Trap RACK1 with Ras to mobilize Src signaling at syndecan-2/p120-GAP upon transformation with oncogenic ras

The Cardiac Syndecan-2 Interactome

The extracellular matrix (ECM) is important in cardiac remodeling and syndecans have gained increased interest in this process due to their ability to convert changes in the ECM to cell signaling. In particular, syndecan-4 has been shown to be important for cardiac remodeling, whereas the role of its close relative syndecan-2 is largely unknown in the heart. To get more insight into the role of syndecan-2, we here sought to identify interaction partners of syndecan-2 in rat left ventricle. By using three different affinity purification methods combined with mass spectrometry (MS) analysis, we identified 30 novel partners and 9 partners previously described in the literature, which together make up the first cardiac syndecan-2 interactome. Eleven of the novel partners were also verified in HEK293 cells (i.e., AP2A2, CAVIN2, DDX19A, EIF4E, JPH2, MYL12A, NSF, PFDN2, PSMC5, PSMD11, and RRAD). The cardiac syndecan-2 interactome partners formed connections to each other and grouped into clusters mainly involved in cytoskeletal remodeling and protein metabolism, but also into a cluster consisting of a family of novel syndecan-2 interaction partners, the CAVINs. MS analyses revealed that although syndecan-2 was significantly enriched in fibroblast fractions, most of its partners were present in both cardiomyocytes and fibroblasts. Finally, a comparison of the cardiac syndecan-2 and -4 interactomes revealed surprisingly few protein partners in common.

PKM2 regulates neural invasion of and predicts poor prognosis for human hilar cholangiocarcinoma

BACKGROUND

The therapeutic and prognostic value of the glycolytic enzymes hexokinase, phosphofructokinase, and pyruvate kinase (PK) has been implicated in a variety of cancers, while their roles in treatment of and prognosis for hilar cholangiocarcinoma (HC) remain unclear. In this study, we determined the expression of PKM2 in and its impact on biology and clinical outcome of human HC.

METHODS

The regulation and function of PKM2 in HC pathogenesis was evaluated using human tissues, molecular and cell biology, and animal models, and its prognostic significance was determined according to its impact on patient survival.

RESULTS

We found that expression of hexokinase 1 and the M2 splice isoform of PK (PKM2) was upregulated in HC tissues and that this expression correlated with tumor recurrence and outcome. PKM2 expression was increased in HC cases with chronic cholangitis as demonstrated by isobaric tags for relative and absolute quantification. High PKM2 expression was highly correlated with high syndecan 2 (SDC2) expression and neural invasion. PKM2 downregulation led to a decrease in SDC2 expression. Treatment with metformin markedly suppressed PKM2 and SDC2 expression at both the transcriptional and posttranscriptional levels and inhibited HC cell proliferation and tumor growth.

CONCLUSIONS

PKM2 regulates neural invasion of HC cells at least in part via regulation of SDC2. Inhibition of PKM2 and SDC2 expression contributes to the therapeutic effect of metformin on HC. Therefore, PKM2 is an independent prognostic factor and potential therapeutic target for human HC.

Targeted inhibition of T-cell factor activity promotes syndecan-2 expression and sensitization to doxorubicin in osteosarcoma cells and bone tumors in mice

