The culture medium of senescent cells in enriched in secreted proteins when compared with the culture medium of quiescent i.e. presenescent cells and these secreted proteins constitute the so-called senescence-associated secretory phenotype (SASP), also known as the senescence messaging secretome (SMS). SASP components include inflammatory and immune-modulatory cytokines (e.g. IL6 and IL8), growth factors (e.g. IGFBPs), shed cell surface molecules (e.g. TNF receptors) and survival factors. While the SASP exhibits a wide ranging profile, it is not significantly affected by the type of senescence trigger (oncogenic signalling, oxidative stress or DNA damage) or the cell type (epithelial vs. mesenchymal) (Coppe et al. 2008). However, as both oxidative stress and oncogenic signaling induce DNA damage, the persistent DNA damage may be a deciding SASP initiator (Rodier et al. 2009). SASP components function in an autocrine manner, reinforcing the senescent phenotype (Kuilman et al. 2008, Acosta et al. 2008), and in the paracrine manner, where they may promote epithelial-to-mesenchymal transition (EMT) and malignancy in the nearby premalignant or malignant cells (Coppe et al. 2008). Interleukin-1-alpha (IL1A), a minor SASP component whose transcription is stimulated by the AP-1 (FOS:JUN) complex (Bailly et al. 1996), can cause paracrine senescence through IL1 and inflammasome signaling (Acosta et al. 2013).
Here, transcriptional regulatory processes that mediate the SASP are annotated. DNA damage triggers ATM-mediated activation of TP53, resulting in the increased level of CDKN1A (p21). CDKN1A-mediated inhibition of CDK2 prevents phosphorylation and inactivation of the Cdh1:APC/C complex, allowing it to ubiquitinate and target for degradation EHMT1 and EHMT2 histone methyltransferases. As EHMT1 and EHMT2 methylate and silence the promoters of IL6 and IL8 genes, degradation of these methyltransferases relieves the inhibition of IL6 and IL8 transcription (Takahashi et al. 2012). In addition, oncogenic RAS signaling activates the CEBPB (C/EBP-beta) transcription factor (Nakajima et al. 1993, Lee et al. 2010), which binds promoters of IL6 and IL8 genes and stimulates their transcription (Kuilman et al. 2008, Lee et al. 2010). CEBPB also stimulates the transcription of CDKN2B (p15-INK4B), reinforcing the cell cycle arrest (Kuilman et al. 2008). CEBPB transcription factor has three isoforms, due to three alternative translation start sites. The CEBPB-1 isoform (C/EBP-beta-1) seems to be exclusively involved in growth arrest and senescence, while the CEBPB-2 (C/EBP-beta-2) isoform may promote cellular proliferation (Atwood and Sealy 2010 and 2011). IL6 signaling stimulates the transcription of CEBPB (Niehof et al. 2001), creating a positive feedback loop (Kuilman et al. 2009, Lee et al. 2010). NF-kappa-B transcription factor is also activated in senescence (Chien et al. 2011) through IL1 signaling (Jimi et al. 1996, Hartupee et al. 2008, Orjalo et al. 2009). NF-kappa-B binds IL6 and IL8 promoters and cooperates with CEBPB transcription factor in the induction of IL6 and IL8 transcription (Matsusaka et al. 1993, Acosta et al. 2008). Besides IL6 and IL8, their receptors are also upregulated in senescence (Kuilman et al. 2008, Acosta et al. 2008) and IL6 and IL8 may be master regulators of the SASP.<p>IGFBP7 is also an SASP component that is upregulated in response to oncogenic RAS-RAF-MAPK signaling and oxidative stress, as its transcription is directly stimulated by the AP-1 (JUN:FOS) transcription factor. IGFBP7 negatively regulates RAS-RAF (BRAF)-MAPK signaling and is important for the establishment of senescence in melanocytes (Wajapeyee et al. 2008).<p>Please refer to Young and Narita 2009 for a recent review.
View original pathway at:Reactome.</div>
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Maertens O, Cichowski K.; ''An expanding role for RAS GTPase activating proteins (RAS GAPs) in cancer.''; PubMedEurope PMCScholia
Raingeaud J, Whitmarsh AJ, Barrett T, Dérijard B, Davis RJ.; ''MKK3- and MKK6-regulated gene expression is mediated by the p38 mitogen-activated protein kinase signal transduction pathway.''; PubMedEurope PMCScholia
Harper JW, Adami GR, Wei N, Keyomarsi K, Elledge SJ.; ''The p21 Cdk-interacting protein Cip1 is a potent inhibitor of G1 cyclin-dependent kinases.''; PubMedEurope PMCScholia
Shimizu H, Mitomo K, Watanabe T, Okamoto S, Yamamoto K.; ''Involvement of a NF-kappa B-like transcription factor in the activation of the interleukin-6 gene by inflammatory lymphokines.''; PubMedEurope PMCScholia
Parisi T, Pollice A, Di Cristofano A, Calabrò V, La Mantia G.; ''Transcriptional regulation of the human tumor suppressor p14(ARF) by E2F1, E2F2, E2F3, and Sp1-like factors.''; PubMedEurope PMCScholia
Mizukami Y, Yoshioka K, Morimoto S, Yoshida Ki.; ''A novel mechanism of JNK1 activation. Nuclear translocation and activation of JNK1 during ischemia and reperfusion.''; PubMedEurope PMCScholia
Zhong YF, Holland PW.; ''The dynamics of vertebrate homeobox gene evolution: gain and loss of genes in mouse and human lineages.''; PubMedEurope PMCScholia
Zhang Y, Xiong Y, Yarbrough WG.; ''ARF promotes MDM2 degradation and stabilizes p53: ARF-INK4a locus deletion impairs both the Rb and p53 tumor suppression pathways.''; PubMedEurope PMCScholia
Trinh BQ, Barengo N, Kim SB, Lee JS, Zweidler-McKay PA, Naora H.; ''The homeobox gene DLX4 regulates erythro-megakaryocytic differentiation by stimulating IL-1β and NF-κB signaling.''; PubMedEurope PMCScholia
Glover JN, Harrison SC.; ''Crystal structure of the heterodimeric bZIP transcription factor c-Fos-c-Jun bound to DNA.''; PubMedEurope PMCScholia
