Different regulation of c-Myc- and E2F-1-induced apoptosis during the ongoing cell cycle

Promoter haplotype structure of solute carrier 22 member 2 (SLC22A2) in the Xhosa population of South Africa and their differential effect on gene expression

SLC22A2 is abundantly expressed in the kidney and facilitates the transport of endogenous and exogenous cationic compounds. It plays a pivotal role in the transport of pharmacologically important compounds such as metformin, cisplatin, lamivudine and cimetidine. Polymorphisms within SLC22A2 could potentially contribute to the inter-individual variable response to drugs. The SLC22A2 gene is known to show polymorphism variability amongst populations of different ethnicities. The present study was undertaken to characterize the promoter haplotype structure of the SLC22A2 gene in the Xhosa population of South Africa. In addition to this, we also investigate the effects of the observed promoter haplotypes on gene expression levels in vitro. We identified six known single nucleotide polymorphisms in the promoter region, namely rs60249401 (G424A), rs113150889 (G289A), rs55920607 (C246T), rs59695691 (A195G), rs572296424 (G156A), rs150063153 (A95C/G) and one novel SNP at location 6:160258967 (A209T). While these polymorphisms appeared in other African and non-African populations, their minor allele frequencies differed considerably from the non-African populations and could be considered to be African specific. A total of nine promoter haplotypes were characterized and the functional significance of each haplotype on promoter activity was determined using a luciferase reporter assay system. Amongst the nine observed haplotypes, three haplotypes (i.e. haplotypes 7, 8 and 9) displayed a significant decrease in expression level when compared to the wild-type with p -values of: 0.0317, <0.0001 and 0.0013 respectively. The data presented here shows African specific promoter haplotypes to cause a decrease in SLC22A2 gene expression levels, which in turn may have an impact on the pharmacokinetic profiles of cationic drugs.

Roscovitine has anti-proliferative and pro-apoptotic effects on glioblastoma cell lines: A pilot study

Purine analogue roscovitine, a cyclin-dependent kinase (CDK) inhibitor, has shown strong anti-proliferative and pro-apoptotic effects in solid and hematologic cancers such as non small-cell lung cancer and lymphomas. It targets CDK2, 7 and 9 preferentially, which are also overexpressed in glioblastoma. Τherefore, the biological effects of roscovitine in glioblastoma cell lines were investigated. Glioblastoma A172 and G28 cell lines were incubated with serial concentrations of roscovitine for 24-120 h. Proliferation was measured using the xCELLigence Real-Time Cell Analyzer, an impedance‑based cell viability system. Cell cycle distribution was assessed by flow cytometry and gene expression was quantified by quantitative RT-PCR and western blot analysis. Roscovitine exhibited a clear dose-dependent anti‑proliferative and pro‑apoptotic effect in the A172 cell line, while G28 cells showed a anti-proliferative effect only at 100 µM. The results of the flow cytometric (FACS) analysis revealed a dose-dependent increase of the G2/M and sub-G1 fractions in A172 cells, while G28 cells responded with an elevated sub-G1 fraction only at the highest concentration. Roscovitine led to a dose‑dependent decrease of transcripts of p53, CDK 7 and cyclins A and E and an increase of >4-fold of p21 in A172 cells. In G28 cells, a dose‑dependent induction of CDK2, p21 and cyclin D was observed between 10 and 50 µM roscovitine after 72 h, however, at the highest concentration of 100 µM, all investigated genes were downregulated. Roscovitine exerted clear dose-dependent anti-proliferative and pro-apoptotic effects in A172 cells and less distinct effects on G28 cells. In A172 cells, roscovitine led to G2/M arrest and induced apoptosis, an effect accompanied by induced p21 and a reduced expression of CDK2, 7 and 9 and cyclins A and E. These effects requre further studies on a larger scale to confirm whether roscovitine can be used as a therapeutic agent against glioblastoma.

