Inhibitory phosphorylation of PP1alpha catalytic subunit during the G(1)/S transition

CRABP1-complexes in exosome secretion

BACKGROUND

Cellular retinoic acid binding protein 1 (CRABP1) mediates rapid, non-canonical activity of retinoic acid (RA) by forming signalosomes via protein-protein interactions. Two signalosomes have been identified previously: CRABP1-MAPK and CRABP1-CaMKII. Crabp1 knockout (CKO) mice exhibited altered exosome profiles, but the mechanism of CRABP1 action was unclear. This study aimed to screen for and identify novel CRABP1 signalosomes that could modulate exosome secretion by using a combinatorial approach involving biochemical, bioinformatic and molecular studies.

METHODS

Immunoprecipitation coupled with mass spectrometry (IP-MS) identified candidate CRABP1-interacting proteins which were subsequently analyzed using GO Term Enrichment, Functional Annotation Clustering; and Pathway Analysis. Gene expression analysis of CKO samples revealed altered expression of genes related to exosome biogenesis and secretion. The effect of CRABP1 on exosome secretion was then experimentally validated using CKO mice and a Crabp1 knockdown P19 cell line.

RESULTS

IP-MS identified CRABP1-interacting targets. Bioinformatic analyses revealed significant association with actin cytoskeletal dynamics, kinases, and exosome secretion. The effect of CRABP1 on exosome secretion was experimentally validated by comparing circulating exosome numbers of CKO and wild type (WT) mice, and secreted exosomes from WT and siCRABP1-P19 cells. Pathway analysis identified kinase signaling and Arp2/3 complex as the major pathways where CRABP1-signalosomes modulate exosome secretion, which was validated in the P19 system.

CONCLUSION

The combinatorial approach allowed efficient screening for and identification of novel CRABP1-signalosomes. The results uncovered a novel function of CRABP1 in modulating exosome secretion, and suggested that CRABP1 could play roles in modulating intercellular communication and signal propagation.

Emerging roles of the Protein Phosphatase 1 (PP1) in the context of viral infections

Protein Phosphatase 1 (PP1) is a major serine/threonine phosphatase in eukaryotes, participating in several cellular processes and metabolic pathways. Due to their low substrate specificity, PP1's catalytic subunits do not exist as free entities but instead bind to Regulatory Interactors of Protein Phosphatase One (RIPPO), which regulate PP1's substrate specificity and subcellular localization. Most RIPPOs bind to PP1 through combinations of short linear motifs (4-12 residues), forming highly specific PP1 holoenzymes. These PP1-binding motifs may, hence, represent attractive targets for the development of specific drugs that interfere with a subset of PP1 holoenzymes. Several viruses exploit the host cell protein (de)phosphorylation machinery to ensure efficient virus particle formation and propagation. While the role of many host cell kinases in viral life cycles has been extensively studied, the targeting of phosphatases by viral proteins has been studied in less detail. Here, we compile and review what is known concerning the role of PP1 in the context of viral infections and discuss how it may constitute a putative host-based target for the development of novel antiviral strategies.

Bromocriptine monotherapy overcomes prostate cancer chemoresistance in preclinical models

Chemoresistance is a major obstacle in the clinical management of metastatic, castration-resistant prostate cancer (PCa). It is imperative to develop novel strategies to overcome chemoresistance and improve clinical outcomes in patients who have failed chemotherapy. Using a two-tier phenotypic screening platform, we identified bromocriptine mesylate as a potent and selective inhibitor of chemoresistant PCa cells. Bromocriptine effectively induced cell cycle arrest and activated apoptosis in chemoresistant PCa cells but not in chemoresponsive PCa cells. RNA-seq analyses revealed that bromocriptine affected a subset of genes implicated in the regulation of the cell cycle, DNA repair, and cell death. Interestingly, approximately one-third (50/157) of the differentially expressed genes affected by bromocriptine overlapped with known p53-p21- retinoblastoma protein (RB) target genes. At the protein level, bromocriptine increased the expression of dopamine D2 receptor (DRD2) and affected several classical and non-classical dopamine receptor signal pathways in chemoresistant PCa cells, including adenosine monophosphate-activated protein kinase (AMPK), p38 mitogen-activated protein kinase (p38 MAPK), nuclear factor kappa B  (NF-κB), enhancer of zeste homolog 2 (EZH2), and survivin. As a monotherapy, bromocriptine treatment at 15 mg/kg, three times per week, via the intraperitoneal route significantly inhibited the skeletal growth of chemoresistant C4-2B-TaxR xenografts in athymic nude mice. In summary, these results provided the first preclinical evidence that bromocriptine is a selective and effective inhibitor of chemoresistant PCa. Due to its favorable clinical safety profiles, bromocriptine could be rapidly tested in PCa patients and repurposed as a novel subtype-specific treatment to overcome chemoresistance.

PP1 catalytic isoforms are differentially expressed and regulated in human prostate cancer

The Ser/Thr-protein phosphatase PP1 (PP1) is a positive regulator of the androgen receptor (AR), which suggests major roles for PP1 in prostate carcinogenesis. However, studies dedicated to the characterization of PP1 in PCa are currently scarce. Here we analyzed the expression and localization of the PP1 catalytic (PP1c) isoforms in formalin-fixed, paraffin-embedded prostate tissue samples, as well as in PCa cell lines. We also analyzed well-characterized PCa cohorts to determine their transcript levels, identify genetic alterations, and assess promoter methylation of PP1c-coding genes. We found that PP-1A was upregulated and relocalized towards the nucleus in PCa and that PPP1CA was frequently amplified in PCa, particularly in advanced stages. PP-1B was downregulated in PCa but upregulated in a subset of tumors with AR amplification. PP-1G transcript levels were found to be associated with Gleason score. PP1c-coding genes were rarely mutated in PCa and were not prone to regulation by promoter methylation. Protein phosphorylation, on the other hand, might be an important regulatory mechanism of PP1c isoforms' activity. Altogether, our results suggest differential expression, localization, and regulation of PP1c isoforms in PCa and support the need for investigating isoform-specific roles in prostate carcinogenesis in future studies.

SPINOPHILIN: a multiplayer tumor suppressor

SPINOPHILIN (SPN, PPP1R9B or NEURABIN-2) is a multifunctional protein that regulates protein-protein interactions in different cell signaling pathways. SPN is also one of the regulatory subunits of protein phosphatase 1 (PP1), implicated in the dephosphorylation of retinoblastoma protein (pRB) during cell cycle. The SPN gene has been described as a tumor suppressor in different human tumor contexts, in which low levels of SPN are correlated with a higher grade and worse prognosis. In addition, mutations of the SPN protein have been reported in human tumors. Recently, an oncogenic mutation of SPN, A566V, was described, which affects both the SPN-PP1 interaction and the phosphatase activity of the holoenzyme, and promotes p53-dependent tumorigenesis by increasing the cancer stem cell (CSC) pool in breast tumors. Thus, the loss or mutation of SPN could be late events that promotes tumor progression by increasing the CSC pool and, eventually, the malignant behavior of the tumor.

Trichodysplasia spinulosa polyomavirus small T antigen synergistically modulates S6 protein translation and DNA damage response pathways to shape host cell environment

TSPyV is a viral agent linked to Trichodysplasia spinulosa, a disfiguring human skin disease which presents with hyperkeratotic spicule eruption in immunocompromised hosts. This proliferative disease state requires extensive modulation of the host cell environment. While the small T (sT) antigen of TSPyV has been postulated to cause widespread cellular perturbation, its specific substrates and their mechanistic connection are unclear. To identify the cellular substrates and pathways perturbed by TSPyV sT and propose a nuanced model that reconciles the multiple arms of TSPyV pathogenesis, changes in expression of several proteins and phospho-proteins in TSPyV sT expressing and TSPyV sT deletion mutant-expressing cell lysates were interrogated using Western blot assays. TSPyV sT expression exploits the DNA damage response pathway, by inducing hyperphosphorylation of ATM and 53BP1 and upregulation of BMI-1. Concurrently, sT dysregulates the S6 protein translation pathway via hyperphosphorylation of CDC2, p70 S6 kinase, S6, and PP1α. The S6^S244/247 and p-PP1α^T320 phospho-forms are points of overlap between the DDR and S6 networks. We propose a mechanistic rationale for previous reports positioning sT antigen as the key driver of TSPyV pathogenesis. We illuminate novel targets in the S6 and DDR pathways and recognize a potential synergy between these pathways. TSPyV may sensitize the cell to both unrestricted translation and genomic instability. This multi-pronged infection model may inform future therapeutic modalities against TSPyV and possibly other viruses with overlapping host substrates.

