Biomedical subjects

F Nuckolls

Publications and source records attributed to F Nuckolls.

5 recordsLinked to original sources

PubMed2001

The E2F1-3 transcription factors are essential for cellular proliferation.

The retinoblastoma tumour suppressor (Rb) pathway is believed to have a critical role in the control of cellular proliferation by regulating E2F activities. E2F1, E2F2 and E2F3 belong to a subclass of E2F factors thought to act as transcriptional activators important for progression through the G1/S transition. Here we show, by taking a conditional gene targeting approach, that the combined loss of these three E2F factors severely affects E2F target expression and completely abolishes the ability of mouse embryonic fibroblasts to enter S phase, progress through mitosis and proliferate. Loss of E2F function results in an elevation of p21Cip1 protein, leading to a decrease in cyclin-dependent kinase activity and Rb phosphorylation. These findings suggest a function for this subclass of E2F transcriptional activators in a positive feedback loop, through down-modulation of p21Cip1, that leads to the inactivation of Rb-dependent repression and S phase entry. By targeting the entire subclass of E2F transcriptional activators we provide direct genetic evidence for their essential role in cell cycle progression, proliferation and development.

Animals↗

PubMed2001

Myc requires distinct E2F activities to induce S phase and apoptosis.

Previous work has shown that the Myc transcription factor induces transcription of the E2F1, E2F2, and E2F3 genes. Using primary mouse embryo fibroblasts deleted for individual E2F genes, we now show that Myc-induced S phase and apoptosis requires distinct E2F activities. The ability of Myc to induce S phase is impaired in the absence of either E2F2 or E2F3 but not E2F1 or E2F4. In contrast, the ability of Myc to induce apoptosis is markedly reduced in cells deleted for E2F1 but not E2F2 or E2F3. From this data, we propose that the induction of specific E2F activities is an essential component in the Myc pathways that control cell proliferation and cell fate decisions.

Adenoviridae↗

PubMed2000

Multiple Ras-dependent phosphorylation pathways regulate Myc protein stability.

Our recent work has shown that activation of the Ras/Raf/ERK pathway extends the half-life of the Myc protein and thus enhances the accumulation of Myc activity. We have extended these observations by investigating two N-terminal phosphorylation sites in Myc, Thr 58 and Ser 62, which are known to be regulated by mitogen stimulation. We now show that the phosphorylation of these two residues is critical for determining the stability of Myc. Phosphorylation of Ser 62 is required for Ras-induced stabilization of Myc, likely mediated through the action of ERK. Conversely, phosphorylation of Thr 58, likely mediated by GSK-3 but dependent on the prior phosphorylation of Ser 62, is associated with degradation of Myc. Further analysis demonstrates that the Ras-dependent PI-3K pathway is also critical for controlling Myc protein accumulation, likely through the control of GSK-3 activity. These observations thus define a synergistic role for multiple Ras-mediated phosphorylation pathways in the control of Myc protein accumulation during the initial stage of cell proliferation.

Amino Acid Sequence↗

PubMed2000

Identification of a novel E2F3 product suggests a mechanism for determining specificity of repression by Rb proteins.

The tumor suppressor function of Rb is intimately related to its ability to interact with E2F and repress the transcription of E2F target genes. Here we describe a novel E2F product that specifically interacts with Rb in quiescent cells. This novel E2F, which we term E2F3b, is encoded by a unique mRNA transcribed from an intronic promoter within the E2F3 locus. The E2F3b RNA differs from the previously characterized E2F3 RNA, which we now term E2F3a, by the utilization of a unique coding exon. In contrast to the E2F3a product that is tightly regulated by cell growth, the E2F3b product is expressed equivalently in quiescent and proliferating cells. But, unlike the E2F4 and E2F5 proteins, which are also expressed in quiescent cells and form complexes with the p130 protein, the E2F3b protein associates with Rb and represents the predominant E2F-Rb complex in quiescent cells. Thus, the previously described specificity of Rb function as a transcriptional repressor in quiescent cells coincides with the association of Rb with this novel E2F product.

Base Sequence↗

PubMed2000

Complex transcriptional regulatory mechanisms control expression of the E2F3 locus.

E2F transcription activity has been shown to play a critical role in cell growth control, regulating the expression of a variety of genes that encode proteins important for the initiation of DNA replication and cell cycle regulation. We have shown that the E2F3 locus encodes two protein products: the E2F3a product, which is tightly regulated by cell growth, and the E2F3b product, which is constitutively expressed throughout the cell cycle. To further explore the mechanism controlling the expression of the two E2F3 gene products, we analyzed the genomic sequences flanking the 5' region of E2F3a and E2F3b. We find that a series of E2F binding sites confer negative control on the E2F3a promoter in quiescent cells, similar to the control of the E2F1 and E2F2 promoters. In addition, a group of E-box elements, which are Myc binding sites, confer responsiveness to Myc and are necessary for full activation of the E2F3a promoter in response to growth stimulation. Based on these results and past experiments, it appears that the E2F1, E2F2, and E2F3a genes are similarly regulated by growth stimulation, involving a combination of E2F-dependent negative control and Myc-mediated positive control. In contrast, the constitutive expression of the E2F3b gene more closely reflects the control of expression of the E2F4 and E2F5 genes.

Base Sequence↗