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Autocrine and Paracrine Signaling Through Neuropeptide Receptors in Human Cancer

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Autocrine and Paracrine Signaling Through Neuropeptide Receptors in Human Cancer Oncogene (2001) 20, 1563 ± 1569 ã 2001 Nature Publishing Group All rights reserved 0950 ± 9232/01 $15.00 www.nature.com/onc Autocrine and paracrine signaling through neuropeptide receptors in human cancer Lynn E Heasley*,1 1Department of Medicine, University of Colorado Health Sciences Center, Denver, Colorado, CO 80262, USA Autocrine and paracrine signaling leading to stimulation lators in the nervous system. In the large majority of of tumor cell growth is a common theme in human cases, the receptors which mediate signaling by cancers. In addition to polypeptide growth factors such neuropeptides are members of the superfamily of G as EGF family members which signal through receptor protein-coupled, seven membrane-spanning receptors tyrosine kinases, accumulating evidence supports the (Burbach and Meijer, 1992). In this review, speci®c autocrine and paracrine involvement of speci®c neuro- examples where physiologic functions of selected peptides with de®ned physiologic actions as neurotrans- neuropeptide families are subverted to roles in mitters and gut hormones in lung, gastric, colorectal, tumorigenesis and hyperplastic growth are explored. pancreatic and prostatic cancers. These neuropeptides, The precise neuropeptide receptor subtypes and the including gastrin-releasing peptide, neuromedin B, neu- dominant intracellular signaling pathways that mediate rotensin, gastrin, cholecystokinin and arginine vasopres- the various proliferative responses are reviewed and the sin bind seven transmembrane-spanning receptors that emerging utilities of both speci®c and broad-spectrum couple to heterotrimeric G proteins. Studies with human neuropeptide antagonists as therapeutics for human small cell lung cancer (SCLC) cells support a require- cancers are considered. ment for balanced signaling through Gq and G12/13 proteins leading to intracellular Ca2+ mobilization, PKC Neuropeptides with putative autocrine or paracrine activation and regulation of the ERK and JNK MAP involvement in human cancers kinase pathways. While speci®c neuropeptide antagonists oer promise for interrupting the single neuropeptide While a large number of peptide hormones have been autocrine systems operating in pancreatic and prostatic identi®ed with putative roles as growth factors in cancers, SCLC is exempli®ed by multiple, redundant human cancer, this review will restrict its focus to the neuropeptide autocrine systems such that tumor growth evidence supporting the autocrine and paracrine cannot be inhibited with a single speci®c antagonist. involvement of the bombesin-related peptides, gastrin, However, a novel class of neuropeptide derivatives based cholecystokinin, neurotensin and arginine vasopressin on the substance P sequence have been de®ned that in human cancer. Physiologically, each of these exhibit broad speci®city for neuropeptide receptors and peptides ful®lls dual functions; as a neurotransmitter induce apoptosis in SCLC by functioning as biased or neuromodulator in the nervous system and as a agonists that stimulate discordant signal transduction. hormone or endocrine factor in the periphery. A brief Thus, interruption of autocrine and paracrine neuropep- overview of the normal functions of the dierent tide signaling with speci®c antagonists or broad-spectrum peptides and their receptors is presented below as well biased agonists oer promising new therapeutic ap- as the evidence for their autocrine and paracrine proaches to the treatment of human cancers. Oncogene involvement in speci®c human cancers. (2001) 20, 1563 ± 1569. Bombesin-like peptides The prototypical neuropep- Keywords: neuropeptides; autocrine; paracrine; cancer tides in this family, bombesin and ranatensin, were cells; G proteins; receptors ®rst isolated from amphibian skin and led to the subsequent identi®cation of the mammalian homolo- gues, gastrin-releasing peptide (GRP) and neuromedin Introduction B (NMB), respectively (reviewed in Battey and Wada, 1991; Ohki-Hamazaki, 2000). In the periphery, GRP Neuropeptides function peripherally as paracrine and and NMB control a wide spectrum of actions including endocrine factors to regulate diverse physiologic smooth muscle contraction and exocrine and endocrine processes and act as neurotransmitters or neuromodu- secretion. GRP is, in fact, named for its ability to stimulate gastrin release from gastrin (G) cells in the antral mucosa. Acting at the central nervous system, these peptides regulate food intake, body temperature *Correspondence: LE Heasley, Division of Renal Medicine, C-281, University of Colorado Health Sciences Center, 4200 E. Ninth Ave., and glucose levels as well as certain behavioral Denver, CO 