Autoregulatory effects of serotonin on proliferation and signaling pathways in lung and small intestine neuroendocrine tumor cell lines

Abstract

BACKGROUND: Survival rates for gastrointestinal (GI) and bronchopulmonary (BP) neuroendocrine tumors (NETs) have not altered significantly (5-year survival rate: GI NETs, 64.1%; BP NETs, 87%-89%) in 30 years (from 1973 to 2004). No effective or specific antineoplastic agents are available to date, although somatostatin analogs inhibit NET 5-hydroxytryptophan (5-HT) secretion. Given the expression of 5-HT receptors on NETs, the authors hypothesized that 5-HT autoregulated NET proliferation. METHODS: Proliferation was evaluated in 3 NET cell lines using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide uptake; in addition, real-time polymerase chain reaction analyses and enzyme-linked immunosorbent assay studies were performed to delineate 5-HT-mediated signaling pathways. To determine the receptor and role of endogenous 5-HT production, the effects of ketanserin (5-HT receptor subtypes 2A and 2C [5-HT2A/2C]); ondansetron (5-HT3); and the suicide inhibitor of the rate-limiting enzyme for 5-HT synthesis, tryptophan hydroxylase (7-HTP) were investigated. RESULTS: Exogenously added 5-HT stimulated proliferation in atypical BP NET NCI-H720 cells (+50%; half-maximal stimulatory concentration [EC50] = 10 nM), in typical BP NET NCI-H727 cells (+40%; EC50 = 0.01 nM), and in GI NET KRJ-I cells (+60%; EC50 = 25 nM). In NCI-H720 cells, proliferation was inhibited by ketanserin (half-maximal inhibitory concentration [IC50] = 0.06 nM) and ondansetron (IC50 = 0.4 nM) and also was inhibited by 7-HTP (IC50 = 0.3 nM). In NCI-H727 cells, ketanserin and 7-HTP inhibited proliferation (IC50 = 0.3 nM and IC50 = 2.3 nM, respectively), whereas ondansetron had no effect. In KRJ-I cells, ketanserin (IC50 = 0.1 nM) and 7-HTP (IC50 = 0.6 nM), but not ondansetron, inhibited proliferation. In all cell lines, 5-HT activated proliferation through extracellular signal-regulated kinase 1 (ERK1) and ERK2 phosphorylation and c-Jun N-terminal kinase (JNK)-mediated pathways (c-JUN and Ki-67 transcription). An autoregulatory effect was indicated by the 7-HTP-mediated inhibition of extracellular 5-HT and downstream effects on NET proliferation. CONCLUSIONS: Lung and GI NET proliferation was autoregulated by 5-HT through alterations in ERK and JNK signaling. Targeting NET cells with 5-HT2 receptor antagonists and 7-HTP reversed proliferation. The current results indicated that 5-HT2 receptor subtype-specific antagonists may represent a viable antiproliferative therapeutic strategy. © 2009 American Cancer Society.