Protein kinase CK2 phosphorylates the Fas-associated factor FAF1 in vivo and influences its transport into the nucleus

Fas-associated factor 1 induces the accumulation of α-synuclein through autophagic suppression in dopaminergic neurons

Impairment of protein clearance mechanisms leads to α-synuclein accumulation in dopaminergic neurons, contributing to the pathogenesis of Parkinson's disease (PD). Based on the finding that Fas-associated factor 1 (FAF1), a positive modulator of PD, colocalizes with α-synuclein in PD patient brains, we investigated the existence of pathological interplay between FAF1 and α-synuclein. Monomeric and high-molecular-weight forms of α-synuclein were increased in FAF1-overexpressing SH-SY5Y cells. In particular, α-synuclein turnover was accelerated by genetic depletion of FAF1 in SH-SY5Y cells. Therefore, we questioned whether FAF1 is involved in the α-synuclein clearance process. Autophagy inhibitors, but not proteasome inhibitors, restored concurrent attenuation of α-synuclein expression by FAF1 depletion in SH-SY5Y cells. Moreover, we found alterations in autophagy markers in SH-SY5Y cells caused by FAF1 overexpression, indicating that FAF1 disturbed α-synuclein clearance through the autophagy-lysosome pathway. Indeed, FAF1 activated the mammalian target of rapamycin (mTOR) pathway, subsequently suppressing autophagosome formation. Consistently, α-synuclein-mediated mitochondrial dysfunction was observed in FAF1-overexpressing SH-SY5Y cells. Furthermore, FAF1 overexpression using stereotaxic injection of adeno-associated virus led to α-synuclein accumulation and autophagy dysregulation in the PD model mice. Taken together, our results reveal a novel role for FAF1: that of a negative regulator of autophagic α-synuclein clearance.

Identification of two independent SUMO-interacting motifs in Fas-associated factor 1 (FAF1): Implications for mineralocorticoid receptor (MR)-mediated transcriptional regulation

Fas-associated factor 1 (FAF1) was originally isolated as a Fas-associated factor and was subsequently found to interact with numerous other proteins that are involved in various cellular events including Fas-mediated apoptosis, nuclear factor (NF)-κB, Wnt/β-catenin, and transforming growth factor (TGF)-β signaling pathways, mineralocorticoid receptor (MR)-mediated transactivation, and ubiquitin-dependent processes. Herein, we defined two small ubiquitin-like modifier (SUMO)-interacting motifs (SIMs) within FAF1 and demonstrated to be crucial for transcriptional modulation of the MR. Our study demonstrated that the SIMs of FAF1 do not play a significant role in regulating its subcellular localization, Fas-mediated apoptosis, or NF-κB or Wnt/β-catenin pathways. Remarkably, FAF1 interacts with the sumoylated MR and represses aldosterone-activated MR transactivation in a SIM-dependent manner. Moreover, silencing of endogenous FAF1 in cells resulted in an increase in the induction of MR target genes by aldosterone, indicating that FAF1 functions as an MR co-repressor. We further provide evidence to suggest that the mechanisms of FAF1/SIM-mediated MR transrepression involve inhibition of MR N/C interactions and promotion of MR polyubiquitination and degradation. Sumoylation has been linked to impacting of repressive properties on several transcription factors and cofactors. Our findings therefore provide mechanistic insights underlying SUMO-dependent transcriptional repression of the MR.

Fas-associated protein factor 1 is involved in meiotic resumption in mouse oocytes

Fas-associated protein factor 1 (FAF1) is a Fas-associated protein that functions in multiple cellular processes. Previous research showed that mutations in Faf1 led to the lethality of cleavage stage embryos in a mouse model. The aim of the present study was to analyze the expression pattern, localization, and function of FAF1 in meiotic resumption of mouse oocytes. FAF1 was exclusively expressed in oocytes at various follicular stages within the ovary and was predominantly localized in the cytoplasm of growing oocytes. Furthermore, Faf1 mRNA and protein were persistently present during oocyte maturation and Faf1 mRNA levels were similar in the germinal vesicle (GV), GV breakdown (GVBD), and metaphase II (MII) stages of oocytes. Moreover, knockdown of Faf1 in GV-stage oocytes led to a significantly decreased rate of GVBD. To our knowledge, these results provide the first evidence regarding a novel function of FAF1 in meiotic resumption in mouse oocytes.

