Pin1 inhibits PP2A-mediated Rb dephosphorylation in regulation of cell cycle and S-phase DNA damage. Cell Death Dis. 2015;6:e1640. [PMID: 25675300 PMCID: PMC4669794 DOI: 10.1038/cddis.2015.3]

Pin1-catalyzed conformational regulation after phosphorylation: A distinct checkpoint in cell signaling and drug discovery

Protein phosphorylation is one of the most common mechanisms regulating cellular signaling pathways, and many kinases and phosphatases are proven drug targets. Upon phosphorylation, protein functions can be further regulated by the distinct isomerase Pin1 through cis-trans isomerization. Numerous protein targets and many important roles have now been elucidated for Pin1. However, no tools are available to detect or target cis and trans conformation events in cells. The development of Pin1 inhibitors and stereo- and phospho-specific antibodies has revealed that cis and trans conformations have distinct and often opposing cellular functions. Aberrant conformational changes due to the dysregulation of Pin1 can drive pathogenesis but can be effectively targeted in age-related diseases, including cancers and neurodegenerative disorders. Here, we review advances in understanding the roles of Pin1 signaling in health and disease and highlight conformational regulation as a distinct signal transduction checkpoint in disease development and treatment.

Expression and significance of pin1 in the wound healing

The aim of this study is to delineate the expression patterns of prolyl cis-trans isomerase NIMA-interacting protein 1 (Pin1), Glial cell-derived neurotrophic factor (GDNF), and Angiotensin II (ANG II) during the process of wound repair, and to ascertain the effects of Pin1, GDNF, and ANG II on the healing of wounds in a rat model. A total of 18 rats were allocated into three groups-sham (control), DMSO (vehicle control), and Pin1 inhibitor (treatment with juglone)-with six animals in each group. An animal model of wound healing was established, followed by the intraperitoneal administration of juglone. Tissue samples from the wounds were subsequently collected for histopathological evaluation. Expression levels of Pin1, GDNF, and Ang II were quantified. In addition, an in vitro model of wound healing was created using human umbilical vein endothelial cells (HUVEC), to assess cell proliferation, migration, and tube formation under conditions of juglone pre-treatment. The expression levels of Pin1, GDNF, and ANG II were notably elevated on 7-, and 10- days post-wound compared to those measured on 3-day. Contrastingly, pre-treatment with juglone significantly inhibited the expression of these molecules. Histological analyses, including HE (Hematoxylin and Eosin), Masson's trichrome, and EVG (Elastic van Gieson) staining, demonstrated that vascular angiogenesis, as well as collagen and elastin deposition, were substantially reduced in the juglone pre-treated group when compared to the normal group. Further, immunohistochemical analysis revealed a considerable decrease in CD31 expression in the juglone pre-treatment group relative to the normal control group. Pin1 serves as a pivotal facilitator of wound repair. The findings indicate that the modulation of Pin1, GDNF, and ANG II expression impacts the wound healing process in rats, suggesting potential targets for therapeutic intervention in human wound repair.

The role of the master cancer regulator Pin1 in the development and treatment of cancer

This review examines the complex role of Pin1 in the development and treatment of cancer. Pin1 is the only peptidyl-prolyl isomerase (PPIase) that can recognize and isomerize phosphorylated Ser/Thr-Pro peptide bonds. Pin1 catalyzes a structural change in phosphorylated Ser/Thr-Pro motifs that can modulate protein function and thereby impact cell cycle regulation and tumorigenesis. The molecular mechanisms by which Pin1 contributes to oncogenesis are reviewed, including Pin1 overexpression and its correlation with poor cancer prognosis, and the contribution of Pin1 to aggressive tumor phenotypes involved in therapeutic resistance is discussed, with an emphasis on cancer stem cells, the epithelial-to-mesenchymal transition (EMT), and immunosuppression. The therapeutic potential of Pin1 inhibition in cancer is discussed, along with the promise and the difficulties in identifying potent, drug-like, small-molecule Pin1 inhibitors. The available evidence supports the efficacy of targeting Pin1 as a novel cancer therapeutic by analyzing the role of Pin1 in a complex network of cancer-driving pathways and illustrating the potential of synergistic drug combinations with Pin1 inhibitors for treating aggressive and drug-resistant tumors.

