Serine protease Omi/HtrA2 targets WARTS kinase to control cell proliferation

Exploring Host-Guest Interactions within a 600 kDa DegP Protease Cage Complex Using Hydrodynamics Measurements and Methyl-TROSY NMR

The DegP protease-chaperone operates within the periplasm of Gram-negative bacteria, where it assists in the regulation of protein homeostasis, promotes virulence, and is essential to survival under stress. To carry out these tasks, DegP forms a network of preorganized apo oligomers that facilitate the capture of substrates within distributions of cage-like complexes which expand to encapsulate clients of various sizes. Although the architectures of DegP cage complexes are well understood, little is known about the structures, dynamics, and interactions of client proteins within DegP cages and the relationship between client structural dynamics and function. Here, we probe host-guest interactions within a 600 kDa DegP cage complex throughout the DegP activation cycle using a model α-helical client protein through a combination of hydrodynamics measurements, methyl-transverse relaxation optimized spectroscopy-based solution nuclear magnetic resonance studies, and proteolytic activity assays. We find that in the presence of the client, DegP cages assemble cooperatively with few intermediates. Our data further show that the N-terminal half of the bound client, which projects into the interior of the cages, is predominantly unfolded and flexible, and exchanges between multiple conformational states over a wide range of time scales. Finally, we show that a concerted structural transition of the protease domains of DegP occurs upon client engagement, leading to activation. Together, our findings support a model of DegP as a highly cooperative and dynamic molecular machine that stabilizes unfolded states of clients, primarily via interactions with their C-termini, giving rise to efficient cleavage.

Protease-independent control of parthanatos by HtrA2/Omi

HtrA2/Omi is a mitochondrial serine protease with ascribed pro-apoptotic as well as pro-necroptotic functions. Here, we establish that HtrA2/Omi also controls parthanatos, a third modality of regulated cell death. Deletion of HtrA2/Omi protects cells from parthanatos while reconstitution with the protease restores the parthanatic death response. The effects of HtrA2/Omi on parthanatos are specific and cannot be recapitulated by manipulating other mitochondrial proteases such as PARL, LONP1 or PMPCA. HtrA2/Omi controls parthanatos in a manner mechanistically distinct from its action in apoptosis or necroptosis, i.e., not by cleaving cytosolic IAP proteins but rather exerting its effects without exiting mitochondria, and downstream of PARP-1, the first component of the parthanatic signaling cascade. Also, previously identified or candidate substrates of HtrA2/Omi such as PDXDC1, VPS4B or moesin are not cleaved and dispensable for parthanatos, whereas DBC-1 and stathmin are cleaved, and thus represent potential parthanatic downstream mediators of HtrA2/Omi. Moreover, mass-spectrometric screening for novel parthanatic substrates of HtrA2/Omi revealed that the induction of parthanatos does not cause a substantial proteolytic cleavage or major alterations in the abundance of mitochondrial proteins. Resolving these findings, reconstitution of HtrA2/Omi-deficient cells with a catalytically inactive HtrA2/Omi mutant restored their sensitivity against parthanatos to the same level as the protease-active HtrA2/Omi protein. Additionally, an inhibitor of HtrA2/Omi's protease activity did not confer protection against parthanatic cell death. Our results demonstrate that HtrA2/Omi controls parthanatos in a protease-independent manner, likely via novel, unanticipated functions as a scaffolding protein and an interaction with so far unknown mitochondrial proteins.

Apoptotic proteins with non-apoptotic activity: expression and function in cancer

Apoptosis is a process of programmed cell death in which a cell commits suicide while maintaining the integrity and architecture of the tissue as a whole. Apoptosis involves activation of one of two major pathways: the extrinsic pathway, where extracellular pro-apoptotic signals, transduced through plasma membrane death receptors, activate a caspase cascade leading to apoptosis. The second, the intrinsic apoptotic pathway, where damaged DNA, oxidative stress, or chemicals, induce the release of pro-apoptotic proteins from the mitochondria, leading to the activation of caspase-dependent and independent apoptosis. However, it has recently become apparent that proteins involved in apoptosis also exhibit non-cell death-related physiological functions that are related to the cell cycle, differentiation, metabolism, inflammation or immunity. Such non-conventional activities were predominantly reported in non-cancer cells although, recently, such a dual function for pro-apoptotic proteins has also been reported in cancers where they are overexpressed. Interestingly, some apoptotic proteins translocate to the nucleus in order to perform a non-apoptotic function. In this review, we summarize the unconventional roles of the apoptotic proteins from a functional perspective, while focusing on two mitochondrial proteins: VDAC1 and SMAC/Diablo. Despite having pro-apoptotic functions, these proteins are overexpressed in cancers and this apparent paradox and the associated pathophysiological implications will be discussed. We will also present possible mechanisms underlying the switch from apoptotic to non-apoptotic activities although a deeper investigation into the process awaits further study.

