Ruth A. Gjerset

DNA Damage Disrupts the p14ARF-B23(Nucleophosmin) Interaction and Triggers a Transient Subnuclear Redistribution of p14ARF

The p14 alternate reading frame (ARF) tumor suppressor plays a central role in cancer by binding to mdm2 (Hdm2 in humans) and enhancing p53-mediated apoptosis following DNA damage and oncogene activation. It is unclear, however, how ARF initiates its involvement in the p53/mdm2 pathway, as p53 and mdm2 are located in the nucleoplasm, whereas ARF is largely nucleolar in tumor cells. We have used immunofluorescence and coimmunoprecipitation to examine how the subnuclear distribution and protein-protein interactions of ARF change immediately after DNA damage and over the time course of the DNA damage response in human tumor cells. We find that DNA damage disrupts the interaction of ARF with the nucleolar protein B23(nucleophosmin) and promotes a transient p53-independent translocation of ARF to the nucleoplasm, resulting in a masking of the ARF NH2 terminus that correlates with the appearance of ARF-Hdm2 complexes. The translocation also results in an unmasking of the ARF COOH terminus, suggesting that redistribution disrupts a nucleolar interaction of ARF involving this region. By 24 hours after irradiation, DNA repair has ceased and the pretreatment immunofluorescence patterns and complexes of ARF have been restored. Although the redistribution of ARF is independent of p53 and likely to be regulated by interactions other than Hdm2, ARF does not promote UV sensitization unless p53 is expressed. The results implicate the nucleolus and nucleolar interactions of the ARF, including potentially novel interactions involving its COOH terminus as sites for early DNA damage and stress-mediated cellular events.

Protein kinase CK2 is a central regulator of topoisomerase I hyperphosphorylation and camptothecin sensitivity in cancer cell lines

Topoisomerase I (topo I) is required to unwind DNA during synthesis and provides the unique target for camptothecin-derived chemotherapeutic agents, including Irinotecan and Topotecan. While these agents are highly effective anticancer agents, some tumors do not respond due to intrinsic or acquired resistance, a process that remains poorly understood. Because of treatment toxicity, there is interest in identifying cellular factors that regulate tumor sensitivity and might serve as predictive biomarkers of therapy sensitivity. Here we identify the serine kinase, protein kinase CK2, as a central regulator of topo I hyperphosphorylation and activity and cellular sensitivity to camptothecin. In nine cancer cell lines and three normal tissue-derived cell lines we observe a consistent correlation between CK2 levels and camptothecin responsiveness. Two other topo I-targeted serine kinases, protein kinase C and cyclin-dependent kinase 1, do not show this correlation. Camptothecin-sensitive cancer cell lines display high CK2 activity, hyperphosphorylation of topo I, elevated topo I activity, and elevated phosphorylation-dependent complex formation between topo I and p14ARF, a topo I activator. Camptothecin-resistant cancer cell lines and normal cell lines display lower CK2 activity, lower topo I phosphorylation, lower topo I activity, and undetectable topo I/p14ARF complex formation. Experimental inhibition or activation of CK2 demonstrates that CK2 is necessary and sufficient for regulating these topo I properties and altering cellular responses to camptothecin. The results establish a cause and effect relationship between CK2 activity and camptothecin sensitivity and suggest that CK2, topo I phosphorylation, or topo I/p14ARF complex formation could provide biomarkers of therapy-responsive tumors.

Comparison of gene therapy with interleukin-2 gene modified fibroblasts and tumor cells in the murine CT-26 model of colorectal carcinoma.

