Altered p53 in microdissected, metachronous, premalignant and malignant oral lesions from the same patients

Aims-To determine whether mutant p53 alleles harboured by malignant tumours of the oral cavity were also present in previous premalignant lesions at the same site.Methods-Paraffin embedded tumour specimens along with their premalignant counterparts were analysed for p53 alterations using immunohistochemistry, microdissection, polymerase chain reaction amplification, and DNA sequencing.Results-Malignant lesions from five of eight patients showed overexpression of p53 protein by immunohistochemistry. Upon DNA sequencing, two of these five specimens had p53 mutations. Of the five patients whose cancers showed p53 overexpression by immunohistochemistry, three had previous premalignant lesions that also had immunohistochemically detectable p53 protein. However, DNA sequencing showed that none of these three had mutations in the p53 gene. The remaining five premalignant lesions had no immunohistochemically detectable p53 protein.Conclusions-Some premalignant lesions have increased p53 protein which can be detected by staining with antibody to p53. This staining is not caused by mutations in p53 that are found in subsequent tumours at the same site.

The novel mouse Polo-like kinase 5 responds to DNA damage and localizes in the nucleolus

Polo-like kinases (Plk1-4) are emerging as an important class of proteins involved in many aspects of cell cycle regulation and response to DNA damage. Here, we report the cloning of a fifth member of the polo-like kinase family named Plk5. DNA and protein sequence analyses show that Plk5 shares more similarities with Plk2 and Plk3 than with Plk1 and Plk4. Consistent with this observation, we show that mouse Plk5 is a DNA damage inducible gene. Mouse Plk5 protein localizes predominantly to the nucleolus, and deletion of a putative nucleolus localization signal (NoLS) within its N-terminal moiety disrupts its nucleolar localization. Ectopic expression of Plk5 leads to cell cycle arrest in G1, decreased DNA synthesis, and to apoptosis, a characteristic it shares with Plk3. Interestingly, in contrast to mouse Plk5 gene, the sequence of human Plk5 contains a stop codon that produces a truncated protein lacking part of the kinase domain.

Preservation of genomic integrity in mouse embryonic stem cells

Embryonic stem (ES) cells and germ cells have the potential to give rise to an entire organism. A common requirement is that both must have very robust mechanisms to preserve the integrity of their genomes. This is particularly true since somatic cells have very high mutation frequencies approaching 10-4 in vivo that would lead to unacceptable levels of fetal lethality and congenital defects. Notably, between 70% and 80% of mutational events monitored at a heterozygous endogenous selectable marker were loss of heterozygosity due to mitotic recombination, a mechanism that affects multiple heterozygous loci between the reporter gene and the site of crossing over. This chapter examines three mechanisms by which mouse embryonic stem cells preserve their genomic integrity. The first entails suppression of mutation and recombination between chromosome homologues by two orders of magnitude when compared with isogenic mouse embryo fibroblasts which had a mutation frequency similar to that seen in adult somatic cells. The second renders mouse ES cells hypersensitive to environmental challenge and eliminates damaged cells from the self-renewing population. Mouse ES cells lack a G1 checkpoint so that cells damaged by exogenous insult such as ionizing radiation do not arrest at the G1/S phase checkpoint but progress into the S phase where the damaged DNA is replicated, the damage exacerbated and the cells driven to apoptosis. The third mechanism examines how mouse ES cells repair double strand DNA breaks. Somatic cells predominantly utilize error prone nonhomologous end joining which, from a teleological perspective, would be disadvantageous for ES cells since it would promote accumulation of mutations. When ES cells were tested for the preferred pathway of double strand DNA break repair, they predominantly utilized the high fidelity homology-mediated repair pathway, thereby minimizing the incurrence of mutations during the repair process. When mouse ES cells are induced to differentiate, the predominant repair pathway switches from homology-mediated repair to nonhomologous end joining that is characteristic of somatic cells.

