Biological relevance of Hsp90-binding immunophilins in cancer development and treatment

Mitochondrial-nuclear communication by FKBP51 shuttling

The HSP90-binding immunophilin FKBP51 is a soluble protein that shows high homology and structural similarity with FKBP52. Both immunophilins are functionally divergent and often show antagonistic actions. They were first described in steroid receptor complexes, their exchange in the complex being the earliest known event in steroid receptor activation upon ligand binding. In addition to steroid-related events, several pleiotropic actions of FKBP51 have emerged during the last years, ranging from cell differentiation and apoptosis to metabolic and psychiatric disorders. On the other hand, mitochondria play vital cellular roles in maintaining energy homeostasis, responding to stress conditions, and affecting cell cycle regulation, calcium signaling, redox homeostasis, and so forth. This is achieved by proteins that are encoded in both the nuclear genome and mitochondrial genes. This implies active nuclear-mitochondrial communication to maintain cell homeostasis. Such communication involves factors that regulate nuclear and mitochondrial gene expression affecting the synthesis and recruitment of mitochondrial and nonmitochondrial proteins, and/or changes in the functional state of the mitochondria itself, which enable mitochondria to recover from stress. FKBP51 has emerged as a serious candidate to participate in these regulatory roles since it has been unexpectedly found in mitochondria showing antiapoptotic effects. Such localization involves the tetratricopeptide repeats domains of the immunophilin and not its intrinsic enzymatic activity of peptidylprolyl-isomerase. Importantly, FKBP51 abandons the mitochondria and accumulates in the nucleus upon cell differentiation or during the onset of stress. Nuclear FKBP51 enhances the enzymatic activity of telomerase. The mitochondrial-nuclear trafficking is reversible, and certain situations such as viral infections promote the opposite trafficking, that is, FKBP51 abandons the nucleus and accumulates in mitochondria. In this article, we review the latest findings related to the mitochondrial-nuclear communication mediated by FKBP51 and speculate about the possible implications of this phenomenon.

Role of FKBP5 and its genetic mutations in stress-induced psychiatric disorders: an opportunity for drug discovery

Stress-induced mental health disorders are affecting many people around the world. However, effective drug therapy for curing psychiatric diseases does not occur sufficiently. Many neurotransmitters, hormones, and mechanisms are essential in regulating the body's stress response. One of the most critical components of the stress response system is the hypothalamus-pituitary-adrenal (HPA) axis. The FKBP prolyl isomerase 51 (FKBP51) protein is one of the main negative regulators of the HPA axis. FKBP51 negatively regulates the cortisol effects (the end product of the HPA axis) by inhibiting the interaction between glucocorticoid receptors (GRs) and cortisol, causing reduced transcription of downstream cortisol molecules. By regulating cortisol effects, the FKBP51 protein can indirectly regulate the sensitivity of the HPA axis to stressors. Previous studies have indicated the influence of FKBP5 gene mutations and epigenetic changes in different psychiatric diseases and drug responses and recommended the FKBP51 protein as a drug target and a biomarker for psychological disorders. In this review, we attempted to discuss the effects of the FKBP5 gene, its mutations on different psychiatric diseases, and drugs affecting the FKBP5 gene.

The Scaffold Immunophilin FKBP51 Is a Phosphoprotein That Undergoes Dynamic Mitochondrial-Nuclear Shuttling

The immunophilin FKBP51 forms heterocomplexes with molecular chaperones, protein-kinases, protein-phosphatases, autophagy-related factors, and transcription factors. Like most scaffold proteins, FKBP51 can use a simple tethering mechanism to favor the efficiency of interactions with partner molecules, but it can also exert more complex allosteric controls over client factors, the immunophilin itself being a putative regulation target. One of the simplest strategies for regulating pathways and subcellular localization of proteins is phosphorylation. In this study, it is shown that scaffold immunophilin FKBP51 is resolved by resolutive electrophoresis in various phosphorylated isoforms. This was evidenced by their reactivity with specific anti-phosphoamino acid antibodies and their fade-out by treatment with alkaline phosphatase. Interestingly, stress situations such as exposure to oxidants or in vivo fasting favors FKBP51 translocation from mitochondria to the nucleus. While fasting involves phosphothreonine residues, oxidative stress involves tyrosine residues. Molecular modeling predicts the existence of potential targets located at the FK1 domain of the immunophilin. Thus, oxidative stress favors FKBP51 dephosphorylation and protein degradation by the proteasome, whereas FK506 binding protects the persistence of the post-translational modification in tyrosine, leading to FKBP51 stability under oxidative conditions. Therefore, FKBP51 is revealed as a phosphoprotein that undergoes differential phosphorylations according to the stimulus.

