The role of serine/threonine protein phosphatase type 5 (PP5) in the regulation of stress-induced signaling networks and cancer

Novel Ser/Thr protein phosphatases in cell death regulation

Cell death is regulated by a myriad of intracellular molecular pathways, with many involving protein phosphorylation and dephosphorylation. In this review, we will focus on Ser/Thr phosphatases-mediated regulation in cell apoptosis as well as on their potential roles in cell necrosis. The emerging functional importance of Ser/Thr protein phosphatases in cell death regulation adds new dimension to the signaling mechanisms of cellular function, physiology, and diseases.

Hsp90: structure and function

Hsp90 is a highly abundant and ubiquitous molecular chaperone which plays an essential role in many cellular processes including cell cycle control, cell survival, hormone and other signalling pathways. It is important for the cell's response to stress and is a key player in maintaining cellular homeostasis. In the last ten years, it has become a major therapeutic target for cancer, and there has also been increasing interest in it as a therapeutic target in neurodegenerative disorders, and in the development of anti-virals and anti-protozoan infections. The focus of this review is the structural and mechanistic studies which have been performed in order to understand how this important chaperone acts on a wide variety of different proteins (its client proteins) and cellular processes. As with many of the other classes of molecular chaperone, Hsp90 has a critical ATPase activity, and ATP binding and hydrolysis known to modulate the conformational dynamics of the protein. It also uses a host of cochaperones which not only regulate the ATPase activity and conformational dynamics but which also mediate interactions with Hsp90 client proteins. The system is also regulated by post-translational modifications including phosphorylation and acetylation. This review discusses all these aspects of Hsp90 structure and function.

Chaperoning steroidal physiology: lessons from mouse genetic models of Hsp90 and its cochaperones

The molecular chaperone Hsp90 is abundant, ubiquitous, and catholic to biological processes in eukaryotes, controlling phosphorylation cascades, protein stability and turnover, client localization and trafficking, and ligand-receptor interactions. Not surprisingly, Hsp90 does not accomplish these activities alone. Instead, an ever-growing number of cochaperones have been identified, leading to an explosion of reports on their molecular and cellular effects on Hsp90 chaperoning of client substrates. Notable among these clients are many members of the steroid receptor family, such as glucocorticoid, androgen, estrogen and progesterone receptors. Cochaperones typically associated with the mature, hormone-competent states of these receptors include p23, the FK506-binding protein 52 (FKBP52), FKBP51, protein phosphatase 5 (PP5) and cyclophilin 40 (Cyp40). The ultimate relevance of these cochaperones to steroid receptor action depends on their physiological effects. In recent years, the first mouse genetic models of these cochaperones have been developed. This work will review the complex and intriguing phenotypes so far obtained in genetically-altered mice and compare them to the known molecular and cellular impacts of cochaperones on steroid receptors. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

Protein phosphatase 5 mediates lipid metabolism through reciprocal control of glucocorticoid receptor and peroxisome proliferator-activated receptor-γ (PPARγ)

Glucocorticoid receptor-α (GRα) and peroxisome proliferator-activated receptor-γ (PPARγ) regulate adipogenesis by controlling the balance between lipolysis and lipogenesis. Here, we show that protein phosphatase 5 (PP5), a nuclear receptor co-chaperone, reciprocally modulates the lipometabolic activities of GRα and PPARγ. Wild-type and PP5-deficient (KO) mouse embryonic fibroblast cells were used to show binding of PP5 to both GRα and PPARγ. In response to adipogenic stimuli, PP5-KO mouse embryonic fibroblast cells showed almost no lipid accumulation with reduced expression of adipogenic markers (aP2, CD36, and perilipin) and low fatty-acid synthase enzymatic activity. This was completely reversed following reintroduction of PP5. Loss of PP5 increased phosphorylation of GRα at serines 212 and 234 and elevated dexamethasone-induced activity at prolipolytic genes. In contrast, PPARγ in PP5-KO cells was hyperphosphorylated at serine 112 but had reduced rosiglitazone-induced activity at lipogenic genes. Expression of the S112A mutant rescued PPARγ transcriptional activity and lipid accumulation in PP5-KO cells pointing to Ser-112 as an important residue of PP5 action. This work identifies PP5 as a fulcrum point in nuclear receptor control of the lipolysis/lipogenesis equilibrium and as a potential target in the treatment of obesity.

