Caveolin-1 expression negatively regulates cell cycle progression by inducing G(0)/G(1) arrest via a p53/p21(WAF1/Cip1)-dependent mechanism

Caveolin-1 is a principal component of caveolae membranes in vivo. Caveolin-1 mRNA and protein expression are lost or reduced during cell transformation by activated oncogenes. Interestingly, the human caveolin-1 gene is localized to a suspected tumor suppressor locus (7q31.1). However, it remains unknown whether caveolin-1 plays any role in regulating cell cycle progression. Here, we directly demonstrate that caveolin-1 expression arrests cells in the G(0)/G(1) phase of the cell cycle. We show that serum starvation induces up-regulation of endogenous caveolin-1 and arrests cells in the G(0)/G(1) phase of the cell cycle. Moreover, targeted down-regulation of caveolin-1 induces cells to exit the G(0)/G(1) phase. Next, we constructed a green fluorescent protein-tagged caveolin-1 (Cav-1-GFP) to examine the effect of caveolin-1 expression on cell cycle regulation. We directly demonstrate that recombinant expression of Cav-1-GFP induces arrest in the G(0)/G(1) phase of the cell cycle. To examine whether caveolin-1 expression is important for modulating cell cycle progression in vivo, we expressed wild-type caveolin-1 as a transgene in mice. Analysis of primary cultures of mouse embryonic fibroblasts from caveolin-1 transgenic mice reveals that caveolin-1 induces 1) cells to exit the S phase of the cell cycle with a concomitant increase in the G(0)/G(1) population, 2) a reduction in cellular proliferation, and 3) a reduction in the DNA replication rate. Finally, we demonstrate that caveolin-1-mediated cell cycle arrest occurs through a p53/p21-dependent pathway. Taken together, our results provide the first evidence that caveolin-1 expression plays a critical role in the modulation of cell cycle progression in vivo.     

Gene Structure of MurineGna11andGna15:Tandemly Duplicated Gq Class G Protein α Subunit Genes

G protein α subunits are encoded by a multigene family of 16 genes that can be grouped into four classes, Gq, Gs, Gi, and G12. The Gq class is composed of four genes in mouse and human, and two of these genes,Gna11andGna15,cosegregate on mouse chromosome 10. We have characterized the gene structures of murineGna11andGna15.The two genes are tandemly duplicated in a head-to-tail array. The upstream gene,Gna11,is ubiquitously expressed, whereas expression of the downstream gene,Gna15,is restricted to hematopoietic cells. The coding sequence of each gene is contained within seven exons, and the two genes together span 43 kb, separated by 6 kb of intergenic region. We have found no evidence for alternative splicing within the coding sequence of either gene. Sequence alignments show that the positions of the six intervening sequences are conserved in the two genes, consistent withGna11andGna15arising by tandem duplication from a common progenitor gene in vertebrates. Phylogenetic trees reveal unequal evolutionary rates among α subunits of the Gq class. The rate of change is approximately six fold higher inGna15than inGna11.     

