Altered cell cycle regulation in the lens of HPV-16 E6 or E7 transgenic mice: implications for tumor suppressor gene function in development

Amphibian limb regeneration: rebuilding a complex structure

The ability to regenerate complex structures is widespread in metazoan phylogeny, but among vertebrates the urodele amphibians are exceptional. Adult urodeles can regenerate their limbs by local formation of a mesenchymal growth zone or blastema. The generation of blastemal cells depends not only on the local extracellular environment after amputation or wounding but also on the ability to reenter the cell cycle from the differentiated state. The blastema replaces structures appropriate to its proximodistal position. Axial identity is probably encoded as a graded property that controls cellular growth and movement through local cell interactions. The molecular basis is not understood, but proximodistal identity in newt blastemal cells may be respecified by signaling through a retinoic acid receptor isoform. The possibility of inducing a blastema on a mammalian limb cannot be discounted, although the molecular constraints are becoming clearer as we understand more about the mechanisms of urodele regeneration.

Analysis of the p53-mediated G1 growth arrest pathway in cells expressing the human papillomavirus type 16 E7 oncoprotein

Cells expressing human papillomavirus type 16 (HPV-16) E7, similar to those which express HPV-16 E6, are resistant to a p53-mediated G1 growth arrest. We examined the p53-mediated DNA damage response pathway in E7-expressing cells to determine the mechanism by which E7-containing cells continue to cycle. In response to DNA damage, no dramatic difference was detected in G1- or S-phase cyclin or cyclin-dependent kinase (Cdk) levels when E7-expressing cells were compared to the parental cell line, RKO. Furthermore, Cdk2 kinase activity was inhibited in both RKO cells and E7-expressing cells, while Cdk2 remained active in E6-expressing cells. However, the steady-state levels of pRB and p107 protein were substantially lower in E7-expressing cells than in the parental RKO cells or E6-expressing cells. There was no reduction in pRB mRNA levels, but the half-life of pRB in E7-expressing cells was markedly shorter. Infection of primary human foreskin keratinocytes with recombinant retroviruses expressing HPV-16 E7 resulted in a decrease in pRB protein levels, indicating this phenomenon is a consequence of E7 expression, not of immortalization or transformation. These data strongly suggest E7 interferes with the stability of pRB and p107 protein. We propose that the removal of these components of the p53-mediated G1 growth arrest pathway in E7-expressing cells contributes to the ability of E7 to overcome a p53-mediated G1 growth arrest.

Double-stranded RNA is a trigger for apoptosis in vaccinia virus-infected cells

The vaccinia virus E3L gene codes for double-stranded RNA (dsRNA) binding proteins which can prevent activation of the dsRNA-dependent, interferon-induced protein kinase PKR. Activated PKR has been shown to induce apoptosis in HeLa cells. HeLa cells infected with vaccinia virus with the E3L gene deleted have also been shown to undergo apoptosis, whereas HeLa cells infected with wild-type vaccinia virus do not. In this report, using virus recombinants expressing mutant E3L products or alternative dsRNA binding proteins, we show that suppression of induction of apoptosis correlates with functional binding of proteins to dsRNA. Infection of HeLa cells with ts23, which leads to synthesis of increased dsRNA at restrictive temperature, induced apoptosis at restrictive but not permissive temperatures. Treatment of cells with cytosine arabinoside, which blocks the late buildup of dsRNA in vaccinia virus-infected cells, prevented induction of apoptosis by vaccinia virus with E3L deleted. Cells transfected with dsRNA in the absence of virus infection also underwent apoptosis. These results suggest that dsRNA is a trigger that can initiate a suicide response in virus-infected and perhaps uninfected cells.

Neuronal Cdc2-like kinase (Nclk) binds and phosphorylates the retinoblastoma protein

The tumor suppressor retinoblastoma protein (RB) plays a central role in cellular growth regulation, differentiation, and apoptosis. Phosphorylation of RB results in a consequent loss of its ability to inhibit cell cycle progression. However, how RB phosphorylation might be regulated in apoptotic or postmitotic cells, such as neurons, remains unclear. Here we report that neuronal Cdc2-like kinase (Nclk), composed of Cdk5 and a neuronal Cdk5 activator (p25(nck5a)), can bind and phosphorylate RB. Since RB has been shown recently to associate with D-type G1 cyclins and viral oncoproteins through a common peptide sequence motif of LXCXE, Nclk binding may be mediated by a related sequence motif (LXCXXE) found in p25(nck5a). We demonstrate (i) in vitro binding of bacterially expressed p25(nck5a) to a GST-RB fusion protein, (ii) coprecipitation of GST-RB and reconstituted Cdk5.p25(nck5a), and (iii) phosphorylation of GST-RB by bacterially expressed Cdk5.p25(nck5a) kinase and by Cdk5.p25(nck5a) kinase purified from bovine brain. Finally, we show that immunoprecipitation of RB from embryonic mouse brain homogenate results in the coprecipitation of Cdk5 and that Cdk5 kinase activity is maximal during late embryonic development, a period when programmed cell death of developing neurons is greatest. Taken together, these results suggest that Nclk can bind to and phosphorylate RB in vitro and in vivo. We infer that Nclk may play an important role in regulating the activity of RB in the brain, including perhaps in apoptosing neurons.