Role of the Holoenzyme PP1-SPN in the Dephosphorylation of the RB Family of Tumor Suppressors During Cell Cycle

Simple Summary

Cell cycle progression is highly regulated by modulating the phosphorylation status of retinoblastoma (RB) family proteins. This process is controlled by a balance in the action of kinases, such as the complexes formed by cyclin-dependent kinases (CDKs) and cyclins, and phosphatases, mainly the protein phosphatase 1 (PP1). However, while the phosphorylation of the RB family has been largely studied, its dephosphorylation is less known. Recently, the PP1-Spinophilin (SPN) holoenzyme has been described as the main phosphatase responsible for the dephosphorylation of RB proteins during the G0/G1 transition and at the end of G1. Here, we describe the regulation of the phosphorylation status of RB family proteins, giving importance not only to their inactivation by phosphorylation but also to their dephosphorylation to restore the cell cycle.

Abstract

Cell cycle progression is highly regulated by modulating the phosphorylation status of the retinoblastoma protein (pRB) and the other two members of the RB family, p107 and p130. This process is controlled by a balance in the action of kinases, such as the complexes formed by cyclin-dependent kinases (CDKs) and cyclins, and phosphatases, mainly the protein phosphatase 1 (PP1). However, while the phosphorylation of the RB family has been largely studied, its dephosphorylation is less known. Phosphatases are holoenzymes formed by a catalytic subunit and a regulatory protein with substrate specificity. Recently, the PP1-Spinophilin (SPN) holoenzyme has been described as the main phosphatase responsible for the dephosphorylation of RB proteins during the G0/G1 transition and at the end of G1. Moreover, SPN has been described as a tumor suppressor dependent on PP1 in lung and breast tumors, where it promotes tumorigenesis by increasing the cancer stem cell pool. Therefore, a connection between the cell cycle and stem cell biology has also been proposed via SPN/PP1/RB proteins.

Mutation of SPINOPHILIN (PPP1R9B) found in human tumors promotes the tumorigenic and stemness properties of cells

Rationale: SPINOPHILIN (SPN, PPP1R9B) is an important tumor suppressor involved in the progression and malignancy of different tumors depending on its association with protein phosphatase 1 (PP1) and the ability of the PP1-SPN holoenzyme to dephosphorylate retinoblastoma (pRB).

Methods: We performed a mutational analysis of SPN in human tumors, focusing on the region of interaction with PP1 and pRB. We explored the effect of the SPN-A566V mutation in an immortalized non-tumorigenic cell line of epithelial breast tissue, MCF10A, and in two different p53-mutated breast cancer cells lines, T47D and MDA-MB-468.

Results: We characterized an oncogenic mutation of SPN found in human tumor samples, SPN-A566V, that affects both the SPN-PP1 interaction and its phosphatase activity. The SPN-A566V mutation does not affect the interaction of the PP1-SPN holoenzyme with pocket proteins pRB, p107 and p130, but it affects its ability to dephosphorylate them during G0/G1 and G1, indicating that the PP1-SPN holoenzyme regulates cell cycle progression. SPN-A566V also promoted stemness, establishing a connection between the cell cycle and stem cell biology via pocket proteins and PP1-SPN regulation. However, only cells with both SPN-A566V and mutant p53 have increased tumorigenic and stemness properties.

Conclusions: SPN-A566V, or other equivalent mutations, could be late events that promote tumor progression by increasing the CSC pool and, eventually, the malignant behavior of the tumor.

Towards Dissecting the Mechanism of Protein Phosphatase-1 Inhibition by Its C-Terminal Phosphorylation

Phosphoprotein phosphatase‐1 (PP1) is a key player in the regulation of phospho‐serine (pSer) and phospho‐threonine (pThr) dephosphorylation and is involved in a large fraction of cellular signaling pathways. Aberrant activity of PP1 has been linked to many diseases, including cancer and heart failure. Besides a well‐established activity control by regulatory proteins, an inhibitory function for phosphorylation (p) of a Thr residue in the C‐terminal intrinsically disordered tail of PP1 has been demonstrated. The associated phenotype of cell‐cycle arrest was repeatedly proposed to be due to autoinhibition of PP1 through either conformational changes or substrate competition. Here, we use PP1 variants created by mutations and protein semisynthesis to differentiate between these hypotheses. Our data support the hypothesis that pThr exerts its inhibitory function by mediating protein complex formation rather than by a direct mechanism of structural changes or substrate competition.

Protein phosphatase 1 in tumorigenesis: is it worth a closer look?

Cancer cells take advantage of signaling cascades to meet their requirements for sustained growth and survival. Cell signaling is tightly controlled by reversible protein phosphorylation mechanisms, which require the counterbalanced action of protein kinases and protein phosphatases. Imbalances on this system are associated with cancer development and progression. Protein phosphatase 1 (PP1) is one of the most relevant protein phosphatases in eukaryotic cells. Despite the widely recognized involvement of PP1 in key biological processes, both in health and disease, its relevance in cancer has been largely neglected. Here, we provide compelling evidence that support major roles for PP1 in tumorigenesis.

Identification and functional analysis of five genes that encode distinct isoforms of protein phosphatase 1 in Nilaparvata lugens

Ten distinct cDNAs encoding five different protein phosphatases 1 (PPP1) were cloned from Nilaparvata lugens. NlPPP1α and NlPPP1β are highly conserved whereas NlPPP1-Y, NlPPP1-Y1 and NlPPP1-Y2 are lowly conserved among insects. NlPPP1α and NlPPP1β exhibited a ubiquitous expression, while NlPPP1-Y, NlPPP1-Y1, and NlPPP1-Y2 were obviously detected from the 4th instar nymph to imago developmental stages in males, especially detected in internal reproductive organ and fat bodies of the male. Injection nymphs with dsRNA of NlPPP1α or NlPPP1β was able to reduce the target gene expression in a range of 71.5-91.0%, inducing a maximum mortality rate of 95.2% or 97.2% at 10th day after injection and eclosion ratio down by 65.5-100.0%. Injection with dsNlPPP1Ys targeted to NlPPP1-Y, NlPPP1-Y1and NlPPP1-Y2 was able to induce a maximum mortality rate of 95.5% at 10th day after injection, eclosion ratio down by 86.4%. Knock-down one of the male-biased NlPPP1 genes has no effect on survival and eclosion ratio. Injection of 4th instar nymph with dsNlPPP1Ys led to reduced oviposition amount and hatchability, down by 44.7% and 19.6% respectively. Knock-down of NlPPP1-Y1 or NlPPP1-Y2 gene did not significantly affect oviposition amount but significantly affected hatchability. The results indicate that the male-biased NlPPP1 genes have overlapping functions in N. lugens development, and NlPPP1-Y1 and NlPPP1-Y2 may play important roles in spermatogenesis and fertilization. The dsNlPPP1β and dsNlPPP1Ys in this study could be the preferred sequence in RNAi and low-conserved male-biased NlPPP1 genes could be potential target for N. lugens control.

The G2-to-M transition from a phosphatase perspective: a new vision of the meiotic division

Cell division is orchestrated by the phosphorylation and dephosphorylation of thousands of proteins. These post-translational modifications underlie the molecular cascades converging to the activation of the universal mitotic kinase, Cdk1, and entry into cell division. They also govern the structural events that sustain the mechanics of cell division. While the role of protein kinases in mitosis has been well documented by decades of investigations, little was known regarding the control of protein phosphatases until the recent years. However, the regulation of phosphatase activities is as essential as kinases in controlling the activation of Cdk1 to enter M-phase. The regulation and the function of phosphatases result from post-translational modifications but also from the combinatorial association between conserved catalytic subunits and regulatory subunits that drive their substrate specificity, their cellular localization and their activity. It now appears that sequential dephosphorylations orchestrated by a network of phosphatase activities trigger Cdk1 activation and then order the structural events necessary for the timely execution of cell division. This review discusses a series of recent works describing the important roles played by protein phosphatases for the proper regulation of meiotic division. Many breakthroughs in the field of cell cycle research came from studies on oocyte meiotic divisions. Indeed, the meiotic division shares most of the molecular regulators with mitosis. The natural arrests of oocytes in G2 and in M-phase, the giant size of these cells, the variety of model species allowing either biochemical or imaging as well as genetics approaches explain why the process of meiosis has served as an historical model to decipher signalling pathways involved in the G2-to-M transition. The review especially highlights how the phosphatase PP2A-B55δ critically orchestrates the timing of meiosis resumption in amphibian oocytes. By opposing the kinase PKA, PP2A-B55δ controls the release of the G2 arrest through the dephosphorylation of their substrate, Arpp19. Few hours later, the inhibition of PP2A-B55δ by Arpp19 releases its opposing kinase, Cdk1, and triggers M-phase. In coordination with a variety of phosphatases and kinases, the PP2A-B55δ/Arpp19 duo therefore emerges as the key effector of the G2-to-M transition.