Overexpression of c-Myc alters G(1)/S arrest following ionizing radiation

Study of the mechanism(s) of genomic instability induced by the c-myc proto-oncogene has the potential to shed new light on its well-known oncogenic activity. However, an underlying mechanism(s) for this phenotype is largely unknown. In the present study, we investigated the effects of c-Myc overexpression on the DNA damage-induced G(1)/S checkpoint, in order to obtain mechanistic insights into how deregulated c-Myc destabilizes the cellular genome. The DNA damage-induced checkpoints are among the primary safeguard mechanisms for genomic stability, and alterations of cell cycle checkpoints are known to be crucial for certain types of genomic instability, such as gene amplification. The effects of c-Myc overexpression were studied in human mammary epithelial cells (HMEC) as one approach to understanding the c-Myc-induced genomic instability in the context of mammary tumorigenesis. Initially, flow-cytometric analyses were used with two c-Myc-overexpressing, nontransformed immortal lines (184A1N4 and MCF10A) to determine whether c-Myc overexpression leads to alteration of cell cycle arrest following ionizing radiation (IR). Inappropriate entry into S phase was then confirmed with a bromodeoxyuridine incorporation assay measuring de novo DNA synthesis following IR. Direct involvement of c-Myc overexpression in alteration of the G(1)/S checkpoint was then confirmed by utilizing the MycER construct, a regulatable c-Myc. A transient excess of c-Myc activity, provided by the activated MycER, was similarly able to induce the inappropriate de novo DNA synthesis following IR. Significantly, the transient expression of full-length c-Myc in normal mortal HMECs also facilitated entry into S phase and the inappropriate de novo DNA synthesis following IR. Furthermore, irradiated, c-Myc-infected, normal HMECs developed a sub-G(1) population and a >4N population of cells. The c-Myc-induced alteration of the G(1)/S checkpoint was also compared to the effects of expression of MycS (N-terminally truncated c-Myc) and p53DD (a dominant negative p53) in the HMECs. We observed inappropriate hyperphosphorylation of retinoblastoma protein and then the reappearance of cyclin A, following IR, selectively in full-length c-Myc- and p53DD-overexpressing MCF10A cells. Based on these results, we propose that c-Myc attenuates a safeguard mechanism for genomic stability; this property may contribute to c-Myc-induced genomic instability and to the potent oncogenic activity of c-Myc.     

The trimeric GTP-binding protein (G(q)/G(11)) alpha subunit is required for insulin-stimulated GLUT4 translocation in 3T3L1 adipocytes

To investigate the potential role of trimeric GTP-binding proteins regulating GLUT4 translocation in adipocytes, wild type and constitutively active G(q) (G(q)/Q209L), G(i) (G(i)/Q205L), and G(s) (G(s)/Q227L) alpha subunit mutants were expressed in 3T3L1 adipocytes. Although expression of neither the wild type nor G(i)/Q205L and G(s)/Q227L alpha subunit mutants had any effect on the basal or insulin-stimulated translocation of a co-expressed GLUT4-enhanced green fluorescent protein (EGFP) fusion protein, expression of G(q)/Q209L resulted in GLUT4-EGFP translocation in the absence of insulin. In contrast, microinjection of an inhibitory G(q)/G(11) alpha subunit-specific antibody but not a G(i) or G(s) alpha subunit antibody prevented insulin-stimulated endogenous GLUT4 translocation. Consistent with a required role for GTP-bound G(q)/G(11), expression of the regulators of G protein signaling (RGS4 and RGS16) also attenuated insulin-stimulated GLUT4-EGFP translocation. To assess the relationship between G(q)/G(11) function with the phosphatidylinositol 3-kinase dependent pathway, expression of a dominant-interfering p85 regulatory subunit, as well as wortmannin treatment inhibited insulin-stimulated but not G(q)/Q209L-stimulated GLUT4-EGFP translocation. Furthermore, G(q)/Q209L did not induce the in vivo accumulation of phosphatidylinositol-3,4,5-trisphosphate (PIP(3)), whereas expression of the RGS proteins did not prevent the insulin-stimulated accumulation of PIP(3). Together, these data demonstrate that insulin stimulation of GLUT4 translocation requires at least two independent signal transduction pathways, one mediated through the phosphatidylinositol 3-kinase and another through the trimeric GTP-binding proteins G(q) and/or G(11).     

Reovirus-induced G(2)/M cell cycle arrest requires sigma1s and occurs in the absence of apoptosis

