The prolyl-isomerase Pin1 activates the mitochondrial death program of p53

Peritoneal expression of Matrilysin helps identify early post-operative recurrence of colorectal cancer

Recurrence of colorectal cancer (CRC) following a potentially curative resection is a challenging clinical problem. Matrix metalloproteinase-7 (MMP-7) is over-expressed by CRC cells and supposed to play a major role in CRC cell diffusion and metastasis. MMP-7 RNA expression was assessed by real-time PCR using specific primers in peritoneal washing fluid obtained during surgical procedure. After surgery, patients underwent a regular follow up for assessing recurrence. transcripts for MMP-7 were detected in 31/57 samples (54%). Patients were followed-up (range 20-48 months) for recurrence prevention. Recurrence was diagnosed in 6 out of 55 patients (11%) and two patients eventually died because of this. Notably, all the six patients who had relapsed were positive for MMP-7. Sensitivity and specificity of the test were 100% and 49% respectively. Data from patients have also been corroborated by computational approaches. Public available coloncarcinoma datasets have been employed to confirm MMP7 clinical impact on the disease. Interestingly, MMP-7 expression appeared correlated to Tgfb-1, and correlation of the two factors represented a poor prognostic factor. This study proposes positivity of MMP-7 in peritoneal cavity as a novel biomarker for predicting disease recurrence in patients with CRC.

MDM4/HIPK2/p53 cytoplasmic assembly uncovers coordinated repression of molecules with anti-apoptotic activity during early DNA damage response

The p53 inhibitor, MDM4 (MDMX) is a cytoplasmic protein with p53-activating function under DNA damage conditions. Particularly, MDM4 promotes phosphorylation of p53 at Ser46, a modification that precedes different p53 activities. We investigated the mechanism by which MDM4 promotes this p53 modification and its consequences in untransformed mammary epithelial cells and tissues. In response to severe DNA damage, MDM4 stimulates p53Ser46^P by binding and stabilizing serine–threonine kinase HIPK2. Under these conditions, the p53-inhibitory complex, MDM4/MDM2, dissociates and this allows MDM4 to promote p53/HIPK2 functional interaction. Comparative proteomic analysis of DNA damage-treated cells versus -untreated cells evidenced a diffuse downregulation of proteins with anti-apoptotic activity, some of which were targets of p53Ser46^P/HIPK2 repressive activity. Importantly, MDM4 depletion abolishes the downregulation of these proteins indicating the requirement of MDM4 to promote p53-mediated transcriptional repression. Consistently, MDM4-mediated HIPK2/p53 activation precedes HIPK2/p53 nuclear translocation and activity. Noteworthy, repression of these proteins was evident also in mammary glands of mice subjected to γ-irradiation and was significantly enhanced in transgenic mice overexpressing MDM4. This study evidences the flexibility of MDM2/MDM4 heterodimer, which allows the development of a positive activity of cytoplasmic MDM4 towards p53-mediated transcriptional function. Noteworthy, this activity uncovers coordinated repression of molecules with shared anti-apoptotic function which precedes active cell apoptosis and that are frequently overexpressed and/or markers of tumour phenotype in human cancer.

The Tyrosine Kinase c-Abl Promotes Homeodomain-interacting Protein Kinase 2 (HIPK2) Accumulation and Activation in Response to DNA Damage

The non-receptor tyrosine kinase c-Abl is activated in response to DNA damage and induces p73-dependent apoptosis. Here, we investigated c-Abl regulation of the homeodomain-interacting protein kinase 2 (HIPK2), an important regulator of p53-dependent apoptosis. c-Abl phosphorylated HIPK2 at several sites, and phosphorylation by c-Abl protected HIPK2 from degradation mediated by the ubiquitin E3 ligase Siah-1. c-Abl and HIPK2 synergized in activating p53 on apoptotic promoters in a reporter assay, and c-Abl was required for endogenous HIPK2 accumulation and phosphorylation of p53 at Ser(46) in response to DNA damage by γ- and UV radiation. Accumulation of HIPK2 in nuclear speckles and association with promyelocytic leukemia protein (PML) in response to DNA damage were also dependent on c-Abl activity. At high cell density, the Hippo pathway inhibits DNA damage-induced c-Abl activation. Under this condition, DNA damage-induced HIPK2 accumulation, phosphorylation of p53 at Ser(46), and apoptosis were attenuated. These data demonstrate a new mechanism for the induction of DNA damage-induced apoptosis by c-Abl and illustrate network interactions between serine/threonine and tyrosine kinases that dictate cell fate.

