Perinuclear localization of the protein-tyrosine phosphatase SHP-1 and inhibition of epidermal growth factor-stimulated STAT1/3 activation in A431 cells

Large-scale phage-based screening reveals extensive pan-viral mimicry of host short linear motifs

Viruses mimic host short linear motifs (SLiMs) to hijack and deregulate cellular functions. Studies of motif-mediated interactions therefore provide insight into virus-host dependencies, and reveal targets for therapeutic intervention. Here, we describe the pan-viral discovery of 1712 SLiM-based virus-host interactions using a phage peptidome tiling the intrinsically disordered protein regions of 229 RNA viruses. We find mimicry of host SLiMs to be a ubiquitous viral strategy, reveal novel host proteins hijacked by viruses, and identify cellular pathways frequently deregulated by viral motif mimicry. Using structural and biophysical analyses, we show that viral mimicry-based interactions have similar binding strength and bound conformations as endogenous interactions. Finally, we establish polyadenylate-binding protein 1 as a potential target for broad-spectrum antiviral agent development. Our platform enables rapid discovery of mechanisms of viral interference and the identification of potential therapeutic targets which can aid in combating future epidemics and pandemics.

SHP-1 knockdown suppresses mitochondrial biogenesis and aggravates mitochondria-dependent apoptosis induced by all trans retinal through the STING/AMPK pathways

BACKGROUND

Oxidative stress-caused damage to the retinal pigment epithelium (RPE) underlies the onset and progression of age-related macular degeneration (AMD). Impaired mitochondrial biogenesis sensitizes RPE cells to mitochondrial dysfunction, energy insufficiency and death. Src-homology 2 domain-containing phosphatase (SHP)-1 is important in regulating immune responses and cell survival. However, its roles in cell survival are not always consistent. Until now, the effects of SHP-1 on RPE dysfunction, especially mitochondrial homeostasis, remain to be elucidated. We sought to clarify the effects of SHP-1 in RPE cells in response to atRAL-induced oxidative stress and determine the regulatory mechanisms involved.

METHODS

In the all trans retinal (atRAL)-induced oxidative stress model, we used the vector of lentivirus to knockdown the expression of SHP-1 in ARPE-19 cells. CCK-8 assay, Annexin V/PI staining and JC-1 staining were utilized to determine the cell viability, cell apoptosis and mitochondrial membrane potential. We also used immunoprecipitation to examine the ubiquitination modification of stimulator of interferon genes (STING) and its interaction with SHP-1. The expression levels of mitochondrial marker, proteins related to mitochondrial biogenesis, and signaling molecules involved were examined by western blotting analysis.

RESULTS

We found that SHP-1 knockdown predisposed RPE cells to apoptosis, aggravated mitochondrial damage, and repressed mitochondrial biogenesis after treatment with atRAL. Immunofluoresent staining and immunoprecipitation analysis confirmed that SHP-1 interacted with the endoplasmic reticulum-resident STING and suppressed K63-linked ubiquitination and activation of STING. Inhibition of STING with the specific antagonist H151 attenuated the effects of SHP-1 knockdown on mitochondrial biogenesis and oxidative damage. The adenosine monophosphate-activated protein kinase (AMPK) pathway acted as the crucial downstream target of STING and was involved in the regulatory processes.

CONCLUSIONS

These findings suggest that SHP-1 knockdown potentiates STING overactivation and represses mitochondrial biogenesis and cell survival, at least in part by blocking the AMPK pathway in RPE cells. Therefore, restoring mitochondrial health by regulating SHP-1 in RPE cells may be a potential therapeutic strategy for degenerative retinal diseases including AMD.

The role of diosgenin in crohn’s disease

Inflammatory bowel disease (IBD) is a chronic idiopathic inflammation that can grossly affect the entire gastrointestinal tract (GIT) from the mouth to the anus. Crohn’s disease is the most known type of IBD and has been the focus of attention due to its increase in prevalence worldwide. Although the etiology is yet to be elucidated, recent studies have pointed out Crohn’s disease to arise from a complex interaction between environmental influences, genetic predisposition, and altered gut microbiota, resulting in dysregulated adaptive and innate responses. The presenting hallmarks of Crohn’s disease may include weight loss, nausea, vomiting, abdominal pain, diarrhea, fever, or chills. Treatment is usually done with many approved immunosuppressive drugs and surgery. However, a promising avenue from natural compounds is a safer therapy due to its safe natural active ingredients and the strong activity it shows in the treatment and management of diseases. Diosgenin, “a major biologically active natural steroidal sapogenin found in Chinese yam,” has been widely reported as a therapeutic agent in the treatment of various classes of disorders such as hyperlipidemia, inflammation, diabetes, cancer, infection, and immunoregulation. In this review, an analysis of literature data on diosgenin employed as a therapeutic agent for the treatment of Crohn’s disease is approached, to strengthen the scientific database and curtail the dreadful impact of Crohn’s disease.

Albendazole Exhibits Anti-Neoplastic Actions against Gastric Cancer Cells by Affecting STAT3 and STAT5 Activation by Pleiotropic Mechanism(s)

Albendazole (ABZ) has been reported to display anti-tumoral actions against various maliganncies, but possible impact of ABZ on gastric cancer has not been deciphered. As aberrant phosphorylation of STAT3 and STAT5 proteins can regulate the growth and progression of gastric cancer, we postulated that ABZ may interrupt the activation of these oncogenic transcription factors. We found that ABZ exposure abrogated STAT3/5 activation, inhibited phosphorylation of Janus-activated kinases 1/2 and Src and enhanced the levels of SHP-1 protein. Silencing of SHP-1 gene by small interfering RNA (siRNA) reversed the ABZ-promoted attenuation of STAT3 as well as STAT5 activation and cellular apoptosis. In addition, these effects were noted to be driven by an augmented levels of reactive oxygen species caused by drug-induced GSH/GSSG imbalance. Thus, the data indicates that ABZ can modulate the activation of STAT3 and STAT5 by pleiotropic mechanisms in gastric cancer cells.

Novel Approaches to Target Mutant FLT3 Leukaemia

Simple Summary

Acute myeloid leukemia (AML) is a haematologic disease in which oncogenic mutations in the receptor tyrosine kinase FLT3 frequently lead to leukaemic development. Potent treatment of AML patients is still hampered by inefficient targeting of leukemic stem cells expressing constitutive active FLT3 mutants. This review summarizes the current knowledge about the regulation of FLT3 activity at cellular level and discusses therapeutical options to affect the tumor cells and the microenvironment to impair the haematological aberrations.

Abstract

Fms-like tyrosine kinase 3 (FLT3) is a member of the class III receptor tyrosine kinases (RTK) and is involved in cell survival, proliferation, and differentiation of haematopoietic progenitors of lymphoid and myeloid lineages. Oncogenic mutations in the FLT3 gene resulting in constitutively active FLT3 variants are frequently found in acute myeloid leukaemia (AML) patients and correlate with patient’s poor survival. Targeting FLT3 mutant leukaemic stem cells (LSC) is a key to efficient treatment of patients with relapsed/refractory AML. It is therefore essential to understand how LSC escape current therapies in order to develop novel therapeutic strategies. Here, we summarize the current knowledge on mechanisms of FLT3 activity regulation and its cellular consequences. Furthermore, we discuss how aberrant FLT3 signalling cooperates with other oncogenic lesions and the microenvironment to drive haematopoietic malignancies and how this can be harnessed for therapeutical purposes.

Blockage of STAT3 Signaling Pathway by Morusin Induces Apoptosis and Inhibits Invasion in Human Pancreatic Tumor Cells

OBJECTIVES

Signal transducer and activator of transcription 3 (STAT3) is an oncogenic transcription factor implicated in carcinogenesis. Here, we investigated the role of morusin, the major prenylflavonoid, isolated from Chinese herbal medicine in abrogating the constitutive STAT3 activation in human pancreatic tumor cells.

METHODS

The effect of morusin on STAT3 activation, associated protein kinases, STAT3-regulated gene products, cellular proliferation, and apoptosis was examined.

RESULTS

Morusin specifically inhibited constitutive STAT3 activation both at tyrosine residue 705 and serine residue 727 in 4 pancreatic tumor cells. The inhibition of STAT3 was mediated through the suppression of activation of upstream JAK1, JAK2, and c-Src kinases. Morusin led to the accumulation of the cells in different phases of the cell cycle and caused induction of apoptosis and loss of mitochondrial membrane potential. Morusin downregulated the expression of various STAT3-regulated gene products; this correlated with induction of caspase-3 activation and anti-invasive effects. Treatment with the protein tyrosine phosphatase inhibitor pervanadate reversed the morusin-induced downregulation of STAT3, thereby suggesting the involvement of a protein tyrosine phosphatase.

CONCLUSIONS

Morusin is a novel blocker of STAT3 activation and thus may have potential in negative regulation of growth and metastasis of pancreatic tumor cells.

Resveratrol attenuates constitutive STAT3 and STAT5 activation through induction of PTPε and SHP-2 tyrosine phosphatases and potentiates sorafenib-induced apoptosis in renal cell carcinoma

BACKGROUND

Signal transducers and activators of transcription (STAT) proteins are critical transcription factor that are aberrantly activated in various types of malignancies, including renal cell carcinoma (RCC).