TKI-addicted ROS1-rearranged cells are destined to survival or death by the intensity of ROS1 kinase activity

ROS1 rearrangement is observed in 1–2% of non-small cell lung cancers (NSCLC). The ROS1 tyrosine kinase inhibitor (TKI) crizotinib has induced marked tumour shrinkage in ROS1-rearranged cancers. However, emergence of acquired resistance to TKI is inevitable within a few years. Previous findings indicate that cabozantinib overcomes secondary mutation–mediated crizotinib-resistance in ROS1-fusion-positive cells. Here we attempted to establish cabozantinib-resistant cells by N-ethyl-N-nitrosourea mutagenesis screening using CD74-ROS1–expressing Ba/F3 cells. Two resistant cell lines with CD74-ROS1 F2004V or F2075C mutations, which are homologous to ALK F1174 or F1245 mutations, survived in the presence of a low dose of ROS1-TKI. Removal of ROS1-TKI from these TKI-addicted cells induced excessive activation of ROS1 tyrosine kinase followed by apoptosis. We succeeded in recapturing the TKI-addicted phenotype using doxycycline-inducible CD74-ROS1 mutant over-expression in Ba/F3 cells, suggesting that excessive ROS1 oncogenic signaling itself induced apoptosis instead of cell growth. Phosphoproteomic analysis and high-throughput inhibitor screening revealed that excessive ROS1 signaling in the TKI-addicted cells phosphorylated or activated apoptosis-related molecules such as FAF1 or p38. Collectively, our findings partly clarify molecular mechanisms of excessive ROS1 oncogenic signaling that mediates paradoxical induction of apoptosis.

FAF1 mediates regulated necrosis through PARP1 activation upon oxidative stress leading to dopaminergic neurodegeneration

Cumulative damage caused by oxidative stress results in diverse pathological conditions. Therefore, elucidating the molecular mechanisms underlying cell death following oxidative stress is important. Here, we describe a novel role for Fas-associated factor 1 (FAF1) as a crucial regulator of necrotic cell death elicited by hydrogen peroxide. Upon oxidative insult, FAF1 translocated from the cytoplasm to the nucleus and promoted the catalytic activation of poly(ADP-ribose) polymerase 1 (PARP1) through physical interaction. Moreover, FAF1 depletion prevented PARP1-linked downstream events involved in the triggering of cell death, including energetic collapse, mitochondrial depolarization and nuclear translocation of apoptosis-inducing factor (AIF), implying that FAF1 has a key role in PARP1-dependent necrosis in response to oxidative stress. We further investigated whether FAF1 might contribute to the pathogenesis of Parkinson's disease through excessive PARP1 activation. Indeed, the overexpression of FAF1 using a recombinant adeno-associated virus system in the mouse ventral midbrain promoted PARP1 activation and dopaminergic neurodegeneration in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease. Collectively, our data demonstrate the presence of an FAF1-PARP1 axis that is involved in oxidative stress-induced necrosis and in the pathology of Parkinson's disease.

FAS-associated factor 1 (FAF1): diverse functions and implications for oncogenesis

FAS-associated factor 1, FAF1, is an evolutionarily conserved protein that has several protein interaction domains. Although FAF1 was initially identified as a member of the FAS death-inducing signaling complex, subsequent work has revealed that FAF1 functions in diverse biological processes. FAF1 has been shown to play an important role in normal development and neuronal cell survival, whereas FAF1 downregulation may contribute to multiple aspects of tumorigenesis. In particular, there is compelling evidence implicating FAF1 as a tumor suppressor involved in the regulation of apoptosis and NFkappaB activity, as well as in ubiquitination and proteasomal degradation. Here, we highlight FAF1's role in NFkappaB signaling and postulate that this pathway has critical connotations for the pathogenesis and treatment of human cancers.

