Background and aim: The role of transforming growth factor beta-1 (TGFbeta-1) in neuroendocrine tumour biology is currently unknown. We therefore examined the expression and biological significance of TGFbeta signalling components in neuroendocrine tumours (NETs) of the gastroenteropancreatic (GEP) tract.
Methods: Expression of TGFbeta-1 and its receptors, Smads and Smad regulated proteins, was examined in surgically resected NET specimens and human NET cell lines by immunohistochemistry, reverse transcriptase-polymerase chain reaction, immunoblotting, and ELISA. Activation of TGFbeta-1 dependent promoters was tested by transactivation assays. Growth regulation was evaluated by cell numbers, soft agar assays, and cell cycle analysis using flow cytometry. The role of endogenous TGFbeta was assessed by a TGFbeta neutralising antibody and stable transfection of a dominant negative TGFbetaR II receptor construct.
Results: Coexpression of TGFbeta-1 and its receptors TGFbetaR I and TGFbetaR II was detected in 67% of human NETs and in all three NET cell lines examined. NET cell lines expressed the TGFbeta signal transducers Smad 2, 3, and 4. In two of the three cell lines, TGFbeta-1 treatment resulted in transactivation of a TGFbeta responsive reporter construct as well as inhibition of c-myc and induction of p21((WAF1)) expression. TGFbeta-1 inhibited anchorage dependent and independent growth in a time and dose dependent manner in TGFbeta-1 responsive cell lines. TGFbeta-1 mediated growth inhibition was due to G1 arrest without evidence of induction of apoptosis. Functional inactivation of endogenous TGFbeta revealed the existence of an autocrine antiproliferative loop in NET cells.
Conclusions: Neuroendocrine tumour cells of the gastroenteropancreatic tract are subject to paracrine and autocrine growth inhibition by TGFbeta-1, which may account in part for the low proliferative index of this tumour entity.