Diacylglycerol kinase δ modulates Akt phosphorylation through pleckstrin homology domain leucine-rich repeat protein phosphatase 2 (PHLPP2)

Diacylglycerol kinases: A look into the future of immunotherapy

Cancer still represents the second leading cause of death right after cardiovascular diseases. According to the World Health Organization (WHO), cancer provoked around 10 million deaths in 2020, with lung and colon tumors accounting for the deadliest forms of cancer. As tumor cells become resistant to traditional therapeutic approaches, immunotherapy has emerged as a novel strategy for tumor control. T lymphocytes are key players in immune responses against tumors. Immunosurveillance allows identification, targeting and later killing of cancerous cells. Nevertheless, tumors evolve through different strategies to evade the immune response and spread in a process called metastasis. The ineffectiveness of traditional strategies to control tumor growth and expansion has led to novel approaches considering modulation of T cell activation and effector functions. Program death receptor 1 (PD-1) and cytotoxic T-lymphocyte antigen 4 (CTLA-4) showed promising results in the early 90s and nowadays are still being exploited together with other drugs for several cancer types. Other negative regulators of T cell activation are diacylglycerol kinases (DGKs) a family of enzymes that catalyze the conversion of diacylglycerol (DAG) into phosphatidic acid (PA). In T cells, DGKα and DGKζ limit the PLCγ/Ras/ERK axis thus attenuating DAG mediated signaling and T cell effector functions. Upregulation of either of both isoforms results in impaired Ras activation and anergy induction, whereas germline knockdown mice showed enhanced antitumor properties and more effective immune responses against pathogens. Here we review the mechanisms used by DGKs to ameliorate T cell activation and how inhibition could be used to reinvigorate T cell functions in cancer context. A better knowledge of the molecular mechanisms involved upon T cell activation will help to improve current therapies with DAG promoting agents.

Emerging roles of PHLPP phosphatases in the nervous system

It has been more than a decade since the discovery of a novel class of phosphatase, the Pleckstrin Homology (PH) domain Leucine-rich repeat Protein Phosphatases (PHLPP). Over time, they have been recognized as crucial regulators of various cellular processes, such as memory formation, cellular survival and proliferation, maintenance of circadian rhythm, and others, with any deregulation in their expression or cellular localization causing havoc in any cellular system. With the ever-growing number of downstream substrates across multiple tissue systems, a web is emerging wherein the central point is PHLPP. A slight nick in the normal signaling cascade of the two isoforms of PHLPP, namely PHLPP1 and PHLPP2, has been recently found to invoke a variety of neurological disorders including Alzheimer's disease, epileptic seizures, Parkinson's disease, and others, in the neuronal system. Improper regulation of the two isoforms has also been associated with various disease pathologies such as diabetes, cardiovascular disorders, cancer, musculoskeletal disorders, etc. In this review, we have summarized all the current knowledge about PHLPP1 (PHLPP1α and PHLPP1β) and PHLPP2 and their emerging roles in regulating various neuronal signaling pathways to pave the way for a better understanding of the complexities. This would in turn aid in providing context for the development of possible future therapeutic strategies.

On the PHLPPside: Emerging roles of PHLPP phosphatases in the heart

PH domain leucine-rich repeat protein phosphatase (PHLPP) is a family of enzymes made up of two isoforms (PHLPP1 and PHLPP2), whose actions modulate intracellular activity via the dephosphorylation of specific serine/threonine (Ser/Thr) residues on proteins such as Akt. Recent data generated in our lab, supported by findings from others, implicates the divergent roles of PHLPP1 and PHLPP2 in maintaining cellular homeostasis since dysregulation of these enzymes has been linked to various pathological states including cardiovascular disease, diabetes, ischemia/reperfusion injury, musculoskeletal disease, and cancer. Therefore, development of therapies to modulate specific isoforms of PHLPP could prove to be therapeutically beneficial in several diseases especially those targeting the cardiovascular system. This review is intended to provide a comprehensive summary of current literature detailing the role of the PHLPP isoforms in the development and progression of heart disease.

