PCAF modulates PTEN activity

Pancreatic Neuroendocrine Tumors: Signaling Pathways and Epigenetic Regulation

Pancreatic neuroendocrine tumors (PNETs) are characterized by dysregulated signaling pathways that are crucial for tumor formation and progression. The efficacy of traditional therapies is limited, particularly in the treatment of PNETs at an advanced stage. Epigenetic alterations profoundly impact the activity of signaling pathways in cancer development, offering potential opportunities for drug development. There is currently a lack of extensive research on epigenetic regulation in PNETs. To fill this gap, we first summarize major signaling events that are involved in PNET development. Then, we discuss the epigenetic regulation of these signaling pathways in the context of both PNETs and commonly occurring-and therefore more extensively studied-malignancies. Finally, we will offer a perspective on the future research direction of the PNET epigenome and its potential applications in patient care.

Regulatory mechanisms triggered by enzyme interactions with lipid membrane surfaces

Recruitment of enzymes to intracellular membranes often modulates their catalytic activity, which can be important in cell signaling and membrane trafficking. Thus, re-localization is not only important for these enzymes to gain access to their substrates, but membrane interactions often allosterically regulate enzyme function by inducing conformational changes across different time and amplitude scales. Recent structural, biophysical and computational studies have revealed how key enzymes interact with lipid membrane surfaces, and how this membrane binding regulates protein structure and function. This review summarizes the recent progress in understanding regulatory mechanisms involved in enzyme-membrane interactions.

Synthetic lethal approaches to target cancers with loss of PTEN function

Phosphatase and tensin homolog (PTEN) is a tumour suppressor gene and has a role in inhibiting the oncogenic AKT signalling pathway by dephosphorylating phosphatidylinositol 3,4,5-triphosphate (PIP3) into phosphatidylinositol 4,5-bisphosphate (PIP2). The function of PTEN is regulated by different mechanisms and inactive PTEN results in aggressive tumour phenotype and tumorigenesis. Identifying targeted therapies for inactive tumour suppressor genes such as PTEN has been challenging as it is difficult to restore the tumour suppressor functions. Therefore, focusing on the downstream signalling pathways to discover a targeted therapy for inactive tumour suppressor genes has highlighted the importance of synthetic lethality studies. This review focuses on the potential synthetic lethality genes discovered in PTEN-inactive cancer types. These discovered genes could be potential targeted therapies for PTEN-inactive cancer types and may improve the treatment response rates for aggressive types of cancer.

Enhanced Transcriptional Signature and Expression of Histone-Modifying Enzymes in Salivary Gland Tumors

Salivary gland tumors (SGTs) are rare and complex neoplasms characterized by heterogenous histology and clinical behavior as well as resistance to systemic therapy. Tumor etiology is currently under elucidation and an interplay of genetic and epigenetic changes has been proposed to contribute to tumor development. In this work, we investigated epigenetic regulators and histone-modifying factors that may alter gene expression and participate in the pathogenesis of SGT neoplasms. We performed a detailed bioinformatic analysis on a publicly available RNA-seq dataset of 94 ACC tissues supplemented with clinical data and respective controls and generated a protein-protein interaction (PPI) network of chromatin and histone modification factors. A significant upregulation of TP53 and histone-modifying enzymes SUV39H1, EZH2, PRMT1, HDAC8, and KDM5B, along with the upregulation of DNA methyltransferase DNMT3A and ubiquitin ligase UHRF1 mRNA levels, as well as a downregulation of lysine acetyltransferase KAT2B levels, were detected in ACC tissues. The protein expression of p53, SUV39H1, EZH2, and HDAC8 was further validated in SGT tissues along with their functional deposition of the repressive histone marks H3K9me3 and H3K27me3, respectively. Overall, this study is the first to detect a network of interacting proteins affecting chromatin structure and histone modifications in salivary gland tumor cells, further providing mechanistic insights in the molecular profile of SGTs that confer to altered gene expression programs.

The regulation of PTEN: Novel insights into functions as cancer biomarkers and therapeutic targets

This review summarizes the implications of the primary tumor suppressor protein phosphatase and tensin homolog (PTEN) in aggressive cancer development. PTEN interacts with other cellular proteins or factors suggesting the existence of an intricate molecular network that regulates their oncogenic function. Accumulating evidence has shown that PTEN exists and plays a role in the cytoplasmic organelles and in the nucleus. PTEN blocks phosphoinositide 3-kinases (PI3K)-protein kinase B-mammalian target of rapamycin signaling pathway by dephosphorylating phosphatidylinositol (PI)-3,4,5-triphosphate to PI-4,5-bisphosphate thus counteracting PI3K function. Studies have shown that PTEN expression is tightly regulated at transcriptional, posttranscriptional, and posttranslational levels (including protein-protein interactions and posttranslational modifications). Despite recent advances in PTEN research, the regulation and function of the PTEN gene remain largely unknown. How mutation or loss of specific exons in the PTEN gene occurs and involves in cancer development is not clear. This review illustrates the regulatory mechanisms of PTEN expression and discusses how PTEN participates in tumor development and/or suppression. Future prospects for the clinical applications are also highlighted.

Estrogen promotes Epithelial ovarian cancer cells proliferation via down-regulating expression and activating phosphorylation of PTEN

Epithelial ovarian cancer (EOC) is the most common of cancer death among malignant tumors in women, its occurrence and development are strongly linked to estrogen. Having identified the phosphatase and tensin homologue (PTEN) is a potent tumor suppressor regulating cell proliferation, migration, and survival. Meanwhile, there is a correlation between PTEN protein expression and estrogen receptor expression in EOC. However, no study has amplified on the molecular regulatory mechanism and function between estrogen and PTEN in the development of EOC. In this research, we found that PTEN shows a low expression level in EOC tissues and estrogen decreased PTEN expression via the estrogen receptor 1 (ESR1) in EOC cells. Knockdown of PTEN enhanced the proliferation and migration level of EOC cells driven by estrogen. Moreover, PTEN was also phosphorylated by G protein-coupled receptor 30 (GPR30)-Protein kinase C (PKC) signaling pathway upon estrogen stimulation. Inhibiting the phosphorylation of PTEN weakened the proliferation and migration of estrogen induced-EOC cells estrogen and decreased the phosphorylation of Protein kinase B (AKT) and Mammalian target of rapamycin (mTOR). These results indicated that estrogen decreased PTEN expression level via the ESR1 genomic pathway and phosphorylated PTEN via the GPR30-PKC non-genomic pathway to activate the PI3K/AKT/mTOR signaling pathway, thereby determining the fate of EOC cells.

Nuclear PTEN's Functions in Suppressing Tumorigenesis: Implications for Rare Cancers

Phosphatase and tensin homolog (PTEN) encodes a tumor-suppressive phosphatase with both lipid and protein phosphatase activity. The tumor-suppressive functions of PTEN are lost through a variety of mechanisms across a wide spectrum of human malignancies, including several rare cancers that affect pediatric and adult populations. Originally discovered and characterized as a negative regulator of the cytoplasmic, pro-oncogenic phosphoinositide-3-kinase (PI3K) pathway, PTEN is also localized to the nucleus where it can exert tumor-suppressive functions in a PI3K pathway-independent manner. Cancers can usurp the tumor-suppressive functions of PTEN to promote oncogenesis by disrupting homeostatic subcellular PTEN localization. The objective of this review is to describe the changes seen in PTEN subcellular localization during tumorigenesis, how PTEN enters the nucleus, and the spectrum of impacts and consequences arising from disrupted PTEN nuclear localization on tumor promotion. This review will highlight the immediate need in understanding not only the cytoplasmic but also the nuclear functions of PTEN to gain more complete insights into how important PTEN is in preventing human cancers.

