Inactivation of p53 gene in human and murine osteosarcoma cells

Proliferative response of different human osteoblast-like cell models to proinflammatory cytokines

Children with inflammatory bowel disease are known to be at risk of osteopenia. The cause of this osteopenia is likely to be multifactorial, but the inflammatory process with its characteristic overproduction of cytokines has been implicated. To investigate this possible contribution of the disease activity to the development of osteopenia, we performed in vitro assays of the proliferation of osteoblast-like cells of differing origins in response to the inflammatory cytokines tumor necrosis factor-alpha and IL-1/beta. Osteoblast-like cells derived from pediatric bone explants, adherent stromal cells derived from bone marrow (osteoprogenitors), MG-63 osteosarcoma cells, and SV-40 virally transformed osteoprogenitor cells (HCC1) were studied. Tumor necrosis factor-alpha stimulated the proliferation of cells in primary cultures (i.e. from explants and marrow samples) in a linear, dose-dependent manner. In contrast, inhibition of proliferation was observed with the established cell lines (MG-63 and HCC1). IL-1beta stimulated proliferation of all cells apart from the immortalized human bone marrow cell line, HCC1, in which case potent inhibition was observed. We conclude that proinflammatory cytokines are potent regulators of osteoblast-like cell proliferation, and that the responses are specific to cell type. The opposite results obtained with established cell lines compared with the primary cultures suggest that careful consideration should be given to choosing the most suitable cell line for in vitro studies relating to in vivo mechanisms predisposing to osteopenia.

The phosphotyrosyl phosphatase activator gene is a novel p53 target gene

The minimal promoter of the phosphotyrosyl phosphatase activator (PTPA) gene, encoding a regulator of protein phosphatase 2A contains two yin-yang 1 (YY1)-binding sites, positively regulating promoter activity. We now describe a role for p53 in the regulation of PTPA expression. Luciferase reporter assays in Saos-2 cells revealed that p53 could down-regulate PTPA promoter activity in a dose-dependent manner, whereas four different p53 mutants could not. The p53-responsive region mapped to the minimal promoter. Overexpression of YY1 reverses the repressive effect of p53, suggesting a functional antagonism between p53 and YY1. The latter does not involve competition for YY1 binding, but rather direct control of YY1 function. Inhibition of PTPA expression by endogenous p53 was demonstrated in UVB-irradiated HepG2 cells, both on the mRNA and protein level. Also basal PTPA levels are higher in p53-negative (Saos-2) versus p53-positive (HepG2, U2OS) cells, suggesting "latent" p53 can control PTPA expression as well. The higher PTPA levels in U2OS cells, programmed to overexpress constitutively a dominant-negative p53 mutant, corroborate this finding. Thus, PTPA expression is negatively regulated by p53 in normal conditions and in conditions where p53 is up-regulated, via an as yet unknown mechanism involving the negative control of YY1.

Similar but distinct effects of the tristetraprolin/TIS11 immediate-early proteins on cell survival

The immediate early protein tristetraprolin (TTP) is required to prevent inappropriate production of the cytokine TNF-alpha, and is a member of a zinc finger protein family that is associated with RNA binding. TTP expression is induced by TNF-alpha, and evidence indicates that TTP can bind and destabilize the TNF-alpha mRNA. TTP and the closely related TIS11b and TIS11d proteins are evolutionarily conserved, however, and induced transiently in various cell types by numerous diverse stimuli, suggesting that they have additional functions. Supporting this idea, continuous expression of each TTP/TIS11 protein at physiological levels causes apoptotic cell death. By various criteria, this cell death appears analogous to apoptosis induced by certain oncoproteins. It is also dependent upon the zinc fingers, suggesting that it involves action on appropriate cellular targets. TTP but not TIS11b or TIS11d also sensitizes cells to induction of apoptosis by TNF-alpha. The data suggest that the TTP and TIS11 immediate early proteins have similar but distinct effects on growth or survival pathways, and that TTP might influence TNF-alpha regulation at multiple levels.

