Biochemical similarity of Schizosaccharomyces pombe ras1 protein with RAS2 protein of Saccharomyces cervisiae

Regulation of cellular senescence by Rb2/p130

Growth regulatory functions of Rb2/p130, which aim at a sustained arrest such as in quiescent or differentiated cells, qualify the protein also to act as a central regulator of growth arrest in cellular senescence. In this respect, Rb2/p130 functions are connected to signaling pathways induced by p53, which is a master regulator in cellular senescence. Here, we summarize the pathways, which specify pRb2/p130 to control this arrest program and distinguish its functions from those of pRb/p105.

P38SAPK2 phosphorylates cyclin D3 at Thr-283 and targets it for proteasomal degradation

Cyclin D3 plays a critical role in maturation of precursor T cells and their levels are tightly regulated during this process. Alteration of cyclin D3 levels has been proposed to be important in the development of different human cancers, including malignancies of the lymphoid system. Thus, we have analysed the mechanisms involved in the regulation of cyclin D3 levels. Our results indicate that cyclin D3 is degraded via proteasome and that Thr-283 is essential for its degradation. Wild-type cyclin D3 but not the Thr-283A mutant accumulated ubiquitylated forms after treatment with proteasome inhibitors. We also observed that different type of stresses promote the Thr-283-dependent in vivo degradation of cyclin D3. The analysis of the kinases involved in Thr-283 phosphorylation indicates that all the members of the p38SAPK family of serine-threonine kinases are able to phosphorylate cyclin D3 at this specific site. Moreover, we found that the overexpression of p38alphaSAPK2 induce the decrease of cyclin D3 in vivo. These results indicate that p38SAPK might be involved in the regulation of cyclin D3 levels and suggest that this mechanism is involved in the maturation of precursor T-cells. Alterations of this mechanism might be important for oncogenesis.

SKP2 associates with p130 and accelerates p130 ubiquitylation and degradation in human cells

p130 is a member of the retinoblastoma family of pocket proteins, which includes pRB and p107. Unlike pRB and p107, p130 protein levels decrease dramatically following its hyperphosphorylation starting in the mid-G1 phase of the cell cycle. However, the mechanism leading to p130 downregulation is unknown. We have found that the proteasome inhibitor, lactacystin, inhibited p130 downregulation in T98G cells progressing through the G1/S transition and S phase and that p130 is multiubiquitylated in 293 cells. We have previously shown that ectopic expression of both cyclin D and E induces phosphorylation and downregulation of p130. Since the SKP1/Cul1/SKP2 E3 ubiquitin ligase complex mediates ubiquitylation of substrates previously phosphorylated by cyclin-dependent kinases, we investigated the potential role of this ubiquitin ligase in mediating p130 downregulation. We found that p130 interacts with SKP1, Cul-1 and SKP2 in human 293 cells. We also found that ectopic coexpression of SKP2 and p130 leads to dose-dependent downregulation of p130, reduces p130 protein half-life and induces p130 ubiquitylation in these cells. Moreover, adenoviral-mediated expression of SKP2 accelerates downregulation of endogenous hyperphosphorylated p130 in mitogen-stimulated T98G cells and primary WI38 fibroblasts. We conclude that p130 is a substrate of the SCF(SKP2) ubiquitin ligase and this E3 ligase regulates p130 abundance during the cell cycle.

P130 and its truncated form mediate p53-induced cell cycle arrest in Rb(-/-) Saos2 cells

In the present study, we investigate the mechanism of how p53 induces growth arrest in Rb-defective Saos2 cells that express temperature-sensitive mutant p53 (ts p53). The activation of p53 at a permissive temperature (32.5 degrees C) induces the cell cycle arrest at both the G1 and G2 stages. The induction of several p53-responsive genes as well as a small form of p130 (S-p130) was detected upon p53 activation. S-p130 retained the functions as a pocket protein and was dominant over p130 at the protein level after 36 h at 32.5 degrees C. A canonical p53 binding site was identified in intron 4 of p130. Furthermore, a novel p53-inducible transcript containing a partial intron 4 sequence downstream of the p53 binding site and exon 5 of p130 was detected by RT-PCR, suggesting S-p130 is induced by p53 at transcriptional level. The results from gel shift assay and immunoprecipitation showed that S-p130 as well as p130 formed complexes with both E2F1 and E2F4 at a permissive temperature. Moreover, the transient expression of E1A (12S) and E2F1 effectively abrogated p53-induced cell cycle arrest. These results strongly suggested that p130 and its truncated form might substitute Rb in mediating p53-induced cell cycle arrest in Rb(-/-) Saos2 cells.