Structure-Guided Exploration of SDS22 Interactions with Protein Phosphatase PP1 and the Splicing Factor BCLAF1

SDS22 is an ancient regulator of protein phosphatase-1 (PP1). Our crystal structure of SDS22 shows that its twelve leucine-rich repeats adopt a banana-shaped fold that is shielded from solvent by capping domains at its extremities. Subsequent modeling and biochemical studies revealed that the concave side of SDS22 likely interacts with PP1 helices α5 and α6, which are distal from the binding sites of many previously described PP1 interactors. Accordingly, we found that SDS22 acts as a "third" subunit of multiple PP1 holoenzymes. The crystal structure of SDS22 also revealed a large basic surface patch that enables binding of a phosphorylated form of splicing factor BCLAF1. Taken together, our data provide insights into the formation of PP1:SDS22 and the recruitment of additional interaction proteins, such as BCLAF1.

A substrate-trapping strategy for protein phosphatase PP1 holoenzymes using hypoactive subunit fusions

The protein Ser/Thr phosphatase PP1 catalyzes an important fraction of protein dephosphorylation events and forms highly specific holoenzymes through an association with regulatory interactors of protein phosphatase one (RIPPOs). The functional characterization of individual PP1 holoenzymes is hampered by the lack of straightforward strategies for substrate mapping. Because efficient substrate recruitment often involves binding to both PP1 and its associated RIPPO, here we examined whether PP1-RIPPO fusions can be used to trap substrates for further analysis. Fusions of an hypoactive point mutant of PP1 and either of four tested RIPPOs accumulated in HEK293T cells with their associated substrates and were co-immunoprecipitated for subsequent identification of the substrates by immunoblotting or MS analysis. Hypoactive fusions were also used to study RIPPOs themselves as substrates for associated PP1. In contrast, substrate trapping was barely detected with active PP1-RIPPO fusions or with nonfused PP1 or RIPPO subunits. Our results suggest that hypoactive fusions of PP1 subunits represent an easy-to-use tool for substrate identification of individual holoenzymes.

Steady-State Levels of Phosphorylated Mitogen-Activated Protein Kinase Kinase 1/2 Determined by Mortalin/HSPA9 and Protein Phosphatase 1 Alpha in KRAS and BRAF Tumor Cells

Although deregulation of MEK/extracellular signal-regulated kinase (ERK) activity is a key feature in cancer, high-magnitude MEK/ERK activity can paradoxically induce growth inhibition. Therefore, additional mechanisms may exist to modulate MEK/ERK activity in favor of tumor cell proliferation. We previously reported that mortalin/HSPA9 can facilitate proliferation of certain KRAS and BRAF tumor cells by modulating MEK/ERK activity. In this study, we demonstrated that mortalin can regulate MEK/ERK activity via protein phosphatase 1α (PP1α). We found that PP1α inhibition increases steady-state levels of phosphorylated MEK1/2 in various tumor cells expressing B-Raf^V600E or K-Ras^G12C/D Intriguingly, coimmunoprecipitation and in vitro binding assays revealed that mortalin facilitates PP1α-mediated MEK1/2 dephosphorylation by promoting PP1α-MEK1/2 interaction in an ATP-sensitive manner. The region spanning Val482 to Glu491 in the substrate-binding cavity and the substrate lid of mortalin were necessary for these physical interactions, which is consistent with conventional heat shock protein 70 (HSP70)-client interaction mechanisms. Nevertheless, mortalin depletion did not affect cellular PP1α levels or its regulatory phosphorylation, suggesting a nonconventional role for mortalin in promoting PP1α-MEK1/2 interaction. Of note, PP1α was upregulated in human melanoma and pancreatic cancer biopsy specimens in correlation with mortalin upregulation. PP1α may therefore have a role in tumorigenesis in concert with mortalin, which affects MEK/ERK activity in tumor cells.

NBS1 Phosphorylation Status Dictates Repair Choice of Dysfunctional Telomeres

Telomeres employ TRF2 to protect chromosome ends from activating the DNA damage sensor MRE11-RAD50-NBS1 (MRN), thereby repressing ATM-dependent DNA damage checkpoint responses. How TRF2 prevents MRN activation at dysfunctional telomeres is unclear. Here, we show that the phosphorylation status of NBS1 determines the repair pathway choice of dysfunctional telomeres. The crystal structure of the TRF2-NBS1 complex at 3.0 Å resolution shows that the NBS1 429YQLSP433 motif interacts specifically with the TRF2TRFH domain. Phosphorylation of NBS1 serine 432 by CDK2 in S/G2 dissociates NBS1 from TRF2, promoting TRF2-Apollo/SNM1B complex formation and the protection of leading-strand telomeres. Classical-NHEJ-mediated repair of telomeres lacking TRF2 requires phosphorylated NBS1S432 to activate ATM, while interaction of de-phosphorylated NBS1S432 with TRF2 promotes alternative-NHEJ repair of telomeres lacking POT1-TPP1. Our work advances understanding of how the TRF2TRFH domain orchestrates telomere end protection and reveals how the phosphorylation status of the NBS1S432 dictates repair pathway choice of dysfunctional telomeres.

Counteracting Protein Kinase Activity in the Heart: The Multiple Roles of Protein Phosphatases

Decades of cardiovascular research have shown that variable and flexible levels of protein phosphorylation are necessary to maintain cardiac function. A delicate balance between phosphorylated and dephosphorylated states of proteins is guaranteed by a complex interplay of protein kinases (PKs) and phosphatases. Serine/threonine phosphatases, in particular members of the protein phosphatase (PP) family govern dephosphorylation of the majority of these cardiac proteins. Recent findings have however shown that PPs do not only dephosphorylate previously phosphorylated proteins as a passive control mechanism but are capable to actively control PK activity via different direct and indirect signaling pathways. These control mechanisms can take place on (epi-)genetic, (post-)transcriptional, and (post-)translational levels. In addition PPs themselves are targets of a plethora of proteinaceous interaction partner regulating their endogenous activity, thus adding another level of complexity and feedback control toward this system. Finally, novel approaches are underway to achieve spatiotemporal pharmacologic control of PPs which in turn can be used to fine-tune misleaded PK activity in heart disease. Taken together, this review comprehensively summarizes the major aspects of PP-mediated PK regulation and discusses the subsequent consequences of deregulated PP activity for cardiovascular diseases in depth.

Detection, Localization and Tyrosine Phosphorylation Status of Ser/Thr Protein Phosphatase1γ in Freshly Ejaculated, In Vitro Capacitated and Cryopreserved Buffalo Spermatozoa

Several recent studies have indicated the important roles of Ser/Thr protein phosphatase1γ (PP1γ) in regulating the motility and capacitation of mammalian spermatozoa. Here, we report the presence and distribution of PP1γ protein in freshly ejaculated, in vitro capacitated and cryopreserved buffalo spermatozoa. The presence of PP1γ and its distribution were assessed by Western blotting and indirect immunofluorescence techniques, whereas the isoforms of PP1γ and their tyrosine phosphorylation status were identified by using 2D electrophoresis. The number of isoforms and the status of tyrosine phosphorylation of PP1γ were increased in capacitated spermatozoa when compared with freshly ejaculated spermatozoa. Differential pattern of expression and tyrosine phosphorylation of PP1γ were observed in cryopreserved spermatozoa, wherein some isoforms were degraded and some were tyrosine phosphorylated. In addition, immunofluorescence technique revealed that PP1γ was localized to principle, mid-piece, post-acrosomal and equatorial regions of buffalo spermatozoa. Differential distribution of tyrosine-phosphorylated proteins were observed in fresh, capacitated and cryopreserved spermatozoa. The tyrosine phosphorylation of several proteins (20, 37, 38, 52, 60, 79 and 100 kDa) were increased when sperm cells were incubated with PP1γ inhibitor, okadaic acid. Together, our results suggest that buffalo spermatozoa express different isoforms of PP1γ protein. The protein expression and tyrosine phosphorylation of PP1γ were increased during capacitation. Furthermore, the differential pattern of expression and tyrosine phosphorylation of PP1γ were observed in cryopreserved spermatozoa. In addition, the inhibition of PP1γ protein increases protein tyrosine phosphorylation in capacitation.

