Aurora-A acts as a tumor suppressor and regulates self-renewal of Drosophila neuroblasts

The choice of self-renewal versus differentiation is a fundamental issue in stem cell and cancer biology. Neural progenitors of the Drosophila post-embryonic brain, larval neuroblasts (NBs), divide asymmetrically in a stem cell-like fashion to generate a self-renewing NB and a Ganglion Mother Cell (GMC), which divides terminally to produce two differentiating neuronal/glial daughters. Here we show that Aurora-A (AurA) acts as a tumor suppressor by suppressing NB self-renewal and promoting neuronal differentiation. In aurA loss-of-function mutants, supernumerary NBs are produced at the expense of neurons. AurA suppresses tumor formation by asymmetrically localizing atypical protein kinase C (aPKC), an NB proliferation factor. Numb, which also acts as a tumor suppressor in larval brains, is a major downstream target of AurA and aPKC. Notch activity is up-regulated in aurA and numb larval brains, and Notch signaling is necessary and sufficient to promote NB self-renewal and suppress differentiation in larval brains. Our data suggest that AurA, aPKC, Numb, and Notch function in a pathway that involved a series of negative genetic interactions. We have identified a novel mechanism for controlling the balance between self-renewal and neuronal differentiation during the asymmetric division of Drosophila larval NBs.     

The adaptor protein SLP-65 acts as a tumor suppressor that limits pre-B cell expansion

Mice deficient in the adaptor protein SLP-65 (also known as BLNK) have reduced numbers of mature B cells, but an increased pre-B cell compartment. We show here that compared to wild-type cells, SLP-65(-/-) pre-B cells show an enhanced ex vivo proliferative capacity. This proliferation requires interleukin 7 and expression of the pre-B cell receptor (pre-BCR). In addition, SLP-65(-/-) mice have a high incidence of pre-B cell lymphoma. Reintroduction of SLP-65 into SLP-65(-/-) pre-B cells led to pre-BCR down-regulation and enhanced differentiation. Our results indicate that SLP-65 regulates a developmental program that promotes differentiation and limits pre-B cell expansion, thereby acting as a tumor suppressor.     

MicroRNA-126 acts as a tumor suppressor in glioma cells by targeting insulin receptor substrate 1 (IRS-1)

MicroRNA (miR-126) was reported to be downregulated and to act as a tumor suppressor in cancers of the lung, cervix, bladder, breast, liver and prostate. However, the precise roles and underling mechanisms of miR-126 in glioma remain largely unknown. This study is aimed to study the role of miR-126 in the progression of glioma and to elucidate underlying miR-126-mediated mechanisms in glioma. Our results revealed that miR-126 was downregulated in the collected glioma specimen, compared with non-cancerous brain tissues. Restored miR-126 expression inhibited cell proliferation, colony formation, migration and invasion, and induced cell cycle arrest at G0/G1 phase and cell apoptosis of U-87 MG glioma cells. Overexpression of miR-126 was also able to suppress the growth of U-87 MG glioma xenografts in mice. Furthermore, insulin receptor substrate 1 (IRS-1) were identified as a target of miR-126, and showed that it was negatively regulated by miR-126 in glioma cells. We also demonstrated that overexpression of miR-126 suppressed PI3K and AKT activation, which contribute to suppress tumor growth of glioma. Taken together, these findings showed that miR-126 functions as a tumor suppressor in glioma cells by targeting IRS-1 expression via the PI3K/AKT signaling pathways, suggesting that miR-126 might be a novel target for therapeutic strategies in glioma.     

MicroRNA-132 is frequently down-regulated in ductal carcinoma in situ (DCIS) of breast and acts as a tumor suppressor by inhibiting cell proliferation

Ductal carcinoma in situ (DCIS) is the most common type of non-invasive breast cancer. The currently accepted step-wise model suggests that breast cancer progressed in the following manner: normal breast cell → usually ductal hyperplasia (UDH) → atypical ductal hyperplasia (ADH) → DCIS → invasive ductal carcinoma (IDC). Therefore, DCIS can serve as a good model to analyze the mechanism underlying invasive breast cancer occurrence. MicroRNAs (miRNAs) are a novel class of small non-coding RNAs (∼22 nt) involved in the regulation of various biological processes. Altered miRNA expression could also contribute to the origination of cancer, including breast cancer. Here, by using miRNA microarray and real time PCR, we analyzed the miRNA expression profile in 21 DCIS and the corresponding normal tissues. miR-10b, miR-125b, miR-132, miR-145, miR-154-3p, miR-382-5p and miR-409-3p were found to be significantly deregulated in DCIS. Results from CCK-8 assay showed that the overexpression of miR-132 could inhibit the proliferation of breast cancer cell line. High expression of miR-132 could also inhibit the colony formation. Our findings will lead to further understanding of the development of breast cancer.     

miR-185 acts as a tumor suppressor by targeting AKT1 in non-small cell lung cancer cells