Impairment of the Pin1/E2F1 axis in the anti-proliferative effect of bortezomib in hepatocellular carcinoma cells

BACKGROUND

The modest efficacy of available therapies for Hepatocellular carcinoma (HCC) indicates the need to develop novel therapeutic approaches. For the proteasome inhibitor Bortezomib (BZB), potentially attractive for HCC treatment, the mechanism of action is largely unknown. The BZB effect on E2Fs and the E2Fs control on the peptidylproline cis-trans isomerase (Pin1), prompted us to explore the BZB effect on the Pin1-E2F1 axis.

METHODS

The tumorigenic cell line HuH7 together with the non-tumorigenic cells IHH and the human pluripotent stem cell derived hepatocytes (hPSC-H), were used as cellular models of HCC and normal liver cells, respectively.

RESULTS

BZB reduces HuH7 growth as shown by cell counting, cell vitality test and cell cycle analysis; this is paralleled by the decrease of Pin1, E2F1, cyclin A2 and of the hyper-phosphorylated pRB. Pin1-E2F1 axis impairment justifies the anti-proliferative effect since Pin-E2F1 depletion decreases HuH7 growth while the over-expression rescues BZB-induced inhibition of proliferation. Moreover, Pin1-E2F1 promote HuH7 growth via the up-regulation of cyclin D1, cyclin E, cyclin A2, E2F2 and in part E2F3. Finally, in the control cells IHH and hPSC-H, BZB effect on cell vitality is not irrelevant, a fact correlated to the cellular proliferation rate. Thus, BZB effect on healthy liver tissue may not be entirely negligible hence caution should be exercised in its use in liver regeneration processes.

CONCLUSION

For the first time we prove the functional involvement of the Pin1-E2F1 axis in the anti-proliferative effect of BZB indicating Pin1-E2F as an attractive target to control HCC cell growth.

Pin1: Intimate involvement with the regulatory protein kinase networks in the global phosphorylation landscape

BACKGROUND

Protein phosphorylation is a universal regulatory mechanism that involves an extensive network of protein kinases. The discovery of the phosphorylation-dependent peptidyl-prolyl isomerase Pin1 added an additional layer of complexity to these regulatory networks.

SCOPE OF REVIEW

We have evaluated interactions between Pin1 and the regulatory kinome and proline-dependent phosphoproteome taking into consideration findings from targeted studies as well as data that has emerged from systematic phosphoproteomic workflows and from curated protein interaction databases.

MAJOR CONCLUSIONS

The relationship between Pin1 and the regulatory protein kinase networks is not restricted simply to the recognition of proteins that are substrates for proline-directed kinases. In this respect, Pin1 itself is phosphorylated in cells by protein kinases that modulate its functional properties. Furthermore, the phosphorylation-dependent targets of Pin1 include a number of protein kinases as well as other enzymes such as phosphatases and regulatory subunits of kinases that modulate the actions of protein kinases.

GENERAL SIGNIFICANCE

As a result of its interactions with numerous protein kinases and their substrates, as well as itself being a target for phosphorylation, Pin1 has an intricate relationship with the regulatory protein kinase and phosphoproteomic networks that orchestrate complex cellular processes and respond to environmental cues. This article is part of a Special Issue entitled Proline-directed Foldases: Cell Signaling Catalysts and Drug Targets.

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.

p53 at the endoplasmic reticulum regulates apoptosis in a Ca2+-dependent manner

The tumor suppressor p53 is a key protein in preventing cell transformation and tumor progression. Activated by a variety of stimuli, p53 regulates cell-cycle arrest and apoptosis. Along with its well-documented transcriptional control over cell-death programs within the nucleus, p53 exerts crucial although still poorly understood functions in the cytoplasm, directly modulating the apoptotic response at the mitochondrial level. Calcium (Ca(2+)) transfer between the endoplasmic reticulum (ER) and mitochondria represents a critical signal in the induction of apoptosis. However, the mechanism controlling this flux in response to stress stimuli remains largely unknown. Here we show that, in the cytoplasm, WT p53 localizes at the ER and at specialized contact domains between the ER and mitochondria (mitochondria-associated membranes). We demonstrate that, upon stress stimuli, WT p53 accumulates at these sites and modulates Ca(2+) homeostasis. Mechanistically, upon activation, WT p53 directly binds to the sarco/ER Ca(2+)-ATPase (SERCA) pump at the ER, changing its oxidative state and thus leading to an increased Ca(2+) load, followed by an enhanced transfer to mitochondria. The consequent mitochondrial Ca(2+) overload causes in turn alterations in the morphology of this organelle and induction of apoptosis. Pharmacological inactivation of WT p53 or naturally occurring p53 missense mutants inhibits SERCA pump activity at the ER, leading to a reduction of the Ca(2+) signaling from the ER to mitochondria. These findings define a critical nonnuclear function of p53 in regulating Ca(2+) signal-dependent apoptosis.

