No abstract

One of the members of the bZIP family of transcriptional activators is NF-IL6/LAP (IL-6 DBP, C/EBP beta, CRP2). NF-IL6/LAP protein is highly expressed in liver nuclei, where it has been implicated as a master regulator of the acute-phase response, induced by interleukin-6 (IL-6) and other inflammatory mediators. Also, NF-IL6/LAP is involved in the activation of the IL-6 promoter in response to IL-1 and bacterial lipopolysaccharide. The control of NF-IL6/LAP expression and activity is complex and poorly understood. Under some conditions the NF-IL6/LAP gene is transcriptionally activated by IL-1 and lipopolysaccharide, whereas in other instances, its binding to cognate DNA sequences is enhanced by cytokines. Additionally, the ability of constitutively expressed NF-IL6/LAP to activate transcription is strongly augmented by IL-6, through an unknown signalling pathway. We now show that stimulation of the protein kinase C pathway increases the phosphorylation of Ser 105 within the activation domain of NF-IL6/LAP, and enhances its transcriptional efficacy.

The p38 mitogen-activated protein kinase (MAPK) pathway plays a critical role in skeletal muscle differentiation. However, the relative contribution of the four p38 MAPKs (p38a, p38b, p38c and p38d) to this process is unknown. Here we show that myoblasts lacking p38a, but not those lacking p38b or p38d, are unable to differentiate and form multinucleated myotubes, whereas p38c-deficient myoblasts exhibit an attenuated fusion capacity. The defective myogenesis in the absence of p38a is caused by delayed cell-cycle exit and continuous proliferation in differentiation-promoting conditions. Indeed, activation of JNK/cJun was enhanced in p38a-deficient myoblasts leading to increased cyclin D1 transcription, whereas inhibition of JNK activity rescued the proliferation phenotype. Thus, p38a controls myogenesis by antagonizing the activation of the JNK proliferation-promoting pathway, before its direct effect on muscle differentiation-specific gene transcription. More importantly, in agreement with the defective myogenesis of cultured p38a D/D myoblasts, neonatal muscle deficient in p38a shows cellular hyperproliferation and delayed maturation. This study provides novel evidence of a fundamental role of p38a in muscle formation in vitro and in vivo.

MKP-1 limits p38 MAPK signaling in macrophages to promote the resolution of the inflammatory response and tissue repair in muscle through an AKT-dependent mechanism.

The activity of c-Jun, the major component of the transcription factor AP-1, is potentiated by amino-terminal phosphorylation on serines 63 and 73 (Ser-63/73). This phosphorylation is mediated by the Jun amino-terminal kinase (JNK) and required to recruit the transcriptional coactivator CREB-binding protein (CBP). AP-1 function is antagonized by activated members of the steroid/thyroid hormone receptor superfamily. Recently, a competition for CBP has been proposed as a mechanism for this antagonism. Here we present evidence that hormone-activated nuclear receptors prevent c-Jun phosphorylation on Ser-63/73 and, consequently, AP-1 activation, by blocking the induction of the JNK signaling cascade. Consistently, nuclear receptors also antagonize other JNK-activated transcription factors such as Elk-1 and ATF-2. Interference with the JNK signaling pathway represents a novel mechanism by which nuclear hormone receptors antagonize AP-1. This mechanism is based on the blockade of the AP-1 activation step, which is a requisite to interact with CBP. In addition to acting directly on gene transcription, regulation of the JNK cascade activity constitutes an alternative mode whereby steroids and retinoids may control cell fate and conduct their pharmacological actions as immunosupressive, anti-inflammatory, and antineoplastic agents. Lipophilic hormones such as steroids, retinoic acid (RA), thyroid hormone (T3), and vitamin D mediate most, if not all, of their actions through specific intracellular receptors that are members of the nuclear receptor superfamily (Mangelsdorf et al. 1995). Nuclear hormone receptors are ligand-regulated sequence-specific transcription factors that may activate or repress gene expression. Ligand-activated gene transcription is generally mediated by binding of nuclear receptors to their cognate DNA elements. Though negative binding elements have been described, repression is mainly conducted by interference with other transcription factors, of which AP-1 is one of the most representative (for review, see Saatcioglu et al. 1994).AP-1 is a sequence-specific transcription factor composed of either homo-or heterodimers among members within the Jun family (c-Jun, JunB, and JunD) or among proteins of the Jun and Fos (c-Fos, FosB, Fra1, and Fra2) families (for review, see Angel and Karin 1991). Among them, c-Jun is the major component of the AP-1 complex (Bohmann et al. 1987;Angel et al. 1988) and c-Fos is its best known partner. AP-1 is activated by mitogens, oncoproteins, cytokines, and stress agents such as ultraviolet (UV) light. AP-1 activation may be mediated by both transcriptionally independent and -dependent mechanisms, involving post-translational modifications of its components or increases in the expression of their corresponding genes, respectively (Angel and Karin 1991;Karin et al. 1997).c-Jun transcriptional activity is enhanced by aminoterminal phosphorylation on Ser-63/73 (Pulverer et al. 1991;Smeal et al. 1991). This inducible phosphorylation is mediated by members of the Jun amino-te...

