Role of Dbl's big sister in the anti-mitogenic pathway from alpha1B-adrenergic receptor to c-Jun N-terminal kinase

DBS is activated by EPHB2/SRC signaling-mediated tyrosine phosphorylation in HEK293 cells

It is well known that Rho family small GTPases (Rho GTPase) has a role of molecular switch in intracellular signal transduction. The switch cycle between GTP-bound and GDP-bound state of Rho GTPase regulates various cell responses such as gene transcription, cytoskeletal rearrangements, and vesicular trafficking. Rho GTPase-specific guanine nucleotide exchange factors (RhoGEFs) are regulated by various extracellular stimuli and activates Rho GTPase such as RhoA, Rac1, and Cdc42. The molecular mechanisms that regulate RhoGEFs are poorly understood. Our studies reveal that Dbl's big sister (DBS), a RhoGEF for Cdc42 and RhoA, is phosphorylated at least on tyrosine residues at 479, 660, 727, and 926 upon stimulation by SRC signaling and that the phosphorylation at Tyr-660 is particularly critical for the serum response factor (SRF)-dependent transcriptional activation of DBS by Ephrin type-B receptor 2 (EPHB2)/SRC signaling. In addition, our studies also reveal that the phosphorylation of Tyr-479 and Tyr-660 on DBS leads to the actin cytoskeletal reorganization by EPHB2/SRC signaling. These findings are thought to be useful for understanding pathological conditions related to DBS such as cancer and non-syndromic autism in future.

Transactivation of the receptor-tyrosine kinase ephrin receptor A2 is required for the low molecular weight hyaluronan-mediated angiogenesis that is implicated in tumor progression

Angiogenesis or the formation of new blood vessels is important in the growth and metastatic potential of various cancers. Therefore, understanding the mechanism(s) by which angiogenesis occurs can have important therapeutic implications in numerous malignancies. We and others have demonstrated that low molecular weight hyaluronan (LMW-HA, ∼2500 Da) promotes endothelial cell (EC) barrier disruption and angiogenesis. However, the mechanism(s) by which this occurs is poorly defined. Our data indicate that treatment of human EC with LMW-HA induced CD44v10 association with the receptor-tyrosine kinase, EphA2, transactivation (tyrosine phosphorylation) of EphA2, and recruitment of the PDZ domain scaffolding protein, PATJ, to the cell periphery. Silencing (siRNA) CD44, EphA2, PATJ, or Dbs (RhoGEF) expression blocked LMW-HA-mediated angiogenesis (EC proliferation, migration, and tubule formation). In addition, silencing EphA2, PATJ, Src, or Dbs expression blocked LMW-HA-mediated RhoA activation. To translate our in vitro findings, we utilized a novel anginex/liposomal targeting of murine angiogenic endothelium with either CD44 or EphA2 siRNA and observed inhibition of LMW-HA-induced angiogenesis in implanted Matrigel plugs. Taken together, these results indicate LMW-HA-mediated transactivation of EphA2 is required for PATJ and Dbs membrane recruitment and subsequent RhoA activation required for angiogenesis. These results suggest that targeting downstream effectors of LMW-HA could be a useful therapeutic intervention for angiogenesis-associated diseases including tumor progression.

Global microRNA elevation by inducible Exportin 5 regulates cell cycle entry

Proper regulation of gene expression during cell cycle entry ensures the successful completion of proliferation, avoiding risks such as carcinogenesis. The microRNA (miRNA) network is an emerging molecular system regulating multiple genetic pathways. We demonstrate here that the global elevation of miRNAs is critical for proper control of gene expression program during cell cycle entry. Strikingly, Exportin 5 (XPO5) is promptly induced during cell cycle entry by a PI3K-dependent post-transcriptional mechanism. Inhibition of XPO5 induction interfered with global miRNA elevation and resulted in a proliferation defect associated with delayed G1/S transition. During cell cycle entry, XPO5 therefore plays a paramount role as a critical molecular hub controlling the gene expression program through global regulation of miRNAs. Our data suggest that XPO5-mediated global miRNA elevation might be involved in a broad range of cellular events associated with cell cycle control.

