On the intracellular trafficking of mouse S5 ribosomal protein from cytoplasm to nucleoli.

The non-ribosomal functions of mammalian ribosomal proteins have recently attracted worldwide attention. The mouse ribosomal protein S5 (rpS5) derived from ribosomal material is an assembled non-phosphorylated protein. The free form of rpS5 protein, however, undergoes phosphorylation. In this study, we have (a) investigated the potential role of phosphorylation in rpS5 protein transport into the nucleus and then into nucleoli and (b) determined which of the domains of rpS5 are involved in this intracellular trafficking. In vitro PCR mutagenesis of mouse rpS5 cDNA, complemented by subsequent cloning and expression of rpS5 truncated recombinant forms, produced in fusion with green fluorescent protein, permitted the investigation of rpS5 intracellular trafficking in HeLa cells using confocal microscopy complemented by Western blot analysis. Our results indicate the following: (a) rpS5 protein enters the nucleus via the region 38-50 aa that forms a random coil as revealed by molecular dynamic simulation. (b) Immunoprecipitation of rpS5 with casein kinase II and immobilized metal affinity chromatography analysis complemented by in vitro kinase assay revealed that phosphorylation of rpS5 seems to be indispensable for its transport from nucleus to nucleoli; upon entering the nucleus, Thr-133 phosphorylation triggers Ser-24 phosphorylation by casein kinase II, thus promoting entrance of rpS5 into the nucleoli. Another important role of rpS5 N-terminal region is proposed to be the regulation of protein's cellular level. The repetitively co-appearance of a satellite C-terminal band below the entire rpS5 at the late stationary phase, and not at the early logarithmic phase, of cell growth suggests a specific degradation balancing probably the unassembled ribosomal protein molecules with those that are efficiently assembled to ribosomal subunits. Overall, these data provide new insights on the structural and functional domains within the rpS5 molecule that contribute to its cellular functions.

Genotoxic and cytotoxic effects of different types of dental cement on normal cultured human lymphocytes.

In this study we have investigated the genotoxic and cytotoxic effects of eluates derived from different types of commercially available dental cements, including glass ionomer cements (GICs) (Ketac Cem/3M ESPE and GC Fuji I/GC Corp), resin-modified glass ionomer cements (RM-GICs) (RelyX Luting/3M ESPE and Vitrebond/3M ESPE) and dual-cure resin cements (RCs) (Variolink II/ Ivoclar-Vivadent and Panavia F 2.0/Kuraray) on normal cultured human lymphocytes. Lymphocyte primary cultures obtained from blood samples of three healthy donors were exposed to serial dilutions of eluates derived from specimens of each material tested. Metaphases were induced with phytohaemagglutinin, collected after 72h treatment by use of colchicine and stained according to the fluorescence plus giemsa (FPG) procedure. Preparations were scored for sister chromatid exchange (SCE) and chromosomal aberrations (CAs), while the proliferation rate index (PRI) was also calculated. Our results show that eluates derived from the RM-GICs and RCs caused severe genotoxic effects by significantly increasing the frequencies of SCEs and CAs in cultures of peripheral blood lymphocytes and by decreasing the relevant PRI values in a dose-dependent manner, whereas the two GICs caused only minor cytogenetic effects. Eluates of the two RM-GICs (Vitrebond and RelyX) were also very cytotoxic, as the first serial dilutions of both materials caused a complete mitotic arrest in lymphocyte cultures. Overall, the degree of genotoxicity and cytotoxicity caused by dental cements decreased as follows: Viterbond>Rely X>Panavia F 2.0>Variolink II>Ketac Cem=GC Fuji I. These results indicate that different types of dental cement differ extensively in their genotoxic and cytotoxic potential and their ability to affect chromosomal integrity, cell-cycle progression, DNA replication and repair. Although these results cannot be directly extrapolated to the clinical situation, the potential occurrence of adverse effects caused by the RM-GICs and RCs tested in this study should be considered when making a clinical decision about dental cements.

Sister-chromatid exchange, chromosomal aberrations and delays in cell-cycle kinetics in human lymphocytes induced by dental composite resin eluates.