Serotype-specific differences in the capacity of reovirus strains to inhibit proliferation of murine L929 cells correlate with the capacity to induce apoptosis. The prototype serotype 3 reovirus strains Abney (T3A) and Dearing (T3D) inhibit cellular proliferation and induce apoptosis to a greater extent than the prototype serotype 1 reovirus strain Lang (T1L). We now show that reovirus-induced inhibition of cellular proliferation results from a G(2)/M cell cycle arrest. Using T1L x T3D reassortant viruses, we found that strain-specific differences in the capacity to induce G(2)/M arrest, like the differences in the capacity to induce apoptosis, are determined by the viral S1 gene. The S1 gene is bicistronic, encoding the viral attachment protein sigma1 and the nonstructural protein sigma1s. A sigma1s-deficient reovirus strain, T3C84-MA, fails to induce G(2)/M arrest, yet retains the capacity to induce apoptosis, indicating that sigma1s is required for reovirus-induced G(2)/M arrest. Expression of sigma1s in C127 cells increases the percentage of cells in the G(2)/M phase of the cell cycle, supporting a role for this protein in reovirus-induced G(2)/M arrest. Inhibition of reovirus-induced apoptosis failed to prevent virus-induced G(2)/M arrest, indicating that G(2)/M arrest is not the result of apoptosis related DNA damage and suggests that these two processes occur through distinct pathways.     

Cloning, chromosome mapping and functional characterization of a human homologue of murine gtse-1 (B99) gene

Murine Gtse-1 (G(2) and S phase expressed protein), previously named B99, is a wt-p53 inducible gene that encodes a microtubule-localized protein which is able to induce G(2)/M phase accumulation when ectopically expressed. Here we report the cloning and characterization of a new cDNA (GTSE-1) encoding a human homologue of the mouse Gtse-1 protein. Chromosome mapping of mouse and human genes assigned Gtse-1 to chromosome 15 and GTSE-1 to chromosome 22q13.2-q13.3 in a region with conserved synteny to that where Gtse-1 mapped. Analysis of the genomic structure revealed that GTSE-1 contains at least 11 exons and 10 introns, spanning approximately 33kb of genomic DNA. Similar to murine Gtse-1, the product of GTSE-1 localized to the microtubules, was able to delay G(2)/M progression when ectopically expressed and was cell cycle regulated. Taken together, these results indicate GTSE-1 as the human functional homologue of murine Gtse-1.     

Genomic Structure and Chromosomal Localization of Mouse Cyclin G1 Gene

Mouse genomic DNA harboring the full coding sequence of cyclin G1 was cloned and analyzed. The locations of five coding exons and the intron–exon boundary sequences were found to be conserved between the mouse and the human genes. Two putative binding sites for thep53tumor suppressor gene product were found around the first exon: one was located in the 5′ regulatory region, and the other was in the first intron. The mouse cyclin G1 gene was mapped to bands A5 to B1 of chromosomes 11 (11A5–B1) by FISH using genomic DNA clone as a biotinylated probe. The location of mouse cyclin G1 is syntenic to that of its human homologue, which we previously mapped to 5q32–q34 of chromosome 5. An additional faint signal was detected on chromosome 4 (4B1–C2), probably indicating the presence of a cyclin G1-related gene or pseudogene in the mouse genome.     

Gene structure and chromosomal localization of mouse cyclin G2 (Ccng2)

Cyclins are essential activators of cyclin-dependent kinases (Cdk) which, in turn, play pivotal roles in controlling transition through cell-cycle checkpoints. Cyclin G2 is a recently discovered second member of the G-type cyclins. The two members of the G-type cyclins, cyclin G1 and cyclin G2, share high structural similarity but their function remains to be defined. Here we characterize the structure of the mouse cyclin G2 gene by first cloning and sequencing the full-length mouse cyclin G2 cDNA. The cyclin G2 cDNA was used to isolate the cyclin G2 gene from a BAC library and to establish that the gene was transcribed from eight exons spanning a total of 8604 bp. The cyclin G2 gene was mapped by fluorescence in situ hybridization (FISH) to mouse chromosome 5E3.3.–F1.3. This region is syntenic to a region on human chromosome 4. The expression of cyclins G1 and G2 was examined in various tissues, but no correlation between expression patterns of the two genes was observed. However, during hepatic ontogenesis the cyclin G2 expression level decreased with age, whereas cyclin G1 expression increased. Transient expression of cyclin G2-green fluorescent protein (GFP) fusion protein in NIH3T3 cells showed that cyclin G2 is essentially a cytoplasmic protein, in contrast to the largely nuclear localization of cyclin G1. Our data suggest that, despite the close structural similarity between mouse cyclins G1 and G2, these proteins most likely perform distinct functions.     

