Degradation of the tumor suppressor PML by Pin1 contributes to the cancer phenotype of breast cancer MDA-MB-231 cells

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.

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.

Reciprocal antagonism of PIN1-APC/CCDH1 governs mitotic protein stability and cell cycle entry

Induced oncoproteins degradation provides an attractive anti-cancer modality. Activation of anaphase-promoting complex (APC/CCDH1) prevents cell-cycle entry by targeting crucial mitotic proteins for degradation. Phosphorylation of its co-activator CDH1 modulates the E3 ligase activity, but little is known about its regulation after phosphorylation and how to effectively harness APC/CCDH1 activity to treat cancer. Peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 (PIN1)-catalyzed phosphorylation-dependent cis-trans prolyl isomerization drives tumor malignancy. However, the mechanisms controlling its protein turnover remain elusive. Through proteomic screens and structural characterizations, we identify a reciprocal antagonism of PIN1-APC/CCDH1 mediated by domain-oriented phosphorylation-dependent dual interactions as a fundamental mechanism governing mitotic protein stability and cell-cycle entry. Remarkably, combined PIN1 and cyclin-dependent protein kinases (CDKs) inhibition creates a positive feedback loop of PIN1 inhibition and APC/CCDH1 activation to irreversibly degrade PIN1 and other crucial mitotic proteins, which force permanent cell-cycle exit and trigger anti-tumor immunity, translating into synergistic efficacy against triple-negative breast cancer.

Regulating the p53 Tumor Suppressor Network at PML Biomolecular Condensates

By forming specific functional entities, nuclear biomolecular condensates play an important function in guiding biological processes. PML biomolecular condensates, also known as PML nuclear bodies (NBs), are macro-molecular sub-nuclear organelles involved in central biological processes, including anti-viral response and cell fate control upon genotoxic stress. PML condensate formation is stimulated upon cellular stress, and relies on protein-protein interactions establishing a PML protein meshwork capable of recruiting the tumor suppressor p53, along with numerous modifiers of p53, thus balancing p53 posttranslational modifications and activity. This stress-regulated process appears to be controlled by liquid-liquid phase separation (LLPS), which may facilitate regulated protein-unmixing of p53 and its regulators into PML nuclear condensates. In this review, we summarize and discuss the molecular mechanisms underlying PML nuclear condensate formation, and how these impact the biological function of p53 in driving the cell death and senescence responses. In addition, by using an in silico approach, we identify 299 proteins which share PML and p53 as binding partners, thus representing novel candidate proteins controlling p53 function and cell fate decision-making at the level of PML nuclear biocondensates.

Targeting peptidyl-prolyl isomerase 1 in experimental pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension (PAH) is a progressive disease characterised by pro-proliferative and anti-apoptotic phenotype in vascular cells, leading to pulmonary vascular remodelling and right heart failure. Peptidyl-prolyl cis/trans isomerase, NIMA interacting 1 (Pin1), a highly conserved enzyme, which binds to and catalyses the isomerisation of specific phosphorylated Ser/Thr-Pro motifs, acts as a molecular switch in multiple coordinated cellular processes. We hypothesised that Pin1 plays a substantial role in PAH, and its inhibition with a natural organic compound, Juglone, would reverse experimental pulmonary hypertension.

RESULTS

We demonstrated that the expression of Pin1 was markedly elevated in experimental pulmonary hypertension (i.e. hypoxia-induced mouse and Sugen/hypoxia-induced rat models) and pulmonary arterial smooth muscle cells of patients with clinical PAH. In vitro Pin1 inhibition by either Juglone treatment or short interfering RNA knockdown resulted in an induction of apoptosis and decrease in proliferation of human pulmonary vascular cells. Stimulation with growth factors induced Pin1 expression, while its inhibition reduced the activity of numerous PAH-related transcription factors, such as hypoxia-inducible factor (HIF)-α and signal transducer and activator of transcription (STAT). Juglone administration lowered pulmonary vascular resistance, enhanced right ventribular function, improved pulmonary vascular and cardiac remodelling in the Sugen/hypoxia rat model of PAH and the chronic hypoxia-induced pulmonary hypertension model in mice.

CONCLUSION

Our study demonstrates that targeting of Pin1 with small molecule inhibitor, Juglone, might be an attractive future therapeutic strategy for PAH and right heart disease secondary to PAH.

Peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 directly binds and stabilizes Nrf2 in breast cancer

Peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 (Pin1) has been frequently overexpressed in many types of malignancy, suggesting its oncogenic function. It recognizes phosphorylated serine or threonine (pSer/Thr) of a target protein and isomerizes the adjacent proline (Pro) residue, thereby altering folding, subcellular localization, stability, and function of target proteins. The oncogenic transcription factor, Nrf2 harbors the pSer/Thr-Pro motif. This prompted us to investigate whether Pin1 could bind to Nrf2 and influence its stability and function in the context of implications for breast cancer development and progression. The correlation between Pin1 and Nrf2 in the triple-negative breast cancer cells was validated by RNASeq analysis as well as immunofluorescence staining. Interaction between Pin1 and Nrf2 was assessed by co-immunoprecipitation and an in situ proximity ligation assay. We found that mRNA and protein levels of Pin1 were highly increased in the tumor tissues of triple-negative breast cancer patients and the human breast cancer cell line. Genetic or pharmacologic inhibition of Pin1 enhanced the ubiquitination and degradation of Nrf2. In contrast, the overexpression of Pin1 resulted in the accumulation of Nrf2 in the nucleus, without affecting its transcription. Notably, the phosphorylation of Nrf2 at serine 215, 408, and 577 is essential for its interaction with Pin1. We also identified phosphorylated Ser104 and Thr277 residues in Keap1, a negative regulator of Nrf2, for Pin1 binding. Pin1 plays a role in breast cancer progression through stabilization and constitutive activation of Nrf2 by competing with Keap1 for Nrf2 binding.

A Pin1/PML/P53 axis activated by retinoic acid in NPM-1c acute myeloid leukemia

Retinoic acid (RA) was proposed to increase survival of chemotherapy- treated patients with nucleophosmin-1 (NPM-1c)-mutated acute myeloid leukemia. We reported that, ex vivo, RA triggers NPM-1c degradation, P53 activation and growth arrest. PML organizes domains that control senescence or proteolysis. Here, we demonstrate that PML is required to initiate RA-driven NPM-1c degradation, P53 activation and cell death. Mechanistically, RA enhances PML basal expression through inhibition of activated Pin1, prior to NPM-1c degradation. Such PML induction drives P53 activation, favoring blast response to chemotherapy or arsenic in vivo. This RA/PML/P53 cascade could mechanistically explain RA-facilitated chemotherapy response in patients with NPM-1c mutated acute myeloid leukemia.

