Effects of HSP90 inhibitor PU-H71 in combination with radiotherapy on metastatic breast carcinoma

Objective: Heat shock protein 90 (HSP90) which is induced by cell stress, stabilizes many cellular proteins and protects them from degradation. HSP90 has distinct expression profiles in normal and malignant cells. Increased expression of HSP90 in carcinomas is closely associated with a poor prognosis and resistance to therapy. HSP90 inhibitors are promising drugs for cancer treatment which are considered as new radiosensitizing agents. The effects of PU-H71, on metastatic breast carcinoma alone and in combination with radiotherapy (RT) are not known. Method: Effect of PU-H71 in mouse metastatic breast carcinoma cells (4T1) and its brain (4TBM), liver (4TLM), heart (4THM) metastatic subsets as well as non-metastatic 67NR cells were determined using WST-1 cell proliferation assay. Changes in radiation sensitivity were determined by clonogenic assays. Changes in client proteins as well as levels of angiogenic and inflammatory mediators following treatments with HSP90 inhibitor and RT were determined in cell lines, ex-vivo cultures and mixed leukocyte cultures. Effect of PU-H71 and RT were investigated on tumor growth and lung metastasis of 4TBM cells injected orthotopic in balb-c mice. Results: HSP90 inhibitors, 17-AAG and PU-H71 potently inhibited growth of metastatic and non-metastatic breast carcinoma cells and increased therapeutic effects of RT. IL-6 levels increased under ex vivo conditions following PU-H71 and radiation co-treatment. On the other hand co-treatment increased the levels of SDF-1, MIP-2 and VEGF only under in vitro conditions. PU-H71 alone inhibited ERK1/2, p38, Akt activation and reduced HSP90's client proteins, N-cadherin and HER2. PU-H71 and RT in combination reduced tumor growth and lung metastasis. PU-H71 and RT increased levels of IL-17, TNF-α, IL-10 and IFN-γ in mixed leukocyte cultures. Conclusion: These results demonstrate for the first time that PU-H71 and RT in combination may reduce tumor growth and lung metastasis especially in orthotopic highly metastatic breast carcinoma model. Key words: PU-H71, radiation therapy, HSP90, breast cancer, metastasis