br Introduction Adapter and scaffold proteins rewire signali

Introduction Adapter and scaffold proteins rewire signaling networks by subcellular compartmentalization and enable spatio-temporal Fer-1 to environmental cues that may be derailed in disease conditions like cancer (Good et al., 2011). Docking protein-1 (DOK1) is a versatile adapter protein and negative regulator of signaling (Mashima et al., 2009). DOK1 belongs to a protein family which controls immune receptors in lymphocytes and myeloid cells and inhibits inflammation in vivo (Shinohara et al., 2005). Dok1-deficient mice suffer from hematopoietic defects (Yasuda et al., 2004) and, together with loss of other DOK members, succumb to aggressive sarcomas (Mashima et al., 2010). DOK1 inhibits cytosolic (CTK) or receptor (RTK) tyrosine kinases and binds p120 RAS GTPase-activating protein (GAP) to dampen proliferation via the extracellular signal-regulated protein kinase-1/2 (ERK1/2) and other signaling cascades in non-hematopoietic cells (Songyang et al., 2001; Shinohara et al., 2004; Zhao et al., 2006). DOK1 sensitizes human cancer cells to apoptosis by etoposide, emphasizing its role in tumor suppression (Siouda et al., 2012). Alternative translation initiation yields several protein isoforms encoded by the same DOK1 mRNA (Kobayashi et al., 2009). Full-length (FL) p62 DOK1 has an N-terminal plextrin homology (PH) domain for membrane binding, a phospho-tyrosine-binding (PTB) domain for interaction with phospho-tyrosine substrates (e.g. growth factor receptors, integrins, etc. (Oxley et al., 2008)) and a C-terminal domain with proline and phospho-tyrosine residues which bind SH3-domains and SRC kinase, respectively. The C-terminal part of DOK1 also interacts with p120RASGAP, SH2 (Songyang et al., 2001; Shinohara et al., 2004) and other PTB domains (e.g. hakai (Mukherjee et al., 2012)) and contains a nuclear export signal (NES) (Niu et al., 2006) that mediates nucleo-cytoplasmic shuttling of DOK1. FL p62 DOK1 localizes to the nucleus in starved or suspended cells, to the plasma membrane and the cytosol in growth factor-stimulated and adherent cells, consistent with tyrosine kinase inhibition at membranes (Niu et al., 2006). N-terminally truncated DOK1 (p37–44) lacks the PH-domain, locates to the perinuclear area and may be responsible for transport between cytosol and nucleus (Kobayashi et al., 2009). Small DOK1 (p19–22) is deficient of both the PTB and the C-terminal domains (Hubert et al., 2000). Polymorphisms and frame shift mutations in human leukemias (Lee et al., 2004; Lee et al., 2007) introduce aberrant stop codons that yield C-terminally deleted DOK1 (p33–35) isoforms with a nuclear localisation signal (NLS) which are confined to the nucleus as well. Splice variants or dominant-negative mutants were also described for other DOK family members (Hosooka et al., 2001; Baldwin et al., 2007; Hamuro et al., 2008). However, the function of subcellular compartmentalization of DOK mutants remains unknown. Loss of FL p62 DOK1 as a tumor suppressor is a common event in human cancers, including solid tumors (Berger et al., 2010; Saulnier et al., 2012). Oncogenic kinases (Janas and Van Aelst, 2011; Miah et al., 2014) and viruses (Siouda et al., 2014) facilitate proteasomal degradation of DOK1 and epigenetic silencing of the DOK1 gene. In contrast, agents that promote stress responses (such as E2F) (Siouda et al., 2012) and differentiation, e.g. ligands for peroxisome proliferator-activated receptor-gamma (PPARγ) (Hosooka et al., 2008; Burgermeister et al., 2011), up-regulate DOK1 expression. DOK1 counteracts inactivation of PPARγ by the RAS-ERK1/2 pathway in human cells (Demers et al., 2009; Burgermeister et al., 2011) and mice (Hosooka et al., 2008; Jiang et al., 2015). DOK1 also triggers apoptosis by recruiting and interacting with SMADs (Yamakawa et al., 2002) and inhibits expression of inflammatory genes driven by NFκB and STATs (Nold-Petry et al., 2015) in hematopoietic cells. Genomic alterations of SMAD3, P53, RAS, and APC are hallmarks of colorectal cancer (CRC) (Kodach et al., 2008). We therefore hypothesized that transcription factors are down-stream effectors of DOK1 mutants in human cancer cells, where FL p62 DOK1 is lost.