Overview
Most chemotherapeutic drugs indiscriminately kill quickly dividing cells. We hypothesize that cancer cells can evade these treatments by persisting for long periods outside of the cell cycle in a quiescent, non-dividing state. Our current research efforts focus on understanding how two unusual proteins, known as Pim-1 and Pim-2 kinases, cause the cell to either quiesce or divide at the molecular level. We have developed a mouse model system to help understand how the Pim kinases function and interact with cellular growth signals. In other studies, we find that Pim-2 also functions to link energy metabolism with DNA synthesis. Our data are consistent with the idea that quiescence is an active state rather than a passive response to the absence of cellular signals that promote cell division. The hope is that this work will contribute to a novel chemotherapeutic strategy aimed at forcing cancer cells to become and remain dormant, thereby allowing cancer to be treated as a chronic disease state.
Research details
The PIM kinase family in hematopoiesis and cancer
The hematopoeitic system tends to be a favorite model for cancer research. One reason for this is that lymphocytes and their precursors have the unique ability to reversibly switch between a state of high proliferation to one of relative quiescence during which cells can persist outside of the cell cycle (referred to as G0) for extended periods. Analogous to studies of yeast and worms, our objective is to test the hypothesis that G0 quiescence is an actively regulated cell fate distinct from G1 arrest and that a "quiescent apparatus" exists to prevent cell cycle entry. Understanding how the transition from quiescence to proliferation is regulated at the molecular level might be the key to preserving immune function in patients being treated for diseases of hematopoietic origin.
We study two oncogenic pathways that are ubiquitously induced in response to prosurvival or mitogenic stimuli: the PI-3K/AKT/mTOR and the JAK/STAT/PIM kinase pathways. Pronounced dwarfism is characteristic of Pim1-/- Pim2-/- Pim3-/- and Akt1-/- Akt2-/- mice as compared to their control littermates, implicating the two pathways as potentially universal regulators of growth in most cell types. Cells from Pim2-deficient mice demonstrate hypersensitivity to the TOR inhibitor rapamycin. Thus, it appears that most cells adapt in the long term to germline deficiency in one pathway as long as the other remains intact in order to preserve normal organ and body architecture. Our efforts focus on understanding how the two pathways regulate 1) mature lymphocyte homeostasis, and 2) blood cell development in fetal, newborn, and adult mice. Describing how the G0 to G1 transition affects the neoplastic potential of hematopoietic cells is another primary goal, one that is critical to our continuing efforts to define and characterize Pim kinase-specific inhibitors.
The G0 program in mature lymphocytes
A highly diverse pool of mature lymphocytes is maintained for life even though almost none of these cells will encounter their cognate antigen. The observation that the prosurvival factor interleukin-7 (IL-7) induces cell cycle entry in memory but not naïve T cells led us to the idea that prosurvival cytokines might work by preventing resting cells from entering the cell cycle. Indeed, PIM1 and PIM2 are primary regulators of a restriction point in early G0 that programs the cellular decision to quiesce or proliferate in mature T cells. G0- are distinguished from G1-arrested cells by virtue of reduced cell size and decreased rates of bioenergetic metabolism, RNA transcription and protein translation. G0 cells can devote as much as 45% of their limited ATP supply to regulated protein turnover. In cytokine-treated resting T cells, PIM2 can maintain the cap-independent translation and cytoplasmic retention of key regulators of cell cycle entry. This list includes members of the NF- κB and STAT families of transcription factors, as well as negative regulators of these proteins including the inhibitor of NF- κB (I κB) and SOCS1 proteins. Many of these proteins appear to be continuously synthesized and degraded in resting cells. Mitogenic signals induce a shift in the expression patterns of PIM1 and PIM2 and their targets, leading to the activation of cap-dependent translation and G1 entry. The combinatorial inhibition of PI-3K/AKT/mTOR and JAK/STAT/PIM prevents the G0 to G1 transition in response to activating stimuli but does not induce apoptosis. We are investigating if similar mechanisms operate in naïve B cells treated with the survival factor BLyS. Another of our goals is to understand how the PIM-dependent control of the G0 to G1 transition affects the differentiation and function of effector T and B cells in vivo and in vitro.
The G0 program in hematopoietic stem cells (HSC)
Unlike their adult counterparts, lymphocytes and long-term-repopulating HSC (LT-HSC) in fetal liver and in newborn mice are highly proliferative. At weaning, most of these cells shift to a G0 phenotype, after which they cycle very infrequently. One idea is that specialized niches might exist within adult animals to keep LT-HSC in G0. Our initial data indicate that LT-HSC from Pim1-/- Pim2-/- adult bone marrow have a decreased ability to transit to G1 in response to differentiative stimuli. This is exacerbated in cells in which PI-3K/AKT/mTOR signaling is inhibited by rapamycin treatment. One of our goals in the next year will be to investigate whether the same molecular mechanisms that regulate lymphocyte homeostasis also control HSC self-renewal. We are also developing a mouse model system to study ex vivo differentiation of fetal liver progenitor cells derived from animals with individual or compound deletions in members of the PIM and AKT families.
Targeting the G0 checkpoint in malignant cells
Since uncontrolled proliferation is one feature that distinguishes normal from neoplastic cells, many chemotherapeutic drugs work by targeting the cell cycle machinery. Tumors can evade these treatments by exiting the cell cycle and persisting in G0. PIM1 and PIM2 are commonly deregulated in established human and murine malignancies, including those arising due to aberrant activation of other oncogenic kinases like BCR-ABL, JAK2 and FLT3-ITD. In contrast to their normal counterparts, some of these transformed cells show stable PIM kinase expression in the absence of mitogenic stimuli. Interfering with PIM1 and PIM2 function leads to deregulation of the G0 apparatus and cell death. To address our long-term goal to understand how the PIM kinases function as oncogenes, we are interested in investigating if the aberrant ability of PIM kinase-deficient fetal liver cells to transit from G0 to G1 affects their neoplastic potential. Since the PIM kinases are both targets and effectors of JAK/STAT-dependent signals, understanding how Pim1 or Pim2 transgenes might complement the developmental deficiencies evident in mice lacking expression of JAK, STAT or SOCS proteins is another aspect of our current efforts. The ability to prevent the shift from G0 to G1 during neoplastic outgrowth, or to force malignant cells to remain in G0, is one that could have profound implications for cancer treatment.
Lab staff
Research Assistant II: James Clark, B.S., B.A.Research Administrative Assistant: Annie McDonnell
Publication listings
Woodland RT, Fox CJ, Schmidt MR, Hammerman PS, Opferman JT, Korsmeyer SJ, Hilbert DM, and Thompson CB. 2007. Multiple signaling pathways promote B Lymphocyte stimulator (BLyS)-dependent B cell growth and survival. Blood 111:750-760.
Adam M, Pogacic V, Bendit M, Chappuis R, Nawijn MC, Duyster J, Fox CJ, Thompson CB, Cools J, Schwaller J. 2006. Targeting PIM kinases impairs survival of hematopoietic cells transformed by kinase inhibitor-sensitive and kinase inhibitor-resistant forms of Fms-like tyrosine kinase 3 and BCR/ABL. Cancer Res 66:3828-3835.