PML Suppresses Influenza Virus Replication by Promoting FBXW7 Expression

Influenza A viruses (IAV) are responsible for seasonal flu epidemics, which can lead to high morbidity and mortality each year. Like other viruses, influenza virus can hijack host cellular machinery for its replication. Host cells have evolved diverse cellular defense to resist the invasion of viruses. As the main components of promyelocytic leukemia protein nuclear bodies (PML-NBs), PML can inhibit the replication of many medically important viruses including IAV. However, the mechanism of PML against IAV is unclear. In the present study, we found PML was induced in response to IAV infection and ectopic expression of PML could inhibit IAV replication, whereas knockdown of endogenous PML expression could enhance IAV replication. Further studies showed that PML increased the expression of FBXW7 by inhibiting its K48-linked ubiquitination and enhanced the interaction between FBXW7 and SHP2, which negatively regulated IAV replication during infection. Moreover, PML stabilized RIG-I to promote the production of type I IFN. Collectively, these data indicated that PML inhibited IAV replication by enhancing FBXW7 expression in the antiviral immunity against influenza virus and extended the mechanism of PML in antiviral immunity.

Protein sumoylation with SUMO1 promoted by Pin1 in glioma stem cells augments glioblastoma malignancy

BACKGROUND

The tumorigenic potential of glioma stem cells (GSCs) is associated with multiple reversible molecular alternations, but the role of posttranslational protein sumoylation in GSCs has not been elucidated. The development of GSC-targeting drugs relies on the discovery of GSC-preferential molecular modifications and the relevant signaling pathways. In this work, we investigated the protein sumoylation status, the major sumoylated substrate, and the key regulatory enzyme in GSCs to explore the therapeutic potential of disrupting protein sumoylation for glioblastoma (GBM) treatment.

METHODS

Patient-derived GSCs, primary GBM sections, and intracranial GBM xenografts were used to determine protein sumoylation and the related molecular mechanisms by immunoblot, quantitative PCR, immunoprecipitation, immunofluorescence, and immunohistochemistry. Orthotopic GBM xenograft models were applied to investigate the inhibition of tumor growth by disrupting protein sumoylation with short hairpin (sh)RNAs or molecular inhibitors.

RESULTS

We show that high levels of small ubiquitin-related modifier 1 (SUMO1)-but not SUMO2/3-modified sumoylation are preferentially present in GSCs. The promyelocytic leukemia (PML) protein is a major SUMO1-sumoylated substrate in GSCs, whose sumoylation facilitates its interaction with c-Myc to stabilize c-Myc proteins. The prolyl-isomerase Pin1 is preferentially expressed in GSCs and functions as the key enzyme to promote SUMO1 sumoylation. Disruption of SUMO1 sumoylation by Pin1 silencing with shRNAs or inhibition with its inhibitor Juglone markedly abrogated GSC maintenance and mitigated GSC-driven tumor growth.

CONCLUSIONS

Our findings indicate that high SUMO1-modified protein sumoylation as a feature of GSCs is critical for GSC maintenance, suggesting that targeting SUMO1 sumoylation may effectively improve GBM treatment.

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.

The Role of ND10 Nuclear Bodies in Herpesvirus Infection: A Frenemy for the Virus?

Nuclear domains 10 (ND10), a.k.a. promyelocytic leukemia nuclear bodies (PML-NBs), are membraneless subnuclear domains that are highly dynamic in their protein composition in response to cellular cues. They are known to be involved in many key cellular processes including DNA damage response, transcription regulation, apoptosis, oncogenesis, and antiviral defenses. The diversity and dynamics of ND10 residents enable them to play seemingly opposite roles under different physiological conditions. Although the molecular mechanisms are not completely clear, the pro- and anti-cancer effects of ND10 have been well established in tumorigenesis. However, in herpesvirus research, until the recently emerged evidence of pro-viral contributions, ND10 nuclear bodies have been generally recognized as part of the intrinsic antiviral defenses that converge to the incoming viral DNA to inhibit the viral gene expression. In this review, we evaluate the newly discovered pro-infection influences of ND10 in various human herpesviruses and analyze their molecular foundation along with the traditional antiviral functions of ND10. We hope to shed light on the explicit role of ND10 in both the lytic and latent cycles of herpesvirus infection, which is imperative to the delineation of herpes pathogenesis and the development of prophylactic/therapeutic treatments for herpetic diseases.

Roles of peptidyl-prolyl isomerase Pin1 in disease pathogenesis

Pin1 belongs to the peptidyl-prolyl cis-trans isomerases (PPIases) superfamily and catalyzes the cis-trans conversion of proline in target substrates to modulate diverse cellular functions including cell cycle progression, cell motility, and apoptosis. Dysregulation of Pin1 has wide-ranging influences on the fate of cells; therefore, it is closely related to the occurrence and development of various diseases. This review summarizes the current knowledge of Pin1 in disease pathogenesis.

H-Ras induces Nrf2-Pin1 interaction: Implications for breast cancer progression

Aberrant activation of H-Ras is often associated with tumor aggressiveness in breast cancer. Peptidyl-prolyl cis-trans isomerase NIMA-interacting 1  (Pin1) is a unique enzyme that interacts with phosphorylated serine or threonine of a target protein and isomerizes the adjacent proline residue. Pin1 is prevalently overexpressed in human cancers, and its overexpression correlates with poor prognosis. Nuclear factor E2-related factor 2 (Nrf2) is a master regulator of cellular redox homeostasis. The sustained activation/accumulation of Nrf2 has been observed in many different types of human malignancies, conferring an advantage for growth and survival of cancer cells. The activated form of H-Ras (GTP-H-Ras) is highly overexpressed in human breast cancer tissues. In our present study, silencing of H-Ras decreased the invasiveness of MDA-MB-231 human breast cancer cells and abrogated the interaction between Pin1 and Nrf2 in these cells. Pin1 knockdown blocked the accumulation of Nrf2, thereby suppressing proliferation and clonogenicity of MCF10A-Ras human mammary epithelial cells. We found that Pin1 binds to Nrf2 which stabilizes this transcription factor by hampering proteasomal degradation. In conclusion, H-Ras activation in cooperation with the Pin1-Nrf2 complex represents a novel mechanism underlying breast cancer progression and constitutive activation of Nrf2 and can be exploited as a therapeutic target.

