Protein Phosphatase 4 Is a Positive Regulator of Hematopoietic Progenitor Kinase 1

An updated review of small-molecule HPK1 kinase inhibitors (2016-present)

Hematopoietic progenitor kinase 1 (HPK1) is a serine-threonine kinase specific to hematopoiesis and a member of the MAP4K family of Ste20-related protein kinases. Targeting HPK1 to ameliorate T cell exhaustion and enhance T cell functions is a promising strategy for clinical immunotherapies. Numerous studies have reported the progress in developing effective HPK1 inhibitors and elucidating their mechanisms of action. However, most inhibitors affect multiple signaling pathways, resulting in unintended side effects that limit their clinical development and application. Herein, we reviewed HPK1-related signaling pathways, clinical candidates and recent advances in small-molecule inhibitors targeting HPK1. Additionally, we present our perspectives on current challenges and potential future research field, hoping to provide inspiration for the development of novel HPK1 inhibitors.

Comprehensive analysis of PPP4C's impact on prognosis, immune microenvironment, and immunotherapy response in lung adenocarcinoma using single-cell sequencing and multi-omics

Background

Elevated PPP4C expression has been associated with poor prognostic implications for patients suffering from lung adenocarcinoma (LUAD). The extent to which PPP4C affects immune cell infiltration in LUAD, as well as the importance of associated genes in clinical scenarios, still requires thorough investigation.

Methods

In our investigation, we leveraged both single-cell and comprehensive RNA sequencing data, sourced from LUAD patients, in our analysis. This study also integrated datasets of immune-related genes from InnateDB into the framework. Our expansive evaluation employed various analytical techniques; these included pinpointing differentially expressed genes, constructing WGCNA, implementing Cox proportional hazards models. We utilized these methods to investigate the gene expression profiles of PPP4C within the context of LUAD and to clarify its potential prognostic value for patients. Subsequent steps involved validating the observed enhancement of PPP4C expression in LUAD samples through a series of experimental approaches. The array comprised immunohistochemistry staining, Western blotting, quantitative PCR, and a collection of cell-based assays aimed at evaluating the influence of PPP4C on the proliferative and migratory activities of LUAD cells.

Results

In lung cancer, elevated expression levels of PPP4C were observed, correlating with poorer patient prognoses. Validation of increased PPP4C levels in LUAD specimens was achieved using immunohistochemical techniques. Experimental investigations have substantiated the role of PPP4C in facilitating cellular proliferation and migration in LUAD contexts. Furthermore, an association was identified between the expression of PPP4C and the infiltration of immune cells in these tumors. A prognostic framework, incorporating PPP4C and immune-related genes, was developed and recognized as an autonomous predictor of survival in individuals afflicted with LUAD. This prognostic tool has demonstrated considerable efficacy in forecasting patient survival and their response to immunotherapeutic interventions.

Conclusion

The involvement of PPP4C in LUAD is deeply intertwined with the tumor's immune microenvironment. PPP4C's over-expression is associated with negative clinical outcomes, promoting both tumor proliferation and spread. A prognostic framework based on PPP4C levels may effectively predict patient prognoses in LUAD, as well as the efficacy of immunotherapy strategy. This research sheds light on the mechanisms of immune interaction in LUAD and proposes a new strategy for treatment.

Functional roles of protein phosphatase 4 in multiple aspects of cellular physiology: a friend and a foe

Protein phosphatase 4 (PP4), one of serine/threonine phosphatases, is involved in many critical cellular pathways, including DNA damage response (DNA repair, cell cycle regulation, and apoptosis), tumorigenesis, cell migration, immune response, stem cell development, glucose metabolism, and diabetes. PP4 has been steadily studied over the past decade about wide spectrum of physiological activities in cells. Given the many vital functions in cells, PP4 has great potential to develop into the finding of key working mechanisms and effective treatments for related diseases such as cancer and diabetes. In this review, we provide an overview of the cellular and molecular mechanisms by which PP4 impacts and also discuss the functional significance of it in cell health.

PP4C facilitates lung cancer proliferation and inhibits apoptosis via activating MAPK/ERK pathway

PURPOSE

Protein phosphatase 4 catalytic subunit (PP4C) has been shown to play crucial regulatory roles in biological process and is frequently upregulated in cancer such as breast and colorectal carcinoma. However, the function and potential molecular mechanism of PP4C in lung cancer remains unclear.

