Sestrin 2 protein regulates platelet-derived growth factor receptor ß (Pdgfrß) expression by modulating proteasomal and Nrf2 transcription factor functions.

We recently identified the antioxidant protein Sestrin 2 (Sesn2) as a suppressor of platelet-derived growth factor receptor ß (Pdgfrß) signaling and Pdgfrß signaling as an inducer of lung regeneration and injury repair. Here, we identified Sesn2 and the antioxidant gene inducer nuclear factor erythroid 2-related factor 2 (Nrf2) as positive regulators of proteasomal function. Inactivation of Sesn2 or Nrf2 induced reactive oxygen species-mediated proteasomal inhibition and Pdgfrß accumulation. Using bacterial artificial chromosome (BAC) transgenic HeLa and mouse embryonic stem cells stably expressing enhanced green fluorescent protein-tagged Sesn2 at nearly endogenous levels, we also showed that Sesn2 physically interacts with 2-Cys peroxiredoxins and Nrf2 albeit under different reductive conditions. Overall, we characterized a novel, redox-sensitive Sesn2/Pdgfrß suppressor pathway that negatively interferes with lung regeneration and is up-regulated in the emphysematous lungs of patients with chronic obstructive pulmonary disease (COPD).

Flotillin-1/reggie-2 protein plays dual role in activation of receptor-tyrosine kinase/mitogen-activated protein kinase signaling.

Our previous work has shown that the membrane microdomain-associated flotillin proteins are potentially involved in epidermal growth factor (EGF) receptor signaling. Here we show that knockdown of flotillin-1/reggie-2 results in reduced EGF-induced phosphorylation of specific tyrosines in the EGF receptor (EGFR) and in inefficient activation of the downstream mitogen-activated protein (MAP) kinase and Akt signaling. Although flotillin-1 has been implicated in endocytosis, its depletion affects neither the endocytosis nor the ubiquitination of the EGFR. However, EGF-induced clustering of EGFR at the cell surface is altered in cells lacking flotillin-1. Furthermore, we show that flotillins form molecular complexes with EGFR in an EGF/EGFR kinase-independent manner. However, knockdown of flotillin-1 appears to affect the activation of the downstream MAP kinase signaling more directly. We here show that flotillin-1 forms a complex with CRAF, MEK1, ERK, and KSR1 (kinase suppressor of RAS) and that flotillin-1 knockdown leads to a direct inactivation of ERK1/2. Thus, flotillin-1 plays a direct role during both the early phase (activation of the receptor) and late (activation of MAP kinases) phase of growth factor signaling. Our results here unveil a novel role for flotillin-1 as a scaffolding factor in the regulation of classical MAP kinase signaling. Furthermore, our results imply that other receptor-tyrosine kinases may also rely on flotillin-1 upon activation, thus suggesting a general role for flotillin-1 as a novel factor in receptor-tyrosine kinase/MAP kinase signaling.

Mitogen-Activated Protein (MAP) Kinase Scaffolding Proteins: A Recount.

The mitogen-activated protein kinase (MAPK) pathway is the canonical signaling pathway for many receptor tyrosine kinases, such as the Epidermal Growth Factor Receptor. Downstream of the receptors, this pathway involves the activation of a kinase cascade that culminates in a transcriptional response and affects processes, such as cell migration and adhesion. In addition, the strength and duration of the upstream signal also influence the mode of the cellular response that is switched on. Thus, the same components can in principle coordinate opposite responses, such as proliferation and differentiation. In recent years, it has become evident that MAPK signaling is regulated and fine-tuned by proteins that can bind to several MAPK signaling proteins simultaneously and, thereby, affect their function. These so-called MAPK scaffolding proteins are, thus, important coordinators of the signaling response in cells. In this review, we summarize the recent advances in the research on MAPK/extracellular signal-regulated kinase (ERK) pathway scaffolders. We will not only review the well-known members of the family, such as kinase suppressor of Ras (KSR), but also put a special focus on the function of the recently identified or less studied scaffolders, such as fibroblast growth factor receptor substrate 2, flotillin-1 and mitogen-activated protein kinase organizer 1.

Cbl-associated protein is tyrosine phosphorylated by c-Abl and c-Src kinases.

BACKGROUND: The c-Cbl-associated protein (CAP), also known as ponsin, localizes to focal adhesions and stress fibers and is involved in signaling events. Phosphorylation has been described for the other two members of the sorbin homology family, vinexin and ArgBP2, but no data exist about the putative phosphorylation of CAP. According to previous findings, CAP binds to tyrosine kinase c-Abl. However, it is not known if CAP is a substrate of c-Abl or other tyrosine kinases or if phosphorylation regulates its localization. RESULTS: We here show that CAP is Tyr phosphorylated by and interacts with both c-Abl and c-Src. One major phosphorylation site, Tyr360, and two minor contributors Tyr326 and Tyr632 were identified as Abl phosphorylation sites, whereas Src preferentially phosphorylates Tyr326 and Tyr360. Phosphorylation of CAP was not necessary for its localization to focal adhesions and stress fibers, but Tyr326Phe substitution alters the function of CAP during cell spreading. CONCLUSION: This is the first demonstration of phosphorylation of CAP by any kinase. Our findings suggest that coordinated action of Src and Abl might regulate the function of CAP and reveal a functional role especially for the Src-mediated Tyr phosphorylation of CAP in cell spreading.

High Efficiency Gene Correction in Hematopoietic Cells by Donor-Template-Free CRISPR/Cas9 Genome Editing.