Pin1-Catalyzed Conformation Changes Regulate Protein Ubiquitination and Degradation

The unique prolyl isomerase Pin1 binds to and catalyzes cis-trans conformational changes of specific Ser/Thr-Pro motifs after phosphorylation, thereby playing a pivotal role in regulating the structure and function of its protein substrates. In particular, Pin1 activity regulates the affinity of a substrate for E3 ubiquitin ligases, thereby modulating the turnover of a subset of proteins and coordinating their activities after phosphorylation in both physiological and disease states. In this review, we highlight recent advancements in Pin1-regulated ubiquitination in the context of cancer and neurodegenerative disease. Specifically, Pin1 promotes cancer progression by increasing the stabilities of numerous oncoproteins and decreasing the stabilities of many tumor suppressors. Meanwhile, Pin1 plays a critical role in different neurodegenerative disorders via the regulation of protein turnover. Finally, we propose a novel therapeutic approach wherein the ubiquitin-proteasome system can be leveraged for therapy by targeting pathogenic intracellular targets for TRIM21-dependent degradation using stereospecific antibodies.

Prolyl isomerase Pin1 promotes autophagy and cancer cell viability through activating FoxO3 signalling

Pin1-directed prolyl isomerization is a central common oncogenic mechanism to drive tumorigenic processes. However, the role of Pin1 in cellular autophagy is still poorly understood. Here we report that pharmacological inhibition of Pin1 decreased the formation of autophagosome/autolysosomes upon nutrient starvation. Inhibition of Pin1 reduced, whereas forced expression of Pin1 increased, the level of LC3 and viability of U2OS and PANC-1 cells. Pin1 could augment the accumulation of LC3 upon chloroquine treatment, while chloroquine also disturbed its function on cell viability. RNA-Seq and qPCR identified altered autophagic pathway upon Pin1 silencing. Mechanistically, FoxO3 was identified critical for Pin1-mediated autophagy. Knockdown of FoxO3 could rescue the changes of LC3 level and cellular viability caused by Pin1 overexpression. In xenograft mouse model, Pin1 reduced the sensitivity of PANC-1 to chloroquine while FoxO3 silencing could inhibit Pin1's function. Moreover, Pin1 could bind FoxO3 via its pS284-P motif, reduce its phosphorylation at T32, facilitate its nuclear retention, and therefore increased its transcriptional activity. S284A mutation of FoxO3 interfered with its T32 phosphorylation, reduced its nuclear localization and disrupted its function to support cell viability upon nutrient starvation. Furthermore, the protein level of Pin1 positively correlated with FoxO3 nuclear localization and LC3 level in pancreatic adenocarcinoma and osteosarcoma samples. Together, this study highlights an important role for Pin1-FoxO3 axis in regulating autophagy and cancer cell viability. Intervening in the Pin1-FoxO3 interaction would serve as an effective therapeutic strategy and the pS284-P motif of FoxO3 provides a potential target for drug design.

PP2A-B55 and its adapter proteins IER2 and IER5 regulate the activity of RB family proteins and the expression of cell cycle-related genes

The retinoblastoma (RB) tumour suppressor protein regulates cell proliferation, motility, differentiation and apoptosis. The phosphorylation state of RB is modulated by kinases and phosphatases, and RB exhibits phosphorylation-sensitive interactions with E2F family transcription factors. Here, we characterize RB dephosphorylation by protein phosphatase 2A (PP2A). The growth factor-inducible immediate early response (IER) proteins IER2 and IER5 possess an adapter-like function in which IER proteins bind to both PP2A and its target proteins and enhance PP2A activity towards the proteins. IER2 interacts with RB and facilitates dephosphorylation of RB at T821/T826 by PP2A. In IER2 knockdown cells, elevated phosphorylation of RB resulted in reduced binding of RB to the promoters and derepression of cyclin D1 and p21. IER5 binds to both RB and RB-like 1 (p107/RBL1), enhances dephosphorylation of these proteins by PP2A and represses the expression of various cell cycle-related genes. However, IER2-regulated dephosphorylation at T821/T826 is not necessary for the repression function of RB in cell mobility-related gene expression. Our data identify PP2A adapter proteins as critical regulators of RB family proteins and suggest that the phosphorylation status of RB differentially affects gene expression.

Reciprocal inhibition of PIN1 and APC/CCDH1 controls timely G1/S transition and creates therapeutic vulnerability

Cyclin-dependent kinases (CDKs) mediated phosphorylation inactivates the anaphase-promoting complex (APC/CCDH1), an E3 ubiquitin ligase that contains the co-activator CDH1, to promote G1/S transition. PIN1 is a phosphorylation-directed proline isomerase and a master cancer signaling regulator. However, little are known about APC/CCDH1 regulation after phosphorylation and about PIN1 ubiquitin ligases. Here we uncover a domain-oriented reciprocal inhibition that controls the timely G1/S transition: The non-phosphorylated APC/CCDH1 E3 ligase targets PIN1 for degradation in G1 phase, restraining G1/S transition; APC/CCDH1 itself, after phosphorylation by CDKs, is inactivated by PIN1-catalyzed isomerization, promoting G1/S transition. In cancer, PIN1 overexpression and APC/CCDH1 inactivation reinforce each other to promote uncontrolled proliferation and tumorigenesis. Importantly, combined PIN1- and CDK4/6-inhibition reactivates APC/CCDH1 resulting in PIN1 degradation and an insurmountable G1 arrest that translates into synergistic anti-tumor activity against triple-negative breast cancer in vivo. Reciprocal inhibition of PIN1 and APC/CCDH1 is a novel mechanism to control timely G1/S transition that can be harnessed for synergistic anti-cancer therapy.