We have investigated eluates derived from commercially available composite resin-based materials used for direct (Tetric Ceram/Ivoclar-Vivadent, Simile/Pentron, Filtek Z-250/3M ESPE) and indirect (Adoro/Ivoclar-Vivadent and Conquest Sculpture/Pentron) dental restorations, with respect to their genotoxic effects on human peripheral lymphocytes. Primary lymphocyte cultures obtained from blood samples of three healthy donors were exposed to eluates of freshly cured specimens of all the materials tested. Metaphases were induced with phytohaemagglutinin, collected after a 72-h treatment using colchicine and stained with the Fluorescence Plus Giemsa (FPG) procedure. Preparations were scored for sister-chromatid exchange (SCE) and chromosomal aberrations (CAs). The proliferation rate index (PRI) and the mitotic index (MI) were also calculated. Our results show that eluates derived from the three direct composites (Filtek Z-250, Simile and Tetric Ceram) increased the frequencies of SCE and CAs and markedly reduced PRI and MI. Tetric Ceram's eluate, being the most genotoxic of all eluates tested, increased the frequencies of SCE up to 24.40 per cell (control, 9.87 per cell) and of CAs up to 424 per 100 metaphases scored (control, 5). Moreover, it caused a pronounced decrease of the PRI down to 1.31 (control, 2.44) and of the MI down to 9.8 per thousand (control, 19.2 per thousand). In contrast, eluates derived from the laboratory-processed composites (Adoro and Conquest Sculpture) induced much less cytogenetic damage. Overall, the degree of genotoxicity and cytotoxicity decreased as follows: Tetric Ceram>Filtek Z-250>Simile>Adoro=Conquest Sculpture. These results indicate that composite resins used for direct and indirect dental restorations differ extensively in their cytotoxic and genotoxic potential and in their ability to affect chromosomal integrity, cell-cycle progression, DNA replication and repair. This underlines the impact of improved polymerization with respect to their biological behavior.

Cloning, sequencing and expression of a cDNA encoding the mouse L35a ribosomal protein during differentiation of murine erythroleukemia (MEL) cells.

In a previous study we reported that ribosomal protein S5 gene is suppressed in differentiating and not in proliferating or apoptotic murine erythroleukaemia (MEL) cells (Vizirianakis et al., 1999). We wish to report here the isolation, characterisation and expression of the full length cDNA for another ribosomal protein, the L35a (rpL35a), in MEL cells. This cDNA shares significant structural homology in both DNA and protein levels to genes encoding the rat and human L35a ribosomal proteins. Northern blot hybridisation analysis has shown that the steady-state level of rpL35a mRNA is progressively reduced during differentiation of MEL cells along the erythrocytic maturation pathway induced by DMSO or UDP-4, two structurally unrelated inducers of differentiation. However, in cells where differentiation was inhibited by N(6)-methyladenosine, the level of rpL35a RNA transcripts was not affected. In addition, rpL35a gene expression was not altered in apoptotic MEL cells. Furthermore, the suppression of L35a gene was not correlated to any change in DNA methylation at CCGG sites located at the rpL35a gene locus in undifferentiated and differentiated MEL cells, as we observed for the rpS5 gene. Overall, these data suggest that the expression of ribosomal genes, the L35a of 60S ribosomal subunit and the S5 of 40S ribosomal subunit, are regulated by a common mechanism in differentiating MEL cells, leading to the observed decrease in ribosomal function.

Protein secretion biotechnology using Streptomyces lividans: large-scale production of functional trimeric tumor necrosis factor alpha.

We evaluated the feasibility of large-scale production of biopharmaceuticals expressed as heterologous polypeptides from the Gram-positive bacterium Streptomyces lividans. As a model protein we used murine tumor necrosis factor alpha (mTNFalpha). mTNFalpha fused C-terminally to the secretory signal peptide of the subtilisin-inhibitor protein from Streptomyces venezuelae. Under appropriate fermentation conditions, significant amounts of mature mTNFalpha (80-120 mg/L) can be recovered from spent growth media. Efficient downstream processing allowing rapid purification of mTNFalpha from culture supernatants was developed. Importantly, the protein is recovered from the spent growth medium in its native trimeric state as judged by biophysical analysis. Further, mTNFalpha secreted by S. lividans is significantly more active in an in vitro apoptosis tissue culture assay than a corresponding polypeptide produced in Escherichia coli. This pilot study provides the first validation of S. lividans protein secretion as an alternative bioprocess for large-scale production of oligomeric proteins of potential therapeutic value.

