Maureen A. Harrington

Phosphorylation of nuclear MyoD is required for its rapid degradation.

ABSTRACT MyoD is a basic helix-loop-helix transcription factor involved in the activation of genes encoding skeletal muscle-specific proteins. Independent of its ability to transactivate muscle-specific genes, MyoD can also act as a cell cycle inhibitor. MyoD activity is regulated by transcriptional and posttranscriptional mechanisms. While MyoD can be found phosphorylated, the functional significance of this posttranslation modification has not been established. MyoD contains several consensus cyclin-dependent kinase (CDK) phosphorylation sites. In these studies, we examined whether a link could be established between MyoD activity and phosphorylation at putative CDK sites. Site-directed mutagenesis of potential CDK phosphorylation sites in MyoD revealed that S200 is required for MyoD hyperphosphorylation as well as the normally short half-life of the MyoD protein. Additionally, we determined that turnover of the MyoD protein requires the proteasome and Cdc34 ubiquitin-conjugating enzyme activity. Results of these studies demonstrate that hyperphosphorylated MyoD is targeted for rapid degradation by the ubiquitin pathway. The targeted degradation of MyoD following CDK phosphorylation identifies a mechanism through which MyoD activity can be regulated coordinately with the cell cycle machinery (CDK2 and CDK4) and/or coordinately with the cellular transcriptional machinery (CDK7, CDK8, and CDK9).

Cutting Edge: Mouse Pellino-2 Modulates IL-1 and Lipopolysaccharide Signaling

Pellino is a Drosophila protein originally isolated in a two-hybrid screen for proteins interacting with the serine/threonine kinase, pelle. Although mammalian homologs have been identified in mouse and man, the function of pellino is as yet unknown. In this study, the cloning, expression pattern, and a preliminary characterization of mouse pellino-2 is described. These studies reveal that mouse pellino-2 is expressed during embryogenesis and in a tissue-restricted manner in the adult. IL-1 induces the association of mouse pellino-2 with the mouse pelle-like kinase/IL-1R-associated kinase protein, a mammalian homolog of pelle. Ectopic pellino-2 expression did not result in NF-κB activation. However, ectopic expression of a mouse pellino-2 antisense construct inhibited IL-1 or LPS-induced activation of NF-κB-dependent IL-8 promoter activity. Our data reveal that mouse pellino-2 is a tissue-restricted component of a signaling pathway that couples the mouse pelle-like kinase/IL-1R-associated kinase protein to IL-1- or LPS-dependent signaling.

Modulation of Tumor Necrosis Factor and Interleukin-1-dependent NF-κB Activity by mPLK/IRAK

The innate immune response is an important defense against pathogenic agents. A component of this response is the NF-κB-dependent activation of genes encoding inflammatory cytokines such as interleukin-8 (IL-8) and cell adhesion molecules like E-selectin. Members of the serine/threonine innate immune kinase family of proteins have been proposed to mediate the innate immune response. One serine/threonine innate immune kinase family member, themouse Pelle-likekinase/human interleukin-1receptor-associated kinase (mPLK/IRAK), has been proposed to play an obligate role in promoting IL-1-mediated inflammation. However, it is currently unknown whether mPLK/IRAK catalytic activity is required for IL-1-dependent NF-κB activation. The present study demonstrates that mPLK/IRAK catalytic activity is not required for IL-1-mediated activation of an NF-κB-dependent signal. Intriguingly, catalytically inactive mPLK/IRAK inhibits type 1 tumor necrosis factor (TNF) receptor-dependent NF-κB activation. The pathway through which mPLK/IRAK mediates this TNF response is TRADD- and TRAF2-independent. Our data suggest that in addition to its role in IL-1 signaling, mPLK/IRAK is a component of a novel signal transduction pathway through which TNF R1 activates NF-κB-dependent gene expression.

Mouse Receptor Interacting Protein 3 Does Not Contain a Caspase-Recruiting or a Death Domain but Induces Apoptosis and Activates NF-κB