Dual-target inhibitors of poly (ADP-ribose) polymerase-1 for cancer therapy: Advances, challenges, and opportunities

PARP1 plays a crucial role in DNA damage repair, making it an essential target for cancer therapy. PARP1 inhibitors are widely used to treat BRCA-deficient malignancies, and six PARP inhibitors have been approved for clinical use. However, excluding the great clinical success of PARP inhibitors, the concomitant toxicity, drug resistance, and limited scope of application restrict their clinical efficacy. To find solutions to these problems, dual-target inhibitors have shown great potential. In recent years, several studies have linked PAPR1 to other primary cancer targets. Many dual-target inhibitors have been developed using structural fusion, linkage, or library construction methods, overcoming the defects of many single-target inhibitors of PARP1 and achieving great success in clinical cancer therapy. This review summarizes the advance of dual-target PARP1 inhibitors in recent years, focusing on their structural optimization process, structure-activity relationships (SARs), and in vitro or in vivo analysis results.

Plasmatic Levels of HSP90α at Diagnosis: A Novel Prognostic Indicator of Clinical Outcome in Advanced Lung Cancer Patients Treated With PD-1/PD-L1 Inhibitors Plus Chemotherapy

Background

The purpose of this study was set to investigate the prognostic role of plasmatic levels of heat shock protein 90 alpha (HSP90α) at diagnosis in advanced lung cancer patients treated with Programmed cell death protein 1 (PD-1)/Programmed cell death-Ligand protein 1 (PD-L1) inhibitors plus chemotherapy.

Methods

A total of 137 advanced lung cancer patients treated with PD-1/PD-L1 inhibitors plus chemotherapy admitted to the Guangxi Medical University Cancer Hospital were enrolled in this study. Smooth curve fitting was conducted to address the nonlinearity of HSP90α and progression-free survival (PFS) and overall survival (OS). We calculated the inflection point using a recursive algorithm. Kaplan–Meier survival analysis and Cox proportional hazards regression model were used to assess the prognostic value of HSP90α for PFS and OS. Subgroup analysis was performed to evaluate the relationship between high HSP90α and disease progression and death risk.

Results

The average age of patients was 58.6 ± 9.8 years, and 73.7% of them were men. We divided patients according to their plasmatic levels of HSP90α into low (HSP90α <52.7 ng/ml) group and high (HSP90α ≥52.7 ng/ml) group. Kaplan–Meier analysis showed a shorter PFS and OS for the high group with log-rank P < 0.05. Univariate and multivariate analyses indicated that high HSP90α was associated with an increased risk of disease progression and death after fully adjusting potential confounders with hazard ratio (HR) 1.8 (95% CI = 1.0–3.2) and HR 2.4 (95% CI = 1.1–5.1), respectively (P < 0.05). After stratification by subgroup analysis, the relationship between high HSP90α and the risk of disease progression and death was consistent across all patient subgroups.

Conclusion

Plasmatic levels of HSP90α at diagnosis can be considered a potential independent prognostic marker of advanced lung cancer patients treated with PD-1/PD-L1 inhibitors plus chemotherapy. A further large-scale prospective validation study is needed to determine whether these results are widely applicable.

[Expression of cyclophilin A in oral squamous cell carcinoma and its effect on cell proliferation and invasion]

OBJECTIVES

To investigate the expression of cyclophilin A (CyPA) in oral squamous cell carcinoma (OSCC) and explore the effect of downregulating the expression of CyPA gene on the proliferation and invasion of SCC-25 cells.

METHODS

A total of 77 cases of patients with OSCC were selected. The expression levels of CyPA proteins in OSCC and adjacent normal tissues were evaluated. SCC-25 cells were cultured and divided into the CyPA interference sequence group, negative control group, and blank group. The expression levels of CyPA mRNA and protein in cells were detected by using real-time fluorescent quantitative polymerase chain reaction and Western blot, respectively. Cell proliferation was detected by using methyl thiazolyl tetrazolium and plate colony formation assays. Cell invasion was detected by using Transwell assay.

