Galiellalactone inhibits the STAT3/AR signaling axis and suppresses Enzalutamide-resistant Prostate Cancer

Daksh Thaper,Ramandeep Kaur,Martin Johansson,Sahil Kumar,Sepideh Vahid,Amina Zoubeidi,Jennifer L. Bishop,Shaghayegh Nouruzi

doi:10.1038/s41598-018-35612-z

Daksh Thaper, Ramandeep Kaur + Show 6 more

Open Access

https://doi.org/10.1038/s41598-018-35612-z

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Abstract

Most prostate cancer patients will progress to a castration-resistant state (CRPC) after androgen ablation therapy and despite the development of new potent anti-androgens, like enzalutamide (ENZ), which prolong survival in CRPC, ENZ-resistance (ENZR) rapidly occurs. Re-activation of the androgen receptor (AR) is a major mechanism of resistance. Interrogating our in vivo derived ENZR model, we discovered that transcription factor STAT3 not only displayed increased nuclear localization but also bound to and facilitated AR activity. We observed increased STAT3 S727 phosphorylation in ENZR cells, which has been previously reported to facilitate AR binding. Strikingly, ENZR cells were more sensitive to inhibition with STAT3 DNA-binding inhibitor galiellalactone (GPA500) compared to CRPC cells. Treatment with GPA500 suppressed AR activity and significantly reduced expression of Cyclin D1, thus reducing cell cycle progression into S phase and hindering cell proliferation. In vivo, GPA500 reduced tumor volume and serum PSA in ENZR xenografts. Lastly, the combination of ENZ and GPA500 was additive in the inhibition of AR activity and proliferation in LNCaP and CRPC cells, providing rationale for combination therapy. Overall, these results suggest that STAT3 inhibition is a rational therapeutic approach for ENZR prostate cancer, and could be valuable in CRPC in combination with ENZ.

Highlights

Kinases such as Src family kinases[16]
Studying STAT3 activity in these LNCaP-derived ENZ resistant cells, we discovered that inhibition of STAT3 by the DNA binding inhibitor galiellalactone (GPA500)[27,28] disrupted the interaction between STAT3 and androgen receptor (AR), reduced AR activity and reduced cell proliferation in PSA producing ENZ-resistant cells (PSAhi ENZR)
Comparison of STAT3 localization in 16DCRPC and PSAhi ENZR cells (49 C and 49 F) showed that is STAT3 more localized to the nucleus in ENZR cells compared to CRPC, and there is a clear co-localization between STAT3 and AR in these cells (Fig. 1A left, Supplementary Fig. S1)

Summary

Introduction

STAT3 activity is significant in PCa progression whether it’s treatment naïve, castrate-resistant or metastatic[17,18,19,20]. Studying STAT3 activity in these LNCaP-derived ENZ resistant cells, we discovered that inhibition of STAT3 by the DNA binding inhibitor galiellalactone (GPA500)[27,28] disrupted the interaction between STAT3 and AR, reduced AR activity and reduced cell proliferation in PSA producing ENZ-resistant cells (PSAhi ENZR). Combination of ENZ and GPA500 in LNCaP and CRPC cells in vitro had an additive effect on AR inhibition and cell proliferation. These effects translated well in vivo; GPA500 treatment reduced tumor volume of ENZR xenografts and reduced serum PSA levels. Our study provides proof-of-principle that STAT3 inhibition using galiellalactone is a viable treatment option as monotherapy in ENZR prostate cancer as well as a logical strategy for combination therapy with ENZ in CRPC

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