METHODS

We investigated the effect of resveratrol (RES), an edible polyphenol phytoalexin on STAT3 and STAT5 activation cascade in both Caki-1 and 786-O RCC cell lines.

RESULTS

We found that RES suppressed both constitutive STAT3 (tyrosine residue 705 and serine residue 727) and STAT5 (tyrosine residue 694 and 699) activation, which correlated with the suppression of the upstream kinases (JAK1, JAK2, and c-Src) in RCC. Also, RES abrogated DNA binding capacity and nuclear translocation of these two transcription factors. RES-induced an increased expression of PTPε and SHP-2 and the deletion of these two genes by small interfering RNA abolished the ability of RES to inhibit STAT3 activation, suggesting the critical role of both PTPε and SHP-2 in its possible mechanism of action. Moreover, RES induced S phase cell cycle arrest, caused induction of apoptosis, loss of mitochondrial membrane potential, and suppressed colony formation in RCC. We also found that RES downregulated the expression of STAT3/5-regulated antiapoptotic, proliferative, and metastatic gene products; and this correlated with induction of caspase-3 activation and anti-invasive activity. Beside, RES potentiated sorafenib induced inhibitory effect on constitutive STAT3 and STAT5 phosphorylation, apoptotic effects in 786-O cells, and this correlated with down-regulation of various oncogenic gene products.

CONCLUSION

Overall, our results suggest that RES is a blocker of both STAT3 and STAT5 activation and thus may exert potential growth inhibitory effects against RCC cells.

Antitumorigenic effect of plumbagin by induction of SH2-containing protein tyrosine phosphatase 1 in human gastric cancer cells

A recent study reported that plumbagin downregulated the activity of Janus kinase 2 (JAK2)-signal transducer and activator of transcription 3 (STAT3) pathway to show various antitumor effects in multiple myeloma cells. We aimed in this in vitro study to demonstrate the inhibition of JAK2/STAT3 pathway by plumbagin through inducing SH2-containing protein tyrosine phosphatase 1 (SHP1) expression in the MKN-28 gastric cancer cell line. We performed western blot analysis to measure SHP1, phosphor-JAK2/STAT3 level, and observed that plumbagin induced SHP1 expression and simultaneously downregulated phosphor-JAK2/STAT3 in MKN-28 cells, with negative SHP1 expression. This effect was consistent when JAK2/STAT3 signaling was activated by interleukin-6 (IL-6), and ameliorated when cells were treated with prevanadate, a protein tyrosin phosphatase inhibitor. Furthermore, plumbagin significantly reduced gene expression of cyclin D1, vascular endothelial growth factor (VEGF)-1, Bcl-xL, survivin and matrix metalloproteinase-9 (MMP-9), known target products of STAT3 activation in gastric carcinogenesis by reverse transcription-polymerase chain reaction (RT-PCR). Several functional studies such as water soluble tetrazolium salt-1 (WST-1) assay, wound closure assay, Matrigel invasion assay and Annexin V assay were also performed, and we validated the functional effect of plumbagin for inhibition of cell proliferation, migration and invasion, and induction of apoptosis. Collectively, our findings suggest that plumbagin is a potential regulator of cellular growth, migration, invasion and apoptosis by inhibiting both constitutive and inducible STAT3 activity through induction of SHP1 in gastric cancer cells.

Role of protein tyrosine phosphatases in the modulation of insulin signaling and their implication in the pathogenesis of obesity-linked insulin resistance

Insulin resistance is a major disorder that links obesity to type 2 diabetes mellitus (T2D). It involves defects in the insulin actions owing to a reduced ability of insulin to trigger key signaling pathways in major metabolic tissues. The pathogenesis of insulin resistance involves several inhibitory molecules that interfere with the tyrosine phosphorylation of the insulin receptor and its downstream effectors. Among those, growing interest has been developed toward the protein tyrosine phosphatases (PTPs), a large family of enzymes that can inactivate crucial signaling effectors in the insulin signaling cascade by dephosphorylating their tyrosine residues. Herein we briefly review the role of several PTPs that have been shown to be implicated in the regulation of insulin action, and then focus on the Src homology 2 (SH2) domain-containing SHP1 and SHP2 enzymes, since recent reports have indicated major roles for these PTPs in the control of insulin action and glucose metabolism. Finally, the therapeutic potential of targeting PTPs for combating insulin resistance and alleviating T2D will be discussed.

Capillarisin inhibits constitutive and inducible STAT3 activation through induction of SHP-1 and SHP-2 tyrosine phosphatases

Signal transducers and activators of transcription (STAT)-3 is a latent cytosolic transcription factor that has been closely associated with survival, proliferation, chemoresistance, and metastasis of tumor cells. Whether the anti-proliferative, pro-apoptotic, and anti-metastatic effects of capillarisin (CPS), derived from Artemisia capillaris (Compositae), are linked to its capability to inhibit STAT3 activation was investigated. We found that CPS specifically inhibited both constitutive and inducible STAT3 activation at tyrosine residue 705 but not at serine residue 727 in human multiple myeloma cells. Besides the inhibition of STAT3 phosphorylation, CPS also abrogated STAT3 constitutive activity and nuclear translocation. The suppression of STAT3 was mediated through the inhibition of activation of upstream JAK1, JAK2, and c-Src kinases. Treatment with the protein tyrosine phosphatase (PTP) inhibitor pervanadate treatment reversed the CPS-induced down-regulation of JAK1/2 and STAT3, thereby suggesting the involvement of a PTP. Indeed, knockdown of the SHP-1 and SHP-2 genes by small interfering RNA suppressed the ability of CPS to inhibit JAK1 and STAT3 activation, suggesting the critical role of both SHP-1 and SHP-2 in its possible mechanism of action. CPS downregulated the expression of STAT3-regulated antiapoptotic and proliferative gene products; and this correlated with suppression of cell viability, the accumulation of cells in sub-G1 phase of cell cycle and induction of apoptosis. Moreover, CPS potentiated bortezomib-induced apoptotic effects in MM cells, and this correlated with down-regulation of various gene products that mediate cell proliferation (Cyclin D1 and COX-2), cell survival (Bcl-2, Bcl-xl, IAP1, IAP2, and Survivin), invasion (MMP-9), and angiogenesis (VEGF). Thus, overall, our results suggest that CPS is a novel blocker of STAT3 activation and thus may have a potential in negative regulation of growth, metastasis, and chemoresistance of tumor cells.

SC-60, a dimer-based sorafenib derivative, shows a better anti-hepatocellular carcinoma effect than sorafenib in a preclinical hepatocellular carcinoma model

Sorafenib is the first approved targeted therapeutic reagent for hepatocellular carcinoma. Here, we report that SC-60, a dimer-based sorafenib derivative, overcomes the resistance of sorafenib and shows a better anti-hepatocellular carcinoma effect in vitro and in vivo. SC-60 substantially increased SH2 domain-containing phosphatase 1 (SHP-1) phosphatase activity in hepatocellular carcinoma cells and purified SHP-1 proteins, suggesting that SC-60 affects SHP-1 directly. Molecular docking and truncated mutants of SHP-1 further confirmed that SC-60 interferes with the inhibitory N-SH2 domain to relieve the closed catalytic protein tyrosine phosphatase domain of SHP-1. Deletion of N-SH2 domain (dN1) or point mutation (D61A) of SHP-1 abolished the effect of SC-60 on SHP-1, p-STAT3, and apoptosis. Importantly, SC-60 exhibited significant survival benefits compared with sorafenib in a hepatocellular carcinoma orthotopic model via targeting the SHP-1/STAT3-related signaling pathway. In summary, dimer derivative of sorafenib, SC-60, is a SHP-1 agonist and may be a potent reagent for hepatocellular carcinoma-targeted therapy.

The ErbB/HER family of protein-tyrosine kinases and cancer

The human epidermal growth factor receptor (EGFR) family consists of four members that belong to the ErbB lineage of proteins (ErbB1-4). These receptors consist of a glycosylated extracellular domain, a single hydrophobic transmembrane segment, and an intracellular portion with a juxtamembrane segment, a protein kinase domain, and a carboxyterminal tail. Seven ligands bind to EGFR including epidermal growth factor and transforming growth factor α, none bind to ErbB2, two bind to ErbB3, and seven ligands bind to ErbB4. The ErbB proteins function as homo and heterodimers. The heterodimer consisting of ErbB2, which lacks a ligand, and ErbB3, which is kinase impaired, is surprisingly the most robust signaling complex of the ErbB family. Growth factor binding to EGFR induces a large conformational change in the extracellular domain, which leads to the exposure of a dimerization arm in domain II of the extracellular segment. Two ligand-EGFR complexes unite to form a back-to-back dimer in which the ligands are on opposite sides of the aggregate. Following ligand binding, EGFR intracellular kinase domains form an asymmetric homodimer that resembles the heterodimer formed by cyclin and cyclin-dependent kinase. The carboxyterminal lobe of the activator kinase of the dimer interacts with the amino-terminal lobe of the receiver kinase thereby leading to its allosteric stimulation. Downstream ErbB signaling modules include the phosphatidylinositol 3-kinase/Akt (PKB) pathway, the Ras/Raf/MEK/ERK1/2 pathway, and the phospholipase C (PLCγ) pathway. Several malignancies are associated with the mutation or increased expression of members of the ErbB family including lung, breast, stomach, colorectal, head and neck, and pancreatic carcinomas and glioblastoma (a brain tumor). Gefitinib, erlotinib, and afatinib are orally effective protein-kinase targeted quinazoline derivatives that are used in the treatment of ERBB1-mutant lung cancer. Lapatinib is an orally effective quinazoline derivative used in the treatment of ErbB2-overexpressing breast cancer. Trastuzumab, pertuzumab, and ado-trastuzumab emtansine, which are given intravenously, are monoclonal antibodies that target the extracellular domain and are used for the treatment of ErbB2-positive breast cancer; ado-trastuzumab emtansine is an antibody-drug conjugate that delivers a cytotoxic drug to cells overexpressing ErbB2. Cetuximab and panitumumab are monoclonal antibodies that target ErbB1 and are used in the treatment of colorectal cancer. Cancers treated with these targeted drugs eventually become resistant to them. The role of combinations of targeted drugs or targeted drugs with cytotoxic therapies is being explored in an effort to prevent or delay drug resistance in the treatment of these malignancies.