Early Drosophila development requires two receptor tyrosine kinase (RTK) pathways: the Torso and the Epidermal growth factor receptor (EGFR) pathways, which regulate terminal and dorsal-ventral patterning, respectively. Previous studies have shown that these pathways, either directly or indirectly, lead to post-transcriptional downregulation of the Capicua repressor in the early embryo and in the ovary. Here, we show that both regulatory effects are direct and depend on a MAPK docking site in Capicua that physically interacts with the MAPK Rolled. Capicua derivatives lacking this docking site cause dominant phenotypes similar to those resulting from loss of Torso and EGFR activities. Such phenotypes arise from inappropriate repression of genes normally expressed in response to Torso and EGFR signaling. Our results are consistent with a model whereby Capicua is the main nuclear effector of the Torso pathway, but only one of different effectors responding to EGFR signaling. Finally, we describe differences in the modes of Capicua downregulation by Torso and EGFR signaling, raising the possibility that such differences contribute to the tissue specificity of both signals.

The NIMA-family kinases Nek9/Nercc1, Nek6 and Nek7 form a signalling module required for mitotic spindle assembly. Nek9, the upstream kinase, is activated during prophase at centrosomes although the details of this have remained elusive. We now identify Plk1 as Nek9 direct activator and propose a two-step activation mechanism that involves Nek9 sequential phosphorylation by CDK1 and Plk1. Furthermore, we show that Plk1 controls prophase centrosome separation through the activation of Nek9 and ultimately the phosphorylation of the mitotic kinesin Eg5 at Ser1033, a Nek6/7 site that together with the CDK1 site Thr926 we establish contributes to the accumulation of Eg5 at centrosomes and is necessary for subsequent centrosome separation and timely mitosis. Our results provide a basis to understand signalling downstream of Plk1 and shed light on the role of Eg5, Plk1 and the NIMA-family kinases in the control of centrosome separation and normal mitotic progression.

Induction of apoptosis in lymphocytes, which may account for the therapeutic effects of glucocorticoids in various diseases including leukemia, depends on the glucocorticoid receptor. However, the events leading from the activated receptor to cell lysis are not understood. A prevailing hypothesis postulates induction of so‐called ‘lysis genes’ by the activated receptor. In this study, we show that an activation‐deficient glucocorticoid receptor mutant is as effective as the wild‐type receptor in repression of AP‐1 activity, inhibition of interleukin‐2 production, inhibition of c‐myc expression and induction of apoptosis. Furthermore, we show that retinoic acid can also induce apoptosis in these cells through the retinoic acid receptor, whose repressive functions but not target site specificity, are similar to those of the glucocorticoid receptor. Therefore, the primary effect of the receptor in glucocorticoid‐mediated apoptosis correlates with transcriptional repression rather than activation and could be mediated by interference with other transcription factors required for cell survival.