Subcellular Localization Relevance and Cancer-Associated Mechanisms of Diacylglycerol Kinases

An increasing number of reports suggests a significant involvement of the phosphoinositide (PI) cycle in cancer development and progression. Diacylglycerol kinases (DGKs) are very active in the PI cycle. They are a family of ten members that convert diacylglycerol (DAG) into phosphatidic acid (PA), two-second messengers with versatile cellular functions. Notably, some DGK isoforms, such as DGKα, have been reported to possess promising therapeutic potential in cancer therapy. However, further studies are needed in order to better comprehend their involvement in cancer. In this review, we highlight that DGKs are an essential component of the PI cycle that localize within several subcellular compartments, including the nucleus and plasma membrane, together with their PI substrates and that they are involved in mediating major cancer cell mechanisms such as growth and metastasis. DGKs control cancer cell survival, proliferation, and angiogenesis by regulating Akt/mTOR and MAPK/ERK pathways. In addition, some DGKs control cancer cell migration by regulating the activities of the Rho GTPases Rac1 and RhoA.

Role of 4-Hexylresorcinol in the Field of Tissue Engineering

4-hexylresorcinol (4-HR), as a derivative of phenolic lipids, has biological and pharmacological properties that are beneficial when used with a biomaterial. It has antimicrobial and antiseptic activity and can thus prevent contamination and infection of biomaterials. 4-HR suppresses the nuclear factor kappa B (NF-κB) signaling pathway related to osteoclast differentiation. The suppression of NF-κB increases the bone formation marker and contributes to new bone formation. The tumor necrosis factor-α (TNF-α) is a pro-inflammatory cytokine produced by macrophages and suppressed by 4-HR. Suppression of TNF-α decreases osteoclast activity and promotes wound healing. 4-HR increases the vascular endothelial growth factor and has an anti-thrombotic effect. When incorporated into silk vascular patches, it promotes endothelium wound healing. Recently, 4-HR has exhibited biological properties and has been successfully incorporated into various biomaterials. Consequently, it is a useful pharmacological chemical that can be used with biomaterials in the field of tissue engineering.

Biological regulation of diacylglycerol kinases in normal and neoplastic tissues: New opportunities for cancer immunotherapy

In the recent years, the arsenal of anti-cancer therapies has evolved to target T lymphocytes and restore their capacity to destroy tumor cells. However, the clinical success is limited, with a large number of patients that never responds and others that ultimately develop resistances. Overcoming the hypofunctional state imposed by solid tumors to T cells has revealed critical but challenging due to the complex strategies that tumors employ to evade the immune system. The Diacylglycerol kinases (DGK) limit DAG-dependent functions in T lymphocytes and their upregulation in tumor-infiltrating T lymphocytes contribute to limit T cell cytotoxic potential. DGK blockade could reinstate T cell attack on tumors, limiting at the same time tumor cell growth, thanks to the DGK positive input into several oncogenic pathways. In this review we summarize the latest findings regarding the regulation of specific DGK isoforms in healthy and anergic T lymphocytes, as well as their contribution to oncogenic phenotypes. We will also revise the latest advances in the search for pharmacological inhibitors and their potential as anti-cancer agents, either alone or in combination with immunomodulatory agents.

Role of Diacylglycerol Kinases in Glucose and Energy Homeostasis

Diacylglycerol kinases (DGKs) catalyze a reaction that converts diacylglycerol (DAG) to phosphatidic acid (PA). DAG and PA act as intermediates of de novo lipid synthesis, cellular membrane constituents, and signaling molecules. DGK isoforms regulate a variety of intracellular processes by terminating DAG signaling and activating PA-mediated pathways. The ten DGK isoforms are unique, not only structurally, but also in tissue-specific expression profiles, subcellular localization, regulatory mechanisms, and DAG preferences, suggesting isoform-specific functions. DAG accumulation has been associated with insulin resistance; however, this concept is challenged by opposing roles of DGK isoforms in the development of type 2 diabetes and obesity despite elevated DAG levels. This review focuses on the tissue- and isoform-specific role of DGK in glucose and energy homeostasis.