The HECT E3 Ligase E6AP/UBE3A as a Therapeutic Target in Cancer and Neurological Disorders

The HECT (Homologous to the E6-AP Carboxyl Terminus)-family protein E6AP (E6-associated protein), encoded by the UBE3A gene, is a multifaceted ubiquitin ligase that controls diverse signaling pathways involved in cancer and neurological disorders. The oncogenic role of E6AP in papillomavirus-induced cancers is well known, with its action to trigger p53 degradation in complex with the E6 viral oncoprotein. However, the roles of E6AP in non-viral cancers remain poorly defined. It is well established that loss-of-function alterations of the UBE3A gene cause Angelman syndrome, a severe neurodevelopmental disorder with autosomal dominant inheritance modified by genomic imprinting on chromosome 15q. Moreover, excess dosage of the UBE3A gene markedly increases the penetrance of autism spectrum disorders, suggesting that the expression level of UBE3A must be regulated tightly within a physiologically tolerated range during brain development. In this review, current the knowledge about the substrates of E6AP-mediated ubiquitination and their functions in cancer and neurological disorders is discussed, alongside with the ongoing efforts to pharmacologically modulate this ubiquitin ligase as a promising therapeutic target.

The peptidyl-prolyl cis/trans isomerase Pin1 interacts with three early regulatory proteins of human cytomegalovirus

Human cytomegalovirus (HCMV) is a ubiquitous human pathogen of high clinical relevance. Despite intensive research of virus-host interaction, crucial details still remain unknown. In this study, the role of the cellular peptidyl-prolyl cis/trans isomerase Pin1 during HCMV infection was investigated. Pin1 is able to recognize phosphorylated serine/threonine-proline motifs and regulates the structural conformation, stability and function of its substrates. Concerning HCMV replication, our recent studies revealed that Pin1 plays an important role in viral nuclear egress by contributing to the depletion of the nuclear lamina at distinct sites through the cis/trans conversion of lamin proteins. Here, novel data illustrate the HCMV-induced upregulation of Pin1 including various cell types being permissive, semi-permissive or non-permissive for productive HCMV replication. Addressing the question of functional impact, Pin1 knock-out (KO) did not show a measurable effect on viral protein expression, at least when assessed by Western blot analysis. Applying highly sensitive methods of qPCR and plaque titration, a pharmacological inhibition of Pin1 activity, however, led to a significant decrease of viral genome equivalents and production of infectious virus, respectively. When focusing on the identification of viral proteins interacting with Pin1 by various coimmunoprecipitation (CoIP) settings, we obtained positive signals for (i) the core nuclear egress complex protein pUL50, (ii) the viral mRNA export factor pUL69 and (iii) the viral DNA polymerase processivity factor pUL44. Confocal immunofluorescence analysis focusing on partial colocalization between Pin1 and the coexpressed viral proteins pUL50, pUL69 or pUL44, respectively, was consistent with the CoIP experiments. Mapping experiments, using transient expression constructs for a series of truncated protein versions and specific replacement mutants, revealed a complex pattern of Pin1 interaction with these three early regulatory HCMV proteins. Data suggest a combination of different modes of Pin1 interactions, involving both classical phosphorylation-dependent Pin1 binding motifs and additional phosphorylation-independent binding sites. Combined, these results support the concept that Pin1 may play an important role in several stages of HCMV infection, thus determining viral replicative efficiency.

Vulnerabilities in mIDH2 AML confer sensitivity to APL-like targeted combination therapy

Although targeted therapies have proven effective and even curative in human leukaemia, resistance often ensues. IDH enzymes are mutated in ~20% of human AML, with targeted therapies under clinical evaluation. We here characterize leukaemia evolution from mutant IDH2 (mIDH2)-dependence to independence identifying key targetable vulnerabilities of mIDH2 leukaemia that are retained during evolution and progression from early to late stages. Mechanistically, we find that mIDH2 leukaemia are metastable and vulnerable at two distinct levels. On the one hand, they are characterized by oxidative and genotoxic stress, in spite of increased 1-carbon metabolism and glutathione levels. On the other hand, mIDH2 leukaemia display inhibition of LSD1 and a resulting transcriptional signature of all-trans retinoic acid (ATRA) sensitization, in spite of a state of suppressed ATRA signalling due to increased levels of PIN1. We further identify GSH/ROS and PIN1/LSD1 as critical nodes for leukaemia maintenance and the combination of ATRA and arsenic trioxide (ATO) as a key therapeutic modality to target these vulnerabilities. Strikingly, we demonstrate that the combination of ATRA and ATO proves to be a powerfully synergistic and effective therapy in a number of mouse and human mIDH1/2 leukemic models. Thus, our findings pave the way towards the treatment of a sizable fraction of human AMLs through targeted APL-like combinatorial therapies.

Synthetic lethality of cytolytic HSV-1 in cancer cells with ATRX and PML deficiency

Cancers that utilize the alternative lengthening of telomeres (ALT) mechanism for telomere maintenance are often difficult to treat and have a poor prognosis. They are also commonly deficient for expression of ATRX protein, a repressor of ALT activity, and a component of promyelocytic leukemia nuclear bodies (PML NBs) that are required for intrinsic immunity to various viruses. Here, we asked whether ATRX deficiency creates a vulnerability in ALT cancer cells that could be exploited for therapeutic purposes. We showed in a range of cell types that a mutant herpes simplex virus type 1 (HSV-1) lacking ICP0, a protein that degrades PML NB components including ATRX, was ten- to one thousand-fold more effective in infecting ATRX-deficient cells than wild-type ATRX-expressing cells. Infection of co-cultured primary and ATRX-deficient cancer cells revealed that mutant HSV-1 selectively killed ATRX-deficient cells. Sensitivity to mutant HSV-1 infection also correlated inversely with PML protein levels, and we showed that ATRX upregulates PML expression at both the transcriptional and post-transcriptional levels. These data provide a basis for predicting, based on ATRX or PML levels, which tumors will respond to a selective oncolytic herpesvirus.

Summary: ATRX deficiency in cancer cells induces downregulation of PML, rendering the cells highly sensitive to lysis with ICP0-null mutant herpes simplex virus-1, with potential therapeutic applications.