Ras palmitoylation is necessary for N-Ras activation and signal propagation in growth factor signalling

Ras GTPases undergo post-translational modifications that govern their subcellular trafficking and localization. In particular, palmitoylation of the Golgi tags N-Ras and H-Ras for exocytotic transport and residency at the PM (plasma membrane). Following depalmitoylation, PM-Ras redistributes to all subcellular membranes causing an accumulation of palmitate-free Ras at endomembranes, including the Golgi and endoplasmic reticulum. Palmitoylation is unanimously regarded as a critical modification at the crossroads of Ras activity and trafficking control, but its precise relevance to native wild-type Ras function in growth factor signalling is unknown. We show in the present study by use of palmitoylation-deficient N-Ras mutants and via the analysis of palmitate content of agonist-activated GTP-loaded N-Ras that only palmitoylated N-Ras becomes activated by agonists. In line with an essential role of palmitoylation in Ras activation, dominant-negative RasS17N loses its blocking potency if rendered devoid of palmitoylation. Live-cell Ras-GTP imaging shows that N-Ras activation proceeds only at the PM, consistent with activated N-Ras-GTP being palmitoylated. Finally, palmitoylation-deficient N-Ras does not sustain EGF (epidermal growth factor) or serum-elicited mitogenic signalling, confirming that palmitoylation is essential for signal transduction by N-Ras. These findings document that N-Ras activation proceeds at the PM and suggest that depalmitoylation, by removing Ras from the PM, may contribute to the shutdown of Ras signalling.

Morin inhibits STAT3 tyrosine 705 phosphorylation in tumor cells through activation of protein tyrosine phosphatase SHP1

The major goal of cancer drug discovery is to find an agent that is safe and affordable, yet effective against cancer. Here we show that morin (3,5,7,2',4'-pentahydroxyflavone) has potential against cancer cells through suppression of the signal transducer and activator of transcription 3 (STAT3) pathway, which is closely linked to the transformation, survival, proliferation, and metastasis of cancer. We found that morin completely suppressed inducible and constitutively activated STAT3 and blocked the nuclear translocation of STAT3 and its DNA binding in multiple myeloma and head and neck squamous carcinoma cells. Morin inhibited activated Src, JAK-1, and JAK-2, all of which are linked to STAT3 activation, while up-regulating a protein inhibitor of activated STAT3, PIAS3. Pervanadate reversed the effects of morin on STAT3 phosphorylation, indicating the role of a protein tyrosine phosphatase. Furthermore, morin induced SHP1 expression at both the mRNA and protein levels, and silencing of SHP1 abrogated the effect of morin on STAT3 phosphorylation, indicating that morin mediates its effects on STAT3 through SHP1. Suppression of STAT3 correlated with the down-regulation of various gene products linked to tumor survival, proliferation, and angiogenesis and led to sensitization of tumor cells to thalidomide and bortezomib. Comparing the activities of morin with those of four structurally related flavonols demonstrated the importance of hydroxyl groups in the B ring in inhibiting STAT3 activation. These findings suggest that morin suppresses the STAT3 pathway, leading to the down-regulation of STAT3-dependent gene expression and chemosensitization of tumor cells.

Computational analysis of the regulation of EGFR by protein tyrosine phosphatases

The tyrosine phosphorylated epidermal growth factor receptor (EGFR) initiates numerous cell signaling pathways. Although EGFR phosphorylation levels are ultimately determined by the balance of receptor kinase and protein tyrosine phosphatase (PTP) activities, the kinetics of EGFR dephosphorylation are not well understood. Previous models of EGFR signaling have generally neglected PTP activity or computed PTP activity by considering data that do not fully reveal the kinetics and compartmentalization of EGFR dephosphorylation. We developed a compartmentalized, mechanistic model to elucidate the kinetics of EGFR dephosphorylation and the coupling of this process to phosphorylation-dependent EGFR endocytosis. Model regression against data from HeLa cells for EGFR phosphorylation response to EGFR activation, PTP inhibition, and EGFR kinase inhibition led to the conclusion that EGFR dephosphorylation occurs at the plasma membrane and in the cell interior with a timescale that is smaller than that for ligand-mediated EGFR endocytosis. The model further predicted that sufficiently rapid dephosphorylation of EGFR at the plasma membrane could potentially impede EGFR endocytosis, consistent with recent experimental findings. Overall, our results suggest that PTPs regulate multiple receptor-level phenomena via their action at the plasma membrane and cell interior and point to new possibilities for targeting PTPs for modulation of EGFR dynamics.

Icariside II induces apoptosis in U937 acute myeloid leukemia cells: role of inactivation of STAT3-related signaling

Background

The aim of this study is to determine anti-cancer effect of Icariside II purified from the root of Epimedium koreanum Nakai on human acute myeloid leukemia (AML) cell line U937.

Methodology/Principal Findings

Icariside II blocked the growth U937 cells in a dose- and time-dependent manner. In this anti-proliferation process, this herb compound rendered the cells susceptible to apoptosis, manifested by enhanced accumulation of sub-G1 cell population and increased the terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive cells. Icariside II was able to activate caspase-3 and cleaved poly (ADP-ribose) polymerase (PARP) in a time-dependent manner. Concurrently, the anti-apoptotic proteins, such as bcl-x_L and survivin in U937 cells, were downregulated by Icariside II. In addition, Icariside II could inhibit STAT3 phosphorylation and function and subsequently suppress the activation of Janus activated kinase 2 (JAK2), the upstream activators of STAT3, in a dose- and time-dependent manner. Icariside II also enhanced the expression of protein tyrosine phosphatase (PTP) SH2 domain-containing phosphatase (SHP)-1, and the addition of sodium pervanadate (a PTP inhibitor) prevented Icariside II-induced apoptosis as well as STAT3 inactivation in STAT3 positive U937 cells. Furthermore, silencing SHP-1 using its specific siRNA significantly blocked STAT3 inactivation and apoptosis induced by Icariside II in U937 cells.

Conclusions/Significance

Our results demonstrated that via targeting STAT3-related signaling, Icariside II sensitizes U937 cells to apoptosis and perhaps serves as a potent chemotherapeutic agent for AML.

Inhibition of STAT3 signaling and induction of SHP1 mediate antiangiogenic and antitumor activities of ergosterol peroxide in U266 multiple myeloma cells

BACKGROUND

Ergosterol peroxide (EP) derived from edible mushroom has been shown to exert anti-tumor activity in several cancer cells. In the present study, anti-angiogenic activity of EP was investigated with the underlying molecular mechanisms in human multiple myeloma U266 cells.

RESULTS

Despite weak cytotoxicity against U266 cells, EP suppressed phosphorylation, DNA binding activity and nuclear translocalization of signal transducer and activator of transcription 3 (STAT3) in U266 cells at nontoxic concentrations. Also, EP inhibited phosphorylation of the upstream kinases Janus kinase 2 (JAK2) and Src in a time-dependent manner. Furthermore, EP increased the expression of protein tyrosine phosphatase SHP-1 at protein and mRNA levels, and conversely silencing of the SHP-1 gene clearly blocked EP-mediated STAT3 inactivation. In addition, EP significantly decreased vascular endothelial growth factor (VEGF), one of STAT3 target genes at cellular and protein levels as well as disrupted in vitro tube formation assay. Moreover, EP significantly suppressed the growth of U266 cells inoculated in female BALB/c athymic nude mice and immunohistochemistry revealed that EP effectively reduced the expression of STAT3 and CD34 in tumor sections compared to untreated control.

CONCLUSION

These findings suggest that EP can exert antitumor activity in multiple myeloma U266 cells partly with antiangiogenic activity targeting JAK2/STAT3 signaling pathway as a potent cancer preventive agent for treatment of multiple myeloma cells.