TAp73 promotes anti-senescence-anabolism not proliferation

TAp73, a member of the p53 family, has been traditionally considered a tumor suppressor gene, but a recent report has claimed that it can promote cellular proliferation. This assumption is based on biochemical evidence of activation of anabolic metabolism, with enhanced pentose phosphate shunt (PPP) and nucleotide biosynthesis. Here, while we confirm that TAp73 expression enhances anabolism, we also substantiate its role in inhibiting proliferation and promoting cell death. Hence, we would like to propose an alternative interpretation of the accumulating data linking p73 to cellular metabolism: we suggest that TAp73 promotes anabolism to counteract cellular senescence rather than to support proliferation.

p53 negatively regulates Pin1 expression under ER stress

Accumulating evidence suggests that endoplasmic reticulum (ER) stress plays a major role in the development of many diseases. A previous study indicated that the apoptotic regulator p53 is significantly increased in response to ER stress and participates in ER stress-induced apoptosis. However, the regulators of p53 expression during ER stress are still not fully understood. Here, we investigated whether p53 contributes to the impairment of Pin1 signaling under ER stress. We found that treatment with thapsigargin, a stimulator of p53 expression and an inducer of ER stress, decreased Pin1 expression in HCT116 cells. Also, we identified functional p53 response elements (p53REs) in the Pin1 promoter. Overexpression of p53 significantly decreased Pin1 expression in HCT116 cells while abolition of p53 gene expression induced Pin1 expression. Pin1 expression was significantly increased by treatment with the p53 inhibitor pifithrin-α or down-regulation of p53 expression. Taken together, ER stress decreased Pin1 expression through p53 activation, and this mechanism may be associated with ER stress-induced cell death. These data reported here support the importance of Pin1 as a potential target molecule mediating tumor development.

Cofactor Strap regulates oxidative phosphorylation and mitochondrial p53 activity through ATP synthase

Metabolic reprogramming is a hallmark of cancer cells. Strap (stress-responsive activator of p300) is a novel TPR motif OB-fold protein that contributes to p53 transcriptional activation. We show here that, in addition to its established transcriptional role, Strap is localised at mitochondria where one of its key interaction partners is ATP synthase. Significantly, the interaction between Strap and ATP synthase downregulates mitochondrial ATP production. Under glucose-limiting conditions, cancer cells are sensitised by mitochondrial Strap to apoptosis, which is rescued by supplementing cells with an extracellular source of ATP. Furthermore, Strap augments the apoptotic effects of mitochondrial p53. These findings define Strap as a dual regulator of cellular reprogramming: first as a nuclear transcription cofactor and second in the direct regulation of mitochondrial respiration.

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.

ShaPINg Cell Fate Upon DNA Damage: Role of Pin1 Isomerase in DNA Damage-Induced Cell Death and Repair

The peptidyl-prolyl cis/trans isomerase Pin1 acts as a molecular timer in proline-directed Ser/Thr kinase signaling and shapes cellular responses based on recognition of phosphorylation marks and implementing conformational changes in its substrates. Accordingly, Pin1 has been linked to numerous phosphorylation-controlled signaling pathways and cellular processes such as cell cycle progression, proliferation, and differentiation. In addition, Pin1 plays a pivotal role in DNA damage-triggered cell fate decisions. Whereas moderate DNA damage is balanced by DNA repair, cells confronted with massive genotoxic stress are eliminated by the induction of programed cell death or cellular senescence. In this review, we summarize and discuss the current knowledge on how Pin1 specifies cell fate through regulating key players of the apoptotic and the repair branch of the DNA-damage response.

