Parkinson’s disease (PD) is a common neurodegenerative disorder primarily characterized by the death of dopaminergic neurons that project from the substantia nigra pars compacta. Although the molecular bases for PD development are still little defined, extensive evidence from human samples and animal models support the involvement of inflammation in onset or progression. However, the exact trigger for this response remains unclear. Here, we provide a systematic review of the cellular mediators, i.e., microglia, astroglia and endothelial cells. We also discuss the genetic and transcriptional control of inflammation in PD and the immunomodulatory role of dopamine and reactive oxygen species. Finally, we summarize the preclinical and clinical approaches targeting neuroinflammation in PD.

Regulation of transcription factor Nrf2 (NF-E2-related factor 2) involves redox-sensitive proteasomal degradation via the E3 ubiquitin ligase Keap1/Cul3. However, Nrf2 is controlled by other mechanisms that have not yet been elucidated. We now show that glycogen synthase kinase 3 (GSK-3) phosphorylates a group of Ser residues in the Neh6 domain of mouse Nrf2 that overlap with an SCF/β-TrCP destruction motif (DSGIS, residues 334 to 338) and promotes its degradation in a Keap1-independent manner. Nrf2 was stabilized by GSK-3 inhibitors in Keap1-null mouse embryo fibroblasts. Similarly, an Nrf2(ΔETGE) mutant, which cannot be degraded via Keap1, accumulated when GSK-3 activity was blocked. Phosphorylation of a Ser cluster in the Neh6 domain of Nrf2 stimulated its degradation because a mutant Nrf2(ΔETGE 6S/6A) protein, lacking these Ser residues, exhibited a longer half-life than Nrf2(ΔETGE). Moreover, Nrf2(ΔETGE 6S/6A) was insensitive to β-TrCP regulation and exhibited lower levels of ubiquitination than Nrf2(ΔETGE). GSK-3β enhanced ubiquitination of Nrf2(ΔETGE) but not that of Nrf2(ΔETGE 6S/6A). The Nrf2(ΔETGE) protein but not Nrf2(ΔETGE 6S/6A) coimmunoprecipitated with β-TrCP, and this association was enhanced by GSK-3β. Our results show for the first time that Nrf2 is targeted by GSK-3 for SCF/β-TrCP-dependent degradation. We propose a "dual degradation" model to describe the regulation of Nrf2 under different pathophysiological conditions.

The phosphatidylinositol 3-kinase (PI3K)/Akt pathway elicits a survival signal against multiple apoptotic insults. In addition, phase II enzymes such as heme oxygenase-1 (HO-1) protect cells against diverse toxins and oxidative stress. In this work, we describe a link between these defense systems at the level of transcriptional regulation of the antioxidant enzyme HO-1. The herb-derived phenol carnosol induced HO-1 expression at both mRNA and protein levels. Luciferase reporter assays indicated that carnosol targeted the mouse ho1 promoter at two enhancer regions comprising the antioxidant response elements (AREs). Moreover, carnosol increased the nuclear levels of Nrf2, a transcription factor governing AREs. Electrophoretic mobility shift assays and luciferase reporter assays with a dominant-negative Nrf2 mutant indicated that carnosol increased the binding of Nrf2 to ARE and induced Nrf2-dependent activation of the ho1 promoter. While investigating the signaling pathways responsible for HO-1 induction, we observed that carnosol activated the ERK, p38, and JNK pathways as well as the survival pathway driven by PI3K. Inhibition of PI3K reduced the increase in Nrf2 protein levels and activation of the ho1 promoter. Expression of active PI3K-CAAX (where A is aliphatic amino acid) was sufficient to activate AREs. The use of dominant-negative mutants of protein kinase Czeta and Akt1, two kinases downstream from PI3K, demonstrated a requirement for active Akt1, but not protein kinase Czeta. Moreover, the long-term antioxidant effect of carnosol was partially blocked by PI3K or HO-1 inhibitors, further demonstrating that carnosol attenuates oxidative stress through a pathway that involves PI3K and HO-1.

Systems medicine has a mechanism-based rather than a symptom- or organ-based approach to disease and identifies therapeutic targets in a nonhypothesis-driven manner. In this work, we apply this to transcription factor nuclear factor (erythroid-derived 2)–like 2 (NRF2) by cross-validating its position in a protein–protein interaction network (the NRF2 interactome) functionally linked to cytoprotection in low-grade stress, chronic inflammation, metabolic alterations, and reactive oxygen species formation. Multiscale network analysis of these molecular profiles suggests alterations of NRF2 expression and activity as a common mechanism in a subnetwork of diseases (the NRF2 diseasome). This network joins apparently heterogeneous phenotypes such as autoimmune, respiratory, digestive, cardiovascular, metabolic, and neurodegenerative diseases, along with cancer. Importantly, this approach matches and confirms in silico several applications for NRF2-modulating drugs validated in vivo at different phases of clinical development. Pharmacologically, their profile is as diverse as electrophilic dimethyl fumarate, synthetic triterpenoids like bardoxolone methyl and sulforaphane, protein–protein or DNA–protein interaction inhibitors, and even registered drugs such as metformin and statins, which activate NRF2 and may be repurposed for indications within the NRF2 cluster of disease phenotypes. Thus, NRF2 represents one of the first targets fully embraced by classic and systems medicine approaches to facilitate both drug development and drug repurposing by focusing on a set of disease phenotypes that appear to be mechanistically linked. The resulting NRF2 drugome may therefore rapidly advance several surprising clinical options for this subset of chronic diseases.

The transcription factor Nrf2 (nuclear factor E2-related factor 2) regulates the expression of antioxidant phase II genes and contributes to preserve redox homeostasis and cell viability in response to oxidant insults. Nrf2 should be coordinated with the canonical cell survival pathway represented by phosphatidylinositol 3-kinase (PI3K) and the Ser/Thr kinase Akt but so far the mechanistic connections remain undefined. Here we identify glycogen synthase kinase-3beta (GSK-3beta), which is inhibited by Akt-mediated phosphorylation, as the link between both processes. Using heme oxygenase-1 (HO-1) as a model phase II gene, we found that both PI3K and Akt increased mRNA and protein levels of this enzyme. Pharmacological inhibitors (LiCl and PDZD-8) and genetic variants of GSK-3beta (constitutively active and dominant negative mutants) indicated that PI3K/Akt activates and GSK-3beta inhibits the antioxidant response elements of the ho1 promoter and pointed Nrf2 as directly involved in this process. Indeed, GSK-3beta phosphorylated Nrf2 in vitro and in vivo. Immunocytochemistry and subcellular fractionation analyses demonstrated that the effect of GSK-3beta-mediated phosphorylation of Nrf2 is to exclude this transcription factor from the nucleus. Nrf2 up-regulated the expression of HO-1, glutathione peroxidase, glutathione S-transferase A1, NAD(P)H: quinone oxidoreductase and glutamate-cysteine ligase and protected against hydrogen peroxide-induced glutathione depletion and cell death, whereas co-expression of active GSK-3beta attenuated both phase II gene expression and oxidant protection. These results contribute to clarify the cross-talk between the survival signal elicited by PI3K/Akt and the antioxidant phase II cell response, and introduce GSK-3beta as the key mediator of this regulation mechanism.