The γ-secretase cleavage product of polycystin-1 regulates TCF and CHOP-mediated transcriptional activation through a p300-dependent mechanism

Mutations in Pkd1, encoding polycystin-1 (PC1), cause autosomal-dominant polycystic kidney disease (ADPKD). We show that the carboxy-terminal tail (CTT) of PC1 is released by γ-secretase-mediated cleavage and regulates the Wnt and CHOP pathways by binding the transcription factors TCF and CHOP, disrupting their interaction with the common transcriptional coactivator p300. Loss of PC1 causes increased proliferation and apoptosis, while reintroducing PC1-CTT into cultured Pkd1 null cells reestablishes normal growth rate, suppresses apoptosis, and prevents cyst formation. Inhibition of γ-secretase activity impairs the ability of PC1 to suppress growth and apoptosis and leads to cyst formation in cultured renal epithelial cells. Expression of the PC1-CTT is sufficient to rescue the dorsal body curvature phenotype in zebrafish embryos resulting from either γ-secretase inhibition or suppression of Pkd1 expression. Thus, γ-secretase-dependent release of the PC1-CTT creates a protein fragment whose expression is sufficient to suppress ADPKD-related phenotypes in vitro and in vivo.

γ-Tocotrienol is a novel inhibitor of constitutive and inducible STAT3 signalling pathway in human hepatocellular carcinoma: potential role as an antiproliferative, pro-apoptotic and chemosensitizing agent

BACKGROUND AND PURPOSE

Activation of signal transducer and activator of transcription 3 (STAT3) play a critical role in the survival, proliferation, angiogenesis and chemoresistance of tumour cells. Thus, agents that suppress STAT3 phosphorylation have potential as cancer therapies. In the present study, we investigated whether the apoptotic, antiproliferative and chemosensitizing effects of γ-tocotrienol are associated with its ability to suppress STAT3 activation in hepatocellular carcinoma (HCC).

EXPERIMENTAL APPROACH

The effect of γ-tocotrienol on STAT3 activation, associated protein kinases and phosphatase, STAT3-regulated gene products, cellular proliferation and apoptosis in HCC cells was investigated.

KEY RESULTS

γ-Tocotrienol inhibited both the constitutive and inducible activation of STAT3 with minimum effect on STAT5. γ-Tocotrienol also inhibited the activation of Src, JAK1 and JAK2 implicated in STAT3 activation. Pervanadate reversed the γ-tocotrienol-induced down-regulation of STAT3, suggesting the involvement of a protein tyrosine phosphatase. Indeed, we found that γ-tocotrienol induced the expression of the tyrosine phosphatase SHP-1 and deletion of the SHP-1 gene by small interfering RNA abolished the ability of γ-tocotrienol to inhibit STAT3 activation. γ-Tocotrienol also down-regulated the expression of STAT3-regulated gene products, including cyclin D1, Bcl-2, Bcl-xL, survivin, Mcl-1 and vascular endothelial growth factor. Finally, γ-tocotrienol inhibited proliferation, induced apoptosis and significantly potentiated the apoptotic effects of chemotherapeutic drugs (paclitaxel and doxorubicin) used for the treatment of HCC.

CONCLUSIONS AND IMPLICATIONS

Overall, these results suggest that γ-tocotrienol is a novel blocker of the STAT3 activation pathway, with a potential role in future therapies for HCC and other cancers.

Embelin suppresses STAT3 signaling, proliferation, and survival of multiple myeloma via the protein tyrosine phosphatase PTEN

Even though embelin, an inhibitor of the XIAP, is known to exhibit anti-inflammatory and anti-cancer activities, very little is known about its mechanism of action. Here, we investigated whether embelin mediates its effect through interference with the signal transducer and activator of transcription 3 (STAT3) pathway. We found that embelin inhibited constitutive STAT3 activation in a variety of human cancer cell lines such as U266, DU-145, and SCC4 cells. The suppression of STAT3 was mediated through inhibition of the activation of JAK2 and c-Src. Pervanadate treatment also reversed the embelin-induced down-regulation of STAT3, suggesting the involvement of a protein tyrosine phosphatase. Indeed, we found that embelin-induced the expression of the tyrosine phosphatase PTEN and deletion of the PTEN gene by small interfering RNA abolished the ability of embelin to inhibit STAT3 activation. Besides, embelin failed to suppress STAT3 activation in PTEN-null PC3 cells, thus indicating that the inhibitory effect of embelin on STAT3 is PTEN-dependent. Embelin down-regulated the expression of STAT3-regulated gene products; this correlated with the suppression of cell proliferation and invasion, and the induction of apoptosis through the activation of caspase-3. Overall, our results indicate that the anti-inflammatory and anti-cancer activities previously assigned to embelin may be mediated in part through the suppression of the STAT3 pathway.

Inhibition of JAK1/STAT3 signaling mediates compound K-induced apoptosis in human multiple myeloma U266 cells

Signal transducer and activator of transcription 3 (STAT3) is an oncogenic transcription factor implicated in carcinogenesis. Here, the role of STAT3 pathway in the antitumor activity of an active ginseng saponin metabolite compound K (CK) was investigated in human multiple myeloma U266 cells. CK increased the cytotoxicity, accumulated the sub-G1 DNA population, cleaved poly (ADP-ribose) polymerase (PARP) and activated caspase-3 in U266 cells. Interestingly, CK inhibited phosphorylation of STAT3 and its upstream activators, the Janus activated kinase 1 (JAK1), but not JAK2. Furthermore, CK enhanced the expression of protein tyrosine phosphatase (PTP) SHP-1, but not PTEN. Additionally, CK down-regulated STAT3 target genes bcl-x(L), bcl-2, survivin, cyclin E and cyclin D1. Conversely, PTP inhibitor pervanadate reversed CK-mediated STAT3 inactivation and cleavages of caspase-3 and PARP. Overall, our findings demonstrate that JAK1/STAT3 signaling mediates CK-induced apoptosis in U266 cells and also suggest the chemopreventive potential of CK for treatment of multiple myeloma.

Thymoquinone inhibits proliferation, induces apoptosis and chemosensitizes human multiple myeloma cells through suppression of signal transducer and activator of transcription 3 activation pathway

BACKGROUND AND PURPOSE

Constitutive activation of the signal transducer and activator of transcription 3 (STAT3) pathway is frequently encountered in several human cancers including multiple myeloma (MM). Thus, agents that suppress STAT3 phosphorylation have a potential for treatment of MM. In the present report, we investigated whether thymoquinone (TQ), the main component isolated from the medicinal plant Nigella sativa, modulated the STAT3 signalling pathway in MM cells.

EXPERIMENTAL APPROACH

The effect of TQ on both constitutive and IL-6-induced STAT3 activation, associated protein kinases, STAT3-regulated gene products involved in proliferation, survival and angiogenesis, cellular proliferation and apoptosis in MM cells, was investigated.

KEY RESULTS

We found that TQ inhibited both constitutive and IL-6-inducible STAT3 phosphorylation which correlated with the inhibition of c-Src and JAK2 activation. Vanadate reversed the TQ-induced down-regulation of STAT3 activation, suggesting the involvement of a protein tyrosine phosphatase. Indeed, we found that TQ can induce the expression of Src homology-2 phosphatase 2 that correlated with suppression of STAT3 activation. TQ also down-regulated the expression of STAT3-regulated gene products, such as cyclin D1, Bcl-2, Bcl-xL, survivin, Mcl-1 and vascular endothelial growth factor. Finally, TQ induced the accumulation of cells in sub-G1 phase, inhibited proliferation and induced apoptosis, as indicated by poly ADP ribose polymerase cleavage. TQ also significantly potentiated the apoptotic effects of thalidomide and bortezomib in MM cells.

CONCLUSIONS AND IMPLICATIONS

Our study has identified STAT3 signalling as a target of TQ and has thus raised its potential application in the prevention and treatment of MM and other cancers.

Prep1 controls insulin glucoregulatory function in liver by transcriptional targeting of SHP1 tyrosine phosphatase

OBJECTIVE

We investigated the function of the Prep1 gene in insulin-dependent glucose homeostasis in liver.

RESEARCH DESIGN AND METHODS

Prep1 action on insulin glucoregulatory function has been analyzed in liver of Prep1-hypomorphic mice (Prep1(i/i)), which express 2-3% of Prep1 mRNA.

RESULTS

Based on euglycemic hyperinsulinemic clamp studies and measurement of glycogen content, livers from Prep1(i/i) mice feature increased sensitivity to insulin. Tyrosine phosphorylation of both insulin receptor (IR) and insulin receptor substrate (IRS)1/2 was significantly enhanced in Prep1(i/i) livers accompanied by a specific downregulation of the SYP and SHP1 tyrosine phosphatases. Prep1 overexpression in HepG2 liver cells upregulated SYP and SHP1 and inhibited insulin-induced IR and IRS1/2 phosphorylation and was accompanied by reduced glycogen content. Consistently, overexpression of the Prep1 partner Pbx1, but not of p160MBP, mimicked Prep1 effects on tyrosine phosphorylations, glycogen content, and on SYP and SHP1 expression. In Prep1 overexpressing cells, antisense silencing of SHP1, but not that of SYP, rescued insulin-dependent IR phosphorylation and glycogen accumulation. Both Prep1 and Pbx1 bind SHP1 promoter at a site located between nucleotides -2,113 and -1,778. This fragment features enhancer activity and induces luciferase function by 7-, 6-, and 30-fold, respectively, in response to Prep1, Pbx1, or both.

CONCLUSIONS

SHP1, a known silencer of insulin signal, is a transcriptional target of Prep1. In liver, transcriptional activation of SHP1 gene by Prep1 attenuates insulin signal transduction and reduces glucose storage.

