Sensitivity of RNA synthesis to actinomycin D inhibition is dependent on the frequency of transcription: a mathematical model

Quantitative analysis of asynchronous transcription-translation and transcription processivity in Bacillus subtilis under various growth conditions

Tight coordination between transcription and translation has long been recognized as the hallmark of gene expression in bacteria. In Escherichia coli cells, disruption of the transcription-translation coordination leads to the loss of transcription processivity via triggering Rho-mediated premature transcription termination. Here we quantitatively characterize the transcription and translation kinetics in Gram-positive model bacterium Bacillus subtilis. We found that the speed of transcription elongation is much faster than that of translation elongation in B. subtilis under various growth conditions. Moreover, a Rho-independent loss of transcription processivity occurs constitutively in several genes/operons but is not subject to translational control. When the transcription elongation is decelerated under poor nutrients, low temperature, or nucleotide depletion, the loss of transcription processivity is strongly enhanced, suggesting that its degree is modulated by the speed of transcription elongation. Our study reveals distinct design principles of gene expression in E. coli and B. subtilis.

A novel defect in mitochondrial p53 accumulation following DNA damage confers apoptosis resistance in Ataxia Telangiectasia and Nijmegen Breakage Syndrome T-cells

We have previously shown that whereas T-cells from normal individuals undergo accumulation of p53 and apoptosis when treated with the genotoxic agent Actinomycin D (ActD), those from Ataxia Telangiectasia (AT) and Nijmegen Breakage Syndrome (NBS) patients resist ActD-induced apoptosis [1]. We have now found similar resistance by the p53-null Jurkat T-cell line and by siRNA p53-knockdown normal T-cells. This evidence that ActD initiates a p53-dependent apoptotic responce prompted us to look for defective p53 accumulation by AT and NBS T-cells. Surprisingly the total p53 level was only slightly reduced compared to normal T cells but its intracellular localization was highly defective: p53 was poorly accumulated in the cytosol and nearly undetectable in mitochondria. In accordance with the dependence of ActD-induced apoptosis on a mitochondrial p53 function, in control T-cells specific inhibition of mitochondrial p53 translocation with μ pifithrin reduced apoptosis by 86%, whereas treatment with α pifithrin, which blocks p53-mediated transcription, had no effect. We also showed that nuclear export is not required for mitochondrial p53 translocation. Observation of an altered p53 ubiquitination pattern and Mdm2 accumulation in ActD-treated AT and NBS T-cells provided a mechanistic link to their defective extranuclear p53 localization. Our results disclose an undescribed defect in mitochondrial p53 accumulation in AT and NBS T-cells that makes them resistant to apoptosis following unrepairable DNA damage.

Gene expression in mouse preimplantation embryos affected by apoptotic inductor actinomycin D

The aim of this study was to test the effect of actinomycin D on the expression of selected genes and to elucidate possible components of its apoptotic pathway in mouse embryos. Selected mRNAs and Trp53 protein were examined in blastocysts cultured for 24 h in vitro with or without the presence of a high concentration of actinomycin D. In all tested genes, the relative quantities of mRNA were significantly lower in treated blastocysts than in controls. The mRNA quantities of H2afz, Actb, Bax, Bad and Bcl2 were reduced at a similar rate, but the decreases in Bcl2l2 and Trp53 mRNA were significantly greater. Treatment with actinomycin D also changed the ratio between the mRNA levels of some pro-apoptotic and anti-apoptotic genes: the Bad/Bcl2l2 and the Bax/Bcl2l2 ratios were on average 4.39 and 2.66 times higher in the treated embryos than in the controls, respectively. Generally, treatment led to developmental arrest and significant increase in the incidence of cells with typical apoptotic features. However, its effect on Trp53 protein expression was not significant. The results suggest that mechanisms beyond the apoptotic effect of actinomycin D might include specific changes in the expression of pro-apoptotic and anti-apoptotic genes, shifting the expression ratio in favor of the pro-apoptotic ones. The results also show that the role of Trp53 is probably not crucial in this apoptotic pathway.

Actinomycin D upregulates proapoptotic protein Puma and downregulates Bcl-2 mRNA in normal peripheral blood lymphocytes

We have examined the ability of actinomycin D to induce apoptosis in human peripheral blood lymphocytes. Run-On assays were performed to specify the primary molecular damage, reverse transcription-PCR, Western blots and flow cytometry studies were performed to ascertain which proteins of the apoptosis machinery were affected to cause actinomycin D-induced cell death. Expression of 23 apoptosis-related genes was investigated. The down-regulation of ribosomal RNA synthesis caused by actinomycin D induced a mitochondria-dependent apoptosis. Although the expression of the majority of examined genes remained indifferent against actinomycin D activity, the cellular level of p53 protein increased, subsequently upregulating both Puma mRNA and protein. Puma-mediated mitochondrial apoptosis was accompanied by nucleolin cleavage and Bcl-2 mRNA destabilization. The stability of the cellular level of Bcl-2 protein independent of a mRNA decrease suggests that protection of Bcl-2 protein against proteasomal degradation can moderate the apoptotic process. In peripheral blood lymphocytes cultured in vitro, the apoptosis induced by a low concentration of actinomycin D (10 nmol/l) is dependent on p53 and Puma activation. This apoptotic pathway is demonstrated in peripheral blood lymphocytes for the first time. A different apoptotic pathway induced in peripheral blood lymphocytes using this drug has, however, been previously revealed by other authors. The combination of cell specificity and dose-dependent effects can likely play a decisive role in apoptosis observed in peripheral blood lymphocytes after genotoxic drug application.

