Analysis of cell cycle checkpoint status in mammalian cells

Immediate transfection of patient-derived leukemia: a novel source for generating cell-based vaccines

Background

The production of cell-based cancer vaccines by gene vectors encoding proteins that stimulate the immune system has advanced rapidly in model systems. We sought to develop non-viral transfection methods that could transform patient tumor cells into cancer vaccines, paving the way for rapid production of autologous cell-based vaccines.

Methods

As the extended culture and expansion of most patient tumor cells is not possible, we sought to first evaluate a new technology that combines electroporation and chemical transfection in order to determine if plasmid-based gene vectors could be instantaneously delivered to the nucleus, and to determine if gene expression was possible in a cell-cycle independent manner. We tested cultured cell lines, a primary murine tumor, and primary human leukemia cells from diagnostic work-up for transgene expression, using both RFP and CD137L expression vectors.

Results

Combined electroporation-transfection directly delivered plasmid DNA to the nucleus of transfected cells, as demonstrated by confocal microscopy and real-time PCR analysis of isolated nuclei. Expression of protein from plasmid vectors could be detected as early as two hours post transfection. However, the kinetics of gene expression from plasmid-based vectors in tumor cell lines indicated that optimal gene expression was still dependent on cell division. We then tested to see if pediatric acute lymphocytic leukemia (ALL) would also display the rapid gene expression kinetics of tumor cells lines, determining gene expression 24 hours after transfection. Six of 12 specimens showed greater than 17% transgene expression, and all samples showed at least some transgene expression.

Conclusion

Given that transgene expression could be detected in a majority of primary tumor samples analyzed within hours, direct electroporation-based transfection of primary leukemia holds the potential to generate patient-specific cancer vaccines. Plasmid-based gene therapy represents a simple means to generate cell-based cancer vaccines and does not require the extensive infrastructure of a virus-based vector system.

Hepatitis C virus core protein stimulates hepatocyte growth: correlation with upregulation of wnt-1 expression

Hepatitis C virus (HCV) core protein has been implicated in the development of human hepatocellular carcinoma (HCC). Here we report that expression of HCV core protein by transient transfection increased cell proliferation, DNA synthesis, and cell cycle progression in Huh-7 cells, a human HCC-derived cell line. Culture supernatant from transfected cells also harbored a growth-promoting effect. Moreover, a full-length HCV replicon, but not a subgenomic replicon devoid of the core gene, significantly stimulated growth of transiently transfected Huh-7.5 cells. However, growth of the subgenomic replicon-containing Huh-7.5 cells could be stimulated by secondary transfection with core gene but not other structural genes present in the full-length replicon. Microarray analysis revealed threefold or more transcriptional changes in 372 of 12,500 known human genes in core protein expressing Huh-7 cells, with most genes involved in cell growth or oncogenic signaling, being upregulated rather than downregulated. Of particular interest is the marked upregulation of both wnt-1 and its downstream target gene WISP-2. Indeed, small interfering RNA against wnt-1 blunted growth stimulation by core gene, whereas transfection of Huh-7 cells with the wnt-1 gene sufficed to promote cell proliferation. Consistent with secretion of the wnt-1 protein, conditioned medium from wnt-1 transfected cells accelerated cell growth. In conclusion, HCV core protein induces Huh-7 cell proliferation whether alone or in the context of HCV replication, which is at least partly mediated by transcriptional upregulation of growth-related genes, in particular wnt-1.

Hyperoxia-induced apoptosis does not require mitochondrial reactive oxygen species and is regulated by Bcl-2 proteins

Exposure of animals to hyperoxia results in lung injury that is characterized by apoptosis and necrosis of the alveolar epithelium and endothelium. The mechanism by which hyperoxia results in cell death, however, remains unclear. We sought to test the hypothesis that exposure to hyperoxia causes mitochondria-dependent apoptosis that requires the generation of reactive oxygen species from mitochondrial electron transport. Rat1a cells exposed to hyperoxia underwent apoptosis characterized by the release of cytochrome c, activation of caspase-9, and nuclear fragmentation that was prevented by the overexpression of Bcl-X(L.) Murine embryonic fibroblasts from bax(-/-) bak(-/-) mice were resistant to hyperoxia-induced cell death. The administration of the antioxidants manganese (III) tetrakis (4-benzoic acid) porphyrin, ebselen, and N-acetylcysteine failed to prevent cell death following exposure to hyperoxia. Human fibrosarcoma cells (HT1080) lacking mitochondrial DNA (rho(0) cells) that failed to generate reactive oxygen species during exposure to hyperoxia were not protected against cell death following exposure to hyperoxia. We conclude that exposure to hyperoxia results in apoptosis that requires Bax or Bak and can be prevented by the overexpression of Bcl-X(L). The mitochondrial generation of reactive oxygen species is not required for cell death following exposure to hyperoxia.

