Altered expression of genes regulating cell growth, proliferation, and apoptosis during adenosine 3',5'-cyclic monophosphate-induced differentiation of neuroblastoma cells in culture

Induction of cell cycle arrest at G1 and S phases and cAMP-dependent differentiation in C6 glioma by low concentration of cycloheximide

Background

Differentiation therapy has been shown effective in treatment of several types of cancer cells and may prove to be effective in treatment of glioblastoma multiforme, the most common and most aggressive primary brain tumor. Although extensively used as a reagent to inhibit protein synthesis in mammalian cells, whether cycloheximide treatment leads to glioma cell differentiation has not been reported.

Methods

C6 glioma cell was treated with or without cycloheximide at low concentrations (0.5-1 μg/ml) for 1, 2 and 3 days. Cell proliferation rate was assessed by direct cell counting and colony formation assays. Apoptosis was assessed by Hoechst 33258 staining and FACS analysis. Changes in several cell cycle regulators such as Cyclins D1 and E, PCNA and Ki67, and several apoptosis-related regulators such as p53, p-JNK, p-AKT, and PARP were determined by Western blot analysis. C6 glioma differentiation was determined by morphological characterization, immunostaining and Western blot analysis on upregulation of GFAP and o p-STAT3 expression, and upregulation of intracellular cAMP.

Results

Treatment of C6 cell with low concentration of cycloheximide inhibited cell proliferation and depleted cells at both G2 and M phases, suggesting blockade at G1 and S phases. While no cell death was observed, cells underwent profound morphological transformation that indicated cell differentiation. Western blotting and immunostaining analyses further indicated that changes in expression of several cell cycle regulators and the differentiation marker GFAP were accompanied with cycloheximide-induced cell cycle arrest and cell differentiation. Increase in intracellular cAMP, a known promoter for C6 cell differentiation, was found to be elevated and required for cycloheximide-promoted C6 cell differentiation.

Conclusion

Our results suggest that partial inhibition of protein synthesis in C6 glioma by low concentration of cycloheximide induces cell cycle arrest at G1 and M phases and cAMP-dependent cell differentiation.

VP16CREB-induced differentiation of neuroblastoma

OBJECTIVE

Drug-induced differentiation is commonly used as a therapeutic modality for the treatment of neuroblastoma tumors. Increased level of cyclic adenosine 3', 5'-monophosphate (cAMP) mediates terminal differentiation in some neuroblastoma cell lines through activation of several signaling networks, including cAMP response element binding protein (CREB). Objective was to test whether cAMP-induced differentiation in a murine neuroblastoma cell line (NBP2) is partly mediated by CREB.

METHODS

Fluorescent microscopy was used to document neuron-like morphological changes imparted by a constitutively active CREB (VP16CREB). Real time PCR (RT-PCR) was performed to verify changes in the expression of cAMP/CREB responsive genes.

RESULTS

It was found that transient expression of VP16CREB into NBP2 cells resulted in morphological changes that were characteristics of terminally differentiated neurons. Furthermore, increased expression of cAMP responsive genes was compromised in cells resisting VP16CREB-mediated differentiation.

CONCLUSION

A constitutively active CREB induces terminal differentiation in a subset of NBP2 cell population. Altered expression of cAMP responsive genes may account for differentiation resistant phenotype in NBP2 cells.

Identifying altered gene expression in neuroblastoma cells preceding apoptosis

PURPOSE

Concomitant differentiation and partial inhibition of proteasome trigger cell death in a neuroblastoma cell line (NBP2). Neither induction of differentiation nor partial inhibition of proteasome alone affects the viability of NBP2 cells. We wanted to identify genes whose expression alters under concomitant conditions and may account for cell death.

METHODS

We used gel electrophoresis to analyze total genomic DNA for the detection of DNA fragmentation. Affymetrix Murine Genome U74A version 2 microarray was used to screen for approximately 6,000 functionally characterized genes and approximately 6,000 expressed sequence tags (ESTs). Real time PCR (RT-PCR) was performed to provide an accurate assessment of changes in gene expression.