We compared the efficacy of gene therapy mediated by interleukin-2 (IL-2) gene-modified tumor cells to gene therapy mediated by IL-2 transduced fibroblasts in the CT-26 model of murine colorectal carcinoma. We transduced CT-26 tumor cells and BALB/c 3T3 fibroblasts with three different retroviral vectors using three different promoters for the human IL-2 gene: DC/TKIL-2 (thymidine kinase promoter), LXSN-iIL2 (long terminal repeat promoter), and LNCX-iIL2 (cytomegalovirus promoter). These transductions resulted in CT-26 and 3T3 subclones that secreted different amounts of IL-2. Immunization of animals with either CT-26/IL-2 cells or with fibroblast/IL-2 cells mixed with CT-26 induced similar levels of immunity that protected 62-82% of animals against a subsequent tumor challenge with parental CT-26. However, mice developed tumors at the site of inoculation in 46% of the animals immunized with CT-26/IL-2 cells. In a separate experiment, CT-26/IL-2 cells were exposed to 6,000 cGy of gamma irradiation to prevent tumor growth at the site of inoculation. Although the CT-26/IL-2 cells continued to secrete IL-2 after irradiation, they were no longer effective at inducing antitumor immunity. In contrast, both irradiated and nonirradiated fibroblast/IL-2 cells, mixed with irradiated CT-26, were equally effective at inducing antitumor immunity. These data suggest that in the CT-26 model, fibroblast-mediated IL-2 gene therapy has advantages for the induction of antitumor immunity and abrogation of tumorigenic potential at the site of inoculation compared with tumor cell-mediated IL-2 gene therapy.

Tumor Suppression and Therapy Sensitization of Localized and Metastatic Breast Cancer by Adenovirus p53

We have examined the effects of a replication-defective adenovirus encoding p53 (RPR/INGN 201 [Ad5CMV-p53]; Adp53), alone or in combination with the breast cancer therapeutic doxorubicin (Adriamycin), to suppress growth and induce apoptosis in breast cancer cells in vitro. We have also examined the in vivo effect of intratumoral administration of Adp53, alone or in combination with doxorubicin, to suppress the growth of established subcutaneous MDA-MB-435 breast cancer tumors. Finally, using the MDA-MB-435 orthotopic model of metastatic breast cancer, we have examined the effect of systemic administration of Adp53, alone or in combination with doxorubicin, to reduce the incidence of metastases. We find that whereas in vitro treatment of cells with Adp53 reduces [(3)H]thymidine incorporation by about 90% at 48 hr, cell viability at 6 days is reduced by only some 50% relative to controls. Although apoptosis is detectable in Adp53-treated cultures, these results suggest that a large fraction of Adp53-treated cells merely undergo reversible cell cycle arrest. Combined treatment with Adp53 and doxorubicin results in a greater than additive loss of viability in vitro and increased apoptosis. In vivo, locally administered Adp53 suppresses growth of established subcutaneous tumors in nude mice and suppression is enhanced by doxorubicin. In the metastatic breast cancer model, systemic administration of Adp53 plus doxorubicin leads to a significant reduction in the incidence of metastases relative to Adp53 or doxorubicin alone. Taken together, these data indicate an additive to synergistic effect of Adp53 and doxorubicin for the treatment of primary and metastatic breast cancer.

Growth suppression by a p14 ARF exon 1β adenovirus in human tumor cell lines of varying p53 and Rb status

We have analyzed the ability of an adenoviral vector encoding the exon 1β region of the p14ARF tumor suppressor (ARF) to suppress the growth and viability of an array of tumor cell lines of various origins and varying p53 and Rb status, in order to establish the clinical potential of ARF. An important activity of ARF is regulation of p53 stability and function through binding to the mdm2 protein. By sequestering mdm2, ARF may promote growth suppression through the Rb pathway as well because mdm2 can bind to Rb and attenuate its function. Whereas the high frequency of ARF gene deletion in human cancers, accounting for some 40% of cancers overall, suggests that ARF would be a strong candidate for therapeutic application, the possible dependence of ARF activity on p53 and Rb function presents a potential limitation to its application, as these functions are often impaired in cancer. We show here that a replication-defective adenovirus, Ad1β, encoding the exon 1β region of ARF is most effective in tumor cells expressing endogenous wild-type p53. Nevertheless, Ad1β suppresses tumor cell growth and viability in vitro and in vivo, inducing G1 or G2 cell cycle arrest and cell death even in tumor cells lacking both functional Rb and p53 pathways, and independently of induction of the p53 downstream targets, p21, bax, and mdm2. These results point to an activity of ARF in human tumor cells that is independent of Rb or p53, and suggest that therapeutic applications based on ARF would have a broad clinical application in cancer.

Construction and characterization of retroviral vectors for interleukin-2 gene therapy.