METHODS

Bioinformatic analysis was used to detect the expression level and prognosis of patients. Western blot, quantitative real-time PCR (qRT-PCR), CCK8, 5-Ethynyl-2'-deoxyuridine (Edu) proliferation assay and flow cytometric were used to explore the function in lung cancer cells.

RESULTS

In this study, we found that PP4C was upregulated in lung cancer tissues as compared with that in normal lung tissues. Furthermore, patients with high expression level of PP4C were correlated with a poor prognosis in lung cancer patients. In vitro, CCK8, Edu proliferation assays and flow cytometry analysis showed that PP4C could promote lung cancer cell growth and inhibit apoptosis. Mechanistic investigations revealed that PP4C may interact with PP4R1 and promote ERK activation. Additionally, PP4C depletion resulted in lower tumor growth in vivo.

CONCLUSIONS

Taken together, these data showed the oncogenic of PP4C in NSCLC tumorigenesis and provide a new insight of PP4C in the progression of NSCLC.

Protein phosphatase 4 (PP4) functions as a critical regulator in tumor necrosis factor (TNF)-α-induced hepatic insulin resistance

Protein phosphatase 4 (PP4) was shown to participate in multiple cellular processes, including DNA damage response, cell cycle and embryo development. Recent studies demonstrated a looming role of PP4 in glucose metabolism. However, whether PP4 is involved in hepatic insulin resistance remains poorly understood. The objective of this study was to estimate the role of PP4 in tumor necrosis factor (TNF)-α-induced hepatic insulin resistance. db/db mice and TNF-α-treated C57BL/6J mice were used as hepatic insulin resistance animal models. In vitro models were established in both HepG2 cells and primary hepatocytes by TNF-α treatment. We found that increased expression and activity of PP4 occurred in the livers of db/db mice and TNF-α-induced hepatic insulin resistance both in vitro and in vivo. Actually, PP4 silencing and suppression of PP4 activity ameliorated TNF-α-induced hepatic insulin resistance, whereas over-expression of PP4 caused insulin resistance. We then further investigated the prodiabetic mechanism of PP4 in TNF-α-induced insulin resistance. We found that PP4 formed a complex with IRS-1 to promote phosphorylation of IRS-1 on serine 307 via JNK activation and reduce the expression of IRS-1. Thus, PP4 is an important regulator in inflammatory related insulin resistance.

High expression of protein phosphatase 4 is associated with the aggressive malignant behavior of colorectal carcinoma

Background

Recent evidence suggests an important role of protein phosphatase 4 (PP4C) in the progression of several cancers, including breast cancer, lung cancer and pancreatic ductal adenocarcinoma. However, the contribution of PP4C to colorectal carcinoma (CRC) remains elusive.

Methods

The expression of PP4C in CRC tissues compared with matched non-tumor tissues and CRC cells was detected using quantitative RT-PCR, immunohistochemistry and western blotting assays. Through univariate and Kaplan-Meier analysis, we correlated the PP4C expression with clinicopathological features and patient survival. A series of experiments, including cell proliferation, lentiviral infection, cell invasion and MMP gelatinase activity assays, were performed to investigate the underlying mechanisms. Through further experiments, tumor growth and metastasis were evaluated in vivo using a xenogenous subcutaneously implant model and a tail vein metastasis model.

Results

In the present study, we found that PP4C expression is frequently increased in human CRC and that the upregulation of PP4C correlates with a more invasive tumor phenotype and poor prognosis. The ectopic expression of PP4C promoted CRC cell proliferation, migration and invasion in vitro and tumor growth and lung metastasis in vivo. Silencing the expression of PP4C resulted in the inhibition of cell proliferation and invasion. Further investigations showed that phosphorylated Akt (p-AKT) is required for the PP4C-mediated upregulation of MMP-2 and MMP-9, which promotes cell invasion.

Conclusions

Our data suggested a potential role of PP4C in tumor progression and provided novel insights into the mechanism of how this factor positively regulated cell proliferation and invasion in CRC cells.