Forecasting cell death dose-response from early signal transduction responses in vitro

The rapid pharmacodynamic response of cells to toxic xenobiotics is primarily coordinated by signal transduction networks, which follow a simple framework: the phosphorylation/dephosphorylation cycle mediated by kinases and phosphatases. However, the time course from initial pharmacodynamic response(s) to cell death following exposure can have a vast range. Viewing this time lag between early signaling events and the ultimate cellular response as an opportunity, we hypothesize that monitoring the phosphorylation of proteins related to cell death and survival pathways at key, early time points may be used to forecast a cell's eventual fate, provided that we can measure and accurately interpret the protein responses. In this paper, we focused on a three-phased approach to forecast cell death after exposure: (1) determine time points relevant to important signaling events (protein phosphorylation) by using estimations of adenosine triphosphate production to reflect the relationship between mitochondrial-driven energy metabolism and kinase response, (2) experimentally determine phosphorylation values for proteins related to cell death and/or survival pathways at these significant time points, and (3) use cluster analysis to predict the dose-response relationship between cellular exposure to a xenobiotic and plasma membrane degradation at 24 h post-exposure. To test this approach, we exposed HepG2 cells to two disparate treatments: a GSK-3β inhibitor and a MEK inhibitor. After using our three-phased approach, we were able to accurately forecast the 24 h HepG2 plasma membrane degradation dose-response from protein phosphorylation values as early as 20 min post-MEK inhibitor exposure and 40 min post-GSK-3β exposure.

The cytoplasmic side of p53's oncosuppressive activities

The tumor suppressor p53 is a transcription factor that in response to a plethora of stress stimuli activates a complex and context-dependent cellular response ultimately protecting genome integrity. In the last two decades, the discovery of cytoplasmic p53 localization has driven an intense research on its extra-nuclear functions. The ability to induce apoptosis acting directly at mitochondria and the related mechanisms of p53 localization and translocation in the cytoplasm and mitochondria have been dissected. However, recent works indicate the involvement of cytoplasmic p53 also in biological processes such as autophagy, metabolism, oxidative stress and drug response. This review will focus on the mechanisms of cytoplasmic p53 activation and the pathophysiological role of p53's transcription-independent functions, highlighting possible therapeutic implications.

Mitochondrial death functions of p53

The p53 tumor suppressor network plays a fundamental surveillance role in both homeostatic and adaptive cell biology. p53 is one of the most important barriers against malignant derailment of normal cells, orchestrating growth arrest, senescence, or cell death by linking many different pathways in response to genotoxic and non-genotoxic insults. p53 is the key broadband sensor for numerous cellular stresses such as DNA damage, hypoxia, oxidative stress, oncogenic signaling, and nucleolar stress. The crucial tumor suppressive and tissue homeostasis activity of p53 is its ability to activate cell death via multiple different pathways. A well-characterized biochemical function of p53 in the regulation of apoptosis is its role as a potent transcriptional regulator. p53 activates a panel of proapoptotic genes from the mitochondrial apoptotic and death receptor programs while repressing antiapoptotic Bcl2 family genes. In addition, over the last 10 y a growing body of evidence has also defined direct extranuclear non-transcriptional p53 activities within mitochondria-mediated cell death pathways that are based on p53 protein accumulation in cytosolic and mitochondrial compartments and protein-protein interactions. To date, transcription-independent p53-mediated cell death regulation has been described for apoptosis, necrosis, and autophagy. Because mitochondrial dysregulation is central to the development of a number of pathologic processes such as cancer and neurodegenerative and age-related diseases, understanding the direct roles of p53 protein in mitochondria has high translational impact and could facilitate the development of novel drug targets to combat these diseases. In this review we will mainly focus on mechanisms of p53-mediated transcription-independent cell death pathways at mitochondria.