Pleiotropy of PP2A Phosphatases in Cancer with a Focus on Glioblastoma IDH Wildtype

Serine/Threonine protein phosphatase 2A (PP2A) is a heterotrimeric (or occasionally, heterodimeric) phosphatase with pleiotropic functions and ubiquitous expression. Despite the fact that they all contribute to protein dephosphorylation, multiple PP2A complexes exist which differ considerably by their subcellular localization and their substrate specificity, suggesting diverse PP2A functions. PP2A complex formation is tightly regulated by means of gene expression regulation by transcription factors, microRNAs, and post-translational modifications. Furthermore, a constant competition between PP2A regulatory subunits is taking place dynamically and depending on the spatiotemporal circumstance; many of the integral subunits can outcompete the rest, subjecting them to proteolysis. PP2A modulation is especially important in the context of brain tumors due to its ability to modulate distinct glioma-promoting signal transduction pathways, such as PI3K/Akt, Wnt, Ras, NF-κb, etc. Furthermore, PP2A is also implicated in DNA repair and survival pathways that are activated upon treatment of glioma cells with chemo-radiation. Depending on the cancer cell type, preclinical studies have shown some promise in utilising PP2A activator or PP2A inhibitors to overcome therapy resistance. This review has a special focus on "glioblastoma, IDH wild-type" (GBM) tumors, for which the therapy options have limited efficacy, and tumor relapse is inevitable.

Post-translational modifications on the retinoblastoma protein

The retinoblastoma protein (pRb) functions as a cell cycle regulator controlling G1 to S phase transition and plays critical roles in tumour suppression. It is frequently inactivated in various tumours. The functions of pRb are tightly regulated, where post-translational modifications (PTMs) play crucial roles, including phosphorylation, ubiquitination, SUMOylation, acetylation and methylation. Most PTMs on pRb are reversible and can be detected in non-cancerous cells, playing an important role in cell cycle regulation, cell survival and differentiation. Conversely, altered PTMs on pRb can give rise to anomalies in cell proliferation and tumourigenesis. In this review, we first summarize recent findings pertinent to how individual PTMs impinge on pRb functions. As many of these PTMs on pRb were published as individual articles, we also provide insights on the coordination, either collaborations and/or competitions, of the same or different types of PTMs on pRb. Having a better understanding of how pRb is post-translationally modulated should pave the way for developing novel and specific therapeutic strategies to treat various human diseases.

Pin1-mediated regulation of articular cartilage stem/progenitor cell aging

Cartilage stem/progenitor cells (CSPCs) was recently isolated and identified from the cartilage tissue. CSPCs is essential for repair and regeneration of cartilage in osteoarthritis (OA). Aging is a primary risk factor for cartilage damage and joint OA. Although studies have confirmed the link between cell aging and OA, the underlying molecular mechanisms regulating CSPCs aging are not fully understood. In this study, we investigated the role of Pin1 in the aging of rat knee joint CSPCs. We isolated CSPCs from rat knee joints and demonstrated that, in long-term in vitro culture, Pin1 protein levels are significantly reduced. At the same time, expression of the senescence-related β-galactosidase and the senescence marker p16^INK4A were markedly elevated. In addition, Pin1 overexpression reversed the progression of cellular senescence, as evidenced by the down-regulation of senescence-related β-galactosidase, increased EdU positive cells and diminished levels of p16^INK4A. In contrast, Pin1 siRNA incorporation promoted CSPCs senescence. In addition, we also observed the distribution of cell cycles through flow cytometry and revealed that Pin1 deficiency results in cell cycle arrest in the G1 phase, suggesting severe lack of proliferation ability, a sign of cellular senescence. Collectively, these results validated that Pin1 is an essential regulator of CSPCs aging.

Phosphorylation and Stabilization of PIN1 by JNK Promote Intrahepatic Cholangiocarcinoma Growth

BACKGROUND AND AIMS

Intrahepatic cholangiocarcinoma (ICC) is a highly aggressive type of liver cancer in urgent need of treatment options. Aberrant activation of the c-Jun N-terminal kinase (JNK) pathway is a key feature in ICC and an attractive candidate target for its treatment. However, the mechanisms by which constitutive JNK activation promotes ICC growth, and therefore the key downstream effectors of this pathway, remain unknown for their applicability as therapeutic targets. Our aim was to obtain a better mechanistic understanding of the role of JNK signaling in ICC that could open up therapeutic opportunities.