DNA repair in murine embryonic stem cells and differentiated cells

Embryonic stem (ES) cells are rapidly proliferating, self-renewing cells that have the capacity to differentiate into all three germ layers to form the embryo proper. Since these cells are critical for embryo formation, they must have robust prophylactic mechanisms to ensure that their genomic integrity is preserved. Indeed, several studies have suggested that ES cells are hypersensitive to DNA damaging agents and readily undergo apoptosis to eliminate damaged cells from the population. Other evidence suggests that DNA damage can cause premature differentiation in these cells. Several laboratories have also begun to investigate the role of DNA repair in the maintenance of ES cell genomic integrity. It does appear that ES cells differ in their capacity to repair damaged DNA compared to differentiated cells. This minireview focuses on repair mechanisms ES cells may use to help preserve genomic integrity and compares available data regarding these mechanisms with those utilized by differentiated cells.

Mutation frequencies and spectra in DNA polymerase eta-deficient mice

The low-fidelity polymerase eta (poleta) is required for bypass of UV-induced pyrimidine dimers inserting adenine nucleotides opposite these lesions. Mutations in the poleta gene are responsible for the genetic defect in xeroderma pigmentosum variant patients. To study if the lack of poleta significantly elevates spontaneous mutation frequency in various organs and tissues of the mouse, we crossed poleta-deficient mice with transgenic mice harboring a chromosomally integrated lacZ-plasmid reporter construct. In cultured embryonic fibroblasts from the lacZ-poleta(-/-) mice, 2.5 J/m(2) UV irradiation induced approximately 5-fold more mutations than in cells from lacZ control mice, in which an approximately 3-fold increase in mutation frequency was found compared with the normal level. Whereas untreated cells harbored mainly 1-bp deletions, UV induced both transitions and transversions, with the latter type more highly represented in the poleta-null cells than in the controls. No difference in mutation induction between the poleta-null cells and the wild-type cells was observed after treatment with N-ethyl-N-nitrosourea. Having shown the validity of the lacZ model to accurately identify poleta-associated mutagenesis, we then determined the mutant frequency at the lacZ locus in liver, spleen, and small intestine of 12-month-old animals. No differences were found between poleta-null, heterozygous, or littermate control mice. We conclude that the poleta defect is specific for UV damage and has no effect on in vivo mutagenesis in mice.

The checkpoint kinases Chk1 and Chk2 regulate the functional associations between hBRCA2 and Rad51 in response to DNA damage

The cellular response to the introduction of double strand DNA breaks involves complexes of protein interactions that govern cell cycle checkpoint arrest and repair of the DNA lesions. The checkpoint kinases Chk1 and Chk2 phosphorylate the carboxy-terminal domain of hBRCA2, a protein involved in recombination-mediated DNA repair (HRR) and replication fork maintenance. Cells deficient in hBRCA2 are hypersensitive to DNA damaging agents. Phosphorylation of the residue in hBRCA2 targeted by the Chk1 and Chk2 kinases regulates its interaction with Rad51. Furthermore, the cell line lex1/lex2, which lacks the carboxy-terminal domain containing the phosphorylated residue, does not support localization of Rad51 to nuclear foci after exposure to UV or treatment with ionizing radiation (IR). The data show that either phosphorylation of Rad51 by Chk1 or phosphorylation of the carboxy-terminal domain of hBRCA2 by Chk1 or Chk2 plays a critical role in the binding of Rad51 to hBRCA2 and the subsequent recruitment of Rad51 to sites of DNA damage. While depletion of Chk1 from cells leads to loss of Rad51 localization to nuclear foci in response to replication arrest, cells lacking Chk2 also show a defect in Rad51 localization, but only in presence of double strand DNA breaks, indicating that each of these kinases may contribute somewhat differently to the formation of Rad51 nucleoprotein filaments depending on the type of DNA damage incurred by the cells.

X-rays induce distinct patterns of somatic mutation in fetal versus adult hematopoietic cells

There are a variety of mechanisms and pathways whereby cells safeguard their genomes in the face of environmental insults that damage DNA. Whether each of these pathways is equally robust at specific developmental stages in mammals and whether they are also modulated in a tissue-specific manner, however, are unclear. Here, we report that ionizing radiation (IR) produces different types of somatic mutations in fetal cells compared with adult cells of the same lineage. While 1 Gy of X-ray significantly induced intragenic point mutations in T cells of adult mice, no point mutational effect was observed when applied to fetuses. Fetal exposure to IR, on the other hand, led to a significant elevation of mitotic recombination in T cells, which was not observed in adults. Base excision repair (BER) activity was significantly lower in fetal hematopoietic cells than in adult cells, due to a low level of DNA polymerase beta, the rate-limiting enzyme in BER. In fetal hematopoietic cells, this low BER activity, together with a high rate of proliferation, causes X-ray-induced DNA lesions, such as base damage, single strand breaks and double strand breaks, to be repaired by homologous recombination, which we observe as mitotic recombination. Higher BER activity and a relatively lower rate of cell proliferation likely contribute to the significant induction of DNA point mutations in adults. Thus, the mutational response to IR is at least partly determined by the availability of specific repair pathways and other developmentally regulated phenotypes, such as mitotic index.