Transfection of MCF-7 carcinoma cells with human integrin alpha7 cDNA promotes adhesion to laminin.

The laminin-binding alpha7beta1 integrin receptor is highly expressed by skeletal and cardiac muscles, and has been suggested to be a crucial molecule during myogenic cell migration and differentiation. Absence of integrin alpha7 subunit contributes to a form of muscular dystrophy in integrin alpha7 null mice, whereas specific mutations in the alpha7 gene are associated in humans with congenital myopathy. To examine in more detail the potential role of integrin alpha7 in human-related muscular disorders, we cloned alpha7 cDNA by RT-PCR from human skeletal muscle mRNA and then expressed the full-length human integrin alpha7 cDNA by transfection in several cell lines including MCF-7, COS-7, and NIH3T3 cells. The isolated cDNA corresponds to the human alpha7X2B alternative splice form. Expression of human alpha7 was further confirmed by transfection of chimeric human/mouse alpha7 cDNA constructs. To demonstrate the functionality of expressed human alpha7, adhesion experiments with transfected MCF-7 cells have confirmed the specific binding of human alpha7 to laminin. In addition, mouse polyclonal and monoclonal antibodies were generated against the extracellular domain of human alpha7 and used to analyze by flow cytometry MCF-7 and NIH3T3 cells transfected with the full-length of human alpha7 cDNA. These results show for the first time the exogenous expression of functional full-length human alpha7 cDNA, as well as the development of monoclonal antibodies against the human alpha7 extracellular domain. Antibodies developed will be useful for further analysis of human disorders involving alpha7 dysfunction and facilitate isolation of muscle stem cells (satellite cells) and thereby expand the opportunities for genetically modified transplantation treatment of human disease.

Transcriptional activation of prion protein gene in growth-arrested and differentiated mouse erythroleukemia and human neoplastic cells.

The prion protein (PrP) is a GPI-anchored sialoglycoprotein that has attracted worldwide attention over the years due to its involvement in the pathogenesis of transmissible spongiform encephalopathies in sheep (scrapie), cattle (BSE), and humans (CJD). To understand the precise role of the Prn-p gene in cell growth and differentiation we investigated the expression pattern of the Prn-p gene in proliferating cells and in cells arrested in growth either by confluency or by induction of terminal differentiation. Viral-transformed mouse spleen hematopoietic cells named murine erythroleukemia (MEL) and other types of inducible cells (human neuroectodermal RD/TE-671, myoid RD cells) were employed. Cells grown exponentially, at confluency, or irreversibly arrested in growth at terminal differentiation state were analyzed by fluorescence cell sorting and Northern blot hybridization to estimate the steady-state level of PrP mRNA at different phases of the cell cycle. MEL cells that failed to differentiate from treatment with N(6)-methyladenosine (N(6)mAdo), an inhibitor of differentiation, were also analyzed for PrP mRNA level. Our results indicate the following: (a) growth arrest of cells at G(1) phase by confluency or by induction of terminal differentiation led to increased accumulation of PrP mRNA transcripts, an event observed also in differentiated MEL, RD/TE-671, and RD cells independent of the inducer used; (b) treatment of MEL cells with N(6)mAdo prevented early activation of the Prn-p gene in cells treated with the inducer; and (c) cell-free nuclear run-off studies showed enhanced expression of the Prn-p gene due to transcriptional activation. These findings indicate, for the first time, that the Prn-p gene, which is thought to be a housekeeping gene, is transcriptionally activated in G(1) phase in confluent and terminally differentiated cells. This information may be valuable in understanding the overaccumulation of PrP in some differentiated tissues and may let us repress Prn-p gene activation by novel agents.

Structural and functional impairment of mitochondria in adriamycin-induced cardiomyopathy in mice: suppression of cytochrome c oxidase II gene expression.