Recent advances in the multitarget-directed ligands approach for the treatment of Alzheimer's disease

With 27 million cases worldwide documented in 2006, Alzheimer's disease (AD) constitutes an overwhelming health, social, economic, and political problem to nations. Unless a new medicine capable to delay disease progression is found, the number of cases will reach 107 million in 2050. So far, the therapeutic paradigm one-compound-one-target has failed. This could be due to the multiple pathogenic mechanisms involved in AD including amyloid β (Aβ) aggregation to form plaques, τ hyperphosphorylation to disrupt microtubule to form neurofibrillary tangles, calcium imbalance, enhanced oxidative stress, impaired mitochondrial function, apoptotic neuronal death, and deterioration of synaptic transmission, particularly at cholinergic neurons. Approximately 100 compounds are presently been investigated directed to single targets, namely inhibitors of β and γ secretase, vaccines or antibodies that clear Aβ, metal chelators to inhibit Aβ aggregation, blockers of glycogen synthase kinase 3β, enhancers of mitochondrial function, antioxidants, modulators of calcium-permeable channels such as voltage-dependent calcium channels, N-methyl-D-aspartate receptors for glutamate, or enhancers of cholinergic neurotransmission such as inhibitors of acetylcholinesterase or butyrylcholinesterase. In view of this complex pathogenic mechanisms, and the successful treatment of chronic diseases such as HIV or cancer, with multiple drugs having complementary mechanisms of action, the concern is growing that AD could better be treated with a single compound targeting two or more of the pathogenic mechanisms leading to neuronal death. This review summarizes the current therapeutic strategies based on the paradigm one-compound-various targets to treat AD. A treatment that delays disease onset and/or progression by 5 years could halve the number of people requiring institutionalization and/or dying from AD. 

Alpha-1 adrenergic receptor transactivates signal transducer and activator of transcription-3 (Stat3) through activation of Src and epidermal growth factor receptor (EGFR) in hepatocytes

Hepatocytes express adrenergic receptors (ARs) that modulate several functions, including liver regeneration, hepatocyte proliferation, glycogenolysis, gluconeogenesis, synthesis of urea and fatty acid metabolism. Adrenergic hepatic function in adults is mainly under the control of alpha(1)-ARs; however, the mechanism through which they influence diverse processes remains incompletely understood. This study describes a novel alpha(1)-AR-mediated transactivation of signal transducer and activator of transcription-3 (Stat3) in primary and transformed hepatocytes. Treatment of primary rat hepatocytes with the alpha(1)-AR agonist, phenylephrine (PE), induced a rapid phosphorylation of Stat3. PE also increased Stat3 phosphorylation, DNA binding and transcription activity in transformed human hepatocellular carcinoma cells (Hep3B). The PE-induced Stat3 phosphorylation, DNA binding and reporter activity were completely blocked by the selective alpha(1)-AR antagonist, prazosin. In addition, transfection of Hep3B cells with human alpha(1B)-AR expression vector also enhanced Stat3 phosphorylation and reporter activity. Moreover, overexpression of RGS2, a protein inhibitor of G(q/11) signaling, blocked PE-induced Stat3 phosphorylation and reporter activity. The observations that PE induced the formation of c-Src-Stat3 binding complex and phosphorylation of epidermal growth factor receptor (EGFR) and that inhibiting Src and EGFR prevented PE-induced Stat3 activation indicate the involvement of Src and EGFR. Taken together, these observations demonstrate a novel alpha(1)-AR-mediated Stat3 activation that involves G(q/11), Src, and EGFR in hepatic cells.

Auto-inhibition of the Dbl family protein Tim by an N-terminal helical motif

Dbl-related oncoproteins are guanine nucleotide exchange factors specific for Rho-family GTPases and typically possess tandem Dbl homology (DH) and pleckstrin homology domains that act in concert to catalyze exchange. Because the ability of many Dbl-family proteins to catalyze exchange is constitutively activated by truncations N-terminal to their DH domains, it has been proposed that the activity of Dbl-family proteins is regulated by auto-inhibition. However, the exact mechanisms of regulation of Dbl-family proteins remain poorly understood. Here we show that the Dbl-family protein, Tim, is auto-inhibited by a short, helical motif immediately N-terminal to its DH domain, which directly occludes the catalytic surface of the DH domain to prevent GTPase activation. Similar to the distantly related Vav isozymes, auto-inhibition of Tim is relieved by truncation, mutation, or phosphorylation of the auto-inhibitory helix. A peptide comprising the helical motif inhibits the exchange activity of Tim in vitro. Furthermore, substitutions within the most highly conserved surface of the DH domain designed to disrupt interactions with the auto-inhibitory helix also activate the exchange process.

Molecular mechanisms mediating the G protein-coupled receptor regulation of cell cycle progression

G protein-coupled receptors are key regulators of cellular communication, mediating the efficient coordination of a cell's responses to extracellular stimuli. When stimulated these receptors modulate the activity of a wide range of intracellular signalling pathways that facilitate the ordered development, growth and reproduction of the organism. There is now a growing body of evidence examining the mechanisms by which G protein-coupled receptors are able to regulate the expression, activity, localization and stability of cell cycle regulatory proteins that either promote or inhibit the initiation of DNA synthesis. In this review, we will detail the intracellular pathways that mediate the G protein-coupled receptor regulation of cellular proliferation, specifically the progression from the G1 phase to the S phase of the cell cycle.