The equilibrium of tumor suppression: DUBs as active regulators of PTEN

PTEN is among the most commonly lost or mutated tumor suppressor genes in human cancer. PTEN, a bona fide lipid phosphatase that antagonizes the highly oncogenic PI3K-AKT-mTOR pathway, is considered a major dose-dependent tumor suppressor. Although PTEN function can be compromised by genetic mutations in inherited syndromes and cancers, posttranslational modifications of PTEN may also play key roles in the dynamic regulation of its function. Notably, deregulated ubiquitination and deubiquitination lead to detrimental impacts on PTEN levels and subcellular partitioning, promoting tumorigenesis. While PTEN can be targeted by HECT-type E3 ubiquitin ligases for nuclear import and proteasomal degradation, studies have shown that several deubiquitinating enzymes, including HAUSP/USP7, USP10, USP11, USP13, OTUD3 and Ataxin-3, can remove ubiquitin from ubiquitinated PTEN in cancer-specific contexts and thus reverse ubiquitination-mediated PTEN regulation. Researchers continue to reveal the precise molecular mechanisms by which cancer-specific deubiquitinases of PTEN regulate its roles in the pathobiology of cancer, and new methods of pharmacologically for modulating PTEN deubiquitinases are critical areas of investigation for cancer treatment and prevention. Here, we assess the mechanisms and functions of deubiquitination as a recently appreciated mode of PTEN regulation and review the link between deubiquitinases and PTEN reactivation and its implications for therapeutic strategies.

Targeting PTEN Regulation by Post Translational Modifications

Phosphatidylinositol-3,4,5-triphosphate (PIP₃) is a lipidic second messenger present at very low concentrations in resting normal cells. PIP₃ levels, though, increase quickly and transiently after growth factor addition, upon activation of phosphatidylinositol 3-kinase (PI3-kinase). PIP₃ is required for the activation of intracellular signaling pathways that induce cell proliferation, cell migration, and survival. Given the critical role of this second messenger for cellular responses, PIP₃ levels must be tightly regulated. The lipid phosphatase PTEN (phosphatase and tensin-homolog in chromosome 10) is the phosphatase responsible for PIP₃ dephosphorylation to PIP₂. PTEN tumor suppressor is frequently inactivated in endometrium and prostate carcinomas, and also in glioblastoma, illustrating the contribution of elevated PIP₃ levels for cancer development. PTEN biological activity can be modulated by heterozygous gene loss, gene mutation, and epigenetic or transcriptional alterations. In addition, PTEN can also be regulated by post-translational modifications. Acetylation, oxidation, phosphorylation, sumoylation, and ubiquitination can alter PTEN stability, cellular localization, or activity, highlighting the complexity of PTEN regulation. While current strategies to treat tumors exhibiting a deregulated PI3-kinase/PTEN axis have focused on PI3-kinase inhibition, a better understanding of PTEN post-translational modifications could provide new therapeutic strategies to restore PTEN action in PIP₃-dependent tumors.

Role of Tumor Suppressor PTEN and Its Regulation in Malignant Transformation of Endometrium

Tumor-suppressive effects of PTEN are well-known, but modern evidence suggest that they are not limited to its ability to inhibit pro-oncogenic PI3K/AKT signaling pathway. Features of PTEN structure facilitate its interaction with substrates of different nature and display its activity in various ways both in the cytoplasm and in cell nuclei, which makes it possible to take a broader look at its ability to suppress tumor growth. The possible mechanisms of the loss of PTEN effects are also diverse - PTEN can be regulated at many levels, leading to change in the protein activity or its amount in the cell, while their significance for the development of malignant tumors has yet to be studied. Here we summarize the current data on the PTEN structure, its functions and changes in its regulatory mechanisms during malignant transformation of the cells, focusing on one of the most sensitive to the loss of PTEN types of malignant tumors - endometrial cancer.

The orchestrated signaling by PI3Kα and PTEN at the membrane interface

The oncogene PI3Kα and the tumor suppressor PTEN represent two antagonistic enzymatic activities that regulate the interconversion of the phosphoinositide lipids PI(4,5)P2 and PI(3,4,5)P3 in membranes. As such, they are defining components of phosphoinositide-based cellular signaling and membrane trafficking pathways that regulate cell survival, growth, and proliferation, and are often deregulated in cancer. In this review, we highlight aspects of PI3Kα and PTEN interplay at the intersection of signaling and membrane trafficking. We also discuss the mechanisms of PI3Kα- and PTEN- membrane interaction and catalytic activation, which are fundamental for our understanding of the structural and allosteric implications on signaling at the membrane interface and may aid current efforts in pharmacological targeting of these proteins.

Signaling pathways and targeted therapies in lung squamous cell carcinoma: mechanisms and clinical trials

Lung cancer is the leading cause of cancer-related death across the world. Unlike lung adenocarcinoma, patients with lung squamous cell carcinoma (LSCC) have not benefitted from targeted therapies. Although immunotherapy has significantly improved cancer patients' outcomes, the relatively low response rate and severe adverse events hinder the clinical application of this promising treatment in LSCC. Therefore, it is of vital importance to have a better understanding of the mechanisms underlying the pathogenesis of LSCC as well as the inner connection among different signaling pathways, which will surely provide opportunities for more effective therapeutic interventions for LSCC. In this review, new insights were given about classical signaling pathways which have been proved in other cancer types but not in LSCC, including PI3K signaling pathway, VEGF/VEGFR signaling, and CDK4/6 pathway. Other signaling pathways which may have therapeutic potentials in LSCC were also discussed, including the FGFR1 pathway, EGFR pathway, and KEAP1/NRF2 pathway. Next, chromosome 3q, which harbors two key squamous differentiation markers SOX2 and TP63 is discussed as well as its related potential therapeutic targets. We also provided some progress of LSCC in epigenetic therapies and immune checkpoints blockade (ICB) therapies. Subsequently, we outlined some combination strategies of ICB therapies and other targeted therapies. Finally, prospects and challenges were given related to the exploration and application of novel therapeutic strategies for LSCC.

Linear ubiquitination of PTEN impairs its function to promote prostate cancer progression

PTEN is frequently mutated in human cancers, which leads to the excessive activation of PI3K/AKT signaling and thus promotes tumorigenesis and drug resistance. Met¹-linked ubiquitination (M1-Ubi) is also involved in cancer progression, but the mechanism is poorly defined. Here we find that HOIP, one important component of linear ubiquitin chain assembly complex (LUBAC), promotes prostate cancer (PCa) progression by enhancing AKT signaling in a PTEN-dependent manner. Mechanistically, PTEN is modified by M1-Ubi at two sites K144 and K197, which significantly inhibits PTEN phosphatase activity and thus accelerates PCa progression. More importantly, we identify that the high-frequency mutants PTEN^R173H and PTEN^R173C in PCa patients showed the enhanced level of M1-Ubi, which impairs PTEN function in inhibition of AKT phosphorylation and cell growth. We also find that HOIP depletion sensitizes PCa cells to therapeutic agents BKM120 and Enzalutamide. Furthermore, the clinical data analyses confirm that HOIP is upregulated and positively correlated with AKT activation in PCa patient specimen, which may promote PCa progression and increase the risk of PCa biochemical relapse. Together, our study reveals a key role of PTEN M1-Ubi in regulation of AKT activation and PCa progression, which may propose a new strategy for PCa therapy.

