Role for the pleckstrin homology domain-containing protein CKIP-1 in AP-1 regulation and apoptosis

N-terminal PH domain and C-terminal auto-inhibitory region of CKIP-1 coordinate to determine its nucleus-plasma membrane shuttling

The pleckstrin homology (PH) domain-containing protein casein kinase 2 interacting protein-1 (CKIP-1) plays an important role in regulation of bone formation and muscle differentiation. How CKIP-1 localization is determined remains largely unclear. We observed that isolated CKIP-1-PH domain was predominantly localized in the nucleus and the C-terminus of CKIP-1 counteracted its nuclear localization. The net charge of basic residues and a serine-rich motif within the PH domain plays a pivotal role in the localization switch of both full-length CKIP-1 and the isolated PH domain. We propose that the N-terminal PH domain and C-terminal auto-inhibitory region of CKIP-1 coordinate to determine its subcellular localization and the nucleus-plasma membrane shuttling.

Activator protein-1 has an essential role in pancreatic cancer cells and is regulated by a novel Akt-mediated mechanism

Activator protein-1 (AP-1) regulates the expression of several genes involved in human tumorigenesis. However, there is little known about this transcription factor in pancreatic ductal adenocarcinoma. We recently found high levels of AP-1-binding activities and multiple AP-1/DNA complexes containing c-Jun, JunD, Fra1, and Fra2 in pancreatic cancer cells. Transient transfection assays indicated that AP-1 was functional and capable of transactivating its gene targets. Furthermore, a c-Jun transactivation mutant inhibited anchorage-dependent and anchorage-independent proliferation, suggesting that AP-1 had an essential role in pancreatic cancer cells. Our study also uncovered a novel mechanism by which protein kinase Akt controls c-Jun activity in pancreatic cancer cells. Indeed, distinct from its known ability to induce c-fos and fra1 and to stabilize c-Jun, Akt appeared to directly regulate the transcriptional activity of c-Jun independently of the phosphorylation sites targeted by c-Jun NH(2)-terminal kinase (Ser(63)/Ser(73)) and glycogen synthase kinase-3 (Thr(239)). Our data also suggest that growth factors might use this Akt-regulated mechanism to potently induce c-Jun targets such as cyclin D1. Collectively, our findings indicate that AP-1 has an important function in pancreatic cancer cells and provide evidence for a previously unknown Akt-mediated mechanism of c-Jun activation.

Two-dimensional blue native/SDS-PAGE analysis reveals heat shock protein chaperone machinery involved in hepatitis B virus production in HepG2.2.15 cells

Hepatitis B virus (HBV) infection is a major health concern with more than two billion individuals currently infected worldwide. Despite the prevalence of infection, gaining a complete understanding of the molecular mechanisms of HBV infection has been difficult because HBV cannot infect common immortalized cell lines. HepG2.2.15, however, is a well established version of the HepG2 cell line that constitutively expresses HBV. Therefore, comparative proteomics analysis of HepG2.2.15 and HepG2 may provide valuable clues for understanding the HBV virus life cycle. In this study, two-dimensional blue native/SDS-PAGE was utilized to characterize different multiprotein complexes from whole cell lysates between HepG2.2.15 and HepG2. These results demonstrate that two unique protein complexes existed in HepG2.2.15 cells. When these complexes were excised from the gel and subjected to the second dimension separation and the proteins were sequenced by mass spectrometry, 20 non-redundant proteins were identified. Of these proteins, almost 20% corresponded to heat shock proteins, including HSP60, HSP70, and HSP90. Antibody-based supershift assays were used to verify the validity of the distinct protein complexes. Co-immunoprecipitation assays confirmed that HSP60, HSP70, and HSP90 proteins physically interacted in HepG2.2.15 but not HepG2 cells. We further demonstrated that down-regulation of HSP70 or HSP90 by small interfering RNA significantly inhibited HBV viral production but did not influence cellular proliferation or apoptosis. Consistent with these results, a significant reduction in HepG2.2.15 HBV secretion was observed when the HSP90 inhibitor 17-allylamino-17-demethoxygeldanamycin was used to treat HepG2.2.15 cells. Collectively these results suggest that the interaction of HSP90 with HSP70/HSP60 contributes to the HBV life cycle by forming a multichaperone machine that may constitute therapeutic targets for HBV-associated diseases.

