Robert J. Ingham

Identification of two novel phenotypically distinct breast cancer cell subsets based on Sox2 transcription activity.

Sox2 (sex-determining region Y-box protein 2) is a transcription factor regulating pluripotency in embryonic stem cells. Sox2 is aberrantly expressed in breast and other cancers, though its biological significance remains widely unexplored. To understand the significance of this aberrancy, we assessed the transcription activity of Sox2 in two Sox2-expressing breast cancer cell lines, MCF7 and ZR751, using a lentiviral Sox2 GFP reporter vector. Surprisingly, Sox2 transcription activity, as measured by GFP expression encoded in a Sox2 reporter construct, was detectable only in a small subset of cells in both cell lines. Purification of GFP+ cells (cells with Sox2 activity) and GFP- cells (cells without Sox2 activity) was enriched for two phenotypically distinct cell populations in both MCF7 and ZR751 cell lines. Specifically, GFP+ cells formed significantly more colonies in methylcellulose and more mammospheres in vitro compared to GFP- cells. These phenotypic differences are directly linked to Sox2 as siRNA knockdown of Sox2 in GFP+ cells abolished these abilities. To provide a mechanistic explanation to our observations, we performed gel shift and chromatin immunoprecipitation studies; Sox2 was found to bind to its DNA binding consensus sequence and the promoters of Cyclin D1 and Nanog (two known Sox2 downstream targets) only in GFP+ cells. GFP+ cells also up-regulated CD49f, phospho-GSK3β, and β-catenin. In summary, we have identified two novel phenotypically distinct cell subsets in two breast cancer cell lines based on their differential Sox2 transcription activity. We demonstrate that Sox2 transcription activity, and not its protein expression alone, underlies the tumorigenicity and cancer stem cell-like phenotypes in breast cancers.

Amot130 Adapts Atrophin-1 Interacting Protein 4 to Inhibit Yes-associated Protein Signaling and Cell Growth

Background: Amot130 regulates cell differentiation and growth signaling. Results: Amot130 binds and activates overexpressed AIP4 to ubiquitinate Amot130 and YAP resulting in Amot130 stabilization and YAP degradation. Conclusion: Amot130 and AIP4 cooperatively inhibit YAP and cell growth. Significance: A mechanism is described whereby Amot130 directs AIP4 to potentially suppress tumor cell growth. The adaptor protein Amot130 scaffolds components of the Hippo pathway to promote the inhibition of cell growth. This study describes how Amot130 through binding and activating the ubiquitin ligase AIP4/Itch achieves these effects. AIP4 is found to bind and ubiquitinate Amot130 at residue Lys-481. This both stabilizes Amot130 and promotes its residence at the plasma membrane. Furthermore, Amot130 is shown to scaffold a complex containing overexpressed AIP4 and the transcriptional co-activator Yes-associated protein (YAP). Consequently, Amot130 promotes the ubiquitination of YAP by AIP4 and prevents AIP4 from binding to large tumor suppressor 1. Amot130 is found to reduce YAP stability. Importantly, Amot130 inhibition of YAP dependent transcription is reversed by AIP4 silencing, whereas Amot130 and AIP4 expression interdependently suppress cell growth. Thus, Amot130 repurposes AIP4 from its previously described role in degrading large tumor suppressor 1 to the inhibition of YAP and cell growth.

Platelet-derived growth factor receptor-α promotes lymphatic metastases in papillary thyroid cancer†