The CUL7/F-box and WD repeat domain containing 8 (CUL7/Fbxw8) ubiquitin ligase promotes degradation of hematopoietic progenitor kinase 1

HPK1, a member of mammalian Ste20-like serine/threonine kinases, is lost in >95% pancreatic cancer through proteasome-mediated degradation. However, the mechanism of HPK1 loss has not been defined. The aims of this study are to identify the ubiquitin ligase and to examine the mechanisms that targets HPK1 degradation. We found that the CUL7/Fbxw8 ubiquitin ligase targeted HPK1 for degradation via the 26 S proteasome. The ubiquitination of HPK1 required its kinase activity and autophosphorylation. Wild-type protein phosphatase 4 (PP4), but not the phosphatase-dead PP4 mutant, PP4-RL, inhibits the interaction of Fbxw8 with HPK1 and Fbxw8-mediated ubiquitination of HPK1. In addition, we showed that Thr-355 of HPK1 is a key PP4 dephosphorylation site, through which CUL7/Fbxw8 ubiquitin ligase and PP4 regulates HPK1 stability. Knockdown of Fbxw8 restores endogenous HPK1 protein expression and inhibits cell proliferation of pancreatic cancer cells. Our study demonstrated that targeted degradation of HPK1 by the CUL7/Fbxw8 ubiquitin ligase constitutes a negative-feedback loop to restrain the activity of HPK1 and that CUL7/Fbxw8 ubiquitin ligase promotes pancreatic cancer cell proliferation. CUL7/Fbxw8 ubiquitin ligase-mediated HPK1 degradation revealed a direct link and novel role of CUL7/Fbxw8 ubiquitin ligase in the MAPK pathway, which plays a critical role in cell proliferation and differentiation.

Attenuation of T cell receptor signaling by serine phosphorylation-mediated lysine 30 ubiquitination of SLP-76 protein

SLP-76 (SH2 domain-containing leukocyte protein of 76 kDa) is an adaptor protein that is essential for T cell development and T cell receptor (TCR) signaling activation. Previous studies have identified an important negative feedback regulation of SLP-76 by HPK1 (hematopoietic progenitor kinase 1; MAP4K1)-induced Ser-376 phosphorylation. Ser-376 phosphorylation of SLP-76 mediates 14-3-3 binding, resulting in the attenuation of SLP-76 activation and downstream signaling; however, the underlying mechanism of this action remains unknown. Here, we report that phosphorylated SLP-76 is ubiquitinated and targeted for proteasomal degradation during TCR signaling. SLP-76 ubiquitination is mediated by Ser-376 phosphorylation. Furthermore, Lys-30 is identified as a ubiquitination site of SLP-76. Loss of Lys-30 ubiquitination of SLP-76 results in enhanced anti-CD3 antibody-induced ERK and JNK activation. These results reveal a novel regulation mechanism of SLP-76 by ubiquitination and proteasomal degradation of activated SLP-76, which is mediated by Ser-376 phosphorylation, leading to down-regulation of TCR signaling.

Negative regulation of pancreatic and duodenal homeobox-1 by somatostatin receptor subtype 5

Somatostatin receptor subtype 5 (SSTR5) mediates the inhibitory effect of somatostatin and its analogs on insulin expression/secretion and islet cell proliferation. We provide biochemical and genetic evidence that SSTR5 exerted its physiological actions via down-regulating pancreatic and duodenal homeobox-1 (PDX-1), a β-cell-specific homeodomain-containing transcription factor. Cotransfection of SSTR5 with PDX-1 resulted in dose-dependent inhibition of PDX-1 expression in human embryonic kidney 293 cells. SSTR5 agonist RPL-1980 inhibited PDX-1 expression and abolished glucagon-like peptide 1-stimulated PDX-1 expression in mouse insulinoma β-TC-6 cells. SSTR5 knockdown by short hairpin RNA led to increased PDX-1 expression that was accompanied by enhanced insulin secretion stimulated by high glucose in β-TC6 cells and alternated expressions of cell cycle proteins that favor cell proliferation in mouse insulinoma MIN6 cells. Quantitative RT-PCR analysis showed that cotransfected SSTR5 inhibited PDX-1 mRNA expression, whereas knockdown of SSTR5 increased PDX-1 mRNA expression. In addition, we found that cotransfected wild-type SSTR5 increased PDX-1 ubiquitination in human embryonic kidney 293 cells, whereas SSTR5 P335L, a hypofunctional single nucleotide polymorphism of SSTR5, inhibited PDX-1 ubiquitination. SSTR5 knockout resulted in increased expression of PDX-1, insulin, and proliferating cell nuclear antigen in the islets of sstr(-/-) mice. Immunohistochemistry analysis showed that SSTR5 P335L was associated with elevated expression of PDX-1 in human pancreatic neuroendocrine tumor. Taken together, our studies demonstrated that SSTR5 is a negative regulator for PDX-1 expression and that SSTR5 may mediate the inhibitory effects of somatostatin and its analogs on insulin expression/secretion and cell proliferation via down-regulating PDX-1 at both transcriptional and posttranslational levels.