Targeting WW domains linker of HECT-type ubiquitin ligase Smurf1 for activation by CKIP-1

E3 ubiquitin ligases are final effectors of the enzyme cascade controlling ubiquitylation. A central issue in understanding their regulation is to decipher mechanisms of their assembly and activity. In contrast with RING-type E3s, fewer mechanisms are known for regulation of HECT-type E3s. Smad ubiquitylation regulatory factor 1 (Smurf1), a C2-WW-HECT-domain E3, is crucial for bone homeostasis, in which it suppresses osteoblast activity. However, whether and how its activity is regulated remains unclear. Here we show that Smurf1, but not Smurf2, interacts with casein kinase-2 interacting protein-1 (CKIP-1), resulting in an increase in its E3 ligase activity. Surprisingly, CKIP-1 targets specifically the linker region between the WW domains of Smurf1, thereby augmenting its affinity for and promoting ubiquitylation of the substrate. Moreover, CKIP-1-deficient mice undergo an age-dependent increase in bone mass as a result of accelerated osteogenesis and decreased Smurf1 activity. These findings provide evidence that the WW domains linker is important in complex assembly and in regulating activity of HECT-type E3s and that CKIP-1 functions as the first auxiliary factor to enhance the activation of Smurf1.

Ceap/BLOS2 interacts with BRD7 and selectively inhibits its transcription-suppressing effect on cellular proliferation-associated genes

The centrosome associated protein Ceap-16 (also termed BLOS2) can accelerate the proliferation of mouse fibroblast NIH3T3 cells, which mechanism remains unclear. Here we identified tumor suppressor candidate BRD7 (bromodomain containing protein 7), which could negatively regulate cell proliferation and growth, as a novel Ceap-16-interacting protein. Ceap-16 and BRD7 interacted with each other both in vitro and in vivo. The C-terminus of BRD7 and the central region of Ceap-16 mediated the interaction. Through this binding, Ceap-16 could translocate from cytoplasm to the nucleus where it selectively inhibited the transcriptional suppression activity of BRD7 towards certain target genes including E2F3 and cyclin A. Moreover, Ceap-16, BRD7 and histone H3/H4 could form a complex and Ceap-16 did not compete with BRD7 binding to histones. These findings suggest a novel function for Ceap-16 in the transcriptional regulation through associating with BRD7.

E3 ubiquitin ligase SIAH1 mediates ubiquitination and degradation of TRB3

Tribbles 3 homolog (TRB3) is recently identified as a scaffold-like regulator of various signal transducers and has been implicated in several processes including insulin signaling, NF-kappaB signaling, lipid metabolism and BMP signaling. To further understand cellular mechanisms of TRB3 regulation, we performed a yeast two-hybrid screen to identify novel TRB3 interacting proteins and totally obtained ten in-frame fused preys. Candidate interactions were validated by co-immunoprecipitation assays in mammalian cells. We further characterized the identified proteins sorted by Gene Ontology Annotation. Its interaction with the E3 ubiquitin ligase SIAH1 was further investigated. SIAH1 could interact with TRB3 both in vitro and in vivo. Importantly, SIAH1 targeted TRB3 for proteasome-dependent degradation. Cotransfection of SIAH1 could withdraw up-regulation of TGF-beta signaling by TRB3, suggesting SIAH1-induced degradation of TRB3 represents a potential regulatory mechanism for TGF-beta signaling.