APPROACH AND RESULTS

Using loss-of-function and gain-of-function studies in vitro and in vivo, we show that activation of the JNK pathway promotes ICC cell proliferation by affecting the protein stability of peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 (PIN1), a key driver of tumorigenesis. PIN1 is highly expressed in ICC primary tumors, and its expression positively correlates with active JNK. Mechanistically, the JNK kinases directly bind to and phosphorylate PIN1 at Ser115, and this phosphorylation prevents PIN1 mono-ubiquitination at Lys117 and its proteasomal degradation. Moreover, pharmacological inhibition of PIN1 through all-trans retinoic acid, a Food and Drug Administration-approved drug, impairs the growth of both cultured and xenografted ICC cells.

CONCLUSIONS

Our findings implicate the JNK-PIN1 regulatory axis as a functionally important determinant for ICC growth, and provide a rationale for therapeutic targeting of JNK activation through PIN1 inhibition.

Targeting Pin1 for Modulation of Cell Motility and Cancer Therapy

Peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 (Pin1) specifically binds and isomerizes the phosphorylated serine/threonine-proline (pSer/Thr-Pro) motif, which leads to changes in protein conformation and function. Pin1 is widely overexpressed in cancers and plays an important role in tumorigenesis. Mounting evidence has revealed that targeting Pin1 is a potential therapeutic approach for various cancers by inhibiting cell proliferation, reducing metastasis, and maintaining genome stability. In this review, we summarize the underlying mechanisms of Pin1-mediated upregulation of oncogenes and downregulation of tumor suppressors in cancer development. Furthermore, we also discuss the multiple roles of Pin1 in cancer hallmarks and examine Pin1 as a desirable pharmaceutical target for cancer therapy. We also summarize the recent progress of Pin1-targeted small-molecule compounds for anticancer activity.

Impact of PIN1 Inhibition on Tumor Progression and Chemotherapy Sensitivity in Colorectal Cancer

BACKGROUND

Deregulated PIN1 is associated with cancer development and progression. Herein, for the first time, we evaluate the roles that PIN1 in tumorigenic characteristics of colorectal cancer (CRC) cells.

METHODS

In this study, PIN1 expression was knocked down in SW-48 cells by synthetic small interfering RNA (siRNA). After confirming the knockdown of PIN1, cell viability, colony formation, apoptosis, autophagy, cancer stem cell (CSC)-related genes, CSC-related signaling pathways, cell migration, and 5-FU chemosensitivity were evaluated in vitro.

RESULTS

Transfection of PIN1 siRNA into SW-48 cells inhibited cancer cell proliferation, migration, and increased apoptosis and autophagy. Transfected SW-48 cells had lower properties of CSCs through the inhibition of β-catenin and Notch1 gene expression. Moreover, inhibition of PIN1 enhanced the inhibitory effect of 5-FU on SW-48 cell proliferation.

CONCLUSION

Our results indicated that targeting of PIN1 serves as a promising therapeutic solution for the suppression of tumor progression processes in CRC.

Targeting of ΔNp63α by miR-522 promotes the migration of breast epithelial cells

The TP63 gene, which encodes the p63 protein, is involved in multiple biological processes, including embryonic development and tumorigenesis. ΔNp63α, the predominant isoform of p63 in epithelial cells, acts as an oncogene in early-stage tumors, but paradoxically acts as a potent antimetastatic factor in advanced cancers. Here, we report that ΔNp63α is a direct target of hsa-miR-522 (miR-522). Induced expression of miR-522 reduced the levels of ΔNp63α, predisposing breast epithelial cells to a loss of epithelial and acquisition of mesenchymal morphology, resulting in accelerated collective and single-cell migration. Restoration of ΔNp63α repressed miR-522-induced migration. Interestingly, overexpression of miR-522 did not affect breast epithelial cell proliferation, suggesting that miR-522 acts specifically through ΔNp63α in this context. Furthermore, expression of miR-522-3p and p63 was negatively correlated in human cancer samples. Thus, miR-522 might be a causative factor for breast tumorigenesis and cancer metastasis. In summary, our results reveal a novel miR-522/p63 axis in cell migration and thus suggest a potential strategy for therapeutic treatment of cancer metastasis.