Cell separation by countercurrent centrifugal elutriation: recent developments

Countercurrent centrifugal elutriation (CCE) is a cell separation technique that separates particles predominantly according to their size, and to some degree according to their specific density, without a need for antibodies or ligands tagging cell surfaces. The principles of this technique have been known for half a century. Still, numerous recent publications confirmed that CCE is a valuable supplement to current cell separation technology. It is mainly applied when homogeneous populations of cells, which mirror an in vivo situation, are required for answering scientific questions or for clinical transplantation, while antibodies or ligands suitable for cell isolation are not available. Currently, new technical developments are expanding its application toward fractionation of healthy and malignant tissue cells and the preparation of dendritic cells for immunotherapy.

Tuberin negatively affects BCL-2’s cell survival function

Uncontrolled cell cycle progression and cell growth are key properties of tumor cells. The tumor suppressor genes responsible for the autosomal dominantly inherited disease tuberous sclerosis (TSC) have been demonstrated to control both, cell cycle and cell size regulation. Hamartin, encoded by TSC1, and tuberin, encoded by TSC2, form a complex, of which tuberin is assumed to be the functional component. Loss of TSC genes function triggers hamartoma development in TSC patients. However, in vivo mostly tumor cell development is rapidly terminated via apoptosis. BCL-2, the founding member of the BCL-2 family of proteins, is well known for its anti-apoptotic properties. Here we show that pro-apoptotic actinomycin D cannot interfere with BCL-2's cell survival functions. However, we found tuberin to negatively regulate BCL-2's anti-apoptotic effects on low serum-induced apoptosis. These findings warrant further investigations to elucidate the molecular mechanism underlying tuberin's negative effects on cell survival.

Cell cycle and cell size regulation in Down syndrome cells

Although the neuropathological features typical for Down Syndrome obviously result from deregulation of both, cell cycle control and differentiation processes, so far research focused on the latter. Considering the known similarities between the neuropathology of Down Syndrome and Alzheimer's disease and the knowledge, that in Alzheimer's disease neuronal degeneration is associated with the activation of mitogenic signals and cell cycle activation, it is tempting to investigate the consequences of an additional chromosome 21 on mammalian cell cycle regulation. We analysed the distribution of cells in different cell cycle phases on the flowcytometer and the cell size of human amniotic fluid cells with normal karyotypes and with trisomy 21. We could not detect any significant differences suggesting that the presence of an additional copy of the about 225 genes on human chromosome 21 does not trigger cell cycle effects in amniotic fluid cells. These data provide new insights into the cell biology of trisomy 21 cells.

BCL-x(L) and BCL2 delay Myc-induced cell cycle entry through elevation of p27 and inhibition of G1 cyclin-dependent kinases

The anti-apoptotic molecules BCL-x(L) and BCL2 delay cell cycle entry from quiescence. We used serum induction and induction of a Myc-estrogen receptor fusion protein (MycER) in quiescent fibroblasts to investigate the mechanisms underlying the cell cycle activity of BCL-x(L) and BCL2. We demonstrate for the first time that BCL-xL and BCL2 delayed serum-induced and Myc-induced, but not E2F-induced, cell cycle entry. The cyclin-dependent kinase inhibitor p27 was elevated during serum deprivation and cell cycle entry in BCL-x(L) or BCL2-expressing NIH3T3 cells and a Rat1MycER cell line. Activation of cyclin-dependent kinase 2 (cdk2) and cyclin-dependent kinase 4 (cdk4) were delayed during progression to S phase, while the induction of cyclin D1 protein, as well as the levels of cyclin E, cdk2, and cdk4 were unaltered by BCL-x(L) or BCL2. Inhibition of cyclin/cdk activities in BCL-x(L) or BCL2 expressing cells was associated with excess p27 in the cyclin/cdk complexes. Neither BCL-x(L) nor BCL2 delayed S phase entry in cells deficient in p27, thus p27 is required for the cell cycle function of BCL-x(L) and BCL2. The cell cycle effects of BCL-x(L) and BCL2 were more profound in Myc-induced than in serum-induced cell cycle entry. Our results suggest that one possible mechanism by which BCL-x(L) and BCL2 delay cell cycle entry may be the inhibition of Myc activity through the elevation of p27.

