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

Increased PHLPP1 expression through ERK-4E-BP1 signaling axis drives nicotine induced oxidative stress related damage of cardiomyocytes

Nicotine, a key constituent of tobacco/electronic cigarettes causes cardiovascular injury and mortality. Nicotine is known to induce oxidative stress and mitochondrial dysfunction in cardiomyocytes leading to cell death. However, the underlying mechanisms remain unclear. Pleckstrin homology domain leucine-rich repeat protein phosphatase (PHLPP) is a member of metal-dependent protein phosphatase (PPM) family and is known to dephosphorylate several AGC family kinases and thereby regulate a diverse set of cellular functions including cell growth, survival, and death. Our lab has previously demonstrated that PHLPP1 removal reduced cardiomyocyte death and cardiac dysfunction following injury. Here, we present a novel finding that nicotine exposure significantly increased PHLPP1 protein expression in the adolescent rodent heart. Building upon our in vivo finding, we determined the mechanism of PHLPP1 expression in cardiomyocytes. Nicotine significantly increased PHLPP1 protein expression without altering PHLPP2 in cardiomyocytes. In cardiomyocytes, nicotine significantly increased NADPH oxidase 4 (NOX4), which coincided with increased reactive oxygen species (ROS) and increased cardiomyocyte apoptosis which were dependent on PHLPP1 expression. PHLPP1 expression was both necessary and sufficient for nicotine induced mitochondrial dysfunction. Mechanistically, nicotine activated extracellular signal-regulated protein kinases (ERK1/2) and subsequent eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1) to increase PHLPP1 protein expression. Inhibition of protein synthesis with cycloheximide (CHX) and 4EGI-1 abolished nicotine induced PHLPP1 protein expression. Moreover, inhibition of ERK1/2 activity by U0126 significantly blocked nicotine induced PHLPP1 expression. Overall, this study reveals a novel mechanism by which nicotine regulates PHLPP1 expression through ERK-4E-BP1 signaling axis to drive cardiomyocyte injury.

Blockage of PHLPP1 protects against myocardial ischemia/reperfusion injury in diabetic mice via activation of STAT3 signaling

Diabetes can exacerbate myocardial ischemia/reperfusion (IR) injury. However, the sensitivity to IR injury and the underlying mechanisms in diabetic hearts remain unclear. Inhibition of PH domain leucine-rich repeating protein phosphatase (PHLPP1) could reduce myocardial IR injury, our previous study demonstrated that the expression of PHLPP1 was upregulated in diabetic myocardial IR model. Thus, this study aimed to investigate the mechanism of PHLPP1 in diabetic myocardial IR injury. Nondiabetic and diabetic C57BL/6 mice underwent 45 min of coronary artery occlusion followed by 2 h of reperfusion. Male C57BL/6 mice were injected with streptozotocin for five consecutive days to establish a diabetes model. H9c2 cells were exposed to normal or high glucose and subjected to 4 h of hypoxia followed by 4 h of reoxygenation. Diabetes or hyperglycemia increased postischemic infarct size, cellular injury, release of creatine kinase-MB, apoptosis, and oxidative stress, while exacerbating mitochondrial dysfunction. This was accompanied by enhanced expression of PHLPP1 and decreased levels of p-STAT3 and p-Akt. These effects were counteracted by PHLPP1 knockdown. Moreover, PHLPP1 knockdown resulted in an increase in mitochondrial translocation of p-STAT3 Ser727 and nuclear translocation of p-STAT3 Tyr705 and p-STAT3 Ser727. However, the effect of PHLPP1 knockdown in reducing posthypoxic cellular damage was nullified by either Stattic or LY294002. Additionally, a co-immunoprecipitation assay indicated a direct interaction between PHLPP1 and p-STAT3 Ser727, but not p-STAT3 Tyr705. The abnormal expression of PHLPP1 plays a significant role in exacerbating myocardial IR injury in diabetic mice. Knockdown of PHLPP1 to activate the STAT3 signaling pathway may represent a novel strategy for alleviating myocardial IR injury in diabetes.

Insights into the role of PHLPP2/Akt/GSK3β/Fyn kinase/Nrf2 trajectory in the reno-protective effect of rosuvastatin against colistin-induced acute kidney injury in rats

OBJECTIVES

Oxidative stress-mediated colistin's nephrotoxicity is associated with the diminished activity of nuclear factor erythroid 2-related factor 2 (Nrf2) that is primarily correlated with cellular PH domain and leucine-rich repeat protein phosphatase (PHLPP2) levels. This study investigated the possible modulation of PHLPP2/protein kinase B (Akt) trajectory as a critical regulator of Nrf2 stability by rosuvastatin (RST) to guard against colistin-induced oxidative renal damage in rats.

METHODS

Colistin (300,000 IU/kg/day; i.p.) was injected for 6 consecutive days, and rats were treated simultaneously with RST orally at 10 or 20 mg/kg.

KEY FINDINGS

RST enhanced renal nuclear Nrf2 translocation as revealed by immunohistochemical staining to boost the renal antioxidants, superoxide dismutase (SOD) and reduced glutathione (GSH) along with a marked reduction in caspase-3. Accordingly, rats treated with RST showed significant restoration of normal renal function and histological features. On the molecular level, RST effectively decreased the mRNA expression of PHLPP2 to promote Akt phosphorylation. Consequently, it deactivated GSK-3β and reduced the gene expression of Fyn kinase in renal tissues.

