Downstream signaling events

Immunreceptor based activation motif. The classical ITAM motif comprises the consensus sequence Yxxl/ Lx(6-12)YxxI/L (where Y stands for tyrosine, I stands for isoleucine, L stands for leucine, and x can be any amino acid). Kinases containing tandem SH2 domains, as for example ZAJP-70 or SYK, recognize the phosphorylated ITAMs, thereby initiating downstream signaling events. 

The major 3 -phosphoinositide products of class I PI3Ks are phosphati-dylinositol 3,4,5-trisphosphate [PI(3,4,5)P3, which is formed primarily from phosphorylation of PI(4,5)P2) and its metabolite phosphatidylinositol 3,4-bisphosphate, PI(3,4)P2]. The basal levels of PI(3,4)P2 and PI(3,4,5)P3 in cells are usually in low abundance but can rise sharply after cell stimulation to interact with an array of protein effectors via pleckstrin homology (PH) domains, modular segments of about 100 amino acids found in many signaling proteins. It is these PH-domain-containing proteins that are able to propagate and drive downstream signaling events. 

The precise mechanism of action of CNTF in the promotion of weight loss has not been defined, but its effects appear to be mediated by various hypothalamic pathways. The weight loss from a low dose of CNTF in rodents and humans does not appear to be caused by cachectic cytokines such as interleukin-1 because no muscle wasting was observed in the CNTF-treated groups. CNTF may mediate some of its effects by coupling to downstream signaling events in the leptin pathway. In a diet-induced model of obesity, CNTF, but not leptin, was shown to stimulate phosphorylation of the cytoplasmic transcription factor... 

The luciferase reporter assay captures the effect of test compounds on downstream signaling events (i.e., activation of NFAT and AP-1). However, such effects could also be due to off-target activity and not (solely) inhibition of the PTP of interest. If the direct substrate(s) of the target PTP is known, and phospho-specific antibodies for this/these substrate(s) are available, immunoblot assays can be employed that directly probe the phosphorylation levels of the substrate(s). If no direct substrate of the target PTP has been... 

Fig. 11.4 Schematic diagram ofthe 11-2 receptor complex. The II-2R is composed of three subunits, IL-2Ra, IL-2R/7 yc. The Box 1 and Box 2 domains are conserved among members ofthe cytokine receptor superfamily, and a variable spacer region V intervenes Boxl and Box2. The Boxl-V-Box2 regions encompass the sites of association with the Janus kinases (Jak). Jakl associates constitutively with IL-2R/> and Jak3 associates constitutively with yc. Both IL-2R/> and yc encode multiple cytoplasmic tyrosine residues with approximated locations shown. At least some of these tyrosines on both chains become phosphorylated following receptor activation whereupon they serve to recruit effector proteins to direct downstream signaling events.

Fig. 11.6 Signaling by the / -subunit of the IL-2 receptor. The cytoplasmic portion ofthe / -subunit is shown with tyrosine phosphorylation sites that mediate further downstream signaling events, jakl is constitutively associated with the Boxl and Box2 regions. Following ligand binding, jakl becomes activated and phosphorylates distinct tyrosine residues on the / -subunit which serve as attachment points for SH2 domains of further signaling proteins as indicated. The Stat5 transcription factor can...

Receptor activation and downstream signaling events are typically followed by receptor desensitization, the process by which the receptor becomes refractory to continued stimulation within seconds to minutes of agonist exposure. This is followed in turn by internalization, a process whereby the receptor is sequestered from the cell surface to intracellular... 

---