Although it is known that small intestinal carcinoids are derived from enterochromaffin (EC) cells, these cells remain poorly characterized and little is known of the growth regulatory mechanisms of these neuroendocrine cells. Down‐regulation or loss of the transforming growth factor‐β1 (TGFβ1) cytostatic program and activation of TGFβ‐mediated transcriptional networks is associated with uncontrolled growth and metastasis in other neural tumors, glioblastomas. Whether this phenomenon is common to small intestinal carcinoid tumors was investigated.

The effects of TGFβ1 on cultured normal EC cells (isolated by FACS sorting) and the neoplastic EC cell line, KRJ‐I, was assessed using the MTT assay. The TGFβRII transcript and protein were identified in tumor cells and the effects of TGFβ1 on SMAD2 phosphorylation and nuclear translocation quantified. The time‐dependent response of SMAD4, SMAD7, c‐Myc, and P21^WAF1/CIP1 protein expression and c‐Myc and p21^WAF1/CIP1 transcript was measured in response to TGFβ1 and the transcript expression of candidate downstream targets, MTA1 and E‐cadherin, were assessed.

TGFβ1 inhibited normal EC cell proliferation (IC₅₀ = 17 pM) but stimulated neoplastic EC cell proliferation (EC₅₀ = 22 pM). In tumor cells, significantly decreased transcript (P < .01) of TGFβRII was identified, but no receptor mutations were identified and protein expression was evident. TGFβ1 (1 ng/mL) resulted in SMAD2 phosphorylation and <7% nuclear expression compared with 93% in normal EC cells. In neoplastic cells, TGFβ1 (1 ng/mL) caused a decrease in SMAD4 (>16%, P < .05), whereas SMAD7 and c‐Myc transcript and protein were respectively increased >21% (P < .05) and ≈40% (P < .002). TGFβ1 (1 ng/mL) also decreased p21^WAF1/CIP1 transcript by 60% (P < .001) and protein that was undetectable at 24 hours. Expression of the downstream targets of the c‐Myc pathway, MTA1, was increased (20%) and E‐cadherin decreased (30%).

The neoplastic EC cell is characterized by loss of TGFβ‐1‐mediated growth inhibition and, similar to glioblastomas, utilizes the TGFβ system to induce gene responses associated with growth promotion (c‐Myc and the ERK pathway), invasion (E‐cadherin), and metastasis (MTA1). Cancer 2007. © 2007 American Cancer Society.