PHLPPs: Emerging players in metabolic disorders

Methyltransferase-like enzyme 14 exacerbates retinal ganglion cell damage and diabetic retinopathy through N6-methyladenosine-dependent upregulation of pleckstrin homology domain and leucine rich repeat protein phosphatase 2

N6-methyladenosine (m6A) modification of pleckstrin homology domain and leucine rich repeat protein phosphatase 2 (PHLPP2), mediated by methyltransferase-like enzyme 14 (METTL14), plays a critical role in regulating PHLPP2 expression across various pathological conditions. This study aims to ascertain whether METTL14 influences m6A methylation of PHLPP2 in diabetic retinopathy (DR) and to delineate the precise function of the METTL14/PHLPP2 axis in disease progression. METTL14 levels were observed to be elevated in retinas of DR rats and in HG-stimulated RGCs, coinciding with an increase in PHLPP2 m6A modification. Knockdown of METTL14 resulted in significant reductions in PHLPP2 expression and its m6A modification. Silencing METTL14 mitigated HG-induced damage in RGCs, which was linked to the inhibition of apoptosis, oxidative stress and inflammation. This protective effect could be negated through the restoration of PHLPP2. METTL14 knockdown modulated the AKT/GSK-3β/Nrf2 signal cascade through PHLPP2. Silencing METTL14 resulted in the downregulation of METTL14 and PHLPP2 in the retinas of DR rats, ameliorated visual function impairment and reduced the pathological alterations. These protective effects of METTL14 silencing against DR were also weakened when PHLPP2 was restored. Overall, these results suggest that suppressing METTL14 improves HG-induced damage in RGCs and protects against DR by downregulating PHLPP2 through m6A modification.

Phosphatase PHLPP1 is an alveolar-macrophage-intrinsic transcriptional checkpoint controlling pulmonary fibrosis

Alveolar macrophages (AMs) are crucial for lung homeostasis, and their dysfunction causes uncontrolled fibrotic responses and pulmonary disorders. Protein phosphatases control multiple cellular events. However, whether nuclear phosphatases cooperate with histone modifiers to affect pulmonary fibrosis progress remains obscure. Here, we identified pleckstrin homology domain and leucine-rich repeat protein phosphatase 1 (PHLPP1) as a key protective factor for pulmonary fibrosis. Transcriptomics and epigenomics data confirmed that PHLPP1 selectively targeted Kruppel-like factor 4 (KLF4) for transcriptional inhibition in AMs. Nuclear PHLPP1 directly bound and dephosphorylated histone deacetylase 8 (HDAC8) at serine 39, thereby enhancing its deacetylase enzyme activity and subsequently suppressing KLF4 expression via the decreased histone acetylation and chromatin accessibility. Thus, loss of PHLPP1 amplified KLF4-centric profibrotic transcriptional program in AMs, while intratracheal administration of Klf4-short hairpin RNA (shRNA) adeno-associated virus ameliorated lung fibrosis in PHLPP1-deficient mice. Our study implies that targeting decreased PHLPP1 in AMs might be a promising therapeutic strategy for pulmonary fibrosis.

Ang-1 and VEGF: central regulators of angiogenesis

Angiopoietin-1 (Ang-1) and Vascular Endothelial Growth Factor (VEGF) are central regulators of angiogenesis and are often inactivated in various cardiovascular diseases. VEGF forms complexes with ETS transcription factor family and exerts its action by downregulating multiple genes. Among the target genes of the VEGF-ETS complex, there are a significant number encoding key angiogenic regulators. Phosphorylation of the VEGF-ETS complex releases transcriptional repression on these angiogenic regulators, thereby promoting their expression. Ang-1 interacts with TEK, and this phosphorylation release can be modulated by the Ang-1-TEK signaling pathway. The Ang-1-TEK pathway participates in the transcriptional activation of VEGF genes. In summary, these elements constitute the Ang-1-TEK-VEGF signaling pathway. Additionally, Ang-1 is activated under hypoxic and inflammatory conditions, leading to an upregulation in the expression of TEK. Elevated TEK levels result in the formation of the VEGF-ETS complex, which, in turn, downregulates the expression of numerous angiogenic genes. Hence, the Ang-1-dependent transcriptional repression is indirect. Reduced expression of many target genes can lead to aberrant angiogenesis. A significant overlap exists between the target genes regulated by Ang-1-TEK-VEGF and those under the control of the Ang-1-TEK-TSP-1 signaling pathway. Mechanistically, this can be explained by the replacement of the VEGF-ETS complex with the TSP-1 transcriptional repression complex at the ETS sites on target gene promoters. Furthermore, VEGF possesses non-classical functions unrelated to ETS and DNA binding. Its supportive role in TSP-1 formation may be exerted through the VEGF-CRL5-VHL-HIF-1α-VH032-TGF-β-TSP-1 axis. This review assesses the regulatory mechanisms of the Ang-1-TEK-VEGF signaling pathway and explores its significant overlap with the Ang-1-TEK-TSP-1 signaling pathway.