Viable head and neck tumor spheroids stimulate in vitro autologous monocyte MCP-1 secretion through soluble substances and CD14/lectin-like receptors

Biopsies from carcinoma tissue and benign control mucosa from head and neck squamous cell carcinoma (HNSCC) patients were used to establish fragment (F)-spheroids in vitro. We have previously shown that autologous monocytes co-cultured with F-spheroids in vitro augment their secretion of monocyte chemotactic protein-1 (MCP-1). Presently, the aims of the present work were to study whether the metabolic activity, secreted products and/or specific receptor/ligand on the surface of the F-spheroids and monocytes are necessary for stimulation of the monocyte MCP-1 secretion upon F-spheroid co-culture. Actinomycin D (1 mug/ml for 24 h) pre-treatment of the F-spheroids abolished the monocyte MCP-1 co-culture response. Co-culture of monocytes and F-spheroids separated by a semi-permeable membrane showed a decreased, but still present, monocyte MCP-1 co-culture response. Conditioned medium from F-spheroids stimulated allogenous monocytes to secrete MCP-1. The addition of glucose or galactose, but not mannose, to co-cultures partially inhibited the monocyte MCP-1 co-culture response. The addition of anti-CD14 antibody diminished the MCP-1 co-culture response. In conclusion, the monocyte MCP-1 co-culture response is dependent on metabolically active spheroids, secreted stimuli, and is augmented by direct contact with F-spheroids, possibly via lectin-like receptors and the CD14 receptor.

Mechanisms for monocyte activation in co-culture with autologous tumor spheroids

Biopsies from carcinoma tissue and benign control mucosa from head and neck squamous cell carcinoma patients were used to establish fragment (F)-spheroids in vitro. We have previously shown that autologous monocytes co-cultured with F-spheroids in vitro secrete interleukin (IL)-6 upon 24h in co-culture. Presently, the aim was to study the mechanisms of this monocyte secretion. Paraformaldehyde (0.1% for 2min) or actinomycin-D (1 microg/ml for 24h) pre-treatment of the F-spheroids abolished the monocyte IL-6 co-culture response. Addition of glucose (100mM) or mannose (100mM), and to some extent galactose (100mM), but not fructose (100mM) to the co-cultures, partly inhibited the monocyte IL-6 co-culture response, but such addition did not inhibit the in vitro monocyte lipopolysaccharide (LPS)-generated IL-6 secretion. When mannose was added to the co-cultures, monocyte IL-6 mRNA expression was eradicated in malignant co-cultures and reduced to a low level in benign co-cultures. Addition of mouse anti-human beta(1)-integrin (anti-CD29) antibody (2 microg/ml) diminished the IL-6 co-culture response but not the monocyte LPS-generated IL-6 response. In conclusion, the monocyte IL-6 co-culture response is dependent on live spheroids and to some extent on direct contact with the F-spheroids, possibly via lectin-like receptor(s), the mannose receptor and beta(1)-integrin.

RNA synthesis inhibitors increase melatonin production in Y79 human retinoblastoma cells

Y79 human retinoblastoma cells synthesize melatonin in cell culture thus providing a unique preparation for studying the regulation of melatonin biosynthesis in mammalian retinas. We have previously demonstrated that Y79 cells express NAT and HIOMT activity and produce melatonin in a cAMP- and protein synthesis-dependent manner by increasing NAT, and not HIOMT activity, as has been demonstrated in other retinal and pineal melatonin synthesizing systems. We have extended these studies to investigate the role of RNA synthesis in melatonin regulation, and report here that RNA synthesis inhibitors do not suppress melatonin production in Y79 retinoblastoma cells. Rather, at intermediate concentrations, the inhibitors actinomycin D and camptothecin increase melatonin levels. Camptothecin, a topoisomerase I inhibitor, also increased NAT activity and accumulated cAMP levels in a calcium-dependent manner. This effect on cAMP did not appear to occur through phosphodiesterase, and other regulators of retinal melatonin such as melatonin degradation or components of the dopamine system were unaffected. These results are in contrast with the suppression of melatonin synthesis by RNA synthesis inhibitors observed in rat and chick pineal glands and in chick retinas.

Effect of actinomycin D on viability, sporulation and nucleotide pool of Bacillus megaterium

A transient 7-fold rise of ppGpp concentration, 2-3-fold increase of pppGpp concentration and 50% drop of the concentration of GTP in Bacillus megaterium cells immediately after their transfer to the sporulation medium were observed. Actinomycin D, in concentrations inhibiting RNA synthesis by 95%, blocked the rise of the (p)ppGpp pool and caused an instant several-fold increase of the GTP level. When the cells were exposed to actinomycin D in the sporulation medium for a 1-h period (time 0-1 h, 1-2 h or 2.20-3.20-h), they were able to form colonies on nutrient agar after being kept, in addition for 1-2 h in the sporulation medium free of the antibiotic. The ability of sporulation was, however, markedly limited. The share of cells that could sporulate increased when the irreversible sporulation phase was reached.