Receptor-type protein tyrosine phosphatase κ directly dephosphorylates CD133 and regulates downstream AKT activation.

Although CD133 has been considered to be a molecular marker for cancer stem cells, its functional roles in tumorigenesis remain unclear. We here examined the molecular basis behind CD133-mediated signaling. Knockdown of CD133 resulted in the retardation of xenograft tumor growth of colon cancer-derived HT-29 and LoVo cells accompanied by hypophosphorylation of AKT, which diminished ß-catenin/T-cell factor-mediated CD44 expression. As tyrosine residues of CD133 at positions 828 and 852 were phosphorylated in HT-29 and SW480 cells, we further addressed the significance of this phosphorylation in the tumorigenesis of SW480 cells expressing mutant CD133, with substitution of these tyrosine residues by glutamate (CD133-EE) or phenylalanine (CD133-FF). Forced expression of CD133-EE promoted much more aggressive xenograft tumor growth relative to wild-type CD133-expressing cells accompanied by hyperphosphorylation of AKT; however, CD133-FF expression had negligible effects on AKT phosphorylation and xenograft tumor formation. Intriguingly, the tyrosine phosphorylation status of CD133 was closely linked to the growth of SW480-derived spheroids. Using yeast two-hybrid screening, we finally identified receptor-type protein tyrosine phosphatase κ (PTPRK) as a binding partner of CD133. In vitro studies demonstrated that PTPRK associates with the carboxyl-terminal region of CD133 through its intracellular phosphatase domains and also catalyzes dephosphorylation of CD133 at tyrosine-828/tyrosine-852. Silencing of PTPRK elevated the tyrosine phosphorylation of CD133, whereas forced expression of PTPRK reduced its phosphorylation level markedly and abrogated CD133-mediated AKT phosphorylation. Endogenous CD133 expression was also closely associated with higher AKT phosphorylation in primary colon cancer cells, and ectopic expression of CD133 enhanced AKT phosphorylation. Furthermore, lower PTPRK expression significantly correlated with the poor prognosis of colon cancer patients with high expression of CD133. Thus, our present findings strongly indicate that the tyrosine phosphorylation of CD133, which is dephosphorylated by PTPRK, regulates AKT signaling and has a critical role in colon cancer progression.

CD133 suppresses neuroblastoma cell differentiation via signal pathway modification.

CD133 (prominin-1) is a transmembrane glycoprotein expressed on the surface of normal and cancer stem cells (tumor-initiating cells), progenitor cells, rod photoreceptor cells and a variety of epithelial cells. Although CD133 is widely used as a marker of various somatic and putative cancer stem cells, its contribution to the fundamental properties of cancer cells, such as tumorigenesis and differentiation, remains to be elucidated. In the present report, we found that CD133 was expressed in several neuroblastoma (NB) cell lines/tumor samples. Intriguingly, CD133 repressed NB cell differentiation, for example neurite extension and the expression of differentiation marker proteins, and was decreased by several differentiation stimuli, but accelerated cell proliferation, anchorage-independent colony formation and in vivo tumor formation of NB cells. NB cell line and primary tumor-sphere experiments indicated that the molecular mechanism of CD133-related differentiation suppression in NB was in part dependent on neurotrophic receptor RET tyrosine kinase regulation. RET transcription was suppressed by CD133 in NB cells and glial cell line-derived neurotrophic factor treatment failed to induce RET in CD133-expressing cells; RET overexpression rescued CD133-related inhibition of neurite elongation. Of note, CD133-related NB cell differentiation and RET repression were mainly dependent on p38MAPK and PI3K/Akt pathways. Furthermore, CD133 has a function in growth and RET expression in NB cell line- and primary tumor cell-derived tumor spheres. To the best of our knowledge, this is the first report of the function of CD133 in cancer cells and our findings may be applied to improve differentiation induction therapy for NB patients.

Bmi1 is a MYCN target gene that regulates tumorigenesis through repression of KIF1Bbeta and TSLC1 in neuroblastoma.