Gears-In-Motion: The Interplay of WW and PPIase Domains in Pin1

Pin1 belongs to the family of the peptidyl-prolyl cis-trans isomerase (PPIase), which is a class of enzymes that catalyze the cis/trans isomerization of the Proline residue. Pin1 is unique and only catalyzes the phosphorylated Serine/Threonine-Proline (S/T-P) motifs of a subset of proteins. Since the discovery of Pin1 as a key protein in cell cycle regulation, it has been implicated in numerous diseases, ranging from cancer to neurodegenerative diseases. The main features of Pin1 lies in its two main domains: the WW (two conserved tryptophan) domain and the PPIase domain. Despite extensive studies trying to understand the mechanisms of Pin1 functions, how these two domains contribute to the biological roles of Pin1 in cellular signaling requires more investigations. The WW domain of Pin1 is known to have a higher affinity to its substrate than that of the PPIase domain. Yet, the WW domain seems to prefer the trans configuration of phosphorylated S/T-P motif, while the PPIase catalyzes the cis to trans isomerasion. Such contradicting information has generated much confusion as to the actual mechanism of Pin1 function. In addition, dynamic allostery has been suggested to be important for Pin1 function. Henceforth, in this review, we will be looking at the progress made in understanding the function of Pin1, and how these understandings can aid us in overcoming the diseases implicated by Pin1 such as cancer during drug development.

Generation of a cell-permeable cycloheptapeptidyl inhibitor against the peptidyl-prolyl isomerase Pin1

Cyclic peptides are capable of binding and modulating challenging drug targets including protein-protein interactions. However, their lack of membrane permeability prevents their application against intracellular targets. In this study, we show that it is possible to design a cell-permeable and biologically active cycloheptapeptide inhibitor against the intracellular enzyme peptidyl-prolyl isomerase Pin1 by integrating cell-penetrating and target-binding sequences.

Arsenic trioxide and angiotensin II have inhibitory effects on HERG protein expression: Evidence for the role of PML SUMOylation

The human ether-a-go-go-related gene (HERG) channel is a novel target for the treatment of drug-induced long QT syndrome, which causes lethal cardiotoxicity. This study is designed to explore the possible role of PML SUMOylation and its associated nuclear bodies (NBs) in the regulation of HERG protein expression. Both arsenic trioxide (ATO) and angiotensin II (Ang II) were able to significantly reduce HERG protein expression, while also increasing PML SUMOylation and accelerating the formation of PML-NBs. Pre-exposure of cardiomyocytes to a SUMOylation chemical inhibitor, ginkgolic acid, or the silencing of UBC9 suppressed PML SUMOylation, subsequently preventing the downregulation of HERG induced by ATO or Ang II. Conversely, knockdown of RNF4 led to a remarkable increase in PML SUMOylation and the function of PML-NBs, further promoting ATO- or Ang II-induced HERG protein downregulation. Mechanistically, an increase in PML SUMOylation by ATO or Ang II dramatically enhanced the formation of PML and Pin1 complexes in PML-NBs, leading to the upregulation of TGF-β1 protein, eventually inhibiting HERG expression through activation of protein kinase A. The present work uncovered a novel molecular mechanism underlying HERG protein expression and indicated that PML SUMOylation is a critical step in the development of drug-acquired arrhythmia.

Pharmacological inhibition of SUMO-1 with ginkgolic acid alleviates cardiac fibrosis induced by myocardial infarction in mice

BACKGROUND AND PURPOSE

Protein modification by small ubiquitin-like modifier (SUMO) plays a critical role in the pathogenesis of heart diseases. The present study was designed to determine whether ginkgolic acid (GA) as a SUMO-1 inhibitor exerts an inhibitory effect on cardiac fibrosis induced by myocardial infarction (MI).

EXPERIMENTAL APPROACH

GA was delivered by osmotic pumps in MI mice. Masson staining, electron microscopy (EM) and echocardiography were used to assess cardiac fibrosis, ultrastructure and function. Expression of SUMO-1, PML, TGF-β1 and Pin1 was measured with Western blot or Real-time PCR. Collagen content, cell viability and myofibroblast transformation were measured in neonatal mouse cardiac fibroblasts (NMCFs). Promyelocytic leukemia (PML) protein was over-expressed by plasmid transfection.

KEY RESULTS

GA improved cardiac fibrosis and dysfunction, and decreased SUMO-1 expression in MI mice. GA (>20 μM) inhibited NMCF viability in a dose-dependent manner. Nontoxic GA (10 μM) restrained angiotensin II (Ang II)-induced myofibroblast transformation and collagen production. GA also inhibited expression of TGF-β1 mRNA and protein in vitro and in vivo. GA suppressed PML SUMOylation and PML nuclear body (PML-NB) organization, and disrupted expression and recruitment of Pin1 (a positive regulator of TGF-β1 mRNA), whereas over-expression of PML reversed that.

CONCLUSIONS AND IMPLICATIONS

Inhibition of SUMO-1 by GA alleviated MI-induced heart dysfunction and fibrosis, and the SUMOylated PML/Pin1/TGF-β1 pathway is crucial for GA-inhibited cardiac fibrosis.

PML: Regulation and multifaceted function beyond tumor suppression

Promyelocytic leukemia protein (PML) was originally identified as a fusion partner of retinoic acid receptor alpha in acute promyelocytic leukemia patients with the (15;17) chromosomal translocation, giving rise to PML–RARα and RARα–PML fusion proteins. A body of evidence indicated that PML possesses tumor suppressing activity by regulating apoptosis, cell cycle, senescence and DNA damage responses. PML is enriched in discrete nuclear substructures in mammalian cells with 0.2–1 μm diameter in size, referred to as alternately Kremer bodies, nuclear domain 10, PML oncogenic domains or PML nuclear bodies (NBs). Dysregulation of PML NB formation results in altered transcriptional regulation, protein modification, apoptosis and cellular senescence. In addition to PML NBs, PML is also present in nucleoplasm and cytoplasmic compartments, including the endoplasmic reticulum and mitochondria-associated membranes. The role of PML in tumor suppression has been extensively studied but increasing evidence indicates that PML also plays versatile roles in stem cell renewal, metabolism, inflammatory responses, neural function, mammary development and angiogenesis. In this review, we will briefly describe the known PML regulation and function and include new findings.