Molecular complexity and gene expression controlling cell turnover during a digestive cycle of carnivorous sponge Lycopodina hypogea

Lycopodina hypogea is a carnivorous sponge that tolerates laboratory husbandry very well. During a digestion cycle, performed without any digestive cavity, this species undergoes spectacular morphological changes leading to a total regression of long filaments that ensure the capture of prey and their reformation at the end of the cycle. This phenomenon is a unique opportunity to analyze the molecular and cellular determinants that ensure digestion in the sister group of all other metazoans. Using differential transcriptomic analysis coupled with cell biology studies of proliferation, differentiation, and programmed cell deaths (i.e., autophagy and the destructive/constructive function of apoptosis), we demonstrate that the molecular and cellular actors that ensure digestive homeostasis in a sister group of all remaining animals are similar in variety and complexity to those controlling tissue homeostasis in higher vertebrates. During a digestion cycle, most of these actors are finely tuned in a coordinated manner. Our data benefits from complementary approaches coupling in silico and cell biology studies and demonstrate that the nutritive function is provided by the coordination of molecular network that impacts the cells turnover in the entire organism.

Mitochondrial serine protease Omi/HtrA2 accentuates brain ischemia/reperfusion injury in rats and oxidative stress injury in vitro by modulating mitochondrial stress proteins CHOP and ClpP and physically interacting with mitochondrial fusion protein OPA1

Serine protease Omi/HtrA2, a member of the HtrA family, is closely related to the maintenance of mitochondrial integrity and participates in apoptosis but its role in cerebral ischemia/reperfusion (I/R) injury and cellular oxidative stress response remains unclear. In this study, we found that I/R injury resulted in a time-dependent increase in Omi/HtrA2 expression in rat brain tissue. Inhibition of Omi/HtrA2 significantly inhibited XIAP cleavage in H₂O₂-induced PC12 cells. In addition, inhibition of Omi/HtrA2 significantly inhibited the up-regulation of mitochondrial stress proteins CHOP and ClpP, significantly reduced mitochondrial aggregation, and attenuated the decline of mitochondrial ΔΨm in PC12 cells. Studies show that there is a physical interaction between Omi/HtrA2 and OPA1. We found that Omi/HtrA2 and OPA1 are closely related to the oxidative stress mitochondrial response in PC12 cells. The current study has demonstrated that Omi/HtrA2 is upregulated in brain I/R injury in vivo and is implicated in mitochondrial response to oxidative stress in vitro by regulating mitochondrial stress proteins CHOP and CLpP and by interacting with mitochondrial cristae remodeling protein OPA1. These findings suggest that Omi/HtrA2 could be a candidate molecular target in diseases that involve oxidative stress such as in I/R injury.

Abbreviation: ATP: Adenosine tripHospHate; Bax: BCL2-Associated X; Bcl-2: B-cell lympHoma-2; BSA: Albumin from bovine serum; DMEM: Dulbecco’s Minimum Essential Medium; DMSO: Dimethyl sulfoxide; HSP60: Heat shock protein60, 70; L-OPA1: Long forms of OPA1; Omi/HtrA2: high-temperature-regulated A2; MCAO: Middle cerebral artery occlusion; OPA1: Optic AtropHy; PBS: PHospHate buffered saline; PMSF: pHenylmethyl sulfonylfluoride; ROS: reactive oxygen species; SDS: Sodium dodecyl sulfate; S-OPA1: Short forms of OPA1; TTC: TripHenyltetrazalium chloride; XIAP: X-linked inhibitor apoptosis protein

