Arrest of G(1)-S progression by the p53-inducible gene PC3 is Rb dependent and relies on the inhibition of cyclin D1 transcription

The p53-inducible gene PC3 (TIS21, BTG2) is endowed with antiproliferative activity. Here we report that expression of PC3 in cycling cells induced accumulation of hypophosphorylated, growth-inhibitory forms of pRb and led to G(1) arrest. This latter was not observed in cells with genetic disruption of the Rb gene, indicating that the PC3-mediated G(1) arrest was Rb dependent. Furthermore, (i) the arrest of G(1)-S transition exerted by PC3 was completely rescued by coexpression of cyclin D1 but not by that of cyclin A or E; (ii) expression of PC3 caused a significant down-regulation of cyclin D1 protein levels, also in Rb-defective cells, accompanied by inhibition of CDK4 activity in vivo; and (iii) the removal from the PC3 molecule of residues 50 to 68, a conserved domain of the PC3/BTG/Tob gene family, which we term GR, led to a loss of the inhibition of proliferation as well as of the down-regulation of cyclin D1 levels. These data point to cyclin D1 down-regulation as the main factor responsible for the growth inhibition by PC3. Such an effect was associated with a decrease of cyclin D1 transcript and of cyclin D1 promoter activity, whereas no effect of PC3 was observed on cyclin D1 protein stability. Taken together, these findings indicate that PC3 impairs G(1)-S transition by inhibiting pRb function in consequence of a reduction of cyclin D1 levels and that PC3 acts, either directly or indirectly, as a transcriptional regulator of cyclin D1.

Identification of a family of human F-box proteins

F-box proteins are an expanding family of eukaryotic proteins characterized by an approximately 40 aminoacid motif, the F box (so named because cyclin F was one of the first proteins in which this motif was identified) [1]. Some F-box proteins have been shown to be critical for the controlled degradation of cellular regulatory proteins [2] [3]. In fact, F-box proteins are one of the four subunits of ubiquitin protein ligases called SCFs. The other three subunits are the Skp1 protein; one of the cullin proteins (Cul1 in metazoans and Cdc53 or Cul A in the yeast Saccharomyces cerevisiae); and the recently identified Roc1 protein (also called Rbx1 or Hrt1). SCF ligases bring ubiquitin conjugating enzymes (either Ubc3 or Ubc4) to substrates that are specifically recruited by the different F-box proteins. The need for high substrate specificity and the large number of known F-box proteins in yeast and worms [2] [4] suggest the existence of a large family of mammalian F-box proteins. Using Skp1 as a bait in a yeast two-hybrid screen and by searching DNA databases, we identified a family of 26 human F-box proteins, 25 of which were novel. Some of these proteins contained WD-40 domains or leucine-rich repeats; others contained either different protein-protein interaction modules or no recognizable motifs. We have named the F-box proteins that contain WD-40 domains Fbws, those containing leucine-rich repeats, Fbls, and the remaining ones Fbxs. We have further characterized representative members of these three classes of F-box proteins.

Cloning of the human interferon-related developmental regulator (IFRD1) gene coding for the PC4 protein, a member of a novel family of developmentally regulated genes

The rat PC4 gene had been initially isolated as a nerve growth factor-inducible sequence in PC12 cells. Although its function remains unknown, recently it has been shown that PC4 is necessary to muscle differentiation and that it might have a role in signal transduction. We report the isolation of the human homolog of the rat PC4 gene, renamed here IFRD1 (interferon-related developmental regulator 1). Several human IFRD1 clones were identified by searching the EST database using the rat IFRD1 (PC4) cDNA as a query. An EST clone containing the entire ORF was chosen for sequencing. Human IFRD1 presented a predicted protein product of 453 amino acids, highly conserved (90.2% identity) compared to the rat IFRD1 (PC4) protein sequences. The mapping assignment of human IFRD1 to chromosome 7q22-q31 was retrieved from the UniGene database maintained at NCBI. A comparison of human IFRD1 (PC4) protein to databases revealed 47% identity to the protein encoded by the human gene SKMc15, originally isolated from a chromosome 3-specific library. Therefore, SKMc15 is a gene related to IFRD1, being the second member of a novel family. We analyzed their expression during murine development, and we found that mouse IFRD1 appears more expressed in specific differentiating structures at midgestation, while mouse SKMc15 is highly expressed soon after gastrulation and in the hepatic primordium, suggesting an involvement in early hematopoiesis.