Overexpression of protein phosphatase 4 correlates with poor prognosis in patients with stage II pancreatic ductal adenocarcinoma

PURPOSE

Protein phosphatase 4 (PP4) has been reported to be overexpressed in breast and lung cancers. PP4 plays an important role in the regulation of centrosome maturation, DNA repair, NF-κB, and c-jun-NH(2)-kinase (JNK) signaling pathways. However, the expression and functions of PP4 in pancreatic cancer have not been studied.

EXPERIMENTAL DESIGN

We examined the expression of PP4 catalytic subunit (PP4C) protein in 133 patients with stage II pancreatic ductal adenocarcinoma (PDAC) and their paired benign pancreatic samples (N = 113) by immunohistochemistry. To confirm the immunohistochemical results, we measured PP4C protein and mRNA levels by Western blotting and real-time reverse transcriptase PCR. Using univariate and multivariate analysis, we correlated PP4C expression with survival and other clinicopathologic features.

RESULTS

PP4C was overexpressed in 75 of 133 (56.4%) stage II PDAC samples, which was significantly higher than the paired benign pancreatic tissue (15%, 17 of 113). PP4C mRNA expression levels were also higher in PDAC samples than the paired benign pancreatic tissue. Overexpression of PP4C in PDAC samples was associated with higher frequencies of distant metastasis (P = 0.02) and poor disease-free and overall survivals in patients with stage II PDAC (P = 0.006 and 0.02) independent of tumor size, margin status, and lymph node status (stage).

CONCLUSIONS

Our study showed that PP4C is overexpressed in PDAC. Overexpression of PP4C in PDAC samples is associated with poor prognosis in patients with stage II PDAC. Therefore, targeting PP4 signaling pathway may represent a new approach for the treatment of PDAC.

IMPACT

Our study showed that PP4C is an independent prognostic factor in patients with stage II PDAC.

Loss of CCDC6, the first identified RET partner gene, affects pH2AX S139 levels and accelerates mitotic entry upon DNA damage

CCDC6 was originally identified in chimeric genes caused by chromosomal translocation involving the RET proto-oncogene in some thryoid tumors mostly upon ionizing radiation exposure. Recognised as a pro-apoptotic phosphoprotein that negatively regulates CREB1-dependent transcription, CCDC6 is an ATM substrate that is responsive to genotoxic stress. Here we report that following genotoxic stress, loss or inactivation of CCDC6 in cancers that carry the CCDC6 fusion, accelerates the dephosphorylation of pH2AX S139, resulting in defective G2 arrest and premature mitotic entry. Moreover, we show that CCDC6 depleted cells appear to repair DNA damaged in a shorter time compared to controls, based on reporter assays in cells. High-troughput proteomic screening predicted the interaction between the CCDC6 gene product and the catalytic subunit of Serin-Threonin Protein Phosphatase 4 (PP4c) recently identified as the evolutionarily conserved pH2AX S139 phosphatase that is activated upon DNA Damage. We describe the interaction between CCDC6 and PP4c and we report the modulation of PP4c enzymatic activity in CCDC6 depleted cells. We discuss the functional significance of CCDC6-PP4c interactions and hypothesize that CCDC6 may act in the DNA Damage Response by negatively modulating PP4c activity. Overall, our data suggest that in primary tumours the loss of CCDC6 function could influence genome stability and thereby contribute to carcinogenesis.