Apoptosis induced by the nuclear death domain protein p84N5 is inhibited by association with Rb protein

Rb protein inhibits both cell cycle progression and apoptosis. Interaction of specific cellular proteins, including E2F1, with Rb C-terminal domains mediates cell cycle regulation. In contrast, the nuclear N5 protein associates with an Rb N-terminal domain with unknown function. The N5 protein contains a region of sequence similarity to the death domain of proteins involved in apoptotic signaling. We demonstrate here that forced N5 expression potently induces apoptosis in several tumor cell lines. Mutation of conserved residues within the death domain homology compromise N5-induced apoptosis, suggesting that it is required for normal function. Endogenous N5 protein is specifically altered in apoptotic cells treated with ionizing radiation. Furthermore, dominant interfering death domain mutants compromise cellular responses to ionizing radiation. Finally, physical association with Rb protein inhibits N5-induced apoptosis. We propose that N5 protein plays a role in the regulation of apoptosis and that Rb directly coordinates cell proliferation and apoptosis by binding specific proteins involved in each process through distinct protein binding domains.

Infectivity and expression of the early adenovirus proteins are important regulators of wild-type and DeltaE1B adenovirus replication in human cells

An adenovirus mutant lacking the expression of the large E1B protein (DeltaE1B) has been reported to replicate selectively in cells lacking the expression of functionally wild-type (wt) p53. Based on these results the DeltaE1B or ONYX-015 virus has been proposed to be an oncolytic virus which might be useful to treat p53-deficient tumors. Recently however, contradictory results have been published indicating that p53-dependent cell death is required for productive adenovirus infection. Since there is an urgent need for new methods to treat aggressive, mutant p53-expressing primary tumors and their metastases we carefully examined adenovirus replication in human cells to determine whether or not the DeltaE1B virus can be used for tumor therapy. The results we present here show that not all human tumor cell lines take up adenovirus efficiently. In addition, we observed inhibition of the expression of adenovirus early proteins in tumor cells. We present evidence that these two factors rather than the p53 status of the cell determine whether adenovirus infection results in lytic cell death. Furthermore, the results we obtained by infecting a panel of different tumor cell lines show that viral spread of the DeltaE1B is strongly inhibited in almost all p53-proficient and -deficient cell lines compared to the wt virus. We conclude that the efficiency of the DeltaE1B virus to replicate efficiently in tumor cells is determined by the ability to infect cells and to express the early adenovirus proteins rather than the status of p53.

p53-Independent and -dependent requirements for E1B-55K in adenovirus type 5 replication

The adenovirus type 5 mutant dl1520 was engineered previously to be completely defective for E1B-55K functions. Recently, this mutant (also known as ONYX-015) has been suggested to replicate preferentially in p53(-) and some p53(+) tumor cell lines but to be attenuated in primary cultured cells (C. Heise, A. Sampson-Johannes, A. Williams, F. McCormick, D. D. F. Hoff, and D. H. Kirn, Nat. Med. 3:639-645, 1997). It has been suggested that dl1520 might be used as a "magic bullet" that could selectively lyse tumor cells without harm to normal tissues. However, we report here that dl1520 replication is independent of p53 genotype and occurs efficiently in some primary cultured human cells, indicating that the mutant virus does not possess a tumor selectivity. Although it was not the sole host range determinant, p53 function did reduce dl1520 replication when analyzed in a cell line expressing temperature-sensitive p53 (H1299-tsp53) (K. L. Fries, W. E. Miller, and N. Raab-Traub, J. Virol. 70:8653-8659, 1996). As found earlier for other E1B-55K mutants in HeLa cells (Y. Ho, R. Galos, and J. Williams, Virology 122:109-124, 1982), dl1520 replication was temperature dependent in H1299 cells. When p53 function was restored at low temperature in H1299-tsp53 cells, it imposed a modest defect in viral DNA replication and accumulation of late viral cytoplasmic mRNA. However, in both H1299 and H1299-tsp53 cells, the defect in late viral protein synthesis appeared to be much greater than could be accounted for by the modest defects in late viral mRNA levels. We therefore propose that in addition to countering p53 function and modulating viral and cellular mRNA nuclear transport, E1B-55K also stimulates late viral mRNA translation.