Exercise-induced Nrf2 activation increases antioxidant defenses in skeletal muscles

Following the discovery that exercise increases the production of reactive oxygen species in contracting skeletal muscles, evidence quickly emerged that endurance exercise training increases the abundance of key antioxidant enzymes in the trained muscles. Since these early observations, knowledge about the impact that regular exercise has on skeletal muscle antioxidant capacity has increased significantly. Importantly, in recent years, our understanding of the cell signaling pathways responsible for this exercise-induced increase in antioxidant enzymes has expanded exponentially. Therefore, the goals of this review are: 1) summarize our knowledge about the influence that exercise training has on the abundance of key antioxidant enzymes in skeletal muscles; and 2) to provide a state-of-the-art review of the nuclear factor erythroid 2-related factor (Nrf2) signaling pathway that is responsible for many of the exercise-induced changes in muscle antioxidant capacity. We begin with a discussion of the sources of reactive oxygen species in contracting muscles and then examine the exercise-induced changes in the antioxidant enzymes that eliminate both superoxide radicals and hydrogen peroxide in muscle fibers. We conclude with a discussion of the advances in our understanding of the exercise-induced control of the Nrf2 signaling pathway that is responsible for the expression of numerous antioxidant proteins. In hopes of stimulating future research, we also identify gaps in our knowledge about the signaling pathways responsible for the exercise-induced increases in muscle antioxidant enzymes.

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.

Phlpp1 Expression in Osteoblasts Plays a Modest Role in Bone Homeostasis

Prior work demonstrated that Phlpp1 deficiency alters limb length and bone mass, but the cell types involved and requirement of Phlpp1 for this effect were unclear. To understand the function of Phlpp1 within bone-forming osteoblasts, we crossed Phlpp1 floxed mice with mice harboring type 1 collagen (Col1a12.3kb)-Cre. Mineralization of bone marrow stromal cell cultures derived from Phlpp1 cKOCol1a1 was unchanged, but levels of inflammatory genes (eg, Ifng, Il6, Ccl8) and receptor activator of NF-κB ligand/osteoprotegerin (RANKL/OPG) ratios were enhanced by either Phlpp1 ablation or chemical inhibition. Micro-computed tomography of the distal femur and L5 vertebral body of 12-week-old mice revealed no alteration in bone volume per total volume, but compromised femoral bone microarchitecture within Phlpp1 cKOCol1a1 conditional knockout females. Bone histomorphometry of the proximal tibia documented no changes in osteoblast or osteoclast number per bone surface but slight reductions in osteoclast surface per bone surface. Overall, our data show that deletion of Phlpp1 in type 1 collagen-expressing cells does not significantly alter attainment of peak bone mass of either males or females, but may enhance inflammatory gene expression and the ratio of RANKL/OPG. Future studies examining the role of Phlpp1 within models of advanced age, inflammation, or osteocytes, as well as functional redundancy with the related Phlpp2 isoform are warranted. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The Complex Genetic and Epigenetic Regulation of the Nrf2 Pathways: A Review

Nrf2 is a major transcription factor that significantly regulates-directly or indirectly-more than 2000 genes. While many of these genes are involved in maintaining redox balance, others are involved in maintaining balance among metabolic pathways that are seemingly unrelated to oxidative stress. In the past 25 years, the number of factors involved in the activation, nuclear translocation, and deactivation of Nrf2 has continued to expand. The purpose of this review is to provide an overview of the remarkable complexity of the tortuous sequence of stop-and-go signals that not only regulate expression or repression, but may also modify transcriptional intensity as well as the specificity of promoter recognition, allowing fluidity of its gene expression profile depending on the various structural modifications the transcription factor encounters on its journey to the DNA. At present, more than 45 control points have been identified, many of which represent sites of action of the so-called Nrf2 activators. The complexity of the pathway and the synergistic interplay among combinations of control points help to explain the potential advantages seen with phytochemical compositions that simultaneously target multiple control points, compared to the traditional pharmaceutical paradigm of "one-drug, one-target".