Disruption of serine/threonine protein phosphatase 5 (PP5:PPP5c) in mice reveals a novel role for PP5 in the regulation of ultraviolet light-induced phosphorylation of serine/threonine protein kinase Chk1 (CHEK1)

PP5 is a ubiquitously expressed Ser/Thr protein phosphatase. High levels of PP5 have been observed in human cancers, and constitutive PP5 overexpression aids tumor progression in mouse models of tumor development. However, PP5 is highly conserved among species, and the roles of PP5 in normal tissues are not clear. Here, to help evaluate the biological actions of PP5, a Cre/loxP-conditional mouse line was generated. In marked contrast to the early embryonic lethality associated with the genetic disruption of other PPP family phosphatases (e.g. PP2A and PP4), intercrosses with mouse lines that ubiquitously express Cre recombinase starting early in development (e.g. MeuCre40 and ACTB-Cre) produced viable and fertile PP5-deficient mice. Phenotypic differences caused by the total disruption of PP5 were minor, suggesting that small molecule inhibitors of PP5 will not have widespread systemic toxicity. Examination of roles for PP5 in fibroblasts generated from PP5-deficient embryos (PP5(-/-) mouse embryonic fibroblasts) confirmed some known roles and identified new actions for PP5. PP5(-/-) mouse embryonic fibroblasts demonstrated increased sensitivity to UV light, hydroxyurea, and camptothecin, which are known activators of ATR (ataxia-telangiectasia and Rad3-related) kinase. Further study revealed a previously unrecognized role for PP5 downstream of ATR activation in a UV light-induced response. The genetic disruption of PP5 is associated with enhanced and prolonged phosphorylation of a single serine (Ser-345) on Chk1, increased phosphorylation of the p53 tumor suppressor protein (p53) at serine 18, and increased p53 protein levels. A comparable role for PP5 in the regulation of Chk1 phosphorylation was also observed in human cells.

Heat-induced chaperone activity of serine/threonine protein phosphatase 5 enhances thermotolerance in Arabidopsis thaliana

• This study reports that Arabidopsis thaliana protein serine/threonine phosphatase 5 (AtPP5) plays a pivotal role in heat stress resistance. A high-molecular-weight (HMW) form of AtPP5 was isolated from heat-treated A. thaliana suspension cells. AtPP5 performs multiple functions, acting as a protein phosphatase, foldase chaperone, and holdase chaperone. The enzymatic activities of this versatile protein are closely associated with its oligomeric status, ranging from low oligomeric protein species to HMW complexes. • The phosphatase and foldase chaperone functions of AtPP5 are associated primarily with the low-molecular-weight (LMW) form, whereas the HMW form exhibits holdase chaperone activity. Transgenic over-expression of AtPP5 conferred enhanced heat shock resistance to wild-type A. thaliana and a T-DNA insertion knock-out mutant was defective in acquired thermotolerance. A recombinant phosphatase mutant (H290N) showed markedly increased holdase chaperone activity. • In addition, enhanced thermotolerance was observed in transgenic plants over-expressing H290N, which suggests that the holdase chaperone activity of AtPP5 is primarily responsible for AtPP5-mediated thermotolerance. • Collectively, the results from this study provide the first evidence that AtPP5 performs multiple enzymatic activities that are mediated by conformational changes induced by heat-shock stress.

Versatile TPR domains accommodate different modes of target protein recognition and function

The tetratricopeptide repeat (TPR) motif is one of many repeat motifs that form structural domains in proteins that can act as interaction scaffolds in the formation of multi-protein complexes involved in numerous cellular processes such as transcription, the cell cycle, protein translocation, protein degradation and host defence against invading pathogens. The crystal structures of many TPR domain-containing proteins have been determined, showing TPR motifs as two anti-parallel α-helices packed in tandem arrays to form a structure with an amphipathic groove which can bind a target peptide. This is however not the only mode of target recognition by TPR domains, with short amino acid insertions and alternative TPR motif conformations also shown to contribute to protein interactions, highlighting diversity in TPR domains and the versatility of this structure in mediating biological events.

Serine/threonine phosphatases: mechanism through structure

The reversible phosphorylation of proteins is accomplished by opposing activities of kinases and phosphatases. Relatively few protein serine/threonine phosphatases (PSPs) control the specific dephosphorylation of thousands of phosphoprotein substrates. Many PSPs, exemplified by protein phosphatase 1 (PP1) and PP2A, achieve substrate specificity and regulation through combinatorial interactions between conserved catalytic subunits and a large number of regulatory subunits. Other PSPs, represented by PP2C and FCP/SCP, contain both catalytic and regulatory domains within the same polypeptide chain. Here, we discuss biochemical and structural investigations that advance the mechanistic understanding of the three major classes of PSPs, with a focus on PP2A.