Increasing evidence has shown that microRNAs play critical roles in the initiation and progression of non-small cell lung cancer (NSCLC). miR-185 is deregulated in various cancers, whereas its functional mechanism in NSCLC is still unclear. Here, we confirmed that the expression of miR-185 was significantly down-regulated in NSCLC tissues and cell lines. miR-185 over-expression caused significant suppression of in vitro cell proliferation, migration and invasion, and in vivo tumor growth. We subsequently identified that AKT1 was a target gene of miR-185. Re-expression of AKT1 could partially rescue the inhibitory effects of miR-185 on the capacity of NSCLC cell proliferation and motility. Collectively, we conclude that miR-185 has a critical function by blocking AKT1 in NSCLC cells, and it may be a novel therapeutic agent for miRNA based NSCLC therapy.     

TRIM29 functions as a tumor suppressor in nontumorigenic breast cells and invasive ER+ breast cancer

Tripartite motif-containing 29 (TRIM29) is a member of the TRIM protein family that has been implicated in hematologic and solid tumor cancers. We found that TRIM29 functions as a tumor suppressor in both the nontumorigenic MCF10A [estrogen receptor (ER)-/TRIM29+] breast cell line and the invasive MCF7 (ER+/TRIM29-) breast cell line. Silencing TRIM29 in MCF10A cells resulted in preneoplastic changes that included loss of polarity in three-dimensional culture, increased proliferation, anchorage-independent growth, and increased migration and invasion. Conversely, the introduction of TRIM29 into MCF7 cells caused reversion to a less aggressive phenotype by antagonizing the growth effect of 17β-estradiol. The interaction between TRIM29 and ER signaling in MCF7 cells was supported by a reduction in ERE binding in the presence of TRIM29 and suppression of ER-dependent gene expression of TFF1, FOS, and GREB1. By microarray analyses, we showed that younger women (<55 years of age) with early-stage, ER+ breast cancer who were given no adjuvant systemic therapy had a significantly lower risk of relapse when their tumor had high TRIM29 expression (P = 0.02). This effect was not observed in older women (>55 years of age) and thus may be due to menopause and loss of circulating estrogens. Our results suggest that loss of TRIM29 expression in normal breast luminal cells can contribute to malignant transformation and lead to progression of ER+ breast cancer in premenopausal women.     

Heat shock protein 60 acts as a receptor for the Listeria adhesion protein in Caco-2 cells

The 104-kDa Listeria adhesion protein (LAP) in Listeria monocytogenes is involved in binding to various mammalian cell lines. However, the receptor that interacts with LAP in eukaryotic cells is unknown. In this study, scanning immunoelectron microscopy qualitatively demonstrated greater binding capacity of wild-type (WT) L. monocytogenes strain (F4244) than a LAP-deficient mutant strain (KB208) to Caco-2 cells. The goal of this study was identification of the host cell receptor for LAP. Using a Western blot ligand overlay assay, we identified a protein of 58 kDa to be the putative receptor for LAP from Caco-2 cells. N-terminal sequencing and subsequent database search identified this protein as heat shock protein 60 (Hsp60). Modified immunoseparation with protein A-Sepharose beads bound to the LAP-specific monoclonal antibody H7 (MAb-H7) and a sequential incubation with LAP preparation and Caco-2 lysate confirmed the receptor to be the same 58-kDa protein. Western blot analysis with anti-Hsp60 MAb of whole-cell adhesion between Caco-2 and WT also revealed the receptor protein to be a 58-kDa protein, thus corroborating the identification of Hsp60 as a host cell receptor for LAP. Furthermore, the anti-Hsp60 antibody also caused approximately 74% reduction in binding of L. monocytogenes WT to Caco-2 cells, whereas a control antibody, C11E9, had no effect on binding. The adhesion mechanism of L. monocytogenes to eukaryotic cells is a complex process, and identification of Hsp60 as a receptor for LAP adds to the list of previously discovered ligand-receptor modules that are essential to achieve successful adhesion.     

The tuberous sclerosis gene on chromosome 9q34 acts as a growth suppressor

We have previously demonstrated allele loss in hamartomas frompatients with tuberous sclerosis for markers spanning the tuberoussclerosis gene on chromosome 16p13.3 (TSC2). Germline deletionsin the TSC2 gene have been shown in 5% of patients with tuberoussclerosis (TSC). These data support our hypothesis that theTSC2 gene acts as a growth suppressor gene, analogous to thetraditional tumour suppressor gene. We now report a TSC hamartomashowing allele loss for markers on chromosome 9q34 in the regionof the TSC1 gene. We studied six hamartomas from four sporadicand two familial cases of TSC, none of which showed allele lossfor markers on chromosome 16p13.3. The hamartomas were paraffinembedded sections of three renal angiomyolipomas, two giantcell astrocytomas, and a cardiac rhabdomyoma. Eight markerswere analysed, comprising from centromeric to telomeric ASS– D9S64 – D9S149 – ABO – D9S150 –DBH – D9S66 – D9S67. One angiomyolipoma showed alleleloss for the markers ABO, DBH and D9S66, but not for D9S149or D9S67. The patient was not informative for D9S150. The familystructure did not permit the phase of the disease and markeralleles to be determined. These finding support the hypothesisthat the TSC1 gene on 9q34, like the TSC2 gene, acts as a growthsuppressor. The data would place the TSC1 gene between D9S149and D9S67. Mapping of allele loss in hamartomas may help inthe refinement of the location of the TSC1 locus.     

