Hsp90-binding immunophilin FKBP51 binds to the hTERT?Hsp90 heterocomplex enhancing telomerase activity

ZGAJNAR NR, LAGADARI M, GALIGNIANA MD

Seeon

Congreso; THE 8th INTERNATIONAL CONFERENCE ON THE HSP90 CHAPERONE MACHINERY; 2016

The reverse transcriptase subunit of telomerase, hTERT, is an Hsp90-interacting protein highly expressed in cancer cells. It compensates the loss of telomeric DNA after each cell division, and also shows antiapoptotic actions. Previously, we have demonstrated that the Hsp90-binding immunophilin FKBP51 is a novel mitochondrial factor with antiapoptotic properties, whose translocation into the organelle involves a TPR domain-dependent mechanism. Inasmuch as FKBP51 and hTERT are both Hsp90-interacting proteins, we explored the potential association of FKBP51 with hTERT?Hsp90 complexes. Co-immunoprecipitation assays performed in HeLa and HEK293 cell extracts demonstrate that endogenous FKBP51 forms complexes with endogenous hTERT?Hsp90 heterocomplexes in an Hsp90-dependent manner. Confocal microscopy images and subcellular fractionation assays demonstrate that high ROS generated by either cell exposure to oxidants or by depletion of endogenous glutathione, favors the translocation of FKBP51 from mitochondria to nuclei in a HSF1-dependent manner. Interestingly, hTERT expression is always induced in the short term in a HSF1-dependent manner. Importantly, the increased hTERT?FKBP51 association greatly enhances telomerase enzymatic activity. Radicicol not only disrupts the hTERT?Hsp90?FKBP51 heterocomplex, but also favors the cytoplasmic relocalization of hTERT. This effect is also observed by overexpression of the TPR-domain peptide, suggesting that Hsp90 is required for hTERT nuclear anchorage. While a fraction of exported hTERT becomes mitochondrial, the cytoplasmic pool of Hsp90-free hTERT is degraded over time via proteasome. These findings support the emerging role of the Hsp90-binding immunophilin FKBP51 as a novel factor able to favor the clonal expansion of cancer cells by enhancing the Hsp90-dependent telomerase activity.