Lin TY, Cheng YC, Yang HC, Lin WC, Wang CC, Lai PL, Shieh SY.; ''Loss of the candidate tumor suppressor BTG3 triggers acute cellular senescence via the ERK-JMJD3-p16(INK4a) signaling axis.''; PubMedEurope PMCScholia
Wu X, Gao H, Bleday R, Zhu Z.; ''Homeobox transcription factor VentX regulates differentiation and maturation of human dendritic cells.''; PubMedEurope PMCScholia
Nelson ML, Kang HS, Lee GM, Blaszczak AG, Lau DK, McIntosh LP, Graves BJ.; ''Ras signaling requires dynamic properties of Ets1 for phosphorylation-enhanced binding to coactivator CBP.''; PubMedEurope PMCScholia
Moiseeva O, Bourdeau V, Roux A, Deschênes-Simard X, Ferbeyre G.; ''Mitochondrial dysfunction contributes to oncogene-induced senescence.''; PubMedEurope PMCScholia
Seidel JJ, Graves BJ.; ''An ERK2 docking site in the Pointed domain distinguishes a subset of ETS transcription factors.''; PubMedEurope PMCScholia
Le Y, Gao H, Bleday R, Zhu Z.; ''The homeobox protein VentX reverts immune suppression in the tumor microenvironment.''; PubMedEurope PMCScholia
Voncken JW, Niessen H, Neufeld B, Rennefahrt U, Dahlmans V, Kubben N, Holzer B, Ludwig S, Rapp UR.; ''MAPKAP kinase 3pK phosphorylates and regulates chromatin association of the polycomb group protein Bmi1.''; PubMedEurope PMCScholia
Matsusaka T, Fujikawa K, Nishio Y, Mukaida N, Matsushima K, Kishimoto T, Akira S.; ''Transcription factors NF-IL6 and NF-kappa B synergistically activate transcription of the inflammatory cytokines, interleukin 6 and interleukin 8.''; PubMedEurope PMCScholia
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Kishimoto T.; ''IL-6: from its discovery to clinical applications.''; PubMedEurope PMCScholia
Banin S, Moyal L, Shieh S, Taya Y, Anderson CW, Chessa L, Smorodinsky NI, Prives C, Reiss Y, Shiloh Y, Ziv Y.; ''Enhanced phosphorylation of p53 by ATM in response to DNA damage.''; PubMedEurope PMCScholia
Reactive oxygen species (ROS), whose concentration increases in senescent cells due to oncogenic RAS-induced mitochondrial dysfunction (Moiseeva et al. 2009) or due to environmental stress, cause DNA damage in the form of double strand breaks (DSBs) (Yu and Anderson 1997). In addition, persistent cell division fueled by oncogenic signaling leads to replicative exhaustion, manifested in critically short telomeres (Harley et al. 1990, Hastie et al. 1990). Shortened telomeres are no longer able to bind the protective shelterin complex (Smogorzewska et al. 2000, de Lange 2005) and are recognized as damaged DNA.
The evolutionarily conserved MRN complex, consisting of MRE11A (MRE11), RAD50 and NBN (NBS1) subunits, binds DSBs (Lee and Paull 2005) and shortened telomeres that are no longer protected by shelterin (Wu et al. 2007). Once bound to the DNA, the MRN complex recruits and activates ATM kinase (Lee and Paull 2005, Wu et al. 2007), leading to phosphorylation of ATM targets, including TP53 (p53) (Banin et al. 1998, Canman et al. 1998, Khanna et al. 1998). TP53, phosphorylated on serine S15 by ATM, binds the CDKN1A (also known as p21, CIP1 or WAF1) promoter and induces CDKN1A transcription (El-Deiry et al. 1993, Karlseder et al. 1999). CDKN1A inhibits the activity of CDK2, leading to G1/S cell cycle arrest (Harper et al. 1993, El-Deiry et al. 1993).
SMURF2 is upregulated in response to telomere attrition in human fibroblasts and induces senecscent phenotype through RB1 and TP53, independently of its role in TGF-beta-1 signaling (Zhang and Cohen 2004). The exact mechanism of SMURF2 involvement is senescence has not been elucidated.
The Interleukin-1 (IL1) family of cytokines comprises 11 members, namely Interleukin-1 alpha (IL1A), Interleukin-1 beta (IL1B), Interleukin-1 receptor antagonist protein (IL1RN, IL1RA), Interleukin-18 (IL18), Interleukin-33 (IL33), Interleukin-36 receptor antagonist protein (IL36RN, IL36RA), Interleukin-36 alpha (IL36A), Interleukin-36 beta (IL36B), Interleukin-36 gamma (IL36G), Interleukin-37 (IL37) and Interleukin-38 (IL38). The genes encoding all except IL18 and IL33 are on chromosome 2. They share a common C-terminal three-dimensional structure and with apart from IL1RN they are synthesized without a hydrophobic leader sequence and are not secreted via the classical reticulum endoplasmic-Golgi pathway.
IL1B and IL18, are produced as biologically inactive propeptides that are cleaved to produce the mature, active interleukin peptide.
The IL1 receptor (IL1R) family comprises 10 members: Interleukin-1 receptor type 1 (IL1R1, IL1RA), Interleukin-1 receptor type 2 (IL1R2, IL1RB), Interleukin-1 receptor accessory protein (IL1RAP, IL1RAcP, IL1R3), Interleukin-18 receptor 1 (IL18R1, IL18RA) , Interleukin-18 receptor accessory protein (IL18RAP, IL18RB), Interleukin-1 receptor-like 1 (IL1RL1, ST2, IL33R), Interleukin-1 receptor-like 2 (IL1RL2, IL36R), Single Ig IL-1-related receptor (SIGIRR, TIR8), Interleukin-1 receptor accessory protein-like 1 (IL1RAPL1, TIGGIR2) and X-linked interleukin-1 receptor accessory protein-like 2 (IL1RAPL2, TIGGIR1). Most of the genes encoding these receptors are on chromosome 2.
IL1 family receptors heterodimerize upon cytokine binding. IL1, IL33 and IL36 bind specific receptors, IL1R1, IL1RL1, and IL1RL2 respectively. All use IL1RAP as a co-receptor. IL18 binds IL18R1 and uses IL18RAP as co-receptor.
The complexes formed by IL1 family cytokines and their heterodimeric receptors recruit intracellular signaling molecules, including Myeloid differentiation primary response protein MyD88 (MYD88), members of he IL1R-associated kinase (IRAK) family, and TNF receptor-associated factor 6 (TRAF6), activating Nuclear factor NF-kappa-B (NFκB), as well as Mitogen-activated protein kinase 14 (MAPK14, p38), c-Jun N-terminal kinases (JNKs), extracellular signal-regulated kinases (ERKs) and other Mitogen-activated protein kinases (MAPKs).