Alterations of Wnt signaling appear to be involved in the pathogenesis of osteosarcoma, presenting mutations of adenomatous polyposis coli (APC) and epigenetic downregulation of Wnt inhibitory factor 1. However, the precise role of Wnt effectors in the bone cancer progression remains unclear. We previously showed that Wnt/β-catenin/T-cell factor (TCF) activation are responsible for the repression of syndecan-2, a key modulator of apoptosis and chemosensitivity in osteosarcoma cells, suggesting a role of Wnt signaling in chemoresistance. In this study, we investigated the functional relationship between syndecan-2, Wnt/β-catenin/TCF signaling and chemosensitivity in these cells. To this goal, we selected resistant osteosarcoma cells from sensitive human cell lines using repeated exposures to doxorubicin. In doxorubicin-responsive but not in doxorubicin-resistant-derived cells syndecan-2 expression was upregulated by doxorubicin treatment. Moreover, syndecan-2 overexpression restored the sensitivity to doxorubicin in resistant-derived cells. We found that syndecan-2 induction by doxorubicin is forkhead box protein O3A (Foxo3a)-dependent. Foxo3a overexpression resulted in increased syndecan-2 expression in sensitive and resistant-derived cells. Doxorubicin modulated Foxo3a binding on syndecan-2 gene promoter and induced Foxo-dependent inhibition of Wnt/TCF activity. Conversely, β-catenin/TCF activation impaired syndecan-2 induction by doxorubicin, indicating that Wnt signaling is competing with the action of the cytotoxic drug. However, β-catenin was also found to be required for Foxo3a activity. Consistently, Dickkopf 1 (DKK1) and secreted frizzled-related protein 1 (sFRP-1) altered doxorubicin action in sensitive cells, whereas inhibition of TCF activity strongly decreased cell viability and increased sensitivity to doxorubicin in sensitive and resistant cells. TCF inhibition also increased the effect of doxorubicin treatment in an orthotopic bone tumor model in mice. Altogether, these data provide evidence that the repression of syndecan-2 by Wnt/β-catenin/TCF signaling contributes to the resistance of osteosarcoma cells to doxorubicin and suggest that TCF inhibition may represent a novel therapeutic strategy in osteosarcoma.

DcR3 binds to ovarian cancer via heparan sulfate proteoglycans and modulates tumor cells response to platinum with corresponding alteration in the expression of BRCA1

BACKGROUND

Overcoming platinum resistance is a major obstacle in the treatment of Epithelial Ovarian Cancer (EOC). In our previous work Decoy Receptor 3 (DcR3) was found to be related to platinum resistance. The major objective of this work was to define the cellular interaction of DcR3 with EOC and to explore its effects on platinum responsiveness.

METHODS

We studied cell lines and primary cultures for the expression of and the cells ability to bind DcR3. Cells were cultured with DcR3 and then exposed to platinum. Cell viability was determined by MTT assay. Finally, the cells molecular response to DcR3 was studied using real time RT-PCR based differential expression arrays, standard RT-PCR, and Western blot.

RESULTS

High DcR3 in the peritoneal cavity of women with EOC is associated with significantly shorter time to first recurrence after platinum based therapy (p = 0.02). None-malignant cells contribute DcR3 in the peritoneal cavity. The cell lines studied do not secrete DcR3; however they all bind exogenous DcR3 to their surface implying that they can be effected by DcR3 from other sources. DcR3s protein binding partners are minimally expressed or negative, however, all cells expressed the DcR3 binding Heparan Sulfate Proteoglycans (HSPGs) Syndecans-2, and CD44v3. DcR3 binding was inhibited by heparin and heparinase. After DcR3 exposure both SKOV-3 and OVCAR-3 became more resistant to platinum with 15% more cells surviving at high doses. On the contrary CaOV3 became more sensitive to platinum with 20-25% more cell death. PCR array analysis showed increase expression of BRCA1 mRNA in SKOV-3 and OVCAR-3 and decreased BRCA1 expression in CaOV-3 after exposure to DcR3. This was confirmed by gene specific real time PCR and Western blot analysis.

CONCLUSIONS

Non-malignant cells contribute to the high levels of DcR3 in ovarian cancer. DcR3 binds readily to EOC cells via HSPGs and alter their responsiveness to platinum chemotherapy. The paradoxical responses seen were related to the expression pattern of HSPGs available on the cells surface to interact with. Although the mechanism behind this is not completely known alterations in DNA repair pathways including the expression of BRCA1 appear to be involved.

