Rapid thymic reconstitution following bone marrow transplantation in neonatal mice is VEGF-dependent.

Age-related differences in thymic function influence the rapidity of T cell reconstitution following hematopoietic stem cell transplantation (HSCT). In adults, thymic reconstitution is delayed until after marrow engraftment is established, and is significantly improved by approaches that increase marrow chimerism, such as pretransplantation irradiation. In contrast, we show that neonatal mice undergo more rapid and efficient thymic reconstitution than adults, even when bone marrow (BM) engraftment is minimal and in the absence of pretransplantation radiation. We have previously shown that the neonatal thymus produces high levels of vascular endothelial growth factor (VEGF) that drives angiogenesis locally. In this report, we show that inhibition of VEGF prior to HSCT prevents rapid thymic reconstitution in neonates, but has no effect on thymic reconstitution in adults. These data suggest that the early radiation-independent thymic reconstitution unique to the neonatal host is mediated through VEGF, and reveals a novel pathway that might be targeted to improve immune reconstitution post-HSCT.

Combined preconditioning and in vivo chemoselection with 6-thioguanine alone achieves highly efficient reconstitution of normal hematopoiesis with HPRT-deficient bone marrow.

Purine analogs such as 6-thioguanine (6TG) cause myelotoxicity upon conversion into nucleotides by hypoxanthine-guanine phosphoribosyltransferase (HPRT). Here we have developed a novel and highly efficient strategy employing 6TG as a single agent for both conditioning and in vivo chemoselection of HPRT-deficient hematopoietic stem cells. The dose-response and time course of 6TG myelotoxicity were first compared in HPRT wild-type mice and HPRT-deficient transgenic mice. Dosage and schedule parameters were optimized to employ 6TG for myelosuppressive conditioning, immediately followed by in vivo chemoselection of HPRT-deficient transgenic donor bone marrow (BM) transplanted into syngeneic HPRT wild-type recipients. At appropriate doses, 6TG induced selective myelotoxicity without any adverse effects on extrahematopoietic tissues in HPRT wild-type mice, while hematopoietic stem cells deficient in HPRT activity were highly resistant to its cytotoxic effects. Combined 6TG conditioning and post-transplantation chemoselection consistently achieved ∼95% engraftment of HPRT-deficient donor BM, with low overall toxicity. Long-term reconstitution of immunophenotypically normal BM was achieved in both primary and secondary recipients. Our results provide proof-of-concept that single-agent 6TG can be used for both myelosuppressive conditioning without requiring irradiation and for in vivo chemoselection of HPRT-deficient donor cells. Our results show that by applying the myelosuppressive effects of 6TG both before (as conditioning) and after transplantation (as chemoselection), highly efficient engraftment of HPRT-deficient hematopoietic stem cells can be achieved.

VEGF-mediated cross-talk within the neonatal murine thymus.

Although the mechanisms of cross-talk that regulate the hematopoietic and epithelial compartments of the thymus are well established, the interactions of these compartments with the thymic endothelium have been largely ignored. Current understanding of the thymic vasculature is based on studies of adult thymus. We show that the neonatal period represents a unique phase of thymic growth and differentiation, marked by endothelium that is organized as primitive, dense networks of capillaries dependent on vascular endothelial growth factor (VEGF). VEGF dependence in neonates is mediated by significantly higher levels of both VEGF production and endothelial VEGF receptor 2 (VEGF-R2) expression than in the adult thymus. VEGF is expressed locally in the neonatal thymus by immature, CD4(-)CD8(-) "double negative" (DN) thymocytes and thymic epithelium. Relative to adult thymus, the neonatal thymus has greater thymocyte proliferation, and a predominance of immature thymocytes and cortical thymic epithelial cells (cTECs). Inhibition of VEGF signaling during the neonatal period results in rapid loss of the dense capillaries in the thymus and a marked reduction in the number of thymocytes. These data demonstrate that, during the early postnatal period, VEGF mediates cross-talk between the thymocyte and endothelial compartments of the thymus.