Targeting the acetylation signaling pathway in cancer therapy

Acetylation represents one of the major post-translational protein modifications, which introduces an acetyl functional group into amino acids such as the lysine residue to yield an acetate ester bond, neutralizing its positive charge. Regulation of protein functions by acetylation occurs in multiple ways, such as affecting protein stability, activity, localization, and interaction with other proteins or DNA. It has been well documented that the recruitment of histone acetyltransferases (HATs) and histone deacetylases (HDACs) to the transcriptional machinery can modulate histone acetylation status, which is directly involved in the dynamic regulation of genes controlling cell proliferation and division. Dysregulation of gene expression is involved in tumorigenesis and aberrant activation of histone deacetylases has been reported in several types of cancer. Moreover, there is growing body of evidence showing that acetylation is widely involved in non-histone proteins to impact their roles in various cellular processes including tumorigenesis. As such, small molecular compounds inhibiting HAT or HDAC enzymatic activities have been developed and investigated for therapeutic purpose. Here we review the recent progress in our understanding of protein acetylation and discuss the therapeutic potential of targeting the acetylation signaling pathway in cancer.

S-palmitoylation of PCSK9 induces sorafenib resistance in liver cancer by activating the PI3K/AKT pathway

Sorafenib is currently the first-line treatment for advanced hepatocellular carcinoma (HCC). However, sorafenib resistance remains a significant challenge. Aberrant AKT signaling activation is a crucial mechanism driving sorafenib resistance in HCC. Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays a vital role in antitumor immune responses. In this study, we demonstrate that aberrant PCSK9 upregulation promotes cell proliferation and sorafenib resistance in HCC by inducing AKT-S473 phosphorylation. After palmitoylation at cysteine 600, the binding affinity between PCSK9 and tensin homolog (PTEN) is dramatically increased, inducing lysosome-mediated PTEN degradation and subsequent AKT activation. We identify zinc finger DHHC-type palmitoyltransferase 16 (ZDHHC16) as a palmitoyltransferase that promotes PCSK9 palmitoylation at cysteine 600. We also develop a biologically active PCSK9-derived peptide that competitively inhibits PCSK9 palmitoylation, suppressing AKT phosphorylation and augmenting the antitumor effects of sorafenib in HCC.

The role of epigenetic modifications in Colorectal Cancer Metastasis

Distant metastasis is the major contributor to the high mortality rate of colorectal cancer (CRC). To overcome the poor prognosis caused by distant metastasis, the mechanisms of CRC metastasis should be further explored. Epigenetic events are the main mediators of gene regulation and further affect tumor progression. Recent studies have found that some epigenetic enzymes are often dysregulated or mutated in multiple tumor types, which prompted us to study the roles of these enzymes in CRC metastasis. In this review, we summarized the alteration of enzymes related to various modifications, including histone modification, nonhistone modification, DNA methylation, and RNA methylation, and their epigenetic mechanisms during the progression of CRC metastasis. Existing data suggest that targeting epigenetic enzymes is a promising strategy for the treatment of CRC metastasis.

Regulation of the tumor suppressor PTEN in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is a subtype of breast cancer (BCa) in which estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER-2) are not expressed. Although TNBC cases account for approximately 15% of all BCa cases, TNBC patients' prognosis is poor compared with that of other BCa subtypes. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) plays an important role in cell proliferation and migration by negatively regulating the PI3K/Akt pathway. PTEN is one of the most commonly inactivated tumor suppressors in BCa. PTEN inactivity is associated with larger tumor sizes, multiple lymph node metastases, and an aggressive triple-negative phenotype. This review primarily focuses on two key points: (1) PTEN and its function. (2) The regulation of tumor suppressor PTEN in TNBC. We provide a summary of genomic alterations of PTEN in BCa. We further discuss the transcriptional regulation of PTEN and how PTEN is regulated by posttranscription and posttranslational modification, as well as by protein interactions. Finally, we discuss the perspectives of the PTEN protein in TNBC.

Promoting axon regeneration in the central nervous system by increasing PI3-kinase signaling

Much research has focused on the PI3-kinase and PTEN signaling pathway with the aim to stimulate repair of the injured central nervous system. Axons in the central nervous system fail to regenerate, meaning that injuries or diseases that cause loss of axonal connectivity have life-changing consequences. In 2008, genetic deletion of PTEN was identified as a means of stimulating robust regeneration in the optic nerve. PTEN is a phosphatase that opposes the actions of PI3-kinase, a family of enzymes that function to generate the membrane phospholipid PIP₃ from PIP₂ (phosphatidylinositol (3,4,5)-trisphosphate from phosphatidylinositol (4,5)-bisphosphate). Deletion of PTEN therefore allows elevated signaling downstream of PI3-kinase, and was initially demonstrated to promote axon regeneration by signaling through mTOR. More recently, additional mechanisms have been identified that contribute to the neuron-intrinsic control of regenerative ability. This review describes neuronal signaling pathways downstream of PI3-kinase and PIP₃, and considers them in relation to both developmental and regenerative axon growth. We briefly discuss the key neuron-intrinsic mechanisms that govern regenerative ability, and describe how these are affected by signaling through PI3-kinase. We highlight the recent finding of a developmental decline in the generation of PIP₃ as a key reason for regenerative failure, and summarize the studies that target an increase in signaling downstream of PI3-kinase to facilitate regeneration in the adult central nervous system. Finally, we discuss obstacles that remain to be overcome in order to generate a robust strategy for repairing the injured central nervous system through manipulation of PI3-kinase signaling.

Lysine acetylation regulates the interaction between proteins and membranes

Lysine acetylation regulates the function of soluble proteins in vivo, yet it remains largely unexplored whether lysine acetylation regulates membrane protein function. Here, we use bioinformatics, biophysical analysis of recombinant proteins, live-cell fluorescent imaging and genetic manipulation of Drosophila to explore lysine acetylation in peripheral membrane proteins. Analysis of 50 peripheral membrane proteins harboring BAR, PX, C2, or EHD membrane-binding domains reveals that lysine acetylation predominates in membrane-interaction regions. Acetylation and acetylation-mimicking mutations in three test proteins, amphiphysin, EHD2, and synaptotagmin1, strongly reduce membrane binding affinity, attenuate membrane remodeling in vitro and alter subcellular localization. This effect is likely due to the loss of positive charge, which weakens interactions with negatively charged membranes. In Drosophila, acetylation-mimicking mutations of amphiphysin cause severe disruption of T-tubule organization and yield a flightless phenotype. Our data provide mechanistic insights into how lysine acetylation regulates membrane protein function, potentially impacting a plethora of membrane-related processes.

Lysine acetylation regulates the function of soluble proteins in vivo, yet it remains largely unexplored whether lysine acetylation regulates the function of membrane proteins. Here, the authors map lysine acetylation predominantly in membrane-interaction regions in peripheral membrane proteins and show with three candidate proteins how lysine acetylation is a regulator of membrane protein function.

