Photoreceptor Neuroprotection: Regulation of Akt Activation Through Serine/Threonine Phosphatases, PHLPP and PHLPPL

The lack of homology domain and leucine rich repeat protein phosphatase 2 ameliorates visual impairment in rats with diabetic retinopathy through regulation of the AKT-GSK-3β-Nrf2 signal cascade

Pleckstrin homology domain and leucine rich repeat protein phosphatase 2 (PHLPP2) is an emerging player in diverse disorders. Our previous findings have documented that reducing PHLPP2 levels in cultured retinal ganglion cells protects against cellular damage caused by high glucose, indicating a possible link between PHLPP2 and diabetic retinopathy (DR). The present work was dedicated to the investigation of PHLPP2 in DR through in vivo experiments with rat models induced by intraperitoneal injection of streptozotocin. Compared to normal rats, the retinas of rats with DR exhibited a notable increase in the level of PHLPP2. The reduction of PHLPP2 levels in the retina was achieved by the intravitreal administration of adeno-associated viruses expressing specific shRNA targeting PHLPP2. Decreasing the expression of PHLPP2 ameliorated visual function impairment and improved the pathological changes of retina in DR rats. Moreover, decreasing the expression of PHLPP2 repressed the apoptosis, oxidative stress and proinflammatory response in the retinas of rats with DR. Reduction of PHLPP2 levels led to an increase in the levels of phosphorylated AKT and glycogen synthase kinase-3β (GSK-3β). Decreasing the expression of PHLPP2 resulted in increased activation of nuclear factor erythroid 2-related factor 2 (Nrf2), which was reversed by suppressing AKT. Notably, the protective effect of reducing PHLPP2 on DR was eliminated when Nrf2 was restrained. These observations show that the down-regulation of PHLPP2 has protective effects on DR by preserving the structure and function of the retina by regulating the AKT-GSK-3β-Nrf2 signal cascade. Therefore, targeting PHLPP2 may hold promise in the treatment of DR.

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.

Diabetic Retinopathy and Insulin Insufficiency: Beta Cell Replacement as a Strategy to Prevent Blindness

Diabetic retinopathy (DR) is a potentially devastating complication of diabetes because it puts patients at risk of blindness. Diabetes is a common cause of blindness in the U.S. and worldwide and is dramatically increasing in global prevalence. Thus new approaches are needed to prevent this dreaded complication. There is extensive data that indicates beta cell secretory failure is a risk factor for DR, independent of its influence on glycemic control. This perspective article will provide evidence for insufficient endogenous insulin secretion as an important factor in the development of DR. The areas of evidence discussed are: (a) Presence of insulin receptors in the retina, (b) Clinical studies that show an association of beta cell insufficiency with DR, (c) Treatment with insulin in type 2 diabetes, a marker for endogenous insulin deficiency, is an independent risk factor for DR, (d) Recent clinical studies that link DR with an insulin deficient form of type 2 diabetes, and (e) Beta cell replacement studies that demonstrate endogenous insulin prevents progression of DR. The cumulative data drive our conclusion that beta cell replacement will have an important role in preventing DR and/or mitigating its severity in both type 1 diabetes and insulinopenic type 2 diabetes.

Signaling roles of phosphoinositides in the retina

The field of phosphoinositide signaling has expanded significantly in recent years. Phosphoinositides (also known as phosphatidylinositol phosphates or PIPs) are universal signaling molecules that directly interact with membrane proteins or with cytosolic proteins containing domains that directly bind phosphoinositides and are recruited to cell membranes. Through the activities of phosphoinositide kinases and phosphoinositide phosphatases, seven distinct phosphoinositide lipid molecules are formed from the parent molecule, phosphatidylinositol. PIP signals regulate a wide range of cellular functions, including cytoskeletal assembly, membrane budding and fusion, ciliogenesis, vesicular transport, and signal transduction. Given the many excellent reviews on phosphoinositide kinases, phosphoinositide phosphatases, and PIPs in general, in this review, we discuss recent studies and advances in PIP lipid signaling in the retina. We specifically focus on PIP lipids from vertebrate (e.g., bovine, rat, mouse, toad, and zebrafish) and invertebrate (e.g., Drosophila, horseshoe crab, and squid) retinas. We also discuss the importance of PIPs revealed from animal models and human diseases, and methods to study PIP levels both in vitro and in vivo. We propose that future studies should investigate the function and mechanism of activation of PIP-modifying enzymes/phosphatases and further unravel PIP regulation and function in the different cell types of the retina.