Protecting genomic integrity in somatic cells and embryonic stem cells

Mutation frequencies at some loci in mammalian somatic cells in vivo approach 10(-4). The majority of these events occur as a consequence of loss of heterozygosity (LOH) due to mitotic recombination. Such high levels of DNA damage in somatic cells, which can accumulate with age, will cause injury and, after a latency period, may lead to somatic disease and ultimately death. This high level of DNA damage is untenable for germ cells, and by extrapolation for embryonic stem (ES) cells, that must recreate the organism. ES cells cannot tolerate such a high frequency of damage since mutations will immediately impact the altered cell, and subsequently the entire organism. Most importantly, the mutations may be passed on to future generations. ES cells, therefore, must have robust mechanisms to protect the integrity of their genomes. We have examined two such mechanisms. Firstly, we have shown that mutation frequencies and frequencies of mitotic recombination in ES cells are about 100-fold lower than in adult somatic cells or in isogenic mouse embryonic fibroblasts (MEFs). A second complementary protective mechanism eliminates those ES cells that have acquired a mutational burden, thereby maintaining a pristine population. Consistent with this hypothesis, ES cells lack a G1 checkpoint, and the two known signaling pathways that mediate the checkpoint are compromised. The checkpoint kinase, Chk2, which participates in both pathways is sequestered at centrosomes in ES cells and does not phosphorylate its substrates (i.e. p53 and Cdc25A) that must be modified to produce a G1 arrest. Ectopic expression of Chk2 does not rescue the p53-mediated pathway, but does restore the pathway mediated by Cdc25A. Wild type ES cells exposed to ionizing radiation do not accumulate in G1 but do so in S-phase and in G2. ES cells that ectopically express Chk2 undergo cell cycle arrest in G1 as well as G2, and appear to be protected from apoptosis.

The breast cancer susceptibility allele CHEK2*1100delC promotes genomic instability in a knock-in mouse model

Allelic variants of CHEK2 contribute to an elevated risk for human breast cancer and possibly other cancer types. In particular, the CHEK2*1100delC polymorphic variant has been identified as a low-penetrance breast cancer susceptibility allele in breast cancer families with wild type BRCA1 and BRCA2. To better understand the molecular basis by which this allele increases risk for disease, we have generated a mouse in which the wild type CHEK2 (Chk2 in mouse) allele has been replaced with the 1100delC variant. Mouse embryo fibroblasts (MEFs) derived from these mice have an altered cell cycle profile in which a far greater proportion of cells are in S-phase and in G2 (4N) compared with wild type cells. The mutant cells show signs of spontaneous genomic instability as indicated by polyploidy and an increase in DNA double strand breaks.

An ageing question: do embryonic stem cells protect their genomes?

Many physiological and cellular processes contribute to the ageing of individuals. One hypothesis argues that the genomes of somatic cells accumulate mutations, which, in turn, alter the metabolism of the cells and contribute to the ageing process. The frequency of somatic mutation approaches 10(-4) and the majority of mutagenic events at heterozygous loci is due to loss of heterozygosity as a consequence of mitotic recombination. A corollary to the argument that somatic cells accumulate mutations is that cells of the germ line and ES cells have a greater requirement for maintaining the integrity of their genomes. In the former case, a high somatic mutation frequency predicts an increase in somatic disease, which limits our lifespan. The corollary is that cells of the germline and ES cells must minimize the mutational burden to limit the frequency of congenital disease and to ensure the proper transmission of undamaged DNA to the gene pool. This report describes two mechanisms utilized by murine ES cells to minimize DNA damage within the proliferative pool. In the first case, murine ES cells display a frequency of mutation and mitotic recombination that is about 100-fold lower than that observed in somatic cells. Second, ES cells lack a G1 checkpoint following DNA damage. When subjected to ionizing radiation, the fraction of apoptotic cells increases to about 40%. Ectopic expression of Chk2 is sufficient to establish a G1 arrest and the concomitant protection from cell death.