Activated Rac1 GTPase translocates protein phosphatase 5 to the cell membrane and stimulates phosphatase activity in vitro

Physiological studies of ion channel regulation have implicated the Ser/Thr protein phosphatase 5 (PP5) as an effector of Rac1 GTPase signaling, but direct biochemical evidence for PP5 regulation by Rac1 is lacking. In this study we used immunoprecipitation, in vitro binding, cellular fractionation, and immunofluorescence techniques to show that the tetratricopeptide repeat domain of PP5 interacts specifically and directly with active Rac1. Consequently, activation of Rac1 promoted PP5 translocation to the plasma membrane in intact cells and stimulated PP5 phosphatase activity in vitro. In contrast, neither constitutively active RhoA-V14 nor dominant negative Rac1N17, which preferentially binds GDP and retains an inactive conformation, bound PP5 or stimulated its activity. In addition, Rac1N17 and Rac1(PBRM), a mutant lacking the C-terminal polybasic region required for Rac1 association with the membrane, both failed to cause membrane translocation of PP5. Mutation of predicted contact residues in the PP5 tetratricopeptide repeat domain or within Rac1 also disrupted co-immunoprecipitation of Rac1-PP5 complexes and membrane translocation of PP5. Specific binding of PP5 to activated Rac1 provides a direct mechanism by which PP5 can be stimulated and recruited to participate in Rac1-mediated signaling pathways.

Decreased ERK and JNK signaling contribute to gene overexpression in "senescent" CD4+CD28- T cells through epigenetic mechanisms

An inflammatory and cytotoxic CD4+CD28- T cell subset infiltrates atherosclerotic plaques and is implicated in plaque rupture and myocardial infarctions. This pathologic subset develops with replicative stress and is found in patients with chronic inflammatory diseases such as RA as well as with aging. CD4+CD28- cells overexpress genes normally suppressed by DNA methylation in CD4+CD28+ T cells, such as KIR, perforin, and CD70. How this subset over expresses methylation-sensitive genes is unknown. DNA methylation patterns are maintained in proliferating cells by Dnmts, which are up-regulated during mitosis by the ERK and JNK signaling pathways. We hypothesized that defects in these signaling pathways contribute to altered gene expression in human CD4+CD28- cells through effects on DNA methylation. We report that signaling through the ERK and JNK pathways is decreased in CD4+CD28- relative to CD4+CD28+ cells from the same individuals and that ERK and JNK pathway inhibition decreases Dnmt1 and -3a levels, which in turn, causes demethylation and overexpression of the TNFSF7 (CD70) gene. We also report that CD4+CD28- T cells overexpress PP5, a stress-induced inhibitor of the ERK and JNK signaling pathways that may contribute to the signaling defects. We conclude that decreased ERK and JNK signaling in the CD4+CD28- subset, arising with replicative stress, can lead to the overexpression of normally suppressed genes through effects on Dnmts and consequently, chromatin structure.

Plasticity of the Hsp90 chaperone machine in divergent eukaryotic organisms

Hsp90 is critical for the regulation and activation of numerous client proteins critical for diverse functions such as cell growth, differentiation, and reproduction. Cytosolic Hsp90 function is dependent on a battery of co-chaperone proteins that regulate the ATPase activity of Hsp90 function or direct Hsp90 to interact with specific client proteins. Little is known about how Hsp90 complexes vary between different organisms and how this affects the scope of clients that are activated by Hsp90. This study determined whether ten distinct Hsp90 co-chaperones were encoded by genes in 19 disparate eukaryotic organisms. Surprisingly, none of the co-chaperones were present in all organisms. The co-chaperone Hop/Sti1 was most widely dispersed (18 out of 19 species), while orthologs of Cdc37, which is critical for the stability and activation of diverse protein kinases in yeast and mammals, were identified in only nine out of 19 species examined. The organism with the smallest proteome, Encephalitozoon cuniculi, contained only three of these co-chaperones, suggesting a correlation between client diversity and the complexity of the Hsp90 co-chaperone machine. Our results suggest co-chaperones are critical for cytosolic Hsp90 function in vivo, but that the composition of Hsp90 complexes varies depending on the specialized protein folding requirements of divergent species.