p53 transactivity during in vitro osteoblast differentiation in a rat osteosarcoma cell line

We previously demonstrated a correlation between wild-type p53 expression and appearance of osteoblastic-specific differentiation characteristics, as evidenced by basal osteocalcin gene expression in a mouse osteosarcoma tumor. The study reported here further explored the possibility of p53's having a distinct transcription-activating role in bone differentiation, in addition to its proposed role in G1 arrest and apoptosis. ROS17/2.3 osteoblastic osteosarcoma cells were stably transfected with a plasmid containing wild-type p53 binding sequences fused to the chloramphenicol acetyltransferase reporter gene. These cells were used to determine the transactivating role of p53 in regulation of osteocalcin gene expression. We chose two conditions under which osteocalcin expression is known to be upregulated: exposure of osteoblastic cells to differentiation-promoting medium and to vitamin D3. Exposure of the transfected cells to differentiation-promoting medium produced an increase in p53 transactivating activity correlating with the appearance of osteocalcin expression after about 1 wk. Vitamin D3 treatment resulted in upregulation of osteocalcin activity without a corresponding change in p53 transactivation activity or expression. In separate experiments, we tested whether changes in osteocalcin expression accompanied changes in p53 activity under conditions of downregulation of cell proliferation mediated by inhibition of DNA synthesis. Hydroxyurea treatment was used to inhibit DNA synthesis and produce growth arrest in osteoblastic cells. Inhibition of osteoblast cell proliferation was associated with a fourfold increase in p53 transactivating activity and a transient increase in osteocalcin steady-state expression. These results demonstrated a close relationship between p53 and osteocalcin and suggested a regulatory role for wild-type p53 in the control of basal osteocalcin gene expression in osteoblasts.

Cell-type-dependent activity of the ubiquitous transcription factor USF in cellular proliferation and transcriptional activation

USF1 and USF2 are basic helix-loop-helix transcription factors implicated in the control of cellular proliferation. In HeLa cells, the USF proteins are transcriptionally active and their overexpression causes marked growth inhibition. In contrast, USF overexpression had essentially no effect on the proliferation of the Saos-2 osteosarcoma cell line. USF1 and USF2 also lacked transcriptional activity in Saos-2 cells when assayed by transient cotransfection with USF-dependent reporter genes. Yet, there was no difference in the expression, subcellular localization, or DNA-binding activity of the USF proteins in HeLa and Saos-2 cells. Furthermore, Gal4-USF1 and Gal4-USF2 fusion proteins activated transcription similarly in both cell lines. Mutational analysis and domain swapping experiments revealed that the small, highly conserved USF-specific region (USR) was responsible for the inactivity of USF in Saos-2 cells. In HeLa, the USR serves a dual function. It acts as an autonomous transcriptional activation domain at promoters containing an initiator element and also induces a conformational change that is required for USF activity at promoters lacking an initiator. Taken together, these results suggest a model in which the transcriptional activity of the USF proteins, and consequently their antiproliferative activity, is tightly controlled by interaction with a specialized coactivator that recognizes the conserved USR domain and, in contrast to USF, is not ubiquitous. The activity of USF is therefore context dependent, and evidence for USF DNA-binding activity in particular cells is insufficient to indicate USF function in transcriptional activation and growth control.

p53 gene mutations and rearrangements in non-Hodgkin's lymphoma

BACKGROUND

Alterations of the p53 gene have been associated with the progression of certain human malignancies. To establish further the correlation between p53 gene alterations and progression of non-Hodgkin's lymphomas (NHLs), the authors analyzed both mutations and rearrangements of the p53 gene in a cohort of 84 NHLs.

METHODS

Eighty-four NHLs were analyzed for p53 gene alterations. Point mutations of exons 5-9 were studied by polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP), and DNA rearrangements were studied by Southern blot analysis. RESULTS. Point mutations and DNA rearrangements of the p53 gene were detected in 6 (7.2%) and 3 (3.6%) patients, respectively. All p53 gene abnormalities were found exclusively in B-cell NHLs. Taken together, patients with p53 gene alterations had poorer survival than other patients (P = 0.024). However, of the three patients with p53 gene rearrangements, the two who appeared to have one normal allele showed a relatively better response to chemotherapy and had longer survival (27 and 47 months). In contrast, the remaining patient who had rearranged bands much stronger than the germline, and thus appeared to have both alleles rearranged, was refractory to chemotherapy and had poorer survival (6 months).

