FKBPL Regulates Estrogen Receptor Signalling and Determines Response to Endocrine Therapy.
Authors: McKeen H. , Byrne C. , Jithesh P. , Donley C. , Yakkundi A. , McCarthy H. , Swanton C. , Hirst D. , Robson T. 
Published: December 2010

Approximately 40% of patients with ER+ breast cancers do not respond to endocrine therapies; furthermore, most tumours eventually become resistant. We have identified a novel Hsp90 co-chaperone and immunophilin, FKBPL, that binds to Hsp90 and affects the stability and signalling of the estrogen receptor (ER) with implications for breast cancer growth and sensitivity to endocrine therapies.Using co-immunoprecipitations, we have demonstrated that FKBPL interacts within Hsp90 complexes associated with the estrogen receptor in MCF-7 cells. Cells stably overexpressing FKBPL become dependent on estrogen for their growth and even in its presence, FKBPL over-expression slows the rate of proliferation of these cells. More importantly, this dependence on estrogen, rendered FKBPL over-expressing cells dramatically more sensitive (up to 90%) to the anti-estrogens, tamoxifen and fulvestrant. Furthermore, knock-down of FKBPL using a targeted siRNA approach, dramatically increased the resistance of these cells to tamoxifen, supporting a role for FKBPL as a determinant of response to endocrine therapies.We have also identified putative estrogen responsive elements within FKBPL's promoter and show that FKBPL is upregulated in response to estrogen suggesting that FKBPL itself is an estrogen responsive gene. As FKBPL levels increased in response to estrogen, ER levels fell; implicating FKBPL in the stabilisation of ER. This is supported by data demonstrating that FKBPL over-expressing cells exhibit decreased levels of ER and cathepsin D, an estrogen responsive gene, critical to breast cancer growth, survival and invasion. Furthermore, knockdown of FKBPL using an siRNA approach increased ER and cathepsin D levels. The regulation of this ER-responsive gene supports a functional role for FKBPL in physiological ER-mediated signalling. FKBPL is also important for the stabilisation of the cyclin dependent kinase inhibitor, p21 (Jascur et al., 2005). We have seen a dramatic fall in p21 levels when FKBPL is knocked down with a targeted siRNA. Loss of p21 has been associated with a tamoxifen growth inducing phenotype and hyperphosphorylation of ER at S118, and with subsequent increased expression of ER-regulated genes. Here we show that ER phosphorylation is increased in FKBPL knockdown cells and decreased in FKBPL over-expressing cells. Together, these data support a model in which high levels of FKBPL would stabilise p21, reducing ER phosphorylation, abrogating tamoxifen-induced agonist potency and so increase sensitivity to the drug.Finally, having established that FKBPL expression may result in p21-mediated growth arrest and sensitisation to endocrine therapies, we hypothesised that FKBPL may have prognostic value that might impact upon tumour proliferative capacity and improve outcome independent of ER status. This was verified in two publically available breast cancer patient cohorts where we demonstrate that high FKBPL expression was correlated with increased overall survival and distant metastasis-free survival.

Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 5126.