Introduction: Carcinoids are neuroendocrine (NE) tumors that secrete hormones causing the carcinoid syndrome. Metastatic carcinoids are not amenable to curative surgery. Our previous research has shown that Notch signaling has a tumor suppressor role in NE tumors. In the present study, we sought to examine Thailandepsin A (TDP-A), a newly discovered HDAC inhibitor as a Notch activating drug and carcinoid cancer agent.

Methods: TDP-A is a bacterial natural product isolated from the fermentation broth of Burkholderia thailandensis E264. The cytotoxicity of TDP-A on human pancreatic (BON) carcinoid cells was measured by determining the IC50 value.The antiproliferative effect of TDP-A on BON cells growth was assessed by the methylthiazolyldiphenyl-tetrazolium bromide (MTT) rapid colorimetric assay.The mechanism of growth inhibition was determined for cell cycle and apoptosis markers by Western blot and flow cytometry analyses. Expression of Notch isoforms 1-3 was assessed at the transcriptional level (real time RT-PCR) and protein level (Western blot) from parental and TDP-A treated BON cells. Functional analysis of Notch Intracellular Domain (NICD) was done by measuring the degree of luciferase activity by CBF1 binding assay. The gene expression of HES and HEY (Notch signaling mediators) was quantified by real time RT-PCR. Effect of TDP-A on NE markers -chromogranin A (CgA) and achaete-scute complex-like1 (ASCL1) - was assessed by Western blot analysis.

Results: The IC50 value for TDP-A treatment of BON cells was determined to be 7 nM. TDP-A treatment decreased cell proliferation in a dose and time dependent manner. Western blot analysis and flow cytometry experiments indicated that the growth inhibition was due to cell cycle arrest (at G2/M phase) followed by apoptosis.Treatment of BON cells with TDP-A led to an induction of Notch isoforms in a dose-dependent manner. Functional Notch signaling was demonstrated by an increase in the CBF1 binding activity and upregualtion of transcriptional levels of HES and HEY families. More importantly, Notch activation led to a dose dependent reduction of NE markers ASCL1 and CgA.

Conclusions: We demonstrated that TDP-A is a potent Notch pathway activator and an inhibitor of BON carcinoid cell proliferation at low nanomolar concentrations. TDP-A suppressed carcinoid cell growth by promoting cell cycle arrest and apoptosis and decreased NET marker expression (ASCL1 and CgA). These results indicate that TDP-A has therapeutic potential for carcinoid cancer.

Citation Format: Renata Jaskula-Sztul, Ajitha Dammalapati, Colin Korlesky, Shaoqin Gong, Yi-Qiang Cheng, Herbert Chen. Thailandepsin A, a new HDAC inhibitor, reduces cellular proliferation and activates the Notch pathway in human carcinoids cancer cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1014. doi:10.1158/1538-7445.AM2013-1014