Interaction of integrin beta1 with cytokeratin 1 in neuroblastoma NMB7 cells

Cytokeratin 1, an intermediate filament keratin, was isolated as a partner of the tyrosine kinase Src from neuroblastoma NMB7 cells. The cytokeratin 1-Src complex was found to be associated with the molecular scaffolder RACK1 (receptor for activated protein kinase C). Interestingly, the cytokeratin 1-Src-RACK1 complex was found to actively bind with membrane receptors such as integrin beta1. We are interested in using this complex to find downstream kinases and phosphatases that bind upon cytokine stimulation, especially during neurogenesis.

An enhanced association of RACK1 with Abl in cells transfected with oncogenic ras

The cellular RACK1 was shown in association with Abl in BALB/3T3 cells transfected with S-ras(Q(61)K) by immunoprecipitation. An identical finding was demonstrated with cells transfected with the embryonic E-ras, but not in cells without transformation. The Abl-RACK1 of transformed cells as resolvable with Triton X-114 was found with little affinity for FAK, PY(397)-FAK and integrin. Of interests, PY(397)-FAK in the membrane skeleton of transformed cells was shown in significant quantities on the Western blot. However the PY(397)-FAK of transformed cells was not functionally able to react with RACK1 and recruit cytokeratin-1, a substrate of Src, indicating that PY(397)-FAK is not operative to transmit integrin signals. In other words, the Abl-RACK1 of transformed cells cannot replace the Src-RACK1 of cells without transformation to bridge PY(397)-FAK and cytokeratin-1 for integrin signals, and the formation of Abl-RACK1 in transformed cells may block the association of PY(397)-FAK-RACK1. We characterized Abl and RACK1 from transformed cells by chromatography on a HiTrap-PEP(Taxol) affinity column, constructed from a beta-tubulin peptide specific for Taxol binding (PEP(Taxol)). However, the Triton X-100 cannot achieve the same resolution of Abl-RACK1 from plasma membrane as is shown with Triton X-114. A significant fraction of Abl was deposited at the membrane skeleton and was therefore not accessible with Triton X-100. Half of Abl resolved with Triton X-100 was demonstrated to have catalytic activity as shown with positive phosphotyrosine staining on the Western blot and competitive elution with a specific phosphate, such as sodium beta-glycerophosphate, from HiTrap-PEP(Taxol), but this was not associated with RACK1. No significant difference of RACK1 was found in Triton X-100 resolvable membrane preparations from cells with and without transformations. Future studies are planned to differentiate the mechanism operative for RACK1 associated and RACK1 freed Abl in cells transformed with oncogenic ras.

Shift syndecan-2 from RACK1 to caveolin-2 upon transformation with oncogenic ras

Syndecan-2 was found to detach from RACK1 and associate with caveolin-2 and Ras in cells transformed with oncogenic ras. Most of syndecan-2 from transformed cells was revealed with negligible phosphorylations at tyrosine residues. We experimented with HeLa cells transfected with plasmids encoding syndecan-2 and its mutants (syndecan-2(Y180F), syndecan-2(Y192F), and syndecan-2(Y180,192F)) to provide evidences that PY180 of syndecan-2 is a binding site for RACK1 and is deprived in cells transfected with oncogenic ras. However, in HeLa cells transfected with syndecan-2(Y180F), RACK1 was found to sustain its reactions with syndecan-2 independent of phosphorylation. The finding of syndecan-2 reactive with caveolin-2/Ras suggests the molecular complex most likely to obstruct RACK1 for functional attachment at syndecan-2, as revealed in cells transfected with oncogenic ras. We provided evidences to reinforce the view that molecular rearrangements upon transformation are specific and interesting.