DNA damage response and mutagenesis in mouse embryonic stem cells

Mutation in embryonic stem (ES) cells can potentially compromise multiple cell lineages and affect the well-being of subsequent generations. Thus, ES cells require sensitive mechanisms to maintain genomic integrity. One mechanism involves suppression of mutation. A complementary mechanism is to regulate the cell cycle checkpoint and facilitate cell death. Here, we describe the detailed protocols we have used to investigate DNA damage response and mutagenesis in mouse ES cells.

Exposure of mice to arsenic and/or benzo[a]pyrene does not increase the frequency of Aprt-deficient cells recovered from explanted skin of Aprt heterozygous mice

Exposure to inorganic arsenic in drinking water is linked to cancer in humans, but the mechanism of arsenic-induced cancer is not clear. Arsenic is not a powerful point mutagen, but can cause chromosome malsegregation and mitotic recombination, two events that can cause loss of tumor suppressor alleles and thereby contribute to the evolution of cancerous cells. To determine whether arsenic increases the frequency of allele loss due to either malsegregation or mitotic recombination in vivo, Aprt(+/-) hybrid mice were exposed to sodium arsenite (10 mg/L) in their drinking water for 10 weeks. To determine whether arsenic enhances the action of a known mutagen, half of the arsenic-treated mice were exposed to benzo[a]pyrene (BaP) for 8 weeks by skin painting (500 nmoles/week). Cells were taken from painted dorsal skin and cultured in the presence of 2,6-diaminopurine (DAP), to select colonies lacking adenosine phosphoribosyl transferase (Aprt) activity. The frequency of DAP-resistant (DAP(r)) colonies varied substantially within the treatment groups, but there was no significant difference between the groups. Analysis of DNA from DAP(r) colonies suggested that mitotic recombination contributed to the loss of wild-type Aprt allele. Whether arsenic or BaP enhanced or diminished the frequency of this process could not be deduced from these data.

Priming phosphorylation of Chk2 by polo-like kinase 3 (Plk3) mediates its full activation by ATM and a downstream checkpoint in response to DNA damage

The tumor suppressor gene Chk2 encodes a serine/threonine kinase that signals DNA damage to cell cycle checkpoints. In response to ionizing radiation, Chk2 is phosphorylated on threonine 68 (T68) by ataxia-telangiectasia mutated (ATM) protein leading to its activation. We have previously shown that polo-like kinase 3 (Plk3), a protein involved in DNA damage checkpoint and M-phase functions, interacts with and phosphorylates Chk2. When Chk2 was immunoprecipitated from Daudi cells (Plk3-deficient), it had weak kinase activity towards Cdc25C compared with Chk2 derived from T47D cells (Plk3-expressing cells). This activity was restored by addition of recombinant Plk3 in a dose-dependent manner. Plk3 phosphorylates Chk2 at two residues, serine 62 (S62) and serine 73 (S73) in vitro, and this phosphorylation facilitates subsequent phosphorylation of Chk2 on T68 by ATM in response to DNA damage. When the Chk2 mutant construct GFP-Chk2 S73A (serine 73 mutated to alanine) is transfected into cells, it no longer associates with a large complex in vivo, and manifests a significant reduction in kinase activity. It is also inefficiently activated by ATM by phosphorylation at T68 and, in turn, is unable to phosphorylate the Cdc25C peptide 200-256, which contains the inhibitory S216 target phosphorylation residue. As a consequence, tyrosine 15 (Y15) on Cdc2 remains hypophosphorylated, and there is a loss of the G2/M checkpoint. These data describe a functional role for Plk3 in a pathway linking ATM, Plk3, Chk2, Cdc25C and Cdc2 in cellular response to DNA damage.