Dual specificity phosphatase 9: A novel binding partner cum substrate of proapoptotic serine protease HtrA2

Human high temperature requirement protease A2 (HtrA2) is a trimeric PDZ bearing proapoptotic serine protease, which is involved in various cellular processes and pathologies. Research in the last decade strongly advocates its role as a potential therapeutic target and therefore warrants the need to minutely investigate its mechanism of action, regulation, interactions with other proteins and its binding specificities. In this particular study, we adopted an in silico approach to predict novel interacting partners and/or substrates of HtrA2 by building a peptide library using a binding pattern search. This library was used to look for novel ligand proteins in the human proteome. Thereafter, the putative interaction was validated using biochemical and cell-based studies. In a first, here we report that HtrA2 shows robust interactions with DUSP9 (Dual specificity phosphatase 9) in GST-pulldown and Co-Immunoprecipitation (Co-IP) experiments and cleaves it in vitro. Besides, we also provided a detailed characterization of the interaction interface. Moreover, this study in general provides an efficient, fast and practical method of candidate ligand library screening for exploring the binding properties of HtrA2.

Role of ALDH1A1 and HTRA2 expression in CCL2/CCR2-mediated breast cancer cell growth and invasion

Chemokines mediate immune cell trafficking during tissue development, wound healing and infection. The chemokine CCL2 is best known to regulate macrophage recruitment during wound healing, infection and inflammatory diseases. While the importance of CCL2/CCR2 signaling in macrophages during cancer progression is well documented, we recently showed that CCL2-mediated breast cancer progression depends on CCR2 expression in carcinoma cells. Using 3D Matrigel: Collagen cultures of SUM225 and DCIS.com breast cancer cells, this study characterized the mechanisms of CCL2/CCR2 signaling in cell growth and invasion. SUM225 cells, which expressed lower levels of CCR2 than DCIS.com cells, formed symmetrical spheroids in Matrigel: Collagen, and were not responsive to CCL2 treatment. DCIS.com cells formed asymmetric cell clusters in Matrigel: Collagen. CCL2 treatment increased growth, decreased expression of E-cadherin and increased TWIST1 expression. CCR2 overexpression in SUM225 cells increased responsiveness to CCL2 treatment, enhancing growth and invasion. These phenotypes corresponded to increased expression of Aldehyde Dehydrogenase 1A1 (ALDH1A1) and decreased expression of the mitochondrial serine protease HTRA2. CCR2 deficiency in DCIS.com cells inhibited CCL2-mediated growth and invasion, corresponding to decreased ALDH1A1 expression and increased HTRA2 expression. ALDH1A1 and HTRA2 expression were modulated in CCR2-deficient and CCR2-overexpressing cell lines. We found that ALDH1A1 and HTRA2 regulates CCR2-mediated breast cancer cell growth and cellular invasion in a CCL2/CCR2 context-dependent manner. These data provide novel insight on the mechanisms of chemokine signaling in breast cancer cell growth and invasion, with important implications on targeted therapeutics for anti-cancer treatment.

This article has an associated First Person interview with the first author of the paper.

Summary: Chemokines are known to regulate immune cell recruitment during inflammation. This report characterizes novel molecular mechanisms through which CCL2/CCR2 chemokine signaling in breast cancer cells regulates growth and invasion.

The multi-subunit GID/CTLH E3 ubiquitin ligase promotes cell proliferation and targets the transcription factor Hbp1 for degradation

In yeast, the glucose-induced degradation-deficient (GID) E3 ligase selectively degrades superfluous gluconeogenic enzymes. Here, we identified all subunits of the mammalian GID/CTLH complex and provide a comprehensive map of its hierarchical organization and step-wise assembly. Biochemical reconstitution demonstrates that the mammalian complex possesses inherent E3 ubiquitin ligase activity, using Ube2H as its cognate E2. Deletions of multiple GID subunits compromise cell proliferation, and this defect is accompanied by deregulation of critical cell cycle markers such as the retinoblastoma (Rb) tumor suppressor, phospho-Histone H3 and Cyclin A. We identify the negative regulator of pro-proliferative genes Hbp1 as a bonafide GID/CTLH proteolytic substrate. Indeed, Hbp1 accumulates in cells lacking GID/CTLH activity, and Hbp1 physically interacts and is ubiquitinated in vitro by reconstituted GID/CTLH complexes. Our biochemical and cellular analysis thus demonstrates that the GID/CTLH complex prevents cell cycle exit in G1, at least in part by degrading Hbp1.