80262, USA responses. Seven membrane-spanning receptors exhibit- Neuropeptide signaling in cancer LE Heasley 1564 ing selectivity for GRP or NMB have been identi®ed assessed (Sun et al., 2000b). However, the ability of through molecular cloning approaches (reviewed in GRP antagonists to inhibit the growth of an ovarian Battey and Wada, 1991; Ohki-Hamazaki, 2000). In carcinoma cell line (Chatzistamou et al., 2000) suggests addition, a distinct receptor referred to as bombesin that a GRP autocrine system may also function in receptor subtype-3 (BRS-3) with extensive homology to ovarian carcinomas. Finally, both central and periph- the GRP and NMB receptors has been cloned, for eral primitive neuroectodermal tumors (PNET) have which the endogenous ligand remains unde®ned (Fathi been shown to express GRP (Fruhawald et al., 1998; et al., 1993; Gorbulev et al., 1992). Interestingly, mice Lawlor et al., 1998) and the in vitro growth of a in which the BRS-3 gene has been disrupted develop peripheral PNET cell line was inhibited by a GRP multiple metabolic defects and mild obesity, indicating receptor antagonist (Lawlor et al., 1998). that BRS-3 and its unde®ned ligand also regulate important endocrine and metabolic processes (Ohki- Gastrin and cholecystokinin Gastrin and cholecystoki- Hamazaki et al., 1997). nin (CCK) serve physiological functions as hormones The bombesin-like peptides, GRP and NMB, were in the gastrointestinal tract and as neuropeptides in the among the ®rst neuropeptides to be implicated as nervous system (reviewed in Wank, 1995, 1998). autocrine growth factors in lung cancer cells. In small Gastrin and CCK are derived from distinct prepro- cell lung cancer (SCLC) cells, GRP (Cuttitta et al., hormones, but are structurally similar peptides that 1985; Moody et al., 1981) and NMB (Cardona et al., share the same C-terminal ®ve amino acids, are a- 1991) are widely synthesized and secreted into the amidated at their C-termini and are partially sulfated medium. In addition, the majority of SCLC cells on tyrosine. CCK is synthesized in neurons throughout express the mRNA for one or more of the bombesin- the central and peripheral nervous system and in like receptors (Corjay et al., 1991; Fathi et al., 1993; intestinal neuroendocrine cells (I cells) while gastrin Toi-Scott et al., 1996). The functionality of the GRP peptides are expressed and released from the duode- and NMB receptors expressed on SCLC cells as num and gastric antrum (gastrin cells) as well as the measured by intracellular Ca2+ mobilization in re- pituitary. Two distinct receptors for gastrin and CCK sponse to applied GRP or NMB has also been have been identi®ed to date; the CCK-A receptor is established (Bunn et al., 1992; Corjay et al., 1991; highly selective for sulfated CCK, while the CCK-B Moody et al., 1992; Woll and Rozengurt, 1989). The receptor exhibits a similar high anity for both ability of bombesin-like peptide antagonists (Mahmoud sulfated and nonsulfated CCK and gastrin (Wank, et al., 1991) or neutralizing antibodies (Cuttitta et al., 1995, 1998). CCK-A receptors are expressed on 1985) to reduce the growth of some cultured SCLC cell multiple cell types in the pancreas, stomach and lines provides evidence for bombesin-like peptides as intestine, but are also expressed in select areas of the autocrine growth factors in SCLC. However, as will central and peripheral nervous system. By contrast, the become clear in this review, SCLC is an example of a CCK-B receptors are predominantly expressed in the cancer where multiple, variably-expressed neuropeptide central nervous system in a pattern similar to CCK and autocrine systems operate in a redundant fashion and on parietal cells and enterochroman-like (ECL) cells their simultaneous disruption will most likely be in the gastric mucosa. required for eective inhibition of cell growth. In addition to regulating gastrointestinal hormone Normal prostate tissue and a substantial proportion secretion, evidence indicates that gastrin and CCK (*70%) of primary and metastatic prostatic tumors exert mitogenic actions on speci®c cells in the gastric contain a dispersed subpopulation of neuroendocrine mucosa and pancreas, respectively. Hypergastrinemia cells that appear to expand in prostate cancers as they resulting from pharmacological blockade of acid become androgen-independent (Aprikian et al., 1998; secretion from parietal cells leads to a modest, but Nelson and Carducci, 2000). These neuroendocrine stable increase in gastrin cell density. The ECL cells in cells express a variety of bioactive neuropeptides the gastric mucosa also respond mitogenically to including GRP. A number of studies document the elevated gastrin levels except that their density steadily elevated expression of GRP receptors in prostate increases during hypergastrinemia and eventually may cancer specimens (Bartholdi et al.,
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