Ccpg1, a novel scaffold protein that regulates the activity of the Rho guanine nucleotide exchange factor Dbs

Dbs is a Rho-specific guanine nucleotide exchange factor (RhoGEF) with in vitro exchange activity specific for RhoA and Cdc42. Like many RhoGEF family members, the in vivo exchange activity of Dbs is restricted in a cell-specific manner. Here we report the characterization of a novel scaffold protein (designated cell cycle progression protein 1 [Ccpg1]) that interacts with Dbs and modulates its in vivo exchange specificity. When coexpressed in mammalian cells, Ccpg1 binds to the Dbl homology/pleckstrin homology domain tandem motif of Dbs and inhibits its exchange activity toward RhoA, but not Cdc42. Expression of Ccpg1 correlates with the ability of Dbs to activate endogenous RhoA in cultured cells, and suppression of endogenous Ccpg1 expression potentiates Dbs exchange activity toward RhoA. The isolated Dbs binding domain of Ccpg1 is not sufficient to suppress Dbs exchange activity on RhoA, thus suggesting a regulatory interaction. Ccpg1 mediates recruitment of endogenous Src kinase into Dbs-containing complexes and interacts with the Rho family member Cdc42. Collectively, our studies suggest that Ccpg1 represents a new class of regulatory scaffold protein that can function as both an assembly platform for Rho protein signaling complexes and a regulatory protein which can restrict the substrate utilization of a promiscuous RhoGEF family member.

Ras activation of a Rac1 exchange factor, Tiam1, mediates neurotrophin-3-induced Schwann cell migration

Endogenous neurotrophins positively and negatively regulate migration of premyelinating Schwann cells before the initiation of myelination. Neurotrophin-3 (NT3) acting through the TrkC receptor tyrosine kinase stimulates Schwann cell migration via the Rho GTPases Rac1 and Cdc42. We previously demonstrated that TrkC directly phosphorylates and activates Dbs, the guanine-nucleotide exchange factor (GEF) for Cdc42, to partially mediate Schwann cell migration. Here, we identify T lymphoma invasion and metastasis (Tiam) 1 as the Rac1-specific guanine-nucleotide exchange factor involved in NT3-induced Schwann cell migration. Furthermore, the interaction between the small GTPase Ras and Tiam1 plays an essential role in the activation of Rac1. Taken together, these results suggest that NT3 activation of TrkC stimulates Schwann cell migration through two parallel signaling units, Ras/Tiam1/Rac1 and Dbs/Cdc42, and that Schwann cell migration is uniquely regulated in the case of Ras and Rac1, by two different types of small GTPases.

The neurotrophin-3 receptor TrkC directly phosphorylates and activates the nucleotide exchange factor Dbs to enhance Schwann cell migration

During the development of the peripheral nervous system, Schwann cells, the myelin-forming glia, migrate along axons before initiating myelination. We previously demonstrated that endogenous neurotrophin-3 (NT3) acting through the TrkC tyrosine kinase receptor enhances migration of premyelinating Schwann cells. This signaling pathway is mediated by the c-Jun N-terminal kinase (JNK) cascade regulated by the Rho GTPases Rac1 and Cdc42. However, missing is the link between TrkC and the GTPases. Here, we show that a guanine-nucleotide exchange factor (GEF), Dbl's big sister (Dbs), couples with TrkC to activate Cdc42 in Schwann cells. Furthermore, TrkC directly phosphorylates Dbs, thereby inducing the Cdc42-GEF activity. Taken together, activation of TrkC triggers Schwann cell migration by regulating Dbs upon direct tyrosine phosphorylation, providing a mechanism whereby a membrane receptor tyrosine kinase can induce the activation of Rho GTPase-GEFs.

Src kinase regulates the activation of a novel FGD-1-related Cdc42 guanine nucleotide exchange factor in the signaling pathway from the endothelin A receptor to JNK

Small GTPases act as binary switches by cycling between an inactive (GDP-bound) and an active (GTP-bound) state. Upon stimulation with extracellular signals, guanine-nucleotide exchange factors (GEFs) stimulate the exchange of GDP to GTP to shift toward the active forms of small GTPases, recognizing the downstream targets. Here we show that KIAA0793, containing substantial sequence homology with the catalytic Dbl homology domain of the faciogenital dysplasia gene product (FGD1), is a specific GEF for Cdc42. We, therefore, tentatively named it FRG (FGD1-related Cdc42-GEF). Src kinase directly phosphorylates and activates FRG, as Vav family GEFs. Additionally, FRG is involved in the signaling pathway from the endothelin A receptor to c-Jun N-terminal kinase, resulting in the inhibition of cell motility. These results suggest that FRG is a member of Cdc42-GEF and plays an important role in the signaling pathway downstream of G protein-coupled receptors.