MiR-125b promotes cell migration and invasion by targeting PPP1CA-Rb signal pathways in gastric cancer, resulting in a poor prognosis

BACKGROUND

MiR-125b functions as an oncogene in many cancers; however, its clinical significance and molecular mechanism in gastric cancers have never been sufficiently investigated. Here, we elucidated the functions and molecular regulated pathways of MiR-125b in gastric cancer.

METHODS

We investigated MiR-125b expression in fresh tissues from 50 gastric cancer patients and 6 gastric cancer cell lines using RT-PCR, and explored its prognostic value by hybridizing MiR-125b in situ for 300 clinical gastric tumor tissues with pathological diagnosis and clinical parameters. The effects of MiR-125b on gastric cancer cells and downstream target genes and proteins were analyzed by MTT, transwell assay, RT-PCR, and western blot on the basis of silencing MiR-125b in vitro. Luciferase reporter plasmid was constructed to demonstrate MiR-125b's direct target.

RESULTS

MiR-125b was upregulated in gastric cancer tissues and cell lines, and significantly promoted cellular proliferation, migration, and invasion by downregulating the expression of PPP1CA and upregulating Rb phosphorylation. MiR-125b expression was significantly correlated with tumor size and depth of invasion, lymph nodes, distant metastasis, and TNM stage. The high-MiR-125b-expression group had a significantly poorer prognosis than the low-expression group (P < 0.05) in stages I, II, and III, and the 5-year survival rate in of the high-expression group was significantly lower than that of the low-expression group.

CONCLUSIONS

MiR-125b functions as an oncogene by targeting downregulated PPP1CA and upregulated Rb phosphorylation in gastric cancer. MiR-125b not only promotes cellular proliferation, migration, and invasion in vitro, but also acts as an independent prognostic factor in gastric cancer.

The natural tumorcide Manumycin-A targets protein phosphatase 1α and reduces hydrogen peroxide to induce lymphoma apoptosis

Numerous compounds for treating human disease have been discovered in nature. Manumycin-A (Man-A) is a natural, well-tolerated microbial metabolite and a potent experimental tumoricide. We recently showed that Man-A stimulated reactive oxygen species (ROS) which were upstream of serine/threonine (Ser/Thr) dephosphorylation and caspase-dependent cleavage of MEK and Akt in lymphoma apoptosis. Conversely, activation-specific, Ser/Thr phosphorylation of MEK and Akt proteins was stable in Man-A-resistant tumors suggesting that stimulation of Ser/Thr PPase activity might be required for Man-A tumoricidal activity. Pre-treatment with Calyculin-A, an equipotent inhibitor of PP1 and PP2A, blocked all downstream effects of Man-A whereas, the PP2A-selective inhibitor, Okadaic acid did not, suggesting that PP1 and not PP2A played a role in Man-A action. Phosphorylation of PP1α on Thr320 inhibits its activity. Hence, we posited that if PP1α was important for Man-A action, then Man-A treatment should promote dephosphorylation of PP1α on Thr320. Indeed, T320 was only dephosphorylated in the tumors that underwent apoptosis. Lastly, stable over-expression of a constitutively active PP1α mimetic (PP1αT320A mutant), elevated basal ROS levels and enhanced Man-A-stimulated apoptosis. Taken together, we conclude that PP1α is an important proximal effector of Man-A mediated lymphoma apoptosis and that the mechanisms of Man-A action warrant further investigation.

Protein phosphatase 1 catalytic isoforms: specificity toward interacting proteins

The coordinated and reciprocal action of serine-threonine protein kinases and protein phosphatases produces transitory phosphorylation, a fundamental regulatory mechanism for many biological processes. Phosphoprotein phosphatase 1 (PPP1), a major serine-threonine phosphatase, in particular, is ubiquitously distributed and regulates a broad range of cellular functions, including glycogen metabolism, cell cycle progression, and muscle relaxation. PPP1 has evolved effective catalytic machinery but in vitro lacks substrate specificity. In vivo, its specificity is achieved not only by the existence of different PPP1 catalytic isoforms, but also by binding of the catalytic moiety to a large number of regulatory or targeting subunits. Here, we will address exhaustively the existence of diverse PPP1 catalytic isoforms and the relevance of their specific partners and consequent functions.

Angiotensin IV upregulates the activity of protein phosphatase 1α in Neura-2A cells

The peptide angiotensin IV (Ang IV) is a derivative of angiotensin II. While insulin regulated amino peptidase (IRAP) has been proposed as a potential receptor for Ang IV, the signalling pathways of Ang IV through IRAP remain elusive. We applied high-resolution mass spectrometry to perform a systemic quantitative phosphoproteome of Neura-2A (N2A) cells treated with and without Ang IV using sta ble-isotope labeling by amino acids in cell culture (SILAC), and identified a reduction in the phosphorylation of a major Ser/Thr protein phosphorylase 1 (PP1) upon Ang IV treatment. In addition, spinophilin (spn), a PP1 regulatory protein that plays important functions in the neural system, was expressed at higher levels. Immunoblotting revealed decreased phosphorylation of p70S6 kinase (p70(S6K)) and the major cell cycle modulator retinoblastoma protein (pRB). These changes are consistent with an observed decrease in cell proliferation. Taken together, our study suggests that Ang IV functions via regulating the activity of PP1.

Role of serine-threonine phosphoprotein phosphatases in smooth muscle contractility

The degree of phosphorylation of myosin light chain 20 (MLC20) is a major determinant of force generation in smooth muscle. Myosin phosphatases (MPs) contain protein phosphatase (PP) 1 as catalytic subunits and are the major enzymes that dephosphorylate MLC20. MP regulatory targeting subunit 1 (MYPT1), the main regulatory subunit of MP in all smooth muscles, is a key convergence point of contractile and relaxatory pathways. Combinations of regulatory mechanisms, including isoform splicing, multiple phosphorylation sites, and scaffolding proteins, modulate MYPT1 activity with tissue and agonist specificities to affect contraction and relaxation. Other members of the PP1 family that do not target myosin, as well as PP2A and PP2B, dephosphorylate a range of proteins that affect smooth muscle contraction. This review discusses the role of phosphatases in smooth muscle contractility with a focus on MYPT1 in uterine smooth muscle. Myometrium shares characteristics of vascular and other visceral smooth muscles yet, during healthy pregnancy, undergoes hypertrophy, hyperplasia, quiescence, and labor as physiological processes. Myometrium presents an accessible model for the study of normal and pathological smooth muscle function, and a better understanding of myometrial physiology may allow the development of novel therapeutics for the many disorders of myometrial physiology from preterm labor to dysmenorrhea.

Molecular characterization of the rice protein RSS1 required for meristematic activity under stressful conditions

Post embryonic growth of plants depends on cell division activity in the shoot and root meristems, in conjunction with subsequent cell differentiation. Under environmental stress conditions, where plant growth is moderately impaired, the meristematic activity is maintained by mechanisms as yet unknown. We previously showed that the rice protein RSS1, whose stability is regulated depending on the cell cycle phases, is a key factor for the maintenance of meristematic activity under stressful conditions. RSS1 interacts with a catalytic subunit of protein phosphatase 1 (PP1), but other molecular characteristics are largely unknown. Here we show that RSS1 interacts with all the PP1 expressed in the shoot apex of rice. This interaction requires one of the conserved regions of RSS1, which is important for RSS1 function. Interestingly, the recombinant RSS1 protein is highly resistant to heat with respect to its anti-coagulability and binding activity to PP1. The features of RSS1 are reminiscent of those of inhibitor-2 of animals, although it is likely that the mode of function of RSS1 is different from that of inhibitor-2. Noticeably, RSS1 binds to PP1 under extremely high ionic strength conditions in vitro. Therefore, RSS1 possibly functions by forming a stable complex with PP1.