MIR-190B Alleviates Cell Autophagy and Burn-Induced Skeletal Muscle Wasting via Modulating PHLPP1/Akt/FoxO3A Signaling Pathway

INTRODUCTION

Cell autophagy is an important material recycling process and is involved in regulating many vital activities under both physiological and pathological conditions. However, the mechanism of autophagy regulating burn-induced skeletal muscle wasting still needs to be elucidated.

METHODS

The rat burn model with 30% total body surface area and L6 cell line were used in this study. An immunofluorescence assay was used to detect autophagic levels. MicroRNA array and real-time PCR were employed to measure miR-190b levels, and its influence on PH domain and leucine-rich repeat protein phosphatase 1 (PHLPP1) protein translation was estimated using luciferase reporter assay. The expression levels of autophagy-related proteins were analyzed by Western blot. Skeletal muscle wasting was evaluated by the ratio of tibias anterior muscle weight to body weight.

RESULTS

Our study demonstrates that burn injury promotes expression of the autophagy-related proteins light chain 3 (LC3) and Beclin-1, suppresses expression of Akt and Forkhead box O (FoxO) 3a protein phosphorylation, and increases PHLPP1 protein level which is required for Akt dephosphorylation. miR-190b, the regulator of PHLPP1 protein translation, also significantly decreases after burn injury. Ectopic expression of miR-190b in L6 myoblast cell downregulates PHLPP1 protein expression, elevates Akt and FoxO3a phosphorylation, and subsequently reduces cell autophagy. Finally, suppressing autophagy with 3-methyladenine represses the protein expression of LC3 and Beclin-1 and mitigates burn-induced skeletal muscle wasting.

CONCLUSION

Burn injury induced skeletal muscle cell autophagy and subsequently resulted in skeletal muscle wasting via regulating miR-190b/PHLPP1/Akt/FoxO3a signaling pathway.

Hyperoside Protected Against Oxidative Stress-Induced Liver Injury via the PHLPP2-AKT-GSK-3β Signaling Pathway In Vivo and In Vitro

Hyperoside, isolated from Drosera rotundifolia L., seeds of Cuscuta chinensis Lam., or Hypericum perforatum L., originally showed to possess an antifungal and antibacterial activity, while recently showed the protective effects against oxidative stress-induced liver injury. This study investigated such a protective effect of hyperoside and the underlying molecular mechanisms in vitro and in carbon tetrachloride (CCl4)-injured rat livers. The data showed that hyperoside was able to prevent the oxidative stress-induced liver morphological changes and CCl4-induced rat liver injury. Hyperoside reversed the decrease of superoxidase dismutase (SOD) level and the increase of malondialdehyde (MDA) level in vivo. Moreover, hyperoside regulated the pleckstrin homology (PH) domain leucine-rich repeat protein phosphatase 2 (PHLPP2)-protein kinase B (AKT)-glycogen synthase kinase 3β (GSK-3β) signaling pathway in tert-butylhydroquinone (t-BHP)-treated liver cells, e.g., Hyperoside reduced PHLPP2 expression to activate AKT phosphorylation, induce GSK-3β phosphorylation, and then increased nuclear factor erythroid-2-related factor 2 (Nrf2) nuclear translocation, reduced nuclear translocation of phosphorylated Fyn, and promoted heme oxygenase-1 (HO-1) expression in vivo and in vitro. In contrast, siRNA-mediated knockdown of PHLPP2 expression enhanced hyperoside-mediated activation of the AKT-GSK-3β kinase pathway in liver cells. In conclusion, the present study demonstrated that hyperoside could protect against oxidative stress-induced liver injury by regulating the PHLPP2-AKT-GSK-3β signaling pathway in vivo and in vitro.