Retinoblastoma protein in growth suppression and death protection

Puzzling new information indicates an inadequacy in our understanding of the retinoblastoma protein (RB). RB and the transcription factor E2F appear to be collaborators. RB-E2F interaction is necessary but not sufficient for growth suppression. Unbecoming of a tumor suppressor, RB has an active role in antagonizing the death response. How RB integrates its multiple functions into a tumor suppression program is still an open issue.

Potential molecular targets for manipulating the radiation response

Recent advances in our understanding of the molecular events that occur following ionizing radiation leading to DNA damage and repair, apoptosis, and cell-cycle arrests suggest new ways in which the radiation response might be manipulated. Specific targets which, if inactivated, might increase radiosensitivity include Ras, which has been implicated in the radioresistant phenotype, and components of DNA-dependent protein kinase or other molecules involved in the recognition or repair of DNA damage. In some tumors, apoptosis is an important mode of cell death following radiation, so agents that promote this may prove useful therapeutically. Conversely, side effects may result from radiation-induced apoptosis of normal tissues: for example, pneumonitis following the destruction of endothelial cells in the pulmonary vasculature. Therefore, decreasing apoptosis in these tissues may reduce late effects. It may also be possible to prevent late effects such as fibrosis by blocking the induction of certain genes such as transforming growth factor beta. Cell-cycle regulation is another area that could be manipulated to increase radiosensitivity. There is evidence that the G2 delay following radiation is important in protecting cells from death. Abolition of this delay may increase radiosensitivity, especially in cells with mutant p53 that have lost the G1 checkpoint.

Detection of group C adenovirus DNA in small-cell lung cancer with the nested polymerase chain reaction

Group C adenovirus is latent in human tissues and can malignantly transform cells. The purpose of this study was to investigate the association between this virus and lung cancer. We investigated latent adenoviral infection using the nested polymerase chain reaction and in situ hybridization in transbronchial biopsy specimens from patients with small-cell lung cancer and non-small-cell lung cancer. The polymerase chain reaction was performed on DNA extracts with two sets of primers directed at a 261-base-pair target sequence of the E1A region of the adenoviral genome. In situ hybridization was performed on histological sections using DNA representing the entire adenovirus type 5 genome. E1A target DNA was present in 11 (31%) of 35 cases of small-cell lung cancer but in none of the 40 cases of non-small-cell lung cancer (P < 0.01). Of the 11 cases found positive by PCR, 8 were positive for adenovirus DNA by in situ hybridization. Adenovirus was prominent in tumor cells in 5 of the 8 cases, and in normal epithelial cells in the 3 remaining cases. Adenovirus DNA was not detected by in situ hybridization in specimens in which E1A DNA was not detected by the polymerase chain reaction. Small-cell lung cancer has mutations or deletions in the p53 and retinoblastoma genes more frequently than are found in non-small-cell lung cancer. Therefore, we speculate that adenovirus infection might participate in the pathogenesis of SCLC by producing mutation in these genes, rather than by inhibiting the function of these proteins.

Tumor suppressor gene mutations in mice

Over the past several years, a number of human tumor suppressor genes have been cloned and characterized. Germline mutations in tumor suppressor genes strongly predispose to cancer, and they are also mutated somatically in sporadic forms of the disease. In order to create animal models for the familial cancer syndromes caused by inherited mutations in these genes as well as to determine their role in embryogenesis, the homologues of several members of this class have been mutated in the mouse. The initial characterization of the heterozygous and homozygous phenotypes caused by these mutations has led to important insights into the mechanisms by which tumor suppressor genes participate in normal development and how their loss contributes to tumorigenesis.

Cellular and molecular features of lens differentiation: a review of recent advances

In this paper, the more recent literature pertaining to differentiation in the developing vertebrate lens is reviewed in relation to previous work. The literature reviewed reveals that the developing lens has been, and will continue to be, a useful model system for the examination of many fundamental processes occurring during embryonic development. Areas of lens development reviewed here include: the induction and early embryology of the lens; lens cell culture techniques; the role of growth factors and cytokines; the involvement of gap junctions in lens cell-cell communication; the role of cell adhesion molecules, integrins, and the extracellular matrix; the role of the cytoskeleton; the processes of programmed cell death (apoptosis) and lens fibre cell denucleation; the involvement of Pax and Homeobox genes; and crystallin gene regulation. Finally, some speculation is provided as to possible directions for further research in lens development.

Apoptosis in ocular disease: a molecular overview

Apoptosis is a form of genetically programmed cell death that can be induced by a variety of different stimuli. It is often referred to as a form of cellular suicide. Typically, apoptosis is characterized by the condensation and shrinkage of the cellular nucleus and cytoplasm, followed by the complete fragmentation of the cell and subsequent phagocytosis of the debris by surrounding cells. Although important during development, and also for maintaining homeostasis in some adult tissues, apoptosis can also be associated with disease processes. Recent laboratory studies indicate that apoptosis is a mechanism of cell death in several important ocular diseases including glaucoma, retinitis pigmentosa, cataract formation, retinoblastoma, retinal ischemia, and diabetic retinopathy. This review summarizes the results of these studies and provides a brief description of some of the key molecules that are involved in the genetic regulation of apoptosis. It is possible that a complete understanding of how these molecules function may someday lead to new treatment options aimed at blocking the death of cells in a variety of ocular diseases.

