Abstract A232: Novel thiosemicarbazones with potent antitumor and antimetastatic activity inhibit the signal transducer and activator of transcription (STAT) pathway.

Abstract

Abstract Background: Novel thiosemicarbazones that bind iron [i.e., di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT) & di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC)] possess marked and selective anti-proliferative and anti-metastatic activities in vitro and in vivo across a range of cancer types [Proc Natl Acad Sci USA (2006) 103,14901-6; Mol Pharmacol (2011) 80,598-609]. However, the mechanisms underlying the anti-tumor effects of these compounds remain incompletely understood. Here, for the first time, we reveal that Dp44mT & DpC inhibit constitutive and interleukin 6 (IL6)-induced activation of signal transducer and activator of transcription 3 (STAT3) signaling, which is hyper-activated in many malignancies and promotes proliferation, survival and metastasis of cancer cells. Methods: The pancreatic cancer cell lines, PANC-1 and MIAPaCa-2, and the prostate cancer cell line, DU145, were incubated with Dp44mT & DpC for 24 h/37°C. Following preparation of whole cell and compartmental protein extracts or native cell lysates, expression levels of proteins involved in STAT signaling were assessed by SDS-PAGE or native PAGE and immunoblotting. Results: Dp44mT & DpC significantly decreased constitutive phosphorylation of the STAT3 transcription factor at Tyr705 in PANC-1, MIAPaCa-2, and DU145 cells. Furthermore, pre-incubation of these cells for 24 h with Dp44mT or DpC inhibited STAT3 phosphorylation and nuclear translocation induced by IL6. In MIAPaCa-2 cells, phosphorylation of the upstream non-receptor tyrosine kinase, Src, was also decreased by these agents. This observation indicated that the inhibition of STAT3 activation by Dp44mT or DpC may be mediated by non-receptor as well as receptor-mediated mechanisms. Using native PAGE, we showed that Dp44mT & DpC significantly decreased levels of dimerized STAT3, which was shown not to be due to direct binding of these compounds to STAT3. In addition, expression of STAT3-regulated gene products, namely cyclin D1 and c-myc, were significantly reduced. Redox-active Dp44mT- and DpC-iron complexes, but not the redox-inactive desferrioxamine-iron complex, also exhibited these inhibitory effects on the STAT pathway in all cell lines similarly to the ligands alone, suggesting that their redox activity was important for their effects. Collectively, these results indicate that Dp44mT & DpC suppress STAT3 transcriptional regulatory activity. Conclusions: Considering the role of IL6/STAT3 signaling in promoting tumorigenesis and tumor progression, these data reveal important insights into the mechanisms underlying the potent anti-cancer activity of Dp44mT and DpC through inhibition of the oncogenic STAT pathway. This knowledge may aid in the development of personalized cancer treatments based on the molecular signature of an individual's cancer. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):A232. Citation Format: Goldie Y. L. Lui, Zaklina Kovacevic, Des R. Richardson. Novel thiosemicarbazones with potent antitumor and antimetastatic activity inhibit the signal transducer and activator of transcription (STAT) pathway. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr A232.