Kyoung Wan Yoon

Negative Regulation of MEKK1-induced Signaling by Glutathione S-Transferase Mu

Mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase 1 (MEKK1) is an important component in the stress-activated protein kinase pathway. Glutathione S-transferase Mu 1-1 (GST M1-1) has now been shown to inhibit the stimulation of MEKK1 activity induced by cellular stresses such as UV and hydrogen peroxide. GST M1-1 inhibited MEKK1 activation in a manner independent of its glutathione-conjugating catalytic activity. In vitro binding and kinase assays revealed that GST M1-1 directly bound MEKK1 and inhibited its kinase activity. Co-immunoprecipitation analysis showed a physical association between endogenous GST M1-1 and endogenous MEKK1 in L929 cells. Overexpressed GST M1-1 interfered with the binding of MEKK1 to SEK1 in transfected HEK293 cells. Furthermore, GST M1-1 suppressed MEKK1-mediated apoptosis. Taken together, our results suggest that GST M1-1 functions as a negative regulator of MEKK1.

CIB1 functions as a Ca2+-sensitive modulator of stress-induced signaling by targeting ASK1

Calcium and integrin binding protein 1 (CIB1) is a Ca2+-binding protein of 22 kDa that was initially identified as a protein that interacts with integrin αIIb. Although it interacts with various proteins and has been implicated in diverse cellular functions, the molecular mechanism by which CIB1 regulates intracellular signaling networks has remained unclear. We now show that, by targeting apoptosis signal-regulating kinase 1 (ASK1), CIB1 negatively regulates stress-activated MAPK signaling pathways. CIB1 was thus shown to bind to ASK1, to interfere with the recruitment of TRAF2 to ASK1, and to inhibit the autophosphorylation of ASK1 on threonine-838, thereby blocking ASK1 activation. Furthermore, CIB1 mitigated apoptotic cell death initiated either by TNF-α in breast cancer MCF7 cells or by 6-hydroxydopamine (6-OHDA) in dopaminergic cells. Ca2+ influx induced by membrane depolarization reversed the inhibitory effect of CIB1 on 6-OHDA-induced ASK1 activation and cell death in dopaminergic neurons. These observations thus suggest that CIB1 functions as a Ca2+-sensitive negative regulator of ASK1-mediated signaling events.

Identification of a novel antiapoptotic protein that antagonizes ASK1 and CAD activities

Diverse stimuli initiate the activation of apoptotic signaling pathways that often causes nuclear DNA fragmentation. Here, we report a new antiapoptotic protein, a caspase-activated DNase (CAD) inhibitor that interacts with ASK1 (CIIA). CIIA, by binding to apoptosis signal-regulating kinase 1 (ASK1), inhibits oligomerization-induced ASK1 activation. CIIA also associates with CAD and inhibits the nuclease activity of CAD without affecting caspase-3–mediated ICAD cleavage. Overexpressed CIIA reduces H2O2- and tumor necrosis factor-α–induced apoptosis. CIIA antisense oligonucleotides, which abolish expression of endogenous CIIA in murine L929 cells, block the inhibitory effect of CIIA on ASK1 activation, deoxyribonucleic acid fragmentation, and apoptosis. These findings suggest that CIIA is an endogenous antagonist of both ASK1- and CAD-mediated signaling.

Negative regulation of SEK1 signaling by serum‐ and glucocorticoid‐inducible protein kinase 1

Serum‐ and glucocorticoid‐inducible protein kinase 1 (SGK1) has been implicated in diverse cellular activities including the promotion of cell survival. The molecular mechanism of the role of SGK1 in protection against cellular stress has remained unclear, however. We have now shown that SGK1 inhibits the activation of SEK1 and thereby negatively regulates the JNK signaling pathway. SGK1 was found to physically associate with SEK1 in intact cells. Furthermore, activated SGK1 mediated the phosphorylation of SEK1 on serine 78, resulting in inhibition of the binding of SEK1 to JNK1, as well as to MEKK1. Replacement of serine 78 of SEK1 with alanine abolished SGK1‐mediated SEK1 inhibition. Oxidative stress upregulated SGK1 expression, and depletion of SGK1 by RNA interference potentiated the activation of SEK1 induced by oxidative stress in Rat2 fibroblasts. Moreover, such SGK1 depletion prevented the dexamethasone‐induced increase in SGK1 expression, as well as the inhibitory effects of dexamethasone on paclitaxel‐induced SEK1‐JNK signaling and apoptosis in MDA‐MB‐231 breast cancer cells. Together, our results suggest that SGK1 negatively regulates stress‐activated signaling through inhibition of SEK1 function.

Daxx mediates activation‐induced cell death in microglia by triggering MST1 signalling

Microglia, the resident macrophages of the mammalian central nervous system, migrate to sites of tissue damage or infection and become activated. Although the persistent secretion of inflammatory mediators by the activated cells contributes to the pathogenesis of various neurological disorders, most activated microglia eventually undergo apoptosis through the process of activation‐induced cell death (AICD). The molecular mechanism of AICD, however, has remained unclear. Here, we show that Daxx and mammalian Ste20‐like kinase‐1 (MST1) mediate apoptosis elicited by interferon‐γ (IFN‐γ) in microglia. IFN‐γ upregulated the expression of Daxx, which in turn mediated the homodimerization, activation, and nuclear translocation of MST1 and apoptosis in microglial cells. Depletion of Daxx or MST1 by RNA interference also attenuated IFN‐γ‐induced cell death in primary rat microglia. Furthermore, the extent of IFN‐γ‐induced death of microglia in the brain of MST1‐null mice was significantly reduced compared with that apparent in wild‐type mice. Our results thus highlight new functions of Daxx and MST1 that they are the key mediators of microglial cell death initiated by the proinflammatory cytokine IFN‐γ.