CONCLUSIONS

Patients with NHLs of intermediate and high grades who carried point mutations or rearrangements of p53 genes had worse outcomes than other patients. Patients with one abnormal p53 allele and one residual normal allele had a more favorable prognosis than those with two abnormal alleles.

Adenovirus E4orf6 oncoprotein modulates the function of the p53-related protein, p73

Recently, several proteins have been identified that are related in their sequence to the p53 tumor-suppressor protein. One of these proteins, which is termed p73, exhibits sequence homology to the p53 transcriptional activation, DNA binding, and oligomerization domains. The adenovirus E1B 55-kDa protein, the adenovirus E4orf6 protein, and SV40 T antigen each can bind to p53 and inhibit p53 function. Here we demonstrate that the adenovirus E4orf6 protein, but not the E1B 55-kDa protein or T antigen, interacts with p73. The E4orf6 protein inhibits p73-mediated transcriptional activation and cell killing in a manner similar to its effect on p53. Thus, only a subset of viral oncoproteins that antagonize p53 function also interacts with the related p73 protein.

The phosphatidylinositol 3-kinase inhibitor wortmannin sensitizes quiescent but not proliferating MG-63 human osteosarcoma cells to radiation

Recent genetic and biochemical studies indicate that DNA-dependent protein kinase (DNA-PK) plays an important role in DNA double-strand break (dsb) repair and V(D)J recombination. Since the catalytic subunit of DNA-PK (DNA-PKcs) has high sequence homology with phosphatidylinositol 3-kinase (PI 3-kinase), we examined the effect of wortmannin, a specific inhibitor of PI 3-kinase, on the survival of human tumor cells after X-irradiation. The present study demonstrates that wortmannin at 20 microM is an effective radiosensitizer of quiescent (Q), but not proliferating (P) cells. In addition, the rejoining of DNA dsb is significantly inhibited in Q, but not in P cells. Finally, we found that Q cell extracts have approximately five-fold less DNA-PK activity than those of P cells. After a 2 h exposure to wortmannin, the DNA-PK activity of Q cell extracts was considerably lower than that of P cells. This can explain why wortmannin sensitizes Q, but not P cells to radiation.

p53-Independent activation of the gadd45 promoter by Delta12-prostaglandin J2

A p53-inducible gene, the growth arrest and DNA damage-inducible gene 45 (gadd45), is associated with cell growth inhibition, DNA damage response and DNA repair. Here we report that Delta12-prostaglandin J2 (Delta12-PGJ2), an in vivo metabolite of arachidonic acid, which inhibits cell proliferation, induces gadd45 mRNA in HeLa cells. Because the p53 protein in HeLa cells is inactivated by papilloma virus E6, this type of gadd45 induction appears to be p53-independent. The induction was dose-dependent, and the maximum induction was observed at a concentration of 7.5 microgram/ml. In a time course study, gadd45 mRNA was induced 3 h after the addition of 7.5 microgram/ml Delta12-PGJ2. To investigate the transcriptional mechanism of gadd45 mRNA induction, we cloned a human genomic DNA fragment containing the gadd45 promoter region, and investigated the effect of Delta12-PGJ2 on the gadd45 promoter activity. In HeLa cells, 7.5 microgram/ml Delta12-PGJ2 markedly stimulated the gadd45 gene promoter about 20-fold or more. On the other hand, Delta12-PGJ2 did not stimulate the promoter activity of a reporter plasmid containing only p53 binding sites in HeLa cells, indicating that the gadd45 promoter activation by Delta12-PGJ2 was not mediated by p53. These results suggest that one of the mechanisms of cell growth arrest by Delta12-PGJ2 is mediated through a p53-independent induction of gadd45.

