FK506 binding proteins and inflammation related signalling pathways; basic biology, current status and future prospects for pharmacological intervention

Abstract

FK506 binding (FKBP) proteins are part of the highly conserved immunophilin family and its members have fundamental roles in the regulation of signalling pathways involved in inflammation, adaptive immune responses, cancer and developmental biology. The original member of this family, FKBP12, is a well-known binding partner for the immunosuppressive drugs tacrolimus (FK506) and sirolimus (rapamycin). FKBP12 and its analog, FKBP12.6, function as cis/trans peptidyl prolyl isomerases (PPIase) and they catalyse the interconversion of cis/trans prolyl conformations. Members of this family uniquely contain a PPIase domain, which may not be functional. The larger FKBPs, such as FKBP51, FKBP52 and FKBPL, contain extra regions, including tetratricopeptide repeat (TPR) domains, which are important for their versatile protein-protein interactions with inflammation-related signalling pathways. In this review we focus on the pivotal role of FKBP proteins in regulating glucocorticoid signalling, canonical and non-canonical NF-κB signalling, mTOR/AKT signalling and TGF-β signalling. We examine the mechanism of action of FKBP based immunosuppressive drugs on these cell signalling pathways and how off target interactions lead to the development of side effects often seen in the clinic. Finally, we discuss the latest advances in the role of FKBPs as therapeutic targets and the development of novel agents for a range of indications of unmet clinical need, including glucocorticoid resistance, obesity, stress-induced inflammation and novel cancer immunotherapy.

Introduction

Immunophilins are a highly conserved intracellular protein superfamily best known for their role in binding to the immunosuppressive drugs, FK506 (tacrolimus), rapamycin (sirolimus) and cyclosporine A (CsA) (Ghartey-Kwansah et al., 2018). In mammals, immunophilins consist of three main groups; cyclophilins, FK506-binding proteins (FKBPs) and parvulins (Ghartey-Kwansah et al., 2018; McClements, Annett, Yakkundi, & Robson, 2015). FKBPs are the principle intracellular target for FK506 and rapamycin, whilst CsA binds to cyclophilins (MacMillan, 2013). In addition, a naturally occurring chimera of FKBPs and cyclophilins, FCBP/CFBP, which can recognise both FK506 and cyclosporine A, was recently discovered in monocellular organisms (Sailen Barik, 2018).

The majority of peptide bonds are in a trans conformation, whereas 6% of all Xaa-Pro peptide bonds have a cis conformation (Sailen Barik, 2018; MacMillan, 2013; Wedemeyer, Welker, & Scheraga, 2002). Immunophilins contain a cis-trans peptidyl-prolyl isomerase (PPIase) domain which catalyses the interconversion of a specific pro-imide bond between cis and trans conformations, resulting in a rate limiting change in protein conformation (Wedemeyer et al., 2002). Furthermore, the PPIase domain is the region responsible for the binding of immunophilin-based drugs and the drug – protein interaction abolishes the PPIase enzymatic activity (Zgajnar et al., 2019). The terms “immunophilins” and “PPIase” are often used interchangeably, however, despite the presence of PPI domains not all members, such as FKBP38 and FKBPL, exhibit PPIase enzymatic activity or bind to immunosuppressant drugs (Tong & Jiang, 2015; Zgajnar et al., 2019). Aberrant PPIase function is associated with numerous age-related diseases including atherosclerosis and cardiovascular disease, type II diabetes, chronic kidney disease and age-related macular degeneration (AMD) (McClements et al., 2015; Ranjan Nath, 2017). The larger immunophilins may contain multiple PPIase domains and/or other functional domains and in this review we will discuss the role of these domains in modulating inflammatory signalling pathways, independent of the binding of immunophilin-based drugs. The most common is a tetratricopeptide (TPR) domain, a sequence motif associated with protein – protein interactions with heat shock protein 70 (Hsp70) and heat shock protein 90 (Hsp90) (Harrell et al., 2002; Tong & Jiang, 2015). Other domains which may be present include calcium binding/calmodulin regions, EF-hand motifs, endoplasmic reticulum (ER) targeting and retention motifs, DNA binding motifs and transmembrane domains (Bonner & Boulianne, 2017; Tong & Jiang, 2015). Immunophilins can be broadly classified based on their cellular distribution into cytoplasmic (e.g. CypA, FKBP12), nuclear (e.g. Cyp33, FKBP25), endoplasmic reticulum (ER) (e.g. Cyp33, FKBP65), multi-domain (e.g. Cyp40, FKBP51), and mitochondrial (e.g. CypD, FKBP38) (Harikishore & Sup Yoon, 2015). In this article, we will review the role of the FKBP protein family in modulating inflammation related signalling pathways and how they can be potentially exploited for the development of novel immunomodulatory agents.