Fibrosis is a cardinal feature of small intestinal neuroendocrine tumors (SI-NETs) both in local peritumoral tissue and systemic sites (cardiac). 5-HT, a commonly secreted NET amine, is a known inducer of fibrosis, although the mechanistic basis for it and growth factors regulating fibrosis and proliferation in the tumor microenvironment are unclear. We hypothesized that targeting 5-HT2B receptors on tumor cells would inhibit SI-NET 5-HT release and, thereby, fibroblast activation in the tumor microenvironment.
We studied the 5-HT2B receptor antagonist PRX-08066 in NET cell lines (KRJ-I, H720) and in the coculture system (KRJ-I cells: fibroblastic HEK293 cells) using real time polymerase chain reaction, ELISA, Ki67 immunostaining, and flow cytometry-based caspase 3 assays to assess antiproliferative and profibrotic signaling pathways.
In the 5-HT2B expressing SI-NET cell line, KRJ-I, PRX-08066 inhibited proliferation (IC50 4.6x10−9M) and 5-HT secretion (6.9 × 10−9M) and decreased ERK1/2 phosphorylation and profibrotic growth factor synthesis and secretion (transforming growth factor beta 1 [TGFβ1], connective tissue growth factor [CTGF] and fibroblast growth factor [FGF2]). In the KRJ-I:HEK293 coculture system, PRX-08066 significantly decreased 5-HT release (>60%), Ki67 (transcript and immunostaining: 20%-80%), TGFβ1, CTGF, and FGF2 transcription (20%-50%) in the KRJ-I cell line. 5-HT itself stimulated HEK293 proliferation (25%) and synthesis of TGFβ1, CTGF and FGF2. PRX-08066 inhibition of KRJ-I function reversed these effects in the coculture system.
Targeting the 5-HT2B receptor may be an effective antiproliferative and antifibrotic strategy for SI-NETs because it inhibits tumor microenvironment fibroblasts as well as NET cells. Fibrosis and proliferation appear to be biologically interfaced neuroendocrine neoplasia domains. Cancer 2010. © 2010 American Cancer Society.