Use of normal and transgenic mice to examine the relationship between terminal differentiation of intestinal epithelial cells and accumulation of their cell cycle regulators

A spatially well organized continuum of proliferation, differentiation, and death is displayed along crypt-villus units in the adult mouse small intestine. This continuum provides an opportunity to examine in vivo the mechanisms by which proliferative status changes as a function of cellular differentiation. Immunohistochemical studies of normal FVB/N mice revealed that as epithelial cells complete their terminal differentiation during a 48-72-h migration up villi, there is a marked and rapid fall in the levels of two important regulators of the G1/S transition, cyclin D1 and cyclin-dependent kinase (cdk) 2. However, cellular levels of their partners, cdk4 and cyclin E, remain unchanged as does the level of pRB. Adult FVB/N transgenic mice were studied that contained an intestinal fatty acid binding protein gene promoter (Fabpi) linked to wild type Simian virus 40 large T antigen (SV40 TAgWt) or a mutant TAg with Lys for Glu substitutions at residues 107 and 108 (SV40 TAgK107/8) that fails to bind pRB and related pocket proteins. Both transgenes are expressed only in villus enterocytes. SV40 TAgWt causes these terminally differentiated cells to re-enter the cycle. Re-entry is accompanied by a reduction in un/hypophosphorylated pRB, an induction of cyclin D1 and cdk2, but no change in cdk4, cyclin E, or E2F-1. In contrast, SV40 TAgK107/8 fails to induce re-entry and does not produce changes in un/hypophosphorylated pRB, cyclin D1, or cdk2 accumulation. These results suggest that un/hypophosphorylated pRB is an important mediator of the cell cycle arrest that normally occurs as enterocytes exit the crypt and complete their differentiation. Fabpi-directed expression of E2F-1 does not cause villus enterocytes to return to the cell cycle, alter their suppression of cyclin D1 or cdk2, or affect their state of differentiation, emphasizing the insensitivity of these cells to the effects of E2F-1. Analyses of p53(-/-) and p53(+/+) mice containing Fabpi-SV40 TAgWt and Fabpi-SV40 TAgK107/8 established that the proliferation induced by SV40 TAgWt does not require p53 and is associated with increased (p53-independent) apoptosis. The presence of cyclin E and cdk4 in differentiating villus enterocytes emphasizes that these cells retain part of their proliferative heritage expressed 24-72 h earlier in the crypt. The data suggest that down-regulation of cdk2 and/or cyclin D1 expression may be important for control of proliferative status and/or execution of terminal differentiation.

Established cell lines transformed by E2F-1 overexpression contain wild-type p53

Overexpression of genes encoding E2F transcription factors can transform some cultured cell lines and cause apoptosis of others. Apoptosis due to E2F overexpression requires the presence of wild-type p53. Cell lines in which stable E2F overexpression is possible might be expected, therefore, to contain mutant p53. In this report, it was asked whether endogenous p53 was mutant or wild-type in four established fibroblast cell lines that this laboratory previously showed stably overexpressed and were transformed by exogenous E2F-1. Unexpectedly, it was found that the p53 in these cells was wild-type by the criteria of immunoprecipitation with conformation-specific, p53 monoclonal antibodies and by transactivation of a p53-dependent reporter gene construct in transient transfection assays. These data indicate that stable overexpression of E2F-1 is possible in the presence of wild-type p53 and may result in cell transformation.

The p53 tumour suppressor gene: a mediator of a G1 growth arrest and of apoptosis

The tumour suppressor gene p53 plays a major role in the protection of cells from DNA damage. Activation of the protein in response to irradiation or genotoxic agents, and possibly by other signals, results in growth arrest at the G1 phase of the cell cycle or in apoptosis. While it has been shown that the ability of p53 to function as a sequence-specific transcriptional activator is necessary for the induction of growth arrest, the mechanism of p53-mediated apoptosis is not yet clear. It appears that under some conditions activation of the G1 checkpoint will prevent apoptosis, but the cellular environment may alter the result of p53 activation towards cell death. p53 may also directly induce apoptosis through several pathways, which may be transcriptionally dependent or independent. The outcome-a G1 arrest or apoptosis-will depend on a complex network of regulatory signals.

Apoptosis -- the story so far

The process of programmed cell death, or apoptosis, has become one of the most intensively studied topics in biological sciences in the last two decades. Apoptosis as a common and universal mechanism of cell death, distinguishable from necrosis, is now a widely accepted concept after the landmark paper by Kerr, Wyllie and Currie in the early seventies [1]. Different components of the death machinery in eukaryotes are discussed in this issue.