RESULTS

The positive expression rate of CyPA protein in OSCC tissues was 76.62%, which was higher than that in adjacent tissues (P<0.05). The positive expression rate of CyPA protein in TNM stage T₃+T₄, clinical stage Ⅲ+Ⅳ, moderately or poorly differentiated lymph node metastasis was increased (P<0.05). Compared with the negative control and blank groups, the CyPA interference sequence group had decreased relative expression levels of CyPA mRNA and protein (P<0.05); optical density va-lues of cells at 24, 48, 72, and 96 h (P<0.05); and number of cell colonies and invasive cells (P<0.05).

CONCLUSIONS

The CyPA protein is highly expressed in OSCC tissues, and the downregulation of CyPA gene expression in SCC-25 cells can reduce cell proliferation and inhibit cell invasion.

Plasma Heat Shock Protein 90 Alpha: A Valuable Predictor of Early Chemotherapy Effectiveness in Advanced Non-Small-Cell Lung Cancer

Background

Heat shock protein-90 alpha (HSP90α) is more abundant in non-small-cell lung cancer (NSCLC) patients than in control individuals. However, whether it can reflect chemotherapy efficacy remains unknown. This study aimed to investigate the association of HSP90α with chemotherapy in advanced NSCLC.

Material/Methods

We retrospectively evaluated data from patients admitted to the Department of Respiratory Medicine, Shaoxing People’s Hospital, from September 2016 to September 2018 with stage IIIB or IV NSCLC and administered 4 cycles of third-generation platinum-based combination chemotherapy (2 drugs simultaneously). Based on the RECIST1.1 criteria, complete remission (CR), partial response (PR), and stable disease (SD) in 60 cases were determined before and after chemotherapy. Before chemotherapy and after 1, 2, and 4 cycles of chemotherapy, plasma HSP90α levels were quantitated by ELISA. Chest CT was performed before and after 2 and 4 cycles of chemotherapy.

Results

After 1–4 cycles of chemotherapy, plasma HSP90α levels were significantly lower than pre-chemotherapy levels (P<0.05). The sums of the longest tumor diameters after 2 and 4 cycles of chemotherapy were decreased compared with pre-chemotherapy values (P<0.05). Plasma HSP90α levels and tumor size showed no significant correlation before and after chemotherapy (r=0.244, P=0.06).

Conclusions

Plasma HSP90α can be considered a valuable predictor of early chemotherapy effectiveness in advanced NSCLC, and is positively correlated with tumor remission after chemotherapy. However, plasma HSP90α level is not correlated with tumor diameter and pathological type.

Hsp90-interacting Co-chaperones and their Family Proteins in Tau Regulation: Introducing a Novel Role for Cdc37L1

Tau is a microtubule-associated protein that serves as a promoter of microtubule assembly and stability in neuron cells. In a collective group of neurodegenerative diseases called tauopathies, tau processing is altered as a result of gene mutations and post-translational modifications. In particular, in Alzheimer's disease (AD) or AD-like conditions, tau becomes hyperphosphorylated and forms toxic aggregates inside the cell. The chaperone heat shock protein 90 (Hsp90) plays an important role in the proper folding, degradation, and recycling of tau proteins and tau kinases. Hsp90 has many co-chaperones that aid in tau processing. In particular, a few of these co-chaperones, such as FK506-binding protein (FKBP) 51, protein phosphatase (PP) 5, cell division cycle 37 (Cdc37), and S100A1 have family members that are reported to affect Hsp90-mediated tau processing in either a similar or an opposite manner. Here, we provide a holistic review of these selected co-chaperones and their family proteins and introduce a novel Hsp90-binding Cdc37 relative, Cdc37-like-1 (Cdc37L1 or L1) in tau regulation. Overall, the proteins discussed here highlight the importance of studying family proteins in order to fully understand the mechanism of tau pathogenesis and to establish drug targets for the treatment of tauopathies.

Regulation of FKBP51 and FKBP52 functions by post-translational modifications

FKBP51 and FKBP52 are two iconic members of the family of peptidyl-prolyl-(cis/trans)-isomerases (EC: 5.2.1.8), which comprises proteins that catalyze the cis/trans isomerization of peptidyl-prolyl peptide bonds in unfolded and partially folded polypeptide chains and native state proteins. Originally, both proteins have been studied as molecular chaperones belonging to the steroid receptor heterocomplex, where they were first discovered. In addition to their expected role in receptor folding and chaperoning, FKBP51 and FKBP52 are also involved in many biological processes, such as signal transduction, transcriptional regulation, protein transport, cancer development, and cell differentiation, just to mention a few examples. Recent studies have revealed that both proteins are subject of post-translational modifications such as phosphorylation, SUMOlyation, and acetylation. In this work, we summarize recent advances in the study of these immunophilins portraying them as scaffolding proteins capable to organize protein heterocomplexes, describing some of their antagonistic properties in the physiology of the cell, and the putative regulation of their properties by those post-translational modifications.