The interleukin-6 (IL6) family of cytokines includes IL6, IL11, IL27, leukemia inhibitory factor (LIF), oncostatin M (OSM), ciliary neurotrophic factor (CNTF), cardiotrophin 1 and 2 (CT-1) and cardiotrophin-like cytokine (CLC) (Heinrich et al. 2003, Pflanz et al. 2002). The latest addition to this family is IL31, discovered in 2004 (Dillon et al. 2004). The family is defined largely by the shared use of the common signal transducing receptor Interleukin-6 receptor subunit beta (IL6ST, gp130). The IL31 receptor uniquely does not include this subunit, instead it uses the related IL31RA. The members of the IL6 family share very low sequence homology but are structurally highly related, forming anti-parallel four-helix bundles with a characteristic “up-up-down-down� topology (Rozwarski et al. 1994, Cornelissen et al. 2012).
Although each member of the IL6 family signals through a distinct receptor complex, their underlying signaling mechanisms are similar. Assembly of the receptor complex is followed by activation of receptor-associated Janus kinases (JAKs), believed to be constitutively associated with the receptor subunits.Activation of JAKs initiates downstream cytoplasmic signaling cascades that involve recruitment and phosphorylation of transcription factors of the Signal transducer and activator of transcription (STAT) family, which dimerize and translocate to the nucleus where they bind enhancer elements of target genes leading to transcriptional activation (Nakashima & Taga 1998).
Negative regulators of IL6 signaling include Suppressor of cytokine signals (SOCS) family members and PTPN11 (SHP-2).
IL6 is a pleiotropic cytokine with roles in processes including immune regulation, hematopoiesis, inflammation, oncogenesis, metabolic control and sleep.
IL6 and IL11 bind their corresponding specific receptors IL6R and IL11R respectively, resulting in dimeric complexes that subsequently associate with IL6ST, leading to IL6ST homodimer formation (in a hexameric or higher order complex) and signal initiation. IL6R alpha exists in transmembrane and soluble forms. The transmembrane form is mainly expressed by hepatocytes, neutrophils, monocytes/macrophages, and some lymphocytes. Soluble forms of IL6R (sIL6R) are also expressed by these cells. Two major mechanisms for the production of sIL6R have been proposed. Alternative splicing generates a transcript lacking the transmembrane domain by using splicing donor and acceptor sites that flank the transmembrane domain coding region. This also introduces a frameshift leading to the incorporation of 10 additional amino acids at the C terminus of sIL6R.A second mechanism for the generation of sIL6R is the proteolytic cleavage or 'shedding' of membrane-bound IL-6R. Two proteases ADAM10 and ADAM17 are thought to contribute to this (Briso et al. 2008). sIL6R can bind IL6 and stimulate cells that express gp130 but not IL6R alpha, a process that is termed trans-signaling. This explains why many cells, including hematopoietic progenitor cells, neuronal cells, endothelial cells, smooth muscle cells, and embryonic stem cells, do not respond to IL6 alone, but show a remarkable response to IL6/sIL6R. It is clear that the trans-signaling pathway is responsible for the pro-inflammatory activities of IL6 whereas the membrane bound receptor governs regenerative and anti-inflammatory IL6 activities
LIF, CNTF, OSM, CTF1, CRLF1 and CLCF1 signal via IL6ST:LIFR heterodimeric receptor complexes (Taga & Kishimoto 1997, Mousa & Bakhiet 2013). OSM signals via a receptor complex consisting of IL6ST and OSMR. These cytokines play important roles in the regulation of complex cellular processes such as gene activation, proliferation and differentiation (Heinrich et al. 1998).
Antibodies have been developed to inhibit IL6 activity for the treatment of inflammatory diseases (Kopf et al. 2010).
Oncogene-induced senescence is triggered by high level of RAS/RAF/MAPK signaling that can be caused, for example, by oncogenic mutations in RAS or RAF proteins, or by oncogenic mutations in growth factor receptors, such as EGFR, that act upstream of RAS/RAF/MAPK cascade. Oncogene-induced senescence can also be triggered by high transcriptional activity of E2F1, E2F2 or E2F3 which can be caused, for example, by the loss-of-function of RB1 tumor suppressor.
Oncogenic signals trigger transcription of CDKN2A locus tumor suppressor genes: p16-INK4A and p14-ARF. p16-INK4A and p14-ARF share exons 2 and 3, but are expressed from different promoters and use different reading frames (Quelle et al. 1995). Therefore, while their mRNAs are homologous and are both translationally inhibited by miR-24 microRNA (Lal et al. 2008, To et al. 2012), they share no similarity at the amino acid sequence level and perform distinct functions in the cell. p16-INK4A acts as the inhibitor of cyclin-dependent kinases CDK4 and CDK6 which phosphorylate and inhibit RB1 protein thereby promoting G1 to S transition and cell cycle progression (Serrano et al. 1993). Increased p16-INK4A level leads to hypophosphorylation of RB1, allowing RB1 to inhibit transcription of E2F1, E2F2 and E2F3-target genes that are needed for cell cycle progression, which results in cell cycle arrest in G1 phase. p14-ARF binds and destabilizes MDM2 ubiquitin ligase (Zhang et al. 1998), responsible for ubiquitination and degradation of TP53 (p53) tumor suppressor protein (Wu et al. 1993, Fuchs et al. 1998, Fang et al. 2000). Therefore, increased p14-ARF level leads to increased level of TP53 and increased expression of TP53 target genes, such as p21, which triggers p53-mediated cell cycle arrest and, depending on other factors, may also lead to p53-mediated apoptosis. CDKN2B locus, which encodes an inhibitor of CDK4 and CDK6, p15-INK4B, is located in the vicinity of CDKN2A locus, at the chromosome band 9p21. p15-INK4B, together with p16-INK4A, contributes to senescence of human T-lymphocytes (Erickson et al. 1998) and mouse fibroblasts (Malumbres et al. 2000). SMAD3, activated by TGF-beta-1 signaling, controls senescence in the mouse multistage carcinogenesis model through regulation of MYC and p15-INK4B gene expression (Vijayachandra et al. 2003). TGF-beta-induced p15-INK4B expression is also important for the senescence of hepatocellular carcinoma cell lines (Senturk et al. 2010).
MAP kinases MAPK1 (ERK2) and MAPK3 (ERK1), which are activated by RAS signaling, phosphorylate ETS1 and ETS2 transcription factors in the nucleus (Yang et al. 1996, Seidel et al. 2002, Foulds et al. 2004, Nelson et al. 2010). Phosphorylated ETS1 and ETS2 are able to bind RAS response elements (RREs) in the CDKN2A locus and stimulate p16-INK4A transcription (Ohtani et al. 2004). At the same time, activated ERKs (MAPK1 i.e. ERK2 and MAPK3 i.e. ERK1) phosphorylate ERF, the repressor of ETS2 transcription, which leads to translocation of ERF to the cytosol and increased transcription of ETS2 (Sgouras et al. 1995, Le Gallic et al. 2004). ETS2 can be sequestered and inhibited by binding to ID1, resulting in inhibition of p16-INK4A transcription (Ohtani et al. 2004).