Anti-DNA autoantibodies reveal toxicity to tumor cell lines

Cytotoxicity of anti-DNA autoantibodies from sera of SLE and CLL patients was assayed on permanent cell lines L929, HL-60, Raji, and K562. L929 cells appeared to be the most sensitive to antibody treatment. DNA-hydrolyzing properties of the same autoantibody preparations were analyzed in parallel. The data obtained outlined the correlation between cytotoxicity and DNA-hydrolyzing properties of these autoantibodies. It was shown that treatment of the cells with cytotoxic anti-DNA autoantibodies induced internucleosomal DNA fragmentation and Annexin V binding to the cell surface characteristic of apoptotic pathway of cell death. A time-dependent profile of antibody-mediated toxicity to L929 cells suggested recruitment of at least two distinct mechanisms of cell death. The first peak of cell death observed in 3 h of incubation was completely inhibited by preincubation of cells with caspase inhibitor YVAD-CHO, while the second increase in cell mortality (18-30 h) persisted. Possible mechanisms for anti-DNA autoantibody cytotoxicity are discussed.

Staurosporine-induced apoptosis of HPV positive and negative human cervical cancer cells from different points in the cell cycle

In the present study, we compare the sensitivity of CaSki and HeLa cells (HPV positive, wild-type p53) and C33A cells (HPV negative, mutated p53) to a protein kinase inhibitor, the staurosporine (ST). We show that ST can reversibly arrest the three cervical-derived cell lines, either in G1 or in G2/M. Beyond certain ST concentrations or/and over 24 h exposure, the cells underwent apoptosis. This process took place in G1 and G2/M for C33A and CaSki plus HeLa cell lines, respectively. By using an in vitro cell-free system, we demonstrated that cytoplasmic extracts from apoptotic cells were sufficient to induce hallmarks of programmed cell death on isolated nuclei. Moreover, we found that only G2/M cytoplasmic extracts from viable CaSki and HeLa cells supplemented with ST, triggered apoptosis while exclusively G1 cytoplasmic fractions from C33A cells were efficient. Our study describes a possible involvement of the HPV infection or/and p53 status in this different ST-induced apoptosis susceptibility.

Induction of cell cycle progression and acceleration of apoptosis are two separable functions of c-Myc: transrepression correlates with acceleration of apoptosis

Analysis of amino-terminus mutants of c-Myc has allowed a systematic study of the interrelationship between Myc's ability to regulate transcription and its apoptotic, proliferative, and transforming functions. First, we have found that c-Myc-accelerated apoptosis does not directly correlate with its ability to transactivate transcription using the endogenous ornithine decarboxylase (ODC) gene as readout for transactivation. Furthermore, deletion of the conserved c-Myc box I domain implicated in transactivation does not inhibit apoptosis. Second, the ability of c-Myc to repress transcription, using the gadd45 gene as a readout, correlates with its ability to accelerate apoptosis. A conserved region of c-Myc implicated in mediating transrepression is absolutely required for c-Myc-accelerated apoptosis. Third, a lymphoma-derived Thr58Ala mutation diminishes c-Myc-accelerated apoptosis through a decreased ability to induce the release of cytochrome c from mitochondria. This mutation in a potential phosphorylation site does not affect cell cycle progression, providing genetic evidence that induction of cell cycle progression and acceleration of apoptosis are two separable functions of c-Myc. Finally, we show that the increased ability of Thr58Ala mutant to elicit cellular transformation correlates with its diminished ability to accelerate apoptosis. Bcl-2 overexpression blocked and the lymphoma-associated Thr58Ala mutation decreased c-Myc-accelerated apoptosis, and both led to a significant increase in the ability of Rat1a cells to form colonies in soft agar. This enhanced transformation was greater in soft agar containing a low concentration of serum, suggesting that protection from apoptosis is a mechanism contributing to the increased ability of these cells to proliferate in suspension. Thus, we show here for the first time that, in addition to mutations in complementary antiapoptotic genes, c-Myc itself can acquire mutations that potentiate neoplastic transformation by affecting apoptosis independently of cell cycle progression.