Janus activated kinase 2/signal transducer and activator of transcription 3 pathway mediates icariside II-induced apoptosis in U266 multiple myeloma cells

Although the flavonoid icariside II exhibits anti-inflammatory and anti-cancer activities, its molecular targets/pathways in human multiple myeloma cells are poorly understood. To analyze the effects on signal transducer and activator of transcription 3 (STAT3) signaling and apoptosis, U266 multiple myeloma cells were treated with icariside II and performed Western blotting, electrophoretic mobility gel shift assay (EMSA), RT-PCR, proliferation assay, cell cycle analysis and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay. Icariside II inhibited STAT3 activation and enhanced the expression of SHP-1 and PTEN through inhibiting Janus activated kinase 2 (JAK2) and c-Src. Icariside II down-regulated the expression of STAT3 target genes Bcl-2, Bcl-x(L), survivin, cyclin D(1), COX-2 and vascular endothelial growth factor (VEGF). Also, icariside II enhanced poly (ADP-ribose) polymerase (PARP) cleavage and caspase-3 activation. Pervanadate reversed the icariside II-mediated STAT3 inactivation and also blocked the cleavages of caspase-3 and PARP, suggesting involvement of STAT3 pathway in icariside II-induced apoptosis. Furthermore, icariside II enhanced the apoptotic effects of clinically used drugs thalidomide and bortezomib in U266 cells. Icariside II could be a potential therapeutic intervention agent alone or in combination with current drugs for multiple myeloma as a novel blocker of STAT3 signaling cascades at multiple levels, contributing to its anti-proliferative and anti-apoptosis.

Exosome release of β-catenin: a novel mechanism that antagonizes Wnt signaling

CD82 and CD9 are tetraspanin membrane proteins that can function as suppressors of tumor metastasis. Expression of CD9 and CD82 in transfected cells strongly suppresses β-catenin-mediated Wnt signaling activity and induces a significant decrease in β-catenin protein levels. Inhibition of Wnt/β-catenin signaling is independent of glycogen synthase kinase-3β and of the proteasome- and lysosome-mediated protein degradation pathways. CD82 and CD9 expression induces β-catenin export via exosomes, which is blocked by a sphingomyelinase inhibitor, GW4869. CD82 fails to induce exosome release of β-catenin in cells that express low levels of E-cadherin. Exosome release from dendritic cells generated from CD9 knockout mice is reduced compared with that from wild-type dendritic cells. These results suggest that CD82 and CD9 down-regulate the Wnt signaling pathway through the exosomal discharge of β-catenin. Thus, exosomal packaging and release of cytosolic proteins can modulate the activity of cellular signaling pathways.

γ-Tocotrienol but not γ-tocopherol blocks STAT3 cell signaling pathway through induction of protein-tyrosine phosphatase SHP-1 and sensitizes tumor cells to chemotherapeutic agents

Although γ-tocotrienol (T3), a vitamin E isolated primarily from palm and rice bran oil, has been linked with anticancer activities, the mechanism of this action is poorly understood. In this study, we investigated whether γ-T3 can modulate the STAT3 cell signaling pathway, closely linked to inflammation and tumorigenesis. We found that γ-T3 but not γ-tocopherol, the most common saturated form of vitamin E, inhibited constitutive activation of STAT3 in a dose- and time-dependent manner, and this inhibition was not cell type-specific. γ-T3 also inhibited STAT3 DNA binding. This correlated with inhibition of Src kinase and JAK1 and JAK2 kinases. Pervanadate reversed the γ-T3-induced down-regulation of STAT3 activation, suggesting the involvement of a protein-tyrosine phosphatase. When examined further, we found that γ-T3 induced the expression of the tyrosine phosphatase SHP-1, and gene silencing of the SHP-1 by small interfering RNA abolished the ability of γ-T3 to inhibit STAT3 activation, suggesting a vital role for SHP-1 in the action of γ-T3. Also γ-T3 down-modulated activation of STAT3 and induced SHP-1 in vivo. Eventually, γ-T3 down-regulated the expression of STAT3-regulated antiapoptotic (Bcl-2, Bcl-xL, and Mcl-1), proliferative (cyclin D1), and angiogenic (VEGF) gene products; and this correlated with suppression of proliferation, the accumulation of cells in sub-G(1) phase of the cell cycle, and induction of apoptosis. This vitamin also sensitized the tumor cells to the apoptotic effects of thalidomide and bortezomib. Overall, our results suggest that γ-T3 is a novel blocker of STAT3 activation pathway both in vitro and in vivo and thus may have potential in prevention and treatment of cancers.

Diosgenin, a steroidal saponin, inhibits STAT3 signaling pathway leading to suppression of proliferation and chemosensitization of human hepatocellular carcinoma cells

Constitutive activation of STAT3 has been shown in several human cancers and transformed cell lines including hepatocellular carcinoma (HCC). In the present report, we investigated whether diosgenin, a steroidal saponin isolated from fenugreek can modulate the STAT3 signaling pathway. We found that diosgenin inhibited both constitutive and inducible activation of STAT3 with no effect on STAT5. The activation of c-Src, JAK1 and JAK2 implicated in STAT3 activation, were also suppressed by this saponin. Pervanadate reversed the diosgenin-induced downregulation of STAT3, suggesting the involvement of a protein tyrosine phosphatase. Indeed, we found that diosgenin can induce the expression of Src homology 2 phosphatase 2 (SH-PTP2) that correlated with downregulation of constitutive STAT3 activation. Diosgenin also downregulated the expression of various STAT3-regulated gene products, inhibited proliferation and potentiated the apoptotic effects of paclitaxel and doxorubicin. Overall, these results suggest that diosgenin is a novel blocker of the STAT3 activation pathway, with a potential role in the treatment of HCC and other cancers.

5-hydroxy-2-methyl-1,4-naphthoquinone, a vitamin K3 analogue, suppresses STAT3 activation pathway through induction of protein tyrosine phosphatase, SHP-1: potential role in chemosensitization

The activation of signal transducers and activators of transcription 3 (STAT3) has been linked with carcinogenesis through survival, proliferation, and angiogenesis of tumor cells. Agents that can suppress STAT3 activation have potential not only for prevention but also for treatment of cancer. In the present report, we investigated whether 5-hydroxy-2-methyl-1,4-naphthoquinone (plumbagin), an analogue of vitamin K, and isolated from chitrak (Plumbago zeylanica), an Ayurvedic medicinal plant, can modulate the STAT3 pathway. We found that plumbagin inhibited both constitutive and interleukin 6-inducible STAT3 phosphorylation in multiple myeloma (MM) cells and this correlated with the inhibition of c-Src, Janus-activated kinase (JAK)1, and JAK2 activation. Vanadate, however, reversed the plumbagin-induced downregulation of STAT3 activation, suggesting the involvement of a protein tyrosine phosphatase. Indeed, we found that plumbagin induced the expression of the protein tyrosine phosphatase, SHP-1, and silencing of the SHP-1 abolished the effect of plumbagin. This agent also downregulated the expression of STAT3-regulated cyclin D1, Bcl-xL, and vascular endothelial growth factor; activated caspase-3; induced poly (ADP ribose) polymerase cleavage; and increased the sub-G(1) population of MM cells. Consistent with these results, overexpression of constitutive active STAT3 significantly reduced the plumbagin-induced apoptosis. When compared with AG490, a rationally designed STAT3/JAK2 inhibitor, plumbagin was found more potent in suppressing the proliferation of cells. Plumbagin also significantly potentiated the apoptotic effects of thalidomide and bortezomib in MM cells. Overall, these results suggest that the plumbagin inhibits STAT3 activation pathway through the induction of SHP-1 and this may mediate the sensitization of STAT3 overexpressing cancers to chemotherapeutic agents.

Knockdown of protein tyrosine phosphatase SHP-1 inhibits G1/S progression in prostate cancer cells through the regulation of components of the cell-cycle machinery

SHP-1, a haematopoietic cell-specific tyrosine phosphatase, is also expressed in human prostate. In this study, we report that SHP-1 depletion in PC-3 cells induced by small interfering RNAs causes G1 phase cell-cycle arrest accompanied by changes in some components of the cell-cycle machinery. SHP-1 knockdown increases p27(Kip1) (p27) protein stability, its nuclear localization and p27 gene transcription. These effects could be mediated by PI3K-AKT pathway as SHP-1 interacts with PI3K regulating its activity and p110 catalytic subunit phosphorylation. The increase in p27 protein stability could also because of reduced cyclin-dependent kinase (CDK2) activity. SHP-1 knockdown decreases the CDK6 levels, inducing retinoblastoma protein hypophosphorylation, downregulation of cyclin E and thereby a decrease in the CDK2 activity. However, the codepletion of SHP-1 and p27 does not produce re-entry into the cycle, implying that p27 is not required to maintain cell-cycle arrest induced by SHP-1 depletion. The maintenance of the PC-3 cell anti-proliferative response after p27 loss could be because of mislocalization of CDK2 induced by SHP-1 knockdown. This study shows that SHP-1 depletion promotes cell-cycle arrest by modulating the activity of cell-cycle regulators and suggests that SHP-1 may be required for the proper functioning of events governing cell-cycle progression.