p53-independent induction of WAF1/Cip1 is correlated with osteoblastic differentiation by vitamin D3

1Alpha,25(OH)2 vitamin D3 (1,25(OH)2D3) can induce differentiation of osteoblastic cells by arresting the cell cycle at G1. The p53-inducible gene, WAF1/Cip1, is one of the inhibitors of cyclin-dependent kinases and can inhibit the phosphorylation of retinoblastoma protein (pRB), thereby keeping pRB functionally active. Here we show that in a p53-null human osteoblastic osteosarcoma MG-63 cell line, 10 nM of 1,25(OH)2D3 completely inhibits cell growth and increases alkaline phosphatase activity, which suggests the induction of osteoblastic differentiation. We also found a p53-independent increase of WAF1/Cip1 mRNA and promoter activation by 1,25(OH)2D3. On the other hand, the expression and the promoter activity of the RB gene decreased after treatment with 1,25(OH)2D3 during the differentiation of MG-63 cells. Our results suggest that the p53-independent WAF1/Cip1 induction by 1,25(OH)2D3 is important for osteoblastic differentiation of MG-63 cells.

Cell cycle-dependent modulation of promoter activities of RB and WAF1/Cip1 genes

A universal inhibitor of cyclin-dependent kinases, WAF1/Cip1 can dephosphorylate the RB gene product to arrest the cell cycle at the G1 phase. Here we show that the mRNA level and the promoter activities of the RB and WAF1/Cip1 genes exhibit cell cycle-dependent change when cells are released from either serum-starvation or the confluent cell state with serum. RB expression and promoter activity are elevated at middle to late G1. In contrast, the mRNA and promoter activity of the WAF1/Cip1 gene increase at early G1. These results suggest that the RB and WAF1/Cip1 expression and promoter activities depend not only on serum, but also on the cell cycle progression itself. Moreover, we identified the responsive region for serum-released cell cycle progression in the RB promoter and mapped it to the region between -4 and -182 relative to the initiating codon of the RB gene. The region in the WAF1/Cip1 promoter responsible for the serum-released cell cycle progression mapped not to the p53 binding site, but to the 374 base-pair region between -1770 and -1396 from the transcription start site.

Inhibition of p53 transactivation function by the human T-cell lymphotropic virus type 1 Tax protein

Human T-cell lymphotropic virus type 1 (HTLV-1) is the etiologic agent for adult T-cell leukemia. HTLV-1 transforms lymphocytes, and there is increasing evidence that the virus-encoded protein, Tax, plays a primary role in viral transformation. We have shown that wild-type p53 in HTLV-1-transformed cells is stabilized. This study was initiated to directly analyze whether the p53 in HTLV-1-transformed cell lines was transcriptionally active and to identify the viral gene product responsible for stabilization and inactivation. Transfection experiments using a p53-responsive reporter plasmid and gamma-irradiation studies demonstrate that the wild-type p53 in HTLV-1-transformed cell lines is not fully active. Further, we demonstrate that the HTLV-1-transforming protein, Tax, stabilizes and inactivates p53 function. Cotransfection of Tax with p53 results in a greater than 10-fold reduction in p53 transcription activity. Using Ga14-p53 fusion proteins, we demonstrate that Tax inhibition of p53 transactivation function is independent of sequence-specific DNA binding. Moreover, Tax inhibits p53 function by interfering with the activity of the N-terminal activation domain (amino acids 1 to 52). We conclude that Tax is involved in the inactivation of p53 function and stabilization of p53 in HTLV-1-infected cells. The functional interference of p53 function by Tax may be important for transformation and leukemogenesis.

