Nuclear reformation after mitosis requires downregulation of the Ran GTPase effector RanBP1 in mammalian cells.

The GTPase Ran regulates nucleocytoplasmic transport in interphase and spindle organisation in mitosis via effectors of the importin beta superfamily. Ran-binding protein 1 (RanBP1) regulates guanine nucleotide turnover on Ran, as well as its interactions with effectors. Unlike other Ran network members that are steadily expressed, RanBP1 abundance is modulated during the mammalian cell cycle, peaking in mitosis and declining at mitotic exit.

RanBP1 downregulation sensitizes cancer cells to taxol in a caspase-3-dependent manner.

Mitotic microtubule (MT)-targeting drugs are widely used to treat cancer. The GTPase Ran regulates multiple processes, including mitotic spindle assembly, spindle pole formation and MT dynamics; Ran activity is therefore essential to formation of a functional mitotic apparatus. The RanBP1 protein, which binds Ran and regulates its interaction with effectors, is overexpressed in many cancer types.

The GTPase Ran: regulation of cell life and potential roles in cell transformation.

The GTPase Ran plays a crucial role in nucleo-cytoplasmic transport of tumor suppressors, proto-oncogenes, signaling molecules and transcription factors. It also plays direct roles in mitosis, through which it regulates faithful chromosome segregation and hence the generation of genetically stable cells. Ran operates through a group of effector proteins.

RANBP1 localizes a subset of mitotic regulatory factors on spindle microtubules and regulates chromosome segregation in human cells.

The GTPase RAN has an established role in spindle assembly and in mitotic progression, although not all mechanisms are fully understood in somatic cells. Here, we have downregulated RAN-binding protein 1 (RANBP1), a RAN partner that has highest abundance in G2 and mitosis, in human cells. RANBP1-depleted cells underwent prolonged prometaphase delay often followed by apoptosis.

Spatial control of mitosis by the GTPase Ran.

Mitosis is the most potentially dangerous event in the life of a cell, during which the cell genetic identity is transmitted to daughters; errors at this stage may yield aneuploid cells that can initiate a genetically unstable clone. The small GTPase Ran is the central element of a conserved signaling network that has a prominent role in mitotic regulation. Pioneering studies with amphibian oocytes indicated that Ran, in the GTP-bound form, activates factors that regulate spindle assembly and dynamics.

A role of p73 in mitotic exit.

The p53-related p73 proteins regulate developmental processes, cell growth, and DNA damage response. p73 function is regulated by post-translational modifications and protein-protein interactions. At the G2/M transition, p73 is phosphorylated at Thr-86 by the p34cdc2/cyclin B complex; this is associated with its exclusion from condensed chromosomes and loss of DNA binding and transcriptional activation ability.

Importin beta is transported to spindle poles during mitosis and regulates Ran-dependent spindle assembly factors in mammalian cells.

Spatial control is a key issue in cell division. The Ran GTPase regulates several fundamental processes for cell life, largely acting through importin molecules. The best understood of these is protein import through the nuclear envelope in interphase, but roles in mitotic spindle assembly are also established.

p53 localization at centrosomes during mitosis and postmitotic checkpoint are ATM-dependent and require serine 15 phosphorylation.

We recently demonstrated that the p53 oncosuppressor associates to centrosomes in mitosis and this association is disrupted by treatments with microtubule-depolymerizing agents. Here, we show that ATM, an upstream activator of p53 after DNA damage, is essential for p53 centrosomal localization and is required for the activation of the postmitotic checkpoint after spindle disruption. In mitosis, p53 failed to associate with centrosomes in two ATM-deficient, ataxiatelangiectasia-derived cell lines.

p73 is regulated by phosphorylation at the G2/M transition.

p73 is a p53 paralog that encodes proapoptotic (transactivation-competent (TA)) and antiapoptotic (dominant negative) isoforms. TAp73 transcription factors mediate cell cycle arrest and/or apoptosis in response to DNA damage and are involved in developmental processes in the central nervous system and the immune system. p73 proteins may also play a role in the regulation of cell growth.

Mammalian RanBP1 regulates centrosome cohesion during mitosis.

