title: Serotonin and the 5-HT7 receptor: the link between hepatocytes, IGF-1 and small intestinal neuroendocrine tumors.
abstract: Platelet-derived serotonin (5-HT) is involved in liver regeneration. The liver is also the metastatic site for malignant enterochromaffin (EC) cell "carcinoid" (neuroendocrine) neoplasms, the principal cellular source of 5-HT. We hypothesized that 5-HT produced by metastatic EC cells played a role in the hepatic tumor-microenvironment principally via 5-HT₇ receptor-mediated activation of hepatocyte IGF-1 synthesis and secretion. Using isolated rat hepatocytes, we evaluated 5-HT₇ receptor expression (using PCR, sequencing and western blot). ELISA, cell transfection and western blots delineated 5-HT-mediated signaling pathways (pCREB, AKT and ERK). IGF-1 synthesis/secretion was evaluated using QPCR and ELISA. IGF-1 was tested on small intestinal neuroendocrine neoplasm proliferation, while IGF-1 production and 5-HT₇ expression were examined in an in vivo SCID metastasis model. Our results demonstrated evidence for a functional 5-HT₇ receptor. 5-HT activated cAMP/PKA activity, pCREB (130-205%, P < 0.05) and pERK/pAKT (1.2-1.75, P < 0.05). Signaling was reversed by the 5-HT₇ receptor antagonist SB269970. IGF-1 significantly stimulated proliferation of two small intestinal neuroendocrine neoplasm cell lines (EC₅₀: 7-70 pg/mL) and could be reversed by the small molecule inhibitor BMS-754807. IGF-1 and 5-HT were elevated (40-300×) in peri-tumoral hepatic tissue in nude mice, while 5-HT₇ was increased fourfold compared to sham-operated animals. We conclude that hepatocytes express a cAMP-coupled 5-HT₇ receptor, which, at elevated 5-HT concentrations that occur in liver metastases, signals via CREB/AKT and is linked to IGF-1 synthesis and secretion. Because IGF-1 regulates NEN proliferation, identification of a role for 5-HT₇ in the hepatic metastatic tumor microenvironment suggests the potential for novel therapeutic strategies for amine-producing mid-gut tumors.
authors.affiliation: Gastrointestinal Pathobiology Research Group, Yale University School of Medicine, New Haven, CT, USA.
authors.forename: Bernhard
authors.initials: B
authors.lastname: Svejda
authors.forename: Mark
authors.initials: M
authors.lastname: Kidd
authors.forename: Andrew
authors.initials: A
authors.lastname: Timberlake
authors.forename: Kathy
authors.initials: K
authors.lastname: Harry
authors.forename: Alexander
authors.initials: A
authors.lastname: Kazberouk
authors.forename: Simon
authors.initials: S
authors.lastname: Schimmack
authors.forename: Ben
authors.initials: B
authors.lastname: Lawrence
authors.forename: Roswitha
authors.initials: R
authors.lastname: Pfragner
authors.forename: Irvin M
authors.initials: IM
authors.lastname: Modlin
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D000242:Cyclic AMP / Q000378:metabolism; D017362:Cyclic AMP Response Element-Binding Protein / Q000378:metabolism; D022781:Hepatocytes / Q000378:metabolism* / Q000473:pathology; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D007414:Intestinal Neoplasms / Q000378:metabolism* / Q000473:pathology; D007421:Intestine, Small / Q000378:metabolism* / Q000473:pathology; D008113:Liver Neoplasms / Q000378:metabolism / Q000473:pathology; D020935:MAP Kinase Signaling System; D051379:Mice; D016513:Mice, SCID; D018358:Neuroendocrine Tumors / Q000378:metabolism* / Q000473:pathology; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D051381:Rats; D011985:Receptors, Serotonin / Q000378:metabolism*; D012701:Serotonin / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology; D059016:Tumor Microenvironment / Q000502:physiology
chemical_list: D017362:Cyclic AMP Response Element-Binding Protein; D011985:Receptors, Serotonin; C104425:serotonin 7 receptor; D012701:Serotonin; D007334:Insulin-Like Growth Factor I; D000242:Cyclic AMP; D051057:Proto-Oncogene Proteins c-akt