Histone deacetylase inhibitor activates the WAF1/Cip1 gene promoter through the Sp1 sites

Treatment of cultured cells with trichostatin A (TSA), a specific histone deacetylase inhibitor, induces the histone hyperacetylation and modulates expression of some mammalian genes. We examined the effects of TSA on cell growth arrest, and its relation to expression of the WAF1/Cip1 gene, a potent inhibitor of cyclin-dependent kinases, in a p53-mutated human osteosarcoma cell line MG63. TSA at 500 ng/ml induced growth arrest at both G1 and G2/M phases, and the expressions of the WAF1/Cip1 mRNA and protein. We also examined the changes of acetylated isoforms of histone H4. Dose-response and kinetic analysis suggest a close correlation between the level of histone acetylation and the induction of the WAF1/Cip1 expressions. Using several mutant WAF1/Cip1 promoter fragments, we found that the TSA responsive elements are two Sp1 sites at -82 and -69 relative to the transcription start site. These findings indicate that TSA induces the WAF1/Cip1 promoter through the typical Sp1 sites, in a p53-independent fashion. Furthermore, the Sp1-luc plasmid, containing SV40 promoter-derived three consensus Sp1 binding sites, was markedly activated by TSA, compared to the mutant Sp1-luc plasmid. These results demonstrate that transcriptional activation through the Sp1 sites of the WAF1/Cip1 promoter by TSA coincides with induced hyperacetylation of histone H4.

Butyrate activates the WAF1/Cip1 gene promoter through Sp1 sites in a p53-negative human colon cancer cell line

Butyrate is a well known colonic luminal short chain fatty acid, which arrests cell growth and induces differentiation in various cell types. We examined the effect of butyrate on the expression of WAF1/Cip1, a potent inhibitor of cyclin-dependent kinases, and its relation to growth arrest in a p53-mutated human colon cancer cell line WiDr. Five millimolar butyrate completely inhibited the growth of WiDr and caused G1-phase arrest. WAF1/Cip1 mRNA was rapidly induced within 3 h by treatment with 5.0 mM butyrate, and drastic WAF1/Cip1 protein induction was detected. Using several mutant WAF1/Cip1 promoter fragments, we found that the butyrate-responsive elements are two Sp1 sites at -82 and -69 relative to the transcription start site. We also found that a TATA element at -46 and two overlapping consensus Sp1 sites at -60 and -55 are essential for the basal promoter activity of WAF1/Cip1. These findings suggest that butyrate arrests the growth of WiDr by activating the WAF1/Cip1 promoter through specific Sp1 sites in a p53-independent fashion.

Co-localization of endogenous and exogenous p53 proteins in nasopharyngeal carcinoma cells

Recently, we have established nine nasopharyngeal carcinoma (NPC) cell lines in which only one cell line showed the p53 mutation. For investigation of the p53 mutation in this line, immunostaining using anti-p53 antibody was applied and showed the presence of p53 protein in the cytoplasm but not in the nucleus. Single strand conformation polymorphism analysis of the p53 gene showed one normal and one additional DNA band. Cloning and sequencing of PCR-amplified DNA showed an AGA (arginine) to ACA (threonine) heterozygous point mutation at codon 280. Transfection of the p53 DNA binding sequence and chloramphenicol acetyltransferase assay revealed loss of transcriptional activation function of endogenous p53 protein. Co-localization of the endogenous and the transfected exogenous p53 protein by polyclonal antibodies to anti-p53 protein revealed strong exogenous p53 staining in the transfected nuclei and weak staining of endogenous p53 protein in the cytoplasm. We concluded that (a) a heterozygous point mutation at codon 280 was identified in the NPC-TW 06 cell line; (b) the point mutation may cause the stagnation of mutant p53 protein in the cytoplasm, and loss of its transcriptional activation function; (c) endogenous and exogenous p53 protein can be co-localized at the same time in the transfected cells; and (d) 280 mutant p53 protein in NPC cells does not cause a decrease or increase in sensitivity to chemotherapy.