Manipulating PML SUMOylation via Silencing UBC9 and RNF4 Regulates Cardiac Fibrosis

The promyelocytic leukemia protein (PML) is essential in the assembly of dynamic subnuclear structures called PML nuclear bodies (PML-NBs), which are involved in regulating diverse cellular functions. However, the possibility of PML being involved in cardiac disease has not been examined. In mice undergoing transverse aortic constriction (TAC) and arsenic trioxide (ATO) injection, transforming growth factor β1 (TGF-β1) was upregulated along with dynamic alteration of PML SUMOylation. In cultured neonatal mouse cardiac fibroblasts (NMCFs), ATO, angiotensin II (Ang II), and fetal bovine serum (FBS) significantly triggered PML SUMOylation and the assembly of PML-NBs. Inhibition of SUMOylated PML by silencing UBC9, the unique SUMO E2-conjugating enzyme, reduced the development of cardiac fibrosis and partially improved cardiac function in TAC mice. In contrast, enhancing SUMOylated PML accumulation, by silencing RNF4, a poly-SUMO-specific E3 ubiquitin ligase, accelerated the induction of cardiac fibrosis and promoted cardiac function injury. PML colocalized with Pin1 (a positive regulator for TGF-β1 mRNA expression in PML-NBs) and increased TGF-β1 activity. These findings suggest that the UBC9/PML/RNF4 axis plays a critical role as an important SUMO pathway in cardiac fibrosis. Modulating the protein levels of the pathway provides an attractive therapeutic target for the treatment of cardiac fibrosis and heart failure.

Neighboring phosphoSer-Pro motifs in the undefined domain of IRAK1 impart bivalent advantage for Pin1 binding

The peptidyl prolyl isomerase Pin1 has two domains that are considered to be its binding (WW) and catalytic (PPIase) domains, both of which interact with phosphorylated Ser/Thr-Pro motifs. This shared specificity might influence substrate selection, as many known Pin1 substrates have multiple sequentially close phosphoSer/Thr-Pro motifs, including the protein interleukin-1 receptor-associated kinase-1 (IRAK1). The IRAK1 undefined domain (UD) contains two sets of such neighboring motifs (Ser131/Ser144 and Ser163/Ser173), suggesting possible bivalent interactions with Pin1. Using a series of NMR titrations with 15N-labeled full-length Pin1 (Pin1-FL), PPIase, or WW domain and phosphopeptides representing the Ser131/Ser144 and Ser163/Ser173 regions of IRAK1-UD, bivalent interactions were investigated. Binding studies using singly phosphorylated peptides showed that individual motifs displayed weak affinities (> 100 μm) for Pin1-FL and each isolated domain. Analysis of dually phosphorylated peptides binding to Pin1-FL showed that inclusion of bivalent states was necessary to fit the data. The resulting complex model and fitted parameters were applied to predict the impact of bivalent states at low micromolar concentrations, demonstrating significant affinity enhancement for both dually phosphorylated peptides (3.5 and 24 μm for peptides based on the Ser131/Ser144 and Ser163/Ser173 regions, respectively). The complementary technique biolayer interferometry confirmed the predicted affinity enhancement for a representative set of singly and dually phosphorylated Ser131/Ser144 peptides at low micromolar concentrations, validating model predictions. These studies provide novel insights regarding the complexity of interactions between Pin1 and activated IRAK1, and more broadly suggest that phosphorylation of neighboring Ser/Thr-Pro motifs in proteins might provide competitive advantage at cellular concentrations for engaging with Pin1.

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.

Estrogen receptor alpha transcriptionally activates casein kinase 2 alpha: A pivotal regulator of promyelocytic leukaemia protein (PML) and AKT in oncogenesis

Protein kinase CK2α is frequently upregulated in different cancers. Alteration of CK2α expression and its activity is sufficient to induce dramatic changes in cell fate. It has been established that CK2α induces oncogenesis through modulation of both AKT and PML. CK2α has been found to be overexpressed in breast cancer. In contrary, statistical reports have shown low level of PML. However, the regulation of CK2α gene expression is not fully understood. In the current study, we found that CK2α and activated AKT positively correlate with ERα, whereas PML follows an inverse correlation in human breast cancer tissues. Modulation of ERα signalling leads to recruitment of activated ERα on the ERE sites of CK2α promoter, resulting in CK2α transactivation. Furthermore, the DMBA induced tumours in rat showed elevated level of active CK2α. Consequently it mediates enhancement of AKT activity and PML degradation, resulting in increased cellular proliferation, migration and metastasis. Syngeneic ERα overexpressing stable mouse 4T1 cells produce larger primary tumours and metastatic lung nodules in mice, corroborating our in vitro findings. Hence, our study provides a novel route of ERα dependent CK2α mediated oncogenesis that causes upregulation and consequent AKT activation along with degradation of tumour suppressor PML.

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.

Exon-centric regulation of ATM expression is population-dependent and amenable to antisense modification by pseudoexon targeting

ATM is an important cancer susceptibility gene that encodes a critical apical kinase of the DNA damage response (DDR) pathway. We show that a key nonsense-mediated RNA decay switch exon (NSE) in ATM is repressed by U2AF, PUF60 and hnRNPA1. The NSE activation was haplotype-specific and was most promoted by cytosine at rs609621 in the NSE 3' splice-site (3'ss), which is predominant in high cancer risk populations. NSE levels were deregulated in leukemias and were influenced by the identity of U2AF35 residue 34. We also identify splice-switching oligonucleotides (SSOs) that exploit competition of adjacent pseudoexons to modulate NSE levels. The U2AF-regulated exon usage in the ATM signalling pathway was centred on the MRN/ATM-CHEK2-CDC25-cdc2/cyclin-B axis and preferentially involved transcripts implicated in cancer-associated gene fusions and chromosomal translocations. These results reveal important links between 3'ss control and ATM-dependent responses to double-strand DNA breaks, demonstrate functional plasticity of intronic variants and illustrate versatility of intronic SSOs that target pseudo-3'ss to modify gene expression.

The function, regulation and therapeutic implications of the tumor suppressor protein, PML

The tumor suppressor protein, promyelocytic leukemia protein (PML), was originally identified in acute promyelocytic leukemia due to a chromosomal translocation between chromosomes 15 and 17. PML is the core component of subnuclear structures called PML nuclear bodies (PML-NBs), which are disrupted in acute promyelocytic leukemia cells. PML plays important roles in cell cycle regulation, survival and apoptosis, and inactivation or down-regulation of PML is frequently found in cancer cells. More than 120 proteins have been experimentally identified to physically associate with PML, and most of them either transiently or constitutively co-localize with PML-NBs. These interactions are associated with many cellular processes, including cell cycle arrest, apoptosis, senescence, transcriptional regulation, DNA repair and intermediary metabolism. Importantly, PML inactivation in cancer cells can occur at the transcriptional-, translational- or post-translational- levels. However, only a few somatic mutations have been found in cancer cells. A better understanding of its regulation and its role in tumor suppression will provide potential therapeutic opportunities. In this review, we discuss the role of PML in multiple tumor suppression pathways and summarize the players and stimuli that control PML protein expression or subcellular distribution.