Structural insights into the activation mechanisms of human HtrA serine proteases

Human HtrA1-4 proteins belong to the HtrA family of evolutionarily conserved serine proteases and function as important modulators of many physiological processes, including maintenance of mitochondrial homeostasis, cell signaling and apoptosis. Disturbances in their action are linked to severe diseases, including oncogenesis and neurodegeneration. The HtrA1-4 proteins share structural and functional features of other members of the HtrA protein family, however there are several significant differences in structural architecture and mechanisms of action which makes each of them unique. Our goal is to present recent studies regarding human HtrAs. We focus on their physiological functions, structure and regulation, and describe current models of activation mechanisms. Knowledge of molecular basis of the human HtrAs' action is a subject of great interest; it is crucial for understanding their relevance in cellular physiology and pathogenesis as well as for using them as targets in future therapies of diseases such as neurodegenerative disorders and cancer.

Staurosporine suppresses survival of HepG2 cancer cells through Omi/HtrA2-mediated inhibition of PI3K/Akt signaling pathway

Staurosporine, which is an inhibitor of a broad spectrum of protein kinases, has shown cytotoxicity on several human cancer cells. However, the underlying mechanism is not well understood. In this study, we examined whether and how this compound has an inhibitory action on phosphatidylinositol 3-kinase (PI3K)/Akt pathway in vitro using HepG2 human hepatocellular carcinoma cell line. Cell viability and apoptosis were determined using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and terminal deoxyribonucleotidyl transferase–mediated dUTP-digoxigenin nick end labeling (TUNEL) assay, respectively. Glutathione S-transferase (GST) pull-down assay and co-immunoprecipitation were performed to detect protein–protein interactions. Small interfering RNA (siRNA) was used to silence the expression of targeted protein. We found that staurosporine significantly decreased cell viability and increased cell apoptosis in a concentration- and time-dependent manner in HepG2 cancer cells, along with the decreased expressions of PDK1 protein and Akt phosphorylation. Staurosporine was also found to enhance Omi/HtrA2 release from mitochondria. Furthermore, Omi/HtrA2 directly bound to PDK1. Pharmacological and genetic inhibition of Omi/HtrA2 restored protein levels of PDK1 and protected HepG2 cancer cells from staurosporine-induced cell death. In addition, staurosporine was found to activate autophagy. However, inhibition of autophagy exacerbated cell death under concomitant treatment with staurosporine. Taken together, our results indicate that staurosporine induced cytotoxicity response by inhibiting PI3K/Akt signaling pathway through Omi/HtrA2-mediated PDK1 degradation, and the process provides a novel mechanism by which staurosporine produces its therapeutic effects.

Mitochondrial Proteome Changes Correlating with β-Amyloid Accumulation

Alzheimer's disease (AD) is a multifactorial disease of wide clinical heterogenity. Overproduction of amyloid precursor protein (APP) and accumulation of β-amyloid (Aβ) and tau proteins are important hallmarks of AD. The identification of early pathomechanisms of AD is critically important for discovery of early diagnosis markers. Decreased brain metabolism is one of the earliest clinical symptoms of AD that indicate mitochondrial dysfunction in the brain. We performed the first comprehensive study integrating synaptic and non-synaptic mitochondrial proteome analysis (two-dimensional differential gel electrophoresis (2D-DIGE) and mass spectrometry) in correlation with Aβ progression in APP/PS1 mice (3, 6, and 9 months of age). We identified changes of 60 mitochondrial proteins that reflect the progressive effect of APP overproduction and Aβ accumulation on mitochondrial processes. Most of the significantly affected proteins play role in the mitochondrial electron transport chain, citric acid cycle, oxidative stress, or apoptosis. Altered expression levels of Htra2 and Ethe1, which showed parallel changes in different age groups, were confirmed also by Western blot. The common regulator bioinformatical analysis suggests the regulatory role of tumor necrosis factor (TNF) in Aβ-mediated mitochondrial protein changes. Our results are in accordance with the previous postmortem human brain proteomic studies in AD in the case of many proteins. Our results could open a new path of research aiming early mitochondrial molecular mechanisms of Aβ accumulation as a prodromal stage of human AD.