Cell cycle genes as targets of retinoid induced ovarian tumor cell growth suppression

We have examined the effect of all-trans-retinoic acid (RA) on cell cycle gene expression in RA sensitive CA-OV3 and RA resistant SK-OV3 ovarian carcinoma cell lines. Gene expression was analysed by multiprobe RNAse protection, Western blotting and in vitro kinase assays. No differences were observed between RA sensitive and RA resistant ovarian carcinoma cells in the levels of expression of many cell cycle genes including cyclin A, B and E, cdk 2,4 and 6, E2F-1, E2F-2, E2F-3, E2F-4, E2F-5, DP-1 and DP-2. However, RA sensitive CA-OV3 cells expressed higher levels of p53, p27, p21, and p16 compared to RA resistant SK-OV3 cells. In addition, RA treatment of CA-OV3 cells resulted in a significant decrease in hyperphosphorylated RB and RB-2/p130 and corresponding significant increases in the levels of hypophosphorylated and/or partially phosphorylated RB-2/p130 protein and hypophosphorylated RB. Also, RA treatment increased expression of the cdk inhibitor p27 and decreased activity of cdk 2, cdk 4 and cdk 6. Finally, amounts of p27-cyclin E and RB-2/p130-E2F4 complexes were found to increase in CA-OV3 cells growth arrested by RA. These results suggest that the pocket protein pathways are critical targets for retinoid suppression of ovarian carcinoma cell growth.

E1A modulates phosphorylation of p130 and p107 by differentially regulating the activity of G1/S cyclin/CDK complexes

We have previously shown that the adenoviral 12S E1A protein modulates the phosphorylation status of p130 and p107 without apparent changes in the cell cycle dependent phosphorylation of the retinoblastoma protein. Here we report on the mechanisms by which E1A modifies differentially the phosphorylation status of pocket proteins. In human U-2 OS osteosarcoma cells transiently expressing E1A, ectopic expression of D-type cyclins alone or combined, but not cyclins E and/or A, fully rescues E1A-mediated block in hyperphosphorylation of p130 to form 3. However, cyclins E and A, individually or together, induce hyperphosphorylation of p130 to species with intermediate mobility. Phosphopeptide maps indicate that E1A inhibits phosphorylation of sites phosphorylatable by CDKs. One of these sites is Ser-1044. The effects of blocking the activities of endogenous and exogenous cyclins with p16 and dominant negative CDK2 in E1A expressing cells further indicate that p130 is phosphorylated by both D-type cyclin and cyclin E/CDK complexes and that E1A modulates the activity of these G1/S CDKs by independent mechanisms. Stable expression of E1A in MC3T3-E1 cells leads to downregulation of D-type cyclins, and upregulation of cyclins E and A. This is accompanied by increased CDK2 kinase activity. Downregulation of D-type cyclins in these cells correlates with a block on both p130 hyperphosphorylation to form 3 and hyperphosphorylation of p107. This is rescued by D-type cyclins but not by cyclin E. In addition, we show that the upregulation of cyclins E and A is at least partially dependent on an intact pocket protein/E2F pathway, but downregulation of D-type cyclins is not. Moreover, we provide evidence that while the lack of a functional pRB pathway also results in a block on hyperphosphorylation of p130 to form 3, this is not sufficient to induce constitutive expression of p130 form 2b.

Accumulation of high levels of the p53 and p130 growth-suppressing proteins in cell lines stably over-expressing cyclin-dependent kinase 6 (cdk6)

Cyclin-dependent kinase 6(cdk6) is present in randomly proliferating cultures of 3T3 cells but has little detectable enzymatic activity. Significant activity is detected only during a short period in early G1 phase. To examine the possible functions of cdk6 in 3T3 cells, lines stably over-expressing cdk6 were constructed and compared to normal 3T3 cells or cell lines with reduced cdk6 levels due to expression of a dominant-negative form of the protein. Over-expression of cdk6 in cells, which led to high levels of activity even in proliferating cultures, had dramatic effects. Cell lines stably over-expressing wild-type cdk6 had a markedly reduced growth rate compared to parental 3T3 cells or lines expressing a dominant-negative form of cdk6. They also over-produced the p53 and p130 proteins and had increased sensitivity to UV-irradiation. Irradiation resulted in accumulation of the Bax protein and rapid cell death. Levels of p53 and p130 proteins were down-regulated and the growth rate of the cells was increased by introduction of the dominant-negative form of cdk6 into cells over-expressing cdk6, indicating that cdk6 is involved in the overproduction of p53 and p130. The results suggest that cdk6, through regulation of growth-suppressing molecules, may play a role in halting cellular growth when proliferation is inappropriate.