Lymph node metastases are common in papillary thyroid cancer (PTC) and can be resistant to surgical extirpation or radioiodine ablation. We examined the role of platelet‐derived growth factor receptor (PDGFR) in mediating lymph node metastases in PTC. Clinical specimens of PTC (n = 137) were surveyed in a tissue array and by western blots to examine the relationship between expression of the α and β subunits of PDGFR and lymph node metastases. PDGFR‐α was found at high levels in primary tumours with known lymphatic metastases but not in those tumours lacking nodal involvement (p < 0.0001). However, PDGFR‐β expression was not linked to metastatic disease (p = 0.78) as it was found in virtually all PTC specimens. A matching analysis in fresh PTC specimens (n = 13) confirmed that PDGFR‐α expression was strongly linked to metastatic spread (p = 0.0047). PDGFR‐α and ‐β were not found in normal thyroid tissue (p < 0.0001). PTC cell lines selectively expressing PDGFR‐α or ‐β were assessed for invasive potential and activation of downstream signal transduction pathways. PTC cell lines expressing PDGFR‐α responded to PDGF‐BB stimulation with increased invasive potential and this process can be blocked by the tyrosine kinase receptor inhibitor sunitinib (p < 0.009). Cell lines with only PDGFR‐β, or no PDGFR, did not show significant changes in invasive potential. Activation of PDGFR‐α led to downstream up‐regulation of both the MAPK/ERK and PI3K/Akt pathways and disruption of either pathway is sufficient to block PDGFR‐mediated increases in invasive potential. Thus, PDGFR‐α is associated with lymph node metastases in papillary thyroid carcinoma and PDGFR‐α promotes increased invasive potential in PTC cell lines. PDGFR‐α is a strong candidate for a diagnostic biomarker to identify patients at risk of nodal metastases. Our results also strengthen the rationale for selection of tyrosine kinase receptor inhibitors that target PDGFR in the treatment of progressive, metastatic PTC. Copyright

Aberrant expression and biological significance of Sox2, an embryonic stem cell transcriptional factor, in ALK-positive anaplastic large cell lymphoma

Sox2 (sex-determining region Y-Box) is one of the master transcriptional factors that are important in maintaining the pluripotency of embryonic stem cells (ESCs). In line with this function, Sox2 expression is largely restricted to ESCs and somatic stem cells. We report that Sox2 is expressed in cell lines and tumor samples derived from ALK-positive anaplastic large cell lymphoma (ALK+ALCL), for which the normal cellular counterpart is believed to be mature T-cells. The expression of Sox2 in ALK+ALCL can be attributed to nucleophosmin-anaplastic lymphoma kinase (NPM-ALK), the oncogenic fusion protein carrying a central pathogenetic role in these tumors. By confocal microscopy, Sox2 protein was detectable in virtually all cells in ALK+ALCL cell lines. However, the transcriptional activity of Sox2, as assessed using a Sox2-responsive reporter construct, was detectable only in a small proportion of cells. Importantly, downregulation of Sox2 using short interfering RNA in isolated Sox2active cells, but not Sox2inactive cells, resulted in a significant decrease in cell growth, invasiveness and tumorigenicity. To conclude, ALK+ALCL represents the first example of a hematologic malignancy that aberrantly expresses Sox2, which represents a novel mechanism by which NPM-ALK mediates tumorigenesis. We also found that the transcriptional activity and oncogenic effects of Sox2 can be heterogeneous in cancer cells.

NPM-ALK: The Prototypic Member of a Family of Oncogenic Fusion Tyrosine Kinases

Anaplastic lymphoma kinase (ALK) was first identified in 1994 with the discovery that the gene encoding for this kinase was involved in the t(2;5)(p23;q35) chromosomal translocation observed in a subset of anaplastic large cell lymphoma (ALCL). The NPM-ALK fusion protein generated by this translocation is a constitutively active tyrosine kinase, and much research has focused on characterizing the signalling pathways and cellular activities this oncoprotein regulates in ALCL. We now know about the existence of nearly 20 distinct ALK translocation partners, and the fusion proteins resulting from these translocations play a critical role in the pathogenesis of a variety of cancers including subsets of large B-cell lymphomas, nonsmall cell lung carcinomas, and inflammatory myofibroblastic tumours. Moreover, the inhibition of ALK has been shown to be an effective treatment strategy in some of these malignancies. In this paper we will highlight malignancies where ALK translocations have been identified and discuss why ALK fusion proteins are constitutively active tyrosine kinases. Finally, using ALCL as an example, we will examine three key signalling pathways activated by NPM-ALK that contribute to proliferation and survival in ALCL.