The use of adriamycin (ADR) in cancer chemotherapy has been limited due to its cumulative cardiovascular toxicity. Earlier observations that ADR interacts with mitochondrial cytochrome c oxidase (COX) and suppresses its enzyme activity led us to investigate ADR's action on the cardiovascular functions and heart mitochondrial morphology in Balb-c mice i.p. treated with ADR for several weeks. At various times during treatment, the animals were assessed for cardiovascular functions by electrocardiography and for heart tissue damage by electron microscopy. In parallel, total RNA was extracted from samples of dissected heart and analyzed by Northern blot hybridization to determine the steady-state level of three RNA transcripts encoded by the COXII, COXIII, and COXIV genes. Similarly, samples obtained from the liver of the same animals were analyzed for comparative studies. Our results indicated that 1) treatment of mice with ADR caused cardiovascular arrhythmias characterized by bradycardia, extension of ventricular depolarization time (tQRS), and failure of QRS at high concentrations (10-14 mg/kg body weight cumulative dose); 2) the heart mitochondria underwent swelling, fusion, dissolution, and/or disruption of mitochondrial cristae after several weeks of treatment. Such abnormalities were not observed in the mitochondria of liver tissue; and 3) among the three genes of COX enzyme examined, only COXII gene expression was suppressed by ADR treatment, mainly after 8 weeks in both heart and liver. Knowing that heart mitochondria represent almost 40% of heart muscle by weight, we conclude that the deteriorating effects of ADR on cardiovascular function involve mitochondrial structural and functional impairment.

Expression of ribosomal protein S5 cloned gene during differentiation and apoptosis in murine erythroleukemia (MEL) cells.

Murine erythroleukemia (MEL) cells have been used as a suitable model system for studying cellular and molecular mechanisms of erythroid differentiation. In an effort to isolate and characterize genes whose expression change during differentiation, we cloned and sequenced a cDNA of 715 bp (rpS5) from a MEL cDNA library. The cloned mouse cDNA showed significant degree of structural homology in both DNA and protein level to known human and rat genes that encode the S5 proteins of 40S ribosomal subunit. The use of 715-bp cDNA as probe revealed the presence of an RNA transcript in the cytoplasm of MEL and human neuroectodermal RD/TE-671 cells. The steady-state accumulation level of this RNA transcript decreased upon induction of differentiation of both cell lines by treatment with DMSO and UDP-4, two structurally different inducers. Blockade of MEL cell differentiation by the inhibitor N6-methyladenosine preserved the constitutive expression of the rpS5 gene. DNA methylation analysis at CCGG sites located at the rpS5 gene locus in undifferentiated and differentiated MEL cells revealed that the suppression of the rpS5 gene during MEL cell differentiation is not related to any change in methylation at these sites. Moreover, the rpS5 gene continued to be expressed in cells undergoing serum-deprived apoptosis, like in control untreated cells. Therefore, we conclude that there may be a disparate pattern of expression of the rpS5 gene in differentiating and apoptotic cells. These data can be valuable in understanding the role of ribosomal proteins during differentiation and cell death (apoptosis) of neoplastic cells, although there is no experimental evidence that the suppression of the rpS5 gene is related mechanistically to the induction of differentiation. It may well be considered as part of the differentiation process.

Effects of hemin on apoptosis, suppression of cytochrome c oxidase gene expression, and bone-marrow toxicity induced by doxorubicin (adriamycin).

We have shown that hemin (iron-protoporphyrin IX) selectively counteracts doxorubicin (Adriamycin, ADR)-induced cytotoxicity on human leukemia K-562 cells by preventing ADR from inhibiting mitochondrial cytochrome c oxidase (COX), a novel target site for anthracyclines. Here, we investigated whether or not (a) treatment with ADR promotes apoptosis and represses the expression of two COX genes (one nuclear and one mitochondrial) in human K-562 cells in the absence and presence of hemin, and (b) injection of hemin preserves bone-marrow cellularity in ADR-myelosuppressed rats. Cultured K-562 cells were incubated with varying concentrations of ADR.HCl (0.2 microM to 5 microM) in the presence and absence of hemin (30 microM) and assessed for DNA degradation, as well as for expression of mitochondrial COXII and nuclear COXIV genes by RNA Northern blot hybridization analysis. In parallel, we investigated whether or not hemin injected i.p. in myelosuppressed rats affected ADR-induced bone-marrow cytotoxicity. These studies have shown the following: (a) ADR caused a dose- and time-dependent DNA fragmentation, characteristic of apoptosis, in K-562 cells; (b) hemin reduced the frequency of cell death caused by ADR: this effect was specific for ADR, because hemin failed to prevent apoptosis induced by methotrexate (MTX) in these cells; (c) ADR suppressed expression of COXIV and COXII genes, and exposure of ADR-treated K-562 cells to hemin did not reverse this suppression; and (d) i.p. injection of hemin in ADR-myelosuppressed rats improved bone-marrow cellularity, promoted colony formation (CFU-GM and CFU-F), and stromal cell outgrowth; moreover, hemin increased WBC counts depressed 12 days after ADR treatment. These studies indicate that hemin is a selective inhibitor of ADR-induced apoptosis of human leukemia cells and preserves bone-marrow cellularity in rats injected with ADR.