Arecoline suppresses epithelial cell viability through the Akt/mTOR signaling pathway via upregulation of PHLPP2

Arecoline, the major active ingredient of the betel nut, is involved in the pathogenesis of oral submucous fibrosis. However, the underlying mechanism of this pathogenesis remains unclear. In this study, we found that arecoline suppresses the cell proliferation of the HaCaT epithelial cell and induces cell cycle arrest at the G1/S phase with an IC50 of 50 μg/mL. Furthermore, we found that arecoline reduces the protein level of cyclin D1, but it has no effect on its mRNA level and protein stability, implying that arecoline may modulate the translation of cyclin D1. We also observed the downregulation of the Akt/mTOR signaling pathway after treatment with arecoline, which may be related to the translation of cyclin D1. RNA-seq analysis identified that PHLPP2, the direct upstream target of Akt, is significantly upregulated after arecoline treatment. siRNA-mediated knockdown of PHLPP2 recovered the phosphorylation state of Akt, as well as attenuated the effect of arecoline on cell viability. Thus, our study revealed the crucial role of PHLPP2 in arecoline-induced cell viability suppression.

Subcellular localization of β-arrestin1 and its prognostic value in lung adenocarcinoma

β-Arrestins play important roles in cancer progression, and the subcellular localization of β-arrestin1 has been receiving increasingly more attention. Intriguingly, several studies, including some of our previous work, showed that the effects of β-arrestin1 on outcomes of cancer patients were controversial.Specimens were obtained from 133 patients with lung adenocarcinoma. Immunohistochemistry was used to detect the expression of β-arrestin1 and p300 in the collected tissues. The Kaplan-Meier analysis and Cox proportional hazards regression were used to examine the relationship between β-arrestin1 and patient survival.We found no significant association between β-arrestin1 and clinicopathological variables. The Kaplan-Meier plot showed that patients with high expression of β-arrestin1 (especially in the nucleus) had a poorer overall survival (OS) and shorter disease-free survival (DFS) (P = .026, P = .015). Additionally, high p300 expression also resulted in worse OS (P = .039). Following the univariate analysis, high expressions of nuclear β-arrestin1 and p300 were classed as poor prognostic factors for both OS (P = .016) and DFS (P = .025).The expression of β-arrestin1 in the nucleus is associated with increased malignant tendency of lung adenocarcinoma, and the predictive value of β-arrestin1 may be optimized by combining information about the expression of p300 acetyltransferase.

PHLPP phosphatases as a therapeutic target in insulin resistance-related diseases

INTRODUCTION

Pleckstrin homology domain leucine-rich repeat protein phosphatases (PHLPPs), originally identified as Akt kinase hydrophobic motif specific phosphatases, have subsequently been shown to regulate several molecules recurring within the insulin signaling pathway. This observation suggests that PHLPP phosphatases may have a clinically relevant role in the pathogenesis of insulin resistance-related diseases and may thus represent suitable targets for the treatment of these conditions.

AREAS COVERED

The literature pertaining to PHLPPs substrates is reviewed herein, along with information on the molecular players involved in regulating the activity and expression of PHLPP phosphatases. In the present review, knowledge of genetic variants in the genes that encode for PHLPP isozymes and the surrounding regulatory regions is also summarized. In addition, data from the studies addressing the role of PHLPPs in insulin resistance-related disorders and from those investigating the possibility to manipulate these phosphatases for therapeutic purposes are presented.

EXPERT OPINION

A number of issues should be resolved before PHLPPs are pursued as therapeutic targets including: the mechanisms regulating the specificity of PHLPP isozymes; the possibility of differentially regulating PHLPP family members and the possible impact of PHLPPs modulation on the risk of cancer.

Roles of lipid-modulating enzymes diacylglycerol kinase and cyclooxygenase under pathophysiological conditions

Lipid not only represents a constituent of the plasma membrane, but also plays a pivotal role in intracellular signaling. Lipid-mediated signaling system is strictly regulated by several enzymes, which act at various steps of the lipid metabolism. Under pathological conditions, prolonged or insufficient activation of this system results in dysregulated signaling, leading to diseases such as cancer or metabolic syndrome. Of the lipid-modulating enzymes, diacylglycerol kinase (DGK) and cyclooxygenase (COX) are intimately involved in the signaling system. DGK consists of a family of enzymes that phosphorylate a second messenger diacylglycerol (DG) to produce phosphatidic acid (PA). Both DG and PA are known to activate signaling molecules such as protein kinase C. COX catalyzes the committed step in prostanoid biosynthesis, which involves the metabolism of arachidonic acid to produce prostaglandins. Previous studies have shown that the DGK and COX are engaged in a number of pathological conditions. This review summarizes the functional implications of these two enzymes in ischemia, liver regeneration, vascular events, diabetes, cancer and inflammation.

