FKBPs and the Akt/mTOR pathway.

FK506-binding proteins (FKBP) belong to the immunophilin family and are best known for their ability to enable the immunosuppressive properties of FK506 and rapamycin. For rapamycin, this is achieved by inducing inhibitory ternary complexes with the kinase mTOR. The essential accessory protein for this gain-of-function was thought to be FKBP12. We recently showed that this view might be too restricted, since larger FK506-binding proteins can functionally substitute for FKBP12 in mammalian cells. Recent studies have also shown that FK506-binding proteins can modulate Akt-mTOR signaling in the absence of rapamycin. Here we discuss the role of FK506-binding proteins for the mechanism of rapamycin as well as their intrinsic actions on the Akt/mTOR pathway.

Increasing the efficiency of ligands for FK506-binding protein 51 by conformational control.

The design of efficient ligands remains a key challenge in drug discovery. In the quest for lead-like ligands for the FK506-binding protein 51 (FKBP51), we designed two new classes of bicyclic sulfonamides to probe the contribution of conformational energy in these ligands. The [4.3.1] scaffold had consistently higher affinity compared to the [3.3.1] or monocyclic scaffolds, which could be attributed to better preorganization of two key recognition motifs. Surprisingly, the binding of the rigid [4.3.1] scaffold was enthalpy-driven and entropically disfavored compared to the flexible analogues. Cocrystal structures at atomic resolution revealed that the sulfonamide nitrogen in the bicyclic scaffolds can accept an unusual hydrogen bond from Tyr(113) that mimics the putative FKBP transition state. This resulted in the first lead-like, functionally active ligand for FKBP51. Our work exemplifies how atom-efficient ligands can be achieved by careful conformational control even in very open and thus difficult binding sites such as FKBP51.

InterAKTions with FKBPs--mutational and pharmacological exploration.

The FK506-binding protein 51 (FKBP51) is an Hsp90-associated co-chaperone which regulates steroid receptors and kinases. In pancreatic cancer cell lines, FKBP51 was shown to recruit the phosphatase PHLPP to facilitate dephosphorylation of the kinase Akt, which was associated with reduced chemoresistance. Here we show that in addition to FKBP51 several other members of the FKBP family bind directly to Akt. FKBP51 can also form complexes with other AGC kinases and mapping studies revealed that FKBP51 interacts with Akt via multiple domains independent of their activation or phosphorylation status. The FKBP51-Akt1 interaction was not affected by FK506 analogs or Akt active site inhibitors, but was abolished by the allosteric Akt inhibitor VIII. None of the FKBP51 inhibitors affected AktS473 phosphorylation or downstream targets of Akt. In summary, we show that FKBP51 binds to Akt directly as well as via Hsp90. The FKBP51-Akt interaction is sensitive to the conformation of Akt1, but does not depend on the FK506-binding pocket of FKBP51. Therefore, FKBP inhibitors are unlikely to inhibit the Akt-FKBP-PHLPP network.

Large FK506-binding proteins shape the pharmacology of rapamycin.

The immunosuppressant and anticancer drug rapamycin works by inducing inhibitory protein complexes with the kinase mTOR, an important regulator of growth and proliferation. The obligatory accessory partner of rapamycin is believed to be FK506-binding protein 12 (FKBP12). Here we show that rapamycin complexes of larger FKBP family members can tightly bind to mTOR and potently inhibit its kinase activity. Cocrystal structures with FKBP51 and FKBP52 reveal the modified molecular binding mode of these alternative ternary complexes in detail. In cellular model systems, FKBP12 can be functionally replaced by larger FKBPs. When the rapamycin dosage is limiting, mTOR inhibition of S6K phosphorylation can be enhanced by FKBP51 overexpression in mammalian cells, whereas FKBP12 is dispensable. FKBP51 could also enable the rapamycin-induced hyperphosphorylation of Akt, which depended on higher FKBP levels than rapamycin-induced inhibition of S6K phosphorylation. These insights provide a mechanistic rationale for preferential mTOR inhibition in specific cell or tissue types by engaging specific FKBP homologs.

Overexpression of tropinone reductases alters alkaloid composition in Atropa belladonna root cultures.

The medicinally applied tropane alkaloids hyoscyamine and scopolamine are produced in Atropa belladonna L. and in a small number of other Solanaceae. Calystegines are nortropane alkaloids that derive from a branching point in the tropane alkaloid biosynthetic pathway. In A. belladonna root cultures, calystegine molar concentration is 2-fold higher than that of hyoscyamine and scopolamine. In this study, two tropinone reductases forming a branching point in the tropane alkaloid biosynthesis were overexpressed in A. belladonna. Root culture lines with strong overexpression of the transcripts contained more enzyme activity of the respective reductase and enhanced enzyme products, tropine or pseudotropine. High pseudotropine led to an increased accumulation of calystegines in the roots. Strong expression of the tropine-forming reductase was accompanied by 3-fold more hyoscyamine and 5-fold more scopolamine compared with control roots, and calystegine levels were decreased by 30-90% of control. In some of the transformed root cultures, an increase of total tropane alkaloids was observed. Thus, transformation with cDNA of tropinone reductases successfully altered the ratio of tropine-derived alkaloids versus pseudotropine-derived alkaloids.