Nuclear Receptors

Many proteins shuttle dynamically between the cytoplasm and the nucleus. Ideally, the biological function of several nuclear factors could be regulated if we can identify the mechanism by which they reach their sites of action. For example, NFκB is persistently nuclear in many cancer cells and this feature has been related to tumour development. On the other hand, p53 activation promotes cell-cycle arrest and apoptotic cell death, and p53 mislocalization in the cytoplasm is responsible for tumour development. Unlike NFκB, localizing p53 to the nucleus would be desirable for the control of cell survival. Consequently, it would be extremely useful from the therapeutic point of view if we could affect the subcellular localization of these proteins by using specific drugs. We analyse the role of high molecular weight immunophilins in the mechanism of transport and anchorage to cell structures of several nuclear factors. Immunophilins are a family of proteins that possesses peptidylprolyl isomerase enzymatic activity. Only the smallest members of this family (FKBP12 and CyP17) acquire immunosuppressive action upon drug binding. It seems that this is not the case for the other members of the family (FKBP52, FKBP51, FKBP38, CyP40, PP5, etc.).
These immunophilins are abundantly expressed and show properties of molecular chaperones. Nonetheless, most of their biological actions are still unknown. We have recently demonstrated that the FKBP52•hsp90 complex binds the dynein/dynactin motor complex, therefore being necessary for the hormone-dependent retrotransport of steroid receptors. Currently, this model is being extended to other members of the nuclear receptor family, such as NFκB, HSF-1 and p53. Our findings suggest that the mechanism of transport first described for steroid receptors is also used by other nuclear factors that are subject of subcellular redistribution upon cell stimulation with specific stimuli. Immunophilins are also present in the nucleus and affect the transcriptional mechanism of the cell. FKBP51 has generally been regarded to be a negative regulator of steroid receptors, whereas FKBP52 is a positive or neutral regulator depending on the receptor and the cell type. Our research is currently focused on the elucidation of the molecular mechanism of these effects and is exploring whether immunophilins participate in the rearrangement of the chromatin architecture where the target-genes are located. Inasmuch as FKBP52 favours the nuclear retention of the glucocorticoid receptor (GR) to nuclear speckles and FKBP51 favours GR nuclear exclusion, we postulate that the balance of expression between these two immunophilins could be one of the reasons for the pleiotropic effects of GR in different tissues. Because we have observed that FKBP51 concentrates in mitochondria and shuttles between this organelle and the nucleus, a possible involvement of the immunophilins with apoptosis is being studied. FKP51 shows antiapoptotic action and all its properties are preserved by several tumour cell lines and cancer tissues (as suggested by analysis of biopsies). Our studies are also extended in an integrated fashion to other factors related to the cycles of cell death and replication such as the telomerase reverse transcriptase (TERT). Our group also studies the capability of molecular chaperones to regulate neurite outgrowth and neuron differentiation of embryonic cells, as well as the trans-differentiation process into neurons. Current studies analyse the phenotypic changes of the cells, molecular markers, modifications of the nuclear and cytoskeletal architecture, and in vivo studies in animal models, where immunophilins are being tested as potential pharmacologic targets for neuronal regeneration and neuroprotection. The laboratory also performs studies on steroid structure-biological activity relationship. Most of the studies attempt to correlate steroid conformation with the binding capacity to steroid receptors and biological activity, in particular (but not exclusively) the mineralocorticoid and glucocorticoid effects. The studies include a broad range of aspects of the effect, i.e., from classical in vivo bioassays to the mechanistic details of the hormone-receptor complex.