Transcription of p14-ARF is stimulated by binding of E2F transcription factors (E2F1, E2F2 or E2F3) in complex with SP1 to p14-ARF promoter (Parisi et al. 2002).
Oncogenic RAS signaling affects mitochondrial metabolism through an unknown mechanism, leading to increased generation of reactive oxygen species (ROS), which triggers oxidative stress induced senescence pathway. In addition, increased rate of cell division that is one of the consequences of oncogenic signaling, leads to telomere shortening which acts as another senescence trigger.
The importance of the RAS/RAF/MAPK cascade in regulating cellular proliferation, differentiation and survival is highlighted by the fact that components of the pathway are mutated with high frequency in a large number of human cancers. Activating mutations in RAS are found in approximately one third of human cancers, while ~8% of tumors express an activated form of BRAF. RAS pathway activation is also achieved in a smaller subset of cancers by loss-of-function mutations in negative regulators of RAS signaling, such as the RAS GAP NF1(reviewed in Prior et al, 2012; Pylayeva-Gupta et al, 2011; Stephen et al, 2014; Lavoie and Therrien, 2015; Lito et al, 2013; Samatar and Poulikakos, 2014; Maertens and Cichowski, 2014).
Oxidative stress, caused by increased concentration of reactive oxygen species (ROS) in the cell, can happen as a consequence of mitochondrial dysfunction induced by the oncogenic RAS (Moiseeva et al. 2009) or independent of oncogenic signaling. Prolonged exposure to interferon-beta (IFNB, IFN-beta) also results in ROS increase (Moiseeva et al. 2006). ROS oxidize thioredoxin (TXN), which causes TXN to dissociate from the N-terminus of MAP3K5 (ASK1), enabling MAP3K5 to become catalytically active (Saitoh et al. 1998). ROS also stimulate expression of Ste20 family kinases MINK1 (MINK) and TNIK through an unknown mechanism, and MINK1 and TNIK positively regulate MAP3K5 activation (Nicke et al. 2005).
MAP3K5 phosphorylates and activates MAP2K3 (MKK3) and MAP2K6 (MKK6) (Ichijo et al. 1997, Takekawa et al. 2005), which act as p38 MAPK kinases, as well as MAP2K4 (SEK1) (Ichijo et al. 1997, Matsuura et al. 2002), which, together with MAP2K7 (MKK7), acts as a JNK kinase.
MKK3 and MKK6 phosphorylate and activate p38 MAPK alpha (MAPK14) and beta (MAPK11) (Raingeaud et al. 1996), enabling p38 MAPKs to phosphorylate and activate MAPKAPK2 (MK2) and MAPKAPK3 (MK3) (Ben-Levy et al. 1995, Clifton et al. 1996, McLaughlin et al. 1996, Sithanandam et al. 1996, Meng et al. 2002, Lukas et al. 2004, White et al. 2007), as well as MAPKAPK5 (PRAK) (New et al. 1998 and 2003, Sun et al. 2007).
Phosphorylation of JNKs (MAPK8, MAPK9 and MAPK10) by MAP3K5-activated MAP2K4 (Deacon and Blank 1997, Fleming et al. 2000) allows JNKs to migrate to the nucleus (Mizukami et al. 1997) where they phosphorylate JUN. Phosphorylated JUN binds FOS phosphorylated by ERK1 or ERK2, downstream of activated RAS (Okazaki and Sagata 1995, Murphy et al. 2002), forming the activated protein 1 (AP-1) complex (FOS:JUN heterodimer) (Glover and Harrison 1995, Ainbinder et al. 1997).
Activation of p38 MAPKs and JNKs downstream of MAP3K5 (ASK1) ultimately converges on transcriptional regulation of CDKN2A locus. In dividing cells, nucleosomes bound to the CDKN2A locus are trimethylated on lysine residue 28 of histone H3 (HIST1H3A) by the Polycomb repressor complex 2 (PRC2), creating the H3K27Me3 (Me3K-28-HIST1H3A) mark (Bracken et al. 2007, Kotake et al. 2007). The expression of Polycomb constituents of PRC2 (Kuzmichev et al. 2002) - EZH2, EED and SUZ12 - and thereby formation of the PRC2, is positively regulated in growing cells by E2F1, E2F2 and E2F3 (Weinmann et al. 2001, Bracken et al. 2003). H3K27Me3 mark serves as a docking site for the Polycomb repressor complex 1 (PRC1) that contains BMI1 (PCGF4) and is therefore named PRC1.4, leading to the repression of transcription of p16-INK4A and p14-ARF from the CDKN2A locus, where PCR1.4 mediated repression of p14-ARF transcription in humans may be context dependent (Voncken et al. 2005, Dietrich et al. 2007, Agherbi et al. 2009, Gao et al. 2012). MAPKAPK2 and MAPKAPK3, activated downstream of the MAP3K5-p38 MAPK cascade, phosphorylate BMI1 of the PRC1.4 complex, leading to dissociation of PRC1.4 complex from the CDKN2A locus and upregulation of p14-ARF transcription (Voncken et al. 2005). AP-1 transcription factor, formed as a result of MAP3K5-JNK signaling, as well as RAS signaling, binds the promoter of KDM6B (JMJD3) gene and stimulates KDM6B expression. KDM6B is a histone demethylase that removes H3K27Me3 mark i.e. demethylates lysine K28 of HIST1H3A, thereby preventing PRC1.4 binding to the CDKN2A locus and allowing transcription of p16-INK4A (Agger et al. 2009, Barradas et al. 2009, Lin et al. 2012).
p16-INK4A inhibits phosphorylation-mediated inactivation of RB family members by CDK4 and CDK6, leading to cell cycle arrest (Serrano et al. 1993). p14-ARF inhibits MDM2-mediated degradation of TP53 (p53) (Zhang et al. 1998), which also contributes to cell cycle arrest in cells undergoing oxidative stress. In addition, phosphorylation of TP53 by MAPKAPK5 (PRAK) activated downstream of MAP3K5-p38 MAPK signaling, activates TP53 and contributes to cellular senescence (Sun et al. 2007).
Prior to mitogen activation, the inhibitory proteins of the INK4 family (p15, p16, p18, and p19) associate with the catalytic domains of free CDK4 and CDK6, preventing their association with D type cyclins (CCND1, CCND2 and CCND3), and thus their activation (Serrano et al. 1993, Hannon and Beach 1994, Guan et al. 1994, Guan et al. 1996, Parry et al. 1995). Inactivation and defects of RB1 strongly upregulate p16-INK4A (Parry et al. 1995).