Functional interaction between DP-1 and p53

The cellular transcription factor DRTF1/E2F and the tumor suppressor protein p53 play important roles in controlling early cell cycle events. DRTF1/E2F is believed to coordinate and integrate the transcription of cell cycle-regulating genes, for example, those involved in DNA synthesis, with the activity of regulatory proteins, such as the retinoblastoma tumor suppressor gene product (pRb), which modulate its transcriptional activity. In contrast, p53 is thought to monitor the integrity of chromosomal DNA and when appropriate interfere with cell cycle progression, for example, in response to DNA damage. Generic DRTF1/E2F DNA binding activity and transcriptional activation arise when members of two distinct families of proteins, such as DP-1 and E2F-1, interact as DP/E2F heterodimers. In many cell types, DP-1 is a widespread component of DRTF1/E2F DNA binding activity which when expressed at high levels oncogenically transforms embryonic fibroblasts. Here, we document an association between DP-1 and p53 and demonstrate its presence in mammalian cell extracts. In vitro p53 interacts with an immunochemically distinct form of DP-1 and in vivo can regulate transcription driven by the DP-1/E2F-1 heterodimer. At the biochemical level, p53 competes with E2F-1 for DP-1, with a consequent reduction in DNA binding activity. Mutational analysis defines within DP-1 a C-terminal region required for the interaction with p53 and within p53 an N-terminal region distinct from that required to bind to MDM2. Our results establish DRTF1/E2F as a common cellular target in growth control mediated through the activities of pRb and p53 and suggest an alternative mechanism through which p53 may regulate cellular proliferation.

The herpes simplex virus major regulatory protein ICP4 blocks apoptosis induced by the virus or by hyperthermia

Cells infected with herpes simplex virus 1 (HSV-1) undergo productive or latent infection without exhibiting features characteristic of apoptosis. In this report, we show that HSV-1 induces apoptosis but has evolved a function that blocks apoptosis induced by infection as well as by other means. Specifically, (i) Vero cells infected with a HSV-1 mutant deleted in the regulatory gene alpha 4 (that encodes repressor and transactivating functions), but not those infected with wild-type HSV-1(F), exhibit cytoplasmic blebbing, chromatin condensation, and fragmented DNA detected as a ladder in agarose gels or by labeling free DNA ends with terminal transferase; (ii) Vero cells infected with wild-type HSV-1(F) or cells expressing the alpha 4 gene and infected with the alpha 4- virus did not exhibit apoptosis; (iii) fragmentation of cellular DNA was observed in Vero cells that were mock-infected or infected with the alpha 4- virus and maintained at 39.5 degrees C, but not in cells infected with wild-type virus and maintained at the same temperature. Wild-type strains of HSV-1 with limited extrahuman passages, such as HSV-1 (F), carry a temperature-sensitive lesion in the alpha 4 gene and at 39.5 degrees C only alpha genes are expressed. These results indicate that the product of the alpha 4 gene is able to suppress apoptosis induced by the virus as well by other means.

Morphological and biochemical changes in neurons: apoptosis versus mitosis

Apoptosis and mitosis are often thought to share certain morphological similarities and therefore to be regulated by similar sets of enzymes. In this study, the Golgi apparatus and nuclear lamina were examined in PC12 cells and rat superior cervical ganglion neurons undergoing apoptosis in response to withdrawal of nerve growth factor or addition of staurosporine. We found that the Golgi apparatus disperses during apoptosis, without obvious degradation, in a manner similar to that occurring in mitosis. In contrast, the nuclear lamina did not become completely solubilized during apoptosis, as occurs in mitosis, but remained as a distinct structure around the nucleus, although some degradation of nuclear lamins was seen. To assess the integrity of the nuclear envelope, fluorescent probes were introduced into the cytoplasm of live and dying cells. High molecular weight tracers were still excluded from the nuclei of apoptotic cells, demonstrating the continued existence of a functional nuclear barrier. These data suggest, therefore, that cell death is unlikely to occur simply as a result of inappropriate activation of cell cycle enzymes.

The rise and fall of apoptosis during multistage tumorigenesis: down-modulation contributes to tumor progression from angiogenic progenitors

In a mouse model of multistage tumorigenesis of islet beta-cells, apoptosis was activated concomitant with T-antigen oncogene-induced cell proliferation, further increased in the angiogenic stage, and markedly reduced in solid tumors. Crosses to p53-null mice confirmed this stage-specific variation as a p53-independent apoptotic process. Several apoptosis regulators were expressed, of which bcl-xL was up-regulated in tumors. When overexpressed throughout the pathway, bcl-xL protected most oncogene-expressing cells from apoptosis, enhancing progression from angiogenic progenitor to tumor without affecting earlier transitions. Further, two classes of solid tumor are described, distinguished by size and apoptotic incidence, implicating apoptosis regulation in expansive tumor growth. Thus, down-modulation of apoptosis selectively contributes to late steps in a tumorigenesis pathway.

Mammalian cells resistant to tumor suppressor genes

Expression of p53 causes growth arrest or apoptosis in many normal and neoplastic cell types, but the relationship between these two effects has remained obscure. To begin to dissect the underlying mechanisms at a genetic level, we have generated mutant cells resistant to the action of wild-type p53. Rat embryo fibroblasts transformed with ras and a temperature-sensitive p53 (tsp53(135val)) gene were chemically mutagenized and selected for growth at a temperature at which p53 adopts a wild-type conformation (31.5 degrees C). Clones that grew exponentially at 31.5 degrees C were selected. Cell fusion experiments demonstrated that the mutations conferring resistance to p53-mediated growth arrest were dominant. The mutagenized clones were resistant not only to p53-mediated growth arrest, but also to the apoptosis induced by E1A in conjunction with p53, and partially resistant to the retinoblastoma tumor suppressor, pRB. The results suggest that a single downstream pathway can control the induction of growth arrest and apoptosis, and that both p53 and RB function through this pathway.

