPK/TFs and HuR is eventually shut-off with the rising levels of MKP-1, a consequence of the action of both MAPK/TFs and HuR (Fig. 1 and below). As MKP-1 dephosphorylates MAPKs (predominantly JNK and p38), it provides an effective mechanism of negative feedback regulation.

Figure 1.

Figure 1

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Feedback Regulation Elicited by MKP-1. Stress agents broadly trigger signal transduction pathways that culminate in the activation of MAPKs (ERK, JNK, p38, green ovals). In turn, MAPKs phosphorylate numerous substrates including several transcription factors driving expression of stress-response genes (yellow box). Stress stimuli also act upon HuR, typically enhancing its cytoplasmic abundance and its association with mRNAs encoding stress-response proteins. Accordingly, stress-response genes are effectively induced via two synergistic processes: transcriptionally through the action of MAPKs and post-transcriptionally through the action of HuR. However, the expression of stress response proteins must be shut off with comparable efficiency. We propose that one of HuR post-transcriptional targets, MKP-1 (orange box), provides negative feedback control. MKP-1 dephosphorylates and thereby inactivates MAPKs (primarily JNK and p38), thereby blocking their ability to activate the transcription factors which induce stress-response genes (orange lines). These dynamic interactions permit the rapid induction and the effective repression of stress-response genes, through joint transcriptional and post-transcriptional processes.

Complexity of the Regulation of MKP-1 Levels

In addition to being regulated by HuR, MKP-1 expression is controlled at multiple steps, broadly divided into three levels: transcriptional, post-transcriptional and post-translational (Fig. 2).

Figure 2.

Figure 2

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Multiple Levels of MKP-1 Regulation. In response to stress, MKP-1 gene expression increases through the action of numerous regulatory factors acting on the MKP-1 DNA, mRNA and protein. Transcription increases through local chromatin modification, accompanied by increased association of transcription factors to the MKP-1 promoter (light blue box). Post-transcriptionally, the stability and translation rate of the MKP-1 mRNA are regulated by various RNA-binding proteins (medium blue box). After translation, the MKP-1 protein half-life is further regulated through phosphorylation that affects its ubiquitinylation and hence its proteolysis (dark blue box).

MKP-1 transcription

The transcriptional component of MKP-1 induction by stress and mitogens has been have studied extensively. In response to various stimuli, transcription factors such as AP-1 (c-Fos, c-Jun, ATF2), SP1, SP3, glucocorticoid receptor and NF-κB were shown to associate with the MKP-1 promoter and induced MKP-1 transcription.30–34 In addition, phosphorylation and acetylation of histone H3 altered the chromatin at the MKP-1 gene locus, increasing the association of RNA polymerase II to the MKP-1 gene promoter and its transcription.35

MKP-1 mRNA metabolism

Following transcription, several RBPs besides HuR have been found to bind the MKP-1 mRNA and modulate its post-transcriptional fate. Two RBPs associating with the MKP-1 mRNA were shown to affect its stability: tristetraprolin (TTP), which promoted MKP-1 mRNA degradation,36 and NF90, which increased its stability.12 The translation of MKP-1 mRNA was promoted by HuR and repressed by NF90.12 Although other translation-repressing RBPs were also found to bind to the MKP-1 mRNA, including TIAR [T-cell intracellular antigen-1 (TIA-1)-related] and TIA-1,12 their influence on MKP-1 expression remains to be studied.

MKP-1 protein stability

Additionally, MKP-1 protein levels can fluctuate through changes in protein stability. MKP-1 is stabilized after phosphorylation by MAPKs and degraded by ubiquitinylation.37–39

Perspective: MKP-1 Regulation and the Stress Response

The multi-leveled, seemingly redundant regulation of MKP-1 underscores the importance of expressing this key phosphatase in response to the appropriate stimuli, at the correct time, and in right abundance. We propose that HuR’s ability to induce MKP-1 expression by associating with the MKP-1 mRNA is advantageous for the cell: it allows the timely production of MKP-1 in response to stress 12 that can damage DNA (causing potentially agents such as H2O2, inactivating mutations) or can temporarily repress transcription. In this manner, the cell ensures that an essential regulator of MAPKs is in place to shut off MAPK signaling as needed, thus avoiding the harmful consequences of unscheduled proliferation or apoptosis. This paradigm highlights the increasingly recognized role for HuR in the maintenance of homeostasis following cellular injury.

Acknowledgments

This work was supported by the Intramural Research Program of the National Institute on Aging, National Institutes of Health.

Abbreviations

Bcl-2

B-cell lymphoma 2

Chk2

checkpoint kinase 2

COX-2

cyclooxygenase-2

ELAV-like 1

embryonic lethal, abnormal vision, drosophila-like

ERK

extracellular regulated kinase

HIF

hypoxia-inducible factor

JNK

c-Jun N-terminal Kinase

MAPK

mitogen-activated protein kinase

Mcl-1

myeloid cell leukemia sequence 1

NPM

nucleophosmin

ProTα

prothymosin-α

SIRT1

mammalian SIR2 (silent information regulator 2) protein ortholog

UVC

short-wavelength ultraviolet light

VEGF

vascular endothelial growth factor

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