Radiation Sensitivity and Tumor Susceptibility in ATM Phospho-Mutant ATF2 Mice.

The transcription factor ATF2 was previously shown to be an ATM substrate. Upon phosphorylation by ATM, ATF2 exhibits a transcription-independent function in the DNA damage response through localization to DNA repair foci and control of cell cycle arrest. To assess the physiological significance of this phosphorylation, we generated ATF2 mutant mice in which the ATM phosphoacceptor sites (S472/S480) were mutated (ATF2(KI)).

Dominant-negative inhibitors of soluble TNF attenuate experimental arthritis without suppressing innate immunity to infection.

TNF is a pleiotropic cytokine required for normal development and function of the immune system; however, TNF overexpression also induces inflammation and is associated with autoimmune diseases. TNF exists as both a soluble and a transmembrane protein. Genetic studies in mice have suggested that inflammation in disease models involves soluble TNF (solTNF) and that maintenance of innate immune function involves transmembrane TNF (tmTNF).

Differential regulation of retinoblastoma tumor suppressor protein by G(1) cyclin-dependent kinase complexes in vivo.

The retinoblastoma tumor suppressor protein (pRB) negatively regulates early-G(1) cell cycle progression, in part, by sequestering E2F transcription factors and repressing E2F-responsive genes. Although pRB is phosphorylated on up to 16 cyclin-dependent kinase (Cdk) sites by multiple G(1) cyclin-Cdk complexes, the active form(s) of pRB in vivo remains unknown. pRB is present as an unphosphorylated protein in G(0) quiescent cells and becomes hypophosphorylated (approximately 2 mol of PO(4) to 1 mol of pRB) in early G(1) and hyperphosphorylated (approximately 10 mol of PO(4) to 1 mol of pRB) in late G(1) phase.

p21cip1 is required for the differentiation of oligodendrocytes independently of cell cycle withdrawal.

Differentiation of most cell types requires both establishment of G1 arrest and the induction of a program related to achieving quiescence. We have chosen to study the differentiation of oligodendrocyte cells to determine the role of p27 and p21 in this process. Here we report that both p27 and p21 are required for the appropriate differentiation of these cells.

Transforming growth factor beta targeted inactivation of cyclin E:cyclin-dependent kinase 2 (Cdk2) complexes by inhibition of Cdk2 activating kinase activity.

Transforming growth factor beta (TGF-beta)-mediated G(1) arrest previously has been shown to specifically target inactivation of cyclin D:cyclin-dependent kinase (Cdk) 4/6 complexes. We report here that TGF-beta-treated human HepG2 hepatocellular carcinoma cells arrest in G(1), but retain continued cyclin D:Cdk4/6 activity and active, hypophosphorylated retinoblastoma tumor suppressor protein. Consistent with this observation, TGF-beta-treated cells failed to induce p15(INK4b), down-regulate CDC25A, or increase levels of p21(CIP1), p27(KIP1), and p57(KIP2).

Transduced p16INK4a peptides inhibit hypophosphorylation of the retinoblastoma protein and cell cycle progression prior to activation of Cdk2 complexes in late G1.

Progression of cells through the G1 phase of the cell cycle requires cyclin D:Cdk4/6 and cyclin E:Cdk2 complexes; however, the duration and ordering of these complexes remain unclear. To address this, we synthesized a peptidyl mimetic of the Cdk4/6 inhibitor, p16INK4a that contained an NH2-terminal TAT protein transduction domain. Transduction of TAT-p16 wild-type peptides into cells resulted in the loss of active, hypophosphorylated pRb and elicited an early G1 cell cycle arrest, provided cyclin E:Cdk2 complexes were inactive.

Hypo-phosphorylation of the retinoblastoma protein (pRb) by cyclin D:Cdk4/6 complexes results in active pRb.

In cycling cells, the retinoblastoma protein (pRb) is un- and/or hypo-phosphorylated in early G1 and becomes hyper-phosphorylated in late G1. The role of hypo-phosphorylation and identity of the relevant kinase(s) remains unknown. We show here that hypo-phosphorylated pRb associates with E2F in vivo and is therefore active.

xid affects events leading to B cell cycle entry.

X-linked agammaglobulinemia patients and X-linked immunodeficient (xid) mice possess mutations in the Bruton's tyrosine kinase (Btk kinase) gene and display defects in B cell development and activation by sIg cross-linking. Btk is an early activation kinase in sIg-cross-linked B cells. xid does not ablate Btk protein kinase activity, and immediate signal transduction events, such as tyrosine phosphorylation, occur in sIg-activated xid B cells.

Role of c-myc and p27 in anti-IgM induced B-lymphoma apoptosis.

Crosslinking membrane IgM receptors on a set of murine B cell lymphomas leads to a rapid increase in c-myc, followed by a decrease in its expression to undetectable levels by 8-24 hours. These cells die soon thereafter via apoptosis. IgD receptor crosslinking also leads to an increase in c-myc expression, but it remains above baseline levels for more than 24 hours; these cells continue to proliferate and do not die.

Role of cyclin A and p27 in anti-IgM induced G1 growth arrest of murine B-cell lymphomas.

Cross-linking surface immunoglobulin (Ig)M on the WEHI-231 B-cell lymphoma results in decreased cell size, G1/S growth arrest, and finally DNA cleavage into oligonucleosomal fragments that are the classical features of apoptotic cells. Treatment of WEHI-231 cells with anti-IgM in early G1 phase prevents phosphorylation of the retinoblastoma gene product (pRb) and inhibits entry into S phase. Using unsynchronized cells, we previously demonstrated that cyclin A-associated and Cdk2-dependent GST-pRb kinase activity were inhibited in WEHI-231 cells treated with anti-IgM.

Lymphoma models for B-cell activation and tolerance: anti-immunoglobulin M treatment induces growth arrest by preventing the formation of an active kinase complex which phosphorylates retinoblastoma gene product in G1.

The product of the retinoblastoma gene, RB-1, is the prototype of a class of tumor suppressor genes that is expressed in most mammalian cells. The RB protein is phosphorylated in a cell cycle-dependent manner and is modulated during cellular differentiation. We have shown previously that anti-immunoglobulin M (anti-mu) treatment of WEHI-231 and CH31 B-lymphoma cells caused cell cycle blockade and apoptosis.

Loosening of cell cycle controls of human lymphocytes under the action of tumour promoter TPA.

The effect of tumour promoter TPA (12-O-tetradecanoylphorbol-13-acetate) on the cell cycle of human peripheral blood lymphocytes stimulated by phytohaemagglutinin (PHA) in vitro was studied and it was found that TPA caused cells to accumulate in all the cell cycle phases. This accumulation took place preferentially at later culture passages, when lymphocytes stimulated by PHA alone stopped mainly in G0/G1 phases. Other effects of TPA were cell induction to enter higher DNA ploidy and to survive and even synthesize DNA under colchicine block of mitosis or under cytochalasin block of cytokinesis.