CONCLUSIONS

RST could attenuate colistin-induced oxidative acute kidney injury via its suppressive effect on PHLPP2 to endorse Nrf2 activity through modulating Akt/GSK3 β/Fyn kinase trajectory.

Loss of pleckstrin homology domain and leucine-rich repeat protein phosphatase 2 has protective effects on high glucose-injured retinal ganglion cells via the effect on the Akt-GSK-3β-Nrf2 pathway

OBJECTIVE

Pleckstrin homology domain and leucine-rich repeat protein phosphatase 2 (PHLPP2) is linked to various pathological states. However, whether PHLPP2 mediates diabetic retinopathy is unaddressed. This work explored the biological function of PHLPP2 in modulating high glucose (HG)-elicited damage of retinal ganglion cells (RGCs), an in vitro model for studying diabetic retinopathy.

METHODS

Mouse RGCs were treated with HG to establish a cell model. PHLPP2 was silenced by transfecting specific shRNAs targeting PHLPP2. RT-qPCR, immunoblotting, CCK-8 assay, flow cytometry, TUNEL assay, and ELISA were carried out.

RESULTS

Significant increases in PHLPP2 levels were observed in cultured RGCs exposed to HG. The severe damages evoked by HG to RGCs were remarkably weakened in PHLPP2-silenced RGCs, including improved cell survival, attenuated cell apoptosis, repressed oxidative stress, and prohibited proinflammatory response. The silencing of PHLPP2 strengthened the activation of Nrf2 in HG-treated RGCs via modulation of the Akt-GSK-3β axis. Interruption of the Akt-GSK-3β axis reversed PHLPP2-silencing-elicited Nrf2 activation. The protective effects of PHLPP2 silencing on HG-induced injury of RGCs were diminished by Nrf2 inhibition.

CONCLUSIONS

The loss of PHLPP2 was beneficial for HG-injured RGCs through the effect on the Akt-GSK-3β-Nrf2 pathway. This work suggests a possible role of PHLPP2 in diabetic retinopathy.

MicroRNA-520c-3p impacts sphingolipid metabolism mediating PI3K/AKT signaling in NSCLC: Systems perspective

Increasing research suggests that sphingolipid metabolism is essential for the progression and metastasis of cancer. The underlying mechanistic insight into the dysregulation of sphingolipid metabolism affecting pathways is poorly investigated. As a result, the goal of the current study was to glean knowledge from the systems biology approach to investigate how the sphingolipid metabolism affects the signal transduction network in non-small cell lung cancer (NSCLC), the most common type of cancer in terms of occurrence and death globally. Our paper includes system-level models representing the diseased and healthy states elucidating that sphingolipids and its enzymes mediate PI3K/AKT pathway. Notably, its activation of downstream signaling mediators has led to cancer growth. Considering the critical role of sphingolipids in NSCLC, our study advocates the target CERS6 which can be potentially inhibited using hsa-miR-520c-3p to combat NSCLC for future precision medicine.

Exercise-induced signaling pathways to counteracting cardiac apoptotic processes

Cardiovascular diseases are the most common cause of death in the world. One of the major causes of cardiac death is excessive apoptosis. However, multiple pathways through moderate exercise can reduce myocardial apoptosis. After moderate exercise, the expression of anti-apoptotic proteins such as IGF-1, IGF-1R, p-PI3K, p-Akt, ERK-1/2, SIRT3, PGC-1α, and Bcl-2 increases in the heart. While apoptotic proteins such as PTEN, PHLPP-1, GSK-3, JNK, P38MAPK, and FOXO are reduced in the heart. Exercise-induced mechanical stress activates the β and α5 integrins and subsequently, focal adhesion kinase phosphorylation activates the Akt/mTORC1 and ERK-1/2 pathways, leading to an anti-apoptotic response. One of the reasons for the decrease in exercise-induced apoptosis is the decrease in Fas-ligand protein, Fas-death receptor, TNF-α receptor, Fas-associated death domain (FADD), caspase-8, and caspase-3. In addition, after exercise mitochondrial-dependent apoptotic factors such as Bid, t-Bid, Bad, p-Bad, Bak, cytochrome c, and caspase-9 are reduced. These changes lead to a reduction in oxidative damage, a reduction in infarct size, a reduction in cardiac apoptosis, and an increase in myocardial function. After exercising in the heart, the levels of RhoA, ROCK1, Rac1, and ROCK2 decrease, while the levels of PKCε, PKCδ, and PKCɑ are activated to regulate calcium and prevent mPTP perforation. Exercise has an anti-apoptotic effect on heart failure by increasing the PKA-Akt-eNOS and FSTL1-USP10-Notch1 pathways, reducing the negative effects of CaMKIIδ, and increasing the calcineurin/NFAT pathway. Exercise plays a protective role in the heart by increasing HSP20, HSP27, HSP40, HSP70, HSP72, and HSP90 along with increasing JAK2 and STAT3 phosphorylation. However, research on exercise and factors such as Pim-1, Notch, and FAK in cardiac apoptosis is scarce, so further research is needed. Future research is recommended to discover more anti-apoptotic pathways. It is also recommended to study the synergistic effect of exercise with gene therapy, dietary supplements, and cell therapy for future research.