Deficiency of microRNA-628-5p promotes the progression of gastric cancer by upregulating PIN1

Gastric cancer is one of the most common cancer and is the second leading cause of cancer-related mortality in the world. PIN1, belonging to peptidyl-prolyl cis-trans isomerase family, uniquely catalyzes the structural transformation of phosphorylated Ser/Thr-Pro motif. It's high expressed in most cancers and promotes their progression. However, the mechanism of PIN1 high expression and its function in gastric cancer progression are still unclear. In this research, we revealed that PIN1 not only promotes the proliferation and colony formation of gastric cancer, but also increases its migration and invasion. The PIN1 expression in metastasis lesion is usually higher than the corresponding primary site. Inhibiting PIN1 by shRNA suppresses the progression of gastric cancer significantly. Besides, we demonstrated that miR-628-5p is a novel PIN1-targeted microRNA, and the expression of miR-628-5p is negatively correlated with PIN1 in gastric cancer. Exogenous expression of miR-628-5p inhibits the progression of gastric cancer that revered by restoring PIN1 expression. However, miR-628-5p is downregulated in majority of gastric cancer tissue especially in metastasis lesion. The lower miR-628-5p level indicates poorer prognosis. In summary, our study demonstrated that deficient miR-628-5p expression facilitates the expression of PIN1, and consequently promotes the progression of gastric cancer.

Prolyl Isomerase Pin1 in Human Cancer: Function, Mechanism, and Significance

Peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 (Pin1) is an evolutionally conserved and unique enzyme that specifically catalyzes the cis-trans isomerization of phosphorylated serine/threonine-proline (pSer/Thr-Pro) motif and, subsequently, induces the conformational change of its substrates. Mounting evidence has demonstrated that Pin1 is widely overexpressed and/or overactivated in cancer, exerting a critical influence on tumor initiation and progression via regulation of the biological activity, protein degradation, or nucleus-cytoplasmic distribution of its substrates. Moreover, Pin1 participates in the cancer hallmarks through activating some oncogenes and growth enhancers, or inactivating some tumor suppressors and growth inhibitors, suggesting that Pin1 could be an attractive target for cancer therapy. In this review, we summarize the findings on the dysregulation, mechanisms, and biological functions of Pin1 in cancer cells, and also discuss the significance and potential applications of Pin1 dysregulation in human cancer.

Oncogenic Hijacking of the PIN1 Signaling Network

Cellular choices are determined by developmental and environmental stimuli through integrated signal transduction pathways. These critically depend on attainment of proper activation levels that in turn rely on post-translational modifications (PTMs) of single pathway members. Among these PTMs, post-phosphorylation prolyl-isomerization mediated by PIN1 represents a unique mechanism of spatial, temporal and quantitative control of signal transduction. Indeed PIN1 was shown to be crucial for determining activation levels of several pathways and biological outcomes downstream to a plethora of stimuli. Of note, studies performed in different model organisms and humans have shown that hormonal, nutrient, and oncogenic stimuli simultaneously affect both PIN1 activity and the pathways that depend on PIN1-mediated prolyl-isomerization, suggesting the existence of evolutionarily conserved molecular circuitries centered on this isomerase. This review focuses on molecular mechanisms and cellular processes like proliferation, metabolism, and stem cell fate, that are regulated by PIN1 in physiological conditions, discussing how these are subverted in and hijacked by cancer cells. Current status and open questions regarding the use of PIN1 as biomarker and target for cancer therapy as well as clinical development of PIN1 inhibitors are also addressed.

PIN1 in Cell Cycle Control and Cancer

Cell cycle progression is tightly controlled by many cell cycle-regulatory proteins that are in turn regulated by a family of cyclin-dependent kinases (CDKs) through protein phosphorylation. The peptidyl-prolyl cis/trans isomerase PIN1 provides a further post-phosphorylation modification and functional regulation of these CDK-phosphorylated proteins. PIN1 specifically binds the phosphorylated serine or threonine residue preceding a proline (pSer/Thr-Pro) motif of its target proteins and catalyzes the cis/trans isomerization on the pSer/Thr-Pro peptide bonds. Through this phosphorylation-dependent prolyl isomerization, PIN1 fine-tunes the functions of various cell cycle-regulatory proteins including retinoblastoma protein (Rb), cyclin D1, cyclin E, p27, Cdc25C, and Wee1. In this review, we discussed the essential roles of PIN1 in regulating cell cycle progression through modulating the functions of these cell cycle-regulatory proteins. Furthermore, the mechanisms underlying PIN1 overexpression in cancers were also explored. Finally, we examined and summarized the therapeutic potential of PIN1 inhibitors in cancer therapy.

Regulation of the Minichromosome Maintenance Protein 3 (MCM3) Chromatin Binding by the Prolyl Isomerase Pin1

Human Pin1 is a peptidyl prolyl cis/trans isomerase with a unique preference for phosphorylated Ser/Thr-Pro substrate motifs. Here we report that MCM3 (minichromosome maintenance complex component 3) is a novel target of Pin1. MCM3 interacts directly with the WW domain of Pin1. Proline-directed phosphorylation of MCM3 at S112 and T722 are crucial for the interaction with Pin1. MCM3 as a subunit of the minichromosome maintenance heterocomplex MCM2-7 is part of the pre-replication complex responsible for replication licensing and is implicated in the formation of the replicative helicase during progression of replication. Our data suggest that Pin1 coordinates phosphorylation-dependently MCM3 loading onto chromatin and its unloading from chromatin, thereby mediating S phase control.

