Transcriptional regulation by the phosphorylation-dependent factor CREB

Signaling by protein phosphatases in the nucleus

The nucleus contains a large variety of protein phosphatases, which function in key processes such as cell-cycle progression, replication, transcription and RNA processing. Here, we review the pleiotropic action of nuclear protein phosphatases and focus in particular on the underlying signaling strategies. It appears that nuclear protein phosphatases can both mediate and antagonize signaling by protein kinases, sometimes as part of feedback loops. Some protein phosphatases shuttle between the cytoplasm and the nucleus, which enables them to act as signal transducers between both compartments. An emerging theme is the contribution of protein phosphatases to cycles of protein phosphorylation and dephosphorylation that steer the assembly and firing of molecular machines in the nucleus.

Repression of activator protein-1-mediated transcriptional activation by the Notch-1 intracellular domain

Developmental decisions that control cell fate are commonly regulated by the Notch signaling pathway. Activation of transmembrane Notch receptors results in proteolytic liberation of the intracellular domain of Notch, which translocates into the nucleus, binds a repressor (C promoter binding factor 1/RBP-Jkappa, Su(H), and Lag-1 (CSL)), and induces target genes. We found that the intracellular domain of human Notch-1 (NIC-1) represses activator protein-1 (AP-1)-mediated transactivation. Because numerous genes that control immune and inflammatory responses are AP-1-dependent and Notch regulates immune cell function, we investigated the underlying molecular mechanisms. Repression of AP-1 by NIC-1 did not represent a general inhibitory effect on transcription because nuclear factor kappaB-dependent transcription and transcription driven by a constitutive promoter and enhancer were not affected by NIC-1. The physiological relevance of the repression was supported by the facts that repression was apparent in multiple cell lines, endogenous AP-1 target genes were repressed, and similar concentrations of NIC-1 were required for CSL-dependent activation and AP-1 repression. The RBP-Jkappa-associated molecule domain of NIC-1 that mediates CSL binding and distinct sequences necessary for transactivation were required for repression. However, there was not a strict correlation between the sequence requirements for CSL-dependent activation and AP-1 repression. Repression correlated with predominant nuclear localization of NIC-1 and was not accompanied by disruption of c-Jun amino-terminal kinase-dependent signaling events required for AP-1 activation or by defective AP-1 DNA binding activity. These results provide evidence for negative cross-talk between Notch and AP-1, which may have important consequences for controlling diverse biological processes.

Cyclic AMP increases rat inhibitor of apoptosis protein (RIAP1) mRNA in renal mesangial cells

BACKGROUND

The cyclic adenosine 3',5'-monophosphate (cAMP) pathway plays a central role in the regulation of cell proliferation, differentiation and apoptosis. Cyclic AMP has been identified as a bifunctional regulator of apoptosis. The inhibitor of apoptosis proteins (IAP) regulates apoptosis by directly inhibiting distinct caspases.

METHODS

Expression levels of rat IAP (RIAP)-1 were investigated by RNase protection assay in rat mesangial cells after stimulation with diverse agents that modulate cellular levels of cAMP.

RESULTS

Rat mesangial cells up-regulated RIAP1 mRNA levels after cAMP stimulation. Membrane-permeable cAMP analogs, as well as cAMP production in response to the beta-adrenergic receptor agonist salbutamol caused a large increase in RIAP1 mRNA level, which could be inhibited by the protein kinase A inhibitors H89 and Rp-cAMPS, or by the nuclear factor-kappaB (NF-kappaB) inhibitor BAY117085. Inhibition of phosphodiesterase type IV by denbufyllin or rolipram potentiated the cAMP-mediated increase in RIAP1 mRNA. In contrast, the cyclic guanosine 3',5'-monophosphate (cGMP) analog Bt2cGMP did not affect the RIAP1 mRNA level.

CONCLUSIONS

These data establish, to our knowledge for the first time, that RIAP1 mRNA levels are regulated by the cAMP-signaling pathway and suggest potential new avenues of therapy to modulate apoptosis.

CCAAT/enhancer binding proteins: do they possess intrinsic cAMP-inducible activity?

CCAAT/enhancer binding proteins (C/EBPs) are transcription factors that are enriched in tissues which play a central role in energy metabolism, such as adipose and liver. Structure/function analyses of these proteins have identified several transactivation domains, some of which can physically interact with general transcription factors present in the preinitiation complex. C/EBPs are generally considered to be constitutively-acting factors, unlike other transcription factors whose activities can be regulated by covalent modification, binding of a specific ligand, etc. However, studies of the regulatory property of the phosphoenolpyruvate carboxykinase gene promoter have uncovered a role for C/EBPs in mediating cAMP responsiveness, and identified specific domains within the proteins, which mediate this effect. Interestingly, a number of other gene promoters that are activated in response to cAMP also contain binding sites for C/EBP, and these binding sites are often located within the region of the promoter that is responsible for mediating the acute responsiveness to cAMP. The evidence presented in this review provides compelling support for the hypothesis that C/EBPs have both constitutive and cAMP-inducible activities, and should be considered as a cAMP-responsive nuclear regulator.

Kinetic modelling approaches to in vivo imaging

The ability to visualize protein dynamics and biological processes by in vivo microscopy is revolutionizing many areas of biology. These methods generate large, kinetically complex data sets, which often cannot be intuitively interpreted. The combination of dynamic imaging and computational modelling is emerging as a powerful tool for the quantitation of biophysical properties of molecules and processes. The new discipline of computational cell biology will be essential in uncovering the pathways, mechanisms and controls of biological processes and systems as they occur in vivo.