PROTACs: Promising Approaches for Epigenetic Strategies to Overcome Drug Resistance

Epigenetic modulation of gene expression is essential for tissue-specific development and maintenance in mammalian cells. Disruption of epigenetic processes, and the subsequent alteration of gene functions, can result in inappropriate activation or inhibition of various cellular signaling pathways, leading to cancer. Recent advancements in the understanding of the role of epigenetics in cancer initiation and progression have uncovered functions for DNA methylation, histone modifications, nucleosome positioning, and non-coding RNAs. Epigenetic therapies have shown some promise for hematological malignancies, and a wide range of epigenetic-based drugs are undergoing clinical trials. However, in a dynamic survival strategy, cancer cells exploit their heterogeneous population which frequently results in the rapid acquisition of therapy resistance. Here, we describe novel approaches in drug discovery targeting the epigenome, highlighting recent advances the selective degradation of target proteins using Proteolysis Targeting Chimera (PROTAC) to address drug resistance.

Long non-coding RNAs and their targets as potential biomarkers in breast cancer

Breast cancer is among the lethal types of cancer with a high mortality rate, globally. Its high prevalence can be controlled through improved analysis and identification of disease-specific biomarkers. Recently, long non-coding RNAs (lncRNAs) have been reported as key contributors of carcinogenesis and regulate various cellular pathways through post-transcriptional regulatory mechanisms. The specific aim of this study was to identify the novel interactions of aberrantly expressed genetic components in breast cancer by applying integrative analysis of publicly available expression profiles of both lncRNAs and mRNAs. Differential expression patterns were identified by comparing the breast cancer expression profiles of samples with controls. Significant co-expression networks were identified through WGCNA analysis. WGCNA is a systems biology approach used to elucidate the pattern of correlation between genes across microarray samples. It is also used to identify the highly correlated modules. The results obtained from this study revealed significantly differentially expressed and co-expressed lncRNAs and their cis- and trans-regulating mRNA targets which include RP11-108F13.2 targeting TAF5L, RPL23AP2 targeting CYP4F3, CYP4F8 and AL022324.2 targeting LRP5L, AL022324.3, and Z99916.3, respectively. Moreover, pathway analysis revealed the involvement of identified mRNAs and lncRNAs in major cell signalling pathways, and target mRNAs expression is also validated through cohort data. Thus, the identified lncRNAs and their target mRNAs represent novel biomarkers that could serve as potential therapeutics for breast cancer and their roles could also be further validated through wet labs to employ them as potential therapeutic targets in future.

Tankyrases as modulators of pro-tumoral functions: molecular insights and therapeutic opportunities

Tankyrase 1 (TNKS1) and tankyrase 2 (TNKS2) are two homologous proteins that are gaining increasing importance due to their implication in multiple pathways and diseases such as cancer. TNKS1/2 interact with a large variety of substrates through the ankyrin (ANK) domain, which recognizes a sequence present in all the substrates of tankyrase, called Tankyrase Binding Motif (TBM). One of the main functions of tankyrases is the regulation of protein stability through the process of PARylation-dependent ubiquitination (PARdU). Nonetheless, there are other functions less studied that are also essential in order to understand the role of tankyrases in many pathways. In this review, we concentrate in different tankyrase substrates and we analyze in depth the biological consequences derived of their interaction with TNKS1/2. We also examine the concept of both canonical and non-canonical TBMs and finally, we focus on the information about the role of TNKS1/2 in different tumor context, along with the benefits and limitations of the current TNKS inhibitors targeting the catalytic PARP domain and the novel strategies to develop inhibitors against the ankyrin domain. Available data indicates the need for further deepening in the knowledge of tankyrases to elucidate and improve the current view of the role of these PARP family members and get inhibitors with a better therapeutic and safety profile.

The mechanism of full activation of tumor suppressor PTEN at the phosphoinositide-enriched membrane

Tumor suppressor PTEN, the second most highly mutated protein in cancer, dephosphorylates signaling lipid PIP₃ produced by PI3Ks. Excess PIP₃ promotes cell proliferation. The mechanism at the membrane of this pivotal phosphatase is unknown hindering drug discovery. Exploiting explicit solvent simulations, we tracked full-length PTEN trafficking from the cytosol to the membrane. We observed its interaction with membranes composed of zwitterionic phosphatidylcholine, anionic phosphatidylserine, and phosphoinositides, including signaling lipids PIP₂ and PIP₃. We tracked its moving away from the zwitterionic and getting absorbed onto anionic membrane that harbors PIP₃. We followed it localizing on microdomains enriched in signaling lipids, as PI3K does, and observed PIP₃ allosterically unfolding the N-terminal PIP₂ binding domain, positioning it favorably for the polybasic motif interaction with PIP₂. Finally, we determined PTEN catalytic action at the membrane, all in line with experimental observations, deciphering the mechanisms of how PTEN anchors to the membrane and restrains cancer.

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PTEN localizes on membrane microdomains enriched in phosphoinositides, as PI3K does

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Full PTEN activation requires both signaling lipids, PIP₂ and PIP₃

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Strong salt bridge interactions sustain stable PTEN membrane localization

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Substrate-induced P loop conformational change implicates PTEN catalytic activity

Redox Regulation of PTEN by Peroxiredoxins

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is known as a tumor suppressor gene that is frequently mutated in numerous human cancers and inherited syndromes. PTEN functions as a negative regulator of PI3K/Akt signaling pathway by dephosphorylating phosphatidylinositol (3, 4, 5)-trisphosphate (PIP3) to phosphatidylinositol (4, 5)-bisphosphate (PIP2), which leads to the inhibition of cell growth, proliferation, cell survival, and protein synthesis. PTEN contains a cysteine residue in the active site that can be oxidized by peroxides, forming an intramolecular disulfide bond between Cys¹²⁴ and Cys⁷¹. Redox regulation of PTEN by reactive oxygen species (ROS) plays a crucial role in cellular signaling. Peroxiredoxins (Prxs) are a superfamily of peroxidase that catalyzes reduction of peroxides and maintains redox homeostasis. Mammalian Prxs have 6 isoforms (I-VI) and can scavenge cellular peroxides. It has been demonstrated that Prx I can preserve and promote the tumor-suppressive function of PTEN by preventing oxidation of PTEN under benign oxidative stress via direct interaction. Also, Prx II-deficient cells increased PTEN oxidation and insulin sensitivity. Furthermore, Prx III has been shown to protect PTEN from oxidation induced by 15s-HpETE and 12s-HpETE, these are potent inflammatory and pro-oxidant mediators. Understanding the tight connection between PTEN and Prxs is important for providing novel therapies. Herein, we summarized recent studies focusing on the relationship of Prxs and the redox regulation of PTEN.

Interactomic affinity profiling by holdup assay: Acetylation and distal residues impact the PDZome-binding specificity of PTEN phosphatase

Protein domains often recognize short linear protein motifs composed of a core conserved consensus sequence surrounded by less critical, modulatory positions. PTEN, a lipid phosphatase involved in phosphatidylinositol 3-kinase (PI3K) pathway, contains such a short motif located at the extreme C-terminus capable to recognize PDZ domains. It has been shown that the acetylation of this motif could modulate the interaction with several PDZ domains. Here we used an accurate experimental approach combining high-throughput holdup chromatographic assay and competitive fluorescence polarization technique to measure quantitative binding affinity profiles of the PDZ domain-binding motif (PBM) of PTEN. We substantially extended the previous knowledge towards the 266 known human PDZ domains, generating the full PDZome-binding profile of the PTEN PBM. We confirmed that inclusion of N-terminal flanking residues, acetylation or mutation of a lysine at a modulatory position significantly altered the PDZome-binding profile. A numerical specificity index is also introduced as an attempt to quantify the specificity of a given PBM over the complete PDZome. Our results highlight the impact of modulatory residues and post-translational modifications on PBM interactomes and their specificity.

