The dependence receptor DCC (deleted in colorectal cancer) defines an alternative mechanism for caspase activation

Abstract

The expression of DCC (deleted in colorectal cancer) is often markedly reduced in colorectal and other cancers. However, the rarity of point mutations identified in DCC coding sequences and the lack of a tumor predisposition phenotype in DCC hemizygous mice have raised questions about its role as a tumor suppressor. DCC also mediates axon guidance and functions as a dependence receptor; such receptors create cellular states of dependence on their respective ligands by inducing apoptosis when unoccupied by ligand. We now show that DCC drives cell death independently of both the mitochondria-dependent pathway and the death receptor͞caspase-8 pathway. Moreover, we demonstrate that DCC interacts with both caspase-3 and caspase-9 and drives the activation of caspase-3 through caspase-9 without a requirement for cytochrome c or Apaf-1. Hence, DCC defines an additional pathway for the apoptosome-independent caspase activation. V ogelstein and his colleagues (1) have shown that the development of colonic carcinoma from normal colonic epithelium is associated with the mutation of a specific set of genes. Allelic deletions (loss of heterozygosity) on chromosome 18q in more than 70% of primary colorectal tumors prompted the search for a tumor suppressor gene at that locus. This search led to the cloning of a putative cell-surface receptor, DCC (deleted in colorectal cancer) (1). DCC expression was then shown to be markedly reduced in more than 50% of colorectal tumors. Moreover, the loss of DCC expression is not restricted to colon carcinoma but has been observed in other tumor types, including carcinoma of the stomach, pancreas, esophagus, prostate, bladder, breast, male germ cell tumors, neuroblastomas, gliomas, and some leukemias (2, 3). However, proof that DCC is a tumor suppressor gene remains inconclusive (4, 5).DCC encodes an approximately 200-kDa type I membrane protein of 1,447 amino acids, which displays homology in its extracellular domain with cell adhesion molecules (2), suggesting that DCC may play a role in cell-cell or cell-matrix interactions (6). However, DCC-mediated cell aggregation has not been firmly established (7). Recently, Tessier-Lavigne and collaborators (8, 9) have suggested that DCC may function as a component of a receptor complex that mediates the effects of the axonal chemoattractant netrin-1. The role of DCC in mediating growth cone extension has been supported by the analysis of the DCC knockout mice, which display abnormal brain development (4). However, the signaling transduction of netrin-1 through DCC that results in axon outgrowth is mainly unknown. In response to netrin-1 binding, DCC has been shown to interact with other netrin-1 receptors like UNC5H (i.e., three members UNC5H1, -2, and -3) (10) or the adenosine A2b receptor shown to transduce cAMP production upon netrin-1 binding (11). Recently, it also has been proposed that Frazzled, the Drosophila ortholog of DCC, is not, in certain circumstances, a transducing receptor but rather a carrier for the cue netrin-1 that allows netr...

Dual neurotoxic and neuroprotective role of metabotropic glutamate receptor 1 in conditions of trophic deprivation – Possible role as a dependence receptor

“…Although very different in structure, dependence receptors share some functional similarities including, the ability to mediate negative signaling in the absence of the agonist and positive signaling in its presence. In many cases the negative signaling is due to a caspase-mediated cleavage of the receptor C-terminal domain (CTD) which either releases a pro-apoptotic C-terminal receptor fragment (Bordeaux et al, 2000;Llambi et al, 2001), or exposes a pro-apoptotic region that presumably remains bound to the cell membrane (Mehlen et al, 1998;Forcet et al, 2001;Thibert et al, 2003). While a specific cleavage of the mGlu1 CDT has not been described, it should be pointed out that a sequence analysis of the long CTD of the mGlu1a splice variant reveals six putative caspase cleavage sites, as well as multiple sequences that may potentially interact with a variety of intracellular proteins including: seven in absentia homolog-1A (Siah-1A) and calmodulin (Ishikawa et al, 1999;Kammermeier and Ikeda, 2001), G-proteincoupled receptor kinases (Dale et al, 2000), alpha-tubulin (Ciruela et al, 1999), tamalin/ cytohesin complex (Kitano et al, 2002), homer proteins (Brakeman et al, 1997;Tu et al, 1999), protein phosphatase 1C (Croci et al, 2003), protein kinase C, regulators of G-protein signaling (RGS) proteins, Src-family protein tyrosine kinase and arrestins (Valenti et al, 2002;Hermans and Challiss, 2001).…”

Dependence receptors: a new paradigm in cell signaling and cancer therapy

“…In some cases, ADD recruits caspase-activating complexes that are different from those implicated in death receptors and in intrinsic mitochondrial classical apoptotic pathways. For example, in the absence of netrin-1, DCC recruits and activates caspase 9, thereby allowing caspase 3 activation, but this process does not require cytochrome c release and subsequent formation of an apoptosome (cytochrome c/apaf-1/caspase 9) complex, as is the case in the classic mitochondrial pathway (Forcet et al, 2001). DCC does not interact directly with caspase 9, hence it may recruit one or more adaptor proteins (Figure 3).…”

“…Another view could be that caspases are never completely inactive, even in nonapoptotic cells, and that this residual caspase activity is sufficient to detect receptors disengaged from their ligand. Interestingly, caspase 3 was observed to interact with DCC, downstream of its cleavage site, only in nonapoptotic conditions, that is, in the presence of netrin-1, or when the DCC caspase cleavage site is mutated (Forcet et al, 2001). It is tempting to speculate that, when dimerized in the presence of its ligand, DCC adopts a conformation that prevents its cleavage by caspases, whereas it can be efficiently cleaved as a monomer in the absence of ligand.…”

Receptors that mediate cellular dependence

“…3 Although such positive survival signals are clearly extremely important, data obtained over the past 10 years argue for a complementary and novel form of signal transduction that is proapoptotic, and is activated or propagated by stimulus withdrawal. [4][5][6][7][8][9][10][11][12][13][14][15][16] Moreover, whereas positive survival signals -such as those mediated by RTKs that bind trophic factors -involve classical signal transduction (i.e., ligand-receptor interaction initiates the signal), negative survival signals (i.e., ligand withdrawal initiates the signal), such as those mediated by the netrin-1 receptors, deleted in colorectal cancer (DCC) and uncoordinated gene 5 (Unc5)H1-3 (see below for a more detailed description), involve nonclassical signal transduction, in which typically the unbound receptor (or possibly the receptor bound by a hypothetical 'antitrophin') is activated to induce cell death by proteolytic processing, generating proapoptotic fragments ( Figure 1). In this latter case, ligand binding blocks the proapoptotic effect of the fragments, at least in some cases by inhibiting the proteolytic processing of the receptor.…”

“…9 Point mutation of this caspase site completely suppressed the proapoptotic effect of DCC, resulting in a modestly antiapoptotic effect of the mutant. 9,13 Functionally, therefore, DCC may amplify cellular caspase activity in the absence of ligand, via exposure of a proapoptotic domain lying in the amino-terminal region of the intracellular domain, proximal to Asp1290. Whether DCC may also initiate apoptosis -as opposed to functioning simply as an amplifier -is a subject of current studies.…”