WTp53 induction does not override MTp53 chemoresistance and radioresistance due to gain-of-function in lung cancer cells.

New molecular cancer treatment strategies aim to reconstitute wild-type p53 (WTp53) function in mutant p53 (MTp53)-expressing tumors as a means of resensitizing cells to chemotherapy or radiotherapy. The success of this approach may depend on whether MTp53 proteins are acting in a dominant-negative or independent gain-of-function mode. Herein, we describe an isogenic, temperature-sensitive p53 model (p53(A138V)) in p53-null human H1299 lung cancer cells in which WTp53 can be selectively coexpressed with a temperature-sensitive MTp53 allele (A138V) during initial DNA damage and subsequent DNA repair. Cells expressing MTp53 alone or coexpressing induced WTp53 and MTp53 were tested for p53 transcription, G(1) and G(2) cell cycle checkpoints, apoptosis, and long-term clonogenic survival following DNA damage. Transient transfection of WTp53 into H1299 cells, or shift-down of H1299-p53(A138V) stable transfectants to 32 degrees C to induce WTp53, led to increased p21(WAF1) expression and G(1) and G(2) arrests following DNA damage but did not increase BAX expression or apoptosis. In contrast, both transient and stable expression of the p53(A138V) mutant in p53-null H1299 cells (e.g. testing gain-of-function) at 37 degrees C blocked p21(WAF1) induction following DNA damage. Cell death was secondary to mitotic catastrophe and/or tumor cell senescence. Overexpression of WTp53 did not resensitize resistant MTp53-expressing cells to ionizing radiation, cisplatinum, or mitomycin C. Our results suggest that human MTp53 proteins can lead to resistant phenotypes independent of WTp53-mediated transcription and checkpoint control. This should be considered when using p53 as a prognostic factor and therapeutic target.

The p53 protein family and radiation sensitivity: Yes or no?

The p53 tumor suppressor protein is a key mediator of an ATM-dependent DNA damage response cascade following cellular exposure to ionizing radiation. The p53-family members, p63 and p73, are highly similar to p53, yet are differentially activated by IR, UV and cis-platinum via ATM and c-abl/ATR signaling pathways. Loss of function of p53 can occur by mutation or degradation; giving rise to alterations in G(1) and G(2) cell cycle checkpoint control, cell death, DNA repair and genetic stability. The end result of these alterations can be the generation of radioresistant mutant tumor cells. Indeed, in isogenic systems, loss of p53 or p73 function has been associated with decreased chemosensitivity and radiosensitivity, in vitro. However, clinical data supporting a role for p53 genotype as an independent predictive factor for radiotherapy outcome continues to be controversial due to variable endpoints in clinical trial design and in methodology in detecting p53 function. Nonetheless, in carefully controlled radiotherapy studies where mutations in p53 have been detected using DNA sequencing or functional assays, the presence of mutant p53 can be associated with decreased local control following radiotherapy. This suggests that novel molecular treatment strategies specifically designed to re-institute normal p53 function within resistant tumors can be used as combined modality protocols to improve local control and maintain a therapeutic ratio. A future challenge lies in the pre-therapy determination of a 'molecular therapeutic ratio' for individual patients which could allow for specific prognostication based on p53 functional status and subsequent individualized therapy.

Coordination of DNA damage responses via the Smc5/Smc6 complex.