1-(5-Isoquinolinesulfonyl)-2-methylpiperazine induces apoptosis in human neuroblastoma cells, SH-SY5Y, through a p53-dependent pathway

We have studied the effect of 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), a protein kinase inhibitor, on the regulation of apoptosis in the human neuroblastoma cell line, SH-SY5Y. H-7 (20-100 microM) induced apoptosis in these cells characterized by DNA fragmentation and chromatin condensation. Immunoblot analyses were performed with specific antibody against BCL-2, BCL-XS/L, BAX, JUNB, c-JUN, ICH-1L, c-FOS, RB, CDK-2, and p53. H-7 treatment did not significantly alter the level of these proteins with the exception of p53. H-7, but not staurosporine, caused a dramatic nuclear accumulation of p53. The kinetics of nuclear accumulation of p53 correlates well with the kinetics of induction of apoptosis. The effect of H-7 was further assessed in a group of human cell lines. Only cell lines harboring the wild-type p53 gene were responsive to the stimulatory effect of H-7 on nuclear accumulation of p53. Furthermore, cell lines carrying a mutated p53 gene were resistant to the cytotoxic effect of H-7. The ability of H-7 in mediating apoptosis in the SH-SY5Y line expressing a dominant negative mutant of p53 was significantly diminished. Taken together, these data strongly suggest that a p53-dependent mechanism contributes to the cytotoxicity of H-7 in human neuroblastoma cells.

Clonal osteoblastic cell lines from p53 null mouse calvariae are immortalized and dependent on bone morphogenetic protein 2 for mature osteoblastic phenotype

p53 protein regulates cell cycle progression and its absence will result in unlimited cell divisions required for immortalization of cells. Immortalized osteoblastic cell lines were established from p53 null mouse calvariae of normal phenotype. The clonal murine cell lines demonstrated osteoblastic phenotype as exemplified by alkaline phosphatase enzyme activity. They also express bone morphogenetic protein 2 (BMP2) mRNA. Addition of recombinant BMP2 to these cells dramatically increased the alkaline phosphatase activity in a dose dependent manner. In the absence of BMP2 these cells do not undergo osteoblastic differentiation. Treatment of these cells with recombinant bone morphogenetic protein 2 stimulated differentiated osteoblast formation, as determined by mineralized nodule formation. Thus, these immortalized cells in culture represent osteoblast progenitors that lack p53 protein and respond to osteogenic stimuli. These cell lines offer a model system to study the role of p53 in osteoblastic differentiation and programmed cell death. Also these cells will be useful in studying the effects of p53 on transcriptional regulation of osteoblast specific gene expression.

Receptor protein tyrosine kinase DDR is up-regulated by p53 protein

We have previously reported on radiation-induction of ptk-3 in rat astrocyte culture [Sakuma et al. (1995) Radiat. Res. 143, 1-7]. Ptk-3 was considered to be a rat version of human DDR (discoidin domain receptor). We cloned and analyzed genomic DNA of the DDR and its promoter region. We discovered that the promoter region contained a consensus sequence of the p53 tumor suppressor binding site. Adenovirus-mediated p53 transfection induced a high level of DDR mRNA in SAOS2 human osteosarcoma cells. These results indicate that DDR is up-regulated by the p53 protein.

Alterations of the p15, p16,and p18 genes in osteosarcoma

Activation of cyclin-dependent kinases (CDKs) by interaction with cyclins regulates progression through cell cycle checkpoints. This process is counterbalanced by CDK inhibitors (CDKIs), which can inhibit progression through the cell cycle. Because CDKI expression acts to inhibit cellular proliferation, CDKIs may have a role as tumor suppressors. One class of CDKIs, characterized by the presence of ankyrin repeats, has at least four members (p15INK4B), p16INK4, p18, and p19). Two of these, p15INK4B, p16INK4, have been mapped to chromosome 9p21, a region of frequent loss in a wide variety of cancers. Alterations of p16INK4 have been detected in various tumors and cell lines. We analyzed p15INK4B, p16INK4, and p18 alterations in 52 osteosarcomas (including 11 explants), and 23 other various sarcomas. Single-stranded conformation polymorphism analysis [polymerase chain reaction (PCR-SSCP)] of the coding regions of these CDKI genes detected a missense mutation of p16INK4 exon 1 in one soft tissue sarcoma. Southern blotting detected complete deletion of p15INK4B and p16INK4 genes in osteosarcomas from 2 patients and a soft tissue sarcoma from another individual. Loss of heterozygosity (LOH) at chromosome 9p21 was observed with a microsatellite probe closely linked to the INK4 genes in the latter case. Deletions of both p15INK4B and p16INK4 genes were detected in five of eight osteosarcoma cell lines. By contrast, no alterations of p18 were detected in any sample. Together these data suggest that alterations of the p15INK4B and p16INK4 genes, but not p18, may occur in approximately 5% of sarcomas. However, deletions of the p15INK4B and P16INK4 genes are frequent in osteosarcoma cell lines and probably have a role in tumor cell growth in culture. Notably, all seven detectable deletions involved both p15INK4B and p16INK4 genes, suggesting that both contribute individual tumor suppressor activity.