Assembly of the FKBP51-PHLPP2-AKT signaling complex in cerebral ischemia/reperfusion injury in rats

The imbalance of cell pro-death and pro-survival signaling pathways determines the neuronal fate during cerebral ischemia/reperfusion (I/R) injury. However, the biological mechanisms regulating the balance between activation of the pro-death or the pro-survival signaling pathways remain unclear. In this study, a rat model of I/R injury was established using four-vessel occlusion followed by different times of reperfusion. I/R injury did not affect the level of FK506 binding protein 51 (FKBP51), PH domain and leucine rich repeat protein phosphatases (PHLPP)-2, and AKT, but induced assembly of the FKBP51-PHLPP2-AKT signaling complex, as indicated by the enhancement of interactions among these compounds following reperfusion. Using an antisense oligonucleotide, PHLPP2 expression was effectively inhibited. Critically, the inhibition of PHLPP2 prohibited the interactions of FKBP51, PHLPP2 and AKT, reversed the decrease of p-AKT expression and increased the expression of p-JNKs and p-c-Jun elicited by I/R injury. In addition, PHLPP2 inhibition reversed I/R-injury-induced Caspase-3 cleavage and loss of pyramid neurons in the CA1 region of hippocampus. The results of the current study indicate that the assembly of the FKBP51-PHLPP2-AKT signaling complex plays a critical role in mediating cell death in I/R injury. The inhibition of PHLPP2 via antisense oligonucleotide treatment may be an effective method to prohibit the assembly of the FKBP51-PHLPP-AKT signaling complex, thus balancing the cell pro-survival and pro-death signaling pathways ultimately mitigating cell death in I/R injury.

Essential role of PH domain and leucine-rich repeat protein phosphatase 2 in Nrf2 suppression via modulation of Akt/GSK3β/Fyn kinase axis during oxidative hepatocellular toxicity

Instances of sustained oxidative activity have been shown to involve dysregulation of Nrf2-mediated transcriptional induction; however, mechanisms warranting Nrf2-repression remain unclear. In this study, using primary rat hepatocytes, we have attempted to identify factors that may negatively influence Nrf2 survival pathway. Though studies indicate a conspicuous association between Akt and Nrf2, a confirmatory link between the two is unaddressed. On inhibiting PI3K/Akt pathway, we observed compromised activities of antioxidant and detoxification enzymes culminating in oxidative cytotoxicity. This was accompanied by reduced nuclear retention of Nrf2 and its ARE binding affinity, increased Nrf2 ubiquitination and concurrent decline in its downstream targets. Moreover, Akt inhibition enhanced nuclear translocation as well as phosphorylation of Fyn kinase, an enzyme linked to Nrf2 degradation, by relieving GSK3β from phosphorylation-mediated repression. The involvement of Akt and Fyn kinase in influencing Nrf2 signaling was further confirmed in oxidatively stressed hepatocytes by using tert-butyl hydroperoxide (tBHP). tBHP-induced decrease in Nrf2 levels was associated with enhanced Fyn kinase phosphorylation, Fyn kinase nuclear translocation and decreased levels of phosphorylated GSK3β(Ser9) in a time-dependent manner. Interestingly, tBHP induced site-specific deactivation of Akt as only Akt(Ser473) phosphorylation was observed to be affected. Further, protein expression as well as nuclear localization of PHLPP2, a phosphatase specific for Akt(Ser473), was found to be significantly enhanced in tBHP-stressed hepatocytes. Silencing of PHLPP2 not only resulted in considerable restoration of Nrf2 signaling, enhanced Nrf2-ARE binding and reduced Nrf2 ubiquitination but also significantly suppressed tBHP-induced ROS generation and alterations in mitochondrial permeability. We infer that cellular PHLPP2 levels may aggravate oxidative toxicity by suppressing Nrf2/ARE transcriptional regulation via Akt(Se473)/GSK3β/Fyn kinase axis. The study indicates that PHLPP2 could serve as a new target for developing strategies to manage pathological conditions exacerbated due to oxidative stress.