Translational control of PML contributes to TNFα-induced apoptosis of MCF7 breast cancer cells and decreased angiogenesis in HUVECs

The tumor suppressor protein promyelocytic leukemia (PML) is a key regulator of inflammatory responses and tumorigenesis and functions through the assembly of subnuclear structures known as PML nuclear bodies (NBs). The inflammation-related cytokine tumor necrosis factor-α (TNFα) is known to induce PML protein accumulation and PML NB formation that mediate TNFα-induced cell death in cancer cells and inhibition of migration and capillary tube formation in endothelial cells (ECs). In this study, we uncover a novel mechanism of PML gene regulation in which the p38 MAPK and its downstream kinase MAP kinase-activated protein kinase 1 (MNK1) mediate TNFα-induced PML protein accumulation and PML NB formation. The mechanism includes the presence of an internal ribosome entry site (IRES) found within the well-conserved 100 nucleotides upstream of the PML initiation codon. The activity of the PML IRES is induced by TNFα in a manner that involves MNK1 activation. It is proposed that the p38–MNK1–PML network regulates TNFα-induced apoptosis in breast cancer cells and TNFα-mediated inhibition of migration and capillary tube formation in ECs.

A Selective, Cell-Permeable Nonphosphorylated Bicyclic Peptidyl Inhibitor against Peptidyl-Prolyl Isomerase Pin1

Pin1 regulates the levels and functions of phosphoproteins by catalyzing phosphorylation-dependent cis/trans isomerization of peptidyl-prolyl bonds. Previous Pin1 inhibitors contained phosphoamino acids, which are metabolically unstable and have poor membrane permeability. In this work, we report a cell-permeable and metabolically stable nonphosphorylated bicyclic peptide as a potent and selective Pin1 inhibitor, which inhibited the intracellular Pin1 activity in cultured mammalian cells but had little effect on other isomerases such as Pin4, FKBP12, or cyclophilin A.

Stress responses from the endoplasmic reticulum in cancer

The endoplasmic reticulum (ER) is a dynamic organelle that is essential for multiple cellular functions. During cellular stress conditions, including nutrient deprivation and dysregulation of protein synthesis, unfolded/misfolded proteins accumulate in the ER lumen, resulting in activation of the unfolded protein response (UPR). The UPR also contributes to the regulation of various intracellular signaling pathways such as calcium signaling and lipid signaling. More recently, the mitochondria-associated ER membrane (MAM), which is a site of close contact between the ER and mitochondria, has been shown to function as a platform for various intracellular stress responses including apoptotic signaling, inflammatory signaling, the autophagic response, and the UPR. Interestingly, in cancer, these signaling pathways from the ER are often dysregulated, contributing to cancer cell metabolism. Thus, the signaling pathway from the ER may be a novel therapeutic target for various cancers. In this review, we discuss recent research on the roles of stress responses from the ER, including the MAM.

Interaction of p53 with prolyl isomerases: Healthy and unhealthy relationships

BACKGROUND

The p53 protein family, comprising p53, p63 and p73, is primarily involved in preserving genome integrity and preventing tumor onset, and also affects a range of physiological processes. Signal-dependent modifications of its members and of other pathway components provide cells with a sophisticated code to transduce a variety of stress signaling into appropriate responses. TP53 mutations are highly frequent in cancer and lead to the expression of mutant p53 proteins that are endowed with oncogenic activities and sensitive to stress signaling.

SCOPE OF REVIEW

p53 family proteins have unique structural and functional plasticity, and here we discuss the relevance of prolyl-isomerization to actively shape these features.

MAJOR CONCLUSIONS

The anti-proliferative functions of the p53 family are carefully activated upon severe stress and this involves the interaction with prolyl-isomerases. In particular, stress-induced stabilization of p53, activation of its transcriptional control over arrest- and cell death-related target genes and of its mitochondrial apoptotic function, as well as certain p63 and p73 functions, all require phosphorylation of specific S/T-P motifs and their subsequent isomerization by the prolyl-isomerase Pin1. While these functions of p53 counteract tumorigenesis, under some circumstances their activation by prolyl-isomerases may have negative repercussions (e.g. tissue damage induced by anticancer therapies and ischemia-reperfusion, neurodegeneration). Moreover, elevated Pin1 levels in tumor cells may transduce deregulated phosphorylation signaling into activation of mutant p53 oncogenic functions.

GENERAL SIGNIFICANCE

The complex repertoire of biological outcomes induced by p53 finds mechanistic explanations, at least in part, in the association between prolyl-isomerases and the p53 pathway. This article is part of a Special Issue entitled Proline-directed foldases: Cell signaling catalysts and drug targets.

Pin1-based diagnostic and therapeutic strategies for breast cancer

Pin1 is the only known cis-to-trans isomerase that recognizes the phosphorylated pThr/pSer-Pro motifs in many signaling molecules, playing unique roles in the pathogenesis of breast cancer. First, Pin1 is prevalently over-expressed in kinds of breast cancer cell lines and tissues, such as MDA-MB-231 cell, MCF-7 cell, Her2+, ERα+, and basal-like breast cancer subtypes. Second, Pin1 amplifies many oncogenic signaling pathways, inhibits multiple tumor suppressors, promotes the angiogenesis and metastasis of breast cancer cells, and enhances the resistance of breast cancer cells to anti-tumor medicines. Third, inhibiting Pin1 blocks most of these detrimental effects in a great number of breast cancer cell lines. These findings suggest Pin1 as a promising diagnostic biomarker as well as an efficient therapeutic target for breast cancer. It is strongly expected that a Pin1-positive subtype of breast cancers should be extremely concerned and that the therapeutic efficacy of Pin1 inhibitors on breast cancer patients should be evaluated as soon as possible. Nonetheless, Pin1-based therapeutic strategies for breast cancer still deserve some debates. Hence, we give the predictions of several important issues, such as application precondition, side effects, and personalized medication, when Pin1 inhibitors are used in the breast cancer therapy. These proposals are meaningful for the further development of Pin1-based diagnostic and therapeutic strategies in order to conquer breast cancer.