Molecular aspects of chemical carcinogenesis: the roles of oncogenes and tumour suppressor genes

The observation that oncogenes are frequently activated in human tumours raises the question of whether these genes are involved in chemical carcinogenesis. H-ras activation is probably an initiating event in mouse skin and rat mammary gland systems. The H-ras oncogene is also important in mouse liver tumours; in mouse lung the K-ras gene is commonly activated. In both, the mutations observed are usually those predicted from the adduct-forming properties of the carcinogen. Among non-ras oncogenes, only raf and neu have been detected in experimental tumours. Tumour suppressor genes are frequently inactivated in human tumours. Searches for such phenomena in animal tumours have generally had disappointing results. p53 and Rb gene alterations are rarely observed in chemically-induced tumours. The reason may be that unknown tumour suppressor genes are involved in animal tumour development. Several novel genes have been identified using animal tumour susceptibility models. Thus, ras genes are important in chemical carcinogenesis, but as the methodology for studying other genes improves, their roles will be seen in perspective.

Rearranged p53 gene with loss of normal allele in a low-grade nonrecurrent glioma

We are studying the p53 gene profile in primary glial tumors by seeking alterations in the hybridization pattern of the tumor DNA probed with a p53 gene probe. This report documents a rearranged p53 gene with loss of the normal allele in a low-grade mixed glioma which has not recurred during 4-year follow-up. The tumor had a low 5-bromodeoxyuridine (BrdU)-labeling index and low AgNOR count. The p53 protein was not detected on immunochemical staining. To our knowledge, this is the first report of an altered p53 gene in a low-grade nonrecurrent glial tumor and highlights the presence of further checks and balances on the control of cell proliferation and other malignancy-associated phenotypes, even in an already-established tumor.

No association between c-myc amplification and TP53 mutation in sarcoma tumorigenesis

Oncogenes and tumor suppressor genes coparticipate in sarcoma tumorigenesis. We tested 43 sarcomas for c-myc amplification by Southern blot and molecular hybridization techniques and TP53 mutations by the polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) technique and direct sequencing of the PCR products. We found eight tumors with c-myc amplification and five different tumors with TP53 mutations but no tumors harbor both c-myc and TP53 alterations. We suggest that c-myc amplification and TP53 mutations do not seem to coparticipate in the tumorigenesis of sarcomas.

Dependence of induction of osteocalcin gene expression on the presence of wild-type p53 in a murine osteosarcoma cell line

The p53 gene undergoes rearrangement in a high percentage of osteosarcomas, resulting in loss of its expression. A p53-null murine osteosarcoma cell line F6 was transfected with either a wild-type or a mutant p53 gene. Stably transfected cell lines were obtained, and their differentiation capabilities were compared in vitro with the parental cell line. Alkaline phosphatase and osteocalcin expression were measured as early and late differentiation markers, respectively. Induction of alkaline phosphatase expression was not affected by the presence of either p53 gene, whereas osteocalcin expression was seen in cells containing the wild-type p53 gene but not in the parental p53-null or mutant-expressing cell lines. That the induction of osteocalcin was intrinsically dependent on the presence of wild-type p53 was also indicated by the use of a temperature-sensitive Val 135 p53 mutant at 32 degrees C; predominant expression of p53 in the wild-type conformation resulted in osteocalcin expression. While the wild-type p53 gene could suppress tumor formation in vivo, the tumors expressing the mutant p53 gene grew two to three times as large as the tumors that did not express p53. Therefore, the absence of end-point differentiation in bone due to p53 rearrangements may contribute to the maintenance of the tumorigenic phenotype in osteosarcomas.