PP1 and PP2A phosphatases--cooperating partners in modulating retinoblastoma protein activation

The retinoblastoma/pocket protein family is one of the master regulators of the eukaryotic cell cycle. It includes the retinoblastoma protein (Rb) and the related p107 and p130 proteins. The importance of the Rb pathway for homeostasis and tumour suppression is evident from the fact that inactivating mutations in Rb are frequently associated with many cancers. Rbs regulate the cell cycle by controlling the activity of the E2F family of transcription factors. The activity of Rb proteins themselves is modulated by their phosphorylation status at several Ser/Thr residues: phosphorylation by cyclin-dependent kinases inactivates Rb proteins and positively influences the transcription of genes necessary for cell cycle progression. Although the mechanisms of cyclin-dependent kinase-mediated inactivation of Rb proteins are understood in great detail, our knowledge of the process that counteracts Rb phosphorylation is still quite limited. The present review focuses on the Ser/Thr phosphatases that are responsible for the dephosphorylation and thus activation of Rb proteins. Two major scenarios are considered: (a) when pocket proteins are dephosphorylated during regular cell cycle progression and (b) when rapid dephosphorylation is dictated by external stress or growth inhibitory conditions, such as oxidative stress, UV radiation or other DNA-damaging stimuli, and cell differentiation factors. It transpires that protein phosphatase 1 and protein phosphatase 2A can efficiently modulate pocket protein activity in a highly context-dependent manner and both are tightly regulated by the presence of different regulatory subunits or interacting proteins.

Spinophilin is required for normal morphology, Ca(2+) homeostasis and contraction but dispensable for β-adrenergic stimulation of adult cardiomyocytes

Spinophilin (SPN) is a ubiquitously expressed scaffolding protein that interacts through several binding modules with a variety of target proteins. Thus, SPN bundles F-actin, targets protein phosphatase 1 to the ryanodine receptor, and targets regulators of G-protein signaling to G-protein coupled receptors in cardiomyocytes. In this work we studied the role of SPN on cardiomyocyte morphology, function, and β-adrenergic responsiveness using a homozygous SPN knock-out mouse model (SPN-/-). We show that spinophilin deficiency significantly (1) reduced cardiomyocyte length, (2) increases both Ca(2+) amplitude and maximal rate of Ca(2+) rise during systole, and (3) decreased shortening amplitude and maximal rate of shortening, while (4) β-adrenergic stimulation remained intact. Our data suggest that spinophilin is an upstream regulator required for normal growth and excitation-contraction coupling, but is dispensable for β-adrenergic stimulation of adult cardiomyocytes.

RSS1 regulates the cell cycle and maintains meristematic activity under stress conditions in rice

Plant growth and development are sustained by continuous cell division in the meristems, which is perturbed by various environmental stresses. For the maintenance of meristematic functions, it is essential that cell division be coordinated with cell differentiation. However, it is unknown how the proliferative activities of the meristems and the coordination between cell division and differentiation are maintained under stressful conditions. Here we show that a rice protein, RSS1, whose stability is controlled by cell cycle phases, contributes to the vigour of meristematic cells and viability under salinity conditions. These effects of RSS1 are exerted by regulating the G1-S transition, possibly through an interaction of RSS1 with protein phosphatase 1, and are mediated by the phytohormone, cytokinin. RSS1 is conserved widely in plant lineages, except eudicots, suggesting that RSS1-dependent mechanisms might have been adopted in specific lineages during the evolutionary radiation of angiosperms.

Protein phosphatase 1-dependent dephosphorylation of Akt is the prime signaling event in sphingosine-induced apoptosis in Jurkat cells

Sphingosine (SPH) is an important bioactive lipid involved in mediating a variety of cell functions including apoptosis. However, the signaling mechanism of SPH-induced apoptosis remains unclear. We have investigated whether SPH inhibits survival signaling in cells by inhibiting Akt kinase activity. This study demonstrates that treatment of Jurkat cells with SPH leads to Akt dephosphorylation as early as 15  min, and the cells undergo apoptosis after 6  h. This Akt dephosphorylation is not mediated through deactivation of upstream kinases, since SPH does not inhibit the upstream phosphoinositide-dependent kinase 1 (PDK1) phosphorylation. Rather, sensitivity to the Ser/Thr protein phosphatase inhibitors (calyculin A, phosphatidic acid, tautomycin, and okadaic acid) indicates an important role for protein phosphatase 1 (PP1) in this process. In vitro phosphatase assay, using Akt immunoprecipitate following treatment with SPH, reveals an increase in Akt-PP1 association as determined by immunoprecipitation analysis. Moreover, SPH-induced dephosphorylation of Akt at Ser(473) subsequently leads to the activation of GSK-3β, caspase 3, PARP cleavage, and ultimately apoptosis. Pre-treatment with caspase 3 inhibitor z-VAD-fmk and Ser/Thr phosphatase inhibitor abrogates the effect of SPH on facilitating apoptosis. Altogether, these results demonstrate that PP1-mediated inhibition of the key anti-apoptotic protein, Akt, plays an important role in SPH-mediated apoptosis in Jurkat cells.

Early growth response 1 (Egr-1) regulates phosphorylation of microtubule-associated protein tau in mammalian brain

In the normal brain, tau protein is phosphorylated at a number of proline- and non-proline directed sites, which reduce tau microtubule binding and thus regulate microtubule dynamics. In Alzheimer disease (AD), tau is abnormally hyperphosphorylated, leading to neurofibrillary tangle formation and microtubule disruption, suggesting a loss of regulatory mechanisms controlling tau phosphorylation. Early growth response 1 (Egr-1) is a transcription factor that is significantly up-regulated in AD brain. The pathological significance of this up-regulation is not known. In this study, we found that lentivirus-mediated overexpression of Egr-1 in rat brain hippocampus and primary neurons in culture activates proline-directed kinase Cdk5, inactivates PP1, promotes tau phosphorylation at both proline-directed Ser(396/404) and non-proline-directed Ser(262) sites, and destabilizes microtubules. Furthermore, in Egr-1(-/-) mouse brain, Cdk5 activity was decreased, PP1 activity was increased, and tau phosphorylation was reduced at both proline-directed and non-proline-directed sites. By using nerve growth factor-exposed PC12 cells, we determined that Egr-1 activates Cdk5 to promote phosphorylation of tau and inactivates PP1 via phosphorylation. When Cdk5 was inhibited, tau phosphorylation at both proline- and non-proline directed sites and PP1 phosphorylation were blocked, indicating that Egr-1 acts through Cdk5. By using an in vitro kinase assay and HEK-293 cells transfected with tau, PP1, and Cdk5, we found that Cdk5 phosphorylates Ser(396/404) directly. In addition, by phosphorylating and inactivating PP1, Cdk5 promotes tau phosphorylation at Ser(262) indirectly. Our results indicate that Egr-1 is an in vivo regulator of tau phosphorylation and suggest that in AD brain increased levels of Egr-1 aberrantly activate an Egr-1/Cdk5/PP1 pathway, leading to accumulation of hyperphosphorylated tau, thus destabilizing the microtubule cytoskeleton.

Protein phosphatase 1 complexes modulate sperm motility and present novel targets for male infertility

Infertility is a growing concern in modern society, with 30% of cases being due to male factors, namely reduced sperm concentration, decreased motility and abnormal morphology. Sperm cells are highly compartmentalized, almost devoid of transcription and translation consequently processes such as protein phosphorylation provide a key general mechanism for regulating vital cellular functions, more so than for undifferentiated cells. Reversible protein phosphorylation is the principal mechanism regulating most physiological processes in eukaryotic cells. To date, hundreds of protein kinases have been identified, but significantly fewer phosphatases (PPs) are responsible for counteracting their action. This discrepancy can be explained in part by the mechanism used to control phosphatase activity, which is based on regulatory interacting proteins. This is particularly true for PP1, a major serine/threonine-PP, for which >200 interactors (PP1 interacting proteins-PIPs) have been indentified that control its activity, subcellular location and substrate specificity. For PP1, several isoforms have been described, among them PP1γ2, a testis/sperm-enriched PP1 isoform. Recent findings support our hypothesis that PP1γ2 is involved in the regulation of sperm motility. This review summarizes the known sperm-specific PP1-PIPs, involved in the acquisition of mammalian sperm motility. The complexes that PP1 routinely forms with different proteins are addressed and the role of PP1/A-kinase anchoring protein complexes in sperm motility is considered. Furthermore, the potential relevance of targeting PP1-PIPs complexes to infertility diagnostics and therapeutics as well as to male contraception is also discussed.