The nuclear localization of 3'-phosphoinositide-dependent kinase-1 is dependent on its association with the protein tyrosine phosphatase SHP-1

3'-Phosphoinositide-dependent protein kinase-1 (PDK1), the direct upstream kinase of Akt, can localize to the nucleus during specific signalling events. The mechanism used for its import into the nucleus, however, remains unresolved as it lacks a canonical nuclear localization signal (NLS). Expression of activated Src kinase in C6 glioblastoma cells promotes the association of tyrosylphosphorylated PDK1 with the NLS-containing tyrosine phosphatase SHP-1 as well as the nuclear localization of both proteins. A constitutive nucleo-cytoplasmic SHP-1:PDK1 shuttling complex is supported by several lines of evidence including (i) the distribution of both proteins to similar subcellular compartments following manipulation of the nuclear pore complex, (ii) the nuclear retention of SHP-1 upon overexpression of a PDK1 protein bearing a disrupted nuclear export signal (NES), and (iii) the exclusion of PDK1 from the nucleus upon overexpression of SHP-1 lacking the NLS or following siRNA-mediated knock-down of SHP-1. The latter case results in a perinuclear distribution of PDK1 that corresponds with the distribution of PIP3 (phosphatidylinositol 3,4,5-triphosphate), while a PDK1 protein bearing a mutated PH domain that abrogates PIP3-binding is excluded from the nucleus. Our data suggest that the SHP-1:PDK1 complex is recruited to the nuclear membrane by binding to perinuclear PIP3, whereupon SHP-1 (and its NLS) facilitates active import. Export from the nucleus relies on PDK1 (and its NES). The intact complex contributes to Src kinase-induced, Akt-sensitive podial formation in C6 cells.

Boswellic acid blocks signal transducers and activators of transcription 3 signaling, proliferation, and survival of multiple myeloma via the protein tyrosine phosphatase SHP-1

Activation of signal transducers and activators of transcription-3 (STAT-3) has been linked with survival, proliferation, chemoresistance, and angiogenesis of tumor cells, including human multiple myeloma (MM). Thus, agents that can suppress STAT3 activation have potential as cancer therapeutics. In our search for such agents, we identified acetyl-11-keto-beta-boswellic acid (AKBA), originally isolated from Boswellia serrata. Our results show that AKBA inhibited constitutive STAT3 activation in human MM cells. AKBA suppressed IL-6-induced STAT3 activation, and the inhibition was reversible. The phosphorylation of both Jak 2 and Src, constituents of the STAT3 pathway, was inhibited by AKBA. Interestingly, treatment of cells with pervanadate suppressed the effect of AKBA to inhibit the phosphorylation of STAT3, thus suggesting the involvement of a protein tyrosine phosphatase. We found that AKBA induced Src homology region 2 domain-containing phosphatase 1 (SHP-1), which may account for its role in dephosphorylation of STAT3. Moreover, deletion of the SHP-1 gene by small interfering RNA abolished the ability of AKBA to inhibit STAT3 activation. The inhibition of STAT3 activation by AKBA led to the suppression of gene products involved in proliferation (cyclin D1), survival (Bcl-2, Bcl-xL, and Mcl-1), and angiogenesis (VEGF). This effect correlated with the inhibition of proliferation and apoptosis in MM cells. Consistent with these results, overexpression of constitutive active STAT3 significantly reduced the AKBA-induced apoptosis. Overall, our results suggest that AKBA is a novel inhibitor of STAT3 activation and has potential in the treatment of cancer.

Inactivation of effector caspases through nondegradative polyubiquitylation

Ubiquitin-mediated inactivation of caspases has long been postulated to contribute to the regulation of apoptosis. However, detailed mechanisms and functional consequences of caspase ubiquitylation have not been demonstrated. Here we show that the Drosophila Inhibitor of Apoptosis 1, DIAP1, blocks effector caspases by targeting them for polyubiquitylation and nonproteasomal inactivation. We demonstrate that the conjugation of ubiquitin to drICE suppresses its catalytic potential in cleaving caspase substrates. Our data suggest that ubiquitin conjugation sterically interferes with substrate entry and reduces the caspase's proteolytic velocity. Disruption of drICE ubiquitylation, either by mutation of DIAP1's E3 activity or drICE's ubiquitin-acceptor lysines, abrogates DIAP1's ability to neutralize drICE and suppress apoptosis in vivo. We also show that DIAP1 rests in an "inactive" conformation that requires caspase-mediated cleavage to subsequently ubiquitylate caspases. Taken together, our findings demonstrate that effector caspases regulate their own inhibition through a negative feedback mechanism involving DIAP1 "activation" and nondegradative polyubiquitylation.

Guggulsterone, a farnesoid X receptor antagonist, inhibits constitutive and inducible STAT3 activation through induction of a protein tyrosine phosphatase SHP-1

Signal transducers and activator of transcription 3 (STAT3) is a transcription factor that has been associated with survival, proliferation, chemoresistance, and angiogenesis of tumor cells. Whether the apoptotic, antiproliferative, and antimetastatic effects of guggulsterone (GS), a farnesoid X receptor antagonist, are linked to its ability to suppress STAT3 activation was investigated. We found that the Z but not the E stereoisomer of GS inhibited both constitutive and interleukin-6-induced STAT3 activation in human multiple myeloma cells. The suppression of STAT3 was mediated through the inhibition of activation of protein tyrosine kinases Janus-activated kinase 2 and c-Src. Vanadate treatment reversed the GS-induced down-regulation of STAT3, suggesting the involvement of a protein tyrosine phosphatase. Indeed, we found that GS induced the expression of both the protein and mRNA for tyrosine protein phosphatase SHP-1 that was not due to demethylation of the SHP-1 promoter previously implicated in the epigenetic silencing of SHP-1. Moreover, knockdown of SHP-1 by small interfering RNA suppressed the effect of GS on induction of SHP-1 and on the inhibition of STAT3 activation, thereby implicating SHP-1 in the action of GS. Finally, GS down-regulated the expression of STAT3-regulated antiapoptotic (Bcl-2, Bcl-xL, and Mcl-1), proliferative (cyclin D1), and angiogenic (VEGF) gene products; and this correlated with suppression of proliferation, the accumulation of cells in sub-G(1) phase of cell cycle, and induction of apoptosis. Overall, these results suggest that GS is a novel blocker of STAT3 activation and thus may have a potential in regulation of growth and metastasis of tumor cells.

MAL decreases the internalization of the aquaporin-2 water channel

Body water homeostasis depends critically on the hormonally regulated trafficking of aquaporin-2 (AQP2) water channels in renal collecting duct epithelial cells. Several types of posttranslational modifications are clearly involved in controlling the distribution of AQP2 between intracellular vesicles and the apical plasma membrane. Little is known, however, about the protein interactions that govern the trafficking of AQP2 between these organelles. MAL is a detergent-resistant membrane-associated protein implicated in apical sorting events. We wondered, therefore, whether MAL plays a role in the regulated trafficking of AQP2 between intracellular vesicles and the apical surface. We find that AQP2 and MAL are coexpressed in epithelial cells of the kidney collecting duct. These two proteins interact, both in the native kidney and when expressed by transfection in cultured cells. The S256-phosphorylated form of AQP2 appears to interact more extensively with MAL than does the water channel protein not phosphorylated at this serine. We find that MAL is not involved in detergent-resistant membrane association or apical delivery of AQP2 in LLC-PK(1) renal epithelial cells. Instead, MAL increases the S256 phosphorylation and apical surface expression of AQP2. Furthermore, internalization experiments show that MAL induces surface expression of AQP2 by attenuating its internalization. Thus, the involvement of MAL in the cell surface retention of apical membrane proteins could play an important role in regulated absorption and secretion in transporting epithelia.

Ursolic acid inhibits STAT3 activation pathway leading to suppression of proliferation and chemosensitization of human multiple myeloma cells

The activation of signal transducers and activators of transcription 3 (STAT3) has been linked with the proliferation of a variety of human cancer cells, including multiple myeloma. Agents that can suppress STAT3 activation have potential for prevention and treatment of cancer. In the present report, we tested an agent, ursolic acid, found in basil, apples, prunes, and cranberries, for its ability to suppress STAT3 activation. We found that ursolic acid, a pentacyclic triterpenoid, inhibited both constitutive and interleukin-6-inducible STAT3 activation in a dose- and time-dependent manner in multiple myeloma cells. The suppression was mediated through the inhibition of activation of upstream kinases c-Src, Janus-activated kinase 1, Janus-activated kinase 2, and extracellular signal-regulated kinase 1/2. Vanadate treatment reversed the ursolic acid-induced down-regulation of STAT3, suggesting the involvement of a tyrosine phosphatase. Indeed, we found that ursolic acid induced the expression of tyrosine phosphatase SHP-1 protein and mRNA. Moreover, knockdown of SHP-1 by small interfering RNA suppressed the induction of SHP-1 and reversed the inhibition of STAT3 activation, thereby indicating the critical role of SHP-1 in the action of this triterpene. Ursolic acid down-regulated the expression of STAT3-regulated gene products such as cyclin D1, Bcl-2, Bcl-xL, survivin, Mcl-1, and vascular endothelial growth factor. Finally, ursolic acid inhibited proliferation and induced apoptosis and the accumulation of cells in G1-G0 phase of cell cycle. This triterpenoid also significantly potentiated the apoptotic effects of thalidomide and bortezomib in multiple myeloma cells. Overall, these results suggest that ursolic acid is a novel blocker of STAT3 activation that may have a potential in prevention and treatment of multiple myeloma and other cancers.