Cell growth patterns and lens geometry: a quantitative study from three-dimensional reconstructions

The growth of the lens of the sea lamprey, Petromyzon marinus, was studied over the 5 years of larval development. Whole lenses (25) and Golgi-impregnated cells (393) were reconstructed with computer-assisted microscopy. Several cellular geometric parameters (length, width, curvature, surface, volume, shape) were correlated with the position of the cell's base on the lens capsular perimeter. Based on these correlations, the cells formed four groups that correspond to the central anterior, germinative, transitional and cortical fiber zones. A fifth zone, containing nuclear fiber cells, never stained. Lens growth is exponential during the 5 years. The anterior epithelium increases in size and in cell number by cell growth and division. The posterior mass increases in cell number by recruitment and increases in size by cell growth. A model is proposed to account for the size and shape of the lens based upon the coupling of anterior and posterior growth patterns. Four zonal boundaries are defined by changes in cell growth patterns. With growth, cells are subsumed into adjacent zones and zonal boundaries move away from the lens center. We find no support for the suggestion that cells migrate centrally.

Interference with p53 protein inhibits hematopoietic and muscle differentiation

The involvement of p53 protein in cell differentiation has been recently suggested by some observations made with tumor cells and the correlation found between differentiation and increased levels of p53. However, the effect of p53 on differentiation is in apparent contrast with the normal development of p53-null mice. To test directly whether p53 has a function in cell differentiation, we interfered with the endogenous wt-p53 protein of nontransformed cells of two different murine histotypes: 32D myeloid progenitors, and C2C12 myoblasts. A drastic inhibition of terminal differentiation into granulocytes or myotubes, respectively, was observed upon expression of dominant-negative p53 proteins. This inhibition did not alter the cell cycle withdrawal typical of terminal differentiation, nor p21(WAF1/CIP1) upregulation, indicating that interference with endogenous p53 directly affects cell differentiation, independently of the p53 activity on the cell cycle. We also found that the endogenous wt-p53 protein of C2C12 cells becomes transcriptionally active during myogenesis, and this activity is inhibited by p53 dominant-negative expression. Moreover, we found that p53 DNA-binding and transcriptional activities are both required to induce differentiation in p53-negative K562 cells. Taken together, these data strongly indicate that p53 is a regulator of cell differentiation and it exerts this role, at least in part, through its transcriptional activity.

Lessons from the p53 mutant mouse

The use of the mouse as a model organism in cancer research has a long and productive history, from the earliest studies of chemical carcinogenesis to the recent advances in gene targeting. Many of the basic principles of tumorigenesis have been formed in whole or in part through the study of tumor development in the mouse. Over the past decade, the major experimental approach has been to generate cancer-prone strains, either through transgenic technologies or, more recently, gene targeting. Here, I will review the state of the field of gene targeting of tumor-suppressor genes and concentrate on the p53 mutant strains and the lessons learned from the p53 mutant mouse.

p53: puzzle and paradigm

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Serum- and calcium-induced differentiation of human keratinocytes is inhibited by the E6 oncoprotein of human papillomavirus type 16

Transfection of the E6 and E7 genes of the high-risk human papillomaviruses (HPVs) into primary genital keratinocytes generates colonies of proliferating cells which are resistant to calcium- and serum-induced terminal differentiation. To genetically map the HPV gene(s) responsible for this cellular phenotype, we cloned cDNAs for full-length E6, full-length E7, four truncated E6 splice variants (E6I to E6IV), and a series of E6 C-terminal truncation mutants into a simian virus 40 expression vector. The E6 proteins were tagged with the AU1 epitope at the C terminus to verify their expression in COS cells by immunoprecipitation and immunofluorescence. Transfection of the full-length E6 protein, either wild type or epitope tagged, induced calcium- and serum-resistant keratinocyte colonies, but the truncated E6 variants and full-length E7 protein did not. E6-induced colonies, while altered in response to serum and calcium, could not be established into cell lines without the combined presence of the E7 protein, further exemplifying the independent roles of E6 and E7 in cell differentiation and cell proliferation. The E6 C-terminal deletion mutants defined two distinct functional domains between amino acids 120 and 151. Amino acids 120 to 151 contained an apparent bipartite nuclear localization signal, whereas amino acids 132 to 141 were required for the induction of resistance to calcium- and serum-induced differentiation and for immortalization of human keratinocytes in conjunction with E7.

Bax and Bcl-xs are induced at the onset of apoptosis in involuting mammary epithelial cells

Mammary gland involution is a physiological process in which the entire organ is remodeled through the process of apoptosis. Apoptosis of secretory alveolar cells is initiated at the time of weaning, followed by the collapse and disappearance of the entire lobuloalveolar compartment. While apoptotic figures were rare in mammary epithelium of lactating mice, their number increased after weaning and reached a maximum on day 3 of involution. Active cell death continued until day 5 after weaning and only little parenchyma remained on day 8, when remodeling of the gland was completed. Bax mRNA levels increased during the first day of involution independent of the presence or absence of p53. Bax protein was detected in an increasing number of cells after weaning, peaking at day 3 and decreasing thereafter. Low levels of bcl-x mRNA and protein were present during lactation, followed by a sharp increase during the first 2 days of involution. The bcl-xS splice variant of bcl-x can promote cell death, and bcl-xL has a protective function in cell culture. The ratio of bcl-xS versus bcl-xL remained stable in the virgin, pregnant and lactating gland. However, during the first 2 days of involution, bcl-xS expression increased six-fold more than bcl-xL. To further evaluate the role of Bcl-xS which was less abundant in the mammary cells than Bcl-xL, cotransfection studies were performed in cell culture. They confirmed that Bcl-xS protein can facilitate apoptosis even when Bcl-xL is present in excess. These findings point to a significant role for Bax and Bcl-xS in the regulation of apoptosis of secretory alveolar cells during involution.

