Abstract 363: STAT5 inhibition induces apoptosis in peripheral T cell lymphoma
Authors: Simpson Haley M. , Furusawa Aki , Sadashivaiah Kavitha , Banerjee Arnob 
Published: July 2017

Peripheral T cell lymphoma (PTCL) is a heterogeneous malignancy with extremely poor prognosis. Five-year overall survival is &lt;40% and can be as low as 12% for specific subtypes. There is, therefore, an urgent need to identify and develop novel therapeutic targets and approaches. A number of recent sequencing studies, including a whole exome sequencing analysis of primary PTCL patients conducted by our laboratory, have revealed recurrent oncogenic driver mutations in the common gamma chain/JAK/STAT signaling pathway, resulting in the aberrant activation of the transcriptional regulator STAT5. STAT5 has additionally been found to be upregulated or contain activating mutations in 30-40% of PTCL cases of specific subtypes. These findings highlight STAT5 as an oncogenic driver of PTCL and suggest a role for it in novel therapeutic approaches. Based on these data, we aimed to evaluate STAT5 mechanistically as a therapeutic target in PTCL.

The drug, pimozide, which is an FDA approved neuroleptic agent, was recently identified by drug screen to be a STAT5 inhibitor. We assessed the efficacy of pimozide in PTCL using resazurin dye assay, Fluorescence Activated Cell Sorting (FACS), and immunoblot analysis. Our data demonstrate a concentration dependent reduction in STAT5 activity and the number of viable cells in PTCL cell lines after culture with pimozide. This is shown to be due to an increase in cell death by apoptosis. To verify that pimozide’s efficacy is due to its inhibition of STAT5 activity, we transduced PTCL cell lines with anti-STAT5 shRNA to knock down STAT5 and assessed PTCL cell survival and proliferation. We show that STAT5 knockdown results in a three-fold reduction in PTCL cell viability, which occurs due to an increase in apoptosis. Furthermore, our data supports a TRAIL dependent mechanism for induction of apoptosis shown by an upregulation of TRAIL by PTCL cells after STAT5 knockdown or culture with pimozide. Mitochondrial membrane potential is also disrupted. These findings have potential clinical implications, as we further demonstrate that pimozide inhibits STAT5 phosphorylation, reduces cell viability, and induces apoptosis in primary PTCL patient samples ex vivo.

This research supports further exploration of STAT5 as a therapeutic target in PTCL and the development of STAT5 inhibition in the treatment of PTCL. To facilitate the development of non-chemotherapeutic dependent therapeutic approaches for PTCL, we are now assessing STAT5 inhibition in combination with small molecule inducers of apoptosis including TRAIL pathway and PARP inhibitors. PARP inhibitors have shown efficacy in malignancies with DNA damage response pathway mutations, which have been identified in up to half of cases of certain subtypes of PTCL. We also aim to further investigate the use of JAK inhibitors, kinases responsible for STAT5 activation, in combination in PTCL, as several have demonstrated clinical efficacy in other hematologic malignancies.

Citation Format: Haley M. Simpson, Aki Furusawa, Kavitha Sadashivaiah, Arnob Banerjee. STAT5 inhibition induces apoptosis in peripheral T cell lymphoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 363. doi:10.1158/1538-7445.AM2017-363