Neuroendocrine tumors (NETs) of GI tract are a heterogeneous group of neoplasms that secrete peptides and amines. Small bowel NETs originate from enterochromaffin (EC) cells and secrete chromogranin A (CgA) and serotonin (5-HT). NETs are highly vascular, a reflection of increased tumour related angiogenesis. Adenosine, the major regulator of angiogenesis, is released by enhanced degradation of ATP, during cellular stress, damage and hypoxia.
Aims: Aims of the study were to investigate expression and function of adenosine receptors (AR) in human NET cell lines, BON-1 (pancreatic) and KRJ-1 (intestinal-EC cell) and to identify AR expression in archival material from NET patients.
In vitro: RT-PCR coupled with sequencing showed that BON-1 and KRJ-1 cells expressed all four AR: A1, A2a, A2b and A3. cAMP stimulation data using selective AR agonists and antagonists showed that A2a and A2b receptors were dominant. ED50 values for adenosine in both cell lines were between 5×10−7 and 10−6 M with a 315 fold increase in cAMP levels (P<0.01). The stable AR agonist NECA and the A2aR selective agonist CGS21680 stimulated cell proliferation (MTS assay) by 2030% at 10−5 M agonist (P<0.05). In BON-1 cells, hypoxia, adenosine and NECA stimulated a 2-fold (P<0.05) secretion in CgA, with ED50 values of respectively 5×10−6 and 10−7 M.
Ex vivo: Eighteen archival human NET histopathological tissue samples (pancreas, small bowel and appendix) were obtained & immunostained for AR. Strong A2bR labelling was seen in 4/4 appendiceal tumours, 4/4 pancreatic tumours and 4/9 intestinal tumours; 15/15 tumours were strongly positive for A2aR. There was very weak or no staining for the A1 or A3R.
Our data show that NETs express predominantly the A2a and A2b receptors and their activation leads to increased proliferation and CgA secretion. Targeting adenosine signal pathways may thus be useful in the therapeutic management of NETs.