Prolyl isomerase Pin1: a promoter of cancer and a target for therapy

Pin1 is the only known peptidyl-prolyl cis–trans isomerase (PPIase) that specifically recognizes and isomerizes the phosphorylated Serine/Threonine-Proline (pSer/Thr-Pro) motif. The Pin1-mediated structural transformation posttranslationally regulates the biofunctions of multiple proteins. Pin1 is involved in many cellular processes, the aberrance of which lead to both degenerative and neoplastic diseases. Pin1 is highly expressed in the majority of cancers and its deficiency significantly suppresses cancer progression. According to the ground-breaking summaries by Hanahan D and Weinberg RA, the hallmarks of cancer comprise ten biological capabilities. Multiple researches illuminated that Pin1 contributes to these aberrant behaviors of cancer via promoting various cancer-driving pathways. This review summarized the detailed mechanisms of Pin1 in different cancer capabilities and certain Pin1-targeted small-molecule compounds that exhibit anticancer activities, expecting to facilitate anticancer therapies by targeting Pin1.

Synthesis and biological evaluation of pyrimidine derivatives as novel human Pin1 inhibitors

Pin1 (Protein interacting with NIMA1) is a cis-trans isomerase and promotes the amide bond rotation of phosphoSer/Thr-Pro motifs in its substrates. Inhibition of Pin1 might be a novel strategy for developing anticancer agents. Herein, a series of pyrimidine derivatives were synthesized and their Pin1 inhibitory activities were evaluated. Among them, four compounds (2a, 2f, 2h and 2l) displayed potent inhibitory activities against Pin1 with IC₅₀ values lower than 3 µM. This series of pyrimidine-based inhibitors presented time-dependent inhibition against Pin1. The structure-activity relationships on the 2-, 4- and 5-positions of the pyrimidine ring were analyzed in details, which would facilitate further exploration of new Pin1 inhibitors.

PP2A as the Main Node of Therapeutic Strategies and Resistance Reversal in Triple-Negative Breast Cancer

Triple negative breast cancer (TNBC), is defined as a type of tumor lacking the expression of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2). The ER, PR and HER2 are usually the molecular therapeutic targets for breast cancers, but they are ineffective for TNBC because of their negative expressions, so chemotherapy is currently the main treatment strategy in TNBC. However, drug resistance remains a major impediment to TNBC chemotherapeutic treatment. Recently, the protein phosphatase 2A (PP2A) has been found to regulate the phosphorylation of some substrates involved in the relevant target of TNBC, such as cell cycle control, DNA damage responses, epidermal growth factor receptor, immune modulation and cell death resistance, which may be the effective therapeutic strategies or influence drug sensitivity to TNBCs. Furthermore, PP2A has also been found that could induce ER re-expression in ER-negative breast cancer cells, and which suggests PP2A could promote the sensitivity of tamoxifen to TNBCs as a resistance reversal agent. In this review, we will summarize the potential therapeutic value of PP2A as the main node in developing targeting agents, disrupting resistance or restoring drug sensitivity in TNBC.

The peptidyl-prolyl isomerase PIN1 relieves cyclin-dependent kinase 2 (CDK2) inhibition by the CDK inhibitor p27

PIN1 is a peptidyl-prolyl isomerase that catalyzes the cis/trans isomerization of peptide bonds between proline and phosphorylated serine/threonine residues. By changing the conformation of its protein substrates, PIN1 increases the activities of key proteins that promote cell cycle progression and oncogenesis. Moreover, it has been shown that PIN1 stabilizes and increases the level of the cyclin-dependent kinase (CDK) inhibitor p27, which inhibits cell cycle progression by binding cyclin A- and cyclin E-CDK2. Notwithstanding the associated increase in the p27 level, PIN1 expression promotes rather than retards cell proliferation. To explain the paradoxical effects of PIN1 on p27 levels and cell cycle progression, we hypothesized that PIN1 relieves CDK2 inhibition by suppressing the CDK inhibitory activity of p27. Here, we confirmed that PIN1-expressing cells exhibit higher p27 levels but have increased CDK2 activities and higher proliferation rates in the S-phase compared with Pin1-null fibroblasts or PIN1-depleted hepatoma cells. Using co-immunoprecipitation and CDK kinase activity assays, we found that PIN1 binds the phosphorylated Thr¹⁸⁷-Pro motif in p27 and reduces p27's interaction with cyclin A- or cyclin E-CDK2, leading to increased CDK2 kinase activity. In conclusion, our results indicate that although PIN1 increases p27 levels, it also attenuates p27's inhibitory activity on CDK2 and thereby contributes to increased G₁-S phase transitions and cell proliferation.

