ABSTRACT
The Notch system in vertebrates comprises four receptors (Notch1-Notch4) and at least five ligands from the families Delta and JAG/Serrate (DSL): JAG1, JAG2, Delta-like (Dll)-1, Dll-3, and Dll-4 (Artavanis-Tsakonas et al. 1999; Miele 2006; Miele et al. 2006). Ligands of Notch receptors can be divided into several groups based on their domain composition. Canonical DSL ligands (JAG1, JAG2 and Dll-1) are type I cell surface proteins, consisting of the Delta/Serrate/LAG-2 (DSL), Delta and OSM-11-like proteins [DOS, which is specialized tandem EGF repeats] and EGF motifs. The other subtypes of DSL canonical ligands include Dll-3 and Dll-4 that lack the DOS motif (Cordle et al. 2008; D’Souza et al. 2008; Komatsu et al. 2008). Both the DSL and DOS domains are crucial for physical binding with Notch receptor (Kopan and Ilagan 2009). However some membranetethered and secreted non canonical ligands lacking DSL and DOS domains have also been documented to activate Notch signaling both in vitro and in vivo (Cui et al. 2004; Leask and Abraham 2006; Gupta et al. 2007; Albig et al. 2008; D’Souza et al. 2008; Heath et al. 2008; Lu et al. 2008), which may explain the diverse and frequent effects of Notch signaling with the small number of canonical DSL ligands and receptors in vertebrate genomes (D’Souza et al. 2008). Notch receptors belong to a large single-pass type 1 transmembrane protein family; the extracellular domain consists of 29-36 tandem arrays of EGF (epidermal growth factor)-like repeats, followed by a conserved negative regulatory region (NRR or LNR) consisting of three cystein-rich Notch Lin12 repeats (N/Lin 12) and a heterodimerization (HD) domain (Milner and Bigas 1999). Notch family members differ in the number of EGF-like repeats, however they share many similarities in structure
Table 1. Function of Notch signaling in human tumors and tumor cell lines
(Weng et al. 2004; Kopan and Ilagan 2009). EGF-like repeats mediate ligand binding, whereas NRR functions to prevent both ligand-dependent and -independent signaling (Weng et al. 2004). The cytoplasmic portion of Notch is composed of a DNA binding protein (RBP-Jk associated molecule or RAM) domain and six ankyrin (ANK) repeats, which are flanked by two nuclear localization signals (NLS), followed by a transactivation domain (TAD) and a domain rich in proline, glutamine, serine and threonine residues (PEST) that controls receptor half life (Okuyama et al. 2008; Kopan and Ilagan 2009; Tien et al. 2009) (Figure 1). Membrane localization of Notch requires S1 cleavage of precursor of the Notch receptor. This event occurs in the Golgi network by the action of a furin-like convertase. Then, the two fragments are re-assembled as a non-covalently linked heterodimeric receptor at the cell surface (Lewis 1998). Mature Notch receptors are heterodimers made up of an extracellular subunit, a transmembrane subunit (NTM) and a cytoplasmic subunit. Activation of Notch comprises two consecutive cleavages of the transmembrane receptor upon the binding of a Notch ligand. Binding of Notch heterodimer to ligand triggers S2 cleavage. This process takes places at the cell surface. NTM subunit is cleaved by ADAM/Tumor necrosis factora-converting enzyme (TACE) metalloprotease family at Site 2 (located ~12 amino acids before the transmembrane domain). S2 cleavage releases the Notch extracelluar domain (NECD) from the heterodimer and creates a membrane-tethered Notch extracellular truncation (NEXT), which becomes a substrate for g-secretase. S3 is cleaved by g-secretase at Site 3 and 4 (Mumm et al. 2000). This last cleavage occurs on the plasma membrane and/or in endosome. The new mobile cytoplasmic subunit [Notch intracellular domain (NICD or NIC)] is translocated to the nucleus, where it interacts with members of the DNA-binding protein, recombination signal binding protein for immunoglobulin kappa J (RBP-Jk) or CBF1/Su(H)/Lag-1 (CSL) family of transcription factors (Borggrefe and Oswald 2009). Activated NICD-RBP-Jk complex displaces co-repressors and recruits coactivator (co-A) mediating the transcription of target genes such as Hes-1 (hairy enhancer of split), cyclin D, Hey-1 (hairy/enhancer-of-split related with YRPW motif) and others (Miele 2006; Miele et al. 2006). In the absence of NICD, CSL may interplay with the ubiquitous corepressor (Co-R) proteins and histone deacetylases (HDACs) to repress transcription of some target genes (Fortini and Artavanis-Tsakonas 1994; Fiuza and Arias 2007).