ABSTRACT The death domain-containing receptor superfamily and their respective downstream mediators control whether or not cells initiate apoptosis or activate NF-κB, events critical for proper immune system function. A screen for upstream activators of NF-κB identified a novel serine-threonine kinase capable of activating NF-κB and inducing apoptosis. Based upon domain organization and sequence similarity, this novel kinase, named mRIP3 (mouse receptor interacting protein 3), appears to be a new RIP family member. RIP, RIP2, and mRIP3 contain an N-terminal kinase domain that share 30 to 40% homology. In contrast to the C-terminal death domain found in RIP or the C-terminal caspase-recruiting domain found in RIP2, the C-terminal tail of mRIP3 contains neither motif and is unique. Despite this feature, overexpression of the mRIP3 C terminus is sufficient to induce apoptosis, suggesting that mRIP3 uses a novel mechanism to induce death. mRIP3 also induced NF-κB activity which was inhibited by overexpression of either dominant-negative NIK or dominant-negative TRAF2. In vitro kinase assays demonstrate that mRIP3 is catalytically active and has autophosphorylation site(s) in the C-terminal domain, but the mRIP3 catalytic activity is not required for mRIP3 induced apoptosis and NF-κB activation. Unlike RIP and RIP2, mRIP3 mRNA is expressed in a subset of adult tissues and is thus likely to be a tissue-specific regulator of apoptosis and NF-κB activity. While the lack of a dominant-negative mutant precludes linking mRIP3 to a known upstream regulator, characterizing the expression pattern and the in vitro functions of mRIP3 provides insight into the mechanism(s) by which cells modulate the balance between survival and death in a cell-type-specific manner.

H2O2-induced egr-1, fra-1, and c-jun gene expression is mediated by tyrosine kinase in aortic smooth muscle cells

Abstract Hydrogen peroxide (H2O2) has recently been shown to have a dual effect on cell growth by stimulating proliferation and triggering apoptosis. Apoptosis induced by H2O2 is a direct consequence of oxidant injury, while the proliferative response to H2O2 is thought to be a protective mechanism against oxidant injury. Signaling of the H2O2-induced proliferative effect has been proposed to occur via the activation of mitogen-activated protein kinase (MAPK) and increase in expression of transcription factors. In the present study, H2O2-induced mitogenic signaling in aortic smooth muscle cells (ASMC) was investigated with a specific focus on the roles of tyrosine kinase and tyrosine phosphatase in the regulation of the H2O2-stimulated egr-1, fra-1, and c-jun transcription. The results show that H2O2-induced increases in egr-1, fra-1, and c-jun mRNA levels, as measured by Northern blot analysis, are time and dose dependent with the peak of the response within 2 h. Tyrosine kinase inhibitors (genistein, amino-genistein, and tyrphostin 51) significantly attenuated H2O2-induced expression of these genes and a tyrosine phosphatase inhibitor (perox-vanadate) stimulated their expression. H2O2 stimulated tyrosine kinase activities and caused protein tyrosine phosphorylation, which was blocked by tyrphostin 51. H2O2 also caused tyrosine phosphorylation of platelet derived growth factor (PDGF) receptor. These data show that H2O2 increases egr-1, fra-1, and c-jun mRNA levels in vascular smooth muscle cells, and the increase in expression of these genes is mediated by activation of tyrosine kinase. Our data also provide evidence that the H2O2-induced mitogenic response is, in part, mediated through the receptor tyrosine kinase, PDGF receptor.

SIMPL Is a Tumor Necrosis Factor-specific Regulator of Nuclear Factor-κB Activity

The IL-1 receptor-associated kinase (IRAK/mPLK) is linked to the regulation of nuclear factor-κB (NF-κB)-dependent gene expression. Here we describe a novel binding partner of IRAK/mPLK that we term SIMPL (signaling molecule that associates with the mouse pelle-like kinase). Overexpression of SIMPL leads to the activation of NF-κB-dependent promoters, and inactivation of SIMPL inhibits IRAK/mPLK as well as tumor necrosis factor receptor type I-induced NF-κB activity. Dominant inhibitory alleles of IκB kinase (IKKα or IKKβ) block the activation of NF-κB by IRAK/mPLK and SIMPL. Furthermore, SIMPL binds IRAK/mPLK and the IKKs in vitro and in vivo. In the presence of antisense mRNA to SIMPL, the physical association between IRAK/mPLK and IKKβ but not IRAK/mPLK and IKKα is greatly diminished. Moreover, dominant-negative SIMPL blocks IKKα- or IKKβ-induced NF-κB activity. These results lead us to propose a model in which SIMPL functions to regulate NF-κB activity by linking IRAK/mPLK to IKKβ/α-containing complexes.

Tumor Necrosis Factor Alpha Induction of NF-κB Requires the Novel Coactivator SIMPL

ABSTRACT A myriad of stimuli including proinflammatory cytokines, viruses, and chemical and mechanical insults activate a kinase complex composed of IκB kinase β (IKK-β), IKK-α, and IKK-γ/N, leading to changes in NF-κB-dependent gene expression. However, it is not clear how the NF-κB response is tailored to specific cellular insults. Signaling molecule that interacts with mouse pelle-like kinase (SIMPL) is a signaling component required for tumor necrosis factor alpha (TNF-α)-dependent but not interleukin-1-dependent NF-κB activation. Herein we demonstrate that nuclear localization of SIMPL is required for type I TNF receptor-induced NF-κB activity. SIMPL interacts with nuclear p65 in a TNF-α-dependent manner to promote endogenous NF-κB-dependent gene expression. The interaction between SIMPL and p65 enhances p65 transactivation activity. These data support a model in which TNF-α activation of NF-κB dependent-gene expression requires nuclear relocalization of p65 as well as nuclear relocalization of SIMPL, generating a TNF-α-specific induction of gene expression.