Heat stress induces apoptosis through transcription-independent p53-mediated mitochondrial pathways in human umbilical vein endothelial cell

Cells apoptosis induced by intense heat stress is the prominent feature of heat-related illness. However, little is known about the biological effects of heat stress on cells apoptosis. Herein, we presented evidence that intense heat stress could induce early apoptosis of HUVEC cells through activating mitochondrial pathway with changes in mitochondrial membrane potential(ΔΨm), release of cytochrome c, and activation of caspase-9 and -3. We further revealed that p53 played a crucial role in heat stress-induced early apoptosis, with p53 protein rapidly translocated into mitochondria. Using pifithrin-α(PFT), a p53's mitochondrial translocation inhibitor, we found that pretreated with PFT, heat stress induced mitochondrial p53 translocation was significantly suppressed, accompanied by a significant alleviation in the loss of ΔΨm, cytochrome c release and caspase-9 activation. Furthermore, we also found that generation of reactive oxygen species (ROS) was a critical mediator in heat stress-induced apoptosis. In addition, the antioxidant MnTMPyP significantly decreased the heat stress-induced p53's mitochondrial translocation, followed by the loss of ΔΨm, cytochrome c release, caspase-9 activation and heat stress-mediated apoptosis. Conclusively, these findings indicate the contribution of the transcription-independent mitochondrial p53 pathway to early apoptosis in HUVEC cells induced by oxidative stress in response to intense heat stress.

DLX5, FGF8 and the Pin1 isomerase control ΔNp63α protein stability during limb development: a regulatory loop at the basis of the SHFM and EEC congenital malformations

Ectrodactyly, or Split-Hand/Foot Malformation (SHFM), is a congenital condition characterized by the loss of central rays of hands and feet. The p63 and the DLX5;DLX6 transcription factors, expressed in the embryonic limb buds and ectoderm, are disease genes for these conditions. Mutations of p63 also cause the ectodermal dysplasia–ectrodactyly–cleft lip/palate (EEC) syndrome, comprising SHFM. Ectrodactyly is linked to defects of the apical ectodermal ridge (AER) of the developing limb buds. FGF8 is the key signaling molecule in this process, able to direct proximo-distal growth and patterning of the skeletal primordial of the limbs. In the limb buds of both p63 and Dlx5;Dlx6 murine models of SHFM, the AER is poorly stratified and FGF8 expression is severely reduced. We show here that the FGF8 locus is a downstream target of DLX5 and that FGF8 counteracts Pin1–ΔNp63α interaction. In vivo, lack of Pin1 leads to accumulation of the p63 protein in the embryonic limbs and ectoderm. We show also that ΔNp63α protein stability is negatively regulated by the interaction with the prolyl-isomerase Pin1, via proteasome-mediated degradation; p63 mutant proteins associated with SHFM or EEC syndromes are resistant to Pin1 action. Thus, DLX5, p63, Pin1 and FGF8 participate to the same time- and location-restricted regulatory loop essential for AER stratification, hence for normal patterning and skeletal morphogenesis of the limb buds. These results shed new light on the molecular mechanisms at the basis of the SHFM and EEC limb malformations.

Toward a new STATe: the role of STATs in mitochondrial function

Signal Transducers and Activators of Transcription (STATs) have been studied extensively and have been associated with virtually every biochemical pathway. Until recently, however, they were thought to exert these effects solely as a nuclear transcription factor. The finding that STAT3 localizes to the mitochondria and modulates respiration has opened up a new avenue through which STATs may regulate the cell. Recently, other members of the STAT family (STAT1, STAT2, STAT5, and STAT6) have also been shown to be present in the mitochondria. Coordinate regulation at the nucleus and mitochondria by these proteins places them in a unique position to drive cellular processes to achieve a specific response. This review summarizes recent findings that have led to our current understanding of how STATs influence mitochondrial function in health and disease.

Prolyl-isomerase Pin1 controls normal and cancer stem cells of the breast

Mammary epithelial stem cells are fundamental to maintain tissue integrity. Cancer stem cells (CSCs) are implicated in both treatment resistance and disease relapse, and the molecular bases of their malignant properties are still poorly understood. Here we show that both normal stem cells and CSCs of the breast are controlled by the prolyl-isomerase Pin1. Mechanistically, following interaction with Pin1, Notch1 and Notch4, key regulators of cell fate, escape from proteasomal degradation by their major ubiquitin-ligase Fbxw7α. Functionally, we show that Fbxw7α acts as an essential negative regulator of breast CSCs' expansion by restraining Notch activity, but the establishment of a Notch/Pin1 active circuitry opposes this effect, thus promoting breast CSCs self-renewal, tumor growth and metastasis in vivo. In human breast cancers, despite Fbxw7α expression, high levels of Pin1 sustain Notch signaling, which correlates with poor prognosis. Suppression of Pin1 holds promise in reverting aggressive phenotypes, through CSC exhaustion as well as recovered drug sensitivity carrying relevant implications for therapy of breast cancers.