Inhibited expression of hematopoietic progenitor kinase 1 associated with loss of jumonji domain containing 3 promoter binding contributes to autoimmunity in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by T cell overactivation and B cell hyper-stimulation. Hematopoietic progenitor kinase 1 (HPK1, also called MAP4K1) negatively regulates T cell-mediated immune responses. However, the role of HPK1 and the mechanisms that regulate HPK1 expression in SLE remain poorly understood. Using chromatin immunoprecipitation (ChIP) microarray data, we identified markedly increased histone H3 lysine 27 trimethylation (H3K27me3) enrichment at the HPK1 promoter of SLE CD4+ T cells relative to controls, and confirmed this observation using ChIP and real-time PCR experiments. We further found that HPK1 mRNA and protein levels were significantly decreased in CD4+ T cells of patients with SLE, and that this decrease was not caused by exposure to standard SLE medications. Down-regulating HPK1 in healthy CD4+ T cells significantly accelerated T cell proliferation and production of IFNγ and IgG. Consistent with these findings, overexpressing HPK1 in SLE CD4+ T cells caused a significant decrease in T cell reactivity. In addition, we observed a striking decrease in jumonji domain containing 3 (JMJD3) binding, but no marked change in enhancer of zeste homolog 2 (EZH2) binding, at the HPK1 promoter region in SLE CD4+ T cells compared to healthy controls. SiRNA knock down of JMJD3 in healthy CD4+ T cells led to decreased JMJD3 binding and increased H3K27me3 enrichment at the HPK1 promoter region, thus inhibiting the expression of HPK1. Concordantly, plasmid-induced overexpression of JMJD3 in SLE CD4+ T cells led to increased JMJD3 binding, decreased H3K27me3 enrichment, and up-regulated HPK1 expression. Our results show for the first time that inhibited HPK1 expression in SLE CD4+ T cells is associated with loss of JMJD3 binding and increased H3K27me3 enrichment at the HPK1 promoter, contributing to T cell overactivation and B cell overstimulation in SLE. These findings suggest that HPK1 may serve as a novel target for effective SLE therapy.

Suppressor of MEK null (SMEK)/protein phosphatase 4 catalytic subunit (PP4C) is a key regulator of hepatic gluconeogenesis

Fasting promotes hepatic gluconeogenesis to maintain glucose homeostasis. The cAMP-response element binding protein (CREB)-regulated transcriptional coactivator 2 (CRTC2) is responsible for transcriptional activation of gluconeogenic genes and is critical for conveying the opposing hormonal signals of glucagon and insulin in the liver. Here, we show that suppressor of MEK null 1 (SMEK1) and SMEK2 [protein phosphatase 4 (PP4) regulatory subunits 3a and 3b, respectively] are directly involved in the regulation of hepatic glucose metabolism in mice. Expression of hepatic SMEK1/2 is up-regulated during fasting or in mouse models of insulin-resistant conditions in a Peroxisome Proliferator-Activated Receptor-gamma Coactivator 1α (PGC-1α)-dependent manner. Overexpression of SMEK promotes elevations in plasma glucose with increased hepatic gluconeogenic gene expression, whereas depletion of the SMEK proteins reduces hyperglycemia and enhances CRTC2 phosphorylation; the effect is blunted by S171A CRTC2, which is refractory to salt-inducible kinase (SIK)-dependent inhibition. Taken together, we would propose that mammalian SMEK/PP4C proteins are involved in the regulation of hepatic glucose metabolism through dephosphorylation of CRTC2.

Protein phosphatase 4 regulates apoptosis in leukemic and primary human T-cells

The control of T-cell survival is of overwhelming importance for preventing leukemia and lymphoma. The present report demonstrates that the serine/threonine protein phosphatase PP4 regulates the survival of both leukemic T-cells and untransformed human peripheral blood T-cells, particularly after treatment with anti-leukemic drugs and other cytotoxic stimuli. PP4-induced apoptosis is mediated, at least in part, through de-phosphorylation of apoptosis regulator PEA-15, previously implicated in the control of leukemic cell survival. PP4 activity significantly affects the mutation rate in leukemic T-cells, indicating that PP4 dysfunction may be important in the development and progression of leukemia.

Protein phosphatase 4 mediates localization of the Miranda complex during Drosophila neuroblast asymmetric divisions