RESULTS

Concomitant differentiation and partial inhibition of proteasome trigger apoptosis, characterized by genomic DNA fragmentation in NBP2 cells. We found that the expression of 41 genes changed 2.5-fold or more primarily under concomitant conditions midway through apoptosis. Based on real time PCR, the expression of galectin-3, glycosylated 96, a leucine zipper protein (LRG-21), and endothelial cell activated protein C receptor (EPCR) increased between 50-500-fold, whereas the expression of Polo serine/threonine kinase, N-myc, and Histone H2A.1 decreased ranging from 8 to 37 fold. Altered expression of galectin-3, EPCR, and LRG-21 was detected as early as 2-8 h post simultaneous conditions.

CONCLUSION

We identified new genes that might be involved in apoptotic events in neuroblastoma cells.

Prostaglandin-induced neurodegeneration is associated with increased levels of oxidative markers and reduced by a mixture of antioxidants

Prostaglandin E2 (PGE2), one product of inflammatory reactions, and PGA1, which is formed during PGE2 extraction, induce degeneration in adenosine 3',5'-cyclic monophosphate (cAMP)-induced differentiated neuroblastoma (NB) cells in culture. The mechanisms of action of PGE2 on neurodegeneration are not well understood. To investigate this, we have utilized PGA(1), which mimics the effect of PGE2 and is very stable in solution. We have assayed selected markers of oxidative stress such as heme oxygenase-1 (HO-1), catalase, glutathione peroxidase (GPx1), mitochondrial superoxide dismutase (Mn-SOD-2) and cytosolic superoxide dismutase (Cu/Zn-SOD-1). The results showed that the treatment of differentiated NB cells with PGA1 for a period of 48 hr increased the expression of HO-1 and catalase, decreased the expression of GPx1 and Mn-SOD-2, and did not change the expression of Cu/Zn-SOD-1 as measured by gene array and confirmed by real-time PCR. The protein levels of HO-1 and GPx1 increased; however, the protein level of Mn-SOD-2 decreased and the levels of catalase and Cu/Zn-SOD-1 did not change as determined by Western blot. The increases in the levels of HO-1 and GPx1 reflected an adaptive response to increased oxidative stress, whereas decrease in the level of Mn-SOD-2 may make cells more sensitive to oxidative damage. These data suggest that one of the mechanisms of action of PGA1 on neurodegeneration may involve increased oxidative stress. This was supported further by the fact that a mixture of antioxidants (alpha-tocopherol, vitamin C, selenomethionine, and reduced glutathione), but not the individual antioxidants, reduced the level of PGA1-induced degeneration in differentiated NB cells. The addition of a single antioxidant at two or four times the concentration used in the mixture was toxic.

Role of the adenosine 3',5'-cyclic monophosphate (cAMP) in enhancing the efficacy of siRNA-mediated gene silencing in neuroblastoma cells