The p90 ribosomal S6 kinases (RSK1-4) comprise a family of serine/threonine kinases that lie at the terminus of the ERK pathway. RSK family members are unusual among serine/threonine kinases in that they contain two distinct kinase domains, both of which are catalytically functional . The C-terminal kinase domain is believed to be involved in autophosphorylation, a critical step in RSK activation, whereas the N-terminal kinase domain, which is homologous to members of the AGC superfamily of kinases, is responsible for the phosphorylation of all known exogenous substrates of RSK. RSKs can be activated by the ERKs (ERK1, 2, 5) in the cytoplasm as well as in the nucleus, they both have cytoplasmic and nuclear substrates, and they are able to move from nucleus to cytoplasm. Efficient RSK activation by ERKs requires its interaction through a docking site located near the RSK C terminus. The mechanism of RSK activation has been studied mainly with regard to ERK1 and ERK2. RSK activation leads to the phosphorylation of four essential residues Ser239, Ser381, Ser398, and Thr590, and two additional sites, Thr377 and Ser749 (the amino acid numbering refers to RSK1). ERK is thought to play at least two roles in RSK1 activation. First, activated ERK phosphorylates RSK1 on Thr590, and possibly on Thr377 and Ser381, and second, ERK brings RSK1 into close proximity to membrane-associated kinases that may phosphorylate RSK1 on Ser381 and Ser398. Moreover, RSKs and ERK1/2 form a complex that transiently dissociates upon growth factor signalling. Complex dissociation requires phosphorylation of RSK1 serine 749, a growth factor regulated phosphorylation site located near the ERK docking site. Serine 749 is phosphorylated by the N-terminal kinase domain of RSK1 itself. ERK1/2 docking to RSK2 and RSK3 is also regulated in a similar way. The length of RSK activation following growth factor stimulation depends on the duration of the RSK/ERK complex, which, in turn, differs among the different RSK isoforms. RSK1 and RSK2 readily dissociate from ERK1/2 following growth factor stimulation stimulation, but RSK3 remains associated with active ERK1/2 longer, and also remains active longer than RSK1 and RSK2.
At the G1/S transition, the Cdh1 (FZR1) subunit of the APC/C:Cdh1 complex is phosphorylated by Cyclin A:Cdk2 (CCNA:CDK2) and dissociates from APC/C. This inactivates APC/C and permits the accumulation of cell cycle proteins required for DNA synthesis and entry into mitosis (Lukas et al. 1999). Activation of the ATM kinase by DNA damage in the form of double strand breaks results in TP53-mediated induction of CDKN1A (p21) expression. CDKN1A binds CCNA:CDK2 complex and prevents it from phosphorylating Cdh1 (Takahashi et al. 2012).
Cyclin A-Cdk2 (CCNA:CDK2) prevents unscheduled APC reactivation during S phase by binding and subsequently phosphorylating FZR1 (Cdh1). Phosphorylation-dependent dissociation of the Cdh1-activating subunit inhibits the APC/C (Sorensen et al. 2001). DNA damage activates ATM kinase, resulting in TP53-mediated induction of CDKN1A (p21) expression. CDKN1A binds CCNA:CDK2 complex and prevents its association with Cdh1 (Takahashi et al. 2012).
Cdh1:APC/C complex, stabilized by the DNA damage-induced ATM-TP53-CDKN1A axis, ubiquitinates EHMT1 (GLP) and EHMT2 (G9a) histone methyltransferases, targeting them for degradation (Takahashi et al. 2012).
Cdh1 (FZR1) is able to bind both G9a (EHMT2) and GLP (EHMT1) (Takahashi et al. 2012). EHMT1 and EHMT2 histone methyltransferases were shown to function as a heterodimer in vivo (Tachibana et al. 2005).
EHMT1 (GLP) and EHMT2 (G9a) histone methyltransferases dimethylate histone H3 (HIST1H3A) on lysine residue 10, creating an H3K9Me2 mark on nucleosomes associated with the IL8 promoter (Takahashi et al. 2012).
EHMT1 (GLP) and EHMT2 (G9a) histone methyltransferases dimethylate histone H3 (HIST1H3A) on lysine residue 10, creating an H3K9Me2 mark on nucleosomes associated with the IL6 promoter (Takahashi et al. 2012).
Methylation of the IL6 promoter by EHMT1:EHMT2 (GLP:G9a) histone methyltransferases inhibits IL6 transcription, while Cdh1:APC/C-mediated degradation of EHTM1:EHTM2 downstream of the ATM-TP53-CDKN1A axis stimulates IL6 transcription (Takahashi et al. 2012). Oncogenic RAS signaling stimulates activation of the CEBPB transcription factor (C/EBP-beta) which binds IL6 promoter and stimulates IL6 transcription (Kuilman et al. 2008, Lee et al. 2010). NF kappa B transcription factor is also activated in senescent cells (Chien et al. 2011) through interleukin-1-alpha (IL1A) signaling (Jimi et al. 1996, Hartupee et al. 2008, Orjalo et al. 2009), and it cooperates with CEBPB in the activation of IL6 transcription (Shimizu et al. 1990, Libermann and Baltimore 1990, Matsusaka et al. 1993, Acosta et al. 2008). Autocrine IL6 signaling stimulates CEBPB expression (Kuilman et al. 2008), creating a positive feedback loop. STAT3, activated by IL6 signaling cascade is necessary for CEBPB transcription, but the direct binding of STAT3 to the CEBPB promoter has not been demonstrated (Niehof et al. 2001).
Methylation of the IL8 promoter by EHMT1:EHMT2 (GLP:G9a) histone methyltransferases inhibits IL8 transcription, while Cdh1:APC/C-mediated degradation of EHTM1:EHTM2 downstream of the ATM-TP53-CDKN1A axis stimulates IL8 transcription (Takahashi et al. 2012). CEBPB transcription factor, activated by oncogenic RAS signaling, binds IL8 promoter and stimulates IL8 transcription (Kuilman et al. 2008). The NF-kappa-B transcription factor is also activated in senescent cells (Chien et al. 2011) through interleukin-1-alpha (IL1A) signaling (Jimi et al. 1996, Hartupee et al. 2008, Orjalo et al. 2009), and it cooperates with CEBPB in the activation of IL8 transcription (Kunsch and Rosen 1993, Stein and Baldwin 1993, Matsusaka et al. 1993, Acosta et al. 2008).
FOS:JUN (AP-1) transcription factor, formed in response to oncogenic RAF-MAPK signaling which also triggers oxidative stress, binds the promoter of IGFBP7 gene (Wajapeyee et al. 2008).