Asymmetric localization of cell fate determinants is a crucial step in neuroblast asymmetric divisions. Whereas several protein kinases have been shown to mediate this process, no protein phosphatase has so far been implicated. In a clonal screen of larval neuroblasts we identified the evolutionarily conserved Protein Phosphatase 4 (PP4) regulatory subunit PP4R3/Falafel (Flfl) as a key mediator specific for the localization of Miranda (Mira) and associated cell fate determinants during both interphase and mitosis. Flfl is predominantly nuclear during interphase/prophase and cytoplasmic after nuclear envelope breakdown. Analyses of nuclear excluded as well as membrane targeted versions of the protein suggest that the asymmetric cortical localization of Mira and its associated proteins during mitosis depends on cytoplasmic/membrane-associated Flfl, whereas nuclear Flfl is required to exclude the cell fate determinant Prospero (Pros), and consequently Mira, from the nucleus during interphase/prophase. Attenuating the function of either the catalytic subunit of PP4 (PP4C; Pp4-19C in Drosophila) or of another regulatory subunit, PP4R2 (PPP4R2r in Drosophila), leads to similar defects in the localization of Mira and associated proteins. Flfl is capable of directly interacting with Mira, and genetic analyses indicate that flfl acts in parallel to or downstream from the tumor suppressor lethal (2) giant larvae (lgl). Our findings suggest that Flfl may target PP4 to the MIra protein complex to facilitate dephosphorylation step(s) crucial for its cortical association/asymmetric localization.

Proteasome-mediated degradation and functions of hematopoietic progenitor kinase 1 in pancreatic cancer

Hematopoietic progenitor kinase 1 (HPK1) regulates stress responses, proliferation, and apoptosis in hematopoietic cells. In this study, we examined the expression, regulation, and functions of HPK1 in pancreatic ductal adenocarcinomas (PDA). We found that loss of HPK1 protein expression correlated significantly with the progression of pancreatic intraepithelial neoplasias (P = 0.001) and development of invasive PDA. Similarly, HPK1 protein was not expressed in any of eight PDA cell lines examined but was expressed in immortalized human pancreatic duct epithelial (HPDE) cells. There was no difference in HPK1 mRNA levels in PDA cell lines or primary PDA compared with those in HPDE cells or ductal epithelium in chronic pancreatitis and normal pancreas, respectively. Treatment of Panc-1 cells with a proteasome inhibitor, MG132, increased the HPK1 protein levels in a dose-dependent manner, suggesting that alteration in proteasome activity contributes to the loss of HPK1 protein expression in pancreatic cancer. Like the endogenous HPK1, both wild-type HPK1 and its kinase-dead mutant, HPK1-M46, overexpressed in Panc-1 cells, were also targeted by proteasome-mediated degradation. After MG132 withdrawal, wild-type HPK1 protein expression was markedly decreased within 24 hours, but kinase-dead HPK1 mutant protein expression was sustained for up to 96 hours. Therefore, HPK1 kinase activities were required for the loss of HPK1 protein in PDAs. Furthermore, restoring wild-type HPK1 protein in PDA cells led to the increase in p21 and p27 protein expression and cell cycle arrest. Thus, HPK1 may function as a novel tumor suppressor and its loss plays a critical role in pancreatic cancer.

Deactivation of sphingosine kinase 1 by protein phosphatase 2A

Sphingosine kinase 1 (SK1) is an important regulator of cellular signaling that has been implicated in a broad range of cellular processes. Cell exposure to a wide array of growth factors, cytokines, and other cell agonists can result in a rapid and transient increase in SK activity via an activating phosphorylation. We have previously identified extracellular signal-regulated kinases 1 and 2 (ERK1/2) as the kinases responsible for the phosphorylation of human SK1 at Ser(225), but the corresponding phosphatase targeting this phosphorylation has remained undefined. Here, we provide data to support a role for protein phosphatase 2A (PP2A) in the deactivation of SK1 through dephosphorylation of phospho-Ser(225). The catalytic subunit of PP2A (PP2Ac) was found to interact with SK1 using both GST-pulldown and coimmunoprecipitation analyses. Coexpression of PP2Ac with SK1 resulted in reduced Ser(225) phosphorylation of SK1 in human embryonic kidney (HEK293) cells. In vitro phosphatase assays showed that PP2Ac dephosphorylated both recombinant SK1 and a phosphopeptide based on the phospho-Ser(225) region of SK1. Finally, both basal and tumor necrosis factor-alpha-stimulated cellular SK1 activity were regulated by molecular manipulation of PP2Ac activity. Thus, PP2A appears to function as an endogenous regulator of SK1 phosphorylation.