Peptidyl-prolyl isomerase 1 (Pin1) preserves the phosphorylation state of tissue factor and prolongs its release within microvesicles

The exposure and release of TF is regulated by post-translational modifications of its cytoplasmic domain. Here, the potential of Pin1 to interact with the cytoplasmic domain of TF, and the outcome on TF function was examined. MDA-MB-231 and transfected-primary endothelial cells were incubated with either Pin1 deactivator Juglone, or its control Plumbagin, as well as transfected with Pin1-specific or control siRNA. TF release into microvesicles following activation, and also phosphorylation and ubiquitination states of cellular-TF were then assessed. Furthermore, the ability of Pin1 to bind wild-type and mutant forms of overexpressed TF-tGFP was investigated by co-immunoprecipitation. Additionally, the ability of recombinant or cellular Pin1 to bind to peptides of the C-terminus of TF, synthesised in different phosphorylation states was examined by binding assays and spectroscopically. Finally, the influence of recombinant Pin1 on the ubiquitination and dephosphorylation of the TF-peptides was examined. Pre-incubation of Pin1 with Juglone but not Plumbagin, reduced TF release as microvesicles and was also achievable following transfection with Pin1-siRNA. This was concurrent with early ubiquitination and dephosphorylation of cellular TF at Ser253. Pin1 co-immunoprecipitated with overexpressed wild-type TF-tGFP but not Ser258→Ala or Pro259→Ala substituted mutants. Pin1 did interact with Ser258-phosphorylated and double-phosphorylated TF-peptides, with the former having higher affinity. Finally, recombinant Pin1 was capable of interfering with the ubiquitination and dephosphorylation of TF-derived peptides. In conclusion, Pin1 is a fast-acting enzyme which may be utilised by cells to protect the phosphorylation state of TF in activated cells prolonging TF activity and release, and therefore ensuring adequate haemostasis.

The Prolyl Isomerase Pin1 Is a Novel Target of 6,7,4'-Trihydroxyisoflavone for Suppressing Esophageal Cancer Growth

Intake of soy isoflavones is inversely associated with the risk of esophageal cancer. Numerous experimental results have supported the anticancer activity of soy isoflavones. This study aimed to determine the anti-esophageal cancer activity of 6,7,4'-trihydroxyisoflavone (6,7,4'-THIF), a major metabolite of daidzein, which is readily metabolized in the human body. Notably, 6,7,4'-THIF inhibited proliferation and increased apoptosis of esophageal cancer cells. On the basis of a virtual screening analysis, Pin1 was identified as a target protein of 6,7,4'-THIF. Pull-down assay results using 6,7,4'-THIF Sepharose 4B beads showed a direct interaction between 6,7,4'-THIF and the Pin1 protein. Pin1 is a critical therapeutic and preventive target in esophageal cancer because of its positive regulation of β-catenin and cyclin D1. The 6,7,4'-THIF compound simultaneously reduced Pin1 isomerase activity and the downstream activation targets of Pin1. The specific inhibitory activity of 6,7,4'-THIF was analyzed using Neu/Pin1 wild-type (WT) and Neu/Pin1 knockout (KO) MEFs. 6,7,4'-THIF effected Neu/Pin1 WT MEFs, but not Neu/Pin1 KO MEFs. Furthermore, the results of a xenograft assay using Neu/Pin1 WT and KO MEFs were similar to those obtained from the in vitro assay. Overall, we found that 6,7,4'-THIF specifically reduced Pin1 activity in esophageal cancer models. Importantly, 6,7,4'-THIF directly bound to Pin1 but not FKBP or cyclophilin A, the same family of proteins. Because Pin1 acts like an oncogene by modulating various carcinogenesis-related proteins, this study might at least partially explain the underlying mechanism(s) of the anti-esophageal cancer effects of soy isoflavones. Cancer Prev Res; 10(5); 308-18. ©2017 AACR.

