Ribonucleotide reductase gene expression during cyclic AMP-induced cell cycle arrest in T lymphocytes

p53 deficiency and defective mitotic checkpoint in proliferating T lymphocytes increase chromosomal instability through aberrant exit from mitotic arrest

During the proliferation of T cells for successful immune responses against pathogens, the fine regulation of cell cycle is important to the maintenance of T cell homeostasis and the prevention of lymphoproliferative disorders. However, it remains to be elucidated how the cell cycle is controlled at the mitotic phase in proliferating T cells. Here, we show that during the proliferation of primary T cells, the disruption of the mitotic spindle leads to cell-cycle arrest at mitosis and that prolonged mitotic arrest results in not only apoptosis but also the form of chromosomal instability observed in human cancers. It is interesting that in response to spindle damage, the phosphorylation of BubR1, a mitotic checkpoint kinase, was significantly induced in proliferating T cells, and the expression of the dominant-negative mutant of BubR1 compromised mitotic arrest and subsequent apoptosis and thus led to the augmentation of polyploidy formation. We also show that in response to prolonged spindle damage, the expression of p53 but not of p73 was significantly induced. In addition, following sustained mitotic arrest, p53-deficient T cells were found to be more susceptible to polyploidy formation than the wild type. These results suggest that during flourishing immune response, mitotic checkpoint and p53 play important roles in the prevention of chromosomal instability and in the maintenance of the genomic integrity of proliferating T cells.

The effect of cyclic-AMP on the regulation of c-myc expression in T lymphoma cells

Myc is implicated in the control of growth in a variety of cell types. I investigated c-myc gene expression in several lymphoid cell lines to determine the response to cyclic AMP. Cyclic AMP causes a precipitous decline in c-myc message concentration that precedes G1 cell cycle arrest in wild type S49 cells but not in KIN- cells that lack cAMP dependent PKA activity. In wild-type S49 cells washout of cyclic AMP restores c-myc message levels within 2 h but does not relieve the G1 arrest until 10 h later. Transcription runoff studies demonstrate inhibition of both transcriptional initiation and prolongation of initiated transcripts. However, the degree of inhibition is insufficient to explain the absence of detectable myc message suggesting that the predominant effect of cyclic AMP is to destabilize the c-myc message. In contrast to wild-type cells, the "Deathless" mutant S49 cell line is viable when arrested in G1 by exposure to cyclic AMP and has preserved c-myc expression. Thus, in S49 cells down regulation of c-myc expression appears to be associated with loss of viability rather than G1 cell cycle arrest. Interestingly, CEM human T lymphoma cells do not arrest in G1 phase when exposed to cyclic AMP in spite of losing detectable c-myc gene expression. This suggests that in some T lymphoma cells c-myc gene expression may not be necessary for cell cycle progression and proliferation.