Casein kinase 2-interacting protein-1, a novel Akt pleckstrin homology domain-interacting protein, down-regulates PI3K/Akt signaling and suppresses tumor growth in vivo

The serine/threonine kinase Akt plays a central role in cell survival and proliferation. Its activation is linked to tumorigenesis in several human cancers. Although many Akt substrates have been elucidated, the Akt-binding proteins that regulate Akt function remain unclear. We report herein having identified casein kinase 2-interacting protein-1 (CKIP-1) as an Akt pleckstrin homology (PH) domain-binding protein with Akt inhibitory function. CKIP-1 formed a complex with each Akt isoform (Akt1, Akt2, and Akt3) via its NH2 terminus. Dimerization of CKIP-1 via its leucine zipper (LZ) motif at the COOH terminus was found to be associated with Akt inactivation because deletion of the LZ motif eliminated Akt inhibitory function, although it could still bind to Akt. Expression of the NH2 terminus-deleted CKIP-1 mutant containing the LZ motif, but lacking Akt-binding ability, induced Akt phosphorylation and activation by sequestering the ability of endogenous CKIP-1 to bind to Akt. Stable CKIP-1 expression caused Akt inactivation and cell growth inhibition in vitro. In addition, the growth of stable CKIP-1 transfectants xenografted into nude mice was slower than that of mock transfectants. These results indicate that CKIP-1, a novel Akt PH domain-interacting protein, would be a candidate of tumor suppressor with an Akt inhibitory function.

The role of CKIP-1 in cell morphology depends on its interaction with actin-capping protein

CKIP-1 is a pleckstrin homology domain-containing protein that induces alterations of the actin cytoskeleton and cell morphology when expressed in human osteosarcoma cells. CKIP-1 interacts with the heterodimeric actin-capping protein in cells, so we postulated that this interaction was responsible for the observed cytoskeletal and morphological effects of CKIP-1. To test this postulate, we used peptide "walking arrays" and alignments of CKIP-1 with CARMIL, another CP-binding protein, to identify Arg-155 and Arg-157 of CKIP-1 as residues potentially required for its interactions with CP. CKIP-1 mutants harboring Arg-155 and Arg-157 substitutions exhibited greatly decreased CP binding, while retaining wild-type localization, the ability to interact with protein kinase CK2, and self-association. To examine the phenotype associated with expression of these mutants, we generated tetracycline-inducible human osteosarcoma cells lines expressing R155E,R157E mutants of CKIP-1. Examination of these cell lines reveals that CKIP-1 R155E,R157E did not induce the distinct changes in cell morphology and the actin cytoskeleton that are characteristic of wild-type CKIP-1 demonstrating that the interaction between CKIP-1 and CP is required for these cellular effects.

The PH domain containing protein CKIP-1 binds to IFP35 and Nmi and is involved in cytokine signaling

The pleckstrin homology domain-containing protein CKIP-1 is implicated in regulation of cell differentiation, apoptosis, cytoskeleton as well as recruitment of CK2 and ATM kinases to plasma membrane. Protein-protein interactions of CKIP-1 were required for these functions. Here we identify the IFN-induced protein IFP35 and its homologue Nmi as two novel CKIP-1 interacting partners. The NID domains of IFP35 and Nmi are required for the interactions. Similar to IFP35 and Nmi, CKIP-1 can be up-regulated dramatically by IFN-gamma and IL-2 and form homodimer and homotrimer in vivo. Nmi stabilizes IFP35, whereas CKIP-1 destabilizes IFP35 via inhibiting IFP35-Nmi interaction. The ratio of Nmi to CKIP-1 determines the stability of IFP35 and control cytokine signaling in a novel mechanism. Importantly, similar to Nmi and contrast to IFP35, CKIP-1 inhibits tumor cell growth and Akt-mediated cell survival. Thus, our results provide a novel role of CKIP-1 in cytokine signaling response and the biochemical mechanism, by which two previously identified modulators IFP35 and Nmi are involved via interactions.