A Phospho-SIM in the Antiviral Protein PML is Required for Its Recruitment to HSV-1 Genomes

Herpes simplex virus type 1 (HSV-1) is a significant human pathogen that infects a large portion of the human population. Cells deploy a variety of defenses to limit the extent to which the virus can replicate. One such factor is the promyelocytic leukemia (PML) protein, the nucleating and organizing factor of nuclear domain 10 (ND10). PML responds to a number of stimuli and is implicated in intrinsic and innate cellular antiviral defenses against HSV-1. While the role of PML in a number of cellular pathways is controlled by post-translational modifications, the effects of phosphorylation on its antiviral activity toward HSV-1 have been largely unexplored. Consequently, we mapped phosphorylation sites on PML, mutated these and other known phosphorylation sites on PML isoform I (PML-I), and examined their effects on a number of PML's activities. Our results show that phosphorylation at most sites on PML-I is dispensable for the formation of ND10s and colocalization between PML-I and the HSV-1 regulatory protein, ICP0, which antagonizes PML-I function. However, inhibiting phosphorylation at sites near the SUMO-interaction motif (SIM) of PML-I impairs its ability to respond to HSV-1 infection. Overall, our data suggest that PML phosphorylation regulates its antiviral activity against HSV-1.

SCP phosphatases suppress renal cell carcinoma by stabilizing PML and inhibiting mTOR/HIF signaling

The tumor-suppressor protein promyelocytic leukemia (PML) is aberrantly degraded in multiple types of human cancers through mechanisms that are incompletely understood. Here, we show that the phosphatase SCP1 and its isoforms SCP2/3 dephosphorylate PML at S518, thereby blocking PML ubiquitination and degradation mediated by the prolyl isomerase Pin1 and the ubiquitin ligase KLHL20. Clinically, SCP1 and SCP3 are downregulated in clear cell renal cell carcinoma (ccRCC) and these events correlated with PMLS518 phosphorylation, PML turnover, and high-grade tumors. Restoring SCP1-mediated PML stabilization not only inhibited malignant features of ccRCC, including proliferation, migration, invasion, tumor growth, and tumor angiogenesis, but also suppressed the mTOR-HIF pathway. Furthermore, blocking PML degradation in ccRCC by SCP1 overexpression or Pin1 inhibition enhanced the tumor-suppressive effects of the mTOR inhibitor temsirolimus. Taken together, our results define a novel pathway of PML degradation in ccRCC that involves SCP downregulation, revealing contributions of this pathway to ccRCC progression and offering a mechanistic rationale for combination therapies that jointly target PML degradation and mTOR inhibition for ccRCC treatment.

Regulation of mitochondrial apoptosis by Pin1 in cancer and neurodegeneration

Mitochondria are sensitive and efficient organelles that regulate essential biological processes including: energy metabolism, decoding and transduction of intracellular signals, and balance between cell death and survival. Of note, dysfunctions in mitochondrial physiology are a general hallmark of cancer cells, leading to transformation-related features such as altered cellular metabolism, survival under stress conditions and reduced apoptotic response to chemotherapy. Mitochondrial apoptosis is a finely regulated process that derives from activation of multiple signaling networks. A crucial biochemical requirement for transducing pro-apoptotic stimuli is represented by kinase-dependent phosphorylation cascades. In this context a pivotal role is played by the prolyl-isomerase Pin1, which translates Ser/Thr-Pro phosphorylation into conformational changes able to modify the activities of its substrates. In this review we will discuss the impact of Pin1 in regulating various aspects of apoptosis in different biological contexts with particular emphasis on cancer and neurodegenerative diseases.

Prolyl isomerase Pin1 in cancer

Proline-directed phosphorylation is a posttranslational modification that is instrumental in regulating signaling from the plasma membrane to the nucleus, and its dysregulation contributes to cancer development. Protein interacting with never in mitosis A1 (Pin1), which is overexpressed in many types of cancer, isomerizes specific phosphorylated Ser/Thr-Pro bonds in many substrate proteins, including glycolytic enzyme, protein kinases, protein phosphatases, methyltransferase, lipid kinase, ubiquitin E3 ligase, DNA endonuclease, RNA polymerase, and transcription activators and regulators. This Pin1-mediated isomerization alters the structures and activities of these proteins, thereby regulating cell metabolism, cell mobility, cell cycle progression, cell proliferation, cell survival, apoptosis and tumor development.

Deacetylation of the tumor suppressor protein PML regulates hydrogen peroxide-induced cell death

The promyelocytic leukemia protein (PML) is a tumor suppressor that is expressed at a low level in various cancers. Although post-translational modifications including SUMOylation, phosphorylation, and ubiquitination have been found to regulate the stability or activity of PML, little is known about the role of its acetylation in the control of cell survival. Here we demonstrate that acetylation of lysine 487 (K487) and SUMO1 conjugation of K490 at PML protein are mutually exclusive. We found that hydrogen peroxide (H₂O₂) promotes PML deacetylation and identified SIRT1 and SIRT5 as PML deacetylases. Both SIRT1 and SIRT5 are required for H₂O₂-mediated deacetylation of PML and accumulation of nuclear PML protein in HeLa cells. Knockdown of SIRT1 reduces the number of H₂O₂-induced PML-nuclear bodies (NBs) and increases the survival of HeLa cells. Ectopic expression of wild-type PML but not the K487R mutant rescues H₂O₂-induced cell death in SIRT1 knockdown cells. Furthermore, ectopic expression of wild-type SIRT5 but not a catalytic defective mutant can also restore H₂O₂-induced cell death in SIRT1 knockdown cells. Taken together, our findings reveal a novel regulatory mechanism in which SIRT1/SIRT5-mediated PML deacetylation plays a role in the regulation of cancer cell survival.

Cell-permeable bicyclic peptide inhibitors against intracellular proteins

Cyclic peptides have great potential as therapeutic agents and research tools but are generally impermeable to the cell membrane. Fusion of cyclic peptides with a cyclic cell-penetrating peptide produces bicyclic peptides that are cell-permeable and retain the ability to recognize specific intracellular targets. Application of this strategy to protein tyrosine phosphatase 1B and a peptidyl-prolyl cis-trans isomerase (Pin1) isomerase resulted in potent, selective, proteolytically stable, and biologically active inhibitors against the enzymes.

Control of antioxidative response by the tumor suppressor protein PML through regulating Nrf2 activity

PML plays a critical role in the maintenance of ROS homeostasis via a unique mechanism in which PML functions as an oxidative sensor to regulate the expression of antioxidant genes through Nrf2. PML is also indispensable for sulforaphane-mediated ROS generation, Nrf2 activation, antiproliferation, antimigration, and antiangiogenesis.