Recent advances in neuroblastoma (NB) research addressed that epigenetic alterations such as hypermethylation of promoter sequences, with consequent silencing of tumor-suppressor genes, can have significant roles in the tumorigenesis of NB. However, the exact role of epigenetic alterations, except for DNA hypermethylation, remains to be elucidated in NB research. In this paper, we clarified the direct binding of MYCN to Bmi1 promoter and upregulation of Bmi1 transcription by MYCN. Mutation introduction into an MYCN binding site in the Bmi1 promoter suggests that MYCN has more important roles in the transcription of Bmi1 than E2F-related Bmi1 regulation. A correlation between MYCN and polycomb protein Bmi1 expression was observed in primary NB tumors. Expression of Bmi1 resulted in the acceleration of proliferation and colony formation in NB cells. Bmi1-related inhibition of NB cell differentiation was confirmed by neurite extension assay and analysis of differentiation marker molecules. Intriguingly, the above-mentioned Bmi1-related regulation of the NB cell phenotype seems not to be mediated only by p14ARF/p16INK4a in NB cells. Expression profiling analysis using a tumor-specific cDNA microarray addressed the Bmi1-dependent repression of KIF1Bbeta and TSLC1, which have important roles in predicting the prognosis of NB. Chromatin immunoprecipitation assay showed that KIF1Bbeta and TSLC1 are direct targets of Bmi1 in NB cells. These findings suggest that MYCN induces Bmi1 expression, resulting in the repression of tumor suppressors through Polycomb group gene-mediated epigenetic chromosome modification. NB cell proliferation and differentiation seem to be partially dependent on the MYCN/Bmi1/tumor-suppressor pathways.

Plk1 regulates liver tumor cell death by phosphorylation of TAp63.

We previously found that Plk1 inhibited the p53/p73 activity through its direct phosphorylation. In this study, we investigated the functional role of Plk1 in modulating the p53 family member TAp63, resulting in the control of apoptotic cell death in liver tumor cells. Immunoprecipitation and in vitro pull-down assay showed that p63 binds to the kinase domain of Plk1 through its DNA-binding region. in vitro kinase assay indicated that p63 is phosphorylated by Plk1 at Ser-52 of the transactivating (TA) domain. Plk1 decreased the protein stability of TAp63 by its phosphorylation and suppressed TAp63-induced cell death. Furthermore, Plk1 knockdown in p53-mutated liver tumor cells transactivated p53 family downstream effectors, PUMA, p21(Cip1/WAF1) and 14-3-3sigma, and induced apoptotic cell death. Double knockdown of Plk1/p63 attenuated Plk1 knockdown-induced apoptotic cell death and transactivation. Intriguingly, both Plk1 and p63 are highly expressed in the side population (SP) fraction of liver tumor cells compared to non-SP fraction cells, suggesting the significance of Plk1/TAp63 in the control of cell death in tumor-initiating SP fraction cells. Thus, Plk1 controls TAp63 by its phosphorylation and regulates apoptotic cell death in liver tumor cells. Plk1/TAp63 may be a suitable candidate as a molecular target of liver tumor treatments.

N-MYC promotes cell proliferation through a direct transactivation of neuronal leucine-rich repeat protein-1 (NLRR1) gene in neuroblastoma.

Neuronal leucine-rich repeat protein-1 (NLRR1) gene encodes a type I transmembrane protein with unknown function. We have previously described that NLRR1 gene is highly expressed in unfavorable neuroblastomas as compared with favorable tumors and its higher expression levels correlate significantly with poor clinical outcome. In this study, we have found that NLRR1 gene is one of direct target genes for N-MYC and its gene product contributes to N-MYC-dependent growth promotion in neuroblastoma. Expression levels of NLRR1 were significantly associated with those of N-MYC in various neuroblastoma cell lines as well as primary neuroblastoma tissues. Indeed, enforced expression of N-MYC resulted in a remarkable induction of the endogenous NLRR1. Consistent with these results, we have identified two functional E-boxes within the promoter region and intron 1 of NLRR1 gene. Intriguingly, c-myc also transactivated NLRR1 gene. Enforced expression of NLRR1 promoted cell proliferation and rendered cells resistant to serum deprivation. In support with these observations, small-interfering RNA-mediated knockdown of the endogenous NLRR1-reduced growth rate and sensitized cells to serum starvation. Collectively, our present findings provide a novel insight into understanding molecular mechanisms behind aggressive neuroblastoma with N-MYC amplification.