Ubiquitin-dependent signaling: the role of ubiquitination in the response of cells to their environment

The response of a cell to its external environment requires rapid and significant alteration of protein amount, localization and/or function. This regulation involves a complex combination of processes that control synthesis, localization and degradation. All of these processes must be properly regulated and are often interrelated. Intracellular proteolysis is largely accomplished by the ubiquitin-dependent system and has been shown to be required for growth control, cell cycle regulation, receptor function, development and the stress response. Substrates subject to regulated degradation by this system include cyclins and cyclin-dependent kinase inhibitors, tumor suppressors, transcription factors and cell surface receptors. In addition, proteins that are damaged by oxidation or that are improperly folded or localized are substrates whose degradation by this system often leads to antigen presentation on the surface of the cell in the context of Class I major histocompatibility complex molecules. A very large body of work in the last fifteen years has shown that degradation by this system requires the covalent attachment of a small protein called ubiquitin and that this modification serves to direct target proteins for degradation by a 26S proteolytic particle, the proteasome. Thus, the attachment of the ubiquitin domain is of vital importance in regulating normal growth and differentiation, as well as in defending against cellular damage caused by xenobiotics, environmental insults, infection and mutation. This review focuses on the role of ubiquitination in the cellular signaling pathways that deal with these external influences.

The MYC dualism in growth and death

Over-expression of the transcription factor c-Myc immortalizes primary cells and transforms in co-operation with activated ras. Therefore, c-myc is considered a proto-oncogene. Since its discovery c-Myc has been shown to render cells growth factor independent, accelerates passage through G1 of the cell cycle, inhibits differentiation and elicits apoptosis. Whereas the effects on immortalization, proliferation and inhibition of differentiation are in conceivable accordance with gain of function, as it is defined for a proto-oncogene, its pro-apoptotic activity disables a straight forward explanation of the physiological role of c-Myc and suggests a highly complex contribution during development. The recent accomplishments in c-Myc research shed some light on the difficile regulatory network which keeps check on c-Myc activity such as by binding to proteins some of which are transcription factors for non-c-Myc targets. Moreover, it was shown that genes are targeted by c-Myc depending on the sequence of flanking regions adjacent to the E-box or in dependence on the availability of binding partners which is most probably specific to the cellular context. Cdc25A and ornithine decarboxylase, both described to be c-Myc targets, have been brought forward as downstream effectors in the induction of proliferation under serum rich conditions, or in the induction of apoptosis when serum factors are limited. These genes seem to be regulated by c-Myc in a cell type-specific manner. H-ferritin, IRP2 and telomerase are the most recently discovered direct targets of c-Myc. The regulation of H-ferritin and IRP2 might explain the potential of c-Myc to promote proliferation and the regulation of telomerase could be responsible for the immortalizing properties of c-Myc. In the future, H-ferritin and telomerase have to be analyzed whether or not these genes are also Myc targets in other cell systems. Although the intense research efforts regarding the function of c-Myc last already two decades the role of this gene is still enigmatic.

Regulation of E2F: a family of transcription factors involved in proliferation control

Members of the E2F family of transcription factors are key participants in orchestration of the cell cycle, cell growth arrest and apoptosis. Therefore, an understanding of the regulation of E2F activity is essential for an understanding of the control of cellular proliferation. E2F activity is regulated by the retinoblastoma family of tumor suppressors and by multiple other mechanisms. This review will describe our current knowledge of these mechanisms which together constitute a highly complex network by which the cell cycle and cellular proliferation can be controlled.