FOS:JUN (AP-1) transcription factor stimulates the transcription of IGFBP7 gene. IGFBP7 is a component of the senescence-associated secretory phenotype (SASP) and is secreted by senescent melanocytes in which the senescence is induced by the expression of oncogenic BRAF V600E. The BRAF V600E-mediated induction of IGFBP7 expression is AP-1 dependent. The conditioned medium harvested from BRAF V600E senescent melanocytes is able to inhibit cellular proliferation and induce senescence of naive melanocytes only when IGFBP7 is present in the medium (Wajapeyee et al. 2008).
RSK6A1/2/3-mediated phosphorylation of CEBPB downstream of activated RAS stimulates CEBPB homodimerization and DNA binding (Lee, Shuman et al. 2010) and, specifically, RAS-induced CEBPB activation stimulates CEBPB binding to the IL6 promoter (Kuilman et al. 2008; Lee, Shuman et al. 2010). RAS-activated CEBPB is able to recruit additional transcription activators, such as EP300, to the IL6 promoter (Lee, Miller et al. 2010). NFKB transcription complex, activated by interleukin-1-alpha (IL1A) signaling (Jimi et al. 1996, Hartupee et al. 2008, Orjalo et al. 2009), also binds the promoter of the IL6 gene (Shimizu et al. 1990, Libermann and Baltimore 1990) and cooperates with CEBPB in the activation of IL6 transcription (Matsusaka et al. 1993).
RAS-mediated activation of CEBPB (C/EBP-beta) stimulates CEBPB binding to the IL8 promoter (Kuilman et al. 2008). NFKB transcription complex, activated by interleukin-1-alpha (IL1A) signaling (Jimi et al. 1996, Hartupee et al. 2008, Orjalo et al. 2009), also binds the promoter of the IL8 gene (Kunsch and Rosen 1993) and cooperates with CEBPB in the activation of IL8 transcription (Matsusaka et al. 1993, Stein and Baldwin 1993).
Phosphorylation of CEBPB (C/EBP-beta) serine residue S321 by ERK1/2-activated RSK1, RSK2 or RSK3, downstream of activated RAS, is necessary for the relief of CEBPB autoinhibiton (Lee et al. 2010). Phosphorylation on other sites may also be involved in CEBPB activation.
Phosphorylation of CEBPB (C/EBP-beta) transcription factor on threonine residue T235 happens downstream of activated RAS, is mediated by MAPKs - likely MAPK3 (ERK1) and MAPK1 (ERK2), and positively affects CEBPB-mediated transcription of IL6 (Nakajima et al. 1993).
RSK1/2/3-mediated phosphorylation of CEBPB promotes the formation of CEBPB homodimers which are active as transcription factors (Lee, Miller et al. 2010; Lee, Shuman et al. 2010).
Once bound to the CDKN2B promoter, CEBPB stimulates CKDN2B transcription, contributing to cell cycle arrest in oncogene induced senescence (Kuilman et al. 2008). In addition, since CEBPB expression is stimulated by IL6 signaling, with IL6 itself being a transcriptional target of CEBPB (Neihof et al. 2001, Kuilman et al. 2008, Lee et al. 2010), IL6-CEBPB-CDKN2B axis provides an autocrine SASP-mediated growth arrest mechanism.
STAT3, activated by IL6 signaling cascade, is necessary for CEBPB transcription, but direct binding of STAT3 to the CEBPB promoter has not been demonstrated (Niehof et al. 2001). As CEBPB activates IL6 transcription in response to oncogenic RAS signaling, and IL6 activates CEBPB transcription (Niehof et al. 2001, Kuilman et al. 2008, Lee et al. 2010), a positive feedback loop exists between CEBPB and IL6.
The promoter of the interleukin 1-alpha (IL1A) gene contains AP-1 binding sites which are occupied by the AP-1 (FOS:JUN) complex, resulting in the stimulation of IL1A transcription (Bailly et al. 1996 Alheim et al. 1996, Burleson et al. 1996, Huang et al. 1999, Niu et al. 2004, Moerman-Herzog and Barger 2012, Murakami et al. 2013).
The AP-1 complex (FOS:JUN) binds IL1A promoter and stimulates IL1A transcription (Bailly et al. 1996 Alheim et al. 1996, Burleson et al. 1996, Huang et al. 1999, Niu et al. 2004, Moerman-Herzog and Barger 2012, Murakami et al. 2013).
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A:Cdk2:p21/p27
complexA:phospho-Cdk(Thr
160):Cdh1:phosho-APC/C complexA:phospho-Cdk2(Thr
160):phospho-Cdh1:phospho-APC/C complexStress Induced
SenescenceThe evolutionarily conserved MRN complex, consisting of MRE11A (MRE11), RAD50 and NBN (NBS1) subunits, binds DSBs (Lee and Paull 2005) and shortened telomeres that are no longer protected by shelterin (Wu et al. 2007). Once bound to the DNA, the MRN complex recruits and activates ATM kinase (Lee and Paull 2005, Wu et al. 2007), leading to phosphorylation of ATM targets, including TP53 (p53) (Banin et al. 1998, Canman et al. 1998, Khanna et al. 1998). TP53, phosphorylated on serine S15 by ATM, binds the CDKN1A (also known as p21, CIP1 or WAF1) promoter and induces CDKN1A transcription (El-Deiry et al. 1993, Karlseder et al. 1999). CDKN1A inhibits the activity of CDK2, leading to G1/S cell cycle arrest (Harper et al. 1993, El-Deiry et al. 1993).
SMURF2 is upregulated in response to telomere attrition in human fibroblasts and induces senecscent phenotype through RB1 and TP53, independently of its role in TGF-beta-1 signaling (Zhang and Cohen 2004). The exact mechanism of SMURF2 involvement is senescence has not been elucidated.
IL1B and IL18, are produced as biologically inactive propeptides that are cleaved to produce the mature, active interleukin peptide.
The IL1 receptor (IL1R) family comprises 10 members: Interleukin-1 receptor type 1 (IL1R1, IL1RA), Interleukin-1 receptor type 2 (IL1R2, IL1RB), Interleukin-1 receptor accessory protein (IL1RAP, IL1RAcP, IL1R3), Interleukin-18 receptor 1 (IL18R1, IL18RA) , Interleukin-18 receptor accessory protein (IL18RAP, IL18RB), Interleukin-1 receptor-like 1 (IL1RL1, ST2, IL33R), Interleukin-1 receptor-like 2 (IL1RL2, IL36R), Single Ig IL-1-related receptor (SIGIRR, TIR8), Interleukin-1 receptor accessory protein-like 1 (IL1RAPL1, TIGGIR2) and X-linked interleukin-1 receptor accessory protein-like 2 (IL1RAPL2, TIGGIR1). Most of the genes encoding these receptors are on chromosome 2. IL1 family receptors heterodimerize upon cytokine binding. IL1, IL33 and IL36 bind specific receptors, IL1R1, IL1RL1, and IL1RL2 respectively. All use IL1RAP as a co-receptor. IL18 binds IL18R1 and uses IL18RAP as co-receptor.