Co-mutagenic activity of arsenic and benzo[a]pyrene in mouse skin

Exposure to inorganic arsenic in drinking water is linked to skin, lung and bladder cancer in humans. The mechanism of arsenic-induced cancer is not clear, but exposure to arsenic and polycyclic arylhydrocarbons (PAH) is more carcinogenic than exposure to either type of carcinogen alone. Arsenic can also generate reactive oxygen species, suggesting that oxidation of DNA may play a role in carcinogenesis. Oxidization of guanosines in polyG tracts is known to cause frameshift mutations, and such events can be detected in situ using the G11 placental alkaline phosphatase (PLAP) transgenic mouse model, which reports frameshift mutations in a run of 11 G:C basepairs by generating cells containing heat-resistant alkaline phosphatase activity. PAH can also induce frameshift mutations. In the study described here, FVB/N mice carrying the G11 PLAP transgene were crossed to C57Bl/6 mice. Half of the hybrid mice were given drinking water with sodium arsenite (10 mg/L) for 10 weeks. Half of the arsenic treated mice were also exposed to benzo[a]pyrene (BaP) by skin painting (500 nmol/week) for 8 weeks. Another group of mice was exposed to BaP but not arsenic. The effect on frameshift mutation was assessed by staining sections of skin tissue to detect cells with PLAP activity. Arsenic alone had no significant effect. On average, mice given BaP alone had approximately three times more PLAP-positive (PLAP+) cells. By contrast, mice exposed to both arsenic and BaP exhibited 10-fold more PLAP+ cells in the skin, and these cells were often arranged in large clusters, suggesting derivation from stem cells. Whereas combined treatment produced more PLAP+ cells, stable BaP adduct levels and arsenic burdens were not higher in mice exposed to both agents compared to mice exposed to either one agent or the other.

Mlh1 mediates tissue-specific regulation of mitotic recombination

Mitotic recombination (MR) between chromosome homologs in somatic cells is a major pathway to the loss of heterozygosity (LOH), which may cause cancer if tumor suppressor genes are involved. MR can be suppressed by DNA sequence heterology (homeology) in hybrid mice from matings between species or between subspecies. We now report that MR is relatively suppressed in F1 hybrids between inbred strains C57BL/6 and 129S2. The frequency of MR in fibroblasts is lower in F1 hybrid mice than in either of the two parental strains. However, MR in T cells is not affected by strain background. Thus, relatively small genetic differences are capable of restricting MR in a tissue-specific manner. Using Mlh1-deficient mice, we tested the role of mismatch repair in MR in two isogenic cell types. In fibroblasts of C57BL/6 x 129S2 F1 mice, the suppression of MR is alleviated in the absence of MLH1. In contrast, MR is not affected by Mlh1 status in T cells. The frequency of point mutations at the reporter gene loci Aprt and Hprt, on the other hand, is significantly increased in both T cells and fibroblasts of Mlh1(-/-) mice. Thus, different cell types respond differently to MLH1 deficiency, and the contribution of MR to tumorigenesis may be tissue-dependent in the absence of mismatch repair.

The Plk3-Cdc25 circuit

Polo-like kinases (Plks) are key regulators of the cell cycle, especially in the G2 phase and mitosis. They are incorporated into signaling networks that regulate many aspects of the cell cycle, including but not limited to centrosome maturation and separation, mitotic entry, chromosome segregation, mitotic exit, and cytokinesis. The Plks have well conserved 30-amino-acid elements, designated polo boxes (PBs), located in their carboxyl-termini, which with their flanking regions constitute a functional Polo-box domain (PBD). Members of the Plk family exist in a variety of organisms including Polo in Drosophila melanogaster; Cdc5 in Saccharomyces cerevisiae; Plo1 in Schizosaccharomyces pombe; Plx1 in Xenopus laevis; and Plk1, Snk/Plk2, Fnk/Prk/Plk3, and Sak in mammals. Polo, Cdc5, and Plo1 are essential for viability. The Plks can be separated into two groups according to their functions. The first group (Polo, Cdc5, plo1, Plx1, and Plk1) primarily performs mitotic functions, whereas the second group (Plk2 and Plk3) appears to have additional functions during the G1, S, and G2 phases of the cell cycle. Several contributions to this issue will discuss different aspects of Plk involvement in cell-cycle regulation. This review, therefore, will focus on the role of Plk3 in regulating Cdc25 phosphatase function and its effect on the cell cycle.