Oxidative stress increases production of beta-amyloid precursor protein and beta-amyloid (Abeta) in mammalian lenses, and Abeta has toxic effects on lens epithelial cells

Many amyloid diseases are characterized by protein aggregations linked to oxidative stress. Such diseases including those of the brain, muscle, and blood vessels exhibit plaques containing beta-amyloid (Abeta). Here we demonstrate that Alzheimer's precursor protein (betaAPP) and A beta are present at low levels in normal lenses and increase in intact cultured monkey lenses treated with H2O2 or UV radiation (known cataractogenic agents), and with phorbol 12-myristate 13-acetate. AP-1 factor binding, shown by others to up-regulate betaAPP expression, increased in the monkey lenses treated with H2O2, UV radiation, or phorbol 12-myristate 13-acetate and paralleled the increase in betaAPP expression. Rat lenses exposed to oxidative stress showed increased betaAPP in the anterior epithelium and cortex. Incubation of cultured rabbit lens N/N1003A epithelial cells with Abeta induced inclusions and vacuoles and was cytotoxic. Abeta cross-reacting protein was readily detected in the cortex of a cataractous human lens. Our data show that betaAPP and Abeta increase in mammalian lenses as part of a response to H2O2 or UV radiation and suggest that they may contribute to the mechanism by which oxidative damage leads to lens opacification.

Chronic estrogen-induced cervical and vaginal squamous carcinogenesis in human papillomavirus type 16 transgenic mice

High-risk human papillomaviruses (HPVs), including type 16, have been identified as factors in cervical carcinogenesis. However, the presence and expression of the virus per se appear to be insufficient for carcinogenesis. Rather, cofactors most likely are necessary in addition to viral gene expression to initiate neoplasia. One candidate cofactor is prolonged exposure to sex hormones. To examine the possible effects of estrogen on HPV-associated neoplasia, we treated transgenic mice expressing the oncogenes of HPV16 under control of the human keratin-14 promoter (K14-HPV16 transgenic mice) and nontransgenic control mice with slow release pellets of 17beta-estradiol. Squamous carcinomas developed in a multistage pathway exclusively in the vagina and cervix of K14-HPV16 transgenic mice. Estrogen-induced carcinogenesis was accompanied by an incremental increase in the incidence and distribution of proliferating cells solely within the cervical and vaginal squamous epithelium of K14-HPV16 mice. Expression of the HPV transgenes in untreated transgenic mice was detectable only during estrus; estrogen treatment resulted in transgene expression that was persistent but not further upregulated, remaining at low levels at all stages of carcinogenesis. The data demonstrate a novel mechanism of synergistic cooperation between chronic estrogen exposure and the oncogenes of HPV16 that coordinates squamous carcinogenesis in the female reproductive tract of K14-HPV16 transgenic mice.

Induction of DNA synthesis and apoptosis in cardiac myocytes by E1A oncoprotein

Beginning during the second half of gestation, increasing numbers of cardiac myocytes withdraw from the cell cycle such that DNA synthesis is no longer detectable in these cells by neonatal day 17 in vivo. The mechanisms that exclude these and other terminally differentiated cells from the cell division cycle are poorly understood. To begin to explore the molecular basis of the barrier to G1/S progression in cardiac myocytes, we used adenoviruses to express wild-type and mutant E1A proteins in primary cultures from embryonic day 20 rats. While most of these cardiac myocytes are ordinarily refractory to DNA synthesis, even in the presence of serum growth factors, expression of wild-type E1A stimulates DNA synthesis in up to 94% or almost all successfully transduced cells. Rather than complete the cell cycle, however, these cells undergo apoptosis. Apoptosis is limited to those cells that engage in DNA synthesis, and the kinetics of the two processes suggest that DNA synthesis precedes apoptosis. Mutations in E1A that disable it from binding Rb and related pocket proteins have little effect on its ability to stimulate DNA synthesis in cardiac myocytes. In contrast, mutants that are defective in binding the cellular protein p300 stimulate DNA synthesis 2.4-4.1-fold less efficiently, even in the context of retained E1A pocket protein binding. In the absence of ElA pocket protein binding, the usual situation in the cell, loss of p300 binding severely decreases the ability of ElA to stimulate DNA synthesis. These results suggest that the barrier to G1/S progression in cardiac myocytes is mediated. at least in part, by the same molecules that gate the G1/S transition in actively cycling cells, and that p300 or related family members play an important role in this process.

