Direct effects of Bmi1 on p53 protein stability inactivates oncoprotein stress responses in embryonal cancer precursor cells at tumor initiation.

Embryonal cancer can arise from postnatally persistent embryonal remnant or rest cells, which are uniquely characterized by the absence of p53 mutations. Perinatal overexpression of the MycN oncoprotein in embryonal cancer precursor cells causes postnatal rests, and later tumor formation through unknown mechanisms. However, overexpression of Myc in adult tissues normally activates apoptosis and/or senescence signals as an organismal defense mechanism against cancer. Here, we show that perinatal neuroblastoma precursor cells exhibited a transiently diminished p53 response to MycN oncoprotein stress and resistance to trophic factor withdrawal, compared with their adult counterpart cells from the TH-MYCN(+/+) transgenic mouse model of neuroblastoma. The adult stem cell maintenance factor and Polycomb group protein, Bmi1 (B-cell-specific Moloney murine leukemia virus integration site), had a critical role at neuroblastoma initiation in the model, by repressing p53 responses in precursor cells. We further show in neuroblastoma tumor cells that Bmi1 could directly bind p53 in a complex with other Polycomb complex proteins, Ring1A or Ring1B, leading to increased p53 ubiquitination and degradation. Repressed p53 signal responses were also seen in precursor cells for other embryonal cancer types, medulloblastoma and acute lymphoblastic leukemia. Collectively, these date indicate a general mechanism for p53 inactivation in some embryonal cell types and consequent susceptibility to MycN oncogenesis at the point of embryonal tumor initiation.

TRIM16 acts as a tumour suppressor by inhibitory effects on cytoplasmic vimentin and nuclear E2F1 in neuroblastoma cells.

The family of tripartite-motif (TRIM) proteins are involved in diverse cellular processes, but are often characterized by critical protein-protein interactions necessary for their function. TRIM16 is induced in different cancer types, when the cancer cell is forced to proceed down a differentiation pathway. We have identified TRIM16 as a DNA-binding protein with histone acetylase activity, which is required for the retinoic acid receptor ß(2) transcriptional response in retinoid-treated cancer cells. In this study, we show that overexpressed TRIM16 reduced neuroblastoma cell growth, enhanced retinoid-induced differentiation and reduced tumourigenicity in vivo. TRIM16 was only expressed in the differentiated ganglion cell component of primary human neuroblastoma tumour tissues. TRIM16 bound directly to cytoplasmic vimentin and nuclear E2F1 in neuroblastoma cells. TRIM16 reduced cell motility and this required downregulation of vimentin. Retinoid treatment and enforced overexpression caused TRIM16 to translocate to the nucleus, and bind to and downregulate nuclear E2F1, required for cell replication. This study, for the first time, demonstrates that TRIM16 acts as a tumour suppressor, affecting neuritic differentiation, cell migration and replication through interactions with cytoplasmic vimentin and nuclear E2F1 in neuroblastoma cells.

The function of melanotransferrin: a role in melanoma cell proliferation and tumorigenesis.

Melanotransferrin (MTf) or melanoma tumor antigen p97 is an iron (Fe) binding transferrin homolog expressed highly on melanomas and at lower levels on normal tissues. It has been suggested that MTf is involved in a variety of processes such as Fe metabolism and cellular differentiation. Considering the crucial role of Fe in many metabolic pathways, for example, DNA synthesis, it is important to understand the function of MTf. To define the roles of MTf, two models were developed: (i) an MTf knockout (MTf-/-) mouse and (ii) downregulation of MTf expression in melanoma cells by post-transcriptional gene silencing (PTGS). Examination of the MTf-/- mice demonstrated no differences compared with wild-type littermates. However, microarray analysis showed differential expression of molecules involved in proliferation such as Mef2a, Tcf4, Gls and Apod in MTf-/- mice compared with MTf+/+ littermates. Considering the role of MTf in melanoma cells, PTGS was used to downregulate MTf mRNA and protein levels by >90 and >80%, respectively. This resulted in inhibition of proliferation and migration. As found in MTf-/- mice, in melanoma cells with suppressed MTf expression, hMEF2A and hTCF4 were upregulated compared with parental cells. Furthermore, when melanoma cells with decreased MTf expression were injected into nude mice, tumor growth was markedly reduced, suggesting a role for MTf in proliferation and tumorigenesis.

Examination of the distribution of the transferrin homologue, melanotransferrin (tumour antigen p97), in mouse and human.

Melanotransferrin (MTf) is a transferrin homologue initially identified in melanoma cells. Serum transferrin (Tf) contains two iron (Fe)-binding sites and plays a vital role in Fe transport. However, human MTf has only a single, high affinity, Fe-binding site. Furthermore, while isolated MTf can bind Fe, it plays little role in Fe uptake by cells and its function remains elusive. To further understand the biological role of this molecule, we examined the expression profile of mouse MTf (mMTf) and human MTf (hMTf) and the splice variant of the latter. Analysis of mMTf in 18 normal mouse tissues and 4 embryonic stages (7-17 days) using an RNA dot blot demonstrated it was expressed at high levels in the pancreas, salivary gland and epididymis of the adult, while embryonic tissues showed low expression. The expression pattern was very different from that of mouse transferrin receptor 1 (TfR1) mRNA, which was found at high levels in the spleen and embryo. Using the more sensitive RT-PCR technique, mMTf expression was demonstrated across all 24 normal mouse tissues assessed. Analysis of the mMTf genomic sequence predicted only one mMTf transcript, although two putative transcripts were found in the testis using Northern blotting. An alternate hMTf transcript, h delta MTf, has been identified by others, although its tissue distribution was not previously examined. In human heart and skeletal muscle, three putative hMTf transcripts were identified at approximately 2, 3 and 4 kb, the smallest transcript being consistent with h delta MTf. The two larger transcripts were also found in 10 other human tissues. The h delta MTf transcript was detected using RT-PCR and Southern blotting in tumour-derived cell lines, with the highest expression being identified in melanoma cells. Immunohistochemistry showed that hMTf was expressed primarily within epithelia. In fact, the most pronounced expression was within the epidermis of the skin, tubules of the kidney and the ducts of sweat and salivary glands. The distribution of MTf and its splice variants may provide clues to their possible biological roles.

Cloning and expression of a prokaryotic enzyme, arginine deiminase, from a primitive eukaryote Giardia intestinalis.

Arginine deiminase (EC 3.5.3.6) catalyzes the irreversible catabolism of arginine to citrulline in the arginine dihydrolase pathway. This pathway has been regarded as restricted to prokaryotic organisms but is an important source of energy to the primitive protozoan Giardia intestinalis. In this paper we report the cloning and expression of the arginine deiminase gene from this parasite. Degenerate oligonucleotides based on amino acid sequences of tryptic peptides from the purified protein were used to amplify a portion of the arginine deiminase gene. This was then used as a probe to screen HindIII and PstI "mini" libraries to obtain two overlapping clones that contained the arginine deiminase gene. The open reading frame encoded 581 amino acids including all of the tryptic peptides that were sequenced and corresponded to a molecular mass of 67 kDa. Northern blot analysis identified a single 1.8-kilobase transcript in both trophozoites and encysting cells. Arginine deiminase was successfully expressed in Escherichia coli and purified to homogeneity. The recombinant protein was found to have characteristics comparable with those of the native enzyme.