Protein kinase CK2 in health and disease: CK2: a key player in cancer biology

Elevated levels of protein kinase CK2 (formerly casein kinase 2 or II) have long been associated with increased cell growth and proliferation both in normal and cancer cells. The ability of CK2 to also act as a potent suppressor of apoptosis offers an important link to its involvement in cancer since deregulation of both cell proliferation and apoptosis are among the key features of cancer cell biology. Dysregulated CK2 may impact both of these processes in cancer cells. All cancers that have been examined show increased CK2 expression, which may also relate to prognosis. The extensive involvement of CK2 in cancer derives from its impact on diverse molecular pathways controlling cell proliferation and cell death. Downregulation of CK2 by various approaches results in induction of apoptosis in cultured cell and xenograft cancer models suggesting its potential as a therapeutic target.

Impact of protein kinase CK2 on inhibitor of apoptosis proteins in prostate cancer cells

We have previously demonstrated that protein kinase CK2 is a potent suppressor of apoptosis in cells subjected to diverse mediators of apoptosis. The process of apoptosis involves a complex series of molecules localized in various cellular compartments. Among the various proteins that modulate apoptotic activity are inhibitors of apoptosis proteins (IAPs) which are elevated in cancers and have been proposed to block caspase activity. We have examined the impact of CK2 signal on these proteins in prostate cancer cells. Cellular IAPs demonstrate distinct localization and responsiveness to altered CK2 expression or activity in the cytoplasmic and nuclear matrix fractions. Modulation of cellular CK2 by various approaches impacts on cellular IAPs such that inhibition or downregulation of CK2 results in reduction in these proteins. Further, IAPs are also reduced when cells are treated with sub-optimal concentrations of chemical inhibitors of CK2 combined with low or sub-optimal levels of apoptosis-inducing agents (such as etoposide) suggesting that downregulation of CK2 sensitizes cells to induction of apoptosis which may be related to attenuation of IAPs. Decreased IAP protein levels in response to apoptotic agents such as TNFalpha or TRAIL were potently blocked upon forced overexpression of CK2 in cells. Together, our results suggest that one of the modes of CK2-mediated modulation of apoptotic activity is via its impact on cellular IAPs.

Phosphorylation of murine caspase-9 by the protein kinase casein kinase 2 regulates its cleavage by caspase-8

Previous studies from our laboratory had indicated that cytochrome c-independent processing and activation of caspase-9 by caspase-8 contributed to early amplification of the caspase cascade in tumor necrosis factor (TNF)-alpha-treated murine cells. Here we show that murine caspase-9 is phosphorylated by casein kinase 2 (CK2) on a serine near the site of caspase-8 cleavage. CK2 has been shown to regulate cleavage of the pro-apoptotic Bid protein by phosphorylating serine residues near its caspase-8 cleavage site. Similarly, CK2 modification of Ser(348) on caspase-9 appears to render the protease refractory to cleavage by active caspase-8. This phosphorylation did not affect the ability of caspase-9 to autoprocess. Substitution of Ser(348) abolished phosphorylation but not cleavage, and a phospho-site mutant promoted apoptosis in TNF-alpha-treated caspase-9 knock-out mouse embryo fibroblasts. Furthermore, inhibition of CK2 activity and RNA interference-mediated knockdown of the kinase accelerated caspase-9 activation, whereas phosphatase inhibition delayed both caspase-9 activation and death in response to TNF receptor occupation. Taken together, these studies show that TNF receptor cross-linking promotes dephosphorylation of caspase-9, rendering it susceptible to processing by activated caspase-8 protein. Thus, our data suggest that modification of procaspase-9 to protect it from inappropriate cleavage and activation is yet another mechanism by which the oncogenic kinase CK2 promotes survival.

