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Treatment options of advanced neuroendocrine tumors (NETs) are unsatisfactory. Hence, innovative therapeutic approaches are urgently needed. Inhibition of histone deacetylases (HDACs) is a promising new approach in cancer therapy. While several HDAC inhibitors have already entered clinical trials, the effect of HDAC inhibition on NET has not been investigated. Therefore, we evaluated the antineoplastic effects of three different HDAC inhibitors, trichostatin A (TSA), sodium butyrate (NaB), and MS-275, on growth and apoptosis of the gastrointestinal NET cell lines CM and BON. We could demonstrate that HDAC inhibition dose-dependently inhibited proliferation of both cell lines with IC 50 values varying from the millimolar (NaB) to the micromolar (MS-275) and the nanomolar range (TSA). Moreover, HDAC inhibition potently induced apoptosis, which was accompanied by DNA-fragmentation, an up to 12-fold caspase-3 activation and downregulated Bcl-2 expression. Furthermore, HDAC inhibition resulted in cell cycle arrest at the G 1 -S-transition, which was associated with the suppression of cyclin D1 expression and induction of p21 and p27 expression. For BON cells, we observed an additional block in the G 2 /M phase, which was aligned with a downregulation of cyclin B1. In addition, combined treatment with MS-275 and somatostatin or the synthetic somatostatin analog octreotide was evaluated. Neither somatostatin nor its stable analog octreotide augmented the antiproliferative effect of MS-275 in NET cells. To conclude, our data show that HDAC inhibition is a promising new approach in the treatment of NET disease, which should be evaluated in clinical studies.

Gastrointestinal neuroendocrine tumours (NET) represent a heterogeneous tumour entity. The anti-neoplastic therapy of advanced NET disease is still unsatisfactory and innovative therapeutic approaches are needed. As NET frequently express insulin-like growth factors (IGFs) and their receptors (IGFR), known to promote survival, oncogenic transformation, tumour growth and spreading, the inhibition of the IGF/IGF-receptor system may offer possibilities for novel targeted treatment strategies of NET. Here, we studied the anti-neoplastic effects of an inhibition of the IGF-I receptor (IGF-1R) signalling in NET cells by the novel IGF-1R tyrosine kinase (TK) inhibitor NVP-AEW541, whose anti-neoplastic potency has not yet been tested in NET disease. Using two human NET cell lines with different growth characteristics, we demonstrated that NVP-AEW541 dose-dependently inhibited the proliferation of NET cells by inducing apoptosis and cell cycle arrest. Anti-neoplastic effects of NVP-AEW541 were also detected in primary cultures of human neuroendocrine gastrointestinal tumours. Apoptosis was characterized by activation of the apoptotic key enzyme, caspase-3, as well as by detection of changes in the expression of the pro-and anti-apoptotic proteins, BAX and Bcl-2, after NVP-AEW541 treatment. Cell cycle was arrested at the G1/S checkpoint. The anti-neoplastic effects of NVP-AEW541 involved the inactivation of ERK1/2. Induction of immediate cytotoxicity did not account for the anti-neoplastic effects of NVP-AEW541, as shown by measurement of lactate dehydrogenase release. Moreover, additive anti-neoplastic effects were observed when NVP-AEW541 was combined with cytostatics such as doxorubicin or the 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, fluvastatin. This is the first report on the induction of apoptosis and cell cycle arrest by the IGF-1R-TK inhibitor, NVP-AEW541, in NET cells. The inhibition of the IGF/IGFR system appears to be a promising novel approach for future treatment strategies of NET disease.