alpha-Sarcin catalytic activity is not required for cytotoxicity

Backgroundα-Sarcin is a protein toxin produced by Aspergillus giganteus. It belongs to a family of cytotoxic ribonucleases that inactivate the ribosome and inhibit protein synthesis. α-Sarcin cleaves a single phosphodiester bond within the RNA backbone of the large ribosomal subunit, which makes the ribosome unrecognizable to elongation factors and, in turn, blocks protein synthesis. Although it is widely held that the protein synthesis inhibition caused by the toxin leads to cell death, it has not been directly shown that catalytically inactive mutants of α-sarcin are non-toxic when expressed directly within the cytoplasm of cells. This is important since recent studies have cast doubt on whether protein synthesis inhibition is sufficient to initiate apoptosis.ResultsIn this report, we assay α-sarcin cytotoxicity and ability to inhibit protein synthesis by direct cytoplasmic expression. We show that mutations in α-sarcin, which impair α-sarcins ability to inhibit protein synthesis, do not affect its cytotoxicity. The mutants are unable to activate JNK, confirming that the sarcin-ricin loop remains intact and that the α-sarcin mutants are catalytically inactive. In addition, both mutant and wildtype variants of α-sarcin localize to the nucleus and cytoplasm, where they co-localize with ribosomal marker RPS6.ConclusionWe conclude that although protein synthesis inhibition likely contributes to cell death, it is not required. Thus, our results suggest that α-sarcin can promote cell death through a previously unappreciated mechanism that is independent of rRNA cleavage and JNK activation.

Aberrant expression of the transcriptional factor Twist1 promotes invasiveness in ALK-positive anaplastic large cell lymphoma

The transcriptional factor Twist1 has been shown to play a key role in regulating epithelial mesenchymal transition, invasiveness and migratory properties in solid tumors. We found that Twist1 is aberrantly expressed in ALK-positive anaplastic large cell lymphoma (ALK+ALCL), a type of T-cell lymphoid malignancy. Using RT-PCR and Western blots, Twist1 was detectable in all 3 ALK+ALCL cell lines examined but absent in normal T-cells. By immunohistochemistry, Twist1 was detectable in all 10 cases of ALK+ALCL examined; benign lymphoid tissues were consistently negative. Twist1 expression in ALK+ALCL cells can be attributed to the NPM-ALK/STAT3 signaling axis, the key oncogenic driving force in this tumor type. Twist1 is biologically important in ALK+ALCL cells, as Twist1 knockdown resulted in a significant decrease in their invasiveness in an in-vitro assay. Further investigation revealed that this increase in invasiveness is linked to the activation of AKT and down-regulation of p66Shc, two signaling proteins known to be involved in NPM-ALK-mediated oncogenesis. Lastly, knockdown of Twist1 sensitizes ALK+ALCL cells to the growth inhibitory effect of PF-2341066 (Crizotinib®), an ALK inhibitor being used in clinical trials. In conclusion, Twist1 expression, owing to the abnormal NPM-ALK/STAT3 signaling, contributes to its invasiveness and decreased sensitivity to PF-2341066 in ALK+ALCL.

Serine phosphorylation of NPM–ALK, which is dependent on the auto-activation of the kinase activation loop, contributes to its oncogenic potential

It is well established that the tumorigenic potential of nucleophosmin (NPM)-anaplastic lymphoma kinase (ALK), an oncogenic tyrosine kinase, is dependent on its tyrosine phosphorylation. Using tandem affinity purification-mass spectrometry, we found evidence of phosphorylation of three serine residues of NPM-ALK (Serine¹³⁵, Serine¹⁶⁴ and Serine⁴⁹⁷) ectopically expressed in GP293 cells. Using a specific anti-phosphoserine antibody and immunoprecipitation, we confirmed the presence of serine phosphorylation of NPM-ALK in all three NPM-ALK-expressing cell lines examined. Similar to the tyrosine phosphorylation, phosphorylation of these serine residues was dependent on the activation status of the kinase activation loop of ALK. All of these three serine residues are biologically important as mutation of any one of these residues resulted in a significant reduction in the tumorigenicity of NPM-ALK (assessed by cell viability and clonogenic assay), which correlated with a substantial reduction in the phosphorylation of extracellular signal-regulated kinase 1/2, c-jun N-terminal kinase and signal transducer and activator of transcription 6. Serine phosphorylation of NPM-ALK appears to be regulated by multiple serine kinases since it was markedly reduced by pharmacologic inhibitors for glycogen synthase kinase-3, casein kinase I or mitogen-activated protein kinases. In summary, our study is the first to identify serine phosphorylation of NPM-ALK and to provide evidence that it enhances the tumorigenic potential of this oncogenic protein.