The Nuclear Receptor Field: A Historical Overview and Future Challenges

In this article we summarize the birth of the field of nuclear receptors, the discovery of untransformed and transformed isoforms of ligand-binding macromolecules, the discovery of the three-domain structure of the receptors, and the properties of the Hsp90-based heterocomplex responsible for the overall structure of the oligomeric receptor and many aspects of the biological effects. The discovery and properties of the subfamily of receptors called orphan receptors is also outlined. Novel molecular aspects of the mechanism of action of nuclear receptors and challenges to resolve in the near future are discussed.

The FKBP51 Glucocorticoid Receptor Co-Chaperone: Regulation, Function, and Implications in Health and Disease

Among the chaperones and co-chaperones regulating the glucocorticoid receptor (GR), FK506 binding protein (FKBP) 51 is the most intensely investigated across different disciplines. This review provides an update on the role of the different co-chaperones of Hsp70 and Hsp90 in the regulation of GR function. The development leading to the focus on FKBP51 is outlined. Further, a survey of the vast literature on the mechanism and function of FKBP51 is provided. This includes its structure and biochemical function, its regulation on different levels—transcription, post-transcription, and post-translation—and its function in signaling pathways. The evidence portraying FKBP51 as a scaffolding protein organizing protein complexes rather than a chaperone contributing to the folding of individual proteins is collated. Finally, FKBP51’s involvement in physiology and disease is outlined, and the promising efforts in developing drugs targeting FKBP51 are discussed.

Hsp90-binding immunophilin FKBP51 forms complexes with hTERT enhancing telomerase activity

FK506-binding proteins are members of the immunophilin family of proteins. Those immunophilins associated to the 90-kDa-heat-shock protein, Hsp90, have been proposed as potential modulators of signalling cascade factors chaperoned by Hsp90. FKBP51 and FKBP52 are the best characterized Hsp90-bound immunophilins first described associated to steroid-receptors. The reverse transcriptase subunit of telomerase, hTERT, is also an Hsp90 client-protein and is highly expressed in cancer cells, where it is required to compensate the loss of telomeric DNA after each successive cell division. Because FKBP51 is also a highly expressed protein in cancer tissues, we analyzed its potential association with hTERT·Hsp90 complexes and its possible biological role. In this study it is demonstrated that both immunophilins, FKBP51 and FKBP52, co-immunoprecipitate with hTERT. The Hsp90 inhibitor radicicol disrupts the heterocomplex and favors the partial cytoplasmic relocalization of hTERT in similar manner as the overexpression of the TPR-domain peptide of the immunophilin. While confocal microscopy images show that FKBP51 is primarily localized in mitochondria and hTERT is totally nuclear, upon the onset of oxidative stress, FKBP51 (but not FKBP52) becomes mostly nuclear colocalizing with hTERT, and longer exposure times to peroxide favors hTERT export to mitochondria. Importantly, telomerase activity of hTERT is significantly enhanced by FKBP51. These observations support the emerging role assigned to FKBP51 as antiapoptotic factor in cancer development and progression, and describe for the first time the potential role of this immunophilin favoring the clonal expansion by enhancing telomerase activity.

FK506 binding protein 51 integrates pathways of adaptation: FKBP51 shapes the reactivity to environmental change

This review portraits FK506 binding protein (FKBP) 51 as "reactivity protein" and collates recent publications to develop the concept of FKBP51 as contributor to different levels of adaptation. Adaptation is a fundamental process that enables unicellular and multicellular organisms to adjust their molecular circuits and structural conditions in reaction to environmental changes threatening their homeostasis. FKBP51 is known as chaperone and co-chaperone of heat shock protein (HSP) 90, thus involved in processes ensuring correct protein folding in response to proteotoxic stress. In mammals, FKBP51 both shapes the stress response and is calibrated by the stress levels through an ultrashort molecular feedback loop. More recently, it has been linked to several intracellular pathways related to the reactivity to drug exposure and stress. Through its role in autophagy and DNA methylation in particular it influences adaptive pathways, possibly also in a transgenerational fashion. Also see the video abstract here.