Quick, Coordinated and Authentic Reprogramming of Ribosome Biogenesis during iPSC Reprogramming

Induction of pluripotent stem cells (iPSC) by OCT4 (octamer-binding transcription factor 4), SOX2 (SR box 2), KLF4 (Krüppel-Like Factor 4), and MYC (cellular Myelocytomatosis, c-MYC or MYC) (collectively OSKM) is revolutionary, but very inefficient, slow, and stochastic. It is unknown as to what underlies the potency aspect of the multi-step, multi-pathway, and inefficient iPSC reprogramming. Mesenchymal-to-epithelial (MET) transition is known as the earliest pathway reprogrammed. Using the recently established concepts of reprogramome and reprogramming legitimacy, the author first demonstrated that ribosome biogenesis (RB) is globally enriched in terms of human embryonic stem cells in comparison with fibroblasts, the popular starting cells of pluripotency reprogramming. It is then shown that the RB network was reprogrammed quickly in a coordinated fashion. Human iPSCs also demonstrated a more robust ribosome biogenesis. The quick and global reprogramming of ribosome biogenesis was also observed in an independent fibroblast line from a different donor. This study additionally demonstrated that MET did not initiate substantially at the time of proper RB reprogramming. This quick, coordinated and authentic RB reprogramming to the more robust pluripotent state by the OSKM reprogramming factors dramatically contrasts the overall low efficiency and long latency of iPSC reprogramming, and aligns well with the potency aspect of the inefficient OSKM reprogramming.

Histone Deacetylase Inhibitors as Multitarget-Directed Epi-Drugs in Blocking PI3K Oncogenic Signaling: A Polypharmacology Approach

Genetic mutations and aberrant epigenetic alterations are the triggers for carcinogenesis. The emergence of the drugs targeting epigenetic aberrations has provided a better outlook for cancer treatment. Histone deacetylases (HDACs) are epigenetic modifiers playing critical roles in numerous key biological functions. Inappropriate expression of HDACs and dysregulation of PI3K signaling pathway are common aberrations observed in human diseases, particularly in cancers. Histone deacetylase inhibitors (HDACIs) are a class of epigenetic small-molecular therapeutics exhibiting promising applications in the treatment of hematological and solid malignancies, and in non-neoplastic diseases. Although HDACIs as single agents exhibit synergy by inhibiting HDAC and the PI3K pathway, resistance to HDACIs is frequently encountered due to activation of compensatory survival pathway. Targeted simultaneous inhibition of both HDACs and PI3Ks with their respective inhibitors in combination displayed synergistic therapeutic efficacy and encouraged the development of a single HDAC-PI3K hybrid molecule via polypharmacology strategy. This review provides an overview of HDACs and the evolution of HDACs-based epigenetic therapeutic approaches targeting the PI3K pathway.

Lysine acetylation of cytoskeletal proteins: Emergence of an actin code

Reversible lysine acetylation of nuclear proteins such as histones is a long-established important regulatory mechanism for chromatin remodeling and transcription. In the cytoplasm, acetylation of a number of cytoskeletal proteins, including tubulin, cortactin, and the formin mDia2, regulates both cytoskeletal assembly and stability. More recently, acetylation of actin itself was revealed to regulate cytoplasmic actin polymerization through the formin INF2, with downstream effects on ER-to-mitochondrial calcium transfer, mitochondrial fission, and vesicle transport. This finding raises the possibility that actin acetylation, along with other post-translational modifications to actin, might constitute an "actin code," similar to the "histone code" or "tubulin code," controlling functional shifts to these central cellular proteins. Given the multiple roles of actin in nuclear functions, its modifications might also have important roles in gene expression.

Role of Ubiquitination in PTEN Cellular Homeostasis and Its Implications in GB Drug Resistance

Glioblastoma (GB) is the most common and aggressive brain malignancy, characterized by heterogeneity and drug resistance. PTEN, a crucial tumor suppressor, exhibits phosphatase-dependent (PI3K-AKT-mTOR pathway)/independent (nucleus stability) activities to maintain the homeostatic regulation of numerous physiological processes. Premature and absolute loss of PTEN activity usually tends to cellular senescence. However, monoallelic loss of PTEN is frequently observed at tumor inception, and absolute loss of PTEN activity also occurs at the late stage of gliomagenesis. Consequently, aberrant PTEN homeostasis, mainly regulated at the post-translational level, renders cells susceptible to tumorigenesis and drug resistance. Ubiquitination-mediated degradation or deregulated intracellular localization of PTEN hijacks cell growth rheostat control for neoplastic remodeling. Functional inactivation of PTEN mediated by the overexpression of ubiquitin ligases (E3s) renders GB cells adaptive to PTEN loss, which confers resistance to EGFR tyrosine kinase inhibitors and immunotherapies. In this review, we discuss how glioma cells develop oncogenic addiction to the E3s-PTEN axis, promoting their growth and proliferation. Antitumor strategies involving PTEN-targeting E3 ligase inhibitors can restore the tumor-suppressive environment. E3 inhibitors collectively reactivate PTEN and may represent next-generation treatment against deadly malignancies such as GB.

Complex functions of Gcn5 and Pcaf in development and disease

A wealth of biochemical and cellular data, accumulated over several years by multiple groups, has provided a great degree of insight into the molecular mechanisms of actions of GCN5 and PCAF in gene activation. Studies of these lysine acetyltransferases (KATs) in vitro, in cultured cells, have revealed general mechanisms for their recruitment by sequence-specific binding factors and their molecular functions as transcriptional co-activators. Genetic studies indicate that GCN5 and PCAF are involved in multiple developmental processes in vertebrates, yet our understanding of their molecular functions in these contexts remains somewhat rudimentary. Understanding the functions of GCN5/PCAF in developmental processes provides clues to the roles of these KATs in disease states. Here we will review what is currently known about the developmental roles of GCN5 and PCAF, as well as emerging role of these KATs in oncogenesis.

Non-histone protein acetylation by the evolutionarily conserved GCN5 and PCAF acetyltransferases

GCN5, conserved from yeast to humans, and the vertebrate specific PCAF, are lysine acetyltransferase enzymes found in large protein complexes. Both enzymes have well documented roles in the histone acetylation and the concomitant regulation of transcription. However, these enzymes also acetylate non-histone substrates to impact diverse aspects of cell physiology. Here, I review our current understanding of non-histone acetylation by GCN5 and PCAF across eukaryotes, from target identification to molecular mechanism and regulation. I focus mainly on budding yeast, where Gcn5 was first discovered, and mammalian systems, where the bulk of non-histone substrates have been characterized. I end the review by defining critical caveats and open questions that apply to all models.