The detection of DNA damage activates DNA repair pathways and checkpoints to allow time for repair. Ultimately, these responses must be coordinated to ensure that cell cycle progression is halted until repair is completed. Several multiprotein complexes containing members of the structural maintenance of chromosomes family of proteins have been described, including the condensin and cohesin complexes, that are critical for chromosomal organization. Here we show that the Smc5/Smc6 (Smc5/6) complex is required for a coordinated response to DNA damage and normal chromosome integrity. Fission yeast cells lacking functional Smc6 initiate a normal checkpoint response to DNA damage, culminating in the phosphorylation and activation of the Chk1 protein kinase. Despite this, cells enter a lethal mitosis, presumably without completion of DNA repair. Another subunit of the complex, Nse1, is a conserved member of this complex and is also required for this response. We propose that the failure to maintain a checkpoint response stems from the lack of ongoing DNA repair or from defective chromosomal organization, which is the signal to maintain a checkpoint arrest. The Smc5/6 complex is fundamental to genome integrity and may function with the condensin and cohesin complexes in a coordinated manner.

Protein kinase inhibitor 2-aminopurine overrides multiple genotoxic stress-induced cellular pathways to promote cell survival.

2-Aminopurine (2-AP) is an adenine analog shown to cause cells to bypass chemical- and radiation-induced cell cycle arrest through as-yet unidentified mechanisms. 2-AP has also been shown to act as a kinase inhibitor. Tumor suppressor p53 plays an important role in the control of cell cycle and apoptosis in response to genotoxic stress. We were interested in examining the effect of 2-AP on p53 phosphorylation and its possible consequences on checkpoint control in cells subjected to various forms of DNA damage. Here, we show that 2-AP suppresses p53 phosphorylation in response to gamma radiation, adriamycin, or ultraviolet treatment. This is partly explained by the ability of the kinase inhibitor to inhibit ATM or ATR activities in vitro and impair ATM- or ATR-dependent p53 phosphorylation in vivo. However, 2-AP is also capable of inhibiting p53 phosphorylation in cells deficient in ATM, DNA-PK, or ATR suggesting the existence of multiple pathways by which this kinase inhibitor modulates p53 activation. Biologically, the 2-AP-mediated inhibition of p53 stabilization enables wild-type p53-containing cells to bypass adriamycin-induced G(2)/M arrest. In the long term, however, 2-AP facilitates cells to resist DNA damage-induced cell death independently of p53.

Cell-cycle responses to DNA damage in G2.

Cellular reproduction, at its basic level, is simply the passing of genetic information from a single parent cell into two daughter cells. As the cellular genome encodes all the information that defines a cell, it is crucial that the genome be accurately replicated. Furthermore, the duplicated genome must be properly segregated so that each daughter cell contains the exact same information as the parent cell. The processes by which this occurs is known as the cell cycle. The failure of either duplication or segregation of the genome can have disastrous consequences for an organism, including cancer and death. This article discusses what is known about checkpoints, the surveillance mechanisms that monitor both the fidelity and accuracy of DNA replication and segregation. Specifically, we will focus on the G2 checkpoint that is responsible for ensuring proper segregation of the duplicated genome into the daughter cells and how this checkpoint functions to arrest entry into mitosis in response to DNA damage.

Normal p53 function in primary cells deficient for Siah genes.

Overexpression studies have suggested that Siah1 proteins may act as effectors of p53-mediated cellular responses and as regulators of mitotic progression. We have tested these hypotheses using Siah gene knockout mice. Siah1a and Siah1b were not induced by activation of endogenous p53 in tissues, primary murine embryonic fibroblasts (MEFs) or thymocytes. Furthermore, primary MEFs lacking Siah1a, Siah1b, Siah2, or both Siah2 and Siah1a displayed normal cell cycle progression, proliferation, p53-mediated senescence, and G(1) phase cell cycle arrest. Primary thymocytes deficient for Siah1a, Siah2, or both Siah2 and Siah1a, E1A-transformed MEFs lacking Siah1a, Siah1b, or Siah2, and Siah1b-null ES cells all underwent normal p53-mediated apoptosis. Finally, inhibition of Siah1b expression in Siah2 Siah1a double-mutant cells failed to inhibit cell division, p53-mediated induction of p21 expression, or cell cycle arrest. Our loss-of-function experiments do not support a general role for Siah genes in p53-mediated responses or mitosis.