Dephosphorylation of nucleophosmin by PP1β facilitates pRB binding and consequent E2F1-dependent DNA repair

We report a new pathway through which PP1β signals to nucleophosmin (NPM) in response to DNA damage. UV induces dephosphorylation of NPM at multiple sites, leading to enhancement of complex formation between NPM and retinoblastoma tumor suppressor protein and the subsequent upregulation of E2F1. Consequently, such signaling pathway potentiates the cellular DNA repair capacity.

Nucleophosmin (NPM) is an important phosphoprotein with pleiotropic functions in various cellular processes. Although phosphorylation has been postulated as an important functional determinant, possible regulatory roles of this modification on NPM are not fully characterized. Here, we find that NPM is dephosphorylated on various threonine residues (Thr199 and Thr234/237) in response to UV-induced DNA damage. Further experiments indicate that the serine/threonine protein phosphatase PP1β is a physiological NPM phosphatase under both the genotoxic stress and growth conditions. As a consequence, NPM in its hypophosphorylated state facilitates DNA repair. Finally, our results suggest that one possible mechanism of this protective response lies in enhanced NPM-retinoblastoma tumor suppressor protein (pRB) interaction, leading to the relief of the repressive pRB–E2F1 circuitry and the consequent transcriptional activation of E2F1 and several downstream DNA repair genes. Thus, this study unveils a key phosphatase of NPM and highlights a novel mechanism by which the PP1β–NPM pathway contributes to cellular DNA damage response.

Basis for MAP4 dephosphorylation-related microtubule network densification in pressure overload cardiac hypertrophy

Increased activity of Ser/Thr protein phosphatases types 1 (PP1) and 2A (PP2A) during maladaptive cardiac hypertrophy contributes to cardiac dysfunction and eventual failure, partly through effects on calcium metabolism. A second maladaptive feature of pressure overload cardiac hypertrophy that instead leads to heart failure by interfering with cardiac contraction and intracellular transport is a dense microtubule network stabilized by decoration with microtubule-associated protein 4 (MAP4). In an earlier study we showed that the major determinant of MAP4-microtubule affinity, and thus microtubule network density and stability, is site-specific MAP4 dephosphorylation at Ser-924 and to a lesser extent at Ser-1056; this was found to be prominent in hypertrophied myocardium. Therefore, in seeking the etiology of this MAP4 dephosphorylation, we looked here at PP2A and PP1, as well as the upstream p21-activated kinase 1, in maladaptive pressure overload cardiac hypertrophy. The activity of each was increased persistently during maladaptive hypertrophy, and overexpression of PP2A or PP1 in normal hearts reproduced both the microtubule network phenotype and the dephosphorylation of MAP4 Ser-924 and Ser-1056 seen in hypertrophy. Given the major microtubule-based abnormalities of contractile and transport function in maladaptive hypertrophy, these findings constitute a second important mechanism for phosphatase-dependent pathology in the hypertrophied and failing heart.

Regulated protein kinases and phosphatases in cell cycle decisions

Many aspects of cell physiology are controlled by protein kinases and phosphatases, which together determine the phosphorylation state of targeted substrates. Some of these target proteins are themselves kinases or phosphatases or other components of a regulatory network characterized by feedback and feed-forward loops. In this review we describe some common regulatory motifs involving kinases, phosphatases, and their substrates, focusing particularly on bistable switches involved in cellular decision processes. These general principles are applied to cell cycle transitions, with special emphasis on the roles of regulated phosphatases in orchestrating progression from one phase to the next of the DNA replication-division cycle.

Identification of CDK2 substrates in human cell lysates

BACKGROUND

Protein phosphorylation regulates a multitude of biological processes. However, the large number of protein kinases and their substrates generates an enormously complex phosphoproteome. The cyclin-dependent kinases--the CDKs--comprise a class of enzymes that regulate cell cycle progression and play important roles in tumorigenesis. However, despite intense study, only a limited number of mammalian CDK substrates are known. A comprehensive understanding of CDK function requires the identification of their substrate network.

RESULTS

We describe a simple and efficient approach to identify potential cyclin A-CDK2 targets in complex cell lysates. Using a kinase engineering strategy combined with chemical enrichment and mass spectrometry, we identified 180 potential cyclin A-CDK2 substrates and more than 200 phosphorylation sites. About 10% of these candidates function within pathways related to cell division, and the vast majority are involved in other fundamental cellular processes. We have validated several candidates as direct cyclin A-CDK2 substrates that are phosphorylated on the same sites that we identified by mass spectrometry, and we also found that one novel substrate, the ribosomal protein RL12, exhibits site-specific CDK2-dependent phosphorylation in vivo.

CONCLUSIONS

We used methods entailing engineered kinases and thiophosphate enrichment to identify a large number of candidate CDK2 substrates in cell lysates. These results are consistent with other recent proteomic studies, and suggest that CDKs regulate cell division via large networks of cellular substrates. These methods are general and can be easily adapted to identify direct substrates of many other protein kinases.

Nuclear inhibitor of protein phosphatase-1 (NIPP1) directs protein phosphatase-1 (PP1) to dephosphorylate the U2 small nuclear ribonucleoprotein particle (snRNP) component, spliceosome-associated protein 155 (Sap155)

Pre-mRNA splicing entails reversible phosphorylation of spliceosomal proteins. Recent work has revealed essential roles for Ser/Thr phosphatases, such as protein phosphatase-1 (PP1), in splicing, but how these phosphatases are regulated is largely unknown. We show that nuclear inhibitor of PP1 (NIPP1), a major PP1 interactor in the vertebrate nucleus, recruits PP1 to Sap155 (spliceosome-associated protein 155), an essential component of U2 small nuclear ribonucleoprotein particles, and promotes Sap155 dephosphorylation. C-terminally truncated NIPP1 (NIPP1-DeltaC) formed a hyper-active holoenzyme with PP1, rendering PP1 minimally phosphorylated on an inhibitory site. Forced expression of NIPP1-WT and -DeltaC resulted in slight and severe decreases in Sap155 hyperphosphorylation, respectively, and the latter was accompanied with inhibition of splicing. PP1 overexpression produced similar effects, whereas small interfering RNA-mediated NIPP1 knockdown enhanced Sap155 hyperphosphorylation upon okadaic acid treatment. NIPP1 did not inhibit but rather stimulated Sap155 dephosphorylation by PP1 in vitro through facilitating Sap155/PP1 interaction. Further analysis revealed that NIPP1 specifically recognizes hyperphosphorylated Sap155 thorough its Forkhead-associated domain and dissociates from Sap155 after dephosphorylation by associated PP1. Thus NIPP1 works as a molecular sensor for PP1 to recognize phosphorylated Sap155.

