The antiapoptotic and oncogenic effects of the Hsp90-binding immunophilin FKBP51 are affected by HSF1 activation

MARIO D. GALIGNIANA; ZGAJNAR, M.; LAGADARI M; GALLO LI

Turku

Conferencia; VIII Internation Congress on Stress Proteins in Biology & Medicine; 2017

Cell Stress Society International

FKBP51 is an Hsp90-binding immunophilin first discovered associated to steroid receptors. Recently, we have provided evidence by confocal imaging that this immunophilin colocalizes with Tom20, cytochrome c, COX IV and MitoTracker stain, suggesting that a substantial fraction of FKBP51 is mitochondrial (mt-51). This was confirmed by electron microscopy and biochemical fractionation. Interestingly, we also found that mt-51 exists alone and complexed with mt-GR, mt-Hsp90 and mt-Hsp70. In the present study we show that upon the onset of various stresses (ROS, UV light, heat-shock, TNF, LPS, glucose deprivation, high ionic strength, toxic drugs, etc.), mt-51 rapidly moves to the nucleus along with mt-Hsp70, and is also concentrated in nucleoli, being excluded from their fibrillar centers. Such mitochondrial-nuclear trafficking is reversible. In view of the heterogeneous nature of those stimuli, we focused our interest on the activation of HSF. While HSF1 knock-out MEF cells do not show such mitochondrial-nuclear/nucleolar translocation of mt-51, the overexpression of HSF1 concentrates the IMM in the nucleus and nucleoli even in the absence of any stimulus. We demonstrate that this unexpected mitochondrial localization of FKBP51 depends on its TPR domain, whereas its release from the organelle is dependent on the membrane depolarization and shares identical kinetics as cytochrome c release. While the overexpression of FKBP51 increases cell viability against several injuries, its knock-down favors activation of the cell death programme. Immunoprecipitation assays demonstrate that the antiapoptotic factor Bcl-2 interacts with FKBP51 in the same complex. Consistent with the high production of ROS and HSF1 activation in tumor cell lines, we found that FKBP51 is highly expressed in these cells and mostly located in nuclei/nucleoli, being both observations also valid for several types of human tumor tissues. The reverse transcriptase subunit of telomerase (hTERT) that compensates the loss of telomeric DNA after each cell division is an Hsp90-interacting protein highly expressed in cancer cells. Co-immunoprecipitation assays demonstrate that endogenous FKBP51 forms complexes with endogenous hTERT?Hsp90 heterocomplexes in an Hsp90-dependent manner. As it may be envisaged due to all these novel findings, FKBP51 does enhance hTERT enzymatic activity. Curiously, we noted that hTERT expression is induced after cell exposure to ROS in a HSF1-dependent manner. The overexpression of HSF1 is sufficient to increase hTERT levels. Radicicol disrupts the hTERT?Hsp90?FKBP51 heterocomplex, and 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 moves in opposite direction than mt-51a and becomes mitochondrial, the cytoplasmic pool of Hsp90-free hTERT is targeted to proteosomal degradation. In summary, we report a novel role for the Hsp90-binding immunophilin FKBP51 as a mitochondrial factor able to exert antiapoptotic actions, as well as to enhance the rapid clonal expansion of cancer cells by increasing telomerase activity, both actions being favored by HSF1 activation.