Serotonin and the 5-HT7 receptor: the link between hepatocytes, IGF-1 and small intestinal neuroendocrine tumors

Cancer Sci. 2013 Jul;104(7):844-55. doi: 10.1111/cas.12174. Epub 2013 May 24.

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.

MeSH terms

  • Animals
  • Cell Line, Tumor
  • Cell Proliferation
  • Cyclic AMP / metabolism
  • Cyclic AMP Response Element-Binding Protein / metabolism
  • Hepatocytes / metabolism*
  • Hepatocytes / pathology
  • Insulin-Like Growth Factor I / metabolism*
  • Intestinal Neoplasms / metabolism*
  • Intestinal Neoplasms / pathology
  • Intestine, Small / metabolism*
  • Intestine, Small / pathology
  • Liver Neoplasms / metabolism
  • Liver Neoplasms / pathology
  • MAP Kinase Signaling System
  • Mice
  • Mice, SCID
  • Neuroendocrine Tumors / metabolism*
  • Neuroendocrine Tumors / pathology
  • Proto-Oncogene Proteins c-akt / metabolism
  • Rats
  • Receptors, Serotonin / metabolism*
  • Serotonin / metabolism*
  • Signal Transduction / physiology
  • Tumor Microenvironment / physiology

Substances

  • Cyclic AMP Response Element-Binding Protein
  • Receptors, Serotonin
  • serotonin 7 receptor
  • Serotonin
  • Insulin-Like Growth Factor I
  • Cyclic AMP
  • Proto-Oncogene Proteins c-akt