Prolyl isomerase Pin1 and protein kinase HIPK2 cooperate to promote cortical neurogenesis by suppressing Groucho/TLE:Hes1-mediated inhibition of neuronal differentiation

The Groucho/transducin-like Enhancer of split 1 (Gro/TLE1):Hes1 transcriptional repression complex acts in cerebral cortical neural progenitor cells to inhibit neuronal differentiation. The molecular mechanisms that regulate the anti-neurogenic function of the Gro/TLE1:Hes1 complex during cortical neurogenesis remain to be defined. Here we show that prolyl isomerase Pin1 (peptidyl-prolyl cis-trans isomerase NIMA-interacting 1) and homeodomain-interacting protein kinase 2 (HIPK2) are expressed in cortical neural progenitor cells and form a complex that interacts with the Gro/TLE1:Hes1 complex. This association depends on the enzymatic activities of both HIPK2 and Pin1, as well as on the association of Gro/TLE1 with Hes1, but is independent of the previously described Hes1-activated phosphorylation of Gro/TLE1. Interaction with the Pin1:HIPK2 complex results in Gro/TLE1 hyperphosphorylation and weakens both the transcriptional repression activity and the anti-neurogenic function of the Gro/TLE1:Hes1 complex. These results provide evidence that HIPK2 and Pin1 work together to promote cortical neurogenesis, at least in part, by suppressing Gro/TLE1:Hes1-mediated inhibition of neuronal differentiation.

Disarming mutant p53 oncogenic function

In the last decade intensive research has confirmed the long standing hypothesis that some p53 point mutants acquire novel activities able to cooperate with oncogenic mechanisms. Particular attention has attracted the ability of several such mutants to actively promote the development of aggressive and metastatic tumors in vivo. This knowledge opens a new dimension on rational therapy design, suggesting novel strategies based on pharmacological manipulation of those neomorphic activities. P53 point mutants have several characteristics that make them attractive targets for anti-cancer therapies. Remarkably, mutant p53 has been found predominantly in tumor cells and may act pleiotropically by interfering with a variety of cellular processes. Therefore, drugs targeting mutant p53 may selectively affect tumor cells, inactivating simultaneously several mechanisms of tumor promotion. Moreover, the high frequency of missense mutations on the p53 gene suggests that interfering with mutant p53 function may provide a valuable approach for the development of efficient therapies able to target a wide range of tumor types.

Autophosphorylation and Pin1 binding coordinate DNA damage-induced HIPK2 activation and cell death

Excessive genome damage activates the apoptosis response. Protein kinase HIPK2 is a key regulator of DNA damage-induced apoptosis. Here, we deciphered the molecular mechanism of HIPK2 activation and show its relevance for DNA damage-induced apoptosis in cellulo and in vivo. HIPK2 autointeracts and site-specifically autophosphorylates upon DNA damage at Thr880/Ser882. Autophosphorylation regulates HIPK2 activity and mutation of the phosphorylation-acceptor sites deregulates p53 Ser46 phosphorylation and apoptosis in cellulo. Moreover, HIPK2 autophosphorylation is conserved between human and zebrafish and is important for DNA damage-induced apoptosis in vivo. Mechanistically, autophosphorylation creates a binding signal for the phospho-specific isomerase Pin1. Pin1 links HIPK2 activation to its stabilization by inhibiting HIPK2 polyubiquitination and modulating Siah-1-HIPK2 interaction. Concordantly, Pin1 is required for DNA damage-induced HIPK2 stabilization and p53 Ser46 phosphorylation and is essential for induction of apotosis both in cellulo and in zebrafish. Our results identify an evolutionary conserved mechanism regulating DNA damage-induced apoptosis.

Another fork in the road--life or death decisions by the tumour suppressor p53

In response to cellular stress signals, the tumour suppressor p53 accumulates and triggers a host of antineoplastic responses. For instance, DNA damage activates two main p53-dependent responses: cell cycle arrest and attendant DNA repair or apoptosis (cell death). It is broadly accepted that, in response to DNA damage, the function of p53 as a sequence-specific transcription factor is crucial for tumour suppression. The molecular determinants, however, that favour the initiation of either a p53-dependent cell cycle arrest (life) or apoptotic (death) transcriptional programme remain elusive. Gaining a clear understanding of the mechanisms controlling cell fate determination by p53 could lead to the identification of molecular targets for therapy, which could selectively sensitize cancer cells to apoptosis. This review summarizes the literature addressing this important question in the field. Special emphasis is given to the role of the p53 response element, post-translational modifications and protein-protein interactions on cell fate decisions made by p53 in response to DNA damage.