FAF1 suppresses IkappaB kinase (IKK) activation by disrupting the IKK complex assembly

This study presents a molecular inhibitory mechanism by Fas-associated factor 1 (FAF1) on IkappaB kinase (IKK) activation, where divergent NF-kappaB-activating stimuli converge. FAF1 interacts with IKKbeta in response to proinflammatory stimuli (such as tumor necrosis factor-alpha, interleukin-1beta, and lipopolysaccharide) and suppresses IKK activation. Interaction of the leucine-zipper domain of IKKbeta with FAF1 affected the IKK heterocomplex (IKKalpha/beta) and homocomplex (IKKalpha/alpha, IKKbeta/beta) formations and attenuated IKKgamma recruitment to IKKbeta. Overexpression of FAF1 reduced the level of IKKbeta activity, whereas FAF1 depletion increased the activity. These results indicate that FAF1 inhibits IKK activation and its downstream signaling by interrupting the IKK complex assembly through physical interaction with IKKbeta. Taken together, FAF1 robustly suppresses NF-kappaB activation through the inhibition of IKK activation in combination with previously reported cytoplasmic retention of NF-kappaB p65 (Park, M. Y., Jang, H. D., Lee, S. Y., Lee, K. J., and Kim, E. (2004) J. Biol. Chem. 279, 2544-2549). Such redundant suppression would prevent inadvertent activation of the NF-kappaB pathway.

Identifying interaction motifs in CK2beta--a ubiquitous kinase regulatory subunit

Casein kinase 2 (CK2) is probably the most ubiquitous serine/threonine kinase found in eukaryotes: it phosphorylates >300 cellular proteins, ranging from transcription factors to proteins involved in chromatin structure and cell division. CK2 is a heterotetrameric enzyme that induces neoplastic growth when overexpressed. The beta subunit of CK2 (CK2beta) functions as the regulator of the catalytic CK2alpha and CK2alpha' subunits, enhancing their stability, activity and specificity. However, CK2beta also functions as a multisubstrate docking platform for several other binding partners. Here, we discuss the organization and roles of interaction motifs of CK2beta, postulate new protein-interaction sites and map these to the known interaction motifs, and show how the resulting complexity of interactions mediated by CK2 gives rise to the versatile functions of this pleiotropic protein kinase.

The PUB domain functions as a p97 binding module in human peptide N-glycanase

The AAA ATPase p97 is a ubiquitin-selective molecular machine involved in multiple cellular processes, including protein degradation through the ubiquitin-proteasome system and homotypic membrane fusion. Specific p97 functions are mediated by a variety of cofactors, among them peptide N-glycanase, an enzyme that removes glycans from misfolded glycoproteins. Here we report the three-dimensional structure of the aminoterminal PUB domain of human peptide N-glycanase. We demonstrate that the PUB domain is a novel p97 binding module interacting with the D1 and/or D2 ATPase domains of p97 and identify an evolutionary conserved surface patch required for p97 binding. Furthermore, we show that the PUB and UBX domains do not bind to p97 in a mutually exclusive manner. Our results suggest that PUB domain-containing proteins constitute a widespread family of diverse p97 cofactors.

The pleckstrin homology domain-containing protein CKIP-1 is involved in regulation of cell morphology and the actin cytoskeleton and interaction with actin capping protein

CKIP-1 is a pleckstrin homology domain-containing protein that interacts with protein kinase CK2. To elucidate the functions of CKIP-1, we generated human osteosarcoma cell lines with tetracycline-regulated expression of Flag-CKIP-1. Flag-CKIP-1 expression resulted in distinct changes in cellular morphology. Therefore, we examined the actin profile by immunofluorescence, quantitative measurement of phalloidin binding, and immunoblot analysis. These studies demonstrate that Flag-CKIP-1 expression resulted in increases in F-actin staining and protein levels of beta-actin. To elucidate the mechanisms behind the observed phenotype, we utilized tandem affinity purification to isolate CKIP-1 interacting proteins. Mass spectrometry analysis led to the identification of the actin capping protein subunits, CPalpha and CPbeta, as novel CKIP-1 interaction partners. Interactions were confirmed by coimmunoprecipitation and by colocalization. Furthermore, we demonstrate that Ser9 of CPalpha is phosphorylated by protein kinase CK2 in vitro, that CPalpha is phosphorylated in vivo, and that treatment with a CK2-specific inhibitor results in a decrease in CPalpha phosphorylation. Finally, we demonstrate that CKIP-1 and CK2 inhibit the activity of actin capping protein at the barbed ends of actin filaments. Overall, our results are consistent with CKIP-1 playing a role in the regulation of the actin cytoskeleton through its interactions with actin capping protein.