Restoration of an absent G1 arrest and protection from apoptosis in embryonic stem cells after ionizing radiation

Response to DNA damage and cell-cycle regulation differ markedly between embryonic stem (ES) cells and somatic cells. ES cells require exquisitely sensitive mechanisms to maintain genomic integrity and do so, in part, by suppressing spontaneous mutation. Spontaneous mutation frequency in somatic cells is approximately 10(-4) compared with 10(-6) for ES cells. ES cells also lack a G(1) checkpoint and are hypersensitive to IR and other DNA-damaging agents. These characteristics facilitate apoptosis and the removal of cells with a mutational burden from the population, thereby keeping the population free of damaged cells. Here, we identify signaling pathways that are compromised and lead to a natural absence of aG(1) arrest in ES cells after DNA damage. The affected pathways are those mediated by p53 and p21 and by ATM, Chk2, Cdc25A, and Cdk2. In ES cells, Chk2 kinase is not intranuclear as in somatic cells but is sequestered at centrosomes and is unavailable to phosphorylate Cdc25A phosphatase and cause its degradation. Although ectopic expression of Chk2 does not rescue the p53/p21 pathway, its expression is sufficient to allow it to phosphorylate Cdc25A, activate downstream targets, restore a G(1) arrest, and protect the cell from apoptosis.

Accelerating drug discovery

Although the evolution of '-omics' methodologies is still in its infancy, both the pharmaceutical industry and patients could benefit from their implementation in the drug development process

Major histocompatibility gene therapy: the importance of haplotype and beta 2-microglobulin

OBJECTIVES/HYPOTHESIS

Alloantigen gene therapy with the genes for the Class I major histocompatibility complex (MHC) HLA-B7 and beta 2-microglobulin in HLA-B7-negative patients has potential efficacy in the treatment of head and neck cancer, although the mechanism of response is unclear. Whether tumor regression is due to a response to HLA-B7 in HLA-B7-negative patients (i.e., due to "foreign" antigen) or simply to MHC overexpression is unknown. Therefore, a mouse model was used to compare tumor growth following syngeneic MHC transfection to alloantigenic MHC transfection. The importance of the beta 2-microglobulin gene was also evaluated.

STUDY DESIGN

Prospective animal study.

METHODS

The head and neck cancer cell line SCC-VII that grows in immunocompetent C3H mice, which are MHC haplotype H2-K, was used. Stable transfections were made with H2-K, H2-K, and beta 2-microglobulin in the SCC-VII cells. To test the importance of MHC "foreignness," mice were injected with SCC-VII cells, SCC-VII plus H2-K plus beta 2-microglobulin transfected cells, and SCC-VII plus H2-K plus beta 2-microglobulin transfected cells. To evaluate beta 2-microglobulin, mice were injected with SCC-VII cells, SCC-VII plus H2-K plus beta 2-microglobulin transfected cells, SCC-VII plusH2-K transfected cells, and SCC-VII plus beta 2-microglobulin transfected cells. Tumor growth in all groups was compared statistically.

RESULTS

Major histocompatibility complex foreignness was a part of the antitumor response. Foreign MHC routinely abrogated tumor growth, whereas syngeneic MHC only slowed tumor growth. beta 2-microglobulin aided the MHC tumor inhibition but did not inhibit tumor without the MHC.

CONCLUSION

The antitumor response was greater when the MHC gene used was foreign. beta 2-microglobulin increased the efficacy of MHC gene therapy. Both of these findings are important when designing clinical trials of immunologically based gene therapies for head and neck cancer.

Cdc25C phosphorylation on serine 191 by Plk3 promotes its nuclear translocation

Mitosis in human cells is initiated at the end of G2 by activation of the Cdc2/cyclin B complex. Activation occurs by dephosphorylation of the inhibitory residues, threonine 14 (T14) and tyrosine 15 (Y15), on Cdc2 by the Cdc25C phosphatase. Entry into mitosis is regulated by the subcellular relocalization of Cdc2/cyclin B, which is rapidly imported into the nucleus at the end of G2. Here, we show that polo-like kinase 3 (Plk3) is able to phosphorylate Cdc25C primarily on S191, and to a lesser extent on S198 in vitro, both of which are within a nuclear exclusion motif. Following transfection, the S191D Cdc25C mutant leads to an enhanced accumulation of Cdc25C in the nucleus, while the S191A mutant facilitated the Cdc25C nuclear exclusion. Furthermore, translocation of Cdc25C to the nucleus was accompanied by a decrease in Cdc2 phosphorylation on Y15. Plk3-WT overexpression led to a sharp increase in Cdc25C nuclear accumulation, while Plk3-KD overexpression failed to do so. The effect of Plk3 overexpression on Cdc25C was reversed by coexpression of a Plk3 siRNA. These results support a role for the polo kinases in coordinating the translocation and perhaps the timing of both Cdc25C and its target Cdc2/cyclin B to the nucleus upon entry into mitosis.