Arginine methylation-dependent regulation of ASK1 signaling by PRMT1

Protein arginine methylation, catalyzed by protein arginine methyltransferases (PRMTs), is implicated in modulation of cellular processes including gene transcription. The role of PRMTs in the regulation of intracellular signaling pathways has remained obscure, however. We now show that PRMT1 methylates apoptosis signal-regulating kinase 1 (ASK1) at arginine residues 78 and 80 and thereby negatively regulates ASK1 signaling. PRMT1-mediated ASK1 methylation attenuated the H2O2-induced stimulation of ASK1, with this inhibitory effect of PRMT1 being abolished by replacement of arginines 78 and 80 of ASK1 with lysine. Furthermore, depletion of PRMT1 expression by RNA interference potentiated H2O2-induced stimulation of ASK1. PRMT1-mediated ASK1 methylation promoted the interaction between ASK1 and its negative regulator thioredoxin, whereas it abrogated the association of ASK1 with its positive regulator TRAF2. Moreover, PRMT1 depletion potentiated paclitaxel-induced ASK1 activation and apoptosis in human breast cancer cells. Together, our results indicate that arginine methylation of ASK1 by PRMT1 contributes to the regulation of stress-induced signaling that controls a variety of cellular events including apoptosis.

Positive regulation of ASK1-mediated c-Jun NH 2 -terminal kinase signaling pathway by the WD-repeat protein Gemin5

Gemin5 is a 170-kDa WD-repeat-containing protein that was initially identified as a component of the survival of motor neurons (SMN) complex. We now show that Gemin5 facilitates the activation of apoptosis signal-regulating kinase 1 (ASK1) and downstream signaling. Gemin5 physically interacted with ASK1 as well as with the downstream kinases SEK1 and c-Jun NH2-terminal kinase (JNK1), and it potentiated the H2O2-induced activation of each of these kinases in intact cells. Moreover, Gemin5 promoted the binding of ASK1 to SEK1 and to JNK1, as well as the ASK1-induced activation of JNK1. In comparison, Gemin5 did not physically associate with MKK7, MKK3, MKK6, or p38. Furthermore, depletion of endogenous Gemin5 by RNA interference (RNAi) revealed that Gemin5 contributes to the activation of ASK1 and JNK1, and to apoptosis induced by H2O2 and tumor necrosis factor-α (TNFα) in HeLa cells. Together, our results suggest that Gemin5 functions as a scaffold protein for the ASK1–JNK1 signaling module and thereby potentiates ASK1-mediated signaling events.

CIIA functions as a molecular switch for the Rac1-specific GEF activity of SOS1

CIIA mediates the TGF-β–induced activation of SOS1–Rac1 signaling and cell migration.

CIIA negatively regulates the Ras-Erk1/2 signaling pathway through inhibiting the Ras-specific GEF activity of SOS1

ABSTRACT Son of sevenless 1 (SOS1) is a Ras-specific guanine-nucleotide-exchange factor (GEF) that mediates intracellular signaling processes induced by receptor tyrosine kinases. In this study, we show that CIIA (also known as VPS28) physically associates with SOS1 and thereby inhibits the GEF activity of SOS1 on Ras, which prevents the epidermal growth factor (EGF)-induced activation of the Ras–Erk1/2 pathway. Furthermore, CIIA inhibited cyclin D1 expression, as well as DNA, synthesis in response to EGF. Intriguingly, CIIA failed to inhibit the Ras-specific GEF activity of Noonan-syndrome-associated SOS1 mutants (M269R, R552G, W729L and E846K). Taken together, our results suggest that CIIA functions as a negative modulator of the SOS1–Ras signaling events initiated by peptide growth factors including EGF.

CIB1 protects against MPTP-induced neurotoxicity through inhibiting ASK1

Calcium and integrin binding protein 1 (CIB1) is a calcium-binding protein that was initially identified as a binding partner of platelet integrin αIIb. Although CIB1 has been shown to interact with multiple proteins, its biological function in the brain remains unclear. Here, we show that CIB1 negatively regulates degeneration of dopaminergic neurons in a mouse model of Parkinson’s disease using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Genetic deficiency of the CIB1 gene enhances MPTP-induced neurotoxicity in dopaminergic neurons in CIB1−/− mice. Furthermore, RNAi-mediated depletion of CIB1 in primary dopaminergic neurons potentiated 1-methyl-4-phenyl pyrinidium (MPP+)-induced neuronal death. CIB1 physically associated with apoptosis signal-regulating kinase 1 (ASK1) and thereby inhibited the MPP+-induced stimulation of the ASK1-mediated signaling cascade. These findings suggest that CIB1 plays a protective role in MPTP/MPP+-induced neurotoxicity by blocking ASK1-mediated signaling.