The role of PARL and HtrA2 in striatal neuronal injury after transient global cerebral ischemia

The presenilin-associated rhomboid-like (PARL) protein and high temperature requirement factor A2 (HtrA2) are key regulators of mitochondrial integrity and play pivotal roles in apoptosis. However, their roles after cerebral ischemia have not been thoroughly elucidated. To clarify these roles, mice were subjected to transient global cerebral ischemia, and striatal neuronal injury was assessed. Western blot and coimmunoprecipitation analyses revealed that PARL and processed HtrA2 localized to mitochondria, and that PARL was bound to HtrA2 in sham animals. Expression of PARL and processed HtrA2 in mitochondria significantly decreased 6 to 72 hours after ischemia, and the binding of PARL to HtrA2 disappeared after ischemia. In contrast, expression of processed HtrA2 increased 24 hours after ischemia in the cytosol, where HtrA2 was bound to X chromosome-linked inhibitor-of-apoptosis protein (XIAP). Administration of PARL small interfering RNA inhibited HtrA2 processing and worsened ischemic neuronal injury. Our results show that downregulation of PARL after ischemia is a key step in ischemic neuronal injury, and that it decreases HtrA2 processing and increases neuronal vulnerability. In addition, processed HtrA2 released into the cytosol after ischemia contributes to neuronal injury via inhibition of XIAP.

The protease Omi cleaves the mitogen-activated protein kinase kinase MEK1 to inhibit microglial activation

Inflammation in Parkinson's disease is closely associated with disease pathogenesis. Mutations in Omi, which encodes the protease Omi, are linked to neurodegeneration and Parkinson's disease in humans and in mouse models. The severe neurodegeneration and neuroinflammation that occur in mnd2 (motor neuron degeneration 2) mice result from loss of the protease activity of Omi by the point mutation S276C; however, the substrates of Omi that induce neurodegeneration are unknown. We showed that Omi was required for the production of inflammatory molecules by microglia, which are the resident macrophages in the central nervous system. Omi suppressed the activation of the mitogen-activated protein kinases (MAPKs) extracellular signal-regulated kinase 1 and 2 (ERK1/2) by cleaving the upstream kinase MEK1 (mitogen-activated or extracellular signal-regulated protein kinase kinase 1). Knockdown of Omi in microglial cell lines led to activation of ERK1/2 and resulted in degradation of IκBα [α inhibitor of nuclear factor κB (NF-κB)], resulting in NF-κB activation and the expression of genes encoding inflammatory molecules, such as tumor necrosis factor-α and inducible nitric oxide synthase. The production of inflammatory molecules induced by the knockdown of Omi was blocked by the MEK1-specific inhibitor U0126. Furthermore, expression of the protease-deficient S276C Omi mutant in a microglial cell line had no effect on MEK1 cleavage or ERK1/2 activation. In the brains of mnd2 mice, we observed increased transcription of several genes encoding inflammatory molecules, as well as activation of astrocytes and microglia. Therefore, our study demonstrates that Omi is an intrinsic cellular factor that inhibits neuroinflammation.

Non-apoptotic functions of apoptosis-regulatory proteins

During the past two decades, apoptotic cell death has been the subject of an intense wave of investigation, leading to the discovery of multiple gene products that govern both its induction and execution. In parallel, it has progressively become evident that most, if not all, proteins that had initially been discovered for their essential role in apoptosis also mediate a wide range of non-apoptotic functions. On the one hand, apoptotic regulators and executioners are involved in non-lethal physiological processes as diverse as cell cycle progression, differentiation, metabolism, autophagy and inflammation. On the other hand, pro-apoptotic proteins can control other modalities of programmed cell death, in particular regulated necrosis. In this review, we summarize the unconventional roles of the apoptotic core machinery from a functional perspective and discuss their pathophysiological implications.