Impaired expression of an organic cation transporter, IMPT1, in a knockout mouse model for kidney stone disease

The imprinted multimembrane-spanning polyspecific transporter-like gene 1 ( IMPT1) encodes a predicted protein with organic cation transport capabilities. As a first step in understanding the function of IMPT1, we identified the renal structures expressing this gene in knockout mice with adenine phosphoribosyltransferase (APRT) deficiency and 2,8-dihydroxyadenine (DHA) nephrolithiasis. IMPT1 mRNA was not detected using a standard in situ hybridization (ISH) protocol, but we observed intense staining in cortico-medullary tubules and glomeruli in wild-type mice using an improved reverse transcription-polymerase chain reaction (RT-PCR) ISH procedure. IMPT1 mRNA expression was significantly decreased in the cortical region in kidney sections from APRT-deficient male mice. APRT-deficient female mice are less severely affected by DHA-induced kidney stone disease, and we observed only a modest reduction in IMPT1 expression in kidneys from these mice. IMPT1 expression in APRT heterozygous mice was comparable to that in wild-type mice, suggesting imprinting of one of the parental alleles. These findings suggest that decreased IMPT1 mRNA expression may contribute to the impaired renal function in APRT-deficient male mice, and that RT-PCR ISH is a valuable tool for localizing the site of expression of transcripts that are not detectable using standard ISH procedures.

Induction of alpha-catenin, integrin alpha3, integrin beta6, and PDGF-B by 2,8-dihydroxyadenine crystals in cultured kidney epithelial cells

BACKGROUND

Homozygous adenine phosphoribosyltransferase (APRT) deficiency is associated with 2,8-dihydroxyadenine (DHA) nephrolithiasis. Using whole kidney RNA from Aprt knockout mice, we previously showed that the renal deposition of DHA leads to changes in the expression of genes involved in tissue injury. To determine the cellular basis for these changes, we investigated gene expression in cultured human kidney (NHK-C) and African green monkey (BSC-1) epithelial cells exposed to DHA or calcium oxalate monohydrate (COM) crystals.

METHODS

First-strand cDNAs, synthesized from mRNA isolated from treated and untreated cells, were hybridized to membrane-bound cDNA arrays containing 588 genes associated with various physiological and pathological processes. Changes in gene expression were confirmed by reverse transcription PCR.

RESULTS

Twenty-seven percent of the array cDNAs were expressed in untreated NHK-C cells at varying levels relative to a housekeeping gene. The expression of three adhesion molecules (alpha-catenin, integrin alpha3, and integrin beta6) and platelet-derived growth factor B (PDGF-B) was elevated following exposure of NHK-C cells to DHA. Increased expression of the adhesion molecules was also observed in BSC-1 cells, but PDGF-B expression could not be detected. COM crystals also stimulated the expression of these four genes in NHK-C cells, but the expression profile was quantitatively different compared with DHA.

CONCLUSIONS

These findings suggest that DHA crystals stimulate the expression of specific genes in kidney epithelial cells and that the pathways for DHA-induced cell injury may be similar to those for COM crystals. The induction of adhesion molecules and PDGF-B may affect cell-cell or cell-matrix interactions and/or alter the actin cytoskeleton. These alterations may ultimately contribute to crystal-induced renal injury.

Alloantigen gene therapy for head and neck cancer: evaluation of animal models

BACKGROUND

Human trials of alloantigen gene therapy, using the class I major histocompatibility complex (MHC) HLA-B7, have demonstrated the potential efficacy of this treatment for head and neck cancer. Its mechanism remains unclear. An immune-competent mouse model of MHC gene therapy to test factors potentially important to the tumor response is needed.

METHODS

Two cell lines were used, B4B8 cells that grow in Balb/c mice and SCC-VII cells that grow in C3H mice. The mouse MHC H2-K(b) was used as the therapeutic gene, because it is an alloantigen to both mice strains. Plasmids that encode the H2-K(b) cDNA were prepared, and the cell lines were transfected. Mice were injected subcutaneously with naive cells to determine the tumor kinetics and serve as controls. Mice were injected with H2-K(b) transfected cells and tumor growth was compared with controls. Mice that did not grow tumor were rechallenged with naive cells to assess for tumor immunity. Mice were injected with transfected and naive cells admixed to determine whether the concentration of the alloantigen is important.

