thapsigargin and Thymoma

The mechanisms by which trichothecene mycotoxins cause immunological effects in leukocytes such as cytokine up-regulation, aberrant IgA production, or apoptotic cell death are not fully understood. In the present study, mRNA differential display analysis was used to evaluate changes in gene expression induced by the trichothecene vomitoxin (VT or deoxynivalenol) in a T-cell model, the murine EL-4 thymoma, that was stimulated with phorbol 12-myristate 13-acetate (PMA) and ionomycin (ION). Ten differentially expressed fragments of cDNA were isolated and sequenced and three of these were identified as the known genes GRP78/BiP, P58(IPK), and RAD17. Most notably, expression of GRP78/BiP (a 78-kDa glucose-regulated protein), a stress-response gene induced by agents or conditions that adversely affect endoplasmic reticulum (ER) function, was found to decrease in VT-exposed cells. Competitive RT-PCR analysis revealed that 250 ng/ml VT decreased GRP78/BiP mRNA expression in both unstimulated and PMA/ION-stimulated EL-4 cells at 6 and 24 h after VT treatment. Western blotting confirmed that VT (50 to 1000 ng/ml) also significantly diminished GRP/BiP protein levels in a dose-response manner in PMA/ION-stimulated cells. GRP78/BiP has been shown to play a role in regulation of protein folding and secretion, and to protect cells from apoptosis. When PMA/ION-stimulated cells were incubated with 50 to 1000 ng/ml VT for 24 h, 200-bp DNA laddering, a hallmark of apoptosis, increased in a dose-dependent manner. In addition to GRP78, mRNA expression of the cochaperone P58(IPK), which is the 58-kDa cellular inhibitor of the double-stranded RNA-regulated protein kinase (PKR), was also shown to be suppressed by VT-treatment. GRP78 and P58(IPK) are critical for maintenance of cell homeostasis and prevention of apoptosis. The down-regulation of these molecular chaperones by VT represent a novel observation and has the potential to impact immune function at multiple levels.

Topics: Animals; Blotting, Western; Calcimycin; Carrier Proteins; DNA Fragmentation; DNA, Complementary; Down-Regulation; Endoplasmic Reticulum; Endoplasmic Reticulum Chaperone BiP; Enzyme Inhibitors; Heat-Shock Proteins; HSP40 Heat-Shock Proteins; Ionomycin; Ionophores; Mice; Molecular Chaperones; Repressor Proteins; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger; T-Lymphocytes; Tetradecanoylphorbol Acetate; Thapsigargin; Thymoma; Trichothecenes

Suppression of apoptosis appears to contribute to the development of various diseases, including autoimmune disorders and cancer. Numerous genes that encode activators and suppressors of apoptosis have been identified; however, such genes have not been shown to be expressed in all cell types. Furthermore, the sensitivity of different cell types to induction of apoptosis varies widely. We have employed a genetic approach using somatic cell hybridization to determine if apoptosis is a dominant or a recessive process in cells. These studies have utilized cell fusion partners with differing sensitivity to induction of apoptosis. The apoptosis-sensitive cells chosen were BW5147 murine thymoma cells. These cells readily undergo apoptosis in response to glucocorticoids and calcium ionophore. The resistant fusion partners were HTC rat hepatoma cells, which possess an intact glucocorticoid signal transduction pathway but are resistant to induction of apoptosis by either agent. Neither cell type expresses detectable Bcl-2 protein. Heterokaryons were identified by their retention of fluorescent cytosolic dyes and by nuclear morphology and cell size. The three types of heterokaryons observed were intratypic HTC/HTC and BW5147/BW5147 heterokaryons and intertypic BW5147/HTC heterokaryons. Glucocorticoid receptor was shown by immunohistochemistry to undergo hormone-dependent translocation to all nuclei in intertypic heterokaryons. BW5147/BW5147 heterokaryons die after treatment with glucocorticoid and calcium ionophore, whereas both HTC/ HTC and BW5147/HTC hybrids survive. The presence of multiple BW5147 cells fused to a single HTC cell did not affect this outcome. This demonstrates that HTC cells are able to dominantly suppress apoptosis in all BW5147/HTC heterokaryons. Thus, HTC cells contain activities that can suppress apoptosis in lymphocytes.

Topics: Animals; Apoptosis; Calcimycin; Cell Division; Cell Fusion; Dexamethasone; Dimethyl Sulfoxide; Hybrid Cells; Liver Neoplasms, Experimental; Lymphocytes; Mice; Rats; Thapsigargin; Thymoma; Tumor Cells, Cultured

In the WEHI7.2 thymoma cell line, cAMP, glucocorticoids, or increases in cytosolic Ca2+ concentration lead to cell death by apoptosis. In the present study, we examined the effects of these compounds on cAMP response element (CRE)-mediated gene expression. Thapsigargin and A23187 were employed to increase cytosolic Ca2+ levels and induce apoptosis. Both compounds enhanced transcription from a CRE preceding apoptotic death. Moreover, the transcriptional response to the combination of forskolin and either thapsigargin or A23187 was synergistic mirroring the effect on cell death. Importantly, dexamethasone treatment, which causes an efflux of Ca2+ from the ER, induced transcription from a CRE alone or in synergy with forskolin. The increase in CRE-controlled gene expression correlated with a decrease in cell viability. Following treatment with forskolin, thapsigargin, or dexamethasone, the CRE binding protein (CREB) was phosphorylated at levels correlating with the level of induced gene expression. These data suggest that transcriptional crosstalk between independent signaling pathways occurs in lymphocytes, and CREB may play a central role in the mediation of CRE-dependent transcription by these diverse set of apoptotic agents.

Topics: Animals; Apoptosis; Calcimycin; Calcium; Cell Division; Chloramphenicol O-Acetyltransferase; Colforsin; Cyclic AMP; Dexamethasone; Gene Expression Regulation, Neoplastic; Genes, Reporter; Glucocorticoids; Lymphocytes; Mice; Signal Transduction; Thapsigargin; Thymoma; Thymus Neoplasms; Transfection; Tumor Cells, Cultured