ABSTRACT: Molecular mechanisms of cell cycle exit are poorly understood. A group of genes required for cell cycle exit and maintenance of cell quiescence in human fibroblasts following serum deprivation has been recently identified. Studies on lymphocytes following growth factor deprivation-induced cell cycle exit have predominantly focused on the initiation of apoptosis. A set of genes involved in lymphocyte quiescence have also been identified among genes highly expressed in resting lymphocytes and down-regulated after cell activation. In our study, proliferating IL-2-dependent human T cells were forced to exit cell cycle by growth factor withdrawal, and their gene expression profiles were examined. The differential gene expression analysis was performed in primary and immortalized IL-2-dependent T lymphocytes. Cell samples were collected directly from the IL-2-containing cultures and 8-hrs following IL-2 withdrawal, before apoptosis could be evidenced by the Annexin-V staining. The three primary T lymphoblast cell populations were obtained from the peripheral blood mononuclear cells (PBMC) stimulated for 24h by wheat germ agglutinin and cultured in the presence of IL-2 up to 4-8 population doublings. As shown by the cell surface analysis, these populations were composed of T cells exclusively. Samples of these cell populations were subsequently analyzed as biological replicates. Two spontaneously immortalized IL-2-dependent T cell lines were derived from normal spleen and from PBMC derived from Nijmegen Breakage Syndrome patient. Gene expression was assessed by the Affymetrix microarray HG-U133 2.0 Plus that detects 38,500 genes. The expression of a selected number of genes was verified by the qRT-PCR method. We have identified a set of 53 genes that we called a “T lymphocyte cell cycle exit signature”, comprised of 13 up-regulated and 40 down-regulated genes. Genes linked to transcription, cell cycle, cell growth, proliferation and differentiation, cell adhesion and immune functions were found to be overrepresented among the differentially expressed, before and after IL-2 deprivation. Among those, PIM1, BCL2, IL-8, HBEGF, DUSP6, OSM, CISH, SOCS2, SOCS3, LIF and IL13 were down-regulated and RPS24, SQSTM1, TMEM1, LRRC8D, ECOP, YY1AP1, C1orf63, ASAH1, SLC25A46 and MIA3 were up-regulated. Identification of genes involved in cell cycle exit and quiescence, may provide new insights into the mechanisms of tissue repair and regeneration as well as of cancer development.