Overexpression of the nerve growth factor-inducible PC3 immediate early gene is associated with growth inhibition

PC3 (pheochromocytoma cell-3) is an immediate early gene isolated as sequence induced in the rat PC12 cell line during neuronal differentiation by nerve growth factor (NGF). PC3, which is expressed in vivo in the neuroblast when it ceases proliferating and differentiates into a neuron, has partial homology with two antiproliferative genes, BTG1 and Tob. Here we report that overexpression of PC3 in NIH3T3 and PC12 cells leads to marked inhibition of cell proliferation. In stable NIH3T3 clones expressing PC3, the transition from G1 to S phase was impaired, whereas the retinoblastoma (RB) protein was detected as multiple isoforms of M(r) 105,000-115,000 (indicative of a hyperphosphorylated state) only in low-density cultures. Such findings are consistent with a condition of growth inhibition. Thus, PC3 might be a negative regulator of cell proliferation, possibly acting as a transducer of factors influencing cell growth and/or differentiation, such as NGF, by a RB-dependent pathway. This is the first evidence of a NGF-inducible immediate early gene displaying antiproliferative activity.

Inhibition of differentiation in myoblasts deprived of the interferon-related protein PC4

PC4 (pheochromocytoma cell-4) is an immediate early gene related to IFN-gamma, the mRNA of which is induced during the course of neuronal differentiation by nerve growth factor in the PC12 cell line. Here we report that PC4 mRNA is also expressed in the myoblast C2C12 cell line and is regulated during differentiation; its expression decreases within 6 h from the onset of differentiation, attains a minimum after 12 h, and returns to basal level within 36 h. This transient down-regulation of PC4 expression in C2C12 myoblasts is prevented by transforming growth factor beta, a molecule which inhibits the differentiation of muscle. Sense and antisense PC4 cDNA transfection strategies in C2C12 cells were then used to clarify the role of PC4 in muscle differentiation. While no effect was seen by over-expression of PC4, stable transfectants underexpressing PC4 exhibited a delay in attaining the differentiated phenotype, with an impairment of myogenin and myosin expression. Myogenin was also inhibited in C2C12 cells microinjected with the anti-PC4 polyclonal antibody A451. We thus postulate a role for PC4 as a positive regulator during muscle differentiation.

Nerve growth factor regulates the subcellular localization of the nerve growth factor-inducible protein PC4 in PC12 cells

The immediate early gene (IEG) PC4, which encodes a protein related to gamma interferon, is activated at the onset of the neuronal differentiation induced by nerve growth factor (NGF) in PC12 cells. With an antibody raised to a bacterial beta gal-PC4 fusion protein, the PC4 protein is detected as an immunoreactive molecular species of 49 kDa, whose synthesis is rapidly induced by NGF in parallel with the induction of its mRNA. Immunofluorescence, electron microscopy and subfractionation studies indicate that the PC4 immunoreactivity is localized in the cytoplasm of PC12 cells, where it is increased transiently by NGF within 3 hr of treatment. In addition, the PC4 immunoreactivity presents an NGF-dependent pattern of intracellular localization. In fact, within 3 hr after addition of NGF, PC4 is also significantly expressed on the inner face of the plasma membrane, to which it is physically associated. After longer NGF treatment, PC4 disappears from the plasma membrane and appears in the nucleus, with reduced cytoplasmic expression. Localization in the nucleus is reversed by removal of NGF and closely parallels changes in the state of differentiation of the cell. The existence within the PC4 protein of a consensus sequence for the addition of myristic acid and of a putative sequence for the nuclear localization suggests possible mechanisms for the NGF-dependent redistribution. For an NGF-inducible IEG product, such growth factor-dependent localization of PC4 is a novel type of regulation in the pathways from the NGF receptor to the adjacent membrane proteins and to the nucleus.