RESULTS

B4B8 tumors grew slowly, whereas SCC-VII tumors grew rapidly. Transfection with H2-K(b) plasmid prevented or inhibited tumor growth of both the B4B8 and SCC-VII tumors. This growth inhibition was independent of the number of cells injected. In the mice that did not grow tumor, tumor immunity was demonstrated after challenge with naive cells in both models. There was no relationship between induction of immunity and the timing of the challenge or initial cell quantity. The mice injected with a mixture of naive and transfected cells grew tumor, although growth was delayed in the B4B8 model.

CONCLUSIONS

The results demonstrate that the two mouse models can serve as a rapid and slow growing tumor model of alloantigen gene therapy. In addition, it was noted that initial tumor cell number is not a significant factor for predicting tumor response and demonstrated that in both of these models alloantigen gene therapy results in significant antitumor immunity.

Antitumor effect of secreted Flt3-ligand can act at distant tumor sites in a murine model of head and neck cancer

The Flt3 ligand (Flt3-L) manifests antitumor activity, presumably due to its capacity to recruit dendritic cells and cause their proliferation. To assess whether local production of Flt3-L can mediate a "distant bystander" effect, murine B4B8 squamous cell carcinoma cells were transfected with a plasmid encoding a secretory form of Flt3-L to produce B4B8FL cells. Similarly, B4B8FL and B4B8 cells were transfected with herpes simplex virus thymidine kinase (HSVTK) to produce B4B8TK and B4B8FL/TK cells, which should be sensitive to ganciclovir (GCV), to know whether the effects of Flt3-L and HSVTK/GCV would be synergistic. To test for a distant bystander effect in vivo, B4B8FL, B4B8TK, and B4B8FL/TK cells were injected subcutaneously into the left flank of syngeneic Balb/c mice, and naïve B4B8 cells were injected into the right flank. The formation of tumors derived from B4B8FL and B4B8FL/TK cells was significantly delayed in both flanks compared with naïve B4B8 and B4B8TK cells. Growth of B4B8TK tumors in the ipsilateral flank was retarded following GCV treatment, but in contrast to B4B8FL and B4B8FL/TK cells, no distant bystander effect in the contralateral flank was observed. Immunohistochemistry showed lymphocytic infiltrates in both flanks of the B4B8FL and B4B8FL/TK groups. The data indicate that in these cells, local secretion of Flt3-L is sufficient to evoke a distant bystander effect but that expression of HSVTK, even after GCV administration, is not. Furthermore, the combination of local Flt3-L and HSVTK production, together with GCV administration, does not enhance the distant bystander effect produced by Flt3-L alone.

Gender- and age-dependent changes in kidney androgen protein mRNA expression in a knockout mouse model for nephrolithiasis

Kidney androgen-regulated protein (Kap) is the most abundant protein in the mouse kidney, but its function is unknown. We previously observed a significant decrease in Kap mRNA expression in whole kidney tissue from male mice with adenine phosphoribosyltransferase (APRT) deficiency and 2,8-dihydroxyadenine (DHA) nephrolithiasis. The disease phenotype is more severe in male mice and is age-dependent. To identify the cellular basis for differential Kap expression, we used in situ hybridization (ISH) and reverse transcription-polymerase chain reaction ISH (RT-PCR ISH) to identify the cell types expressing this mRNA in paraffin-embedded kidney sections. In 1-month-old wild-type male mice, Kap was detected primarily in S3 proximal tubule segments, but expression was very low in female mice. In 1-month-old APRT-deficient male mice, Kap expression was decreased significantly and was undetectable in female mice. Kap mRNA was not detected in 3- or 6-month-old mice using our standard ISH protocol, but we observed intense cytoplasmic staining in S3 proximal tubules in wild-type male mice of these age groups using an improved RT-PCR ISH procedure. Our studies demonstrate age-, gender-, and APRT genotype-dependent changes in Kap mRNA expression in mouse kidney. Kap expression is under multihormonal control, and hormonal changes in DHA-induced nephrolithiasis may account for the decreased Kap expression in APRT-deficient mice.