The structural basis of mode of activation and functional diversity: a case study with HtrA family of serine proteases

HtrA (High temperature requirement protease A) proteins that are primarily involved in protein quality control belong to a family of serine proteases conserved from bacteria to humans. HtrAs are oligomeric proteins that share a common trimeric pyramidal architecture where each monomer comprises a serine protease domain and one or two PDZ domains. Although the overall structural integrity is well maintained and they exhibit similar mechanism of activation, subtle conformational changes and structural plasticity especially in the flexible loop regions and domain interfaces lead to differences in their active site conformation and hence in their specificity and functions.

The serine protease HtrA2 cleaves UCH-L1 and inhibits its hydrolase activity: implication in the UCH-L1-mediated cell death

Ubiquitin (Ub) carboxyl-terminal hydrolase L1 (UCH-L1) has dual functions, such as hydrolase activity on the chemical bonds formed by the C-terminal Gly of Ub and dimerization-dependent ubiquitin ligase activity. Accumulating evidence suggests that dual activities of UCH-L1 were intimately associated with Parkinson's diseases (PD) and cancer. However, the molecular mechanism that regulates UCH-L1 enzymatic activity has not yet been fully elucidated. The serine protease high temperature requirement A2 (HtrA2), a PD-associated gene, is important in regulating cell survival as well as apoptosis. Using in vitro and in vivo cleavage assays, we have demonstrated that UCH-L1 is a natural substrate for the serine protease HtrA2 in the apoptotic pathway. Notably, we show that released, cytosolic HtrA2 decreases UCH-L1 protein level and its hydrolase activity through HtrA2-mediated cleavage of UCH-L1 under apoptotic conditions. These findings suggest that the HtrA2-mediated cleavage of UCH-L1 may play important roles in regulating the fine balance between cell growth and cell death.

HtrA2, taming the oncogenic activities of WT1

Wilms' tumour is a paediatric malignancy of the kidneys and is one of the most common solid childhood cancers. The Wilms' tumour 1 protein (WT1) is a transcription factor that can either activate or repress genes involved in growth, apoptosis and differentiation. It is frequently mutated or aberrantly expressed in Wilms' tumour, where the wild type protein would normally act as a tumour suppressor. Several studies, however, have found that wild type WT1 acts as an oncogene in adult tumours, primarily through the inhibition of apoptosis. The expression of WT1 correlates with the aggressiveness of several adult cancers, and its continued expression following treatment is indicative of a poor outcome.We recently found that the treatment of tumour cell lines with cytotoxic drugs leads to the cleavage of WT1 by the serine protease HtrA2. HtrA2 binds to a specific region of WT1, the suppression domain, and then cleaves WT1 at multiple sites. The HtrA2-mediated proteolysis of WT1 leads to its removal from gene promoter regions and changes in gene expression. Cleavage of WT1 by HtrA2 enhances apoptosis. This event is advantageous to the treatment of adult tumours where WT1 acts as an oncogene. However, when WT1 is acting as a tumour suppressor in paediatric malignancies, proteolysis by HtrA2 would be antagonistic to therapy.

A kinase shRNA screen links LATS2 and the pRB tumor suppressor

pRB-mediated inhibition of cell proliferation is a complex process that depends on the action of many proteins. However, little is known about the specific pathways that cooperate with the Retinoblastoma protein (pRB) and the variables that influence pRB's ability to arrest tumor cells. Here we describe two shRNA screens that identify kinases that are important for pRB to suppress cell proliferation and pRB-mediated induction of senescence markers. The results reveal an unexpected effect of LATS2, a component of the Hippo pathway, on pRB-induced phenotypes. Partial knockdown of LATS2 strongly suppresses some pRB-induced senescence markers. Further analysis shows that LATS2 cooperates with pRB to promote the silencing of E2F target genes, and that reduced levels of LATS2 lead to defects in the assembly of DREAM (DP, RB [retinoblastoma], E2F, and MuvB) repressor complexes at E2F-regulated promoters. Kinase assays show that LATS2 can phosphorylate DYRK1A, and that it enhances the ability of DYRK1A to phosphorylate the DREAM subunit LIN52. Intriguingly, the LATS2 locus is physically linked with RB1 on 13q, and this region frequently displays loss of heterozygosity in human cancers. Our results reveal a functional connection between the pRB and Hippo tumor suppressor pathways, and suggest that low levels of LATS2 may undermine the ability of pRB to induce a permanent cell cycle arrest in tumor cells.