Endogenous insulin signaling in the RPE contributes to the maintenance of rod photoreceptor function in diabetes

In diabetes, there are two major physiological aberrations: (i) Loss of insulin signaling due to absence of insulin (type 1 diabetes) or insulin resistance (type 2 diabetes) and (ii) increased blood glucose levels. The retina has a high proclivity to damage following diabetes, and much of the pathology seen in diabetic retinopathy has been ascribed to hyperglycemia and downstream cascades activated by increased blood glucose. However, less attention has been focused on the direct role of insulin on retinal physiology, likely due to the fact that uptake of glucose in retinal cells is not insulin-dependent. The retinal pigment epithelium (RPE) is instrumental in maintaining the structural and functional integrity of the retina. Recent studies have suggested that RPE dysfunction is a precursor of, and contributes to, the development of diabetic retinopathy. To evaluate the role of insulin on RPE cell function directly, we generated a RPE specific insulin receptor (IR) knockout (RPEIRKO) mouse using the Cre-loxP system. Using this mouse, we sought to determine the impact of insulin-mediated signaling in the RPE on retinal function under physiological control conditions as well as in streptozotocin (STZ)-induced diabetes. We demonstrate that loss of RPE-specific IR expression resulted in lower a- and b-wave electroretinogram amplitudes in diabetic mice as compared to diabetic mice that expressed IR on the RPE. Interestingly, RPEIRKO mice did not exhibit significant differences in the amplitude of the RPE-dependent electroretinogram c-wave as compared to diabetic controls. However, loss of IR-mediated signaling in the RPE reduced levels of reactive oxygen species and the expression of pro-inflammatory cytokines in the retina of diabetic mice. These results imply that IR-mediated signaling in the RPE regulates photoreceptor function and may play a role in the generation of oxidative stress and inflammation in the retina in diabetes.

A serine/threonine phosphatase 1 of Streptococcus suis type 2 is an important virulence factor

Streptococcus suis is regarded as one of the major pathogens of pigs, and Streptococcus suis type 2 (SS2) is considered a zoonotic bacterium based on its ability to cause meningitis and streptococcal toxic shock-like syndrome in humans. Many bacterial species contain genes encoding serine/threonine protein phosphatases (STPs) responsible for dephosphorylation of their substrates in a single reaction step. This study investigated the role of stp1 in the pathogenesis of SS2. An isogenic stp1 mutant (Δstp1) was constructed from SS2 strain ZJ081101. The Δstp1 mutant exhibited a significant increase in adhesion to HEp-2 and bEnd.3 cells as well as increased survival in RAW264.7 cells, as compared to the parent strain. Increased survival in macrophage cells might be related to resistance to reactive oxygen species since the Δstp1 mutant was more resistant than its parent strain to paraquat-induced oxidative stress. However, compared to parent strain virulence, deletion of stp1 significantly attenuated virulence of SS2 in mice, as shown by the nearly double lethal dose 50 value and the lower bacterial load in organs and blood in the murine model. We conclude that Stp1 has an essential role in SS2 virulence.

MiR-760 overexpression promotes proliferation in ovarian cancer by downregulation of PHLPP2 expression

OBJECTIVE

Ovarian cancer is one of the most lethal gynecologic malignancies worldwide and with poor prognosis and survival rate in women. Identifying sensitive and specific molecular in carcinogenesis may improve diagnostic and therapeutic strategies for this malignancy and achieve a better clinical outcome.

METHODS

miR-760 expression in ovarian cancer cell lines and patient tissues were determined using Real-time PCR. 145 human ovarian cancer tissue samples were analyzed by RT-PCR to investigate the association between miR-760expression and the clinicopathological characteristics of ovarian cancer patients. Functional assays, such as MTT, anchorage-independent growth, colony formation and BRDU assay were used to determine the oncogenic role of miR-760 in human ovarian cancer progression. Furthermore, western blotting and luciferase assay were used to determine the mechanism of miR-760 promotes proliferation in ovarian cancer cells.

RESULT

The expression of miR-760 was markedly upregulated in ovarian cancer cell lines and tissues, and high miR-760 expression was associated with an aggressive phenotype and poor prognosis with ovarian cancer patients. Upregulation of miR-760 promoted, whereas downregulation of miR-760 inhibited the proliferation of ovarian cancer cells in vitro. Additionally, we identified PHLPP2 as a direct target of miR-760, and silencing the expression of PHLPP2 is the essential biological function of miR-760 during ovarian cancer cell proliferation. Finally, we showed a significant correlation between miR-760 and PHLPP2 expression in ovarian cancer tissues.

CONCLUSION

Our findings suggest that miR-760 represents a potential onco-miR and participates in ovarian cancer carcinogenesis, which highlight its potential as a target for ovarian cancer therapy.