Depletion of protein phosphatase 4 in human cells reveals essential roles in centrosome maturation, cell migration and the regulation of Rho GTPases

The mechanisms that co-ordinate centrosome maturation and the migration of human cells remain elusive. Protein phosphatase 4 (Ppp4) is a ubiquitous protein serine/threonine phosphatase in eukaryotes that is enriched at centrosomes. HEK293 cells cultures depleted to 30% Ppp4c levels by lentivirus-delivered stable gene silencing were delayed in mitosis at the prometaphase/metaphase boundary and displayed cells with aberrant chromosome organisation and microtubules unconnected to the centrosomes. The levels of alpha- and gamma-tubulin and aurora A were decreased; in mitotic cells, the cytological localisations of polo-like kinase 1, alpha- and gamma-tubulin and aurora A were aberrant and the phosphorylation of Aurora A-Thr 288 was decreased. The novel localisation of endogenous Ppp4 regulatory subunit, R3A, to centrosomes in human mitotic cells suggests that a Ppp4c-R2-R3 trimeric complex mediates centrosome maturation. We demonstrate for the first time that human cells depleted to 30% Ppp4c showed severely decreased migration and exhibit decreased levels of both total beta-actin and filamentous actin in cell extensions, filopodia and lamellopodia-like structures. Our studies show that Ppp4c is required for the organisation of the actin cytoskeleton at the leading edge of human cells during migration. We also demonstrate that the active forms of the RhoGTPases, Rac1 and Cdc42, are substantially decreased in the presence and absence of growth factor in Ppp4c depleted cells, implicating Ppp4c in the regulation of these GTPases. The results suggest that Ppp4c-R2-R3 complexes may co-ordinate centrosome maturation and cell migration via regulation of RhoGTPases and that Ppp4 may be a useful anticancer target.

Differential regulation and properties of MAPKs

Mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs including embryogenesis, proliferation, differentiation and apoptosis based on cues derived from the cell surface and the metabolic state and environment of the cell. In mammals, there are more than a dozen MAPK genes. The best known are the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun N-terminal kinase (JNK(1-3)) and p38(alpha, beta, gamma and delta) families. ERK3, ERK5 and ERK7 are other MAPKs that have distinct regulation and functions. MAPK cascades consist of a core of three protein kinases. Despite the apparently simple architecture of this pathway, these enzymes are capable of responding to a bewildering number of stimuli to produce exquisitely specific cellular outcomes. These responses depend on the kinetics of their activation and inactivation, the subcellular localization of the kinases, the complexes in which they act, and the availability of substrates. Fine-tuning of cascade activity can occur through modulatory inputs to cascade component from the primary kinases to the scaffolding accessory proteins. Here, we describe some of the properties of the three major MAPK pathways and discuss how these properties govern pathway regulation and activity.

Conditional knockout mice reveal an essential role of protein phosphatase 4 in thymocyte development and pre-T-cell receptor signaling

Okadaic acid-sensitive serine/threonine phosphatases have been shown to regulate interleukin-2 transcription and T-cell activation. Okadaic acid inhibits protein phosphatase 4 (PP4), a novel PP2A-related serine/threonine phosphatase, at a 50% inhibitory concentration (IC(50)) comparable to that for PP2A. This raises the possibility that some cellular functions of PP2A, determined in T cells by using okadaic acid, may in fact be those of PP4. To investigate the in vivo roles of PP4 in T cells, we generated conventional and T-cell-specific PP4 conditional knockout mice. We found that the ablation of PP4 led to the embryonic lethality of mice. PP4 gene deletion in the T-cell lineage resulted in aberrant thymocyte development, including T-cell arrest at the double-negative 3 stage (CD4(-) CD8(-) CD25(+) CD44(-)), abnormal thymocyte maturation, and lower efficacy of positive selection. PP4-deficient thymocytes showed decreased proliferation and enhanced apoptosis in vivo. Analysis of pre-T-cell receptor (pre-TCR) signaling further revealed impaired calcium flux and phospholipase C-gamma1-extracellular signal-regulated kinase activation in the absence of PP4. Anti-CD3 injection in PP4-deficient mice led to enhanced thymocyte apoptosis, accompanied by increased proapoptotic Bim but decreased antiapoptotic Bcl-xL protein levels. In the periphery, antigen-specific T-cell proliferation and T-cell-mediated immune responses in PP4-deficient mice were dramatically compromised. Thus, our results indicate that PP4 is essential for thymocyte development and pre-TCR signaling.

