John M. Sedivy

Telomere Shortening Triggers Senescence of Human Cells through a Pathway Involving ATM, p53, and p21CIP1, but Not p16INK4a

Cellular senescence can be triggered by telomere shortening as well as a variety of stresses and signaling imbalances. We used multiparameter single-cell detection methods to investigate upstream signaling pathways and ensuing cell cycle checkpoint responses in human fibroblasts. Telomeric foci containing multiple DNA damage response factors were assembled in a subset of senescent cells and signaled through ATM to p53, upregulating p21 and causing G1 phase arrest. Inhibition of ATM expression or activity resulted in cell cycle reentry, indicating that stable arrest requires continuous signaling. ATR kinase appears to play a minor role in normal cells but in the absence of ATM elicited a delayed G2 phase arrest. These pathways do not affect expression of p16, which was upregulated in a telomere- and DNA damage-independent manner in a subset of cells. Distinct senescence programs can thus progress in parallel, resulting in mosaic cultures as well as individual cells responding to multiple signals.

Suppression of Raf-1 kinase activity and MAP kinase signalling by RKIP

Raf-1 phosphorylates and activates MEK-1, a kinase that activates the extracellular signal regulated kinases (ERK). This kinase cascade controls the proliferation and differentiation of different cell types. Here we describe a Raf-1-interacting protein, isolated using a yeast two-hybrid screen. This protein inhibits the phosphorylation and activation of MEK by Raf-1 and is designated RKIP (Raf kinase inhibitor protein). In vitro, RKIP binds to Raf-1, MEK and ERK, but not to Ras. RKIP co-immunoprecipitates with Raf-1 and MEK from cell lysates and colocalizes with Raf-1 when examined by confocal microscopy. RKIP is not a substrate for Raf-1 or MEK, but competitively disrupts the interaction between these kinases. RKIP overexpression interferes with the activation of MEK and ERK, induction of AP-1-dependent reporter genes and transformation elicited by an oncogenically activated Raf-1 kinase. Downregulation of endogenous RKIP by expression of antisense RNA or antibody microinjection induces the activation of MEK-, ERK- and AP-1-dependent transcription. RKIP represents a new class of protein-kinase-inhibitor protein that regulates the activity of the Raf/MEK/ERK module.

Stat3-mediated Myc expression is required for Src transformation and PDGF-induced mitogenesis

Signal transducer and activator of transcription (STAT) proteins perform key roles in mediating signaling by cytokines and growth factors, including platelet-derived growth factor (PDGF). In addition, Src family kinases activate STAT signaling and are required for PDGF-induced mitogenesis in normal cells. One STAT family member, Stat3, has been shown to have an essential role in cell transformation by the Src oncoprotein. However, the mechanisms by which STAT-signaling pathways contribute to mitogenesis and transformation are not fully defined. We show here that disruption of Stat3 signaling by using dominant-negative Stat3β protein in NIH 3T3 fibroblasts suppresses c-Myc expression concomitant with inhibition of v-Src-induced transformation. Ectopic expression of c-Myc is able to partially reverse this inhibition, suggesting that c-Myc is a downstream effector of Stat3 signaling in v-Src transformation. Furthermore, c-myc gene knockout fibroblasts are refractory to transformation by v-Src, consistent with a requirement for c-Myc protein in v-Src transformation. In normal NIH 3T3 cells, disruption of Stat3 signaling with dominant-negative Stat3β protein inhibits PDGF-induced mitogenesis in a manner that is reversed by ectopic c-Myc expression. Moreover, inhibition of Src family kinases with the pharmacologic agent, SU6656, blocks Stat3 activation by PDGF. These findings, combined together, delineate the signaling pathway, PDGF → Src → Stat3 → Myc, that is important in normal PDGF-induced mitogenesis and subverted in Src transformation.

Accumulation of Senescent Cells in Mitotic Tissue of Aging Primates

Cellular senescence, a stress induced growth arrest of somatic cells, was first documented in cell cultures over 40 years ago, however its physiological significance has only recently been demonstrated. Using novel biomarkers of cellular senescence we examined whether senescent cells accumulate in tissues from baboons of ages encompassing the entire lifespan of this species. We show that dermal fibroblasts, displaying markers of senescence such as telomere damage, active checkpoint kinase ATM, high levels of heterochromatin proteins and elevated levels of p16, accumulate in skin biopsies from baboons with advancing age. The number of dermal fibroblasts containing damaged telomeres reaches a value of over 15% of total fibroblasts, whereas 80% of cells contain high levels of the heterochromatin protein HIRA. In skeletal muscle, a postmitotic tissue, only a small percentage of myonuclei containing damaged telomeres were detected regardless of animal age. The presence of senescent cells in mitotic tissues might therefore be a contributing factor to aging and age related pathology and provides further evidence that cellular senescence is a physiological event.

Regulation of cyclin-Cdk activity in mammalian cells.

Abstract. Cell cycle progression is driven by the coordinated regulation of the activities of cyclin-dependent kinases (Cdks). Of the several mechanisms known to regulate Cdk activity in response to external signals, regulation of cyclin gene expression, post-translational modification of Cdks by phosphorylation-dephosphorylation cascades, and the interaction of cyclin/Cdk complexes with protein inhibitors have been thoroughly studied. During recent years, much attention has also been given to mechanisms that regulate protein degradation by the ubiquitin/proteasome pathway, as well as to the regulation of subcellular localization of the proteins that comprise the intrinsic cell cycle clock. The purpose of the present review is to summarize the most important aspects of the various mechanisms implicated in cell cycle regulation.

