Id proteins facilitate self-renewal and proliferation of neural stem cells

The APC/C Ubiquitin Ligase: From Cell Biology to Tumorigenesis

The ubiquitin proteasome system (UPS) is required for normal cell proliferation, vertebrate development, and cancer cell transformation. The UPS consists of multiple proteins that work in concert to target a protein for degradation via the 26S proteasome. Chains of an 8.5-kDa protein called ubiquitin are attached to substrates, thus allowing recognition by the 26S proteasome. Enzymes called ubiquitin ligases or E3s mediate specific attachment to substrates. Although there are over 600 different ubiquitin ligases, the Skp1-Cullin-F-box (SCF) complexes and the anaphase promoting complex/cyclosome (APC/C) are the most studied. SCF involvement in cancer has been known for some time while APC/C's cancer role has recently emerged. In this review we will discuss the importance of APC/C to normal cell proliferation and development, underscoring its possible contribution to transformation. We will also examine the hypothesis that modulating a specific interaction of the APC/C may be therapeutically attractive in specific cancer subtypes. Finally, given that the APC/C pathway is relatively new as a cancer target, therapeutic interventions affecting APC/C activity may be beneficial in cancers that are resistant to classical chemotherapy.

Inhibitor of DNA-binding 4 contributes to the maintenance and expansion of cancer stem cells in 4T1 mouse mammary cancer cell line

The cancer stem cell (CSC) hypothesis proposes that CSCs are the root of cancer. CSC-targeted therapies may prevent cancer relapse and provide more effective treatment. The expression of aldehyde dehydrogenase 1, as assessed by the Aldefluor assay, has been recognized as a marker of CSCs in breast cancer. Inhibitors of DNA-binding proteins (IDs) have an important role in stem cell differentiation. In this study, we examined IDs necessary for the regulation of stem properties in Aldefluor^pos 4T1 cells. When the expression profile of IDs in Aldefluor^neg and Aldefluor^pos 4T1 cells was compared, qRT-PCR analysis showed that ID4 expression was highly upregulated in Aldefluor^pos 4T1 cells. In addition, knockdown of ID4 expression suppressed the properties of CSCs, including their sphere-forming ability and side population phenotype. The findings suggest that ID4 may be a therapeutic target for the treatment of advanced breast cancer.

p53 directly represses Id2 to inhibit the proliferation of neural progenitor cells

Neural progenitor cells (NPCs) have the capacity to proliferate and give rise to all major central nervous system cell types and represent a possible cell of origin in gliomagenesis. Deletion of the tumor suppressor gene Tp53 (p53) results in increased proliferation and self-renewal of NPCs and is a common genetic mutation found in glioma. We have identified inhibitor of DNA binding 2 (Id2) as a novel target gene directly repressed by p53 to maintain normal NPC proliferation. p53((-/-)) NPCs express elevated levels of Id2 and suppression of Id2 expression is sufficient to inhibit the increased proliferation and self-renewal which results from p53 loss. Elevated expression of Id2 in wild-type NPCs phenocopies the behavior of p53((-/-)) NPCs by enhancing NPC proliferation and self-renewal. Interestingly, p53 directly binds to a conserved site within the Id2 promoter to mediate these effects. Finally, we have identified elevated Id2 expression in glioma cell lines with mutated p53 and demonstrated that constitutive expression of Id2 plays a key role in the proliferation of glioma stem-like cells. These findings indicate that Id2 functions as a proproliferative gene that antagonizes p53-mediated cell cycle regulation in NPCs and may contribute to the malignant proliferation of glioma-derived tumor stem cells.

Malignant rhabdoid tumors express stem cell factors, which relate to the expression of EZH2 and Id proteins

