S J Orr

Commitment Point during G0→G1 That Controls Entry into the Cell Cycle

ABSTRACT Initiation of T-lymphocyte-mediated immune responses involves two cellular processes: entry into the cell cycle (G0→G1) for clonal proliferation and coordinated changes in surface and secreted molecules that mediate effector functions. However, a point during G0→G1 beyond which T cells are committed to enter the cell cycle has not been defined. We define here a G0→G1 commitment point that occurs 3 to 5 h after CD3 and CD28 stimulation of human CD4 or CD8 T cells. Transition through this point requires cdk6/4-cyclin D, since inhibition with TAT-p16INK4A during the first 3 to 5 h prevents cell cycle entry and maintains both naive and memory T cells in G0. Transition through the G0→G1 commitment point is also necessary for T cells to increase in size, i.e., to enter the cellular growth cycle. However, transition through this point is not required for the induction of effector functions. These can be initiated while cells are maintained in G0 with TAT-p16INK4A. We have termed this quiescent, activated state G0(A). Our data provide proof of the principle that entry of T cells into the cell cycle and cellular growth cycles are coupled at the G0→G1 commitment point but that these processes can be uncoupled from the early expression of molecules of effector functions.

Histone Deacetylase Inhibitors (HDI) Cause DNA Damage in Leukemia Cells: A Mechanism for Leukemia-Specific HDI-Dependent Apoptosis?

Histone deacetylase inhibitors (HDI) increase gene expression through induction of histone acetylation. However, it remains unclear whether increases in specific gene expression events determine the apoptotic response following HDI administration. Herein, we show that a variety of HDI trigger in hematopoietic cells not only widespread histone acetylation and DNA damage responses but also actual DNA damage, which is significantly increased in leukemic cells compared with normal cells. Thus, increase in H2AX and ataxia telangiectasia mutated (ATM) phosphorylation, early markers of DNA damage, occurs rapidly following HDI administration. Activation of the DNA damage and repair response following HDI treatment is further emphasized by localizing DNA repair proteins to regions of DNA damage. These events are followed by subsequent apoptosis of neoplastic cells but not normal cells. Our data indicate that induction of apoptosis by HDI may result predominantly through accumulation of excessive DNA damage in leukemia cells, leading to activation of apoptosis. (Mol Cancer Res 2006;4(8):563–73)

Reducing MCM levels in human primary T cells during the G(0)-->G(1) transition causes genomic instability during the first cell cycle.

DNA replication is tightly regulated, but paradoxically there is reported to be an excess of MCM DNA replication proteins over the number of replication origins. Here, we show that MCM levels in primary human T cells are induced during the G0→G1 transition and are not in excess in proliferating cells. The level of induction is critical as we show that a 50% reduction leads to increased centromere separation, premature chromatid separation (PCS) and gross chromosomal abnormalities typical of genomic instability syndromes. We investigated the mechanisms involved and show that a reduction in MCM levels causes dose-dependent DNA damage involving activation of ATR & ATM and Chk1 & Chk2. There is increased DNA mis-repair by non-homologous end joining (NHEJ) and both NHEJ and homologous recombination are necessary for Mcm7-depleted cells to progress to metaphase. Therefore, a simple reduction in MCM loading onto DNA, which occurs in cancers as a result of aberrant cell cycle control, is sufficient to cause PCS and gross genomic instability within one cell cycle.

Proteomic and protein interaction network analysis of human T lymphocytes during cell‐cycle entry

Regulating the transition of cells such as T lymphocytes from quiescence (G0) into an activated, proliferating state involves initiation of cellular programs resulting in entry into the cell cycle (proliferation), the growth cycle (blastogenesis, cell size) and effector (functional) activation. We show the first proteomic analysis of protein interaction networks activated during entry into the first cell cycle from G0. We also provide proof of principle that blastogenesis and proliferation programs are separable in primary human T cells. We employed a proteomic profiling method to identify large‐scale changes in chromatin/nuclear matrix‐bound and unbound proteins in human T lymphocytes during the transition from G0 into the first cell cycle and mapped them to form functionally annotated, dynamic protein interaction networks. Inhibiting the induction of two proteins involved in two of the most significantly upregulated cellular processes, ribosome biogenesis (eIF6) and hnRNA splicing (SF3B2/SF3B4), showed, respectively, that human T cells can enter the cell cycle without growing in size, or increase in size without entering the cell cycle.

p27Kip1 and p130 Cooperate To Regulate Hematopoietic Cell Proliferation In Vivo

ABSTRACT To investigate the potential functional cooperation between p27Kip1 and p130 in vivo, we generated mice deficient for both p27Kip1 and p130. In p27Kip1−/−; p130−/− mice, the cellularity of the spleens but not the thymi is significantly increased compared with that of their p27Kip1−/− counterparts, affecting the lymphoid, erythroid, and myeloid compartments. In vivo cell proliferation is significantly augmented in the B and T cells, monocytes, macrophages, and erythroid progenitors in the spleens of p27Kip1−/−; p130−/− animals. Immunoprecipitation and immunodepletion studies indicate that p130 can compensate for the absence of p27Kip1 in binding to and repressing CDK2 and is the predominant CDK-inhibitor associated with the inactive CDK2 in the p27Kip1−/− splenocytes. The finding that the p27Kip1−/−; p130−/− splenic B cells are hypersensitive to mitogenic stimulations in vitro lends support to the concept that the hyperproliferation of splenocytes is not a result of the influence of their microenvironment. In summary, our findings provide genetic and molecular evidence to show that p130 is a bona fide cyclin-dependent kinase inhibitor and cooperates with p27Kip1 to regulate hematopoietic cell proliferation in vivo.

High efficiency protein transduction of quiescent and proliferating primary hematopoietic cells

Primary hematopoietic cells are relatively refractory to DNA transfection methodologies. This is particularly so when they are quiescent or terminally differentiated and no longer able to divide. However, whole proteins can be introduced into such cells by protein transduction. We have modified the protein transduction domain (PTD) from the HIV-TAT protein used by other investigators. Using green fluorescent protein (GFP) as a reporter, we show that this new sequence allows more efficient transduction of recombinant fusion protein into a variety of hematopoietic cells tested compared with the native HIV TAT domain. This is true for peripheral blood CD34+ cells, dendritic cells, granulocytes, monocytes and lymphocytes all of which are quiescent or terminally differentiated. Furthermore, we were able to transduce myeloblasts from patients with acute myeloid leukemia (AML). In all cell types tested transduction efficiency was almost 100%. Transduction is maximal 15-30 s after addition of PTD or TAT-GFP fusion proteins as tested on quiescent T lymphocytes. This method will allow us to study of the effects of a variety of gene products in cell types that were previously resistant to gene transfection studies.