Lack of a p21waf1/cip-Dependent G1/S Checkpoint in Neural Stem and Progenitor Cells After DNA Damage In Vivo

The cyclin-dependent kinase inhibitor p21waf1/cip mediates the p53-dependent G1/S checkpoint, which is generally considered to be a critical requirement to maintain genomic stability after DNA damage. We used staggered 5-ethynyl-2′deoxyuridine/5-bromo-2′-deoxyuridine double-labeling in vivo to investigate the cell cycle progression and the role of p21waf1/cip in the DNA damage response of neural stem and progenitor cells (NSPCs) after exposure of the developing mouse cortex to ionizing radiation. We observed a radiation-induced p21-dependent apoptotic response in migrating postmitotic cortical cells. However, neural stem and progenitor cells (NSPCs) did not initiate a p21waf1/cip1-dependent G1/S block and continued to enter S-phase at a similar rate to the non-irradiated controls. The G1/S checkpoint is not involved in the mechanisms underlying the faithful transmission of the NSPC genome and/or the elimination of critically damaged cells. These processes typically involve intra-S and G2/M checkpoints that are rapidly activated after irradiation. p21 is normally repressed in neural cells during brain development except at the G1 to G0 transition. Lack of activation of a G1/S checkpoint and apoptosis of postmitotic migrating cells after DNA damage appear to depend on the expression of p21 in neural cells, since substantial cell-to-cell variations are found in the irradiated cortex. This suggests that repression of p21 during brain development prevents the induction of the G1/S checkpoint after DNA damage. Stem Cells 2012;30:537–54

Upregulation of bfl-1 is a potential mechanism of chemoresistance in B-cell chronic lymphocytic leukaemia

B-cell chronic lymphocytic leukaemia (B-CLL) is characterised by the progressive accumulation of monoclonal CD5+ B cells. In a previous study, we have analysed the expression profile of apoptosis-regulating genes using a cDNA-based microarray and found overexpression of the antiapoptotic bcl-2 family member, bfl-1, in B-CLL cells with an apoptosis-resistant phenotype. In this study, bfl-1 mRNA levels have been determined by competitive PCR in an extended population of B-CLL patients to characterise its role in disease progression and development of chemoresistance. bfl-1 levels were significantly higher in patients with no response (NR) to last chemotherapy than in patients responding (partial response (PR)) to last chemotherapy (P<0.05) and in patients who had not required treatment (P<0.05). We found no correlation between bfl-1 mRNA levels and disease progression, IGHV mutational status or other clinical parameters. In addition, bfl-1 mRNA levels were inversely correlated with apoptotic response to in vitro fludarabine treatment of B-CLL cells. Specific downregulation of bfl-1 using siRNA induced apoptosis in resistant cells. Our data suggest that bfl-1 contributes to chemoresistance and might be a therapeutic target in B-CLL

PDGF and PDGF receptors in glioma

The family of platelet-derived growth factors (PDGFs) plays a number of critical roles in normal embryonic development, cellular differentiation, and response to tissue damage. Not surprisingly, as it is a multi-faceted regulatory system, numerous pathological conditions are associated with aberrant activity of the PDGFs and their receptors. As we and others have shown, human gliomas, especially glioblastoma, express all PDGF ligands and both the two cell surface receptors, PDGFR-α and -β. The cellular distribution of these proteins in tumors indicates that glial tumor cells are stimulated via PDGF/PDGFR-α autocrine and paracrine loops, while tumor vessels are stimulated via the PDGFR-β. Here we summarize the initial discoveries on the role of PDGF and PDGF receptors in gliomas and provide a brief overview of what is known in this field

The activation region of the Tat protein of human immunodeficiency virus type-1 functions in yeast.

The N-terminal 48 amino acids of the Tat protein of human immunodeficiency virus type (HIV)-1 constitute its activation region. This region can autonomously activate transcription when targeted to the HIV-1 long terminal repeat or certain heterologous promoters either through DNA binding sites located upstream of the transcription initiation site or via downstream RNA binding sites in mammalian cells. To determine whether the Tat activation region can function in yeast, we have assayed the effect of a chimeric gene (GAL-Tat48) expressing the DNA binding domain of the yeast transcription factor Gal4 (residues 1-147) and the activation region of Tat on GAL1 promoter-directed expression of the lacZ reporter gene in Saccharomyces cerevisiae. Our results indicate that the Gal-Tat48 fusion protein can induce significant activation of the GAL1 promoter. Analysis of a number of Tat mutants located within the activation region indicate that the amino acid residues of Tat essential for trans-activation in mammalian cells are also required for transactivation in yeast. Our results suggest that Tat-mediated transcriptional activation may involve a mechanism conserved among yeast and mammalian cells