Mammalian Hippo pathway: from development to cancer and beyond

The Hippo pathway was discovered as a signal transduction pathway that regulates organ size in Drosophila melanogaster. It is composed of three components: cell surface upstream regulators including cell adhesion molecules and cell polarity complexes; a kinase cascade comprising two serine-threonine kinases with regulators and adaptors; and a downstream target, a transcription coactivator. The coactivator mediates the transcription of cell proliferation-promoting and anti-apoptotic genes. The pathway negatively regulates the coactivator to restrict cell proliferation and to promote cell death. Thus, the pathway prevents tissue overgrowth and tumourigenesis. The framework of the pathway is conserved in mammals. A dysfunction of the pathway is frequently detected in human cancers and correlates with a poor prognosis. Recent works indicated that the Hippo pathway plays an important role in tissue homoeostasis through the regulation of stem cells, cell differentiation and tissue regeneration.

The regulatory crosstalk between kinases and proteases in cancer

Kinases and proteases are responsible for two fundamental regulatory mechanisms--phosphorylation and proteolysis--that orchestrate the rhythms of life and death in all organisms. Recent studies have highlighted the elaborate interplay between both post-translational regulatory systems. Many intracellular or pericellular proteases are regulated by phosphorylation, whereas multiple kinases are activated or inactivated by proteolytic cleavage. The functional consequences of this regulatory crosstalk are especially relevant in the different stages of cancer progression. What are the clinical implications derived from the fertile dialogue between kinases and proteases in cancer?

THAP5 is a human cardiac-specific inhibitor of cell cycle that is cleaved by the proapoptotic Omi/HtrA2 protease during cell death

Omi/HtrA2 is a mitochondrial serine protease that has a dual function: while confined in the mitochondria, it promotes cell survival, but when released into the cytoplasm, it participates in caspase-dependent as well as caspase-independent cell death. To investigate the mechanism of Omi/HtrA2's function, we set out to isolate and characterize novel substrates for this protease. We have identified Thanatos-associated protein 5 (THAP5) as a specific interactor and substrate of Omi/HtrA2 in cells undergoing apoptosis. This protein is an uncharacterized member of the THAP family of proteins. THAP5 has a unique pattern of expression and is found predominantly in the human heart, although a very low expression is also seen in the human brain and muscle. THAP5 protein is localized in the nucleus and, when ectopically expressed, induces cell cycle arrest. During apoptosis, THAP5 protein is degraded, and this process can be blocked using a specific Omi/HtrA2 inhibitor, leading to reduced cell death. In patients with coronary artery disease, THAP5 protein levels substantially decrease in the myocardial infarction area, suggesting a potential role of this protein in human heart disease. This work identifies human THAP5 as a cardiac-specific nuclear protein that controls cell cycle progression. Furthermore, during apoptosis, THAP5 is cleaved and removed by the proapoptotic Omi/HtrA2 protease. Taken together, we provide evidence to support that THAP5 and its regulation by Omi/HtrA2 provide a new link between cell cycle control and apoptosis in cardiomyocytes.

Human tissue factor pathway inhibitor-2 is internalized by cells and translocated to the nucleus by the importin system

Tissue factor pathway inhibitor-2 (TFPI-2) is a serine proteinase inhibitor that induces caspase-mediated apoptosis when offered to a variety of tumor cells. In order to investigate the mechanism of TFPI-2-induced apoptosis, we initially studied the uptake and trafficking of TFPI-2 by HT-1080 cells. Exogenously offered TFPI-2 was rapidly internalized and distributed in both the cytosolic and nuclear fractions. Nuclear localization of TFPI-2 was also detected in a variety of endothelial cells constitutively expressing TFPI-2. Nuclear localization of TFPI-2 required a NLS sequence located in its Lys/Arg-rich C-terminal tail comprising residues 191-211, as a TFPI-2 construct lacking the C-terminal tail failed to localize to the nucleus. Complexes of TFPI-2 and importin-alpha were co-immunoprecipitated from cell lysates of HT-1080 cells either offered or overexpressing this protein, providing evidence that TFPI-2 was shuttled to the nucleus by the importin system. Our results provide the initial description of TFPI-2 internalization and translocation to the nucleus in a number of cells.