The murine GABA(B) receptor 1: cDNA cloning, tissue distribution, structure of the Gabbr1 gene, and mapping to chromosome 17

GABA (gamma-aminobutyric acid) is a major inhibitory neurotransmitter in the central nervous system (CNS) which activates both ionotropic (GABA(A)/GABA(C)) and metabotropic (GABA(B)) receptor systems. We identified two alternatively spliced cDNA variants of the murine GABA(B) receptor 1 that are predominantly expressed in the CNS. Deduced protein structures are highly homologous to the previously characterized rat and human receptors. Comparison of the genomic structures of mouse and human revealed that alternative splicing occurred at the same position, whereas the mouse gene has an additional 5' exon. Radiation hybrid mapping, combined with database searches, indicated that the GABA(B) receptor gene (Gabbr1) is located on mouse chromosome 17, adjacent to the marker D17Mit24 in a region homologous to human chromosome 6p21.3.     

Human NDR kinases control G(1)/S cell cycle transition by directly regulating p21 stability

The G(1) phase of the cell cycle is an important integrator of internal and external cues, allowing a cell to decide whether to proliferate, differentiate, or die. Multiple protein kinases, among them the cyclin-dependent kinases (Cdks), control G(1)-phase progression and S-phase entry. With the regulation of apoptosis, centrosome duplication, and mitotic chromosome alignment downstream of the HIPPO pathway components MST1 and MST2, mammalian NDR kinases have been implicated to function in cell cycle-dependent processes. Although they are well characterized in terms of biochemical regulation and upstream signaling pathways, signaling mechanisms downstream of mammalian NDR kinases remain largely unknown. We identify here a role for human NDR in regulating the G(1)/S transition. In G(1) phase, NDR kinases are activated by a third MST kinase (MST3). Significantly, interfering with NDR and MST3 kinase expression results in G(1) arrest and subsequent proliferation defects. Furthermore, we describe the first downstream signaling mechanisms by which NDR kinases regulate cell cycle progression. Our findings suggest that NDR kinases control protein stability of the cyclin-Cdk inhibitor protein p21 by direct phosphorylation. These findings establish a novel MST3-NDR-p21 axis as an important regulator of G(1)/S progression of mammalian cells.     

Gene structure and chromosomal localization of mouse cyclin G2 (Ccng2)

Cyclins are essential activators of cyclin-dependent kinases (Cdk) which, in turn, play pivotal roles in controlling transition through cell-cycle checkpoints. Cyclin G2 is a recently discovered second member of the G-type cyclins. The two members of the G-type cyclins, cyclin G1 and cyclin G2, share high structural similarity but their function remains to be defined. Here we characterize the structure of the mouse cyclin G2 gene by first cloning and sequencing the full-length mouse cyclin G2 cDNA. The cyclin G2 cDNA was used to isolate the cyclin G2 gene from a BAC library and to establish that the gene was transcribed from eight exons spanning a total of 8604 bp. The cyclin G2 gene was mapped by fluorescence in situ hybridization (FISH) to mouse chromosome 5E3.3.–F1.3. This region is syntenic to a region on human chromosome 4. The expression of cyclins G1 and G2 was examined in various tissues, but no correlation between expression patterns of the two genes was observed. However, during hepatic ontogenesis the cyclin G2 expression level decreased with age, whereas cyclin G1 expression increased. Transient expression of cyclin G2-green fluorescent protein (GFP) fusion protein in NIH3T3 cells showed that cyclin G2 is essentially a cytoplasmic protein, in contrast to the largely nuclear localization of cyclin G1. Our data suggest that, despite the close structural similarity between mouse cyclins G1 and G2, these proteins most likely perform distinct functions.     