Posttranslational Regulation and Conformational Plasticity of PTEN

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is a tumor suppressor that is frequently down-modulated in human cancer. PTEN inhibits the phosphatidylinositol 3-phosphate kinase (PI3K)/AKT pathway through its lipid phosphatase activity. Multiple PI3K/AKT-independent actions of PTEN, protein-phosphatase activities and functions within the nucleus have also been described. PTEN, therefore, regulates many cellular processes including cell proliferation, survival, genomic integrity, polarity, migration, and invasion. Even a modest decrease in the functional dose of PTEN may promote cancer development. Understanding the molecular and cellular mechanisms that regulate PTEN protein levels and function, and how these may go awry in cancer contexts, is, therefore, key to fully understanding the role of PTEN in tumorigenesis. Here, we discuss current knowledge on posttranslational control and conformational plasticity of PTEN, as well as therapeutic possibilities toward reestablishment of PTEN tumor-suppressor activity in cancer.

Redox regulation of tumor suppressor PTEN in cell signaling

Phosphatase and tensin homologs deleted on chromosome 10 (PTEN) is a potent tumor suppressor and often dysregulated in cancers. Cellular PTEN activity is restrained by the oxidation of active-site cysteine by reactive oxygen species (ROS). Recovery of its enzymatic activity predominantly depends on the availability of cellular thioredoxin (Trx) and peroxiredoxins (Prx), both are important players in cell signaling. Trx and Prx undergo redox-dependent conformational changes through the oxidation of cysteine residues at their active sites. Their dynamics are essential for protein functionality and regulation. In this review, we summarized the recent advances regarding the redox regulation of PTEN, with a specific focus on our current state-of-the-art understanding of the redox regulation of PTEN. We also proposed a tight association of the redox regulation of PTEN with Trx dimerization and Prx hyperoxidation, providing guidance for the identification of novel therapeutic targets.

Histone Acetyltransferase (HAT) P300/CBP Inhibitors Induce Synthetic Lethality in PTEN-Deficient Colorectal Cancer Cells through Destabilizing AKT

PTEN, a tumor suppressor, is found loss of function in many cancers, including colorectal cancer. To identify the synthetic lethal compounds working with PTEN deficiency, we performed a synthetic lethality drug screening with PTEN-isogenic colorectal cancer cells. From the screening, we found that PTEN-/- colorectal cancer cells were sensitive to anacardic acid, a p300/CBP histone acetyltransferase (HAT) inhibitor. Anacardic acid significantly reduced the viability of PTEN-/- cells not in PTEN+/+ cells via inducing apoptosis. Inhibition of HAT activity of p300/CBP by anacardic acid reduced the acetylation of histones at the promoter region and inhibited the transcription of Hsp70 family of proteins. The down-regulation of Hsp70 family proteins led to the reduction of AKT-Hsp70 complex formation, AKT destabilization and decreased the level of phosphorylated AKT at Ser473, all of which are vital for the survival of PTEN-/- colorectal cells. The synthetic lethality effect of anacardic acid was further validated in tumor xenograft mice models, where PTEN-/- colorectal tumors showed greater sensitivity to anacardic acid treatment than PTEN+/+ tumors. These data suggest that anacardic acid induced synthetic lethality by inhibiting HAT activity of p300/CBP, thereby reducing Hsp70 transcription and destabilizing AKT in PTEN deficient colorectal cancer cells.

Structural Mechanisms of PTEN Regulation

The tumor suppressor phosphatase and tensin homolog on chromosome 10 (PTEN) is a tightly regulated enzyme responsible for dephosphorylating the progrowth lipid messenger molecule phosphatidylinositol 3,4,5-trisphosphate (PIP₃) on the plasma membrane. The carboxy-terminal tail (CTT) of PTEN is key for regulation of the enzyme. When phosphorylated, the unstructured CTT interacts with the phosphatase-C2 superdomain to inactivate the enzyme by preventing membrane association. PTEN mutations associated with cancer also inactivate the enzyme. Alternate translation-initiation sites generate extended isoforms of PTEN, such as PTEN-L that has multiple roles in cells. The extended amino-terminal region bears a signal sequence and a polyarginine sequence to facilitate exit from and entry into cells, respectively, and a membrane-binding helix that activates the enzyme. This amino-terminal region also facilitates mitochondrial and nucleolar localization. This review explores PTEN structure and its impact on localization and regulation.

PCAF-mediated acetylation of ISX recruits BRD4 to promote epithelial-mesenchymal transition

Epigenetic regulation is important for cancer progression; however, the underlying mechanisms, particularly those involving protein acetylation, remain to be fully understood. Here, we show that p300/CBP‐associated factor (PCAF)‐dependent acetylation of the transcription factor intestine‐specific homeobox (ISX) regulates epithelial–mesenchymal transition (EMT) and promotes cancer metastasis. Mechanistically, PCAF acetylation of ISX at lysine 69 promotes the interaction with acetylated bromodomain‐containing protein 4 (BRD4) at lysine 332 in tumor cells, and the translocation of the resulting complex into the nucleus. There, it binds to promoters of EMT genes, where acetylation of histone 3 at lysines 9, 14, and 18 initiates chromatin remodeling and subsequent transcriptional activation. Ectopic ISX expression enhances EMT marker expression, including TWIST1, Snail1, and VEGF, induces cancer metastasis, but suppresses E‐cadherin expression. In lung cancer, ectopic expression of PCAF–ISX–BRD4 axis components correlates with clinical metastatic features and poor prognosis. These results suggest that the PCAF–ISX–BRD4 axis mediates EMT signaling and regulates tumor initiation and metastasis.

Functions and mechanisms of non-histone protein acetylation

Nε-lysine acetylation was discovered more than half a century ago as a post-translational modification of histones and has been extensively studied in the context of transcription regulation. In the past decade, proteomic analyses have revealed that non-histone proteins are frequently acetylated and constitute a major portion of the acetylome in mammalian cells. Indeed, non-histone protein acetylation is involved in key cellular processes relevant to physiology and disease, such as gene transcription, DNA damage repair, cell division, signal transduction, protein folding, autophagy and metabolism. Acetylation affects protein functions through diverse mechanisms, including by regulating protein stability, enzymatic activity, subcellular localization and crosstalk with other post-translational modifications and by controlling protein-protein and protein-DNA interactions. In this Review, we discuss recent progress in our understanding of the scope, functional diversity and mechanisms of non-histone protein acetylation.

Axon regeneration induced by environmental enrichment- epigenetic mechanisms

Environmental enrichment is known to be beneficial for cognitive improvement. In many animal models of neurological disorders and brain injury, EE has also demonstrated neuroprotective benefits in neurodegenerative diseases and in improving recovery after stroke or traumatic brain injury. The exact underlying mechanism for these phenomena has been unclear. Recent findings have now indicated that neuronal activity elicited by environmental enrichment induces Ca²⁺ influx in dorsal root ganglion neurons results in lasting enhancement of CREB-binding protein-mediated histone acetylation. This, in turn, increases the expression of pro-regeneration genes and promotes axonal regeneration. This mechanism associated with neuronal activity elicited by environmental enrichment-mediated pathway is one of several epigenetic mechanisms which modulate axon regeneration upon injury that has recently come to light. The other prominent mechanisms, albeit not yet directly associated with environmental enrichment, include DNA methylation/demethylation and N⁶-methyladenosine modification of transcripts. In this brief review, I highlight recent work that has shed light on the epigenetic basis of environmental enrichment-based axon regeneration, and discuss the mechanism and pathways involved. I further speculate on the implications of the findings, in conjunction with the other epigenetic mechanisms, that could be harness to promote axon regeneration upon injury.