The Tyrosine 343 Residue of Nucleophosmin (NPM)-Anaplastic Lymphoma Kinase (ALK) Is Important for Its Interaction with SHP1, a Cytoplasmic Tyrosine Phosphatase with Tumor Suppressor Functions

The cytoplasmic tyrosine phosphatase SHP1 has been shown to inhibit the oncogenic fusion protein nucleophosmin (NPM)-anaplastic lymphoma kinase (ALK), and loss of SHP1 contributes to NPM-ALK-mediated tumorigenesis. In this study, we aimed to further understand how SHP1 interacts and regulates NPM-ALK. We employed an in vitro model in which GP293 cells were transfected with various combinations of NPM-ALK (or mutants) and SHP1 (or mutants) expression vectors. We found that SHP1 co-immunoprecipitated with NPM-ALK, but not the enzymatically inactive NPM-ALKK210R mutant, or the mutant in which all three functionally important tyrosine residues (namely, Tyr338, Tyr342, and Tyr343) in the kinase activation loop (KAL) of ALK were mutated. Interestingly, whereas mutation of Tyr338 or Tyr342 did not result in any substantial change in the NPM-ALK/SHP1 binding (assessed by co-immunoprecipitation), mutation of Tyr343 abrogated this interaction. Furthermore, the NPM-ALK/SHP1 binding was readily detectable when each of the remaining 8 tyrosine residues known to be phosphorylated were mutated. Although the expression of SHP1 effectively reduced the level of tyrosine phosphorylation of NPM-ALK, it did not affect that of the NPM-ALKY343F mutant. In soft agar clonogenic assay, SHP1 expression significantly reduced the tumorigenicity of NPM-ALK but not that of NPM-ALKY343F. In conclusion, we identified Tyr343 of NPM-ALK as the crucial site for mediating the NPM-ALK/SHP1 interaction. Our results also support the notion that the tumor suppressor effects of SHP1 on NPM-ALK are dependent on its ability to bind to this oncogenic protein.

Disheveled proteins promote cell growth and tumorigenicity in ALK-positive anaplastic large cell lymphoma.

Our previous oligonucleotide array studies revealed that ALK-positive anaplastic large cell lymphoma (ALK(+)ALCL) express high levels of the disheveled proteins (Dvls), a family of proteins that is integral to the Wnt signaling pathways. In this study, we assessed whether the Dvls are important in the pathogenesis of ALK(+)ALCL. By Western blotting, Dvl-2 and Dvl-3 were found to be highly expressed in ALK(+)ALCL cell lines and patient samples. The higher molecular weight forms, consistent with phosphorylated/active Dvl proteins, were observed in these lysates. siRNA knock-down of Dvls did not affect the Wnt canonical pathway, as assessed by the β-catenin protein levels and nuclear localization. In contrast, the same treatment led to changes in the transcriptional activity of NFAT and the phosphorylation status of Src, both of which are known to be regulated by the Wnt non-canonical signaling pathways in other cell types. Coupled with these biochemical changes, there was a significant decrease in cell growth and soft agar colony formation. NPM-ALK, the oncogenic tyrosine kinase characteristic of ALK(+)ALCL, was found to bind to the Dvls and enhance their tyrosine phosphorylation. In conclusion, our data suggest that the Dvls contribute to the pathogenesis of ALK(+)ALCL via signaling in the Wnt non-canonical pathways. To our knowledge, this is the first report demonstrating a physical and functional interaction between the Dvls and an oncogenic tyrosine kinase.