FAK signaling pathways are modulated by HSPB8 and BAG3 in breast cancer

Breast cancer (BC) is a widespread and heterogeneous disease in which autophagy plays an essential role in tumor development and progression. It has been suggested that autophagy activation may prevent tumor development in the initial stages of the disease, while in more advanced stages, autophagy might activate survival mechanisms for cancer cells. Moreover, autophagy may be involved in developing therapies resistance and in forming metastases. Some Heat Shock Proteins (HSPs) play an important role in autophagy. The small HSPB8 draws attention because it is generally highly expressed in Estrogen Receptor positive (ER+) BC and its over-expression increases autophagic flux, proliferation, migration and survival of BC cells under stress conditions. HSPB8 mediates the autophagic degradation of client proteins via the chaperone- assisted selective autophagy (CASA) complex in which it binds the BAG cochaperone 3 (BAG3), the HSP70 and the E3-ubiquitin ligase STUB1. Similarly to HSPB8, BAG3 is also highly expressed in BC cells, and its unregulated expression is linked to a poor prognosis. In our previous studies, we showed that HSPB8 and BAG3 silencing reduces proliferation and migratory capacities of hormone-sensitive MCF-7 BC cells. Here, we analyzed the signal transduction mechanisms involved in HSPB8- and BAG3-mediated regulation of metastatic potential. We evaluated the function of protein tyrosine kinase 2 (PTK2 also known as FAK), known to activate several downstream signals controlling PI3K, AKT and MAP kinases. We demonstrated that HSPB8 and BAG3 downregulation correlated with a significant reduction in the phosphorylated and active form of FAK which, in a cascade mechanism, induces a decrease in the proliferative, migratory, and adhesive capacity of MCF-7 and T47D ER+ human BC cell lines. Furthermore, co-immunoprecipitation studies demonstrated a co-localization between BAG3 and FAK, also confirmed by Immunofluorescence staining in BC specimen sections. Based on our results, we suggest that BAG3 associates with FAK and, together with HSPB8, modulates the signal transduction mechanisms controlling the survival of the hormone-sensitive human ER+, PR positive (PR+), HER2 negative (HER2−) BC cell lines MCF-7 and T47D.