Differential expression and regulation of the retinoblastoma family of proteins during testicular development and spermatogenesis: roles in the control of germ cell proliferation, differentiation and apoptosis

Normal spermatogenesis is highly dependent on well-balanced germ cell proliferation, differentiation, and apoptosis. However, the molecular mechanisms that govern these processes are largely unknown. Retinoblastoma family proteins (pRb, p107 and p130) are potentially important regulators of cell growth, differentiation and apoptosis. pRb has been shown to be expressed in the rat testis and involved in the regulation of spermatogenesis. In the present study, the expression and localization of the other two pRb family members, p107 and p130, were analysed at both mRNA and protein levels during testicular development and spermatogenesis using Northern, Western blotting, immunohistochemistry, and in situ hybridization. Furthermore, changes of levels and phosphorylation status of pRb family proteins in response to growth suppression and/or apoptosis induction were investigated using a seminiferous tubule culture system and three animal models. Our data suggest that: (1) pRb family proteins are differentially expressed in the rat testis and they function in a cell-type-specific manner during testicular development and spermatogenesis; (2) they participate in the control of germ cell cycle and act in a cell cycle-phase-specific fashion during germ cell proliferation, and (3) they are also involved in the regulation of apoptosis of germ cells and Leydig cells.

Modeling ischemia in vitro: selective depletion of adenine and guanine nucleotide pools

Intracellular ATP depletion is a hallmark event in ischemic injury. It has been extensively characterized in models of chemical anoxia in vitro. In contrast, the fate of GTP during ischemia remains unknown. We used LLC-PK proximal tubular cells to measure GTP and ATP changes during anoxia. In 45 min, antimycin A decreased ATP and GTP to 8% and 2% of controls, respectively. Ischemia in vivo resulted in comparable reductions in GTP and ATP. After 2 h of recovery, GTP levels in LLC-PK cells increased to 65% while ATP increased to 29%. We also investigated steady-state models of selective ATP or GTP depletion. Combinations of antimycin A and mycophenolic acid selectively reduced GTP to 51% or 25% of control. Similarly, alanosine selectively reduced ATP to 61% or 26% of control. Selective GTP depletion resulted in significant apoptosis. Selective ATP depletion caused mostly necrosis. These models of ATP or GTP depletion can prove useful in dissecting the relative contribution of the two nucleotides to the ischemic phenotype.

pRB and p107 have distinct effects when expressed in pRB-deficient tumor cells at physiologically relevant levels

A key difference among the three structurally similar pRB family members is that only pRB is a tumor suppressor. Identification of distinctive functional differences between pRB and p107/p130 therefore holds promise for a better understanding of the tumor suppression mechanisms of pRB. Enigmatically, pRB and p107 have been shown to have indistinguishable growth suppression activities when studied in the pRB-deficient Saos-2 cell system. In this study, we discovered that, when expressed at physiologically relevant levels, pRB and p107 had distinctive effects in causing growth suppression. pRB induced cellular p130 levels while p107 repressed them. p107, but not pRB, blocked cells inside S phase in addition to G1 arrest. In contrast, no qualitative differences were identified in their abilities to repress the expression of a set of suspected pRB/E2F repression target genes. These results indicate that pRB and p107 possess different growth suppression effects, despite the fact that they have similar E2F repression effects.

Granulocytic differentiation of myeloid progenitor cells by p130, the retinoblastoma tumor suppressor homologue

The retinoblastoma protein (pRB) and the related pocket proteins, p107 and p130, play crucial roles in mammalian cell cycle control. Recent studies indicate that these pocket proteins are also involved in cellular differentiation processes. We demonstrate in this work that the pRB-related p130 selectively accumulates during the in vitro differentiation of the myeloid progenitor cell, 32Dcl3, into granulocyte in response to granulocyte-colony stimulating factor (G-CSF). This G-CSF-dependent granulocytic differentiation is blocked by the adenovirus E1A oncoprotein, which binds to and inactivates the pRB family of pocket proteins including p130. Furthermore, enforced overexpression of p130 but not pRB inhibits the myeloid cell proliferation that is concomitantly associated with granulocytic differentiation morphologically characterized by nuclear segmentation. However, simple G1-cell cycle arrest induced by cytokine deprivation or ectopic overexpression of the p27 cyclin-dependent kinase inhibitor, or inhibition of E2F activities by dominant negative DP-1 is not sufficient to trigger granulocytic differentiation. The differentiation-promoting activity of p130 in myeloid cells requires both the pocket domain and the spacer domain. Our results indicate that the pRB-related p130 plays a critical role in myeloid cell differentiation and suggest that coupling of cell cycle exit with the cellular differentiation program may be specifically achieved by p130.