In situ detection of phosphorylated platelet-derived growth factor receptor beta using a generalized proximity ligation method

Improved methods are needed for in situ characterization of post-translational modifications in cell lines and tissues. For example, it is desirable to monitor the phosphorylation status of individual receptor tyrosine kinases in samples from human tumors treated with inhibitors to evaluate therapeutic responses. Unfortunately the leading methods for observing the dynamics of tissue post-translational modifications in situ, immunohistochemistry and immunofluorescence, exhibit limited sensitivity and selectivity. Proximity ligation assay is a novel method that offers improved selectivity through the requirement of dual recognition and increased sensitivity by including DNA amplification as a component of detection of the target molecule. Here we therefore established a generalized in situ proximity ligation assay to investigate phosphorylation of platelet-derived growth factor receptor beta (PDGFRbeta) in cells stimulated with platelet-derived growth factor BB. Antibodies specific for immunoglobulins from different species, modified by attachment of DNA strands, were used as secondary proximity probes together with a pair of primary antibodies from the corresponding species. Dual recognition of receptors and phosphorylated sites by the primary antibodies in combination with the secondary proximity probes was used to generate circular DNA strands; this was followed by signal amplification by replicating the DNA circles via rolling circle amplification. We detected tyrosine phosphorylated PDGFRbeta in human embryonic kidney cells stably overexpressing human influenza hemagglutinin-tagged human PDGFRbeta in porcine aortic endothelial cells transfected with the beta-receptor, but not in cells transfected with the alpha-receptor, and also in immortalized human foreskin fibroblasts, BJ hTert, endogenously expressing the PDGFRbeta. We furthermore visualized tyrosine phosphorylated PDGFRbeta in tissue sections from fresh frozen human scar tissue undergoing wound healing. The method should be of great value to study signal transduction, screen for effects of pharmacological agents, and enhance the diagnostic potential in histopathology.

Capsaicin is a novel blocker of constitutive and interleukin-6-inducible STAT3 activation

PURPOSE

Capsaicin, a constituent of green and red peppers, has been linked with suppression of tumorigenesis through a mechanism that is not well understood. Because the transcription factor signal transducer and activator of transcription 3 (STAT3) has been closely linked with tumorigenesis, we investigated the effect of this vanilloid on the STAT3 pathway in human multiple myeloma cells.

EXPERIMENTAL DESIGN

The effect of capsaicin on both constitutive and interleukin-6-induced STAT3 activation, associated protein kinases, and STAT3-regulated gene products involved in proliferation, survival and angiogenesis, cellular proliferation, and apoptosis in multiple myeloma cells was investigated.

RESULTS

We found that capsaicin inhibited constitutive activation of STAT3 in multiple myeloma cells in a dose- and time-dependent manner, with minimum effect on STAT5. Capsaicin also inhibited the interleukin-6-induced STAT3 activation. The activation of Janus-activated kinase 1 and c-Src, implicated in STAT3 activation, was also inhibited by the vanilloid, with no effect on extracellular signal-regulated kinase 1/2 activation. Pervanadate reversed the capsaicin-induced down-regulation of STAT3, suggesting the involvement of a protein tyrosine phosphatase. Capsaicin down-regulated the expression of the STAT3-regulated gene products, such as cyclin D1, Bcl-2, Bcl-xL, survivin, and vascular endothelial growth factor. Finally, capsaicin induced the accumulation of cells in G(1) phase, inhibited proliferation, and induced apoptosis, as indicated by caspase activation. Capsaicin also significantly potentiated the apoptotic effects of Velcade and thalidomide in multiple myeloma cells. When administered i.p., capsaicin inhibited the growth of human multiple myeloma xenograft tumors in male athymic nu/nu mice.

CONCLUSION

Overall, these results suggest that capsaicin is a novel blocker of the STAT3 activation pathway, with a potential role in the prevention and treatment of multiple myeloma and other cancers.

Proteolytic processing of the receptor-type protein tyrosine phosphatase PTPBR7

The single-copy mouse gene Ptprr gives rise to different protein tyrosine phosphatase (PTP) isoforms in neuronal cells through the use of distinct promoters, alternative splicing, and multiple translation initiation sites. Here, we examined the array of post-translational modifications imposed on the PTPRR protein isoforms PTPBR7, PTP-SL, PTPPBSgamma42 and PTPPBSgamma37, which have distinct N-terminal segments and localize to different parts of the cell. All isoforms were found to be short-lived, constitutively phosphorylated proteins. In addition, the transmembrane isoform, PTPBR7, was subject to N-terminal proteolytic processing, in between amino acid position 136 and 137, resulting in an additional, 65-kDa transmembrane PTPRR isoform. Unlike for some other receptor-type PTPs, the proteolytically produced N-terminal ectodomain does not remain associated with this PTPRR-65. Shedding of PTPBR7-derived polypeptides at the cell surface further adds to the molecular complexity of PTPRR biology.

Genomic sequence correction by single-stranded DNA oligonucleotides: role of DNA synthesis and chemical modifications of the oligonucleotide ends

BACKGROUND

Single-stranded oligonucleotides (ssODN) can induce site-specific genetic alterations in selected mammalian cells, but the involved mechanisms are not known.

METHODS

We corroborate the potential of genomic sequence correction by ssODN using chromosomally integrated mutated enhanced green fluorescent protein (mEGFP) reporter genes in CHO cell lines. The role of integration site was studied in a panel of cell clones with randomly integrated reporters and in cell lines with site-specific single copy integration of the mEGFP reporter in opposite orientations. Involvement of end modification was examined on ssODN with unprotected or phosphorothioate (PS) protected ends. Also ssODN containing octyl or hexaethylene glycol (HEG) end blocking groups were tested. The significance of DNA synthesis was investigated by cell cycle analysis and by the DNA polymerases alpha, delta and epsilon inhibitor aphidicolin.

RESULTS

Correction rates of up to 5% were observed upon a single transfection of ssODN. Independent of the mEGFP chromosomal integration site and of its orientation towards the replication fork, antisense ssODN were more effective than sense ssODN. When ssODN ends were blocked by either octyl or HEG groups, correction rates were reduced. Finally, we demonstrate a dependence of the process on DNA synthesis.

CONCLUSIONS

We show that, on a chromosomal level, the orientation of the replication fork towards the targeted locus is not central in the strand bias of ssODN-based targeted sequence correction. We demonstrate the importance of accessible ssODN ends for sequence alteration. Finally, we provide evidence for the involvement of DNA synthesis in the process.

DEP-1 protein tyrosine phosphatase inhibits proliferation and migration of colon carcinoma cells and is upregulated by protective nutrients

The transmembrane protein-tyrosine phosphatase (PTP) DEP-1 (density-enhanced phosphatase) is a candidate tumor suppressor in the colon epithelium. We have explored the function of DEP-1 in colon epithelial cells by inducible re-expression in a DEP-1-deficient human colon cancer cell line. Density-enhanced phosphatase-1 re-expression led to profound inhibition of cell proliferation and cell migration, and was associated with cytoskeletal rearrangements. These effects were dependent on the PTP activity of DEP-1 as they were not observed with cells expressing the catalytically inactive DEP-1 C1239S variant. shRNA-mediated suppression of DEP-1 in a colon epithelial cell line with high endogenous DEP-1 levels enhanced proliferation, further supporting the antiproliferative function of DEP-1. Nutrients, which are considered to be chemoprotective with respect to colon cancer development, including butyrate, green tea and apple polyphenols, had the capacity to elevate transcription of endogenous DEP-1 mRNA and expression of DEP-1 protein. Upregulation of DEP-1 expression, and in turn inhibition of cell growth and migration may present a previously unrecognized mechanism of chemoprevention by nutrients.

The SHP-1 protein tyrosine phosphatase negatively modulates glucose homeostasis

The protein tyrosine phosphatase SHP-1 is a well-known inhibitor of activation-promoting signaling cascades in hematopoietic cells but its potential role in insulin target tissues is unknown. Here we show that Ptpn6(me-v/me-v) (also known as viable motheaten) mice bearing a functionally deficient SHP-1 protein are markedly glucose tolerant and insulin sensitive as compared to wild-type littermates, as a result of enhanced insulin receptor signaling to IRS-PI3K-Akt in liver and muscle. Downregulation of SHP-1 activity in liver of normal mice by adenoviral expression of a catalytically inert mutant of SHP-1, or after small hairpin RNA-mediated SHP-1 silencing, further confirmed this phenotype. Tyrosine phosphorylation of CEACAM1, a modulator of hepatic insulin clearance, and clearance of serum [125I]-insulin were markedly increased in SHP-1-deficient mice or SHP-1-deficient hepatic cells in vitro. These findings show a novel role for SHP-1 in the regulation of glucose homeostasis through modulation of insulin signaling in liver and muscle as well as hepatic insulin clearance.

