Changes in c-myc expression and the kinetics of dexamethasone-induced programmed cell death (apoptosis) in human lymphoid leukaemia cells

Abstract

The kinetics of dexamethasone-induced death of CCRF CEM clone C7A human lymphoblastic leukaemia cells was determined with respect to changes in the expression of the c-myc protein. Cell death was characterised as being by apoptosis: cells with an intact plasma membrane had condensed chromatin and were characterised as having approximately 300 kbp fragments when DNA integrity was analysed by pulsed-field electrophoresis. Onset of apoptosis required a minimum of 36 h exposure to 5 microM dexamethasone; before this time no apoptotic cells were observed. This 36 h incubation period appeared to be necessary to prime the cells for subsequent death by apoptosis. In the continued presence of dexamethasone the percentage of apoptotic cells increased to 60% apoptotic cells by 54 h. Investigation of changes in c-myc protein showed that it was undetectable after 12 h of incubation with dexamethasone, although cells were not committed to die at this time. Cells were treated with dexamethasone for 54 h and for various pulsed periods with a non-toxic concentration of cycloheximide (200 nM). When cycloheximide was present during the first 36 h priming period of dexamethasone treatment, there was an immediate loss of c-myc protein and apoptosis at 54 h was completely inhibited. In contrast, there was no inhibition of apoptosis when dexamethasone-treated cells were incubated with an 18 h pulse of cycloheximide added after 36 h. Cells exposed to dexamethasone for 36 h ('primed') were given various periods of dexamethasone-free incubation before readdition of dexamethasone for a further 18 h. The longer the cells were free of drug after priming, the less susceptible they became to apoptosis, suggesting a slow decay of their 'memory' of the initial 36 h period of exposure. Cycloheximide inhibited the decay of this memory. Removal of dexamethasone after a 36 h exposure was characterised by a subsequent 24 h suppression of c-myc protein expression. Despite this, 90% of cells became refractory to apoptosis before the reappearance of c-myc protein. The evidence does not support the hypothesis that changes in c-myc expression are required for the engagement of apoptosis of CEM cells. Images Figure 1 Figure 2 Figure 4 Figure 5

c-Myc degradation induced by DNA damage results in apoptosis of CHO cells

“…Furthermore, flow cytometrical analysis of sub-G1 DNA region in TC-treated UV41 cells showed that the accumulation of apoptotic cells was also time-dependent ( Figure 2c). Several studies found that lowered c-Myc content in different cell systems such as Jurkat cells or B-lymphoid cells led to apoptosis (Wood et al, 1994;Sonenshein, 1997; and reviewed by Thompson, 1998a). Taken together, the present results indicate that the reduction of c-Myc level by DNA damaging agents TC is closely correlated with cellular apoptosis.…”

c-Myc does not prevent glucocorticoid-induced apoptosis of human leukemic lymphoblasts

“…The discrepancies might be due to the di erent technologies applied (transient transfection and antisense oligonucleotide technology versus doxycyclineregulated expression in stably-transfected cell lines), clonal divergence between the CCRF-CEM subclones used by the two groups, or other reasons. In support of our conclusion, Wood et al (1994) reported that removal of GC from the medium after 36 h prevented apoptosis in CEM-C7A cells even though c-Myc protein remained repressed for another 24 h suggesting that sustained Myc downregulation does not entail cell death. Moreover, in P1798 mouse thymoma cells, GC-induced G1 cell cycle arrest as well as apoptosis elicited by the combined action of GC and serum withdrawal was also una ected by forced expression of c-myc (Rhee et al, 1995).…”

CCL2 Inhibits the Apoptosis Program Induced by Growth Factor Deprivation, Rescuing Functional T Cells

“…We cannot formally exclude, at this time, that other apoptotic signals dependent of transcription and translation, such as glucocorticoid-induced apoptosis (15,42), could also induce a CCR2 overexpression. Importantly, CCL2 can effectively interfere with apoptosis induced by IL-2 deprivation.…”