Insulin-like growth factor I controls a mutually exclusive association of RACK1 with protein phosphatase 2A and beta1 integrin to promote cell migration

The WD repeat scaffolding protein RACK1 can mediate integration of the insulin-like growth factor I receptor (IGF-IR) and integrin signaling in transformed cells. To address the mechanism of RACK1 function, we searched for regulatory proteins that associate with RACK1 in an IGF-I-dependent manner. The serine threonine phosphatase protein phosphatase 2A (PP2A) was found associated with RACK1 in serum-starved cells, and it dissociated immediately upon stimulation with IGF-I. This dissociation of PP2A from RACK1 and an IGF-I-mediated decrease in cellular PP2A activity did not occur in cells expressing either the serine 1248 or tyrosine 1250/1251 mutants of the IGF-IR that do not interact with RACK1. Recombinant RACK1 could bind to PP2A in vitro and restore phosphatase activity to PP2A from IGF-I-stimulated cells. Ligation of integrins with fibronectin or Matrigel was sufficient to facilitate IGF-I-mediated dissociation of PP2A from RACK1 and also to recruit beta1 integrin as PP2A dissociated. By using TAT-fused N-terminal and C-terminal deletion mutants of RACK1, we determined that both PP2A and beta1 integrin interact in the C terminus of RACK1 within WD repeats 4 to 7. This suggests that integrin ligation displaces PP2A from RACK1. MCF-7 cells overexpressing RACK1 exhibited enhanced motility, which could be reversed by the PP2A inhibitor okadaic acid. Small interfering RNA-mediated suppression of RACK1 also decreased the migratory capacity of DU145 cells. Taken together, our findings indicate that RACK1 enhances IGF-I-mediated cell migration through its ability to exclusively associate with either beta1 integrin or PP2A in a complex at the IGF-IR.

K-ras as a target for cancer therapy

The central role K-, H- and N-Ras play in regulating diverse cellular pathways important for cell growth, differentiation and survival is well established. Dysregulation of Ras proteins by activating mutations, overexpression or upstream activation is common in human tumors. Of the Ras proteins, K-ras is the most frequently mutated and is therefore an attractive target for cancer therapy. The complexity of K-ras signaling presents many opportunities for therapeutic targeting. A number of different approaches aimed at abrogating K-ras activity have been explored in clinical trials. Several of the therapeutic agents tested have demonstrated clinical activity, supporting ongoing development of K-ras targeted therapies. However, many of the agents currently being evaluated have multiple targets and their antitumor effects may not be due to K-Ras inhibition. To date, no selective, specific inhibitor of K-ras is available for routine clinical use. In this review, we will summarize the structure and function of K-ras with attention to its role in tumorigenesis and discuss the successes and failures of the various strategies designed to therapeutically target this important oncogene.

Dimerize RACK1 upon transformation with oncogenic ras

From our previous studies, we learned that syndecan-2/p120-GAP complex provided docking site for Src to prosecute tyrosine kinase activity upon transformation with oncogenic ras. And, RACK1 protein was reactive with syndecan-2 to keep Src inactivated, but not when Ras was overexpressed. In the present study, we characterized the reaction between RACK1 protein and Ras. RACK1 was isolated from BALB/3T3 cells transfected with plasmids pcDNA3.1-[S-ras(Q61K)] of shrimp Penaeus japonicus and RACK1 was revealed to react with GTP-K(B)-Ras(Q61K), not GDP-K(B)-Ras(Q61K). This selective interaction between RACK1 and GTP-K(B)-Ras(Q61K) was further confirmed with RACK1 of human placenta and mouse RACK1-encoded fusion protein. We found that RACK1 was dimerized upon reaction with GTP-K(B)-Ras(Q61K), as well as with 14-3-3beta and geranylgeranyl pyrophosphate, as revealed by phosphorylation with Src tyrosine kinase. We reported the complex of RACK1/GTP-K(B)-Ras(Q61K) reacted selectively with p120-GAP. This interaction was sufficient to dissemble RACK1 into monomers, a preferred form to compete for the binding of syndecan-2. These data indicate that the reaction of GTP-K(B)-Ras(Q61K) with RACK1 in dimers may operate a mechanism to deplete RACK1 from reaction with syndecan-2 upon transformation by oncogenic ras and the RACK1/GTP-Ras complex may provide a route to react with p120-GAP and recycle monomeric RACK1 to syndecan-2.