Myosin phosphatase interacts with and dephosphorylates the retinoblastoma protein in THP-1 leukemic cells: its inhibition is involved in the attenuation of daunorubicin-induced cell death by calyculin-A

Reversible phosphorylation of the retinoblastoma protein (pRb) is an important regulatory mechanism in cell cycle progression. The role of protein phosphatases is less understood in this process, especially concerning the regulatory/targeting subunits involved. It is shown that pretreatment of THP-1 leukemic cells with calyculin-A (CL-A), a cell-permeable phosphatase inhibitor, attenuated daunorubicin (DNR)-induced cell death and resulted in increased pRb phosphorylation and protection against proteolytic degradation. Protein phosphatase-1 catalytic subunits (PP1c) dephosphorylated the phosphorylated C-terminal fragment of pRb (pRb-C) slightly, whereas when PP1c was complexed to myosin phosphatase target subunit-1 (MYPT1) in myosin phosphatase (MP) holoenzyme dephosphorylation was stimulated. The pRb-C phosphatase activity of MP was partially inhibited by anti-MYPT1(1-296) implicating MYPT1 in targeting PP1c to pRb. MYPT1 became phosphorylated on both inhibitory sites (Thr695 and Thr850) upon CL-A treatment of THP-1 cells resulting in the inhibition of MP activity. MYPT1 and pRb coprecipitated from cell lysates by immunoprecipitation with either anti-MYPT1 or anti-pRb antibodies implying that pRb-MYPT1 interaction occurred at cellular levels. Surface plasmon resonance-based experiments confirmed binding of pRb-C to both PP1c and MYPT1. In control and DNR-treated cells, MYPT1 and pRb were predominantly localized in the nucleus exhibiting partial colocalization as revealed by immunofluorescence using confocal microscopy. Upon CL-A treatment, nucleo-cytoplasmic shuttling of both MYPT1 and pRb, but not PP1c, was observed. The above data imply that MP, with the targeting role of MYPT1, may regulate the phosphorylation level of pRb, thereby it may be involved in the control of cell cycle progression and in the mediation of chemoresistance of leukemic cells.

Role(s) of the serine/threonine protein phosphatase 1 on mammalian sperm motility

Mammalian spermatozoa acquire the capacity for motility and fertilization during the transit through the epididymis under the control of different factors, such as cAMP, intracellular pH, intracellular calcium and phosphorylation of sperm proteins. As the acquisition of functional competence including gaining motility during epididymal transit occurs in the complete absence of contemporaneous gene transcription and translation on the part of the spermatozoa, it is widely accepted that post-translational modifications are the only means by which spermatozoa can acquire functionality. Serine-threonine protein phosphatase 1 (PP1) together with their testis/sperm-specific interacting proteins might be involved in this regulatory mechanism. PP1alpha, PP1beta/delta, PP1gamma1 and PP1gamma2 are all expressed in the testis whereas PP1gamma2 is the only isoform expressed on spermatozoa. I2, I3, sds22, 14-3-3 and hsp90 are associated with PP1gamma2 in spermatozoa located on the sperm head and tail. Activity of PP1gamma2 and the binding pattern to these regulatory proteins changes in spermatozoa recruited from the caput and those from the cauda part of the epididymis. In this review, we summarize the possible roles of PP1 on spermatozoa during spermatogenesis and flagellar motility control. We suggest that PP1 might take part in the inhibition of the sperm motility activation by interacting with AKAPs and CAMKII. A hypothesized signaling pathway of mammalian sperm motility activation and PP1's function has been proposed.

Phactr4 regulates neural tube and optic fissure closure by controlling PP1-, Rb-, and E2F1-regulated cell-cycle progression

Here we identify the humpty dumpty (humdy) mouse mutant with failure to close the neural tube and optic fissure, causing exencephaly and retinal coloboma, common birth defects. The humdy mutation disrupts Phactr4, an uncharacterized protein phosphatase 1 (PP1) and actin regulator family member, and the missense mutation specifically disrupts binding to PP1. Phactr4 is initially expressed in the ventral cranial neural tube, a region of regulated proliferation, and after neural closure throughout the dorsoventral axis. humdy embryos display elevated proliferation and abnormally phosphorylated, inactive PP1, resulting in Rb hyperphosphorylation, derepression of E2F targets, and abnormal cell-cycle progression. Exencephaly, coloboma, and abnormal proliferation in humdy embryos are rescued by loss of E2f1, demonstrating the cell cycle is the key target controlled by Phactr4. Thus, Phactr4 is critical for the spatially and temporally regulated transition in proliferation through differential regulation of PP1 and the cell cycle during neurulation and eye development.

Phosphorylation of Protein Phosphatase 1 by Cyclin-dependent Protein Kinase 5 during Nerve Growth Factor-induced PC12 Cell Differentiation

The transcription factor Egr-1 activates cyclin-dependent protein kinase 5 (Cdk5) during nerve growth factor (NGF)-induced differentiation of PC12 cells into neurons (Harada, T. Morooka, T., Ogawa, S., and Nishida, E. (2001) Nat. Cell Biol. 3, 453-459). The downstream target of Cdk5 in the Egr-1/Cdk5 pathway is not clear. In this study, we observed that phosphorylation of protein phosphatase 1 (PP1) on Thr(320) is reduced in brain extracts from Egr-1(-/-) mice, indicating that a kinase downstream of Egr-1 phosphorylates PP1. In HEK 293 cells co-transfected with PP1 and Cdk5, Cdk5 phosphorylates PP1. In vitro, Cdk5 purified from bovine brain phosphorylates bacterially expressed recombinant PP1. In NGF-treated PC12 cells, inhibition of Cdk5 by olomoucine or silencing Cdk5 expression by small interfering RNA strategy, suppresses PP1 phosphorylation. Silencing Cdk5 expression by small interfering RNA also blocks NGF-induced neurite outgrowth. Overexpression of PP1 (wild type) promotes NGF-induced differentiation of PC12 cells, whereas that of PP1 (T320A) has no effect. Our data indicate that PP1 is a downstream target of the NGF/Egr-1/Cdk5 pathway during NGF-induced differentiation of PC12 cells and suggest that PP1 phosphorylation promotes neuronal differentiation.

Protein phosphatase 1alpha activity prevents oncogenic transformation

Cyclin-dependent kinase 2 (Cdk2) phosphorylates Thr320 of protein phosphatase 1alpha (PP1alpha) in late G(1), thereby inhibiting its activity. Phosphorylation-resistant PP1alphaT320A, acting as a constitutively active (CA) mutant, causes a late G(1) arrest by preventing the phosphorylation and inactivation of the retinoblastoma protein (pRb). Both PP1alpha-mediated G(1) arrest and PP1alpha phosphorylation in late G(1) require the presence of pRb, indicating that PP1alpha is a crucial regulator of the pRb pathway, which is almost invariably mutated in human cancer. These findings prompted us to investigate whether PP1alpha interferes with oncogenic transformation. The ability of NIH 3T3 cells to form foci after transformation with ras/cyclin D1 was significantly inhibited by co-transfection with PP1alphaT320A, but not PP1alpha. Likewise, cells expressing PP1alphaT320A or PP1alphaT320A fused to green fluorescent protein (GFP) were unable to form colonies in soft agar, regardless of whether PP1alpha constructs were co-transfected with ras/cyclin D1 or transfected into stably transformed cells. Overexpressed wild-type (Wt) PP1alpha and GFP-PP1alpha were phosphorylated in Thr320, most likely explaining its lack of effect. Expression of GFP-PP1alphaT320A was associated with caspase-cleaved pRb in Western blots (WB) and morphological signs of cell death. These findings demonstrate that PP1alpha activity can override oncogenic signaling by causing cell-cycle arrest and/or apoptosis rather than restoring contact inhibition or anchorage dependence.

Identification of PP1alpha as a caspase-9 regulator in IL-2 deprivation-induced apoptosis

One of the mechanisms that regulate cell death is the reversible phosphorylation of proteins. ERK/MAPK phosphorylates caspase-9 at Thr(125), and this phosphorylation is crucial for caspase-9 inhibition. Until now, the phosphatase responsible for Thr(125) dephosphorylation has not been described. Here, we demonstrate that in IL-2-proliferating cells, phosphorylated serine/threonine phosphatase type 1alpha (PP1alpha) associates with phosphorylated caspase-9. IL-2 deprivation induces PP1alpha dephosphorylation, which leads to its activation and, as a consequence, dephosphorylation and activation of caspase-9 and subsequent dissociation of both molecules. In cell-free systems supplemented with ATP caspase-9 activation is induced by addition of cytochrome c and we show that in this process PP1alpha is indispensable for triggering caspase-9 as well as caspase-3 cleavage and activation. Moreover, PP1alpha associates with caspase-9 in vitro and in vivo, suggesting that it is the phosphatase responsible for caspase-9 dephosphorylation and activation. Finally, we describe two novel phosphatase-binding sites different from the previously described PP1alpha consensus motifs, and we demonstrate that these novel sites mediate the interaction of PP1alpha with caspase-9.