The complexes formed by IL1 family cytokines and their heterodimeric receptors recruit intracellular signaling molecules, including Myeloid differentiation primary response protein MyD88 (MYD88), members of he IL1R-associated kinase (IRAK) family, and TNF receptor-associated factor 6 (TRAF6), activating Nuclear factor NF-kappa-B (NFκB), as well as Mitogen-activated protein kinase 14 (MAPK14, p38), c-Jun N-terminal kinases (JNKs), extracellular signal-regulated kinases (ERKs) and other Mitogen-activated protein kinases (MAPKs).Although each member of the IL6 family signals through a distinct receptor complex, their underlying signaling mechanisms are similar. Assembly of the receptor complex is followed by activation of receptor-associated Janus kinases (JAKs), believed to be constitutively associated with the receptor subunits.Activation of JAKs initiates downstream cytoplasmic signaling cascades that involve recruitment and phosphorylation of transcription factors of the Signal transducer and activator of transcription (STAT) family, which dimerize and translocate to the nucleus where they bind enhancer elements of target genes leading to transcriptional activation (Nakashima & Taga 1998).
Negative regulators of IL6 signaling include Suppressor of cytokine signals (SOCS) family members and PTPN11 (SHP-2).
IL6 is a pleiotropic cytokine with roles in processes including immune regulation, hematopoiesis, inflammation, oncogenesis, metabolic control and sleep.
IL6 and IL11 bind their corresponding specific receptors IL6R and IL11R respectively, resulting in dimeric complexes that subsequently associate with IL6ST, leading to IL6ST homodimer formation (in a hexameric or higher order complex) and signal initiation. IL6R alpha exists in transmembrane and soluble forms. The transmembrane form is mainly expressed by hepatocytes, neutrophils, monocytes/macrophages, and some lymphocytes. Soluble forms of IL6R (sIL6R) are also expressed by these cells. Two major mechanisms for the production of sIL6R have been proposed. Alternative splicing generates a transcript lacking the transmembrane domain by using splicing donor and acceptor sites that flank the transmembrane domain coding region. This also introduces a frameshift leading to the incorporation of 10 additional amino acids at the C terminus of sIL6R.A second mechanism for the generation of sIL6R is the proteolytic cleavage or 'shedding' of membrane-bound IL-6R. Two proteases ADAM10 and ADAM17 are thought to contribute to this (Briso et al. 2008). sIL6R can bind IL6 and stimulate cells that express gp130 but not IL6R alpha, a process that is termed trans-signaling. This explains why many cells, including hematopoietic progenitor cells, neuronal cells, endothelial cells, smooth muscle cells, and embryonic stem cells, do not respond to IL6 alone, but show a remarkable response to IL6/sIL6R. It is clear that the trans-signaling pathway is responsible for the pro-inflammatory activities of IL6 whereas the membrane bound receptor governs regenerative and anti-inflammatory IL6 activities
LIF, CNTF, OSM, CTF1, CRLF1 and CLCF1 signal via IL6ST:LIFR heterodimeric receptor complexes (Taga & Kishimoto 1997, Mousa & Bakhiet 2013). OSM signals via a receptor complex consisting of IL6ST and OSMR. These cytokines play important roles in the regulation of complex cellular processes such as gene activation, proliferation and differentiation (Heinrich et al. 1998).
Antibodies have been developed to inhibit IL6 activity for the treatment of inflammatory diseases (Kopf et al. 2010).
Oncogenic signals trigger transcription of CDKN2A locus tumor suppressor genes: p16-INK4A and p14-ARF. p16-INK4A and p14-ARF share exons 2 and 3, but are expressed from different promoters and use different reading frames (Quelle et al. 1995). Therefore, while their mRNAs are homologous and are both translationally inhibited by miR-24 microRNA (Lal et al. 2008, To et al. 2012), they share no similarity at the amino acid sequence level and perform distinct functions in the cell. p16-INK4A acts as the inhibitor of cyclin-dependent kinases CDK4 and CDK6 which phosphorylate and inhibit RB1 protein thereby promoting G1 to S transition and cell cycle progression (Serrano et al. 1993). Increased p16-INK4A level leads to hypophosphorylation of RB1, allowing RB1 to inhibit transcription of E2F1, E2F2 and E2F3-target genes that are needed for cell cycle progression, which results in cell cycle arrest in G1 phase. p14-ARF binds and destabilizes MDM2 ubiquitin ligase (Zhang et al. 1998), responsible for ubiquitination and degradation of TP53 (p53) tumor suppressor protein (Wu et al. 1993, Fuchs et al. 1998, Fang et al. 2000). Therefore, increased p14-ARF level leads to increased level of TP53 and increased expression of TP53 target genes, such as p21, which triggers p53-mediated cell cycle arrest and, depending on other factors, may also lead to p53-mediated apoptosis. CDKN2B locus, which encodes an inhibitor of CDK4 and CDK6, p15-INK4B, is located in the vicinity of CDKN2A locus, at the chromosome band 9p21. p15-INK4B, together with p16-INK4A, contributes to senescence of human T-lymphocytes (Erickson et al. 1998) and mouse fibroblasts (Malumbres et al. 2000). SMAD3, activated by TGF-beta-1 signaling, controls senescence in the mouse multistage carcinogenesis model through regulation of MYC and p15-INK4B gene expression (Vijayachandra et al. 2003). TGF-beta-induced p15-INK4B expression is also important for the senescence of hepatocellular carcinoma cell lines (Senturk et al. 2010).
MAP kinases MAPK1 (ERK2) and MAPK3 (ERK1), which are activated by RAS signaling, phosphorylate ETS1 and ETS2 transcription factors in the nucleus (Yang et al. 1996, Seidel et al. 2002, Foulds et al. 2004, Nelson et al. 2010). Phosphorylated ETS1 and ETS2 are able to bind RAS response elements (RREs) in the CDKN2A locus and stimulate p16-INK4A transcription (Ohtani et al. 2004). At the same time, activated ERKs (MAPK1 i.e. ERK2 and MAPK3 i.e. ERK1) phosphorylate ERF, the repressor of ETS2 transcription, which leads to translocation of ERF to the cytosol and increased transcription of ETS2 (Sgouras et al. 1995, Le Gallic et al. 2004). ETS2 can be sequestered and inhibited by binding to ID1, resulting in inhibition of p16-INK4A transcription (Ohtani et al. 2004).