Mammary epithelial cell-cycle progression via the alpha(2)beta(1) integrin: unique and synergistic roles of the alpha(2) cytoplasmic domain

The alpha(2)beta(1) integrin supports cell-cycle progression of mammary epithelial cells adherent to type I collagen matrices. Integrin collagen receptors containing the alpha(2) cytoplasmic domain stimulated expression of cyclin E and cyclin-dependent kinase (cdk)2, resulting in cyclin E/cdk2 activation in the absence of growth factors other than insulin. Integrin collagen receptors in which the alpha(2) cytoplasmic domain was replaced by the alpha(1) cytoplasmic domain or an alpha(2) subunit cytoplasmic domain truncated after the GFFKR sequence failed to stimulate cyclin E/cdk2 activation or entry into S phase in the absence of growth factors. Although overexpression of cyclins D or E or cdk2 in cells expressing the integrin collagen receptor with the alpha(1)-integrin cytoplasmic domain did not restore G(1) progression when mammary epithelial cells adhered to type I collagen, co-expression of cyclin E and cdk2 did rescue the ability of the transfectants to enter S phase. Activation of cyclin E/cdk2 complex by mammary epithelial cells required synergy between adhesion mediated by an integrin collagen receptor containing the alpha(2)-integrin subunit cytoplasmic domain and the insulin receptor.

Specific residues within the alpha 2 integrin subunit cytoplasmic domain regulate migration and cell cycle progression via distinct MAPK pathways

The alpha(2) integrin subunit cytoplasmic domain is necessary for epidermal growth factor (EGF)-stimulated chemotactic migration and insulin-dependent entry into S-phase of mammary epithelial cells adherent to type I collagen. Truncation mutants revealed that the seven amino acids, KYEKMTK, in addition to the GFFKR motif were sufficient for these functions. Mutation of tyrosine 1134 to alanine inhibited the ability of the cells to phosphorylate p38 MAPK and to migrate in response to EGF but had only a modest effect on the ability of the cells to induce sustained phosphorylation of the ERK MAPK, to up-regulate cyclin E and cdk2 expression, and to enter S-phase when adherent to type I collagen. Conversely, mutation of the lysine 1136 inhibited the ability of the cells to increase cyclin E and cdk2 expression, to maintain long term phosphorylation of the ERK MAPK, and to enter S-phase but had no effect on the ability of the cells to phosphorylate the p38 MAPK or to migrate on type I collagen in response to EGF. Methionine 1137 was essential for both migration and entry into S-phase. Thus, distinctly different structural elements of the alpha(2) integrin cytoplasmic domain are required to engage the signaling pathways leading to cell migration or cell cycle progression.

TGF-beta1 regulates the expression of multiple max-interacting transcription factors in Balb/MK cells: implications for understanding the mechanism of action of TGF-beta1