Active ERK contributes to protein translation by preventing JNK-dependent inhibition of protein phosphatase 1

Human alveolar macrophages, central to immune responses in the lung, are unique in that they have an extended life span in contrast to precursor monocytes. We have shown previously that the ERK MAPK (ERK) pathway is constitutively active in human alveolar macrophages and contributes to the prolonged survival of these cells. We hypothesized that ERK maintains survival, in part, by positively regulating protein translation. In support of this hypothesis, we have found novel links among ERK, JNK, protein phosphatase 1 (PP1), and the eukaryotic initiation factor (eIF) 2alpha. eIF2alpha is active when hypophosphorylated and is essential for initiation of protein translation (delivery of initiator tRNA charged with methionine to the ribosome). Using [(35)S]methionine labeling, we found that ERK inhibition significantly decreased protein translation rates in alveolar macrophages. Decreased protein translation resulted from phosphorylation (and inactivation) of eIF2alpha. We found that ERK inhibition increased JNK activity. JNK in turn inactivated (via phosphorylation) PP1, the phosphatase responsible for maintaining the hypophosphorylated state of eIF2alpha. As a composite, our data demonstrate that in human alveolar macrophages, constitutive ERK activity positively regulates protein translation via the following novel pathway: active ERK inhibits JNK, leading to activation of PP1alpha, eIF2alpha dephosphorylation, and translation initiation. This new role for ERK in alveolar macrophage homeostasis may help to explain the survival characteristic of these cells within their unique high oxygen and stress microenvironment.

Dephosphorylation of Rb (Thr-821) in response to cell stress

The retinoblastoma tumor suppressor Rb is regulated by reversible phosphorylation that is dependent upon cyclin-dependent kinase (CDK) and protein phosphatase type 1 (PP1) activity in replicating cells. Hyperphosphorylated Rb allows cells to proliferate, whereas the hypophosphorylated isoform of Rb inhibits proliferation. Of the many phosphorylation sites of Rb, there is functional information available for a very few. In this report, we show that threonine-821 (Thr-821) of Rb is dephosphorylated earlier than other phosphorylation sites when cells are grown under hypoxic conditions which leads to Rb activation and G(1) arrest. This finding is interesting because Thr-821 of Rb remains phosphorylated throughout the cell division cycle in replicating cells. We hypothesized that the phosphorylation state of Thr-821 of Rb may depend on cellular stress. We report in this study that, when nontransformed CV1 epithelial cells and Hs578T breast cancer cells are treated with the chemotherapeutic agent cytosine arabinoside (Ara-C), Thr-821 of Rb is rapidly dephosphorylated concomitant with dissociation of the PP1 regulatory subunit PNUTS (phosphatase nuclear targeting subunit) from PP1 enzyme. These data are consistent with the concept that differential regulation of Rb-directed phosphatase activity exists when cells are progressing through the cell cycle compared to that observed when cells are under stress.

ATM regulates ionizing radiation-induced disruption of HDAC1:PP1:Rb complexes

Ionizing radiation elicits signaling events that coordinate DNA repair and interruption of cell cycle progression. We previously demonstrated that ionizing radiation (IR) of cells activates nuclear protein phosphatase-1 (PP1) by promoting dephosphorylation of Thr320, an inhibitory site in the enzyme and that the ATM kinase is required for this response. We sought to identify potential targets of IR-activated PP1. Untreated and IR-treated Jurkat cells were labeled with (32)P orthophosphate, and nuclear extracts were subjected to microcystin affinity chromatography to recover phosphatase complexes that were analyzed by 2D-PAGE and mass spectrometry. Several proteins associated with protein phosphatases demonstrated a significant decrease in (32)P intensity following IR, and one of these was identified as HDAC1. Co-immunoprecipitation revealed complexes containing PP1 with HDAC1 and Rb in cell extracts. In response to IR, there was an ATM-dependent activation of PP1, dephosphorylation of HDAC1, dissociation of HDAC1-PP1-Rb complexes and increased HDAC1 activity. These results suggest that IR regulates HDAC1 phosphorylation and activity through ATM-dependent activation of PP1.

Okadaic acid overcomes the blocked cell cycle caused by depleting Cdc2-related kinases in Trypanosoma brucei

Mitosis and cytokinesis are highly coordinated in eukaryotic cells. But procyclic-form Trypanosoma brucei under G1 or mitotic arrest is still capable of dividing, resulting in anucleate daughter cells (zoids). Okadaic acid (OKA), an inhibitor of protein phosphatases PP1 and PP2A, is known to inhibit kinetoplast replication and cell division yielding multinucleate cells with single kinetoplasts. However, when OKA was applied to cells arrested in G1 or G2/M phase via RNAi knockdown of specific cdc2-related kinases (CRKs), DNA synthesis and nuclear division were resumed without kinetoplast replication or cell division, resulting in multinucleate cells as in the wild type. Cells arrested in G2/M via depleting the mitotic cyclin CycB2 or an aurora B kinase homologue TbAUK1 were, however, not released by OKA treatment. The phenomenon is thus similar to the OKA activation of Cdc2 in Xenopus oocyte by inhibiting PP2A [Maton, et al., Differential regulation of Cdc2 and Aurora-A in Xenopus oocytes: a crucial role of phosphatase 2A. J. Cell Sci. 118 (2005) 2485-2494]. A simultaneous knockdown of the seven PP1s or the PP2A catalytic subunit in T. brucei by RNA interference did not, however, result in multinucleate cells. This could be explained by assuming a negative regulation, either directly or indirectly, of CRK by an OKA-sensitive phosphatase, which could be a PP2A as in the Xenopus oocyte and a positive regulation of kinetoplast replication by an OKA-susceptible protein(s). Test of a PP2A-specific inhibitor, fostriecin, on cells arrested in G2/M via CRK depletion or a knockdown of the PP2A catalytic subunit from the CRK-depleted cells both showed a partial lift of the G2/M block without forming multinucleate cells. These observations support the abovementioned assumption and suggest the presence of a novel OKA-sensitive protein(s) regulating kinetoplast replication that still remains to be identified.

Pocket protein function in melanocyte homeostasis and neoplasia

Melanoma is the most lethal of human skin cancers and its incidence is increasing worldwide [L.K. Dennis (1999). Arch. Dermatol. 135, 275; C. Garbe et al. (2000). Cancer 89, 1269]. Melanomas often metastasize early during the course of the disease and are then highly intractable to current therapeutic regimens [M.F. Demierre and G. Merlino (2004). Curr. Oncol. Rep. 6, 406]. Consequently, understanding the factors that maintain melanocyte homeostasis and prevent their neoplastic transformation into melanoma is of utmost interest from the perspective of therapeutic interdiction. This review will focus on the role of the pocket proteins (PPs), Rb1 (retinoblastoma protein), retinoblastoma-like 1 (Rbl1 also known as p107) and retinoblastoma-like 2 (Rbl2 also known as p130), in melanocyte homeostasis, with particular emphasis on their functions in the cell cycle and the DNA damage repair response. The potential mechanisms of PP deregulation in melanoma and the possibility of PP-independent pathways to melanoma development will also be considered. Finally, the role of the PP family in ultraviolet radiation (UVR)-induced melanoma and the precise contribution that each PP family member makes to melanocyte homeostasis will be discussed in the context of a number of genetically engineered mouse models.

Downregulation of Fer induces PP1 activation and cell-cycle arrest in malignant cells

Fer is a nuclear and cytoplasmic intracellular tyrosine kinase. Herein we show that Fer is required for cell-cycle progression in malignant cells. Decreasing the level of Fer using the RNA interference (RNAi) approach impeded the proliferation of prostate and breast carcinoma cells and led to their arrest at the G0/G1 phase. At the molecular level, knockdown of Fer resulted in the activation of the retinoblastoma protein (pRB), and this was reflected by profound hypo-phosphorylation of pRB on both cyclin-dependent kinase CDK4 and CDK2 phosphorylation sites. Dephosphorylation of pRB was not seen upon the direct targeting of either CDK4 or CDK2 expression, and was only partially achieved by the simultaneous depletion of these two kinases. Amino-acid sequence analysis revealed two protein phosphatase 1 (PP1) binding motifs in the kinase domain of Fer and the association of Fer with the pRB phosphatase PP1alpha was verified using co-immunoprecipitation analysis. Downregulation of Fer potentiated the activation of PP1alpha and overexpression of Fer decreased the enzymatic activity of that phosphatase. Our findings portray Fer as a regulator of cell-cycle progression in malignant cells and as a potential target for cancer intervention.