Adenosine diphosphate ribosyl transferase and x-ray repair cross-complementing 1 polymorphisms in gastric cardia cancer

BACKGROUND & AIMS

Adenosine diphosphate ribosyl transferase (ADPRT) and x-ray repair cross-complementing 1 (XRCC1) are major DNA base excision repair proteins acting interactively in repair processes. This study examined the effects of ADPRT Val762Ala and XRCC1 Arg399Gln polymorphisms on ADPRT-XRCC1 interaction in vitro in cells and their contributions to gastric cardia adenocarcinoma (GCA) risk.

METHODS

The ADPRT-XRCC1 interaction in cells transfected with ADPRT and XRCC1 variant complementary DNA (cDNA) constructs were examined by immunoprecipitation and immunoblotting analysis. Genotypes were analyzed in 500 patients and 1000 controls, and odds ratios (ORs) were estimated by logistic regression.

RESULTS

Interactions between ADPRT-762Val and XRCC1-399Arg or XRCC1-399Gln were robust, but interactions between ADPRT-762Ala and either XRCC1-399Arg or XRCC1-399Gln were very weak. A case-control analysis showed ORs of 2.17 (95% CI, 1.55-3.04) and 1.61 (95% CI, 1.06-2.44) for GCA in the ADPRT Ala/Ala or XRCC1 Gln/Gln genotype carriers, respectively, compared with noncarriers. Gene-gene interaction of ADPRT and XRCC1 polymorphisms increased the OR of GCA in a multiplicative manner (OR for the presence of both ADPRT Ala/Ala and XRCC1 Gln/Gln genotypes, 6.43; 95% CI, 1.80-22.97). A supermultiplicative joint effect between the ADPRT polymorphism and smoking was observed. The ORs (95% CIs) of the Ala/Ala genotype for nonsmokers and smokers who smoked < or = 24 or > 24 pack-years were 1.44 (0.89-2.32), 2.00 (1.09-3.67), or 3.19 (1.59-6.42), respectively (Ptrend test = .008).

CONCLUSIONS

The ADPRT and XRCC1 polymorphisms confer host susceptibility to GCA, which might result from reduced ADPRT-XRCC1 interaction and attenuated base excision repair capacity.

ErbB-4 s80 intracellular domain abrogates ETO2-dependent transcriptional repression

ErbB-4 is cleaved by alpha- and gamma-secretases to release a soluble 80-kDa intracellular domain, termed s80, which translocates to the nucleus. s80 is present in the nucleus of normal and cancerous mammary cells and is predicted to have a role in cell differentiation. To further investigate the mechanism by which s80 may mediate differentiation, we tested whether s80 regulates Eto2, a transcriptional corepressor that is involved in erythrocyte differentiation and is also implicated in human breast cancer. Here we show that ligand binding to ErbB-4 causes s80 translocation to the nucleus, where it colocalizes and interacts with Eto2. Expression of s80 blocks Eto2-mediated transcriptional repression of a heterologous promoter. This effect on Eto2 does not require s80 kinase activity and is mediated by the carboxyl-terminal region of s80. Although other cell surface receptors regulate transcription by activating signal transduction cascades, these data present a novel mechanism of corepressor regulation and suggest a role for Eto2 in ErbB-4-dependent differentiation.

The shape of things to come: an emerging role for protein kinase CK2 in the regulation of cell morphology and the cytoskeleton

Protein kinase CK2 is a highly conserved, pleiotropic, protein serine/threonine kinase that is essential for life in eukaryotes. CK2 has been implicated in diverse cellular processes such as cell cycle regulation, circadian rhythms, apoptosis, transformation and tumorigenesis. In addition, there is increasing evidence that CK2 is involved in the maintenance of cell morphology and cell polarity, and in the regulation of the actin and tubulin cytoskeletons. Accordingly, this review will highlight published evidence in experimental models ranging from yeast to mammals documenting the emerging roles of protein kinase CK2 in the regulation of cell polarity, cell morphology and the cytoskeleton.