Acción biológica de la inmunofilina de alto peso molecular FKBP51 en procesos malignos

Abstract

Las inmunofilinas son proteínas con actividad enzimática de peptidilprolyl-isomerasa que unen drogas inmunosupresoras. La inmunofilina FKBP51 forma complejos con la chaperona HSP90 y une al macrólido FK506. En este trabajo, demostramos que FKBP51 es una fosfoproteína cuyo grado de fosforilación condiciona su localización subcelular, siendo ello regulado por balance hormonal. Una importante fracción subcelular de FKBP51 es mitocondrial, migra al núcleo por estrés, cumple funciones antiapoptóticas, y es sobreexpresada por células tumorales. La subunidad hTERT de telomerasa también es una proteína-cliente de HSP90 fuertemente inducida en células cancerosas. Demostramos que FKBP51 co-inmunoprecipita con complejos hTERT?HSP90. Tanto el inhibidor de HSP90 radicicol como la sobreexpresión del dominio TPR de FKBP51 (el interactuante con HSP90) favorecen la relocalización citoplásmica de hTERT. La actividad enzimática de telomerasa es significativamente mejorada por FKBP51. Tratamientos de células tumorales con FK506 muestran una disminución de la vibilidad celular, y tratamientos de ratones Scid/Nod portando tumores xenográficos con FK506 mostraron una significativa reducción del crecimiento tumoral. Como FK506 muestra afinidad por otras inmunofilinas, ensayamos a una nueva droga sintética que es selectiva para FKBP51, SAFit1, la que mostró resultados similares. Estos estudios demuestran que FKBP51 es un nuevo factor antiapoptótico relacionado con el desarrollo tumoral.

Immunophilins are a family of proteins with peptidylprolyl-isomerase enzymatic activity. They also bind immunosuppressive drugs, a phenomenon that inhibits their isomerase activity. FKBP51 is an HSP90-binding cochaperone able to bind the immunosuppressant macrolide FK506. Nonetheless, FKBP51 is not related to immunosuppression, a mechanism that depends on the immunophilin FKBP12. FKP51 forms complexes with the molecular chaperone Hsp90 via TPR (tetratricopetide repeats) domains, and via TPR domains FKBP51 relates to HSP90-binding proteins such as sterid receptors, tyrosine-kinases and transcription factors. In this study, it is demonstrated for the first time that FKBP51 is a phosphoprotein whose phosphorylation status affects its subcellular localization. This is regulated by hormonal balance. An important fraction of FKBP51 is mitochondrial, which is unexpected since FKBP51 lacks of a known mitochondrial localization signal. Upon the onset of several situations of stress, FKBP51 translocates to the nucleus in an HSF1-dependent mechanism. Interestingly, FKBP51 shows antiapoptotic properties and is overexpressed in cancer cells, where it is constitutively located in the nuclei. The hTERT subunit of telomerase is also an HSP90 client‐protein highly expressed in cancer cells that favors their clonal expansion. In this work we demonstrate that FKBP51 co‐immunoprecipitates with hTERT•HSP90 complexes in an HSP90-dependent fashion. Thus, the HSP90 inhibitor radicicol or the overexpression of the immunophilin TPR domain (required for such association) relocalizes hTERT to the cytoplasm. Importantly, telomerase enzymatic activity is significantly enhanced by FKBP51, a process where the peptidyl-prolyl isomerase activity of the immunophilin is essential. In vitro treatment of tumor cells with FK506 decreases cell viability, whereas in vivo treatment of Scid/Nod mice carrying xenographic tumors, significantly decreases tumor growth. Inasmuch as FK506 could also be bound by other immunophilins, SAFit1, a new selective compound specifically designed for FKBP51 was assayed. Since the experimental results where similar to those shown by FK506, we conclude that they are mediated by FKBP51 and not by other immunophilin. The exposure of cells to oxidative stress leads to the activation of HSF1, which in turn favors the induction of hTERT expression as it is demonstrated by the lack of such induction of HSF1 KO cells. These studies demonstrate that FKBP51 is a novel antiapoptotic factor that migrates to the nucleus and enhances telomerase activity, which in turn favors tumor development.