Fkbp51, a Novel T-Cell-Specific Immunophilin Capable of Calcineurin Inhibition

Abstract

The immunosuppressive drugs FK506 and cyclosporin A block T-lymphocyte proliferation by inhibiting calcineurin, a critical signaling molecule for activation. Multiple intracellular receptors (immunophilins) for these drugs that specifically bind either FK506 and rapamycin (FK506-binding proteins [FKBPs]) or cyclosporin A (cyclophilins) have been identified. We report the cloning and characterization of a new 51-kDa member of the FKBP family from murine T cells. The novel immunophilin, FKBP51, is distinct from the previously isolated and sequenced 52-kDa murine FKBP, demonstrating 53% identity overall. Importantly, Western blot (immunoblot) analysis showed that unlike all other FKBPs characterized to date, FKBP51 expression was largely restricted to T cells. Drug binding to recombinant FKBP51 was demonstrated by inhibition of peptidyl prolyl isomerase activity. As judged from peptidyl prolyl isomerase activity, FKBP51 had a slightly higher affinity for rapamycin than for FK520, an FK506 analog. FKBP51, when complexed with FK520, was capable of inhibiting calcineurin phosphatase activity in an in vitro assay system. Inhibition of calcineurin phosphatase activity has been implicated both in the mechanism of immunosuppression and in the observed toxic side effects of FK506 in nonlymphoid cells. Identification of a new FKBP that can mediate calcineurin inhibition and is restricted in its expression to T cells suggests that new immunosuppressive drugs may be identified that, by virtue of their specific interaction with FKBP51, would be targeted in their site of action.Cyclosporin A (CsA), FK506, and rapamycin are potent immunosuppressive drugs that inhibit T-lymphocyte proliferation. The action of these drugs is mediated by intracellular receptors, termed immunophilins, that bind either CsA (cyclophilins) or FK506 and rapamycin (FK506-binding proteins [FKBPs]) (for reviews, see references 43 and 47). Previous studies identified these receptors as abundant, cytosolic proteins possessing an inherent peptidyl prolyl cis-trans isomerase (PPIase; rotamase) activity that is inhibited by drug ligand binding (12,16,46).The first FKBP to be described in detail at the protein (16, 46) and cDNA (25, 53) levels was FKBP12, a ubiquitous immunophilin of 11.8 kDa highly conserved in eukaryotes. More recently, additional members of this family have been identified on the basis of their ability to bind FK506 and rapamycin. Sequence analysis of the cloned genes corresponding to FKBP12.6, FKBP13, FKBP25, and FKBP52 (named to reflect their molecular weights) from mammalian sources reveals extensive conservation of amino acid sequence, in particular in the protein domain responsible for drug ligand binding and PPIase activity (for reviews, see references 15 and 58).Although the actions of FK506 and rapamycin are mediated by the same family of intracellular receptors, these drugs achieve immunosuppression by different mechanisms (5). Rapamycin inhibits a calcium-independent event involved in the proliferative response of T cells to growth ...

Role of FK506-binding protein 51 in the control of apoptosis of irradiated melanoma cells

“…The relevant data from this analysis are that FKBP51 (i) was expressed in all CMMs analyzed but not in normal skin; (ii) expression was higher in melanocytes during the vertical growth phase compared with the radial growth phase; (iii) expression correlated with the thickness of the tumor lesion; (iv) expression was maximal in metastatic melanoma. In conclusion, we show that FKBP51, a protein with important biological roles in normal cells, 28,29,33 acquires a pro-oncogenic potential when its expression is deregulated, which is what occurs in melanoma. Such deregulation might sustain survival networks that protect the melanoma against killing.…”

Overexpression of FKBP51 in idiopathic myelofibrosis regulates the growth factor independence of megakaryocyte progenitors

“…21,39 Its overexpression in a cytokinedependent MK cell line and normal CD34 ϩ cells leads to an increased cell survival after cytokine deprivation. FKBP51 is known as an immunophillin that can regulate FK506-induced calcineurin inhibition, a calcium-calmodulin-regulated serine/ threonine phosphatase 21,39,40 and binds to the HSP70/HSP90 complex. [41][42][43][44][45] In our report, we show that FKBP51 overexpression per se inhibits calcineurin activity.…”

“…FKBP51 can inhibit calcineurin when complexed with FK506. 21 In addition, FKBP51 binds to HSP90/HSP70 and associates to steroid receptors. 22 The precise function of FKBP51 in hematopoietic cells is presently unknown; however, as a consequence of its protein interactions, FKBP51 may play an important role in cell survival, proliferation, and differentiation.…”

mTOR, A New Therapeutic Target in Acute Myeloid Leukemia

“…Rapamycin specifically binds to FKBP51 and inhibits its activity in a PI3K/Akt independent manner, leading to the enhanced pro-apoptotic effect of doxorubicin in melanoma and acute lymphoblastic leukemia cells. [125][126][127][128] This novel mechanism may explain why rapamycin is also efficient in PTEN +/+ , NFκB dependent tumors. In this context, the rapamycin-anthracyclin combination could be toxic for the leukemic stem cell in which NF-κB has been found to be constitutively activated.…”