Hyaluronic acid induces activation of the κ-opioid receptor

INTRODUCTION

Nociceptive pain is one of the most common types of pain that originates from an injury involving nociceptors. Approximately 60% of the knee joint innervations are classified as nociceptive. The specific biological mechanism underlying the regulation of nociceptors is relevant for the treatment of symptoms affecting the knee joint. Intra-articular administration of exogenous hyaluronic acid (HA) in patients with osteoarthritis (OA) appears to be particularly effective in reducing pain and improving patient function.

METHODS

We performed an in vitro study conducted in CHO cells that expressed a panel of opioid receptors and in primary rat dorsal root ganglion (DRG) neurons to determine if HA induces the activation of opioid peptide receptors (OPr) using both aequorin and the fluorescent dye Fura-2/AM.

RESULTS

Selective agonists and antagonists for each OPr expressed on CHO cells were used to test the efficacy of our in vitro model followed by stimulation with HA. The results showed that HA induces stimulatory effects on the κ receptor (KOP). These effects of HA were also confirmed in rat DRG neurons, which express endogenously the OPr.

CONCLUSIONS

HA activates the KOP receptor in a concentration dependent manner, with a pEC(50) value of 7.57.

Genome wide analysis of Cyclophilin gene family from rice and Arabidopsis and its comparison with yeast

Cyclophilin proteins are the members of immunophillin group of proteins, known for their property of binding to the immune-suppressant drug cyclosporin A, hence named as cyclophilins. These proteins are characterized by the presence of peptidyl prolyl isomerase (PPIase) domain which catalyzes the cis-trans isomerisation process of proline residues. In the present study, an in-silico based approach was followed to identify and characterize the cyclophilin family from rice, Arabidopsis and yeast. We were able to identify 28 rice, 35 Arabidopsis and 8 yeast cyclophilin genes from their respective genomes on the basis of their annotation as well as the presence of highly conserved PPIase domain. The evolutionary relationship of the cyclophilin genes from the three genomes was analyzed using the phylogenetic tree. We have also classified the rice cyclophilin genes on the basis of localization of the protein in cell. The structural similarity of the cyclophilins was also analyzed on the basis of their homology model. The expression analysis performed using Genevestigator revealed a very strong stress responsive behavior of the gene family which was more prominent in later stages of stress. The study indicates the importance of the gene family in stress response as well as several developmental stages thus opening up many avenues for future study on the cyclophilin proteins.

Complete thermodynamic and kinetic characterization of the isomer-specific interaction between Pin1-WW domain and the amyloid precursor protein cytoplasmic tail phosphorylated at Thr668

Peptidyl prolyl cis-trans isomerization acts as an effective molecular timer that plays significant roles in biological and pathological processes. Enzymes such as Pin1 catalyze cis-trans isomerization, accelerating the otherwise slow isomerization rate into time scales relevant for cellular signaling. Here we have combined NMR line shape analysis, fluorescence spectroscopy, and isothermal titration calorimetry to determine the kinetic and thermodynamic parameters describing the trans-specific interaction between the binding domain of Pin1 (WW domain) and a key cis-trans molecular switch in the amyloid precursor protein cytoplasmic tail. A three-state model, in which the cis-trans isomerization equilibrium is coupled to the binding equilibrium through the trans isomer, was found to fit the data well. The trans isomer binds the WW domain with ∼22 μM affinity via very fast association (approaching the diffusion limit) and dissociation rates. The common structural and electrostatic characteristics of Pin1 substrates, which contain a phosphorylated serine/threonine-proline motif, suggest that very rapid binding kinetics are a general feature of Pin1 interactions with other substrates. The fast binding kinetics of the WW domain allows rapid response of Pin1 to the dynamic events of phosphorylation and dephosphorylation in the cell that alter the relative populations of diverse Pin1 substrates. Furthermore, our results also highlight the vastly different rates at which slow uncatalyzed cis-trans isomerization and fast isomer-specific binding events occur. These results, along with the experimental methods presented herein, should guide future experiments aimed at the thermodynamic and kinetic characterization of cis-trans molecular switches and isomer-specific interactions involved in various biological processes.