Malignant rhabdoid tumors (MRTs) are highly aggressive pediatric tumors associated with loss of expression of SMARCB1, commonly occurring in the central nervous system [referred to as atypical teratoid/rhabdoid tumors (AT/RTs)] and in the kidney and soft tissues. Histologically, MRTs are characterized by immunohistochemical evidence of primitive neuroectodermal, mesenchymal, and epithelial differentiation. The ability of MRTs to differentiate along multiple lines, as evidenced by both histologic features and polyphenotypic immunohistochemical staining, and the proliferative nature of MRT cells are characteristics shared with the self-renewal and plasticity of embryonic stem cells (ES). To test the hypothesis that MRTs share similarities with ES, we used immunohistochemistry to evaluate the expression of various stem cell markers in a tissue microarray containing 26 AT/RTs and 16 non-central nervous system MRTs (NCMRTs). Staining intensity was scored as negative (0), low (1+), moderate (2+), and strong (3+) and was multiplied by the percentage of positive tumor cells to establish a semiquantitative measure for each marker. In AT/RT, strong-to-low expression was noted with glypican-3 (20 of 26, 77%), Sall4 (23 of 26, 88%), T-cell leukemia/lymphoma 1 (25 of 26, 96%), and undifferentiated embryonic cell transcription factor 1 (19 of 26, 73%). Markers that showed low expression in AT/RT were Sox2 (8 of 26, 31%), Nanog (7 of 26, 27%), Klf4 (10 of 26, 38%), Zfp206 (5 of 26, 19%), and musashi-1 (21 of 26, 81%). Similarly, in NCMRT, expression was noted with glypican-3 (12 of 16, 75%), Sall4 (13 of 16, 81%), T-cell leukemia/lymphoma 1 (16 of 16, 100%), undifferentiated embryonic cell transcription factor 1 (12 of 16, 75%), Sox2 (5 of 16, 31%), Nanog (8 of 16, 50%), Klf4 (8 of 16, 50%), Zfp206 (13 of 16, 81%), and musashi-1 (11 of 16, 75%). Placental alkaline phosphatase, Oct4, c-KIT, CD30, α-fetoprotein, and β- -human chorionic gonadotrophin were not expressed in all cases. Markers that regulate the expression of stem cell transcription factors were also expressed in MRT. AT/RT cases showed expression of Id proteins: Id1 (17 of 26, 65%), Id2 (24 of 26, 92%), Id3 (22 of 26, 85%), and Id4 (22 of 26, 85%). Low expression was observed with EZH2 (15 of 26, 58%). Similarly, NCMRT cases showed expression of Id1 (15 of 16, 94%), Id2 (16 of 16, 100%), Id3 (16 of 16, 100%), Id4 (13 of 16, 81%), and EZH2 (13 of 16, 81%). Finally, regression analysis revealed a significant relationship between the expression of stem cell markers and EZH2 (P<0.0001), Id1 (P=0.0087), Id2 (P=0.0002), Id3 (P=0.0033), and Id4 (P<0.0001). These data suggest that MRTs express many stem cell-associated transcription factors, which may be regulated by the expression of EZH2 and the Id family of proteins. This study underscores similarities between MRTs and stem cells and may help elucidate common biologic pathways that could serve in advancing more effective therapeutic strategies to treat MRTs.

Ochratoxin A at nanomolar concentration perturbs the homeostasis of neural stem cells in highly differentiated but not in immature three-dimensional brain cell cultures

Ochratoxin A (OTA), a fungal contaminant of basic food commodities, is known to be highly cytotoxic, but the pathways underlying adverse effects at subcytotoxic concentrations remain to be elucidated. Recent reports indicate that OTA affects cell cycle regulation. Therefore, 3D brain cell cultures were used to study OTA effects on mitotically active neural stem/progenitor cells, comparing highly differentiated cultures with their immature counterparts. Changes in the rate of DNA synthesis were related to early changes in the mRNA expression of neural stem/progenitor cell markers. OTA at 10nM, a concentration below the cytotoxic level, was ineffective in immature cultures, whereas in mature cultures it significantly decreased the rate of DNA synthesis together with the mRNA expression of key transcriptional regulators such as Sox2, Mash1, Hes5, and Gli1; the cell cycle activator cyclin D2; the phenotypic markers nestin, doublecortin, and PDGFRα. These effects were largely prevented by Sonic hedgehog (Shh) peptide (500ngml(-1)) administration, indicating that OTA impaired the Shh pathway and the Sox2 regulatory transcription factor critical for stem cell self-renewal. Similar adverse effects of OTA in vivo might perturb the regulation of stem cell proliferation in the adult brain and in other organs exhibiting homeostatic and/or regenerative cell proliferation.

Id2a influences neuron and glia formation in the zebrafish retina by modulating retinoblast cell cycle kinetics

Inhibitor of differentiation (Id) family helix-loop-helix proteins regulate the proliferation, survival and differentiation of numerous cell types during development; however, their functions during retinal development have not been analyzed. Using loss-of-function and overexpression assays in zebrafish, we demonstrate that Id2a levels modulate retinoblast cell cycle kinetics and thereby influence neuron and glia formation in the retina. Id2a-deficient retinas possess increased numbers of cells occupying S phase, at the expense of mitotic cells, and kinetic analyses demonstrate that Id2a is required for S-phase progression and/or the transition from S to M phase. Id2a-dependent defects in retinoblast proliferation lead to microphthalmia and to an absence of nearly all differentiated inner and outer nuclear layer cell types. Overexpression of id2a has the opposite effect on retinoblast cell cycle kinetics: id2a-overexpressing retinoblasts progress from S to M phase more rapidly and they undergo mitosis more frequently, which results in macrophthalmia. Mosaic analyses reveal that Id2a function in facilitating both cell cycle progression and neuronal differentiation in the retina is non-cell-autonomous, suggesting that Id2a functions upstream of the extrinsic pathways that regulate retinogenesis.