Induction of murine erythroleukemia cell differentiation is associated with methylation and differential stability of poly(A)+ RNA transcripts.

Murine erythroleukemia (MEL) cells exposed to DMSO were assessed for their ability to methylate poly(A)+ RNA and accumulate RNA transcripts of globin and nonglobin genes (c-myc, beta-actin and MER5). Cells were pulse-labeled with L-[methyl-3H]methionine, cytoplasmic RNA was isolated, selected for poly(A)+ RNA and analyzed by HPLC chromatography for methylated nucleosides. When MEL cells were exposed to inhibitors of RNA methylation (neplanocin A, 3-deazaneplanocin A and cycloleucine) and assessed for their ability to differentiate by DMSO, accumulate RNA transcripts, produce hemoglobin, methylate poly(A)+ and poly(A)- RNA and synthesize S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH), we observed the following: (a) MEL cells treated with DMSO underwent hypermethylation in poly(A)+ RNA that preferentially occurred at the 5'-cap structures (7-methylguanosine and 2'-O-methylcytidine and 2'-O-methyluridine); (b) inducer-treated MEL cells exhibited a decrease in the intracellular level of SAH that led to a lower ratio of SAH/SAM, an event that favors methylation; and (c) treatment of MEL cells with inhibitors of RNA methylation suppressed methylation of poly(A)- and poly(A)+ RNA, reversed the ratio SAH/SAM seen in differentiated MEL cells and prevented differentiation to occur. Moreover, we observed that treatment of MEL cells with selective inhibitors of RNA methylation caused fragmentation of beta major globin and c-myc mRNAs, two RNA transcripts coded by developmentally regulated genes, while had no detectable effect on the structural integrity of poly(A)+ RNA transcripts transcribed by two housekeeping genes (beta-actin and MER5). These data indicate that induction of erythroid cell differentiation of MEL cells is associated with changes in methylation of poly(A)+ RNA and selective differential stability of RNA transcripts, two events that might be related to each other.

Expression of memory, differentiation, and repression of c-myc and p53 genes in human RD/TE-671 cells induced by a ureido-derivative of pyridine (UDP-4).

Human TE-671 cells have been used to study several aspects of neuroectodermal tumors in culture. Since the human TE-671 cell lines has been re-identified as a rhabdomyosarcoma (RD) rather than a medulloblastoma due to the presence of muscle-type nicotinic acetylcholine receptors, we re-investigated the nature of RD/TE-671 cells and characterized their differentiation induced by 2-(3-ethylureido)-6-methylpyridine (UDP-4), a potent inducer of differentiation of neoplastic cells. RD cells were also used for comparative studies. RD/TE-671 cells exposed to UDP-4 were differentiated irreversibly into postmitotic cells expressing mainly neurofilaments and, to a lesser extent, myoid proteins. In contrast to RD cells that expressed preferentially myoid and not neurofilament proteins (NFPs) upon treatment with UDP-4, differentiated RD/TE-671 cells exhibited characteristic dendritic processes and expressed NFPs (NFP68, NFP160, and NFP200), parvalbumin (calcium-binding protein), and neuron-specific enolase, as well as a small amount of vimentin and desmin. In addition, differentiated RD/TE-671 cells expressed memory for differentiation and underwent an irreversible limitation of proliferation, loss of clonogenic potential, selective repression of c-myc and p53 proto-oncogenes, and changes in cell surface architecture. Treatment of RD/ TE-671 cells with nerve growth factor or epidermal growth factor in the presence of UDP-4 did not alter the phenotype of differentiated cells, whereas co-treatment with 12-O-tetradecanoylphorbol-13-acetate and UDP-4 enhanced morphological differentiation. Therefore, we conclude that: (a) RD/TE-671 cells challenged with UDP-4 express memory to differentiate in the absence of inducer; (b) in contrast to RD cells, RD/TE-671 cells appear to be multipotent cells of neuroectodermal origin capable of differentiation into cells expressing neuronal rather than myoid proteins upon treatment with UDP-4; and (c) differentiation of RD/TE-671 cells leads to selective cessation of cell proliferation and repression of c-myc and p53 proto-oncogenes.