Understanding the role of PIN1 in hepatocellular carcinoma

PIN1 is a peptidyl-prolyl cis/trans isomerase that binds and catalyses isomerization of the specific motif comprising a phosphorylated serine or threonine residue preceding a proline (pSer/Thr-Pro) in proteins. PIN1 can therefore induce conformational and functional changes of its interacting proteins that are regulated by proline-directed serine/threonine phosphorylation. Through this phosphorylation-dependent prolyl isomerization, PIN1 fine-tunes the functions of key phosphoproteins (e.g., cyclin D1, survivin, β-catenin and x-protein of hepatitis B virus) that are involved in the regulation of cell cycle progression, apoptosis, proliferation and oncogenic transformation. PIN1 has been found to be over-expressed in many cancers, including human hepatocellular carcinoma (HCC). It has been shown previously that overexpression of PIN1 contributes to the development of HCC in-vitro and in xenograft mouse model. In this review, we first discussed the aberrant transcription factor expression, miRNAs dysregulation, PIN1 gene promoter polymorphisms and phosphorylation of PIN1 as potential mechanisms underlying PIN1 overexpression in cancers. Furthermore, we also examined the role of PIN1 in HCC tumourigenesis by reviewing the interactions between PIN1 and various cellular and viral proteins that are involved in β-catenin, NOTCH, and PI3K/Akt/mTOR pathways, apoptosis, angiogenesis and epithelial-mesenchymal transition. Finally, the potential of PIN1 inhibitors as an anti-cancer therapy was explored and discussed.

The role of Pin1 in the development and treatment of cancer

Protein phosphorylation and post-phosphorylation events regulate many cellular signaling pathways. Peptidyl-prolyl isomerase (Pin1) is the only peptidyl-prolyl cis/trans isomerase that interacts with numerous oncogenic or tumor suppressive phosphorylated proteins, causes conformational changes in target proteins, and eventually regulates the activities of such proteins. These alterations in activity play a pivotal role in tumorigenesis. Since Pin1 is overexpressed and/or activated in various types of cancers, and the dysregulation of proline-directed phosphorylation contributes to tumorigenesis, Pin1 represents an attractive target for cancer therapy. This review will describe the role of Pin1 in cancer and the current status of Pin1 inhibitor development.

The isomerase PIN1 controls numerous cancer-driving pathways and is a unique drug target

Targeted drugs have changed cancer treatment but are often ineffective in the long term against solid tumours, largely because of the activation of heterogeneous oncogenic pathways. A central common signalling mechanism in many of these pathways is proline-directed phosphorylation, which is regulated by many kinases and phosphatases. The structure and function of these phosphorylated proteins are further controlled by a single proline isomerase: PIN1. PIN1 is overactivated in cancers and it promotes cancer and cancer stem cells by disrupting the balance of oncogenes and tumour suppressors. This Review discusses the roles of PIN1 in cancer and the potential of PIN1 inhibitors to restore this balance.

Peptidyl Prolyl Isomerase PIN1 Directly Binds to and Stabilizes Hypoxia-Inducible Factor-1α

Peptidyl prolyl isomerase (PIN1) regulates the functional activity of a subset of phosphoproteins through binding to phosphorylated Ser/Thr-Pro motifs and subsequently isomerization of the phosphorylated bonds. Interestingly, PIN1 is overexpressed in many types of malignancies including breast, prostate, lung and colon cancers. However, its oncogenic functions have not been fully elucidated. Here, we report that PIN1 directly interacts with hypoxia-inducible factor (HIF)-1α in human colon cancer (HCT116) cells. PIN1 binding to HIF-1α occurred in a phosphorylation-dependent manner. We also found that PIN1 interacted with HIF-1α at both exogenous and endogenous levels. Notably, PIN1 binding stabilized the HIF-1α protein, given that their levels were significantly increased under hypoxic conditions. The stabilization of HIF-1α resulted in increased transcriptional activity, consequently upregulating expression of vascular endothelial growth factor, a major contributor to angiogenesis. Silencing of PIN1 or pharmacologic inhibition of its activity abrogated the angiogenesis. By utilizing a bioluminescence imaging technique, we were able to demonstrate that PIN1 inhibition dramatically reduced the tumor volume in a subcutaneous mouse xenograft model and angiogenesis as well as hypoxia-induced transcriptional activity of HIF-1α. These results suggest that PIN1 interacting with HIF-1α is a potential cancer chemopreventive and therapeutic target.

The role of Pin1 protein in aging of human tendon stem/progenitor cells

Aging of tendon stem/progenitor cells (TSPCs) can lead to tissue degeneration and subsequent injury. However, the molecular mechanisms controlling TSPC aging are not completely understood. In the present study, we investigated the role of Pin1 in aging of human TSPCs. Pin1 mRNA and protein expression levels were significantly decreased during prolonged in vitro culture of human TSPCs. Furthermore, overexpression of Pin1 delayed the progression of cellular senescence, as confirmed by downregulation of senescence-associated β-galactosidase, increased telomerase activity and decreased levels of the senescence marker, p16(INK4A). Conversely, Pin1 siRNA transfection promoted senescence in TSPCs. In addition, miR-140-5p regulated Pin1 expression at the translational level via directly targeting its 3'UTR. Our results collectively demonstrate that Pin1 acts as an important regulator of TSPC aging.