Precise Immunodetection of PTEN Protein in Human Neoplasia

PTEN is a major tumor-suppressor protein whose expression and biological activity are frequently diminished in sporadic or inherited cancers. PTEN gene deletion or loss-of-function mutations favor tumor cell growth and are commonly found in clinical practice. In addition, diminished PTEN protein expression is also frequently observed in tumor samples from cancer patients in the absence of PTEN gene alterations. This makes PTEN protein levels a potential biomarker parameter in clinical oncology, which can guide therapeutic decisions. The specific detection of PTEN protein can be achieved by using highly defined anti-PTEN monoclonal antibodies (mAbs), characterized with precision in terms of sensitivity for the detection technique, specificity for PTEN binding, and constraints of epitope recognition. This is especially relevant taking into consideration that PTEN is highly targeted by mutations and posttranslational modifications, and different PTEN protein isoforms exist. The precise characterization of anti-PTEN mAb reactivity is an important step in the validation of these reagents as diagnostic and prognostic tools in clinical oncology, including their routine use in analytical immunohistochemistry (IHC). Here, we review the current status on the use of well-defined anti-PTEN mAbs for PTEN immunodetection in the clinical context and discuss their potential usefulness and limitations for a more precise cancer diagnosis and patient benefit.

Catalpol suppresses osteoclastogenesis and attenuates osteoclast-derived bone resorption by modulating PTEN activity

Excessive activation of osteoclast activity is responsible for many bone diseases, such as osteoporosis, rheumatoid arthritis, periprosthetic osteolysis, and periodontitis. Natural compounds that inhibit osteoclast formation and/or function have therapeutic potential for treating these diseases. Catalpol, a bioactive iridoid extracted from a traditional herbal medicine Rehmannia glutinosa, exhibits various pharmacological properties, including anti-inflammatory, antioxidant, antidiabetic, and antitumor effects. However, its effects on osteoclast formation and function remain unknown. In the present study, we showed that catalpol inhibited receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL)-induced osteoclast formation and bone resorption, as well as the expression of osteoclast-related marker genes. The investigation of molecular mechanisms showed that catalpol upregulated phosphatase and tensin homolog (PTEN) activity by reducing its ubiquitination and degradation, subsequently suppressing RANKL-induced NF-κB and AKT signaling pathways, leading to an inhibition on NFATc1 induction. Furthermore, catalpol protected mice against inflammation- and ovariectomy-induced bone loss by inhibiting osteoclast activity in vivo. These results suggest that catalpol might be developed as a promising candidate for treating osteoclast-related bone diseases.

The Mechanisms Underlying PTEN Loss in Human Tumors Suggest Potential Therapeutic Opportunities

In this review, we will first briefly describe the diverse molecular mechanisms associated with PTEN loss of function in cancer. We will then proceed to discuss the molecular mechanisms linking PTEN loss to PI3K activation and demonstrate how these mechanisms suggest possible therapeutic approaches for patients with PTEN-null tumors.

PTEN Alterations as a Potential Mechanism for Tumor Cell Escape from PD-1/PD-L1 Inhibition

The recent approval of immune checkpoint inhibitors drastically changed the standard treatments in many advanced cancer patients, but molecular changes within the tumor can prevent the activity of immunotherapy drugs. Thus, the introduction of the inhibitors of the immune checkpoint programmed death-1/programmed death ligand-1 (PD-1/PD-L1), should prompt deeper studies on resistance mechanisms, which can be caused by oncogenic mutations detected in cancer cells. PTEN, a tumor suppressor gene, dephosphorylates the lipid signaling intermediate PIP₃ with inhibition of AKT activity, one of the main effectors of the PI3K signaling axis. As a consequence of genetic or epigenetic aberrations, PTEN expression is often altered, with increased activation of PI3K axis. Interestingly, some data confirmed that loss of PTEN expression modified the pattern of cytokine secretion creating an immune-suppressive microenvironment with increase of immune cell populations that can promote tumor progression. Moreover, PTEN loss may be ascribed to reduction of tumor infiltrating lymphocytes (TILs), which can explain the absence of activity of immune checkpoint inhibitors. This review describes the role of PTEN loss as a mechanism responsible for resistance to anti PD-1/PD-L1 treatment. Moreover, combinatorial strategies between PD-1/PD-L1 inhibitors and PI3K/AKT targeting drugs are proposed as a new strategy to overcome resistance to immune checkpoint inhibition.

MicroRNA-532-3p Suppresses Malignant Behaviors of Tongue Squamous Cell Carcinoma via Regulating CCR7

To provide better therapeutic avenues for treating tongue squamous cell carcinoma (TSCC), a series of experiments about the effects of microRNA (miR)-532-3p on TSCC malignant behaviors were carried out. The result showed that miR-532-3p was down-regulated and C-C chemokine receptor 7 (CCR7) was up-regulated in the tumor tissues compared with those in the paired paratumor tissues. Further, expression of miR-532-3p was detected in four TSCC cell lines, TSCCA, TCA8113, CAL-27, and SCC-25. The miR-532-3p mimics and inhibitor were transfected into the CAL-27 and TCA8113 cell lines which were the relatively lowest and highest miR-532-3p expressions, respectively. It was found that the overexpression of miR-532-3p suppressed TSCC cell proliferation, migration, invasion, and promoted apoptosis in vitro, whilst the knockdown of miR-532-3p reversed these behaviors. The bioinformatics predicted that CCR7 was a downstream gene of miR-532-3p, which was confirmed via luciferase assay. Following, the decline of CCR7 in the miR-532-3p mimics group and the rise of CCR7 in the miR-532-3p inhibitor group were also verified. In addition, enhanced cell proliferation, migration and invasion induced by CCR7 were partly restrained by miR-532-3p in TSCC cell. Meanwhile, miR-532-3p attenuated tumourigenesis in vivo due to the reduction of tumor volume and Ki-67 positive rate and the increase of apoptotic cells. Taken together, these findings reveal a pivotal role for the miR-532-3p/CCR7 axis in regulating TSCC, and this novel axis could be suitable for therapeutic intervention in TSCC disease.

Epigenetically Down-Regulated Acetyltransferase PCAF Increases the Resistance of Colorectal Cancer to 5-Fluorouracil

Only 10%–20% of colorectal cancer (CRC) patients observe effective responses to 5-fluorouracil (5-FU) based chemo-treatment. We used real-time PCR array and Western blot analysis to examine the expression alteration of acetyltransferases and deacetylases in 5-FU resistant CRC cell lines as compared to their respective parental CRC cell lines. Unlike other acetyltransferases and deacetylases, we found that the expression of acetyltransferase P300/CBP-associated factor (PCAF) is consistently decreased in three 5-FU resistant CRC cell lines. Similarly, knockdown of PCAF in HCT116 CRC parental cell line also increases the resistance to 5-FU and attenuates 5-FU-induced apoptosis. Mechanistically, we demonstrated that increased binding of trimethylated histone H3K27 in the promoter region of PCAF attenuated its transcription in 5-FU resistant HCT116/5-FU cells. Decreased PCAF impairs the acetylation of p53 and attenuates the p53-dependent transcription of p21, which results in the increased cyclin D1 and phosphorylation of Retinoblastoma 1. Conversely, overexpression of PCAF in CRC cell lines increases p21 and their susceptibility to 5-FU in vitro and in vivo. However, knockdown of p21 abolishes the beneficial effects of PCAF overexpression on increasing the sensitivity of HCT116/5-FU cells to 5-FU. Also, the reduced intensity of PCAF immunostaining was observed in the precancerous lesion, and microarray data from the public database further demonstrated the association between PCAF down-regulation and poor survival outcome. Our data suggest that PCAF-mediated p53 acetylation is an essential regulatory mechanism for increasing the susceptibility of CRC to 5-FU.