A SHPing tale: perspectives on the regulation of SHP-1 and SHP-2 tyrosine phosphatases by the C-terminal tail

Protein tyrosine phosphorylation is a ubiquitous signalling mechanism and is regulated by a balance between the action of kinases and phosphatases. The SH2 domain-containing phosphatases SHP-1 and SHP-2 are the best studied of the classical non-receptor tyrosine phosphatases, but it is intriguing that despite their close sequence and structural homology these two phosphatases play quite different cellular roles. In particular, whereas SHP-1 plays a largely negative signalling role suppressing cellular activation, SHP-2 plays a largely positive signalling role. Major sequence differences between the two molecules are apparent in the approximately 100 amino acid residues at the extreme C-terminus of the proteins, beyond the phosphatase catalytic domain. Here we review how the differences in the tails of these proteins may regulate their activities and explain some of their functional differences.

Tyrosine phosphorylation regulates maturation of receptor tyrosine kinases

Constitutive activation of receptor tyrosine kinases (RTKs) is a frequent event in human cancer cells. Activating mutations in Fms-like tyrosine kinase 3 (FLT-3), notably, internal tandem duplications in the juxtamembrane domain (FLT-3 ITD), have been causally linked to acute myeloid leukemia. As we describe here, FLT-3 ITD exists predominantly in an immature, underglycosylated 130-kDa form, whereas wild-type FLT-3 is expressed predominantly as a mature, complex glycosylated 150-kDa molecule. Endogenous FLT-3 ITD, but little wild-type FLT-3, is detectable in the endoplasmic reticulum (ER) compartment. Conversely, cell surface expression of FLT-3 ITD is less efficient than that of wild-type FLT-3. Inhibition of FLT-3 ITD kinase by small molecules, inactivating point mutations, or coexpression with the protein-tyrosine phosphatases (PTPs) SHP-1, PTP1B, and PTP-PEST but not RPTPalpha promotes complex glycosylation and surface localization. However, PTP coexpression has no effect on the maturation of a surface glycoprotein of vesicular stomatitis virus. The maturation of wild-type FLT-3 is impaired by general PTP inhibition or by suppression of endogenous PTP1B. Enhanced complex formation of FLT-3 ITD with the ER-resident chaperone calnexin indicates that its retention in the ER is related to inefficient folding. The regulation of RTK maturation by tyrosine phosphorylation was observed with other RTKs as well, defines a possible role for ER-resident PTPs, and may be related to the altered signaling quality of constitutively active, transforming RTK mutants.

Implications of cell cycle progression on functional sequence correction by short single-stranded DNA oligonucleotides

Oligonucleotide-based sequence alteration in living cells is a substantial methodological challenge in gene therapy. Here, we demonstrate that using corrective single-stranded oligonucleotides (ssODN), high and reproducible sequence correction rates can be obtained. CHO cell lines with chromosomally integrated multiple copy EGFP reporter genes routinely show rates of 4.5% targeted sequence correction after transfection with ssODN. We demonstrate that the cell cycle influences the rates of targeted sequence correction in vivo, with a peak in the early S phase during ssODN exposure. After cell division, the altered genomic sequence is predominantly passed to one daughter cell, indicating that targeted sequence alteration occurs after the replication fork has passed over the targeted site. Although high initial correction rates can be obtained by this method, we show that a majority of the corrected cells arrest in the G2/M cell cycle phase, although 1-2% of the corrected cells form viable colonies. The G2/M arrest observed after targeted sequence correction can be partially released by caffeine, pentoxifylline or Go6976 exposure. Despite substantial remaining challenges, targeted sequence alteration based on ssODN increasingly promises to become a powerful tool for functional gene alterations.

Visualization of SHP-1-target interaction

Signaling of receptor tyrosine kinases (RTKs) is regulated by protein-tyrosine phosphatases (PTPs). We previously discovered the efficient downregulation of Ros RTK signaling by the SH2 domain PTP SHP-1, which involves a direct interaction of both molecules. Here, we studied the mechanism of this interaction in detail. Phosphopeptides representing the SHP-1 candidate binding sites in the Ros cytoplasmic domain, pY2267 and pY2327, display high affinity binding to the SHP-1 N-terminal SH2 domain (Kd=217 nM and 171 nM, respectively). Y2327 is, however, a poor substrate of Ros kinase and, therefore, contributes little to SHP-1 binding in vitro. To explore the mechanism of association in intact cells, functional fluorescent fusion proteins of Ros and SHP-1 were generated. Complexes of both molecules could be detected by Förster resonance energy transfer (FRET) in intact HEK293 and COS7 cells. As expected, the association required the functional SHP-1 N-terminal SH2 domain. Unexpectedly, pY2267 and pY2327 both contributed to the association. Mutation of Y2327 reduced constitutive association in COS7 cells. Ligand-dependent association was abrogated upon mutation of Y2267 but remained intact when Y2327 was mutated. A phosphopeptide representing the binding site pY2267 was a poor substrate for SHP-1, whereas Ros activation loop phosphotyrosines were effectively dephosphorylated. We propose a model for SHP-1-Ros interaction in which ligand-stimulated phosphorylation of Ros Y2267 by Ros, phosphorylation of Y2327 by a heterologous kinase, and inactivation of Ros by SHP-1-mediated dephosphorylation play a role in the regulation of complex stability.

EGF-stimulation activates the nuclear localization signal of SHP-1

Protein tyrosine phosphatase SHP-1 plays a critical role in the regulation of a variety of intracellular signaling pathways. SHP-1 is predominantly expressed in the cells of hematopoietic origin, and is recognized as a negative regulator of lymphocyte development and activation. SHP-1 consists of two Src homology 2 (SH2) domains and one protein tyrosine phosphatase (PTP) domain followed by a highly basic C-terminal tail containing tyrosyl phosphorylation sites. It is unclear how the C-terminal tail regulates SHP-1 function. We report the examination of the subcellular localization of a variety of truncated or mutated SHP-1 proteins fused with enhanced green fluorescent protein (EGFP) protein at either the N-terminal or the C-terminal end in different cell lines. Our data demonstrate that a nuclear localization signal (NLS) is located in the C-terminal tail of SHP-1 and the signal is primarily defined by three amino-acid residues (KRK) at the C-terminus. This signal is generally blocked in the native protein and can be exposed by fusing EGFP at the appropriate position or by domain truncation. We have also revealed that this NLS of SHP-1 is triggered by epidermal growth factor (EGF) stimulation and mediates translocation of SHP-1 from the cytosol to the nucleus in COS7 cell lines. These results not only demonstrate the importance of the C-terminal tail of SHP-1 in the regulation of nuclear localization, but also provide insights into its role in SHP-1-involved signal transduction pathways.

IAPs are functionally non-equivalent and regulate effector caspases through distinct mechanisms

Some members of the inhibitor of apoptosis (IAP) family suppress apoptosis by neutralizing caspases. The current model suggests that all caspase-regulatory IAPs function as direct enzyme inhibitors, blocking effector caspases by binding to their catalytically active pockets. Here we show that IAPs are functionally non-equivalent and regulate effector caspases through distinct mechanisms. Whereas XIAP binds directly to the active-site pockets of effector caspases, we find that regulation of effector caspases by Drosophila IAP1 (DIAP1) requires an evolutionarily conserved IAP-binding motif (IBM) at the neo-amino terminus of the large caspase subunit. Remarkably, unlike XIAP, DIAP1-sequestered effector caspases remain catalytically active, suggesting that DIAP1 does not function as a bona fide enzyme inhibitor. Moreover, we demonstrate that the mammalian IAP c-IAP1 interacts with caspase-7 in an exclusively IBM-dependent, but active site pocket-independent, manner that is mechanistically similar to DIAP1. The importance of IBM-mediated regulation of effector-caspases in vivo is substantiated by the enhanced apoptotic potency of IBM-mutant versions of drICE, DCP-1 and caspase-7.

Dysmyelination and reduced myelin basic protein gene expression by oligodendrocytes of SHP-1-deficient mice

We have shown previously that myelin-forming oligodendrocytes express the protein tyrosine phosphatase SHP-1 and that myelin formation was decreased in SHP-1-deficient motheaten mice compared to that in normal littermates. These studies suggested a potential importance for SHP-1 in oligodendrocyte and myelin development. To address further this possibility, we analyzed myelin formation by microscopy and myelin basic protein (MBP) gene expression in motheaten mice at ages when myelination occurs in the developing central nervous system (CNS). Furthermore, we correlate these findings with MBP gene expression in oligodendrocytes grown in vitro. We have found that CNS myelination was significantly reduced in SHP-1-deficient mice relative to their normal littermates at multiple times during the active period of myelination. Under electron microscopy, greater numbers of axons in spinal cords of motheaten mice were either unmyelinated or had thinner myelin sheathes compared to those in matched areas of normal littermates. Accordingly, MBP protein and mRNA levels were reduced in SHP-1-deficient mice compared to that in the CNS of normal littermates. In vitro, O1(+) oligodendrocytes from motheaten mice expressed much less MBP than O1(+) oligodendrocytes of normal littermates indicating an alteration in oligodendrocyte differentiation. The latter correlated with reduced MBP mRNA relative to cerebroside galactosyl transferase (CGT) gene mRNA in SHP-1-deficient oligodendrocytes in purified cultures. We propose that SHP-1 is a critical regulator of developmental signals leading to terminal differentiation and myelin sheath formation by oligodendrocytes.