The Citrus leaf blotch virus movement protein acts as silencing suppressor

To counteract plant antiviral defense based on RNA silencing, many viruses express proteins that inhibit this mechanism at different levels. The genome of Citrus leaf blotch virus (CLBV) encodes a 227-kDa protein involved in replication, a 40-kDa movement protein (MP), and a 41-kDa coat protein (CP). To determine if any of these proteins might have RNA silencing suppressor activities, we have used Agrobacterium-mediated transient assays in the green fluorescent protein (GFP)-expressing Nicotiana benthamiana line 16c. Only CLBV MP was able to suppress intracellular GFP silencing induced by expression of either single- or double-stranded (ds) GFP RNA, but not cell-to-cell or long distance spread of the silencing signal. The MP suppressor activity was weak compared to other characterized viral suppressor proteins. Overall our data indicate that MP acts as a suppressor of local silencing probably by interfering in the silencing pathway downstream of the steps of dsRNA and small RNAs generation.     

Defining CD95 as a tumor suppressor gene

The CD95 (Apo-1/Fas) receptor-ligand system is one of the key regulators of apoptosis and is particularly important for the maintenance of lymphocyte homeostasis. There is now broad evidence that susceptibility of tumor cells towards CD95-mediated apoptosis is largely reduced. In the human, germline and somatic mutations of the CD95 gene are associated with a high risk of both lymphoid and solid tumors. Based on these observations a new concept defining CD95 as a tumor suppressor gene is discussed. In addition to CD95, its natural ligand (CD95L) is also implicated in malignant progression. Compared to their nonmalignant counterparts, malignant cells frequently exhibit aberrant de novo expression of CD95L and are able to induce CD95L-mediated apoptosis in bystander cells. The role for neoplastic CD95L expression in local tissue destruction, invasion, and metastatic spread has been established for many tumor types. CD95L expression by malignant cells may counteract the host's antitumor immunity and favors immune escape of the tumor. On this basis, the significance of loss of CD95 and gain of CD95L expression for tumor progression is discussed.     

Dicer1 functions as a haploinsufficient tumor suppressor

While the global down-regulation of microRNAs (miRNAs) is a common feature of human tumors, its genetic basis is largely undefined. To explore this question, we analyzed the consequences of conditional Dicer1 mutation (Dicer1 “floxed” or Dicer1^fl) on several mouse models of cancer. Here we show Dicer1 functions as a haploinsufficient tumor suppressor gene. Deletion of a single copy of Dicer1 in tumors from Dicer1^fl/+ animals led to reduced survival compared with controls. These tumors exhibited impaired miRNA processing but failed to lose the wild-type Dicer1 allele. Moreover, tumors from Dicer1^fl/fl animals always maintained one functional Dicer1 allele. Consistent with selection against full loss of Dicer1 expression, enforced Dicer1 deletion caused inhibition of tumorigenesis. Analysis of human cancer genome copy number data reveals frequent deletion of DICER1. Importantly, however, the gene has not been reported to undergo homozygous deletion, suggesting that DICER1 is haploinsufficient in human cancer. These findings suggest Dicer1 may be an important haploinsufficient tumor suppressor gene and, furthermore, that other factors controlling miRNA biogenesis may also function in this manner.     

Wounding acts as a tumor promoter in chickens inoculated with avian sarcoma virus 17.

Avian sarcoma virus 17 (ASV17) is an acutely transforming retrovirus which carries the oncogene v-jun. The virus induces fibrosarcomas in chickens at the site of inoculation. Here we describe wound-related tumor formation in 77% of chickens inoculated with ASV17 in one wing and wounded by metal clip insertion in the opposite wing. Tumors from both wound-related and inoculation-related sites were histologically diagnosed as fibrosarcomas. Tissues cultured from both tumor sites produced infectious virus in culture and expressed high levels of the v-Jun oncoprotein detectable by immunofluorescent staining. By varying the time of wounding relative to virus inoculation we defined the early stages of wound healing (2-7 days postinoculation) as favoring wound-related tumor formation. Three other acutely transforming retroviruses containing oncogenes coding for nonreceptor protein tyrosine kinases (v-src, v-yes, and v-fps), inoculated in the same manner, induced wound-related tumors in all cases. We conclude that in chickens, ASV17 collaborates with wound healing to promote tumorigenesis by a process which may relate either to a biochemical function of Jun or to a more general, shared characteristic of transforming retroviruses.