Why is NMNAT Protective against Neuronal Cell Death and Axon Degeneration, but Inhibitory of Axon Regeneration?

Nicotinamide mononucleotide adenylyltransferase (NMNAT), a key enzyme for NAD⁺ synthesis, is well known for its activity in neuronal survival and attenuation of Wallerian degeneration. Recent investigations in invertebrate models have, however, revealed that NMNAT activity negatively impacts upon axon regeneration. Overexpression of Nmnat in laser-severed Drosophila sensory neurons reduced axon regeneration, while axon regeneration was enhanced in injured mechanosensory axons in C. elegansnmat-2 null mutants. These diametrically opposite effects of NMNAT orthologues on neuroprotection and axon regeneration appear counterintuitive as there are many examples of neuroprotective factors that also promote neurite outgrowth, and enhanced neuronal survival would logically facilitate regeneration. We suggest here that while NMNAT activity and NAD⁺ production activate neuroprotective mechanisms such as SIRT1-mediated deacetylation, the same mechanisms may also activate a key axonal regeneration inhibitor, namely phosphatase and tensin homolog (PTEN). SIRT1 is known to deacetylate and activate PTEN which could, in turn, suppress PI3 kinase–mTORC1-mediated induction of localized axonal protein translation, an important process that determines successful regeneration. Strategic tuning of Nmnat activity and NAD⁺ production in axotomized neurons may thus be necessary to promote initial survival without inhibiting subsequent regeneration.

PTEN arginine methylation by PRMT6 suppresses PI3K-AKT signaling and modulates pre-mRNA splicing

Arginine methylation is a ubiquitous posttranslational modification that regulates critical cellular processes including signal transduction and pre-mRNA splicing. Here, we report that the tumor-suppressor PTEN is methylated by protein arginine methyltransferase 6 (PRMT6). Mass-spectrometry analysis reveals that PTEN is dimethylated at arginine 159 (R159). We found that PTEN is mutated at R159 in cancers, and the PTEN mutant R159K loses its capability to inhibit the PI3K-AKT cascade. Furthermore, PRMT6 is physically associated with PTEN, promotes asymmetrical dimethylation of PTEN, and regulates the PI3K-AKT cascade through PTEN R159 methylation. In addition, using transcriptome analyses, we found that PTEN R159 methylation is involved in modulation of pre-mRNA alternative splicing. Our results demonstrate that PTEN is functionally regulated by arginine methylation. We propose that PTEN arginine methylation modulates pre-mRNA alternative splicing and influences diverse physiologic processes.

Mechanisms of PTEN loss in cancer: It's all about diversity

PTEN is a phosphatase which metabolises PIP₃, the lipid product of PI 3-Kinase, directly opposing the activation of the oncogenic PI3K/AKT/mTOR signalling network. Accordingly, loss of function of the PTEN tumour suppressor is one of the most common events observed in many types of cancer. Although the mechanisms by which PTEN function is disrupted are diverse, the most frequently observed events are deletion of a single gene copy of PTEN and gene silencing, usually observed in tumours with little or no PTEN protein detectable by immunohistochemistry. Accordingly, with the exceptions of glioblastoma and endometrial cancer, mutations of the PTEN coding sequence are uncommon (<10%) in most types of cancer. Here we review the data relating to PTEN loss in seven common tumour types and discuss mechanisms of PTEN regulation, some of which appear to contribute to reduced PTEN protein levels in cancers.

SIRT4 regulates PTEN stability through IDE in response to cellular stresses

Tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN) plays a critical role in regulating cell survival, cell growth, and proliferation by antagonizing the PI3K-AKT-mTOR pathway. The regulatory mechanism of PTEN protein is still not completely understood. Here, we found that Sirtuin 4 (SIRT4) interacts with PTEN and regulates its stability. Overexpression of SIRT4 in cells causes down-regulation of PTEN. This regulation is independent of PTEN acetylation and ubiquitination. We further found that SIRT4 degrades PTEN through lysosome pathway mediated by insulin degrading enzyme (IDE). SIRT4 bridges PTEN and IDE for degradation in response to nutritional starvation stresses. Our results suggest that when cells were exposed to nutritional starvation, SIRT4 was induced and cooperated with IDE to degrade PTEN; low levels of PTEN promote cells to survive from cellular stress. Our findings provide a new regulation of PTEN in response to cellular stresses.-Liu, M., Wang, Z., Ren, M., Yang, X., Liu, B., Qi, H., Yu, M., Song, S., Chen, S., Liu, L., Zhang, Y., Zou, J., Zhu, W.-G., Yin, Y., Luo, J. SIRT4 regulates PTEN stability through IDE in response to cellular stresses.

PCAF fine-tunes hepatic metabolic syndrome, inflammatory disease, and cancer

The P300/CBP‐associating factor (PCAF), a histone acetyltransferase, is involved in metabolic and pathogenic diseases, particularly of the liver. The effects of PCAF on fine‐tuning liver diseases are extremely complex and vary according to different pathological conditions. This enzyme has dichotomous functions, depending on differently modified sites, which regulate the activities of various enzymes, metabolic functions, and gene expression. Here, we summarize the most recent findings on the functions and targets of PCAF in various metabolic and immunological processes in the liver and review these new discoveries and models of PCAF biology in three areas: hepatic metabolic syndrome, inflammatory disease, and cancer. Finally, we discuss the potential implications of these findings for therapeutic interventions in liver diseases.

Multifunctional antioxidant nanoliposome-mediated delivery of PTEN plasmids restore the expression of tumor suppressor protein and induce apoptosis in prostate cancer cells

Prostate cancer is the second leading cause of cancer death in men and about one in nine will be diagnosed in his lifetime. Loss of PTEN has been considered as one of the major factors leading to the origin of prostate cancer through modulating PI3K/AKT signaling pathways. In this study, we have prepared a multifunctional antioxidant nanoliposome containing PTEN plasmid and cerium oxide nanoparticles (CeNPs). The efficient delivery of PTEN plasmid to human prostate cancer cells (PC-3) leads to restoration of the expression of lost PTEN protein in the cell cytoplasm. The delivered superoxide dismutase (SOD)-mimetic CeNPs were also found to decrease the cytoplasmic free radical levels in prostate cancer cells. The above two activities induced DNA fragmentation and micronucleus formation in prostate cancer cells. Furthermore, it was also found that these multifunctional antioxidant nanoliposomes inhibit the PI3K/AKT signaling pathway to negatively regulate the cell viability of prostate cancer cells. The mRNA expression pattern of other relevant proteins predominantly involved in cancer cell proliferation and apoptosis suggested that the high PTEN expression could control the synthesis of oncogenic proteins. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 3152-3164, 2018.