Bleomycin resistance in Staphylococcus aureus clinical isolates.

Among clinical isolates of Staphylococcus aureus tested for resistance to the antibiotic bleomycin, 197 were found to be resistant; most of them were also resistant to tobramycin and contained plasmids. DNA dot-blot hybridization analysis of the bleomycin resistant isolates with an 171 bp probe derived from the plasmid pUB110 indicated that 43 strains (22%) carried pUB110-like bleomycin resistance DNA sequences. Analysis of bacterial cell lysates derived from the bleomycin resistant isolates indicated that many contained bleomycin-binding properties (BBP), that prevent DNA damage by the antibiotic. Of 13 strains that were analysed by DNA gel electrophoresis and Southern blot DNA hybridization, six were found to carry pUB110-like bleomycin resistance DNA sequences. These studies indicate that there may be more than one genetic determinant for bleomycin resistance in S. aureus whose DNA is protected.

N6-methyladenosine inhibits murine erythroleukemia cell maturation by blocking methylation of RNA and memory via conversion to S-(N6-methyl)-adenosylhomocysteine.

We have shown earlier that N6-methyladenosine (N6mAdo) and other methylated derivatives block commitment of murine erythroleukemia (MEL) cells to terminal erythroid maturation. In this study, we further investigated the mechanism of this blockade. Treatment of MEL cells with N6mAdo inhibited cell growth, prevented accumulation of committed cells, suppressed methylation of total cytoplasmic RNA, and erased the expression of "memory" response, an event that precedes initiation of commitment. Furthermore, N6mAdo increased the level of S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) and altered the SAH/SAM ratio that influences methylation of ribonucleic acid (RNA). Moreover, analysis of the intracellular extracts revealed that N6-mAdo is converted into S-(N6-methyl)-adenosylhomocysteine (N6-SAH) in MEL cells, an active intermediate that affects methylation of RNA. Therefore, we conclude that N6-mAdo prevents induction of MEL cell differentiation by affecting methylation of critical RNA transcripts involved in expression of "memory" and initiation of commitment. It is likely that this inhibition occurs via conversion of N6mAdo into N6-SAH.

Mitochondrial cytochrome c oxidase as a target site for daunomycin in K-562 cells and heart tissue.

Daunomycin and other structurally related anthracyclines can cause myelosuppression and cardiomyopathy. We explored the possible mechanism(s) by which daunomycin (DAU) interacts with target sites in neoplastic hemopoietic cells and heart tissue. We observed that [3H(G)]DAU interacts selectively with mitochondrial hemoproteins isolated from K-562 cells and rat and bovine heart and forms relatively stable protein complexes. Isolation, purification, and chromatographic analysis of the mitochondrial components complexed with [3H(G)]DAU revealed that one of the major components involved is cytochrome c oxidase (COX). Both DAU and ADR caused a dose-dependent inhibition of COX activity in vitro, an event prevented by exogenous hemin. The interaction of DAU with COX appears to occur via more than one site, one of which at least appears to be the prosthetic group of heme. Therefore, mitochondrial COX, a pivotal mitochondrial enzyme for cell respiration, may serve as a potential target site for DAU and other related anthracyclines.

Analysis of the inhibition of commitment of murine erythroleukemia (MEL) cells to terminal maturation by N6-methyladenosine.