The Ran GTPase plays a central function in control of nucleo-cytoplasmic transport in interphase. Mitotic roles of Ran have also been firmly established in Xenopus oocyte extracts. In this system, Ran-GTP, or the RCC1 exchange factor for Ran, drive spindle assembly by regulating the availability of 'aster-promoting activities'.

Mouse early embryos obtained by natural breeding or in vitro fertilization display a differential sensitivity to extremely low-frequency electromagnetic fields.

We have investigated the sensitivity of pre-implantation embryos obtained by natural breeding (NB) or in vitro fertilization (IVF) to extremely low-frequency magnetic fields (ELF-MF). Fertilized eggs obtained by NB were removed from mothers 12h after mating and cultured in vitro for 5 days under continuous ELF-MF exposure (constant strength of 50Hz and various intensities, i.e.

Exposure of normal and transformed cells to nevirapine, a reverse transcriptase inhibitor, reduces cell growth and promotes differentiation.

Endogenous, nontelomeric reverse transcriptase (RT) is encoded by two classes of repeated elements: retrotransposons and endogenous retroviruses. Expression of RT-coding genes is generally repressed in differentiated nonpathological tissues, yet is active in the mammalian germ line, embryonic tissues and tumor cells. Nevirapine is a non-nucleoside RT inhibitor with a well-characterized inhibitory activity on RT enzymes of retroviral origin.

E1A deregulates the centrosome cycle in a Ran GTPase-dependent manner.

By means of the yeast two-hybrid system, we have discovered a novel physical interaction between the adenovirus E1A oncoprotein and Ran, a small GTPase which regulates nucleocytoplasmic transport, cell cycle progression, and mitotic spindle organization. Expression of E1A elicits induction of S phase and centrosome amplification in a variety of rodent cell lines. The induction of supernumerary centrosomes requires functional RCC1, the nucleotide exchange factor for Ran and, hence, a functional Ran network.

p53 displacement from centrosomes and p53-mediated G1 arrest following transient inhibition of the mitotic spindle.

Growing evidence indicates a central role for p53 in mediating cell cycle arrest in response to mitotic spindle defects so as to prevent rereplication in cells in which the mitotic division has failed. Here we report that a transient inhibition of spindle assembly induced by nocodazole, a tubulin-depolymerizing drug, triggers a stable activation of p53, which can transduce a cell cycle inhibitory signal even when the spindle-damaging agent is removed and the spindle is allowed to reassemble. Cells transiently exposed to nocodazole continue to express high levels of p53 and p21 in the cell cycle that follows the transient exposure to nocodazole and become arrested in G(1), regardless of whether they carry a diploid or polyploid genome after mitotic exit.

Normal and cancer-prone human cells respond differently to extremely low frequency magnetic fields.

Human lymphoblastoid cells of normal origin and from genetic instability syndromes, i.e. Fanconi anemia (FA) group C and ataxia telangectasia, were continuously exposed to extremely low frequency magnetic field (ELF-MF).

E2F transcription factors are differentially expressed in murine gametes and early embryos.

We have examined the murine genes encoding transcription factors E2F1, -3, -5 and -6 in gametes and early embryos. All genes are expressed as maternal transcripts and all are efficiently transcribed after the blastocyst stage. Between those two stages, each E2F mRNA is transcribed with a distinctive and unique pattern.

p53-independent apoptosis and p53-dependent block of DNA rereplication following mitotic spindle inhibition in human cells.

We have studied the response of human transformed cells to mitotic spindle inhibition. Two paired cell lines, K562 and its parvovirus-resistant KS derivative clone, respectively nonexpressing and expressing p53, were continuously exposed to nocodazole. Apoptotic cells were observed in both lines, indicating that mitotic spindle impairment induced p53-independent apoptosis.

TCR engagement regulates differential responsiveness of human memory T cells to Fas (CD95)-mediated apoptosis.

In this work, we have tried to establish whether human memory T cells may be protected from Fas (CD95)-induced apoptosis when correctly activated by Ag, and not protected when nonspecifically or incorrectly activated. In particular, we wanted to investigate the molecular mechanisms that regulate the fate of memory T cells following an antigenic challenge. To address this issue, we chose an experimental system that closely mimics physiological T cell activation such as human T cell lines and clones specific for viral peptides or alloantigens.