Appropriate transforming growth factor-beta1 (TGF-beta1) signaling is required to preserve homeostasis of diverse tissues during development. At the cellular level, one function of TGF-beta1 that is critical for preserving homeostasis is the ability to arrest cell growth. TGF-beta1 arrests growth by blocking the function of the c-myc proto-oncogene. c-myc function is determined by the level of c-myc expression relative to other Max-interacting transcription factors, and TGF-beta1 has been shown to inhibit c-myc expression by inhibiting c-myc transcription. However, whether TGF-beta1 might also increase the expression of a Max-interacting factor that blocks myc function by competing with myc for Max binding is not known. Therefore, we determined the effect of TGF-beta1 on the expression of Max-interacting transcription factors in Balb/MK cells. We found unexpectedly that Balb/MK cells express both N-myc and c-myc. The pattern of N-myc expression during the cell cycle differs from that of c-myc, indicating that mRNA accumulation is controlled by mechanisms specific to each gene. TGF-beta1 rapidly inhibits N-myc mRNA expression; thus N-myc is a novel target of TGF-beta1 in Balb/MK cells. More importantly, we found that TGF-beta1 induces the expression of the putative tumor suppressor genes Mad4 and Mxi1 in both the Balb/MK and Mv1Lu cell lines. Mad4 and Mxi1 are novel targets of TGF-beta1, known to inhibit cell growth by antagonizing the interaction of Myc with Max. Thus, our results suggest that the induction of Mad4 and Mxi1 may function in tandem with the inhibition of N-myc and c-myc to mediate the growth inhibitory function of TGF-beta1.

TGF-beta1 inhibits BRCA1 expression through a pathway that requires pRb

TGF-beta1 inhibits BRCA1 expression, which contradicts the model that TGF-beta1 prevents carcinogenesis by activating tumor suppressor genes. To resolve this apparent contradiction, we examined BRCA1 expression in Mv1Lu cells, a well-established model system for studying the TGF-beta1 tumor suppressor pathway. We found that inactivation of pRb by the papillomavirus type 16 E7 protein increased BRCA1 expression and abolished the ability of TGF-beta1 to inhibit BRCA1 expression. We conclude that TGF-beta1 inhibits BRCA1 expression through a pathway that requires pRb. We propose a model to explain the inhibition of BRCA1 as a target in the TGF-beta1 tumor suppressor signaling pathway. Our results suggest that the tumor suppressor functions of BRCA1 are initiated by the inactivation of pRb, and therefore that the activation of pRb by TGF-beta1 might alleviate the requirement for BRCA1 function.

Inhibition of S-phase progression by adeno-associated virus Rep78 protein is mediated by hypophosphorylated pRb

Adeno-associated virus (AAV) has an antiproliferative action on cells. We investigated the effect of the AAV replication proteins (Rep) on the cell division cycle using retroviral vectors. Rep78 and Rep68 inhibited the growth of primary, immortalized and transformed cells, while Rep52 and Rep40 did not. Rep68 induced cell cycle arrest in phases G(1) and G(2), with elevated CDK inhibitor p21 and reduced cyclin E-, A- and B1-associated kinase activity. Rep78-expressing cells were also impaired in S-phase progression and accumu lated almost exclusively with hypophosphorylated retinoblastoma protein (pRb). The differences between Rep78 and Rep68 were mapped to the C-terminal zinc finger domain of Rep78. Rep78-induced S-phase arrest could be bypassed by adenoviral E1A or papillomaviral E7 proteins but not by E1A or E7 mutants unable to bind pRb. Rb(-/-) primary mouse embryonic fibroblasts displayed a strongly reduced S-phase arrest when challenged with Rep78, compared with matched Rb(+/+) controls. These results suggest that physiological levels of active pRb can interfere with S-phase progression. We propose that the AAV Rep78 protein arrests cells within S-phase by a novel mechanism involving the ectopic accumulation of active pRb.

Radiation-induced instability and its relation to radiation carcinogenesis

PURPOSE

A model that identifies radiation-induced genetic instability as the earliest cellular event in the multi-step sequence leading to radiation-induced cancer was previously proposed. In this paper ongoing experiments are discussed which are designed to test this model and its predictions in mouse mammary epithelial cells.

RESULTS

Several lines of evidence are presented that appear to support this model: first, the development of delayed mutations in p53 following irradiation in altered growth variants; secondly, the high frequencies for the induction of both instability and transformation following irradiation in mammary epithelial cells; and finally, the demonstration that susceptibility to the induction of cytogenetic instability is a heritable trait that correlates with susceptibility to transformation and radiation-induced mammary cancer. Mice resistant to transformation and mammary cancer development are also resistant to the development of instability after irradiation. In contrast, mice sensitive to transformation and cancer are also sensitive to the development of cytogenetic instability.

CONCLUSIONS

Data from this laboratory and from the studies cited above suggest a specific, and perhaps unique, role for radiation-induced instability as a critical early event associated with initiation of the carcinogenic process.