Transcription of p14-ARF is stimulated by binding of E2F transcription factors (E2F1, E2F2 or E2F3) in complex with SP1 to p14-ARF promoter (Parisi et al. 2002).
Oncogenic RAS signaling affects mitochondrial metabolism through an unknown mechanism, leading to increased generation of reactive oxygen species (ROS), which triggers oxidative stress induced senescence pathway. In addition, increased rate of cell division that is one of the consequences of oncogenic signaling, leads to telomere shortening which acts as another senescence trigger.
MAP3K5 phosphorylates and activates MAP2K3 (MKK3) and MAP2K6 (MKK6) (Ichijo et al. 1997, Takekawa et al. 2005), which act as p38 MAPK kinases, as well as MAP2K4 (SEK1) (Ichijo et al. 1997, Matsuura et al. 2002), which, together with MAP2K7 (MKK7), acts as a JNK kinase.
MKK3 and MKK6 phosphorylate and activate p38 MAPK alpha (MAPK14) and beta (MAPK11) (Raingeaud et al. 1996), enabling p38 MAPKs to phosphorylate and activate MAPKAPK2 (MK2) and MAPKAPK3 (MK3) (Ben-Levy et al. 1995, Clifton et al. 1996, McLaughlin et al. 1996, Sithanandam et al. 1996, Meng et al. 2002, Lukas et al. 2004, White et al. 2007), as well as MAPKAPK5 (PRAK) (New et al. 1998 and 2003, Sun et al. 2007).
Phosphorylation of JNKs (MAPK8, MAPK9 and MAPK10) by MAP3K5-activated MAP2K4 (Deacon and Blank 1997, Fleming et al. 2000) allows JNKs to migrate to the nucleus (Mizukami et al. 1997) where they phosphorylate JUN. Phosphorylated JUN binds FOS phosphorylated by ERK1 or ERK2, downstream of activated RAS (Okazaki and Sagata 1995, Murphy et al. 2002), forming the activated protein 1 (AP-1) complex (FOS:JUN heterodimer) (Glover and Harrison 1995, Ainbinder et al. 1997).
Activation of p38 MAPKs and JNKs downstream of MAP3K5 (ASK1) ultimately converges on transcriptional regulation of CDKN2A locus. In dividing cells, nucleosomes bound to the CDKN2A locus are trimethylated on lysine residue 28 of histone H3 (HIST1H3A) by the Polycomb repressor complex 2 (PRC2), creating the H3K27Me3 (Me3K-28-HIST1H3A) mark (Bracken et al. 2007, Kotake et al. 2007). The expression of Polycomb constituents of PRC2 (Kuzmichev et al. 2002) - EZH2, EED and SUZ12 - and thereby formation of the PRC2, is positively regulated in growing cells by E2F1, E2F2 and E2F3 (Weinmann et al. 2001, Bracken et al. 2003). H3K27Me3 mark serves as a docking site for the Polycomb repressor complex 1 (PRC1) that contains BMI1 (PCGF4) and is therefore named PRC1.4, leading to the repression of transcription of p16-INK4A and p14-ARF from the CDKN2A locus, where PCR1.4 mediated repression of p14-ARF transcription in humans may be context dependent (Voncken et al. 2005, Dietrich et al. 2007, Agherbi et al. 2009, Gao et al. 2012). MAPKAPK2 and MAPKAPK3, activated downstream of the MAP3K5-p38 MAPK cascade, phosphorylate BMI1 of the PRC1.4 complex, leading to dissociation of PRC1.4 complex from the CDKN2A locus and upregulation of p14-ARF transcription (Voncken et al. 2005). AP-1 transcription factor, formed as a result of MAP3K5-JNK signaling, as well as RAS signaling, binds the promoter of KDM6B (JMJD3) gene and stimulates KDM6B expression. KDM6B is a histone demethylase that removes H3K27Me3 mark i.e. demethylates lysine K28 of HIST1H3A, thereby preventing PRC1.4 binding to the CDKN2A locus and allowing transcription of p16-INK4A (Agger et al. 2009, Barradas et al. 2009, Lin et al. 2012).
p16-INK4A inhibits phosphorylation-mediated inactivation of RB family members by CDK4 and CDK6, leading to cell cycle arrest (Serrano et al. 1993). p14-ARF inhibits MDM2-mediated degradation of TP53 (p53) (Zhang et al. 1998), which also contributes to cell cycle arrest in cells undergoing oxidative stress. In addition, phosphorylation of TP53 by MAPKAPK5 (PRAK) activated downstream of MAP3K5-p38 MAPK signaling, activates TP53 and contributes to cellular senescence (Sun et al. 2007).
Annotated Interactions
A:Cdk2:p21/p27
complexA:Cdk2:p21/p27
complexA:phospho-Cdk(Thr
160):Cdh1:phosho-APC/C complexA:phospho-Cdk(Thr
160):Cdh1:phosho-APC/C complexA:phospho-Cdk(Thr
160):Cdh1:phosho-APC/C complexA:phospho-Cdk2(Thr
160):phospho-Cdh1:phospho-APC/C complexRSKs can be activated by the ERKs (ERK1, 2, 5) in the cytoplasm as well as in the nucleus, they both have cytoplasmic and nuclear substrates, and they are able to move from nucleus to cytoplasm. Efficient RSK activation by ERKs requires its interaction through a docking site located near the RSK C terminus. The mechanism of RSK activation has been studied mainly with regard to ERK1 and ERK2. RSK activation leads to the phosphorylation of four essential residues Ser239, Ser381, Ser398, and Thr590, and two additional sites, Thr377 and Ser749 (the amino acid numbering refers to RSK1). ERK is thought to play at least two roles in RSK1 activation. First, activated ERK phosphorylates RSK1 on Thr590, and possibly on Thr377 and Ser381, and second, ERK brings RSK1 into close proximity to membrane-associated kinases that may phosphorylate RSK1 on Ser381 and Ser398.
Moreover, RSKs and ERK1/2 form a complex that transiently dissociates upon growth factor signalling. Complex dissociation requires phosphorylation of RSK1 serine 749, a growth factor regulated phosphorylation site located near the ERK docking site. Serine 749 is phosphorylated by the N-terminal kinase domain of RSK1 itself. ERK1/2 docking to RSK2 and RSK3 is also regulated in a similar way. The length of RSK activation following growth factor stimulation depends on the duration of the RSK/ERK complex, which, in turn, differs among the different RSK isoforms. RSK1 and RSK2 readily dissociate from ERK1/2 following growth factor stimulation stimulation, but RSK3 remains associated with active ERK1/2 longer, and also remains active longer than RSK1 and RSK2.