Tissue-specific inactivation of p53 tumor suppression in the mouse

The p53 gene is the most frequent target of structural and functional genetic mutations in human cancer. Thus, considerable effort has been devoted to mapping the functional domains of p53 with regard to their impact on tumorigenesis in vivo. Studies have shown that the carboxy-terminal domain of p53 is sufficient for transformation in vitro. To determine whether a transdominant-negative p53 protein could be used to elicit a tissue-specific p53-null effect in vivo, we tested whether a carboxy-terminal p53 fragment (amino acids 302-390) could abolish p53-dependent apoptosis in an established tumor progression model. We showed previously that loss of p53-dependent apoptosis accelerates brain tumorigenesis in a transgenic mouse model. Here, we show that the same effect can be elicited by expressing a dominant-negative p53 protein tissue specifically in the presence of wild-type p53. Transgenic mice in which pRb function has been disrupted and that coexpress a p53 carboxy-terminal dominant-negative fragment (p53DD) develop aggressive brain tumors mimicking genetic loss of p53 in this model. Inactivation of endogenous p53, which we show to be complexed with p53DD, results in a reduction in apoptosis and acceleration of tumorigenesis. These studies establish a mechanism for tissue-specific knock out of p53 function in vivo.

Bcl-x(S) anatagonizes the protective effects of Bcl-x(L)

Bcl-x, a member of the Bcl-2 family, has two alternatively spliced forms, Bcl-x(L) and Bcl-x(S). Bcl-x(L), like Bcl-2, is able to protect cells from a wide variety of apoptotic stimuli. Bcl-x(S), as a result of alternative splicing, lacks 63 amino acids that comprise the region of greatest amino acid identity between Bcl-x(L) and Bcl-2. These amino acids contain the highly conserved BH1 and BH2 regions, which have been used to define the Bcl-2 family. We show that both Bel-x(L) and Bcl-x(S) are able to regulate cell survival in a dose-dependent fashion. Bcl-x(L) is able to increase the cellular apoptotic threshold and is able to form stable complexes with Bax both in vitro and in vivo. In contrast, Bcl-x(S) can effectively inhibit the protective effects of Bcl-x(L) following growth factor withdrawal and chemotherapeutic drug treatment. However, compared with Bax, Bcl-x(S) binds to Bcl-x(L) weakly when assessed by in vitro binding assays. Coimmunoprecipitation from mammalian cells demonstrates that Bcl-x(S) does not show an observable ability to form heterodimers with other Bcl-2 family members. In addition, overexpression of Bel-x(S) does not alter the ability of Bax to heterodimerize with Bcl-x(L) in vivo. Thus, Bcl-x(S) does not appear to function by competitively disrupting the formation of dimers composed of other Bcl-2 family members. This suggests that Bcl-x(S) can enhance cellular sensitivity to apoptosis via a mechanism of action distinct from other Bc1-2 family members that promote apoptosis.

Human papillomavirus type 16 (HPV 16) E7 and major histocompatibility complex (MHC) class I and II expression in human keratinocytes in culture

The low expression of major histocompatibility complex (MHC) class I antigens on human papillomavirus (HPV)-infected cervical carcinoma cells may be responsible for an insufficient cytotoxic T cell response against these cells. To investigate in vitro whether the HPV type 16 early gene product E7 influences cell surface expression of MHC class I and II molecules the HPV negative keratinocyte cell line HaCaT was either stably transfected with the E7 gene or infected with E7-recombinant vaccinia viruses. No difference in MHC class I transcription was detected between E7-transfected and untransfected HaCaT cells. MHC class I cell surface expression as determined by FACS analysis was stronger in some of the transfectants and less intensive in others when compared to untransfected HaCaT cells. In wildtype as well as in E7-recombinant vaccinia virus infected HaCaT cells downregulation of MHC class I molecules on protein and transcriptional level was observed. The alterations in MHC class I expression were independent of the presence and amount of E7-specific transcripts. None of the transfectants or infected HaCaT cells had MHC class II molecules on their cell surface. Hence, our data did not show a correlation between HPV 16 E7 and MHC expression in vitro.

Squamous epithelial hyperplasia and carcinoma in mice transgenic for the human papillomavirus type 16 E7 oncogene

The human papillomavirus type 16 (HPV-16) genome is commonly present in human cervical carcinoma, in which a subset of the viral genes, E6 and E7, are expressed. The HPV-16 E6 and E7 gene products can associated with and inactivate the tumor suppressor proteins p53 and Rb (the retinoblastoma susceptibility gene product), and in tissue culture cells, these viral genes display oncogenic properties. These findings have led to the hypothesis that E6 and E7 contribute to cervical carcinogenesis. This hypothesis has recently been tested by using transgenic mice as an animal model. HPV-16 E6 and E7 together were found to induce cancers in multiple tissues in which they were expressed, including squamous cell carcinoma, the cancer type most commonly associated with HPV-16 in the human cervix. We have extended these studies to investigate the in vivo activities of HPV-16 E7 when expressed in squamous epithelia of transgenic mice. Grossly, E7 transgenic mice had multiple phenotypes, including wrinkled skin that was apparent prior to the appearance of hair on neonates, thickened ears, and loss of hair in adults. In lines of mice expressing higher levels of E7, we observed stunted growth and mortality at an early age, potentially caused by an incapacity to feed. Histological analysis demonstrated that E7 causes epidermal hyperplasia in multiple transgenic lineages with high penetrance. This epithelial hyperplasia was characterized by an expansion of the proliferating compartment and an expansion of the keratin 10-positive layer of cells and was associated with hyperkeratosis. Hyperplasia was found at multiple sites in the animals in addition to the skin, including the mouth palate, esophagus, forestomach, and exocervix. In multiple transgenic lineages, adult animals developed skin tumors late in life with low penetrance. These tumors arose from the squamous epithelia and from sebaceous glands and were characterized histologically to be highly differentiated, locally invasive, and aggressive in their growth properties. On the basis of these phenotypes, we conclude that HPV-16 E7 can alter epithelial cell growth parameters sufficiently to potentiate tumorigenesis in mice.