Treatment of cultured murine erythroleukemia (MEL or Friend) cells with N6-methylated derivatives of adenosine inhibited erythroid cell differentiation induced by various agents. N6-Methyladenosine (N6mAdo) inhibited initiation of commitment to terminal maturation and prevented accumulation of hemoglobin in a concentration-dependent manner. Treatment with N6mAdo slowed cell growth without causing substantial inhibition in the rate of DNA synthesis and a marked decrease in viability and clonogenic potential of MEL cells. Furthermore, N6mAdo decreased the cytoplasmic accumulation of beta(major) globin mRNA and affected its structural integrity in MEL cells. Cells pre-exposed to N6mAdo failed to initiate commitment as early as control cells upon challenge with the inducer dimethyl sulfoxide. N6mAdo-induced inhibition of commitment was not reversed but rather was potentiated by the presence of adenine, L-homocysteine and/or L-methionine, agents involved in the active methylation cycle. To this respect, N6mAdo-induced inhibition of commitment was found to be different from that caused by cordycepin (3'-deoxyadenosine, an inhibitor of RNA methylation and mRNA polyadenylation). The latter inhibition was fully reversed by the addition of L-methionine. These findings indicate that N6-methyladenosine: (a) blocks a central process that is required for initiation of commitment; and (b) decreases accumulation of beta (major) globin mRNA, causes mRNA degradation and prevents hemoglobin synthesis. Due to the differential sensitivity of N6mAdo- and cordycepin-induced blockade of commitment to L-methionine, these agents inhibit commitment by acting via two different mechanisms impinging on the final pathway of MEL erythroid cell maturation.

Ureido derivatives of pyridine: a new class of inducers of murine erythroleukemia cell differentiation.

A series of ureido derivatives of pyridine (UDPs) were developed as inducers of leukemic cell differentiation. Fifteen agents prepared by coupling aminopyridines with appropriate isocyanates were structurally identified and tested for both antiproliferative and differentiation inducing activity in cultures of murine and human leukemic cells. Five of these lipophilic compounds [2-(3-ethylureido)-pyridine (1), 2-(3-ethylureido)-6-methylpyridine (4), 2,6-bis-(3-ethylureido)-pyridine (7), 2-(3-ethylureido)-5-methylpyridine (14) and 2-(3-ethylureido)-4,5-dimethylpyridine (15)], promoted terminal erythroid maturation of murine erythroleukemia cells (MEL) (95% hemoglobin producing cells) and stimulated hemoglobin synthesis at concentrations as low as 0.075-0.5 mM. These concentrations are 50-70 fold lower than the optimum inducing concentration of hexamethylene bisacetamide (HMBA), a potent known inducer of differentiation. The proportion of cells induced by each ureido derivative of pyridine was concentration-dependent. Moderate inhibition of cell growth was obtained by these agents at optimum inducing concentrations. Agent 1 also stimulated hemoglobin synthesis in 18% of human erythroleukemia K-562 cells and promoted granulocytic differentiation in 24% of human promyelocytic leukemia HL-60 cells. Structure-activity relationships indicate that 7 was the most potent inducer of all UDPs which contain an ethylureido group attached at 2 position of the pyridine ring as a major structural feature needed for inducing activity. These findings indicate that the ureido derivatives of pyridine are a new class of inducers of murine erythroleukemia cells and, to a lesser extent of granulocytic differentiation, of leukemic HL-60 cells in vitro.

Cooperative effects of hemin and anthracyclines in promoting terminal erythroid maturation in K562 human erythroleukemia cells.

Simultaneous exposure to 30 microM hemin and 3 x 10(-8) M aclacinomycin (ACL) or mussetamycin for 6 days led to terminal differentiation of K562 cells. The number of hemoglobinized cells and the total hemoglobin content of cells treated with both ACL and hemin exceeded the sum of the corresponding values induced in response to each of these two agents when used alone. Although neither ACL nor hemin alone induced substantial morphological maturation, 40%-45% of cells treated with both agents developed the morphological characteristics of orthochromatophilic normoblasts, a level of maturation not previously reported for this highly malignant cell line. Subcloning of K562 cells that had been treated with both ACL and hemin in inducer-free plasma clots revealed a 50% decrease in the clonogenic potential of these cells, even though the cells in the original cultures were still growing at only a moderately decreased rate. Despite the apparent terminal maturation of K562 cells induced with both ACL and hemin, with an advanced level of morphologic maturation and hemoglobin synthesis accompanied by a loss of proliferative capacity, these cells remained incapable of producing beta-globin chains and hence hemoglobin A.