Ras-mediated signaling pathway regulates the expression of a low-molecular-weight heat-shock protein in fission yeast

In fission yeast, Schizosaccharomyces pombe, deficiency of ras1 gene causes an abnormal cell shape and abolishes mating ability. However, target genes of this signaling pathway are largely unknown because of the lack of an appropriate analysis system. To overcome this problem, we have started a novel project to categorize entire genes based on their expression levels under different growth conditions. Using this strategy, we screened genes whose expression levels were affected in the presence or absence of the ras1 gene product. For this purpose, we utilized high-density arrays of clones covering the entire genome of the fission yeast, and probed with labelled cDNA derived from various strains and growth conditions. Here, we demonstrate the detection of a low-molecular-weight heat-shock protein gene, hsp16, whose expression is very likely to be regulated by a ras-mediated signaling pathway, but not by the heat-shock response.

Rapid dephosphorylation of p107 following UV irradiation

In response to UV irradiation, mouse NIH3T3 fibroblasts transiently arrest predominantly in the G1 phase of the cell cycle. Here, we investigate the role of the retinoblastoma-related pocket proteins in this biological process. We report here that UV induces an increase in p107/E2F complexes, shown previously to be repressors of E2F-dependent transcriptional activity. Several lines of evidence indicate that the increase of p107/E2F complexes following UV irradiation is a consequence of rapid dephosphorylation of p107. First, UV-mediated p107 dephosphorylation could be abolished by pretreatment of NIH3T3 fibroblasts with the serine/threonine phosphatase inhibitors calyculin A and okadaic acid. Second, alteration of protein phosphatase 2A holoenzyme composition by over-expression of specific B subunits interfered with UV-mediated dephosphorylation of p107. Consistent with this, p107 could be dephosphorylated in vitro with PP2A. Moreover, dephosphorylation of p107 was shown to be independent of the activity of p53 and p21, as it occurred also in UV-treated p53-null as well as p21-null mouse fibroblasts. We observed a close correlation between the UV dosages required for G1 cell cycle arrest and p107 dephosphorylation. Our data suggest a model in which UV radiation-induced cell cycle arrest depends, at least in part, on the induction of a PP2A-like phosphatase that acts on p107.

Cell cycle regulation of the double stranded RNA activated protein kinase, PKR

The interferon (IFN)-induced, double stranded RNA (dsRNA)-activated serine/threonine kinase, PKR, is a potent negative regulator of cell growth when overexpressed in yeast or mammalian cells. To determine whether endogenous PKR plays a role in cell growth control, we have investigated the regulation of PKR levels and activity during the cell cycle in human glioblastoma T98G cells. The steady-state level of PKR mRNA in T98G cells was highest in growth arrested cells, dropped sharply within 3 h of serum stimulation then gradually increased as cells progressed through G1, reaching a plateau in early S phase. PKR protein level increased following serum stimulation reaching a peak at the G2+M boundary and declining thereafter. In contrast, PKR kinase activity exhibited two peaks, in early G1 and at the G1/S boundary, declining sharply in early S phase. Thus, the activity profile did not follow the protein profile indicating a tight regulation of PKR at the level of activity. In T98G cells expressing the catalytically inactive PKRK296R the dsRNA-induced activation of NF-kappaB and IRF-1 was suppressed and the mutant cells exhibited resistance to stress induced apoptosis. Cell cycle distribution analysis showed that the mutant expressing cells exhibited longer G1 phase and fewer cells engaged in S phase. Furthermore, early passage mouse embryo fibroblasts derived from PKR knockout mice grew more slowly compared with the control cells. Taken together these results suggest that PKR may play a role in cell cycle progression.

Inosine-5'-monophosphate dehydrogenase is a rate-determining factor for p53-dependent growth regulation

We have proposed that reduced activity of inosine-5'-monophosphate dehydrogenase (IMPD; IMP:NAD oxidoreductase, EC 1.2.1.14), the rate-limiting enzyme for guanine nucleotide biosynthesis, in response to wild-type p53 expression, is essential for p53-dependent growth suppression. A gene transfer strategy was used to demonstrate that under physiological conditions constitutive IMPD expression prevents p53-dependent growth suppression. In these studies, expression of bax and waf1, genes implicated in p53-dependent growth suppression in response to DNA damage, remains elevated in response to p53. These findings indicate that under physiological conditions IMPD is a rate-determining factor for p53-dependent growth regulation. In addition, they suggest that the impd gene may be epistatic to bax and waf1 in growth suppression. Because of the role of IMPD in the production and balance of GTP and ATP, essential nucleotides for signal transduction, these results suggest that p53 controls cell division signals by regulating purine ribonucleotide metabolism.