Cancer associated fibroblasts induce proliferation and therapeutic resistance to everolimus in neuroendocrine tumors through STAT3 activation

Introduction
Cancer-associated fibroblasts (CAF) have been identified as relevant contributors to cancer progression and drug resistance in many tumors. Although neuroendo-crine tumors (NET) are often associated with a strong stromal reaction, no study has addressed whether CAF are involved in progression and therapeutic resistance in NET. The aim of this study was to characterize the role of CAF in NET. 
Methods
We established primary CAF cultures derived from NET liver metastases to study the effect on NET cell lines NT-3 and BON. Immunohistochemistry was performed on tissue sections of primary and metastatic NET tissue. 
Results
Immunohistochemistry identified CAF dispersed in between tumor cells and within fibrotic bands separating tumor cell clusters in NET. Stimulating NET cells with CAF decreased expression of SSTR2 and chromogranin A and induced expression of CXCR4. CAF induced a 2.3-fold increase in proliferation and completely reversed the response to everolimus in NT-3 cells. We identified STAT3 as the main signal-ing pathway induced by CAF. STAT3 targeting by small interfering RNA (siRNA) knockdown and inhibitors prevented CAF induced proliferation and restored evero-limus responsiveness. STAT3 activation in NET tissue was associated with de-creased chromogranin A expression, increased Ki-67 index and decreased 5-year overall and progression free survival. CAF directly influence proliferation and thera-peutic response in NET cells. 
Conclusion
Identifying STAT3 as the contributing pathway of this so far neglected tumor-stroma interaction has the potential to become a new therapeutic target to halt tumor growth and to restore therapeutic responsiveness in NET. 
 