Oxidative stress is a consequence of an imbalance between reactive oxygen species (ROS) production and the ability of the cytoprotective system to detoxify the reactive intermediates. The tumor suppressor promyelocytic leukemia protein (PML) functions as a stress sensor. Loss of PML results in impaired mitochondrial complex II activity, increased ROS, and subsequent activation of nuclear factor erythroid 2–related factor 2 (Nrf2) antioxidative pathway. We also demonstrate that sulforaphane (SFN), an antioxidant, regulates Nrf2 activity by controlling abundance and subcellular distribution of PML and that PML is essential for SFN-mediated ROS increase, Nrf2 activation, antiproliferation, antimigration, and antiangiogenesis. Taking the results together, we have uncovered a novel antioxidative mechanism by which PML regulates cellular oxidant homeostasis by controlling complex II integrity and Nrf2 activity and identified PML as an indispensable mediator of SFN activity.

Inhibition of Sp1 functions by its sequestration into PML nuclear bodies

Promyelocytic leukemia nuclear bodies (PML NBs) are comprised of PML and a striking variety of its associated proteins. Various cellular functions have been attributed to PML NBs, including the regulation of gene expression. We report here that induced expression of PML recruits Sp1 into PML NBs, leading to the reduction of Sp1 transactivation function. Specifically, Chromatin immunoprecipitation (ChIP) assay demonstrated that induced expression of PML significantly diminishes the amount of Sp1 binding to its target gene promoter, immunofluorescence staining showed dramatic increase in the co-localization between PML and Sp1 upon induction of PML expression, moreover, PML and Sp1 co-fractionated in the core nuclear matrix. Our study further showed that PML promotes SUMOylation of Sp1 in a RING-motif-dependent manner, SUMOylation of Sp1 facilitates physical interaction between Sp1 and PML and recruitment of Sp1 into the PML NBs, the SUMO binding motif of PML was also important for its interaction with Sp1. The results of this study demonstrate a novel mechanism by which PML regulates gene expression through sequestration of the transcription factor into PML NBs.

The epigenetic regulator UHRF1 promotes ubiquitination-mediated degradation of the tumor-suppressor protein promyelocytic leukemia protein

The promyelocytic leukemia (PML) protein is a tumor suppressor originally identified in acute promyelocytic leukemia and implicated in tumorigenesis in multiple forms of cancer. Here, we demonstrate that the PML protein undergoes ubiquitination-mediated degradation facilitated by an E3 ligase UHRF1 (ubiquitin-like with PHD and RING finger domains 1), which is commonly upregulated in various human malignancies. Furthermore, UHRF1 negatively regulates PML protein accumulation in primary human umbilical vein endothelial cells (HUVECs), HEK 293 cells and cancer cells. Knockdown of UHRF1 upregulates whereas ectopic overexpression of UHRF1 downregulates protein abundance of endogenous or exogenous PML, doing so through its binding to the N-terminus of PML. Overexpression of wild-type UHRF1 shortens PML protein half-life and promotes PML polyubiquitination, whereas deletion of the RING domain or coexpression of the dominant-negative E2 ubiquitin-conjugating enzyme, E2D2, attenuates this modification to PML. Finally, knockdown of UHRF1 prolongs PML half-life and increases PML protein accumulation, yet inhibits cell migration and in vitro capillary tube formation, whereas co-knockdown of PML compromises this inhibitory effect. These findings suggest that UHRF1 promotes the turnover of PML protein, and thus targeting UHRF1 to restore PML-mediated tumor suppression represents a promising, novel, anticancer strategy.

β-Transducin repeat-containing protein 1 (β-TrCP1)-mediated silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) protein degradation promotes tumor necrosis factor α (TNFα)-induced inflammatory gene expression

Cytokine modulation of the endothelium is considered an important contributor to the inflammation response. TNFα is an early response gene during the initiation of inflammation. However, the detailed mechanism by which TNFα induces proinflammatory gene expression is not completely understood. In this report, we demonstrate that silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) represses the expression of a subset of TNFα target genes in human umbilical vein endothelial cells. Upon TNFα stimulation, we observed an increase in the E3 ubiquitin ligase β-TrCP1 and a decrease in SMRT protein levels. We show that β-TrCP1 interacts with SMRT in a phosphorylation-independent manner and cooperates with the E2 ubiquitin-conjugating enzyme E2D2 to promote ubiquitination-dependent SMRT degradation. Knockdown of β-TrCP1 increases SMRT protein accumulation, increases SMRT association with its targeted promoters, and decreases SMRT target gene expression. Taken together, our results support a model in which TNFα-induced β-TrCP1 accumulation promotes SMRT degradation and the subsequent induction of proinflammatory gene expression.

Is PML a Tumor Suppressor?

The role of the promyelocytic leukemia (PML) protein has been widely tested in many different contexts, as attested by the hundreds of papers present in the literature. In most of these studies, PML is regarded as a tumor suppressor, a notion on the whole accepted by the scientific community. In this review, we examine how the concept of tumor-suppressor gene has evolved until now and then systematically assess whether this assumption for PML is supported by unambiguous experimental evidence.

PML, SUMOylation, and Senescence

Since its discovery, 25 years ago, promyelocytic leukemia (PML) has been an enigma. Implicated in the oncogenic PML/RARA fusion, forming elusive intranuclear domains, triggering cell death or senescence, controlled by and perhaps controlling SUMOylation… there are multiple PML-related issues. Here we review the reciprocal interactions between PML, senescence, and SUMOylation, notably in the context of cellular transformation.

Pin1 inhibitors: Pitfalls, progress and cellular pharmacology

Compelling data supports the hypothesis that Pin1 inhibitors will be useful for the therapy of cancer: Pin1 deficient mice resist the induction of breast cancers normally evoked by expression of MMTV-driven Ras or Erb2 alleles. While Pin1 poses challenges for drug discovery, several groups have identified potent antagonists by structure based drug design, significant progress has been made designing peptidic inhibitors and a number of natural products have been found that blockade Pin1, notably epigallocatchechin gallate (EGCG), a major flavonoid in green tea. Here we critically discuss the modes of action and likely specificity of these compounds, concluding that a suitable chemical biology tool for probing the function of Pin1 has yet to be found. We conclude by outlining some open questions regarding the target validation of Pin1 and the prospects for identification of improved inhibitors in the future.