Bcl-2 inhibits retinoic acid-induced apoptosis during the neural differentiation of embryonal stem cells

We report here that all trans-retinoic acid (RA), a classical morphogen, induces apoptosis during the neural differentiation of the embryonic stem cell line P19. The apoptotic cells showed, in addition to DNA cleavage, typical morphological changes including chromatin condensation, nuclear fragmentation, and cytoplasmic vacuolation. These apoptotic changes became obvious by 12 h after the addition of RA. The endogenous expression of bcl-2 in surviving cells was down-regulated during this process, and the compelled expression of bcl-2 by retroviral vectors reduced the number of apoptotic cells. Apoptosis was partially inhibited by adding antisense oligonucleotides against RA receptors (RARs) simultaneously or by transfecting a plasmid vector flanked with a RA-responsive element. Antisense oligonucleotides against retinoid X receptors (RXRs), the receptors for 9 cis-RA, did not inhibit apoptosis induced by all trans-RA. Cycloheximide and actinomycin D, inhibitors of protein and RNA syntheses, respectively, suppressed apoptosis. No changes were seen in the expression of tumor necrosis factors, their receptors, Fas, FasL, p53, or c-myc, molecules which have been suggested to participate in the apoptotic process. Addition of neurotrophins to the culture medium did not affect apoptosis. These findings suggest that the signals themselves, promote expression of molecules essential for apoptosis. Furthermore, we observed that RA induced apoptosis of cerebral neurons from murine embryos in primary culture, which suggests that RA might participate in cell death which occurs during neural development.

HIV type 1 Nef perturbs eye lens development in transgenic mice

HIV transgenic mice often display lens cataracts as a consequence of viral-specific expression of HIV gene products in the developing lens. Cataractous mouse lines encoding either HIV-1 proviral DNA, HIV delta Gag/Pol] proviral DNA, or the HIV-1 nef gene alone were examined to ascertain the effect of Nef on murine lens development. Ocular disease was characterized by a progressive architectural disorganization within the lens fiber cell compartment developing in 100% of HIV-positive mice in five reported transgenic lines. Late embryonic stage transgenic lenses featured a mild microphthalmia, pyknotic nuclei within the lens fiber department, ballooning lens fiber cells, and elongated lens epithelial cells. Increased DNA fragmentation was evident in transgenic embryonic lenses, suggesting that cell death occurred by apoptosis. As studied in HIV delta Gag/Pol] transgenic mice, HIV transcription was developmentally linked to alpha A- and alpha B-crystallin gene expression, preceded disease development (in E14.5-E16.5 embryos), and persisted for weeks after birth. HIV-1 Nef was the predominant HIV gene product detected in the lens fiber cells of this line and was expressed almost to the exclusion of other HIV gene products. Nef was implicated as a major determinant of disease because (1) cataracts developed in mice transgenic for Nef alone and (2) the expression of other HIV gene products in wild-type HIV provirus transgenic mice occurred without a concomitant change in lens pathology.

Lens complementation system for the genetic analysis of growth, differentiation, and apoptosis in vivo

A genetic approach has been established that combines the advantages of blastocyst complementation with the experimental attributes of the developing lens for the functional analysis of genes governing cellular proliferation, terminal differentiation, and apoptosis. This lens complementation system (LCS) makes use of a mutant mouse strain, aphakia (ak), homozygotes of which fail to develop an ocular lens. We demonstrate that microinjection of wild-type embryonic stem (ES) cells into ak/ak blastocysts produces chimeras with normal ES-cell-derived lenses and that microinjection of Rb-/- ES cells generates an aberrant lens phenotype identical to that obtained through conventional gene targeting methodology. Our determination that a cell autonomous defect underlies the aphakia condition assures that lenses generated through LCS are necessarily ES-cell-derived. LCS provides for the rapid phenotypic analysis of loss-of-function mutations, circumvents the need for germ-line transmission of null alleles, and, most significantly, facilitates the study of essential genes whose inactivation is associated with early lethal phenotypes.

Mechanisms of carcinogenesis and the mutant mouse

Gene targeting in embryonic stem cells has allowed the creation of several new mouse strains carrying mutations in the murine homologs of human tumor suppressor genes. The phenotypes of these mutant strains are reviewed. In addition, recent experiments utilizing the ApcMin and p53 mutant animals to address specific mechanisms of carcinogenesis are discussed.

p53 in signaling checkpoint arrest or apoptosis

The cell cycle arrest and apoptotic functions of p53 both contribute to the role of this tumour suppressor protein in preventing replication of cells suffering DNA damage. Although the ability of p53 to function as a sequence-specific transcription factor appears to be directly and causally linked to the implementation of an arrest at the G1 stage of the cell cycle, the contribution of transcriptional activation to the apoptotic response is less clear. It seems likely that several p53 activities, both transcriptionally dependent and transcriptionally independent, can play a role in mediating cell death. The requirement for each of these functions appears to depend on the cell type, the cell environment and other genetic alterations already sustained by the cell in which p53 function is activated.