Mechanism of Suppression of the Raf/MEK/Extracellular Signal-Regulated Kinase Pathway by the Raf Kinase Inhibitor Protein

ABSTRACT We have recently identified the Raf kinase inhibitor protein (RKIP) as a physiological endogenous inhibitor of the Raf-1/MEK/extracellular signal-regulated kinase (ERK) pathway. RKIP interfered with MEK phosphorylation and activation by Raf-1, resulting in the suppression of both Raf-1-induced transformation and AP-1-dependent transcription. Here we report the molecular mechanism of RKIPs inhibitory function. RKIP can form ternary complexes with Raf-1, MEK, and ERK. However, whereas MEK and ERK can simultaneously associate with RKIP, Raf-1 binding to RKIP and that of MEK are mutually exclusive. RKIP is able to dissociate a Raf-1–MEK complex and behaves as a competitive inhibitor of MEK phosphorylation. Mapping of the binding domains showed that MEK and Raf-1 bind to overlapping sites in RKIP, whereas MEK and RKIP associate with different domains in Raf-1, and Raf-1 and RKIP bind to different sites in MEK. Both the Raf-1 and the MEK binding sites in RKIP need to be destroyed in order to relieve RKIP-mediated suppression of the Raf-1/MEK/ERK pathway, indicating that binding of either Raf-1 or MEK is sufficient for inhibition. The properties of RKIP reveal the specific sequestration of interacting components as a novel motif in the cells repertoire for the regulation of signaling pathways.

Raf Kinase Inhibitor Protein Interacts with NF-κB-Inducing Kinase and TAK1 and Inhibits NF-κB Activation

ABSTRACT The Raf kinase inhibitor protein (RKIP) acts as a negative regulator of the mitogen-activated protein (MAP) kinase (MAPK) cascade initiated by Raf-1. RKIP inhibits the phosphorylation of MAP/extracellular signal-regulated kinase 1 (MEK1) by Raf-1 by disrupting the interaction between these two kinases. We show here that RKIP also antagonizes the signal transduction pathways that mediate the activation of the transcription factor nuclear factor kappa B (NF-κB) in response to stimulation with tumor necrosis factor alpha (TNF-α) or interleukin 1 beta. Modulation of RKIP expression levels affected NF-κB signaling independent of the MAPK pathway. Genetic epistasis analysis involving the ectopic expression of kinases acting in the NF-κB pathway indicated that RKIP acts upstream of the kinase complex that mediates the phosphorylation and inactivation of the inhibitor of NF-κB (IκB). In vitro kinase assays showed that RKIP antagonizes the activation of the IκB kinase (IKK) activity elicited by TNF-α. RKIP physically interacted with four kinases of the NF-κB activation pathway, NF-κB-inducing kinase, transforming growth factor beta-activated kinase 1, IKKα, and IKKβ. This mode of action bears striking similarities to the interactions of RKIP with Raf-1 and MEK1 in the MAPK pathway. Emerging data from diverse organisms suggest that RKIP and RKIP-related proteins represent a new and evolutionarily highly conserved family of protein kinase regulators. Since the MAPK and NF-κB pathways have physiologically distinct roles, the function of RKIP may be, in part, to coordinate the regulation of these pathways.

c-Myc Regulates Cyclin D-Cdk4 and -Cdk6 Activity but Affects Cell Cycle Progression at Multiple Independent Points

ABSTRACT c-myc is a cellular proto-oncogene associated with a variety of human cancers and is strongly implicated in the control of cellular proliferation, programmed cell death, and differentiation. We have previously reported the first isolation of a c-myc-null cell line. Loss of c-Myc causes a profound growth defect manifested by the lengthening of both the G1and G2 phases of the cell cycle. To gain a clearer understanding of the role of c-Myc in cellular proliferation, we have performed a comprehensive analysis of the components that regulate cell cycle progression. The largest defect observed in c-myc −/− cells is a 12-fold reduction in the activity of cyclin D1-Cdk4 and -Cdk6 complexes during the G0-to-S transition. Downstream events, such as activation of cyclin E-Cdk2 and cyclin A-Cdk2 complexes, are delayed and reduced in magnitude. However, it is clear that c-Myc affects the cell cycle at multiple independent points, because restoration of the Cdk4 and -6 defect does not significantly increase growth rate. In exponentially cycling cells the absence of c-Myc reduces coordinately the activities of all cyclin–cyclin-dependent kinase complexes. An analysis of cyclin-dependent kinase complex regulators revealed increased expression of p27 KIP1 and decreased expression of Cdk7 in c-myc −/− cells. We propose that c-Myc functions as a crucial link in the coordinate adjustment of growth rate to environmental conditions.

Cellular senescence and organismal aging

Cellular senescence, first observed and defined using in vitro cell culture studies, is an irreversible cell cycle arrest which can be triggered by a variety of factors. Emerging evidence suggests that cellular senescence acts as an in vivo tumor suppression mechanism by limiting aberrant proliferation. It has also been postulated that cellular senescence can occur independently of cancer and contribute to the physiological processes of normal organismal aging. Recent data have demonstrated the in vivo accumulation of senescent cells with advancing age. Some characteristics of senescent cells, such as the ability to modify their extracellular environment, could play a role in aging and age-related pathology. In this review, we examine current evidence that links cellular senescence and organismal aging.

Mysterious liaisons: the relationship between c-Myc and the cell cycle

A large body of physiological evidence shows that either upregulation or downregulation of intracellular c-Myc activity has profound consequences on cell cycle progression. Recent work suggests that c-Myc may stimulate the activity of cyclin E/cyclin-dependent kinase 2 (Cdk2) complexes and antagonize the action of the Cdk inhibitor p27KIP1. Cyclin D/Cdk4/6 complexes have also been implicated as targets of c-Myc activity. However, in spite of considerable effort, the mechanisms by which c-Myc interacts with the intrinsic cyclin/Cdk cell cycle machinery remain undefined.