Structure and localization of mouse Pmscl1 and Pmscl2 genes

Sera from some patients with polymyositis-scleoderma overlap syndrome (PM-SCL) recognize two antigenically unrelated proteins, PMSCL1 and PMSCL2. Complete mouse Pmscl1 and Pmscl2 cDNA sequences, chromosomal localizations, exon/intron structure, and promoter region sequences of the mouse Pmscl2 gene are presented. The PMSCL1 gene was found to overlap significantly with cyclin A2 in both human and mouse. As such, it may be deduced that PMSCL1 sequences map to human chromosome 4q27 and the proximal portion of mouse chromosome (Chr) 3 where human and mouse cyclin A2 genes reside. Analysis of human and mouse PMSCL1 cDNA sequences provides evidence that the PMSCL1 protein is 68 amino acids longer than previously thought. A BAC containing mouse Pmscl2 was localized to distal mouse Chr 4 by FISH. This BAC contains the microsatellite D4Mit310. D4Mit310 colocalizes with a number of genes that map to human 1p36. In fact, a STS (G25404) located 54.6 cR from the top of human chromosome 1 was found to contain PMSCL2 sequence upon BLAST search.     

8-chloroadenosine induced HL-60 cell growth inhibition, differentiation, and G(0)/G(1) arrest involves attenuated cyclin D1 and telomerase and up-regulated p21(WAF1/CIP1)

8-Chloroadenosine, an active dephosphorylated metabolite of the antineoplastic agent 8-chloroadenosine 3',5'-monophosphate (8-Cl-cAMP), induces growth inhibition in multiple carcinomas. Here we report that 8-chloroadenosine inhibits growth in human promyelocytic leukemia HL-60 cells by a G(0)/G(1) phase arrest and terminates cell differentiation along the granulocytic lineage. The mechanism of 8-chloroadenosine-induced G(0)/G(1) arrest is independent of apoptosis. The expressions of cyclin D1 and c-myc in HL-60 are suppressed by 8-chloroadenosine, whereas the cyclin-dependent kinases inhibitor p21(WAF1/CIP1) is up-regulated. 8-Chloroadenosine has less effect on the expressions of cyclin-dependent kinase (cdk)2 and cdk4, G(1) phase cyclin-dependent kinases, and only moderately induces the expression of transforming growth factor beta1 (TGFbeta1) and the mitotic inhibitor p27(KIP1). Telomerase activity is reduced in extracts of 8-chloroadenosine treated HL-60 cells, but 8-chloroadenosine does not directly inhibit the catalytic activity of telomerase in vitro. Therefore, anti-proliferation of HL-60 cells by 8-chloroadenosine involves coordination of cyclin D1 suppression, reduction of telomerase activity, and up-regulation of p21(WAF1/CIP1) that arrest cell-cycle progression at G(0)/G(1) phase and terminate cell differentiation.     

Inhibitory phosphorylation of PP1alpha catalytic subunit during the G(1)/S transition.

We have shown earlier that, in cells expressing the retinoblastoma protein (pRB), a protein phosphatase (PP) 1alpha mutant (T320A) resistant to inhibitory phosphorylation by cyclin-dependent kinases (Cdks) causes G(1) arrest. In this study, we examined the cell cycle-dependent phosphorylation of PP1alpha in vivo using three different antibodies. PP1alpha was phosphorylated at Thr-320 during M-phase and again in late G(1)- through early S-phase. Inhibition of Cdk2 led to a small increase in PP1 activity and also prevented PP1alpha phosphorylation. In vitro, PP1alpha was a substrate for Cdk2 but not Cdk4. In pRB-deficient cells, phosphorylation of PP1alpha occurred in M-phase but not at G(1)/S. G(1)/S phosphorylation was at least partially restored after reintroduction of pRB into these cells. Consistent with this result, PP1alpha phosphorylated at Thr-320 co-precipitated with pRB during G(1)/S but was found in extracts immunodepleted of pRB in M-phase. In conjunction with earlier studies, these results indicate that PP1alpha may control pRB function throughout the cell cycle. In addition, our new results suggest that different subpopulations of PP1alpha regulate the G(1)/S and G(2)/M transitions and that PP1alpha complexed to pRB requires inhibitory phosphorylation by G(1)-specific Cdks in order to prevent untimely reactivation of pRB and permit transition from G(1)- to S-phase and/or complete S-phase.