Arrestins as regulatory hubs in cancer signalling pathways

Non-visual arrestins were initially appreciated for the roles they play in the negative regulation of G protein-coupled receptors through the processes of desensitisation and endocytosis. The arrestins are also now known as protein scaffolding platforms that act downstream of multiple types of receptors, ensuring relevant transmission of information for an appropriate cellular response. They function as regulatory hubs in several important signalling pathways that are often dysregulated in human cancers. Interestingly, several recent studies have documented changes in expression and localisation of arrestins that occur during cancer progression and that correlate with clinical outcome. Here, we discuss these advances and how changes in expression/localisation may affect functional outputs of arrestins in cancer biology.

Arrestins in apoptosis

Programmed cell death (apoptosis) is a coordinated set of events eventually leading to the massive activation of specialized proteases (caspases) that cleave numerous substrates, orchestrating fairly uniform biochemical changes than culminate in cellular suicide. Apoptosis can be triggered by a variety of stimuli, from external signals or growth factor withdrawal to intracellular conditions, such as DNA damage or ER stress. Arrestins regulate many signaling cascades involved in life-or-death decisions in the cell, so it is hardly surprising that numerous reports document the effects of ubiquitous nonvisual arrestins on apoptosis under various conditions. Although these findings hardly constitute a coherent picture, with the same arrestin subtypes, sometimes via the same signaling pathways, reported to promote or inhibit cell death, this might reflect real differences in pro- and antiapoptotic signaling in different cells under a variety of conditions. Recent finding suggests that one of the nonvisual subtypes, arrestin-2, is specifically cleaved by caspases. Generated fragment actively participates in the core mechanism of apoptosis: it assists another product of caspase activity, tBID, in releasing cytochrome C from mitochondria. This is the point of no return in committing vertebrate cells to death, and the aspartate where caspases cleave arrestin-2 is evolutionary conserved in vertebrate, but not in invertebrate arrestins. In contrast to wild-type arrestin-2, its caspase-resistant mutant does not facilitate cell death.

Diacylglycerol kinase delta promotes lipogenesis

We have studied the relationship between diacylglycerol kinase delta (DGKδ) and lipogenesis. There is a marked increase in the expression of DGKδ during the differentiation of 3T3-L1 cells to adipocytes, as well as in the synthesis of neutral and polar lipids. When 3T3-L1 undifferentiated fibroblasts are transfected to express DGKδ, there is increased triglyceride synthesis without differentiation to adipocytes. Hence, expression of DGKδ promotes lipogenesis. Lipid synthesis is decreased in DGKδ knockout mouse embryo fibroblasts, especially for lipids with shorter acyl chains and limited unsaturation. This reduction occurs for both neutral and polar lipids. These findings suggest reduced de novo lipid synthesis. This is confirmed by measuring the incorporation of glycerol into polar and neutral lipids, which is higher in the wild type cells than in the DGKδ knockouts. In comparison, there was no change in lipid synthesis in DGKε knockout mouse embryo fibroblasts. We also demonstrate that the DGKδ knockout cells had a lower expression of acetyl-CoA carboxylase and fatty acid synthase as well as a lower degree of activation by phosphorylation of ATP citrate lyase. These three enzymes are involved in the synthesis of long chain fatty acids. Our results demonstrate that DGKδ markedly increases lipid synthesis, at least in part as a result of promoting the de novo synthesis of fatty acids.

Spatiotemporal dynamics of phosphorylation in lipid second messenger signaling

The plasma membrane serves as a dynamic interface that relays information received at the cell surface into the cell. Lipid second messengers coordinate signaling on this platform by recruiting and activating kinases and phosphatases. Specifically, diacylglycerol and phosphatidylinositol 3,4,5-trisphosphate activate protein kinase C and Akt, respectively, which then phosphorylate target proteins to transduce downstream signaling. This review addresses how the spatiotemporal dynamics of protein kinase C and Akt signaling can be monitored using genetically encoded reporters and provides information on how the coordination of signaling at protein scaffolds or membrane microdomains affords fidelity and specificity in phosphorylation events.