The Saccharomyces cerevisiae orthologue of the human protein phosphatase 4 core regulatory subunit R2 confers resistance to the anticancer drug cisplatin

The anticancer agents cisplatin and oxaliplatin are widely used in the treatment of human neoplasias. A genome-wide screen in Saccharomyces cerevisiae previously identified PPH3 and PSY2 among the top 20 genes conferring resistance to these anticancer agents. The mammalian orthologue of Pph3p is the protein serine/threonine phosphatase Ppp4c, which is found in high molecular mass complexes bound to a regulatory subunit R2. We show here that the putative S. cerevisiae orthologue of R2, which is encoded by ORF YBL046w, binds to Pph3p and exhibits the same unusually high asymmetry as mammalian R2. Despite the essential function of Ppp4c-R2 in microtubule-related processes at centrosomes in higher eukaryotes, S. cerevisiae diploid strains with homozygous deletion of YBL046w and two or one functional copies of the TUB2 gene were viable and no more sensitive to microtubule-depolymerizing drugs than the control strain. The protein encoded by YBL046w exhibited a predominantly nuclear localization. These studies suggest that the centrosomal function of Ppp4c-R2 is not required or may be performed by a different phosphatase in yeast. Homozygous diploid deletion strains of S. cerevisiae, pph3Delta, ybl046wDelta and psy2Delta, were all more sensitive to cisplatin than the control strain. The YBL046w gene therefore confers resistance to cisplatin and was termed PSY4 (platinum sensitivity 4). Ppp4c, R2 and the putative orthologue of Psy2p (termed R3) are shown here to form a complex in Drosophila melanogaster and mammalian cells. By comparison with the yeast system, this complex may confer resistance to cisplatin in higher eukaryotes.

Protein phosphatase 4 - from obscurity to vital functions

Protein phosphatase 4 (Ppp4) is a ubiquitous serine/threonine phosphatase in the PPP family that is now recognised to regulate a variety of cellular functions independently of protein phosphatase 2A (PP2A). Regulatory subunits (R1 and R2) have been identified in mammals that interact with the catalytic subunit of Ppp4 (Ppp4c) and control its activity. Ppp4c-R2 complexes play roles in organelle assembly; not only are they essential for maturation of the centrosome, but they are also involved in spliceosomal assembly via interaction with the survival of motor neurons (SMNs) complex. Several cellular signalling routes, including NF-kappaB and the target of rapamycin (TOR) pathways appear to be regulated by Ppp4. Emerging evidence indicates that Ppp4 may play a role in the DNA damage response and that Ppp4c-R1 complexes decrease the activity of a histone deacetylase, implicating Ppp4 in the regulation of chromatin activities. Antitumour agents, cantharidin and fostriecin, potently inhibit the activity of Ppp4. Orthologues of mammalian Ppp4 subunits in Saccharomyces cerevisiae confer resistance to the anticancer, DNA-binding drugs, cisplatin and oxaliplatin.

Histone deacetylase 3 (HDAC3) activity is regulated by interaction with protein serine/threonine phosphatase 4

Histone deacetylase 3 (HDAC3) is one of four members of the human class I HDACs that regulates gene expression by deacetylation of histones and nonhistone proteins. Early studies have suggested that HDAC3 activity is regulated by association with the corepressors N-CoR and SMRT. Here we demonstrate that, in addition to protein-protein interactions with NCoR/SMRT, the activity of HDAC3 is regulated by both phosphorylation and dephosphorylation. A protein kinase CK2 phosphoacceptor site in the HDAC3 protein was identified at position Ser424, which is a nonconserved residue among the class I HDACs. Mutation of this residue was found to reduce deacetylase activity. Interestingly, unlike other class I HDACs, HDAC3 uniquely copurifies with the catalytic and regulatory subunits of the protein serine/threonine phosphatase 4 complex (PP4c/PP4R1). Furthermore, HDAC3 complexes displayed protein phosphatase activity and a series of subsequent mutational analyses revealed that the N terminus of HDAC3 (residues 1-122) was both necessary and sufficient for HDAC3-PP4c interactions. Significantly, both overexpression and siRNA knock-down approaches, and analysis of cells devoid of PP4c, unequivocally show that HDAC3 activity is inversely proportional to the cellular abundance of PP4(c). These findings therefore further highlight the importance of protein-protein interactions and extend the significance of dephosphorylation in the regulation of HDAC activity, as well as present a novel alternative pathway by which HDAC3 activity is regulated.