Activation of NF-κB by fluid shear stress, but not TNF-α, requires focal adhesion kinase in osteoblasts

When bone is mechanically loaded fluid shear stress (FSS) is generated as a result of the movement of interstitial fluid across the membranes of osteoblasts and osteocytes. This external mechanical loading stimulates changes in the activity of cytoplasmic signaling molecules and alters gene expression in bone cells. This process, referred to as mechanotransduction, is vital for maintaining bone health in vivo by regulating the balance between bone formation and bone resorption. This current study focuses on the role of focal adhesions, sites of integrin-mediated cellular attachment to the extracellular matrix, and their proposed function as mechanosensors in bone cells. We examined the role of a key component of focal adhesions and of mechanotransduction, focal adhesion kinase (FAK) in regulation of FSS- and tumor necrosis factor-alpha (TNF-alpha)-induced activation of nuclear factor-kappa B (NF-kappaB) signaling in osteoblasts. Immortalized FAK(+/+) and FAK(-)(/)(-) osteoblasts were exposed to periods of oscillatory fluid shear stress (OFF) and NF-kappaB activation was analyzed. We determined that FAK is required for OFF-induced nuclear translocation and activation of NF-kappaB in osteoblasts. In addition we found that OFF-induced phosphorylation of the IkappaB kinases (IKKalpha/beta) in both FAK(+/+) and FAK(-/-) osteoblasts, but only FAK(+/+) osteoblasts demonstrated the resulting degradation of NF-kappaB inhibitors IkappaBalpha and IkappaBbeta. OFF did not induce the degradation of IkappaBepsilon or the processing of p105 in either FAK(+/+) and FAK(-/-) osteoblasts. To compare the role of FAK in mediating OFF-induced mechanotransduction to the well characterized activation of NF-kappaB by inflammatory cytokines, we exposed FAK(+/+) and FAK(-/-) osteoblasts to TNF-alpha. Interestingly, FAK was not required for TNF-alpha induced NF-kappaB activation in osteoblasts. In addition we determined that TNF-alpha treatment did not induce the degradation of IkappaBbeta as did OFF. These data indicate a novel relationship between FAK and NF-kappaB activation in osteoblast mechanotransduction and demonstrates that the mechanism of FSS-induced NF-kappaB activation in osteoblasts differs from the well characterized TNF-alpha-induced activation.

Transcriptional regulation of the mouse CSF-1 gene.

Research in our laboratory is aimed at understanding the cellular and molecular mechanisms that govern colony stimulating factor‐1 (CSF‐1) gene expression. Our hypothesis is that a basal set of trans‐acting factors is bound to the CSF‐1 gene during fibroblast proliferation, resulting in constitutive CSF‐1 gene expression. Modulation of CSF‐1 gene transcription by growth‐arrest (decrease) or stimulation of growth‐arrested fibroblasts (re‐initiate) is mediated by changes in the basal set of factors bound and/or by the addition of stimulus‐specific factors. We have extended our hypothesis to include other cell types (monocytes) to determine if mechanisms used to control CSF‐1 gene expression in fibroblasts are unique or represent common nontissue‐specific regulatory mechanisms. Analysis of CSF‐1‐CAT reporter constructs in transiently transfected fibroblasts and monocytes was used to identify CSF‐1 genomic sequences that affect transcriptional activity. DNase 1 protection, electrophoretic mobility shift, and methylation interference assays were used to identify the putative cis‐acting elements. Results of our study suggest multiple trans‐acting factors may regulate CSF‐1 gene expression; some may be tissue specific, while others, such as AP1, CTF/NF1, Spl, and Sp3, are shared in common. Mol Reprod Dev 46:39–45, 1997.

Effect of myogenic and adipogenic differentiation on expression of colony-stimulating factor genes.

The influence of cellular differentiation on colony-stimulating factor gene expression was examined in myogenically and adipogenically determined cell lines derived from 5-azacytidine-treated C3H10T1/2 C18 (10T1/2) mouse embryo fibroblasts. These studies demonstrate that colony-stimulating factor gene expression can be modulated by myogenic and adipogenic determination and terminal differentiation.