No death without life: vital functions of apoptotic effectors

As a result of the genetic experiments performed in Caenorhabditis elegans, it has been tacitly assumed that the core proteins of the 'apoptotic machinery' (CED-3, -4, -9 and EGL-1) would be solely involved in cell death regulation/execution and would not exert any functions outside of the cell death realm. However, multiple studies indicate that the mammalian orthologs of these C. elegans proteins (i.e. caspases, Apaf-1 and multidomain proteins of the Bcl-2 family) participate in cell death-unrelated processes. Similarly, loss-of-function mutations of ced-4 compromise the mitotic arrest of DNA-damaged germline cells from adult nematodes, even in a context in which the apoptotic machinery is inoperative (for instance due to mutations of egl-1 or ced-3). Moreover, EGL-1 is required for the activation of autophagy in starved nematodes. Finally, the depletion of caspase-independent death effectors, such as apoptosis-inducing factor (AIF) and endonuclease G, provokes cell death-independent consequences, both in mammals and in yeast (Saccharomyces cerevisiae). These results corroborate the conjecture that any kind of protein that has previously been specifically implicated in apoptosis might have a phylogenetically conserved apoptosis-unrelated function, most likely as part of an adaptive response to cellular stress.

The mitochondrial serine protease HtrA2/Omi: an overview

The HtrA family refers to a group of related oligomeric serine proteases that combine a trypsin-like protease domain with at least one PDZ interaction domain. Mammals encode four HtrA proteases, named HtrA1-4. The protease activity of the HtrA member HtrA2/Omi is required for mitochondrial homeostasis in mice and humans and inactivating mutations associated with neurodegenerative disorders such as Parkinson's disease. Moreover, HtrA2/Omi is released in the cytosol, where it contributes to apoptosis through both caspase-dependent and -independent pathways. Here, we review the current knowledge of HtrA2/Omi biology and discuss the signaling pathways that underlie its mitochondrial and apoptotic functions from an evolutionary perspective.

Mammalian NDR protein kinases: from regulation to a role in centrosome duplication

The NDR (nuclear Dbf2-related) family of kinases is highly conserved from yeast to human, and has been classified as a subgroup of the AGC group of protein kinases based on the sequence of the catalytic domain. Like all other members of the AGC class of protein kinases, NDR kinases require the phosphorylation of conserved Ser/Thr residues for activation. Importantly, NDR family members have two unique stretches of primary sequence: an N-terminal regulatory (NTR) domain and an insert of several residues between subdomains VII and VIII of the kinase domain. The kinase domain insert functions as an auto-inhibitory sequence (AIS), while binding of the co-activator MOB (Mps-one binder) proteins to the NTR domain releases NDR kinases from inhibition of autophosphorylation. However, despite such advances in our understanding of the molecular activation mechanism(s) and physiological functions of NDR kinases in yeast and invertebrates, most biological NDR substrates still remain to be identified. Nevertheless, by showing that the centrosomal subpopulation of human NDR1/2 is required for proper centrosome duplication, the first biological role of human NDR1/2 kinases has been defined recently. How far NDR-driven centrosome overduplication could actually contribute to cellular transformation will also be discussed.

Lats2 is an essential mitotic regulator required for the coordination of cell division

Tumor suppressor Lats2 is a member of the conserved Dbf2 kinase family. It localizes to the centrosome and has been implicated in regulation of the cell cycle and apoptosis. However, the in vivo function of this kinase remains unclear. Here, we show that complete disruption of the gene encoding Lats2 in mice causes developmental defects in the nervous system and embryonic lethality. Furthermore, mutant cells derived from total LATS2-knock-out embryos exhibit mitotic defects including centrosome fragmentation and cytokinesis defects, followed by nuclear enlargement and multinucleation. We show that the Mob1 family, a regulator of mitotic exit, associates with Lats2 to induce its activation. We also show that the complete LATS2-knock-out cells exhibit an acceleration of exit from mitosis and marked down-regulation of critical mitotic regulators. These results suggest that Lats2 plays an essential mitotic role in coordinating accurate cytokinesis completion, governing the stabilization of other mitotic regulators.