The CRISPR/Cas9 prokaryotic adaptive immune system and its swift repurposing for genome editing enables modification of any prespecified genomic sequence with unprecedented accuracy and efficiency, including targeted gene repair. We used the CRISPR/Cas9 system for targeted repair of patient-specific point mutations in the Cytochrome b-245 heavy chain gene (CYBB), whose inactivation causes chronic granulomatous disease (XCGD)-a life-threatening immunodeficiency disorder characterized by the inability of neutrophils and macrophages to produce microbicidal reactive oxygen species (ROS). We show that frameshift mutations can be effectively repaired in hematopoietic cells by non-integrating lentiviral vectors carrying RNA-guided Cas9 endonucleases (RGNs). Because about 25% of most inherited blood disorders are caused by frameshift mutations, our results suggest that up to a quarter of all patients suffering from monogenic blood disorders could benefit from gene therapy employing personalized, donor template-free RGNs.

Ltbp4 regulates Pdgfrß expression via TGFß-dependent modulation of Nrf2 transcription factor function.

Latent transforming growth factor beta binding protein 4 (LTBP4) belongs to the fibrillin/LTBP family of proteins and plays an important role as a structural component of extracellular matrix (ECM) and local regulator of TGFß signaling. We have previously reported that Ltbp4S knock out mice (Ltbp4S-/-) develop centrilobular emphysema reminiscent of late stage COPD, which could be partially rescued by inactivating the antioxidant protein Sestrin 2 (Sesn2). More recent studies showed that Sesn2 knock out mice upregulate Pdgfrß-controlled alveolar maintenance programs that protect against cigarette smoke induced pulmonary emphysema. Based on this, we hypothesized that the emphysema of Ltbp4S-/- mice is primarily caused by defective Pdgfrß signaling. Here we show that LTBP4 induces Pdgfrß signaling by inhibiting the antioxidant Nrf2/Keap1 pathway in a TGFß-dependent manner. Overall, our data identified Ltbp4 as a major player in lung remodeling and injury repair.

Sestrin-2, a repressor of PDGFRß signalling, promotes cigarette-smoke-induced pulmonary emphysema in mice and is upregulated in individuals with COPD.

Chronic obstructive pulmonary disease (COPD) is a leading cause of morbidity and mortality worldwide. COPD is caused by chronic exposure to cigarette smoke and/or other environmental pollutants that are believed to induce reactive oxygen species (ROS) that gradually disrupt signalling pathways responsible for maintaining lung integrity. Here we identify the antioxidant protein sestrin-2 (SESN2) as a repressor of PDGFRß signalling, and PDGFRß signalling as an upstream regulator of alveolar maintenance programmes. In mice, the mutational inactivation of Sesn2 prevents the development of cigarette-smoke-induced pulmonary emphysema by upregulating PDGFRß expression via a selective accumulation of intracellular superoxide anions (O2(-)). We also show that SESN2 is overexpressed and PDGFRß downregulated in the emphysematous lungs of individuals with COPD and to a lesser extent in human lungs of habitual smokers without COPD, implicating a negative SESN2-PDGFRß interrelationship in the pathogenesis of COPD. Taken together, our results imply that SESN2 could serve as both a biomarker and as a drug target in the clinical management of COPD.

Molecular networks in FGF signaling: flotillin-1 and cbl-associated protein compete for the binding to fibroblast growth factor receptor substrate 2.

Fibroblast growth factor receptor substrate 2 (FRS2α) is a signaling adaptor protein that regulates downstream signaling of many receptor tyrosine kinases. During signal transduction, FRS2 can be both tyrosine and threonine phosphorylated and forms signaling complexes with other adaptor proteins and tyrosine phosphatases. We have here identified flotillin-1 and the cbl-associated protein/ponsin (CAP) as novel interaction partners of FRS2. Flotillin-1 binds to the phosphotyrosine binding domain (PTB) of FRS2 and competes for the binding with the fibroblast growth factor receptor. Flotillin-1 knockdown results in increased Tyr phosphorylation of FRS2, in line with the inhibition of ERK activity in the absence of flotillin-1. CAP directly interacts with FRS2 by means of its sorbin homology (SoHo) domain, which has previously been shown to interact with flotillin-1. In addition, the third SH3 domain in CAP binds to FRS2. Due to the overlapping binding domains, CAP and flotillin-1 appear to compete for the binding to FRS2. Thus, our results reveal a novel signaling network containing FRS2, CAP and flotillin-1, whose successive interactions are most likely required to regulate receptor tyrosine kinase signaling, especially the mitogen activated protein kinase pathway.

Functional aspects of membrane association of reggie/flotillin proteins.

Flotillin-2 and flotillin-1, also called reggie-1 and reggie-2, are ubiquitously expressed and highly conserved proteins. Originally, they were described as neuronal regeneration proteins, but they appear to function in a wide variety of cellular processes, such as membrane receptor signaling, endocytosis, phagocytosis and cell adhesion. The molecular details of the function of flotillins in these processes have only been partially clarified. Flotillins are associated with cholesterol and sphingolipid enriched membrane microdomains known as rafts, and some findings even suggest that they define their own kind of a microdomain. The mechanism of the membrane association of flotillins appears to rely mainly on acylation (myristoylation and/or palmitoylation), localizing flotillins onto the cytosolic side of the membranes, whereas no transmembrane domains are present. In addition, flotillins show a strong tendency to form homo- and hetero-oligomers with each other. In this review, we will summarize the recent findings on the function of flotillins and discuss the mechanisms that might regulate their function, such as membrane association, oligomerization and phosphorylation.