Gene-silencing activity mediated by siRNA has been demonstrated in mammalian cells; however, the mechanism of its regulation is not well understood. Since downregulation of a number of genes occurs during adenosine 3',5'-cyclic monophosphate (cAMP)-induced differentiation of neuroblastoma (NB) cells, it is possible that cAMP may play a role in regulating siRNA activity during differentiation. To study this, we utilized an NB cell line (NBP2-PN25) that expresses a short-lived green fluorescent protein (d2EGFP) under the CMV promoter. These cells were transfected with a retroviral plasmid that expresses U6 promoter-driven expression of siRNA targeted to d2EGFP and then were treated with cAMP-elevating agents (200 microg/ml RO20-1724, an inhibitor of cyclic nucleotide phosphodiesterase, and 1 microg/ml prostaglandin A1, a stimulator of adenylate cyclase) for 2 or 24 h. The siRNA activity was measured by determining the level of intensity of d2EGFP protein by flow cytometry, and the level of d2EGFP mRNA by real-time PCR. The results showed that cAMP-elevating agents enhanced U6-driven siRNA activity directed towards d2EGFP in NB cells 24 h after treatment. One of the mechanisms of action of cAMP is mediated via phosphatidylinositol 3-kinase (PI3K) inhibition; therefore, we have investigated the effect of a PI3K inhibitor on siRNA activity. This study showed that inhibition of PI3K also enhanced U6-driven siRNA activity towards d2EGFP. cAMP-stimulating agents increased U6 transcript levels, perhaps suggesting that increased siRNA activity may in part be due to an increase in transcriptional activity. When NB cells were transfected with a synthetic siRNA directed to d2EGFP, both cAMP elevation and PI3K inhibition similarly enhanced siRNA activity. Sodium butyrate, which inhibits the growth of NB cells similar to the effect produced by cAMP, did not affect U6-driven siRNA activity towards d2EGFP. Protein kinase C (PKC) activation or inhibition also failed to affect siRNA activity in NB cells. This study also showed that cAMP elevation and PI3K inhibition increases U6-driven siRNA activity directed towards an endogenous gene, p53. Our data suggest a role for the cAMP pathway in affecting the efficacy of siRNA system during differentiation of NB cells.

Regulated expression of VP16CREB in neuroblastoma cells: analysis of differentiation and apoptosis

Highly malignant neuroblastoma tumors generally have defects in differentiation and apoptotic pathways. For a better understanding of these events, we use a murine neuroblastoma cell line (NBP2) that terminally differentiates into mature neurons in response to elevated levels of cAMP. Because one of the main downstream effectors of the cAMP signaling pathway is cAMP-response element binding (CREB), we reasoned that it might affect the expression of genes associated with differentiation and apoptotic events in NBP2 cells. To investigate this, we established tetracycline-regulated expression (TetOff) of VP16CREB, which constitutively transactivates promoters containing the CRE sequence motif. Using this system, we found that inducible expression of VP16CREB in NBP2 cells results in 1) morphological differentiation that is characterized by the formation of neurites and growth cones, 2) reversible cell differentiation unlike cAMP-induced terminal differentiation, 3) cell cycle arrest at G1, 4) no apoptosis in the presence of partial inhibition of proteasome unlike an increase in cAMP levels, and 5) changes in the expression of many genes, including down-regulation of N-myc, cyclin B1, Dickkopf-1, and Mad-2 and up-regulation of tyrosine hydroxylase, c-fos, N10, and ICER genes. Although VP16CREB expression and activation of the cAMP pathway impart many similar effects in NBP2 cells, they also bear some distinct genetic and morphological differences. Our data suggest that increased levels of cAMP function through not only CREB but also other signaling pathways that account for the additional cAMP-induced effects, including irreversible differentiation and onset of apoptosis during partial inhibition of proteasome in NBP2 cells.

Pheromone-mediated gene expression in the honey bee brain

We tested the hypothesis that queen mandibular pheromone (QMP) causes changes in gene expression in the brain of the adult worker honey bee, and that these changes can be correlated to the downstream behavioral responses induced by QMP. In support of the first hypothesis, cage experiments revealed that QMP transiently regulated expression of several hundred genes and chronically regulated the expression of 19 genes. Several of these genes were also affected by QMP in experiments with bee colonies in the field, demonstrating robust gene regulation by pheromone. To evaluate the second hypothesis, we focused on one function of QMP: delaying the transition from working in the hive (e.g., brood care, or "nursing") to foraging. We compared the list of QMP-regulated genes with the lists of genes differentially regulated in nurse and forager brains generated in a separate study. QMP consistently activated "nursing genes" and repressed "foraging genes," suggesting that QMP may delay behavioral maturation by regulating genes in the brain that produce these behavioral states. We also report here on an ortholog of the Drosophila transcription factor kruppel homolog 1 that was strongly regulated by QMP, especially in the mushroom bodies of the bee brain. These results demonstrate chronic gene regulation by a primer pheromone and illustrate the potential of genomics to trace the actions of a pheromone from perception to action, and thereby provide insights into how pheromones regulate social life.