Summary Several investigators have employed interleukin-2 (IL-2) gene transfer to enhance the immunogenicity of tumor cell vaccines. We describe in this report the construction and characterization of retroviral vectors for IL-2 gene therapy. Human IL-2 cDNA with a chimeric rat preproinsulin/IL-2 DNA leader sequence was sub-cloned into the pLXSN (long terminal repeat promoter) and pLNCX (cytomegalovirus [CMV] promoter) vectors to generate the plasmids pLXSN-iIL2 and pLNCX-iIL2, respectively. Human IL-2 cDNA with a chimeric human tissue factor/IL-2 DNA leader sequence was utilized to construct the vector pLXSN-tIL2. The levels of IL-2 secreted by transduced tumor cells and fibroblasts were evaluated by enzyme-linked immunosorbent assay (ELISA) of culture supernatants and compared with those of normal peripheral blood mononuclear cells (PBMC) activated in vitro with calcium ionophore and phorbol 12-myristate 13-acetate. The highest levels of IL-2 secreted by transduced tumor cells (760 units/106 cells/24 h), adult fibroblasts (625 units/106 cells/24 h), and embryonic fibroblasts (3,975 units/106 cells/24 h) were 150− to 1,000-fold higher than that secreted by the activated PBMC (4 units/106 cells/24 h). Similar levels of IL-2 were expressed by human fibroblasts transduced with pLXSN vectors employing the preproinsulin (pLXSN-iIL2) or tissue factor (pLXSN-tIL2) leader sequences (range in IL-2 units/106 cells/24 h pLXSN-iIL2 = 375–625 vs. pLXSN-tIL2 = 90–440). Because IL-2-transduced cells for clinical applications are generally irradiated to prevent cellular proliferation, we evaluated the effects of radiation on IL-2 production. Radiation doses between 1,500 and 10,000 cGy resulted in gradual decreases in IL-2 secretion by transduced cells. The range of the decrease in IL-2 secretion was 7–11% by day 7, 0–29% by day 14, and 25–50% by day 35. For clinical applications, stable production of the vector in high concentrations is an important consideration. The retroviral vector pLXSN-tIL2 produced the highest viral titer and was chosen for further characterization. Southern blot analysis of SacI-digested genomic DNA from the LXSN-tIL2 producer cell line and SacI-digested pLXSN-tIL2 plasmid DNA revealed the expected 3.2-kbp fragment, suggesting the absence of transgene rearrangement and the suitability of this vector as a candidate for clinical applications.

Characterization of a new human glioblastoma cell line that expresses mutant P53 and lacks activation of the PDGF pathway

SummaryWe have established and characterized a new glioblastoma cell line, termed GT9, from a biopsy sample of a female adult patient with glioblastoma multiforme. The line has now undergone over 60 passages and has been successfully cultured after cryopreservation. Immunofluorescence analyses with a panel of monoclonal antibodies were positive for glial fibrillary acidic protein and vimentin, and negative for neurofilament, galactocerebroside, and fibronectin, a pattern typical of glial cells. Based on a tetraploid, the composite karyotype of GT9 cells included the loss of chromosome 10, gain of chromosome 7, and the presence of double minute chromosomes, three of the most common karyotypic abnormalities in glioblastoma. Sequence analysis of p53 cDNA revealed a homozygous double mutation at codon 249 (commonly mutated in aflatoxin-associated hepatocellular carcinoma) and codon 250. Moreover, there was a complete absence of wild-type p53. However, unlike the majority of human glioblastomas previously described, the expression of platelet-derived growth factor-B (PDGF-B), a potent mitogenic autocrine factor, was low in GT9 cells. The expression and phosphorylation of c-Jun and Jun-B, downstream mediators of the PDGF pathway, were also low. Thus, deregulation of the PDGF pathway does not appear to be involved in the pathogenesis of the GT9 glioblastoma. Conversely, Jun-D, a negative regulator of cell growth, was also low. In addition, Phosphorylated Egr-1, a recently reported suppressor of PDGF-B/v-sis-transformed cells, was also low, suggesting that the lack of activation of the PDGF pathway was not due to these suppressive mechanisms. The circumstance of a weak or inactive PDGF-B autocrine mechanism in human glioblastoma paired with a homozygously altered p53 suggests that the loss of suppressor function of p53 may be a major contribution to the transformed phenotype of these cells.